time travel string theory

Stephen Hawking’s final book suggests time travel may one day be possible – here’s what to make of it

Research Fellow in the Particle Cosmology Group, School of Physics and Astronomy, University of Nottingham

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“If one made a research grant application to work on time travel it would be dismissed immediately,” writes the physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions . He was right. But he was also right that asking whether time travel is possible is a “very serious question” that can still be approached scientifically.

Arguing that our current understanding cannot rule it out, Hawking, it seems, was cautiously optimistic. So where does this leave us? We cannot build a time machine today, but could we in the future?

Let’s start with our everyday experience. We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now . But this is something we can never actually know. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us.

Our inability to access the “now” of someone far away is at the heart of Albert Einstein’s theories of space and time .

Light speed

Einstein told us that space and time are parts of one thing – spacetime – and that we should be as willing to think about distances in time as we are distances in space. As odd as this might sound, we happily answer “about two and half hours”, when someone asks how far Birmingham is from London. What we mean is that the journey takes that long at an average speed of 50 miles per hour.

Mathematically, our statement is equivalent to saying that Birmingham is about 125 miles from London. As physicists Brian Cox and Jeff Forshaw write in their book Why does E=mc²? , time and distance “can be interchanged using something that has the currency of a speed”. Einstein’s intellectual leap was to suppose that the exchange rate from a time to a distance in spacetime is universal – and it is the speed of light.

The speed of light is the fastest any signal can travel, putting a fundamental limit on how soon we can know what is going on elsewhere in the universe. This gives us “causality” – the law that effects must always come after their causes. It is a serious theoretical thorn in the side of time-travelling protagonists. For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth).

Now, if the speed of light is universal (in the vacuum of empty space), we must measure it to be the same – 299,792,458 metres per second – however fast we ourselves are moving. Einstein realised that the consequence of the speed of light being absolute is that space and time itself cannot be. And it turns out that moving clocks must tick slower than stationary ones.

If I were to fly off at incredible speed in a spaceship and return to Earth , less time would pass for me than it would for everyone I left behind. Everyone I returned to would conclude that my life had run as if in slow motion – I would have aged more slowly than them – and I would conclude that theirs had run as if in fast forward. The faster I travelled, the slower my clock would tick relative to clocks on Earth. And if I made the trip at the speed of light, I would return as if I had been frozen in time.

So what if we were to travel faster than light, would time run backwards as science fiction has taught us?

Unfortunately, it takes infinite energy to accelerate a human being to the speed of light, let alone beyond it. But even if we could , time wouldn’t simply run backwards. Instead, it would no longer make sense to talk about forward and backward at all. The law of causality would be violated and the concept of cause and effect would lose its meaning.

Einstein also told us that the force of gravity is a consequence of the way mass warps space and time . The more mass we squeeze into a region of space, the more spacetime is warped and the slower nearby clocks tick. If we squeeze in enough mass, spacetime becomes so warped that even light cannot escape its gravitational pull and a black hole is formed. And if you were to approach the edge of the black hole – its event horizon – your clock would tick infinitely slowly relative to those far away from it.

So could we warp spacetime in just the right way to close it back on itself and travel back in time?

The answer is maybe, and the warping we need is a traversable wormhole . But we also need to produce regions of negative energy density to stabilise it, and the classical physics of the 19th century prevents this. The modern theory of quantum mechanics , however, might not.

According to quantum mechanics, empty space is not empty. Instead, it is filled with pairs of particles that pop in and out of existence. If we can make a region where fewer pairs are allowed to pop in and out than everywhere else, then this region will have negative energy density.

However, finding a consistent theory that combines quantum mechanics with Einstein’s theory of gravity remains one of the biggest challenges in theoretical physics. One candidate, string theory (more precisely M-theory ) may offer up another possibility.

M-theory requires spacetime to have 11 dimensions: the one of time and three of space that we move in and seven more, curled up invisibly small. Could we use these extra spatial dimensions to shortcut space and time? Hawking, at least, was hopeful.

Saving history

So is time travel really a possibility? Our current understanding can’t rule it out, but the answer is probably no.

Einstein’s theories fail to describe the structure of spacetime at incredibly small scales. And while the laws of nature can often be completely at odds with our everyday experience, they are always self-consistent – leaving little room for the paradoxes that abound when we mess with cause and effect in science fiction’s take on time travel.

Despite his playful optimism, Hawking recognised that the undiscovered laws of physics that will one day supersede Einstein’s may conspire to prevent large objects like you and I from hopping casually (not causally) back and forth through time. We call this legacy his “ chronology protection conjecture ”.

Whether or not the future has time machines in store, we can comfort ourselves with the knowledge that when we climb a mountain or speed along in our cars, we change how time ticks.

So, this “ pretend to be a time traveller day ” (December 8), remember that you already are, just not in the way you might hope.

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Science News

Can time travel survive a theory of everything.

It’s complicated. But studying ways to visit the past could help us understand the cosmos

By Tom Siegfried

Contributing Correspondent

September 20, 2019 at 8:00 am

The TARDIS and other fictional time machines make time travel look relatively easy (Peter Capaldi and Jenna Coleman are shown promoting the BBC’s Doctor Who ). But just because it’s permitted by the laws of physics, doesn’t mean it’s practical.

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In many universes, typically those on TV shows or in movies, time travel is not much more difficult than driving downtown in any major city during rush hour. Sure, the traffic can get gnarly, but no law of physics prevents you from reaching your destination eventually.

In real life, time travel isn’t so easy. In fact, it’s probably impossible, a fantasy more farfetched than visiting Alice’s Wonderland, finding gold at the end of a rainbow or cleansing all the hate speech off of Facebook.

Yet time travel does not necessarily violate the laws of physics. In Einstein’s theory of gravity — general relativity — space and time are merged as spacetime, which allows for the possibility of pathways that could bend back to the past and loop back to the future.

Such paths are known as closed timelike curves. They’re a little like great circles around the surface of the Earth — if you start out in one direction and keep going straight, eventually you come back to where you started from. In that case the Earth’s curvature guides you back to your previous point in space; with closed timelike curves, the geometry of spacetime guides you back to an earlier moment in time.

Nobody thinks that general relativity’s time loops would be practical for time travel even if they are possible. For one thing, they might exist only under certain circumstances — the universe would have to be rotating, and not expanding — as the mathematician Kurt Gödel showed in the 1940s. But the universe is expanding, and probably isn’t rotating, so that dampens the prospects for revisiting the Stone Age or acquiring a pet dinosaur.

Besides, even if such pathways did exist, building a ship to traverse them would cost more than all the DeLoreans (and all other transportation vehicles) ever made. It would need a cruising speed of 140,000 miles per second. And with no place to stop for gas (or whatever), the fuel tank would have to be more than a trillion times the size of an oil tanker.

So for practical purposes, time travel’s time has not yet arrived. But even if it’s possible only in principle, the potential ramifications for the basic physics of the universe might make it worth the time to investigate it. Time loops might not enable you to traverse the cosmos in a TARDIS, but perhaps could still help you understand the cosmos more deeply.

A first step would be to attempt to figure out exactly what the relevant laws of physics really are. Einstein’s general relativity is great, but indubitably not the last word about the physics of the universe. After all, it coexists uneasily with quantum mechanics, which rules the subatomic world and presumably, since everything is made of subatomic stuff, the rest of the universe as well. Whether the quantum–general relativity combo truly permits time travel might depend on what the ultimate correct theory combining the two turns out to be.

Several candidate theories have been developed for merging general relativity and quantum mechanics into a unified theory. It’s an open question whether these candidates would allow time travel in something like the way general relativity does, philosopher Christian Wüthrich of the University of Geneva notes in a new paper.

It’s possible, he says, that a theory that supersedes general relativity might still in some way include the equivalent of general relativity’s timelike loops. And even if the basic theory does not include such loops, they still might emerge in practice.

“Although the fundamental theory would then remain inhospitable to time travel itself, it would tolerate the possibility of time travel at some other, less fundamental, scale,” Wüthrich writes in his paper , posted online in June. “Depending on what the relationship between the fundamental theory and emergent spacetime may be in each case, we may find that the emergent, macroscopic spacetime structure permits time travel.”

Reviewing the major proposals for quantum gravity theories does not provide a lot of hope, though. One approach, known as causal set theory, requires sets of events to be ordered in a proper cause-and-effect relationship. So its central idea seems to rule out closed timelike curves.

Another popular approach, known as loop quantum gravity, envisions space to be constructed of fundamental loops (kind of like “atoms of space”). This view has encountered technical difficulties, one of which is how to work time into the picture with space. “Thus, we seem to be faced with a temporally innocuous structure in which no meaningful sense of time travel is permitted,” Wüthrich writes.

It’s possible that the networks of these “atoms of space” could produce high-level spacetime that did contain closed timelike curves. But analysis of the details at this stage of loop quantum gravity’s development does not offer much reason for optimism, Wüthrich concludes.

Time travel’s future might look a little brighter if the correct approach to quantum gravity turns out to be string theory, currently the most popular contender. In string theory, matter’s basic particles are tiny vibrating snippets of energy, called “strings” because they extend in one dimension. Multiple versions of string theory have been constructed, suggesting that they are different manifestations of a more fundamental master theory known as M-theory.

“As M-theory does not yet exist, it is impossible to determine its verdict on time travel,” writes Wüthrich. But investigations of various string theory scenarios do suggest that the ultimate theory would, in fact, naturally incorporate closed timelike curves.

Even if time loops exist in the fundamental theory, though, there’s still no guarantee that they would be preserved in the emergent large-scale spacetime that would be relevant in real life. For that matter, Wüthrich points out, predicting the existence of time travel loops might be taken as evidence against the theory, considering the serious likelihood that time travel really isn’t possible at all.

So whether general relativity’s time loops will survive in a deeper theory remains an open question. “A more fundamental theory may well admit structures amounting to closed timelike curves and thus permit time travel,” Wüthrich asserts. “This clearly remains a live option at the present stage of knowledge.”

In any case, investigating whether quantum gravity theories retain general relativity’s time travel loophole can illuminate many tough questions that must be answered to develop a successful theory and understand how it relates to general relativity. “For this reason alone,” Wüthrich writes, “the question of time travel beyond general relativity is worth our while.”

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What is string theory? An astrophysicist explains one way the universe might work at the fundamental level

Science What is string theory? An astrophysicist explains one way the universe might work at the fundamental level

What is the stuff in the universe made of?

Our high-school physics tells us atoms, with 90 natural elements building the world around us. But what are these atoms made of?

Over the last century, we've ripped atoms apart, eventually finding electrons and quarks, the fundamental building blocks of stuff. But are they really fundamental? Have we truly reached the bottom?

Not according to string theory.

String theory is the idea that everything in the universe, every particle of light and matter, is comprised of miniscule vibrating strings.

These strings are truly tiny, many billions of times smaller than an individual proton within an atomic nucleus.

And they vibrate at countless billions of times per second in ten dimensions of space, or maybe eleven. Or it might be twenty-six dimensions.

Why would physicists, a seemingly sensible bunch, think this is the way the universe operates?

To understand why, we need to step back more than 100 years.

Two beautiful, incompatible theories

At the end of the 1800s, physics was riding high. Scientists thought they understood gravity, electricity, magnetism, heat and gases.

In 1900, Lord Kelvin apparently retorted that "there is nothing new to be discovered in physics" .

But soon after the dawn of the 20th century, this cosy situation began to fall apart.

Einstein rewrote the very notions of space and time with his special and general theories of relativity, whilst Planck, Bohr and Heisenberg revealed that the world of the very small obeyed the apparently nonsensical rules of quantum mechanics.

By the mid-1900s, our view of the universe had been utterly revolutionised. And these new strange physical theories were yielding incredibly accurate predictions for experimental tests.

But despite this success, physicists were unhappy.

The problem was with forces. It was realised that there were four fundamental forces that underpin the universe: gravity, electromagnetism, strong nuclear force and weak nuclear force.

Of these, the strong and weak forces only operate at subatomic scales. At a larger scale, the rest of the universe is a never-ending battle between gravity and electromagnetism.

Gravity is described by Einstein's beautiful mathematics of curved space and time. But the other three forces are written in the language of quantum mechanics. And these two methods are completely incompatible.

This situation frustrates physicists as they have to remember two independent sets of mathematics to describe the physics of the universe.

It also worries them, as they are unable to describe physical processes in situations when the fundamental forces are battling for dominance, such as at the birth of the universe or in the centre of black holes.

So, they have searched for one set of mathematics to describe all of the forces, a Grand Unified Theory that will mean that there is less to remember.

The search for this Grand Unified Theory is not new, and many have tried and failed. Einstein was searching for a way to unify gravity and electromagnetism until his dying days.

This search for grand unification brings us to string theory.

