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Why FTL is Time Travel

Science Fiction is rife with FTL (faster-than-light) travel and communication, But what that often overlooks is how that automatically allows time travel. Let’s have a look.

The speed of light

I wrote about the speed of light before. That’s because the speed of light is both fascinating science, and central to science fiction.

To understand the rest of this post, there are two important things to know: the speed of light is the maximum speed of our universe , and it is constant, regardless of your frame of reference .

To summarize, nothing can go faster than light, because that would stop time and cost infinite energy. This is a consequence of the same rule of physics that says that regardless of how fast you travel, light will always travel at the speed of light for you.

Phew, that was a mouthful. Let me explain that again with an example. Imagine you travel at half the speed of light away from the sun. If you measured the speed of the light coming from the sun, it would still be the same as if you were stationary. You’d think the light from the sun would take more time to reach you, but it doesn’t, because time flows differently for you .

Keep that in mind for what comes next.

An FTL communication example

So, imagine we manage to set up a colony on Alpha Centauri , four light years away. We’d have ships that can travel at a significant fraction of the speed of light. Unfortunately, that means they still take years to cross the distance. Then, one day, we also invent an FTL radio. Whoop, whoop, we can talk to our colony in real-time!

That doesn’t sound so problematic, does it?

Now let’s add the spaceship. Talking is nice and all, but we still need to send goods and services from Earth to our colony. Well, at least we can install one of our fancy new FTL radios and talk to each other on the way.

And that’s where causality starts to break down.

Let’s do a thought experiment: the ship is well under way at near light speed, and we set up a little FTL message chain. Earth says a word to Alpha Centauri over the FTL radio. Alpha Centauri tells it to our spaceship, and our spaceship relays it back to Earth:

Earth -> Alpha Centauri: “Word” Alpha Centauri -> Spaceship: “Word” Spaceship -> Earth: “Word”

Cool, right. Stupidly simple experiment. Unfortunately, that’s not what would happen…

Frame of Reference

Since Earth and Alpha Centauri move at roughly the same speed, they are more or less in the same frame of reference light-speed wise. However, our spaceship is traveling from one to the other at near light speed. That means time is contracting according to the Lorentz factor .

From the points of view of our spaceship, time on earth appears to be going slower than on Alpha Centauri. And that means from their point of view the following happens:

Alpha Centauri -> Spaceship: “Word” Earth -> Alpha Centauri: “Word”

Wait, what? From their point of view, they receive the message before Earth sends it? Weird. But… but… okay, yeah, they see it later, but of course it actually happened the other way around. You can see lightning before you hear it too. Doesn’t matter, right?

You have to realize though, this is not just some trick of the light. For the ship, the messages are actually sent out of chronological order because time itself flows differently on the ship as opposed to Earth and Alpha Centauri. And one frame of reference is no more valid than any other. And, this out-of-order stuff gets worse, because the ship sends the message to earth too….

You see, when the ship sends the message to Earth, it does so when it receives it, so before it’s sent (see above). It actually arrives back on Earth before Earth sends it out. Oh shit, time travel. And we just created a grandfather paradox!

I won’t bore you with the math about this problem, also because I’d probably mess it up. If you want, you can calculate it yourself using the correct formulas and the theory of relativity.

Regardless, this is a problem in science fiction most writers gloss over. In part because it would complicate many scifi stories, but also because it’s hard to get it right. Unless you’re an astrophysicist, and then you might still mess it up.

An interesting question remains, though: does this mean faster-than-light travel just isn’t possible in our universe? Or does it mean that time travel is possible?

The word’s still out on this, as far as I know. Maybe we’ll find out some day. But hopefully not before it actually happens.

Categories Science

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I'm a science fiction and fantasy author/blogger from the Netherlands

Relativity and FTL Travel

By jason w. hinson ( [email protected] ), part iv: faster than light travel--concepts and their "problems", edition: 5.1 last modified: april 8, 2003 url: http://www.physicsguy.com/ftl/ ftp (text version): ftp://ftp.cc.umanitoba.ca/startrek/relativity/.

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“Why FTL Will End the Universe—and Six Ways to Avoid It in an SF Story” by John Lambshead

Every schoolboy with