Vibrating strings each playing a cosmic note

Like a lot of science, the birth of string theory was messy.

It was born in the post-war explosion in particle physics which led to the discovery that the universe appeared to be built of a small family of fundamental particles — quarks, leptons and force-carrying bosons.

This helped make sense of the ever-growing zoo of particles flung out of high-powered accelerators, but physicists asked whether the apparently fundamental quarks, leptons and bosons were themselves made of similar stuff?

Scrabbling within the mathematics, physicists started to find similarities in the particles, representing them as one-dimensional loops of stringy-stuff.

Different vibrations of this stringy stuff correspond to each of the fundamental particles; one note played on a fundamental string is an electron, another note is a quark, another is a photon, the particle of light.

The strings themselves are not made of anything smaller —they are the true fundamental pieces of the universe.

But the mathematics of string theory is a little strange, and in putting the pieces together, physicists needed to add more and more dimensions of space to make their theories work, many more than the three we experience in our everyday lives.

If string theory is correct, more convoluted mathematical trickery is required to hide these extra dimensions from us.

String theorists are built of stern stuff and working with complex vibrations in multiple dimensions didn't daunt them.

With its simplicity as the underlying idea that can explain everything in the universe, string theory has proven very seductive.

Since its crystallisation in the 1980s, it has continued to grow and evolve into a group of ideas — known as "M-Theory" — although no-one seems to know what M stands for.

M-theorists are confident that they are on the correct road towards grand unification, and soon will be able to pull together all of the individual threads and declare victory over the fundamental forces.

Mathematically elegant, but impossible to prove

Not everyone is convinced.

Whilst scientific journals contain many thousands and thousands of pages of mathematical investigations of string theory, they all lack one important thing — predictions that can be tested through experiments.

To probe the tiny scale of strings we would need to build a huge version of the Large Hadron Collider — at least as large as our Milky Way galaxy, if not the observable universe.

As you can imagine, unless something radically changes in the mathematics of string theory, experimental verification will remain forever out of reach.

Again, string theorists remain undaunted. Even if we cannot test the theory, they say, we should continue our efforts on M-theory as the idea is so beautiful, it just cannot be wrong.

With a bit more effort, they say, we will hold in our hands the theory to describe everything.

But to others, this is going too far. Science without predictions is not science, it's simply mathematics.

And if string theory is never tested against nature, then it will never be science.

String theory has made other physicists grumpy, with its fame overshadowing their own search for a Grand Unified Theory

But their approaches can seem as strange and weird as string theory, with ideas such as " Loop Quantum Gravity " which suggests even space and time are built from fundamental bits.

These physicists feel that the theorists on the string theory bandwagon are heading down a dead-end path in the search for ultimate physics.

Is string theory, or its many descendants in " M-theory ", on the right track towards grand unification?

In truth, we simply don't know. We don't know if any of our ideas are truly inching a way towards the ultimate theory, or if a completely different approach is required.

In fact, we simply don't know if a grand unified theory even exists.

But this will not stop physicists from continuing their search.

Professor Geraint Lewis is an astrophysicist at the University of Sydney with expertise in dark energy, gravitational lensing and galactic cannibalism.

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Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second. 

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town. 

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines). 

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth. 

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more. 

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line. 

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space. 

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference. 

Read more: Can we stop time?

The Global Positioning System , or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles (20,100 kilometers) away, moving at 8,700 mph (14,000 km/h). 

According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to the American Physical Society publication Physics Central .  

Read more: Could Star Trek's faster-than-light warp drive actually work?

Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds. 

This means that in order to maintain the accuracy needed to pinpoint your car or phone — or, since the system is run by the U.S. Department of Defense, a military drone — engineers must account for an extra 38 microseconds in each satellite's day. The atomic clocks onboard don’t tick over to the next day until they have run 38 microseconds longer than comparable clocks on Earth.

Given those numbers, it would take more than seven years for the atomic clock in a GPS satellite to un-sync itself from an Earth clock by more than a blink of an eye. (We did the math: If you estimate a blink to last at least 100,000 microseconds, as the Harvard Database of Useful Biological Numbers does, it would take thousands of days for those 38 microsecond shifts to add up.) 

This kind of time travel may seem as negligible as the Kelly brothers' age gap, but given the hyper-accuracy of modern GPS technology, it actually does matter. If it can communicate with the satellites whizzing overhead, your phone can nail down your location in space and time with incredible accuracy. 

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake. 

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life. 

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported . 

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute. 

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .  

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements. 

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades. 

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff." 

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over. 

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

• Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
• A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
• The Time Machine (William Heinemann, 1895) by H. G. Wells
• A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
• The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
• A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
• The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
• Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
• Thief of Time (Doubleday, 2001) by Terry Pratchett
• The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
• All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

• Planet of the Apes (1968)
• Superman (1978)
• Time Bandits (1981)
• The Terminator (1984)
• Back to the Future series (1985, 1989, 1990)
• Star Trek IV: The Voyage Home (1986)
• Bill & Ted's Excellent Adventure (1989)
• Groundhog Day (1993)
• Galaxy Quest (1999)
• The Butterfly Effect (2004)
• 13 Going on 30 (2004)
• The Lake House (2006)
• Meet the Robinsons (2007)
• Hot Tub Time Machine (2010)
• Midnight in Paris (2011)
• Looper (2012)
• X-Men: Days of Future Past (2014)
• Edge of Tomorrow (2014)
• Interstellar (2014)
• Doctor Strange (2016)
• A Wrinkle in Time (2018)
• The Last Sharknado: It's About Time (2018)
• Avengers: Endgame (2019)
• Tenet (2020)
• Palm Springs (2020)
• Zach Snyder's Justice League (2021)
• The Tomorrow War (2021)

Television about time travel:

• Doctor Who (1963-present)
• The Twilight Zone (1959-1964) (multiple episodes)
• Star Trek (multiple series, multiple episodes)
• Samurai Jack (2001-2004)
• Lost (2004-2010)
• Phil of the Future (2004-2006)
• Steins;Gate (2011)
• Outlander (2014-2023)
• Loki (2021-present)

Games about time travel:

• Chrono Trigger (1995)
• TimeSplitters (2000-2005)
• Kingdom Hearts (2002-2019)
• Prince of Persia: Sands of Time (2003)
• God of War II (2007)
• Ratchet and Clank Future: A Crack In Time (2009)
• Sly Cooper: Thieves in Time (2013)
• Dishonored 2 (2016)
• Titanfall 2 (2016)
• Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Get the Space.com Newsletter

Breaking space news, the latest updates on rocket launches, skywatching events and more!

Ailsa is a staff writer for How It Works magazine, where she writes science, technology, space, history and environment features. Based in the U.K., she graduated from the University of Stirling with a BA (Hons) journalism degree. Previously, Ailsa has written for Cardiff Times magazine, Psychology Now and numerous science bookazines. 

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October 24, 2014

18 min read

String Theory Predicts a Time before the Big Bang

String theory suggests that the big bang was not the origin of the universe but simply the outcome of a preexisting state

By Gabriele Veneziano

Alfred T. Kamajian

Was the big bang really the beginning of time? or did the universe exist before then? Such a question seemed almost blasphemous only decades ago. Most cosmologists insisted that it simply made no sense—that to contemplate a time before the big bang was like asking for directions to a place north of the North Pole. But developments in theoretical physics, especially the rise of string theory, have changed their perspective. The pre-bang universe has become the latest frontier of cosmology.

The new willingness to consider what might have happened before the bang is the latest swing of an intellectual pendulum that has rocked back and forth for millennia. In one form or another, the issue of the ultimate beginning has engaged philosophers and theologians in nearly every culture. It is entwined with a grand set of concerns, one famously encapsulated in an 1897 painting by Paul Gauguin: D'ou venons-nous? Que sommes-nous? Ou allons-nous? “Where do we come from? What are we? Where are we going?” The piece depicts the cycle of birth, life and death—origin, identity and destiny for each individual—and these personal concerns connect directly to cosmic ones. We can trace our lineage back through the generations, back through our animal ancestors, to early forms of life and protolife, to the elements synthesized in the primordial universe, to the amorphous energy deposited in space before that. Does our family tree extend forever backward? Or do its roots terminate? Is the cosmos as impermanent as we are?

The ancient Greeks debated the origin of time fiercely. Aristotle, taking the no-beginning side, invoked the principle that out of nothing, nothing comes. If the universe could never have gone from nothingness to somethingness, it must always have existed. For this and other reasons, time must stretch eternally into the past and future. Christian theologians tended to take the opposite point of view. Augustine contended that God exists outside of space and time, able to bring these constructs into existence as surely as he could forge other aspects of our world. When asked, “What was God doing before he created the world?” Augustine answered, “Time itself being part of God's creation, there was simply no before!”

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Einstein's general theory of relativity led modern cosmologists to much the same conclusion. The theory holds that space and time are soft, malleable entities. On the largest scales, space is naturally dynamic, expanding or contracting over time, carrying matter like driftwood on the tide. Astronomers confirmed in the 1920s that our universe is currently expanding: distant galaxies move apart from one another. One consequence, as physicists Stephen Hawking and Roger Penrose proved in the 1960s, is that time cannot extend back indefinitely. As you play cosmic history backward in time, the galaxies all come together to a single infinitesimal point, known as a singularity—almost as if they were descending into a black hole. Each galaxy or its precursor is squeezed down to zero size. Quantities such as density, temperature and spacetime curvature become infinite. The singularity is the ultimate cataclysm, beyond which our cosmic ancestry cannot extend.

The unavoidable singularity poses serious problems for cosmologists. In particular, it sits uneasily with the high degree of homogeneity and isotropy that the universe exhibits on large scales. For the cosmos to look broadly the same everywhere, some kind of communication had to pass among distant regions of space, coordinating their properties. Yet the idea of such communication contradicts the old cosmological paradigm.

Strange Coincidence

To be specific, consider what has happened over the 13.8 billion years since the release of the cosmic microwave background radiation. The distance between galaxies has grown by a factor of about 1,000 (because of the expansion), while the radius of the observable universe has grown by the much larger factor of about 100,000 (because light outpaces the expansion). We see parts of the universe today that we could not have seen 13.8 billion years ago. Indeed, this is the first time in cosmic history that light from the most distant galaxies has reached the Milky Way.

Nevertheless, the properties of the Milky Way are basically the same as those of distant galaxies. It is as though you showed up at a party only to find you were wearing exactly the same clothes as a dozen of your closest friends. If just two of you were dressed the same, it might be explained away as coincidence, but a dozen suggests that the partygoers had coordinated their attire in advance. In cosmology, the number is not a dozen but tens of thousands—the number of independent yet statistically identical patches of sky in the microwave background.

Credit: Samuel Velasco

One possibility is that all those regions of space were endowed at birth with identical properties—in other words, that the homogeneity is mere coincidence. Physicists, however, have thought about two more natural ways out of the impasse: the early universe was much smaller or much older than in standard cosmology. Either (or both, acting together) would have made intercommunication possible.

The most popular choice follows the first alternative. It postulates that the universe went through a period of accelerating expansion, known as inflation, early in its history. Before this phase, galaxies or their precursors were so closely packed that they could easily coordinate their properties. During inflation, they fell out of contact because light was unable to keep pace with the frenetic expansion. After inflation ended, the expansion began to decelerate, so galaxies gradually came back into one another's view.

Physicists ascribe the inflationary spurt to the potential energy stored in a new quantum field, the inflaton, about 10 −35 second after the big bang. Potential energy, as opposed to rest mass or kinetic energy, leads to gravitational repulsion. Rather than slowing down the expansion, as the gravitation of ordinary matter would, the inflaton accelerated it. Proposed in 1981, inflation has explained a wide variety of observations with precision. A number of possible theoretical problems remain, though, beginning with the questions of what exactly the inflaton was and what gave it such a huge initial potential energy.

A less widely known way to solve the puzzle follows the second alternative by getting rid of the singularity. If time did not begin at the bang, if a long era preceded the onset of the present cosmic expansion, matter could have had plenty of time to arrange itself smoothly. Therefore, researchers have reexamined the reasoning that led them to infer a singularity.

One of the assumptions—that relativity theory is always valid—is questionable. Close to the putative singularity, quantum effects must have been important, even dominant. Standard relativity takes no account of such effects, so accepting the inevitability of the singularity amounts to trusting the theory beyond reason. To know what really happened, physicists need to subsume relativity in a quantum theory of gravity. The task has occupied theorists from Albert Einstein onward, but progress was almost zero until the mid-1980s.

Evolution of a Revolution

Today two approaches stand out. One, going by the name of loop quantum gravity, retains Einstein's theory essentially intact but changes the procedure for implementing it in quantum mechanics [see “ Atoms of Space and Time ,” by Lee Smolin]. Practitioners of loop quantum gravity have taken great strides and achieved deep insights over the past several years. Still, their approach may not be revolutionary enough to resolve the fundamental problems of quantizing gravity. A similar problem faced particle theorists after Enrico Fermi introduced his effective theory of the weak nuclear force in 1934. All efforts to construct a quantum version of Fermi's theory failed miserably. What was needed was not a new technique but the deep modifications brought by the electroweak theory of Sheldon L. Glashow, Steven Weinberg and Abdus Salam in the late 1960s.

The second approach, which I consider to be more promising, is string theory—a truly revolutionary modification of Einstein's theory. This article will focus on it, although proponents of loop quantum gravity claim to reach many of the same conclusions.

String theory grew out of a model that I wrote down in 1968 to describe the world of nuclear particles (such as protons and neutrons) and their interactions. Despite much initial excitement, the model failed. It was abandoned several years later in favor of quantum chromodynamics, which describes nuclear particles in terms of more elementary constituents: quarks. Quarks are confined inside a proton or a neutron, as if they were tied together by elastic strings. In retrospect, the original string theory had captured those stringy aspects of the nuclear world. Only later was it revived as a candidate for combining general relativity and quantum theory.

The basic idea is that elementary particles are not pointlike but rather infinitely thin, one-dimensional objects—the strings. The large zoo of elementary particles, each with its own characteristic properties, reflects the many possible vibration patterns of a string. How can such a simple-minded theory describe the complicated world of particles and their interactions? The answer can be found in what we may call quantum string magic. Once the rules of quantum mechanics are applied to a vibrating string—just like a miniature violin string, except that the vibrations propagate along it at the speed of light—new properties appear. All have profound implications for particle physics and cosmology.

First, quantum strings have a finite size. Were it not for quantum effects, a violin string could be cut in half, cut in half again, and so on, all the way down, finally becoming a massless, pointlike particle. Heisenberg's uncertainty principle eventually intrudes, however, and prevents the lightest strings from being sliced smaller than about 10 −34 meter. This irreducible quantum of length, denoted l s , is a new constant of nature introduced by string theory side by side with the speed of light, c , and Planck's constant, h . It plays a crucial role in almost every aspect of string theory, putting a finite limit on quantities that otherwise could become either zero or infinite.

Second, quantum strings may have angular momentum even if they lack mass. In classical physics, angular momentum is a property of an object that rotates with respect to an axis. The formula for angular momentum multiplies together velocity, mass and distance from the axis; hence, a massless object can have no angular momentum. But quantum fluctuations change the situation. A tiny string can acquire up to two units of h of angular momentum without gaining any mass. This feature is very welcome because it precisely matches the properties of the carriers of all known fundamental forces, such as the photon (for electromagnetism) and the graviton (for gravity). Historically, angular momentum is what clued in physicists to the quantum-gravitational implications of string theory.

Third, quantum strings demand the existence of extra dimensions of space, in addition to the usual three. Whereas a classical violin string will vibrate no matter what the properties of space and time are, a quantum string is more finicky. The equations describing the vibration become inconsistent unless spacetime either is highly curved (in contradiction with observations) or contains six extra spatial dimensions.

Fourth, physical constants—such as Newton's and Coulomb's constants, which appear in the equations of physics and determine the properties of nature—no longer have arbitrary, fixed values. They occur in string theory as fields, rather like the electromagnetic field, that can adjust their values dynamically. These fields may have taken different values in different cosmological epochs or in remote regions of space, and even today the physical “constants” may vary by a small amount. Observing any variation would provide an enormous boost to string theory.

One such field, called the dilaton, is the master key to string theory; it determines the overall strength of all interactions. The dilaton fascinates string theorists because its value can be reinterpreted as the size of an extra dimension of space, giving a grand total of 11 spacetime dimensions.

Tying Down the Loose Ends

Finally, quantum strings have introduced physicists to some striking new symmetries of nature known as dualities, which alter our intuition for what happens when objects get extremely small. I have already alluded to a form of duality: typically a short string is lighter than a long one, but if we attempt to squeeze down its size below the fundamental length l s , the string gets heavier again.

Another form of the symmetry, T-duality, holds that small and large extra dimensions are equivalent. This symmetry arises because strings can move in more complicated ways than pointlike particles can. Consider a closed string (a loop) located on a cylindrically shaped space, whose circular cross section represents one finite extra dimension. Besides vibrating, the string can either turn as a whole around the cylinder or wind around it, one or several times, like a rubber band wrapped around a rolled-up poster [ see box below ].

Credit: Samuel Velasco

The energetic cost of these two states of the string depends on the size of the cylinder. The energy of winding is directly proportional to the cylinder radius: larger cylinders require the string to stretch more as it wraps around, so the windings contain more energy than they would on a smaller cylinder. The energy associated with moving around the circle, on the other hand, is inversely proportional to the radius: larger cylinders allow for longer wavelengths (smaller frequencies), which represent less energy than shorter wavelengths do. If a large cylinder is substituted for a small one, the two states of motion can swap roles. Energies that had been produced by circular motion are instead produced by winding, and vice versa. An outside observer notices only the energy levels, not the origin of those levels. To that observer, the large and small radii are physically equivalent.

Although T-duality is usually described in terms of cylindrical spaces, in which one dimension (the circumference) is finite, a variant of it applies to our ordinary three dimensions, which appear to stretch on indefinitely. One must be careful when talking about the expansion of an infinite space. Its overall size cannot change; it remains infinite. But it can still expand in the sense that bodies embedded within it, such as galaxies, move apart from one another. The crucial variable is not the size of the space as a whole but its scale factor—the factor by which the distance between galaxies changes, manifesting itself as the galactic redshift that astronomers observe. According to T-duality, universes with small scale factors are equivalent to ones with large scale factors. No such symmetry is present in Einstein's equations; it emerges from the unification that string theory embodies, with the dilaton playing a central role.

For years string theorists thought that T-duality applied only to closed strings, as opposed to open strings, which have loose ends and thus cannot wind. In 1995 the late Joseph Polchinski of the University of California, Santa Barbara, realized that T-duality did apply to open strings, provided that the switch between large and small radii was accompanied by a change in the conditions at the end points of the string. Until then, physicists had postulated boundary conditions in which no force acted on the ends of the strings, leaving them free to flap around. Under T-duality, these conditions become so-called Dirichlet boundary conditions, whereby the ends stay put.

Any given string can mix both types of boundary conditions. For instance, electrons may be strings whose ends can move around freely in three of the 10 spatial dimensions but are stuck within the other seven. Those three dimensions form a subspace known as a Dirichlet membrane, or D-brane. In 1996 Petr Horava of the University of California, Berkeley, and Edward Witten of the Institute for Advanced Study in Princeton, N.J., proposed that our universe resides on such a brane. The partial mobility of electrons and other particles explains why we are unable to perceive the full 10-dimensional glory of space.

All the magic properties of quantum strings point in one direction: strings abhor infinity. They cannot collapse to an infinitesimal point, so they avoid the paradoxes that collapse entails. Their nonzero size and novel symmetries set upper bounds to physical quantities that increase without limit in conventional theories, and they set lower bounds to quantities that decrease. String theorists expect that when one plays the history of the universe backward in time, the curvature of spacetime starts to increase. But instead of going all the way to infinity (at the traditional big bang singularity), it eventually hits a maximum and shrinks once more. Before string theory, physicists were hard-pressed to imagine any mechanism that could so cleanly eliminate the singularity.

Taming the Infinite

Conditions near the zero time of the big bang were so extreme that no one yet knows how to solve the equations. Nevertheless, string theorists have hazarded guesses about the pre-bang universe. Two popular models are floating around.

The first, known as the pre–big bang scenario, which my colleagues and I began to develop in 1991, combines T-duality with the better-known symmetry of time reversal, whereby the equations of physics work equally well when applied backward and forward in time. The combination gives rise to new possible cosmologies in which the universe, say, five seconds before the big bang expanded at the same pace as it did five seconds after the bang. Yet the rate of change of the expansion was opposite at the two instants: if it was decelerating after the bang, it was accelerating before. In short, the big bang may not have been the origin of the universe but simply a violent transition from acceleration to deceleration.

The beauty of this picture is that it automatically incorporates the great insight of standard inflationary theory—namely, that the universe had to undergo a period of acceleration to become so homogeneous and isotropic. In the standard theory, acceleration occurs after the big bang because of an ad hoc inflaton field. In the pre–big bang scenario, it happens before the bang as a natural consequence of the novel symmetries of string theory.

According to the scenario, the pre-bang universe was almost a perfect mirror image of the post-bang one [ see box below ]. If the universe is eternal into the future, its contents thinning to a meager gruel, it is also eternal into the past. Infinitely long ago it was nearly empty, filled only with a tenuous, widely dispersed, chaotic gas of radiation and matter. The forces of nature, controlled by the dilaton field, were so feeble that particles in this gas barely interacted.

As time went on, the forces gained in strength and pulled matter together. Randomly, some regions accumulated matter at the expense of their surroundings. Eventually the density in these regions became so high that black holes started to form. Matter inside those regions was then cut off from the outside, breaking up the universe into disconnected pieces.

Inside a black hole, space and time swap roles. The center of the black hole is not a point in space but an instant in time. As the infalling matter approached the center, it reached higher and higher densities. But when the density, temperature and curvature reached the maximum values allowed by string theory, these quantities bounced and started decreasing. The moment of that reversal, called a big bang, was later renamed a “big bounce.” The interior of one of those black holes became our universe.

Not surprisingly, such an unconventional scenario has provoked controversy. Andrei Linde of Stanford University has argued that for this scenario to match observations, the black hole that gave rise to our universe would have to have formed with an unusually large size—much larger than the length scale of string theory. An answer to this objection is that the equations predict black holes of all possible sizes. Our universe just happened to form inside a sufficiently large one.

A more serious objection, raised by Thibault Damour of the Institute of Advanced Scientific Studies in Bures-sur-Yvette, France, and Marc Henneaux of the Free University of Brussels, is that matter and spacetime would have behaved chaotically near the moment of the bang, in possible contradiction with the observed regularity of the early universe. I have proposed that a chaotic state would produce a dense gas of miniature “string holes”—strings that were so small and massive that they were on the verge of becoming black holes. The behavior of these holes could solve the problem identified by Damour and Henneaux. A similar proposal has been put forward by Thomas Banks, now at the University of California, Santa Cruz, and Willy Fischler of the University of Texas at Austin. Other critiques also exist, and whether they have uncovered a fatal flaw in the scenario remains to be determined.

The other leading model for the universe before the big bang is the so-called ekpyrotic (“conflagration”) scenario. Developed by a team of cosmologists and string theorists—Justin Khoury, now at the University of Pennsylvania, Paul J. Steinhardt of Princeton University, Burt A. Ovrut of the University of Pennsylvania, Nathan Seiberg of the Institute for Advanced Study and Neil Turok, now at the Perimeter Institute for Theoretical Physics in Ontario—the ekpyrotic scenario relies on the previously mentioned Horava-Witten idea that our universe sits at one end of a higher-dimensional space and a “hidden brane” sits at the opposite end. The two branes exert an attractive force on each other and occasionally collide, making the extra dimension shrink to zero before growing again. The big bang would correspond to the time of collision [ see box below ].

In a variant of this scenario, the collisions occur cyclically. Two branes might hit, bounce off each other, move apart, pull each other together, hit again, and so on. In between collisions, the branes behave like Silly Putty, expanding as they recede and contracting somewhat as they come back together. During the turnaround, the expansion rate accelerates; indeed, the present accelerating expansion of the universe may augur another collision.

The pre–big bang and ekpyrotic scenarios share some common features. Both begin with a large, cold, nearly empty universe, and both share the difficult (and unresolved) problem of making the transition between the pre- and the post-bang phase. Mathematically, the main difference between the scenarios is the behavior of the dilaton field. In the pre–big bang, the dilaton begins with a low value—so that the forces of nature are weak—and steadily gains strength. The opposite is true for the ekpyrotic scenario, in which the collision occurs when forces are at their weakest.

The developers of the ekpyrotic theory initially hoped that the weakness of the forces would allow the bounce to be analyzed more easily, but they were still confronted with a difficult high-curvature situation, so the jury is out on whether the scenario truly avoids a singularity. Also, the ekpyrotic scenario must entail very special conditions to solve the usual cosmological puzzles. For instance, the about-to-collide branes must have been almost exactly parallel to one another, or else the collision could not have given rise to a sufficiently homogeneous bang. The cyclic version may be able to take care of this problem because successive collisions would allow the branes to straighten themselves.

Leaving aside the difficult task of fully justifying these two scenarios mathematically, physicists must ask whether they have any observable physical consequences. At first sight, both scenarios might seem like an exercise not in physics but in metaphysics—interesting ideas that observers could never prove right or wrong. That attitude is too pessimistic. Like the details of the inflationary phase, those of a possible pre-bangian epoch could have observable consequences, especially for the small variations observed in the cosmic microwave background temperature.

First, observations show that the temperature fluctuations were shaped by acoustic waves for several hundred thousand years. The regularity of the fluctuations indicates that the waves were synchronized. Cosmologists have discarded many cosmological models over the years because they failed to account for this synchrony. The inflationary, pre–big bang and ekpyrotic scenarios all pass this first test. In these three models, the waves were triggered by quantum processes amplified during the period of accelerating cosmic expansion. The phases of the waves were aligned.

Second, each model predicts a different distribution of the temperature fluctuations with respect to angular size. Observers have found that fluctuations of all sizes have approximately the same amplitude. (Discernible deviations occur only on very small scales, for which the primordial fluctuations have been altered by subsequent processes.) Inflationary models neatly reproduce this distribution. During inflation, the curvature of space changed relatively slowly, so fluctuations of different sizes were generated under much the same conditions. In both the stringy models, the curvature evolved quickly, increasing the amplitude of small-scale fluctuations, but other processes boosted the large-scale ones, leaving all fluctuations with the same strength. For the ekpyrotic scenario, those other processes involved the extra dimension of space, the one that separated the colliding branes. For the pre–big bang scenario, they involved a quantum field, the axion, related to the dilaton. In short, all three models match the data.

Third, variations in temperature can arise from two distinct processes in the early universe: fluctuations in the density of matter and rippling caused by gravitational waves. Inflation involves both processes, whereas the pre–big bang and ekpyrotic scenarios involve density variations for the most part. Gravitational waves of certain sizes would leave a distinctive signature in the polarization of the cosmic microwave background. Satellite and ground-based observatories may be able to see that signature, if it exists—providing a nearly definitive test.

Credit: Samuel Velasco ( illustration ); Courtesy of NASA/WMAP Science Team ( map )

A fourth test pertains to the statistics of the fluctuations. In inflation the fluctuations follow a bell-shaped curve, which is known to physicists as a Gaussian. The same may be true in the ekpyrotic case, whereas the pre–big bang scenario allows for sizable deviation from Gaussianity.

Analysis of the microwave background is not the only way to verify these theories. The pre–big bang scenario should also produce a random background of gravitational waves in a range of frequencies that, though irrelevant for the microwave background, should be detectable by future gravitational-wave observatories. Moreover, because the pre–big bang and ekpyrotic scenarios involve changes in the dilaton field, which is coupled to the electromagnetic field, they would both lead to large-scale magnetic field fluctuations. Vestiges of these fluctuations might show up in galactic and intergalactic magnetic fields.

So when did time begin? Science does not have a conclusive answer yet, but at least two potentially testable theories plausibly hold that the universe—and therefore time—existed well before the big bang. If either scenario is right, the cosmos has always been in existence and, even if it recollapses one day, will never end.

Gabriele Veneziano , a theoretical physicist, formerly at CERN near Geneva, is now an emeritus professor at the College of France in Paris. He was the father of string theory in the late 1960s and one of the first to later apply the theory to black holes and cosmology.

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Cosmic String

We've blown through black holes and wormholes, but there's yet another possible means of time traveling via theoretic cosmic phenomena. For this scheme, we turn to physicist J. Richard Gott, who introduced the idea of cosmic string back in 1991. As the name suggests, these are stringlike objects that some scientists believe were formed in the early universe.

These strings may weave throughout the entire universe, thinner than an atom and under immense pressure. Naturally, this means they'd pack quite a gravitational pull on anything that passes near them, enabling objects attached to a cosmic string to travel at incredible speeds and benefit from time dilation. By pulling two cosmic strings close together or stretching one string close to a black hole, it might be possible to warp space-time enough to create what's called a closed timelike curve .

Using the gravity produced by the two cosmic strings (or the string and black hole), a spaceship theoretically could propel itself into the past. To do this, it would loop around the cosmic strings.

Quantum strings are highly speculative, however. Gott himself said that in order to travel back in time even one year, it would take a loop of string that contained half the mass-energy of an entire galaxy. In other words, you'd have to split half the atoms in the galaxy to power time machines. And, as with any time machine, you couldn't go back farther than the point at which the time machine was created.

Oh yes, and then there are the time paradoxes.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

String Theory

Claimed by Julie Heil Spring 2022

• 1.1 Relevant Mathematical Equations (Bekenstein-Hawking Formula)
• 1.2 Theoretical Foundation
• 2 History of String Theory
• 3.1 Wormholes
• 3.2 Computational Models
• 5 Connectedness
• 6 Scientific Development Timeline
• 7.1 Further wiki reading
• 7.2.1 Articles
• 7.2.2 Videos
• 7.2.3 Books
• 8 References

Introduction to String Theory

String theory is a theoretical framework to address the shortcomings of Einstein's theory of general relativity and quantum physics - two primary tools for understanding modern physics. String theory is often referred to as the theory of everything - a just identification given its ability to bridge the gap between gravity and quantum physics. The theory of general relativity and quantum mechanics are the primary, established explanations for how the universe works on a macroscopic and microscopic scale, respectively. However, these two physical explanations address different ends of the spectrum of matter in terms of relativistic effects, fundamental forces, and physical properties. String theory provides explanations where modern physics fails. These fields of interest include the early universe, black holes, and atomic nuclei. To understand the purpose of the theory of everything, it is vital to have a sound understanding of general relativity's purpose and the role of relativistic quantum mechanics - that is, in terms of gravitational effects on the matter, the fundamental physics of the universe. General relativity serves an essential purpose in determining the gravitational effects on planetary-sized objects and observable particles. However, the "gap" in Einstein's theory lies within its inability to address gravity's impact on elementary particles. General relativity successfully explains macro-scaled particles, given its ability to incorporate gravitational effects in its theoretical and mathematical foundation. However, the same cannot be said for elementary particles. Accordingly, these elementary particles require relativistic quantum mechanics to explain their physical properties, assuming that gravity's effect on this scale is negligible. This is the disconnect between modern physics' primary schools of thought. As a result, the purpose of the string theory is realized in its ability to successfully incorporate gravitational effects, independent of the scale of particles. Thus, the theory of everything emerged.

See reference 3.

String theory is a theory that utilizes and brings together electromagnetism, the weak force, gravity, and the strong forces. These are the four fundamental forces of nature. The particles that are identified within the string theory all have a particular vibration pattern of a 1D object, a string. String theory falls under the category of quantum theory and is an example of (the quantum) theory of gravity. String theory is highly constrained due to it being confined by the parameters of 1D, that parameter being the string length. While string theory is far from being complete, efforts from modern research in string theory have to advance our current understanding of other related fields of study like black hole physics, algebraic geometry, cosmology, and condensed matter physics.

See reference 5.

Relevant Mathematical Equations (Bekenstein-Hawking Formula)

The Bekenstein-Hawking formula for the entropy of a black hole, shown above, provides a theoretical value for the entropy of a black hole. While this equation provides an expected value that correlates to the entropy of a black hole, according to macroscopic features resulting from its microstates, the formula lacks a derivation for a black hole's entropy-based on counting microstates on a quantum scale with consideration of gravitational aspects. In 1996, Andrew Strominger and Cumrun Vafa provided a derivation for a black hole's entropy with respect to the number of microstates of a black hole and in terms of the string theory. The results obtained from these calculations provided solutions matching Berkenstein and Hawking's original formula. Thus, string theory was validated as a mode of quantum gravity.

See reference 1 .

Theoretical Foundation

String theory is founded on the basis that elementary particles are components of microscopically small strings - to the degree that technology is not available to visualize. A valid and common depiction of strings lined with elementary particles is a visualization of these identical particles laying on a violin string. String theory advocates propose that elementary particles are believed to be in "excited" states due to vibrations in these strings. This is significant given the bending of spacetime and the existence of black holes and wormholes - solutions to Einstein's equations that are not explained entirely by quantum physics or general relativity in their own right. On a theoretical basis, string theory provides solutions based on gravity acting on elementary particles (theories of quantum gravity), a unification of quantum mechanics, and Einstein's theory of general relativity.

See reference 2 . Oscillations of the closed string.

History of String Theory

The origin of string theory came from particle physics back in the 1960s. A series of experiments related to the interaction of the strong forces showed there are infinite hadrons, and their masses and spins become more prominent and higher. They are called resonance states because most of the particles are unstable particles. If an infinite number of particles engage in the interaction, the particle-particle scattering amplitude satisfies a property: duality. Gabriele Veneziano, an Italian physicist, found a simple function that helps duality. This is known as the Veneziano formula. While no experiment could fully satisfy his formula, researchers soon discovered that the formula could be naturally explained as the string and the string's scattering amplitude. So, string theory did not originate from one or a series of experiments but rather from a formula.

The theory of relativity comes into play when talking about string theory. Einstein's theory of relativity can be discussed and considered as string theory's origin. The space-time view of general relativity reformed Newton's space-time view due to the limitations of Newton’s space-time view. Einstein replaced Newton's absolute time and space causality with the law of relativity. Those who believe in the theory of string theory believe gravity has been quantized (successfully). It is debated that there is no successful theory of quantum gravity, but it is still acknowledged to an extent the success of string theory.

String theory was first proposed to describe strong forces. The quark model and quantum chromodynamics were not widely accepted as theories explaining strong forces. Based on current understanding, the meson comprises quarks and antiquarks due to strong forces. However, in string theory, it is characterized as an open string. The ends of the string correspond to quarks and antiquarks, and the string itself corresponds to the strong pulling force created between the two forces. Quantum chromodynamics is relatively prosperous due to string theory’s inability to explain many phenomena of strong forces. Due to this, string theory was abandoned by most physicists for some time.

According to popular saying, the string itself has undergone two revolutions. After the first revolution, string became popular. Some string theory experts, in1985, shortly after the first revolution, believed that the ultimate theory was in sight. Some people say that this is the Theory of Everything (TOE). John Ellis, director of the European Nuclear Center's Theory Department, is a representative of this school. These people were overly optimistic at that time, or that their understanding of strings was more superficial. Schwarz and Green 1980 developed the first superstring theory model. This model dealt with open string vibrations in ten-dimensional space, which can be connected or broken.

The first superstring revolution in 1984 was led by Schwarz and Green’s discovery that there is a symmetry capable of eliminating all distortion and infinity. This brought forth a new candidate for the theory of everything. He brought attention back to string theory and led physicists to pay more attention to string theory.

The second superstring revolution in 1994 brought forth many questions. There were five different versions and explanations revolving around string theory at the time. The traditional methods scientists use could not be used to prove the authenticity of string theory. This led to the desire for new techniques and technologies. The claim that string theory is the theory of everything raises the question of why there are five different versions; they’re like plotholes. Also, suppose point particles can be regarded as string vibrations. What is the reason they cannot be caused by two-dimensional membranes, three-dimensional squares, or higher-dimensional physical vibrations? In the early 1990s, physicists started to understand the duality between versions. Edward Witten made the most notable breakthrough in 1995, where he unified the various dualities under the eleven-dimensional M theory

Potential Discoveries and Applications

The potential usage of string theory is to provide the quantum gravitational solutions that Einstein's theory of general relativity fails to recognize at the center of black holes or wormholes. This inability is due to its lack of consideration of quantum forces alongside gravity. String theory is integral to discovering wormholes' (derived solutions to Einstein's equations) level of stability. Specifically, string theory's quantum forces and gravity enables the determination of the radiative effects and stability of this unexplained astrophysical phenomenon. In conclusion, these recently gained understandings of string theory may ultimately yield answers to questions such as...

1. Given Kerr wormholes and their potential to connect distant points in the universe, is it theoretically feasible to use these solutions to travel vast lengths through the universe?

See reference 2 .

[ Wormhole Simulation: Unknown Source ]

Computational Models

Could not find any good PHET simulations or interactive models other than the wormhole that would help achieve a better understanding on the topic.

[ Computational Exploration of String Theory ]

Example One

Question: Explain how the circle S^1 is equivalent to the real line R with the identification x ~ x + 2πR

Solution: The given identification identifies 0 with 2πR. All points x between 0 and 2πR are identified with points 2πR with a magnitude larger than 2πR. The identification has made the real line periodic with period 2πR. One can think of this as identifying the interval [0,2πR) with the circumference of the circle of radius 1. In this case, the compact space S^1 is found by taking the quotient of the non-compact space R by the discrete symmetry group Z, the group of integers. This corresponds to the fact that points shifted by integer multiples of 2πR are identified.

[ Reference 5 ]

Example Two

Question: Explain how the complex plane C with the identification z∼e^(2πi/3z defines an orbifold compactification.

Solution: The given identification identifies points in the complex plane with themselves rotated by 2π/3 and by 4π/3. The rotation of the complex plane about the origin leaves the origin fixed, so there exists an orbifold singularity at the origin. The resulting space is a cone with an opening angle 2π/3. Since the rotations of the complex plane by 2π/3 correspond to a representation of the cyclic group Zsubscript3 of order three, this space is called the C/Zsubscript3 orbifold.

Connectedness

• How is this topic connected to something that you are interested in?

String Theory is a particularly intriguing topic due to far-reaching applications that today are widely considered science fiction. (i.e., interstellar space travel utilizing wormholes). This topic is something that not only makes you think but still has the scientific world asking questions. A general understanding of the basics yields a profound knowledge of how the physical world operates

• How is it connected to your major?

String theory falls into the category of Physics, Astrophysics, and Theoretical Physics. My major is Physics with a concentration in Astrophysics, and it mends nicely into my career path.

• Is there an interesting industrial application?

Scientists may not realize industrial applications for string theory in the near future. Still, in the long run, an understanding of string theory could have a profound, yet unforeseen, impact on the world - in a similar revolutionary and unpredicted manner as that of quantum mechanics and the development of modern communication and flow of information.

Scientific Development Timeline

1968-1974 : Dual resonance model

1970 : String theory is proposed to understand the quantum mechanics of oscillating strings.

1976 : Supergravity is proposed as a means of explaining the interdependence of gravity and sub-atomic particles' spectrum of excitations - an integral component in string theory.

1974-1984 : Bosonic string theory & superstring theory

1980 : Initially perceived to discredit string theory as a unification of quantum mechanics, classical physics, and particle physics, the inconsistencies of the theory were found to nullify each other when considered as special cases. This is the year that string theory becomes widely accepted as a potentially unifying explanation in the scientific community of the time.

1984-1994 : 1st superstring revolution

1991-1995 : String theory exploration, in terms of black holes, results in development towards the understanding of the different forms of string theory, in terms of their relationship.

1994-2003 : 2nd supervising revolution

1996 : String theory provides a microscopic understanding of black hole entropy and the nature of black hole quantum physics.

See reference 3 & 4 .

See reference 4 for in-depth explanation .

Additional Resources

Further wiki reading.

Elementary Particles and Particle Physics Theory

Quantum Theory

Big Bang Theory

External links

Wormholes and Blackholes

String theory beyond the Planck scale

String theory and noncommutative geometry

String Theory, University of Cambridge Part III Mathematical Tripos

Brian Greene: Making Sense of String Theory (YouTube)

String Theory Explained – What is The True Nature of Reality?

ScienceClic English: String Theory

String Theory and the End of Space and Time with Robbert Dijkgraaf

Steven Gubser, The Little Book of String Theory

P. Di Francesco, P. Mathieu and D. S´en´echal, Conformal Field Theory

B. Zwiebach, A First Course in String Theory

M. Green, J. Schwarz and E. Witten, Superstring Theory

J. Polchinski, String Theory

Reference 1 : "SUPERSTRINGS! Black Holes ." Web.physics.ucsb.edu . UCSB, n.d. Web. 03 Dec. 2015. < http://web.physics.ucsb.edu/~strings/superstrings/bholes.htm >.

Reference 2 : Kaku, Micho, Dr. "Blackholes, Wormholes and the Tenth Dimension." Mkaku.org . N.p., n.d. Web. 03 Dec. 2015. < http://mkaku.org/home/articles/blackholes-wormholes-and-the-tenth-dimension/ >.

Reference 3 : Shwarz, Patricia, Dr. "The Official String Theory Web Site." Superstringtheory.com . N.p., n.d. Web. 03 Dec. 2015. < http://www.superstringtheory.com/index.html >.

Reference 4 : Beibei Chen 2021 IOP Conf. Ser.: Earth Environ. Sci. 658 012002 https://iopscience.iop.org/article/10.1088/1755-1315/658/1/012002/pdf . IOP Publishing;.

Reference 5 : Matt DeCross, Christopher Williams, Tejas Suresh, Satyabrata Dash, Eli Ross. String Theory. Brilliant. https://brilliant.org/wiki/string-theory/

Photo References :

Giribet, Gaston & Celis, Emilio & Simeone, Claudio. (2019). Traversable wormholes in five-dimensional Lovelock theory. Physical Review D. 100. 10.1103/PhysRevD.100.044011.

Vyas, Rakshit & Joshi, Mihir. (2019). STRING THEORY: A MATHEMATICAL FRAMEWORK BEHIND COMPATIBILITY AND RECONCILEMENT BETWEEN GENERAL RELATIVITY AND QUANTUM MECHANICS. ISBN:9788192952147. 60-65.

American Journal of Physics 83, 486 (2015); doi: 10.1119/1.4916949

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String Theorist Brian Greene Wants to Help You Understand the Cold, Cruel Universe

I f you’re feeling all dreamy about the universe, here’s a pro tip: don’t tell Brian Greene. That guy can chill your cosmic buzz fast. I recently swung by the office of the Columbia University theoretical physicist full of happy, giddy questions and came away pretty much empty. Is there such a thing as a natural moral order? I wondered. Not in this universe, there isn’t. What about a purpose to the universe, then–the reason the whole 13.8 billion-year-old shebang with its hundreds of billions of galaxies and trillions of planets happened in the first place? Nope, Greene says, no such purpose, adding, “And that’s O.K.” Maybe for him it is.

Surely, though, Greene will grant the existence of free will–that first item on the wish list of every freshman-year philosophy student who ever lived. Sorry, not a chance.

“Your particles are just obeying their quantum-mechanical marching orders,” Greene says. “You have no ability to intercede in that quantum-mechanical unfolding. None whatsoever.”

But here’s the thing about Greene, founder of the World Science Festival; host of multiple TV series on PBS; and the author of five books, including the blockbuster The Elegant Universe and the just-released Until the End of Time : he says it all with such ebullience, such ingenuous enthusiasm, that if he told you the whole cold, amoral universe was ending tomorrow you’d roll with it the way he would–as just one more dramatic chapter in an extraordinary tale in which we all have a precious if fleeting role. That’s not to say everyone embraces his cosmic view so easily.

“I’ll be frank,” Greene says. “I have some students come in crying. And they say, ‘This is kind of shaking my world up,’ and I say to them, ‘That’s not a bad thing. It’s fine to have your world shook. The pieces may fall back in the end to where you were, and they may not.'”

On the day I saw him, the man who has made himself the master of some of the most abstruse aspects of physics–superstring theory, spatial topography–was instead being mastered by one of the more basic ones: gravity. He was struggling about on crutches, the result of two ruptured spinal disks, which can give out over time whether or not you’re the kind of person who can explain the attraction between the mass of the earth and the mass of your back.

When he makes his way from desk to couch, he drops down gratefully. Behind him is a whiteboard with a storm of equations written on it. The numbers and glyphs frame his face in a perfect metaphor for the impossibly complex ideas that play out in his head, then somehow emerge comprehensibly and coherently on the page.

It’s a busy time for Greene. His World Science Festival will begin its 13th season in May in New York City and its fifth year in its satellite venue in Brisbane, Australia, in March. The Down Under version attracted a total of 700,000 visitors in its first four years. The New York edition has drawn a cumulative 2 million people and more than 40 million online views of its content.

Greene, 57, is also preparing for a promotional tour for his new book, and keeps up a full schedule of teaching, holding office hours and advising graduate students. During our conversation, he mentioned that he was booked to give an evening talk on superstring theory to a gathering of the university’s Society of Physics Students. It’s a Friday night, a party night, but for the students and Greene, talking superstrings is a party.

“I’ve found that the theoretical physicists I’ve spent the most time with are the ones who are just enthralled by the ideas and the minutiae of an equation working out,” he says. “The only difference I have seen relative to my colleagues is I’ve never found pure research to be enough. I’ve always felt like the world is so big and rich that I need to engage with it in different ways. And that can be the books, it can be the TV shows.”

Greene comes by his love of performance rightly. His father was a vaudeville entertainer as well as a composer and voice coach. But Greene’s own passion was math and science and then big science–the kind that seduces you with questions that both demand and defy answers, that can cross the line from science to something else entirely. Here, too, a close family member helped.

“My brother is a Hare Krishna devotee,” Greene says. “He’s 13 years older than I am. When I was little and getting interested in math and physics, he’d say, ‘What are you learning?’ I’d describe the Big Bang, and he’d pull out the Vedas and read to me from them. It was a very interesting back-and-forth over the decades between the scientific pathway toward a certain kind of truth and the spiritual, religious pathway to a certain kind of truth.”

That tension plays out elegantly in Greene’s new book, and to make sure no one misses the dialectic, the chapter names make it clear: “Duration and Impermanence,” “Origins and Entropy,” “Particles and Consciousness.” Greene takes one of his most powerful whacks at entropy, attacking the nettlesome business of the second law of thermodynamics–the broad truth that all systems tend to disorder, which is often used to challenge the truth of evolution itself: that profoundly complex order can emerge from the chaos.

“I resolve that tension in Chapter 3,” Greene says, a boast that could pass as arrogant except that, well, he does resolve the tension in Chapter 3. “It relies on the force of gravity. Without gravity, everything just spreads out, diffuses, and that’s all there would be. But gravity has this wonderful capacity as a universally inward-pulling force which can undertake the following magic trick: it can pull things together, making it more orderly here, at the expense of releasing heat that makes it more disorderly out there. I call it ‘the entropic two-step.'”

There’s a lot of satisfaction in such neat solutions to head-cracking problems. But there is an equivalent neatness to the ostensibly dispiriting conclusions Greene reaches in his books and in his research: that unhappy business of a cold universe, an insentient universe, of the individual as just a quantum contraption, behaving as a product not of choice but of probabilities and randomness. It’s where the free-will thing comes in: the universe is guided by quantum probabilities, and your “choices” are simply a part of that, the way a local breeze is part of the global weather system.

“My feeling is that the reductionist, materialist, physicalist approach to the world is the right one,” Greene says. “There isn’t anything else; these grand mysteries will evaporate over time.” But despite such empirical bravado, Greene says more too–and whether he likes it or not, it’s not reductionist, and if it’s written in a book like Until the End of Time , it could be written in the Vedas as well.

“Rather than feeling, ‘Damn, there’s no universal morality,’ ‘Damn, there’s no universal consciousness,'” he says, “how wondrous is it that I am able to have this conscious experience and it’s nothing more than stuff? That stuff can produce Beethoven’s Ninth Symphony, that stuff can produce the Mona Lisa , that stuff can produce Romeo and Juliet ? Holy smokes, that’s wondrous.” The rational physicist with the deeply spiritual brother surely meant the holy as just a figure of speech–but if so, he picked an apt one.

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Ep. 31: String Theory, Time Travel, White Holes, Warp Speed, Multiple Dimensions, and Before the Big Bang

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We get questions every week about string theory and topics popularized by science fiction. Here’s the problem. There’s just no evidence. Each of these is based on wonderful and well-formed mathematical equations, or wishful thinking, but they’re very hard (if not impossible) to test in the real Universe.

The Problem with String Theory

• Is String Theory Even Wrong? – American Scientist Online
• String Theory: An Evaluation

White Holes & Wormholes

• What is a white hole? – Ask an Astronomer, Cornell University
• White Holes and Wormholes

Faster Than Light Travel

• Faster than Light Travel

Transcript: String Theory, Time Travel, White Holes, Warp Speed, Multiple Dimensions, and Before the Big Bang

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Fraser Cain: So here’s the problem: we get questions sent in every week about the topics in the title of the show, especially string theory. We try to kind of put them aside or hold off, but we need to address it.   One of our goals with Astronomy Cast is to present evidence that supports our current understanding of the Universe. If you think about our Big Bang show early on, we provided four different lines of evidence that support our understanding of the Big Bang. Thanks to science fiction, comic books, there’s a lot of ideas there that do relate to space and astronomy but which don’t quite have any evidence whatsoever, and yet they’re in the popular thinking, so we get questions about them all the time.   So Pamela, what is the evidence to support all those topics?

Dr. Pamela Gay: Absolutely none.

Fraser: None. All right, but let’s try and cover some of these topics as much as we can to explain at least where there could be evidence. So let’s start with string theory. What is string theory?

Pamela: String theory is a mathematical attempt to unify all the different forces. It starts off with the premise that our Universe doesn’t exist just in the three spatial dimensions we experience, and time, but rather it has other dimensions as well. When we look at sub-atomic particles, what we’re seeing is just one aspect of a particle that is a string if you’re able to see it in the multiple different dimensions – this is where the “string” in string theory comes from.   One of the problems with string theory is, because it’s a mathematical invention, you can tweak the parameters to get it to fit almost anything. There are a lot of different versions of string theory that all build off of one another in slightly different ways and come up with slightly different predictions and the things that they’re predicting aren’t unique to just string theory, there are other theories based on the more standard model of particle physics that make similar predictions.

Fraser: But we don’t want to disparage mathematical predictions, I mean many of the great discoveries of the Universe have been done in precisely that way. Think about relativity: Einstein worked out the math before anyone was able to see his predictions in reality. In fact, they’re only just now starting to understand.

Pamela: But the thing with relativity is it made a unique set of predictions based on a unique set of observables. For one set of data in, you get one set of data out. With string theory you can throw in a bazillion different inputs and get an infinite number of outputs, it seems like. So in this case, you have non-unique solutions and physics requires at least probabilities of what your unique solution should be. This is where string theory gets itself into trouble: it’s currently not testable.   A good scientific model has three specific characteristics: it is built on existing observations – you look at something, you see what happens, you write a theory to describe it. You then have to make predictions and then once you’ve made the observations of those predictions you can modify your theory to incorporate them but the theory itself shouldn’t fall apart. Newton’s theory of gravity didn’t fall apart in the face of relativity, it was just modified and added on to. With string theory you currently – yeah, they’re incorporating lots of present observations, but there’s no one test that will prove or disprove string theory and what tests exist aren’t unique to string theory.

Fraser: But isn’t that a sign of all of the different thinkings going into string theory – isn’t that a sign of its health, of the possibilities that could exist there?

Pamela: I think it’s more a sign of it’s youth. It’s a theory that still isn’t fully put together and it needs more work to reach the level of maturity where its able to stand up and stand with the other full-fledged scientific theories and make its unique predictions. It’s not there yet, and some people are actually questioning will it ever get there, or is this simply a unique trick you can play with math that can get you in lots of different directions but doesn’t actually tell you any fundamental truths about the Universe?

Fraser: Right. There are some great books out there, especially the one by Brian Green

Pamela: An Elegant Universe

Fraser: Yeah – which definitely explains the concept in a way you can understand it (and he also did a special on NOVA) so you can explain it. But I think that as it goes back to it, this isn’t something that’s been seen, this isn’t something that’s been observed, some evidence supports certain lines of string theory other evidence supports others, so it’s almost all comes out as a wash in the end.  

Pamela: It’s a tool, it’s just not necessarily a tool that is able to help us build a house.

Fraser: Right, so when you’re thinking about the Big Bang for example, something that has so much evidence, and something like string theory as two different ways to understand the Universe… one is really giving some insight into how the Universe really looks, while the other one is so far purely math. I’m sure it’s wonderful, elegant math that really connects together, but so far there’s no evidence.

Pamela: And without the evidence, we can’t really discuss it in detail in our facts-based show.

Fraser: The moment there’s any evidence, we’ll do a whole show about it.   Okay, time travel: What’s wrong with time travel? Aren’t we moving forward in time right now, one second per second?

Pamela: The problem with time travel is definitional. Most people when they say “let’s travel in time!” want to go back in the past, they want to go on Bill and Ted’s Excellent Adventure and bring Socrates to the future. But, time travel only moves in one direction, and that’s forward.   It’s possible for us to skip forward in time by moving extremely fast. If you or I were to take off running at 80%, 90% the speed of light, we’d see the people around us seem to age progressively faster and the Universe would go through vast amounts of history up until the point that we stopped running and our watch and the watch of the planet Earth started ticking at the same rate.

Fraser: Oh so if we ran around in a circle at 90% the speed of light, it would be like all around us was one of those sped-up movies.

Pamela: Yeah.

Fraser: Right.

Pamela: And we could be Buck Rogers in the 21st Century, although I’m thinking nowadays it would have to be like the 24th Century, but that’s still only moving forward in time. Backwards in time we can’t do. Physics says this arrow only points in one direction.

Fraser: But I’ve heard that many theories, or many formulas in physics have no need for time to move in any one direction; if you put time in the opposite direction the math still works out.

Pamela: Well, and any one thing, you can show that mathematically it is possible. Within the framework of the entire Universe, the Universe itself has this underlying “you will only go forward in time” rule to it. So, while one or two equations (or probably more than that) say yes, we can have things that progress the same way forward or backward or completely reversible, that doesn’t mean that you can reverse the equation and set yourself going backwards in time. There are reversible reactions, things that you set them forward and then you can set them in reverse but when they’re going in reverse, time is still moving forward; you’re just seeing the reaction happening in reverse.

Fraser: I think it’s a great – is it a paradox? – that if there is time travel in the future (it’s almost like the one we did with all of the aliens), then there would be time travellers crawling around everywhere you looked because in theory there’s an infinite amount of time in the future for time travelers to hop into their time machines and go back to any time that they choose and enough of them would choose to come back right now that we’d probably have five or six time travellers looking over our shoulders as we record this historic episode.   [laughter]

Pamela: There was in fact a conference at MIT where they said “okay, anyone out there time-travelling, show up here on this day and time” and no one showed up. So, there you go.

Fraser: There you go.   Okay, so what’s a white hole?

Pamela: A white hole is the opposite of a black hole, in terms of… black holes, material gets into them and can’t get back out. White holes (if they were able to physically exist) would be spewing light and energy out of them. The catch is that sort of breaks the second law of thermodynamics, and not only that but the second you get the smallest fraction of a dust mote, a single atom, a single electron near a white hole, suddenly the white hole becomes a black hole.   White holes are a mathematical invention of what happens if you look at all the geometry of a black hole and get rid of the mass in the centre. So mathematically, they’re pretty cool. They do totally exist in mathematical formulas and they might have existed when the Universe formed, but we have no mechanism for forming one in the modern Universe. You can’t collapse a star and get something with no mass in the centre. Even if any were created during the formation of the Universe, in the 13.7 ± 0.2 Billion years since then, it’s safe to assume that some fraction of a dust mote has gotten near every possible white hole that was out there and converted it to a black hole.   So, we don’t think they could have existed, because they go against the second law of thermodynamics, and even if they did exist, we don’t think they exist now.

Fraser: I think with the staple of science fiction, they’re used as some mechanism for allowing our heroes to travel back in time, or travel across the Universe or move to other dimensions

Pamela: Thus breaking other rules.

Fraser: [laughing] Yeah, but of course the moment they encounter the white hole it would turn into a black hole, I guess, so there it goes. So white holes once again, we have what’s a mathematical construct that’s just some physicists saying “I wonder what the math looks like if I remove something”

Pamela: [laughter] And that actually makes the mathematics a lot easier, it just makes the meaning of the mathematics a lot more nonsensical.

Fraser: Right, just because you can take that interesting mathematical formula doesn’t mean that when you create a science fiction show based on it that you’re basing it on anything.   All right. Warp speed and moving faster than the speed of light. Oh please let this be true…

Pamela: Well, and again it all depends on your definitions. Light travels at a constant speed in a given medium. So, if I have a vacuum, light is going to travel through that vacuum at 300 thousand km/s. If I have a bunch of rubidium gas at just the right temperature in just the right laboratory set-up, I can slow light down. In fact, there’s a bunch of neat ways to slow light down and one of them is just a pair of glasses like you wear to correct your vision. Anytime light transforms from one medium to another, say from air to glass, from air to water, the light appears to bend and the reason it appears to bend is because its speed gets altered, it gets slowed down in a lot of cases.   So in theory, you can shoot a light beam through some sort of media that causes it to slow down and walk beside the media going faster than the speed of light. But you and light can’t go the exact same speed in the exact same media.   Say you go out to outer space and fire your flashlight off and take off after it. You are going to be limited in that case by the rules of general relativity that say the closer to the speed of light you go, the larger your mass gets and if you try to move at the speed of light you’re going to require infinite amounts of energy and that’s never going to happen.

Fraser: Okay, that’s just trying to move yourself faster than the speed of light. So, for example, in Star Trek they said clearly nobody can actually travel faster than the speed of light, we’re just going to warp space. Why can’t we do that?

Pamela: Well, first of all there’s a few gravitational and energy constraints on that… the amount of energy needed to bend the Universe is slightly vast, you might say.   [laughter]

Fraser: How vast? Like all the energy in the Universe?

Pamela: Something like that.. and how exactly do you grab on to one section of the Universe and bend it into the current section of the Universe?

Fraser: Like, line up a whole bunch of black holes maybe, which bend…

Pamela: Yeah. So you’re just creating a bunch of holes that you have to travel through when you do that. So yes, you can bend space but you’re not going to be changing the distance between two points, you’re just changing the geometry of the distance between two points. So we’re sort of stuck and can’t really warp the Universe.

Fraser: Right. Okay.   Now, one of the things that I think goes along with string theory, is talk about multiple dimensions

Pamela: Yes.

Fraser: Or with quantum mechanics is the concept of multiple dimensions, so here we are but there’s another dimension just that we can’t see but where other Fraser and Pamelas are recording Astronomy Show and maybe we could reach them.

Pamela: Well there’s different ways to look at multiple dimensions. You can either look at it as perhaps there are multiple parallel universes. That model, we just can’t test. We have no way of looking into the other universes.   The other way of looking at multiple dimensions is perhaps there are multiple spatial dimensions of which we are simply confined to the three dimensions that we experience. Think of it as being trapped on a sheet of paper: that sheet of paper has two dimensions plus time and it exists in a three dimensions plus time Universe. What if we simply live on a three dimensional surface within some Universe that has ten, eleven, thirty different dimensions, but all those other dimensions are either compressed such that we can’t see them or we’re simply confined to the dimensions that we’re within and we have no way of getting out of those dimensions to experience the others.   There are some theories that are attempting to find ways to make predictions based on multiple dimensions existing. Some of these involve the decay rates of microscopic black holes, where they say “if we have more dimensions, perhaps these things can last longer, and if we ever find one maybe that will be proof” but beyond that there’s no real ways of testing if there’s multiple dimensions and no way of reaching out and touching the extra dimension.

Fraser: Isn’t that one of the thoughts that (it goes back to string theory), that gravity is so weak because it is moving between multiple dimensions?

Pamela: That’s one possible way, or it could just be that the Universe was created so that gravity is a significantly weaker force. The problem is how do we sort that out right now? We can build models so that the evidence fits both different scenarios.

Fraser: When I think multiple dimensions, I’m thinking of going through the worm hole to the other dimension, to the alternate reality.

Pamela: Right.

Fraser: I think that’s a construct of quantum mechanics, right? Where with the uncertainty, every moment particles can do different things and then in one Universe the particle goes one way and then in an alternate reality the particle goes another way.

Pamela: That gets you into multiple universes where you have branching pasts and futures. So, every time you make a decision there is one view of quantum mechanics that says that to fit that decision causes the universe to branch. Where there’s a universe where neither of us were sick, there’s a universe where my voice didn’t come back, there’s a universe where you sniffled your way much more violently through this episode, and every possibility that could exist, does exist, in one of these multiple universes.   Multiple dimensions gets you into a slightly different picture though, where everything is chewing forward in the same universe. Where my decision only effects the dimensions that I exist within, but some of the dimensions I exist within may be dimensions I have no way of contacting, measuring, experiencing.

Fraser: So this is more out of phase, right? If we’re talking about multiple dimensions. Like, instead of being in the 1, 2 and 3 dimensions that I’m used to, I’m instead transported somehow to the 4th, 5th and 6th dimensions.

Pamela: Exactly, and this again crops up in different Star Trek episodes where they’re making contact with different life forms that have the majority of their reality in alternate dimensions so you see them as shadow beings flickering in and out.

Fraser: Do we have any evidence that there are additional dimensions that we can’t –

Pamela: No.

Fraser: Okay, oka—

Pamela: No. We have none. There are people working to find predictions. If we find microscopic black holes that may be proof that we live in a Universe with more than 5 dimensions, but no one’s found a microscopic black hole yet.

Fraser: I’ll bet that someone’s going to be looking for it in the Cosmic Microwave Background Radiation.

Pamela: Yeah, but that’s where they look for everything

Fraser: Yeah that’s what I’m saying – if you look hard enough, you can find everything you need in the Cosmic Microwave Background Radiation.

Pamela: The thing with these microscopic black holes is the numbers that they should exist in, imply that there might be some floating around in our own Solar System, so it might be the Mars Rovers (no, it won’t be)… it might be in our own backyard that, (if they exist) we will find these microscopic black holes. But again, we have no reason to think that they’re out there right now. But it might be one way of testing one set of theories that points toward multiple dimensions.

Fraser: All right. And I think the last one that I mentioned at the top of the show was what came before the Big Bang?

Pamela: And again this is one of those places that science just can’t quite get to. The way I like to explain it is, the places that ancient mapmakers had no clue what was there, they wrote “there be dragons”. In cosmology, the moment of the Big Bang is where we have to put “there be dragons”.   Our scientific theories can’t get beyond the cosmic microwave background in terms of observational data, so any theory that we build has to make predictions for features that we’re going to see in the cosmic microwave background. These predictions that we make really can’t go beyond the first moment in time to say what was before the first moment in time. First of all, it breaks the mathematics. If you have something before the Big Bang, you have something before time came into existence.   Beyond just breaking in terms of time, it also means that we have to start making approximations about what caused the Big Bang (we don’t know). What if it was multiple universes? We can’t prove that. What if it was just some sort of a quantum fluctuation and the entire Universe is a wave function that collapsed? Well, really we have no way of addressing that one either.   So there are theories that work mathematically, but beyond things that they can predict that we’ll find in the cosmic microwave background, these theories don’t make testable predictions. We can’t go back and see what happened before, so now we’ve wandered into a mathematical subdivision of philosophy where you have to say “based on the evidence before me, I have faith that this is the theory that makes the most sense.” At that point you’re not longer talking about science.

Fraser: But I think that with the Big Bang, when someone asks me or someone wants to have a problem with the Big Bang and says “yeah, but you can’t explain what caused the Big Bang” I just say who cares? It’s not my problem. The Big Bang does a wonderful job of explaining the Universe from the moment of the Big Bang through expansion to where we are today. It’s not its job to explain what happened before then. Some other theory – and people are hard at work with other theories (which right now are untestable and purely math) that work on that.   There’s a really good analogy, I think, with evolution. Evolution perfectly and wonderfully explains how we get life the complexity of life on Earth, and yet obviously if you think all life on Earth is connected at a genetic level, you can trace it all back to some original ancestor. So the question of where that first ancestor came from… who cares? I mean, obviously one of the most important questions we could ask, and of course I care, but that’s not evolution’s job, to answer that question. Evolution only explains the moment that first creature came about, and same with the Big Bang. It explains where we got from the Big Bang on, before that is some other theory’s job (and probably an untestable, only mathematical theory).

Pamela: A good way to think of it is science has to explain why a ball I toss into the air goes up and comes back down. Science doesn’t have to be able to explain why I decided to throw the ball.

Fraser: Right. And I think that’s a good thing about science; we don’t need to know the answer, we can change our mind, we can really enjoy the process, we can be grateful of the predictions and the evidence that we’re able to see, but we don’t necessarily need to have any kind of hard and fast faith on what we’re going to find or what the Universe tells us, you just kick back and enjoy the discoveries that are made and our increases of understanding. If it turns out that something is completely wrong… oh well. It was fun while it lasted, now we’ve got a new theory.

Pamela: Scientists are one of the few breeds of creatures that get jubilantly ecstatic when everything they thought was true turns out to be wrong, because then it’s a new problem, a new puzzle to have to try and figure out.

Fraser: So to wrap up, and what I don’t want to do with this episode is I don’t want to be too flippant about this. The problem is that a lot of the concepts we talked about this episode came out by some theorist somewhere, interesting idea but it was a hook that a science fiction writer could use to tell a story about people. “Wouldn’t it be cool if…” So from that point on, they popularized something that wasn’t mainstream, wasn’t ready, had no evidence. The fact that you know about it, and the fact that many people are so interested in it is not necessarily because of the evidence that supports the theory, it’s purely just that science fiction movie makers have popularized them and there isn’t much more we can do until more evidence shows up.

Pamela: So we’re left in a Universe with a lot of really hard to understand mathematics that makes really funky cool things possible within the framework of the math, but we’re left with a physical reality that denies a lot of the math the ability to do its funky, cool things.

Fraser: So unfortunately, no good news for any of those topics, but as we’ve said, we’re happy to be wrong, we can’t wait for the evidence to show up that tells us that any of these theories is nice and well supported, and as soon as it is we’ll report on it and move it firmly from pure math theory to evidence.   All right. Thanks Pamela, that was fun. Hope you get better, we’ll see you next week.

Pamela: Thanks Fraser, it’s been my pleasure.

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A Smithsonian magazine special report

Why String Theory Still Offers Hope We Can Unify Physics

Evidence that the universe is made of strings has been elusive for 30 years, but the theory’s mathematical insights continue to have an alluring pull

Brian Greene

Contributing Writer

In October 1984 I arrived  at Oxford University, trailing a large steamer trunk containing a couple of changes of clothing and about five dozen textbooks. I had a freshly minted bachelor’s degree in physics from Harvard, and I was raring to launch into graduate study. But within a couple of weeks, the more advanced students had sucked the wind from my sails. Change fields now while you still can, many said. There’s nothing happening in fundamental physics.

Then, just a couple of months later, the prestigious (if tamely titled) journal  Physics Letters B  published an article that ignited the first superstring revolution, a sweeping movement that inspired thousands of physicists worldwide to drop their research in progress and chase Einstein’s long-sought dream of a unified theory. The field was young, the terrain fertile and the atmosphere electric. The only thing I needed to drop was a neophyte’s inhibition to run with the world’s leading physicists. I did. What followed proved to be the most exciting intellectual odyssey of my life.

That was 30 years ago this month, making the moment ripe for taking stock: Is string theory revealing reality’s deep laws? Or, as some detractors have claimed, is it a mathematical mirage that has sidetracked a generation of physicists?

Unification has become  synonymous with Einstein, but the enterprise has been at the heart of modern physics for centuries. Isaac Newton united the heavens and Earth, revealing that the same laws governing the motion of the planets and the Moon described the trajectory of a spinning wheel and a rolling rock. About 200 years later, James Clerk Maxwell took the unification baton for the next leg, showing that electricity and magnetism are two aspects of a single force described by a single mathematical formalism. 

The next two steps, big ones at that, were indeed vintage Einstein. In 1905, Einstein linked space and time, showing that motion through one affects passage through the other, the hallmark of his special theory of relativity. Ten years later, Einstein extended these insights with his general theory of relativity, providing the most refined description of gravity, the force governing the likes of stars and galaxies. With these achievements, Einstein envisioned that a grand synthesis of all of nature’s forces was within reach. 

But by 1930, the landscape of physics had thoroughly shifted. Niels Bohr and a generation of intrepid explorers ventured deep into the microrealm, where they encountered quantum mechanics, an enigmatic theory formulated with radically new physical concepts and mathematical rules. While spectacularly successful at predicting the behavior of atoms and subatomic particles, the quantum laws looked askance at Einstein’s formulation of gravity. This set the stage for more than a half-century of despair as physicists valiantly struggled, but repeatedly failed, to meld general relativity and quantum mechanics, the laws of the large and small, into a single all-encompassing description.

Such was the case until December 1984, when John Schwarz, of the California Institute of Technology, and Michael Green, then at Queen Mary College, published a once-in-a-generation paper showing that string theory could overcome the mathematical antagonism between general relativity and quantum mechanics, clearing a path that seemed destined to reach the unified theory. 

The idea underlying string unification is as simple as it is seductive. Since the early 20th century, nature’s fundamental constituents have been modeled as indivisible particles—the most familiar being electrons, quarks and neutrinos—that can be pictured as infinitesimal dots devoid of internal machinery. String theory challenges this by proposing that at the heart of every particle is a tiny, vibrating string-like filament. And, according to the theory, the differences between one particle and another—their masses, electric charges and, more esoterically, their spin and nuclear properties—all arise from differences in how their internal strings vibrate. 

Much as the sonorous tones of a cello arise from the vibrations of the instrument’s strings, the collection of nature’s particles would arise from the vibrations of the tiny filaments described by string theory. The long list of disparate particles that had been revealed over a century of experiments would be recast as harmonious “notes” comprising nature’s score.

Most gratifying, the mathematics revealed that one of these notes had properties precisely matching those of the “graviton,” a hypothetical particle that, according to quantum physics, should carry the force of gravity from one location to another. With this, the worldwide community of theoretical physicists looked up from their calculations. For the first time, gravity and quantum mechanics were playing by the same rules. At least in theory.

I began learning the mathematical underpinnings of string theory during an intense period in the spring and summer of 1985. I wasn’t alone. Graduate students and seasoned faculty alike got swept up in the potential of string theory to be what some were calling the “final theory” or the “theory of everything.” In crowded seminar rooms and flyby corridor conversations, physicists anticipated the crowning of a new order. 

But the simplest and most important question loomed large. Is string theory right? Does the math explain our universe? The description I’ve given suggests an experimental strategy. Examine particles and if you see little vibrating strings, you’re done. It’s a fine idea in principle, but string theory’s pioneers realized it was useless in practice. The math set the size of strings to be about a million billion times smaller than even the minute realms probed by the world’s most powerful accelerators. Save for building a collider the size of the galaxy, strings, if they’re real, would elude brute force detection.

Making the situation seemingly more dire, researchers had come upon a remarkable but puzzling mathematical fact. String theory’s equations require that the universe has extra dimensions beyond the three of everyday experience—left/right, back/forth and up/down. Taking the math to heart, researchers realized that their backs were to the wall. Make sense of extra dimensions—a prediction that’s grossly at odds with what we perceive—or discard the theory.

String theorists pounced on an idea first developed in the early years of the 20th century. Back then, theorists realized that there might be two kinds of spatial dimensions: those that are large and extended, which we directly experience, and others that are tiny and tightly wound, too small for even our most refined equipment to reveal. Much as the spatial extent of an enormous carpet is manifest, but you have to get down on your hands and knees to see the circular loops making up its pile, the universe might have three big dimensions that we all navigate freely, but it might also have additional dimensions so minuscule that they’re beyond our observational reach.

In a paper submitted for publication a day after New Year’s 1985, a quartet of physicists—Philip Candelas, Gary Horowitz, Andrew Strominger and Edward Witten—pushed this proposal one step further, turning vice to virtue. Positing that the extra dimensions were minuscule, they argued, would not only explain why we haven’t seen them, but could also provide the missing bridge to experimental verification.

Strings are so small that when they vibrate they undulate not just in the three large dimensions, but also in the additional tiny ones. And much as the vibrational patterns of air streaming through a French horn are determined by the twists and turns of the instrument, the vibrational patterns of strings would be determined by the shape of the extra dimensions. Since these vibrational patterns determine particle properties like mass, electric charge and so on—properties that can be detected experimentally—the quartet had established that if you know the precise geometry of the extra dimensions, you can make predictions about the results that certain experiments would observe.

For me, deciphering the paper’s equations was one of those rare mathematical forays bordering on spiritual enlightenment. That the geometry of hidden spatial dimensions might be the universe’s Rosetta stone, embodying the secret code of nature’s fundamental constituents—well, it was one of the most beautiful ideas I’d ever encountered. It also played to my strength. As a mathematically oriented physics student, I’d already expended great effort studying topology and differential geometry, the very tools needed to analyze the mathematical form of extra-dimensional spaces.

And so, in the mid-1980s, with a small group of researchers at Oxford, we set our sights on extracting string theory’s predictions. The quartet’s paper had delineated the category of extra-dimensional spaces allowed by the mathematics of string theory and, remarkably, only a handful of candidate shapes were known. We selected one that seemed most promising, and embarked on grueling days and sleepless nights, filled with arduous calculations in higher dimensional geometry and fueled by grandiose thoughts of revealing nature’s deepest workings.

The final results that we found successfully incorporated various established features of particle physics and so were worthy of attention (and, for me, a doctoral dissertation), but were far from providing evidence for string theory. Naturally, our group and many others turned back to the list of allowed shapes to consider other possibilities. But the list was no longer short. Over the months and years, researchers had discovered ever larger collections of shapes that passed mathematical muster, driving the number of candidates into the thousands, millions, billions and then, with insights spearheaded in the mid-1990s by Joe Polchinski, into numbers so large that they’ve never been named. 

Against this embarrassment of riches, string theory offered no directive regarding which shape to pick. And as each shape would affect string vibrations in different ways, each would yield different observable consequences. The dream of extracting unique predictions from string theory rapidly faded.

From a public relations standpoint, string theorists had not prepared for this development. Like the Olympic athlete who promises eight gold medals but wins “only” five, theorists had consistently set the bar as high as it could go. That string theory unites general relativity and quantum mechanics is a profound success. That it does so in a framework with the capacity to embrace the known particles and forces makes the success more than theoretically relevant. Seeking to go even further and uniquely explain the detailed properties of the particles and forces is surely a noble goal, but one that lies well beyond the line dividing success from failure.

Nevertheless, critics who had bristled at string theory’s meteoric rise to dominance used the opportunity to trumpet the theory’s demise, blurring researchers’ honest disappointment of not reaching hallowed ground with an unfounded assertion that the approach had crashed. The cacophony grew louder still with a controversial turn articulated most forcefully by one of the founding fathers of string theory, the Stanford University theoretical physicist Leonard Susskind.

In August 2003, I was sitting with Susskind at a conference in Sigtuna, Sweden, discussing whether he really believed the new perspective he’d been expounding or was just trying to shake things up. “I do like to stir the pot,” he told me in hushed tones, feigning confidence, “but I do think this is what string theory’s been telling us.”

Susskind was arguing that if the mathematics does not identify one particular shape as the right one for the extra dimensions, perhaps there isn’t a single right shape. That is, maybe all of the shapes are right shapes in the sense that there are many universes, each with a different shape for the extra dimensions.

Our universe would then be just one of a vast collection, each with detailed features determined by the shape of their extra dimensions. Why, then, are we in this universe instead of any other? Because the shape of the hidden dimensions yields the spectrum of physical features that allow us to exist. In another universe, for example, the different shape might make the electron a little heavier or the nuclear force a little weaker, shifts that would cause the quantum processes that power stars, including our sun, to halt, interrupting the relentless march toward life on Earth.

Radical though this proposal may be, it was supported by parallel developments in cosmological thinking that suggested that the Big Bang may not have been a unique event, but was instead one of innumerable bangs spawning innumerable expanding universes, called the multiverse. Susskind was suggesting that string theory augments this grand cosmological unfolding by adorning each of the universes in the multiverse with a different shape for the extra dimensions.

With or without string theory, the multiverse is a highly controversial schema, and deservedly so. It not only recasts the landscape of reality, but shifts the scientific goal posts. Questions once deemed profoundly puzzling—why do nature’s numbers, from particle masses to force strengths to the energy suffusing space, have the particular values they do?—would be answered with a shrug. The detailed features we observe would no longer be universal truths; instead, they’d be local bylaws dictated by the particular shape of the extra dimensions in our corner of the multiverse.

Most physicists, string theorists among them, agree that the multiverse is an option of last resort. Yet, the history of science has also convinced us to not dismiss ideas merely because they run counter to expectation. If we had, our most successful theory, quantum mechanics, which describes a reality governed by wholly peculiar waves of probability, would be buried in the trash bin of physics. As Nobel laureate Steven Weinberg has said, the universe doesn’t care about what makes theoretical physicists happy.    

This spring, after nearly two years of upgrades, the Large Hadron Collider will crackle back to life, smashing protons together with almost twice the energy achieved in its previous runs. Sifting through the debris with the most complex detectors ever built, researchers will be looking for evidence of anything that doesn’t fit within the battle-tested “Standard Model of particle physics,” whose final prediction, the Higgs boson, was confirmed just before the machine went on hiatus. While it is likely that the revamped machine is still far too weak to see strings themselves, it could provide clues pointing in the direction of string theory. 

Many researchers have pinned their hopes on finding a new class of so-called “supersymmetric” particles that emerge from string theory’s highly ordered mathematical equations. Other collider signals could show hints of extra-spatial dimensions, or even evidence of microscopic black holes, a possibility that arises from string theory’s exotic treatment of gravity on tiny distance scales.

While none of these predictions can properly be called a smoking gun—various non-stringy theories have incorporated them too—a positive identification would be on par with the discovery of the Higgs particle, and would, to put it mildly, set the world of physics on fire. The scales would tilt toward string theory.

But what happens in the event—likely, according to some—that the collider yields no remotely stringy signatures?

Experimental evidence is the final arbiter of right and wrong, but a theory’s value is also assessed by the depth of influence it has on allied fields. By this measure, string theory is off the charts. Decades of analysis filling thousands of articles have had a dramatic impact on a broad swath of research cutting across physics and mathematics. Take black holes, for example. String theory has resolved a vexing puzzle by identifying the microscopic carriers of their internal disorder, a feature discovered in the 1970s by Stephen Hawking. 

Looking back, I’m gratified at how far we’ve come but disappointed that a connection to experiment continues to elude us. While my own research has migrated from highly mathematical forays into extra-dimensional arcana to more applied studies of string theory’s cosmological insights, I now hold only modest hope that the theory will confront data during my lifetime. 

Even so, string theory’s pull remains strong. Its ability to seamlessly meld general relativity and quantum mechanics remains a primary achievement, but the allure goes deeper still. Within its majestic mathematical structure, a diligent researcher would find all of the best ideas physicists have carefully developed over the past few hundred years. It’s hard to believe such depth of insight is accidental.

I like to think that Einstein would look at string theory’s journey and smile, enjoying the theory’s remarkable geometrical features while feeling kinship with fellow travelers on the long and winding road toward unification. All the same, science is powerfully self-correcting. Should decades drift by without experimental support, I imagine that string theory will be absorbed by other areas of science and mathematics, and slowly shed a unique identity. In the interim, vigorous research and a large dose of patience are surely warranted. If experimental confirmation of string theory is in the offing, future generations will look back on our era as transformative, a time when science had the fortitude to nurture a remarkable and challenging theory, resulting in one of the most profound steps toward understanding reality.

Editor's Note: The web headline has been changed to better reflect the article's content.

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Brian Greene | | READ MORE

Science columnist Brian Greene is a mathematician and physicist at Columbia University, the author of bestselling cosmology books such as The Hidden Reality , co-founder of the World Science Festival and the prime mover behind the online education resource World Science U . Photo: Lark Elliott.

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The Collider, the Particle and a Theory About Fate

By Dennis Overbye

• Oct. 12, 2009

More than a year after an explosion of sparks, soot and frigid helium shut it down, the world’s biggest and most expensive physics experiment, known as the Large Hadron Collider, is poised to start up again. In December, if all goes well, protons will start smashing together in an underground racetrack outside Geneva in a search for forces and particles that reigned during the first trillionth of a second of the Big Bang.

Then it will be time to test one of the most bizarre and revolutionary theories in science. I’m not talking about extra dimensions of space-time, dark matter or even black holes that eat the Earth. No, I’m talking about the notion that the troubled collider is being sabotaged by its own future. A pair of otherwise distinguished physicists have suggested that the hypothesized Higgs boson, which physicists hope to produce with the collider, might be so abhorrent to nature that its creation would ripple backward through time and stop the collider before it could make one, like a time traveler who goes back in time to kill his grandfather.

Holger Bech Nielsen, of the Niels Bohr Institute in Copenhagen, and Masao Ninomiya of the Yukawa Institute for Theoretical Physics in Kyoto, Japan, put this idea forward in a series of papers with titles like “Test of Effect From Future in Large Hadron Collider: a Proposal” and “Search for Future Influence From LHC,” posted on the physics Web site arXiv.org in the last year and a half.

According to the so-called Standard Model that rules almost all physics, the Higgs is responsible for imbuing other elementary particles with mass.

“It must be our prediction that all Higgs producing machines shall have bad luck,” Dr. Nielsen said in an e-mail message. In an unpublished essay, Dr. Nielson said of the theory, “Well, one could even almost say that we have a model for God.” It is their guess, he went on, “that He rather hates Higgs particles, and attempts to avoid them.”

This malign influence from the future, they argue, could explain why the United States Superconducting Supercollider, also designed to find the Higgs, was canceled in 1993 after billions of dollars had already been spent, an event so unlikely that Dr. Nielsen calls it an “anti-miracle.”

You might think that the appearance of this theory is further proof that people have had ample time — perhaps too much time — to think about what will come out of the collider, which has been 15 years and $9 billion in the making.

The collider was built by CERN, the European Organization for Nuclear Research, to accelerate protons to energies of seven trillion electron volts around an 18-mile underground racetrack and then crash them together into primordial fireballs.

For the record, as of the middle of September, CERN engineers hope to begin to collide protons at the so-called injection energy of 450 billion electron volts in December and then ramp up the energy until the protons have 3.5 trillion electron volts of energy apiece and then, after a short Christmas break, real physics can begin.

Dr. Nielsen and Dr. Ninomiya started laying out their case for doom in the spring of 2008. It was later that fall, of course, after the CERN collider was turned on, that a connection between two magnets vaporized, shutting down the collider for more than a year.

Dr. Nielsen called that “a funny thing that could make us to believe in the theory of ours.”

He agreed that skepticism would be in order. After all, most big science projects, including the Hubble Space Telescope, have gone through a period of seeming jinxed. At CERN, the beat goes on: Last weekend the French police arrested a particle physicist who works on one of the collider experiments, on suspicion of conspiracy with a North African wing of Al Qaeda.

Dr. Nielsen and Dr. Ninomiya have proposed a kind of test: that CERN engage in a game of chance, a “card-drawing” exercise using perhaps a random-number generator, in order to discern bad luck from the future. If the outcome was sufficiently unlikely, say drawing the one spade in a deck with 100 million hearts, the machine would either not run at all, or only at low energies unlikely to find the Higgs.

Sure, it’s crazy, and CERN should not and is not about to mortgage its investment to a coin toss. The theory was greeted on some blogs with comparisons to Harry Potter. But craziness has a fine history in a physics that talks routinely about cats being dead and alive at the same time and about anti-gravity puffing out the universe.

As Niels Bohr, Dr. Nielsen’s late countryman and one of the founders of quantum theory, once told a colleague: “We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct.”

Dr. Nielsen is well-qualified in this tradition. He is known in physics as one of the founders of string theory and a deep and original thinker, “one of those extremely smart people that is willing to chase crazy ideas pretty far,” in the words of Sean Carroll, a Caltech physicist and author of a coming book about time, “From Eternity to Here.”

Another of Dr. Nielsen’s projects is an effort to show how the universe as we know it, with all its apparent regularity, could arise from pure randomness, a subject he calls “random dynamics.”

Dr. Nielsen admits that he and Dr. Ninomiya’s new theory smacks of time travel, a longtime interest, which has become a respectable research subject in recent years. While it is a paradox to go back in time and kill your grandfather, physicists agree there is no paradox if you go back in time and save him from being hit by a bus. In the case of the Higgs and the collider, it is as if something is going back in time to keep the universe from being hit by a bus. Although just why the Higgs would be a catastrophe is not clear. If we knew, presumably, we wouldn’t be trying to make one.

We always assume that the past influences the future. But that is not necessarily true in the physics of Newton or Einstein. According to physicists, all you really need to know, mathematically, to describe what happens to an apple or the 100 billion galaxies of the universe over all time are the laws that describe how things change and a statement of where things start. The latter are the so-called boundary conditions — the apple five feet over your head, or the Big Bang.

The equations work just as well, Dr. Nielsen and others point out, if the boundary conditions specify a condition in the future (the apple on your head) instead of in the past, as long as the fundamental laws of physics are reversible, which most physicists believe they are.

“For those of us who believe in physics,” Einstein once wrote to a friend, “this separation between past, present and future is only an illusion.”

In Kurt Vonnegut’s novel “Sirens of Titan,” all of human history turns out to be reduced to delivering a piece of metal roughly the size and shape of a beer-can opener to an alien marooned on Saturn’s moon so he can repair his spaceship and go home.

Whether the collider has such a noble or humble fate — or any fate at all — remains to be seen. As a Red Sox fan my entire adult life, I feel I know something about jinxes.

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Tom Cheshire

String theory: Lauren Beukes plots her time-travel murder-mystery

This article was taken from the May 2013 issue of Wired magazine. Be the first to read Wired's articles in print before they're posted online, and get your hands on loads of additional content by <span class="s1">subscribing online .

Lauren Beukes has a murder wall. "It's full of crazy pictures, three different timelines, murder dates..." The elaborate web of string, photos and objects above her desk helped Beukes to plot her latest book, The Shining Girls , about a time-travelling serial killer. "It's been completely insane trying to keep track of all of this," she says. Her other essential tool: "Scrivener."

The novel, published this month, began as a tweet: she mentioned the idea in passing before a friend told her to delete the tweet and write a book (the tweet is indeed now deleted, lost to posterity). The story opens in 30s Chicago, where Harper Curtis finds a key to a strange house. The house lets him travel through time and points out his victims (the girls who "shine"). He embarks on a killing spree with a twist: he visits each of his victims as children, teenagers and adults, leaving them with a memento to mark them out, then returns years later to kill them (it turns out that a house-shaped time machine is the perfect getaway vehicle). Each victim's backstory is meticulously researched and the body count soon hits double figures: "I had a bunch of others I was going to kill as well, but I got murder fatigue," the Cape Town-based writer says.

Fate catches up with Curtis when Kirby Mazrachi survives her murder and comes after him. "I'm going with linear, fatalistic time travel, which is when you can't change it. It's Greek tragedy -- Oedipus trying to resist his fate and, no matter what happens, reinforcing it."

Beukes's previous two books,

Moxyland and Zoo City , featured South African cities but her next, Broken Monsters , is set in Detroit, with killings even stranger, she says, than The Shining Girls '. "There's this crazy insane hoarder room in the back of my mind, all kinds of weird stuff I've kept back," the 36-year-old says. "Actually, it's more like a mad science experiment, with all sorts of horrible mutations."

The Shining Girls is published on 25 April

This article was originally published by WIRED UK

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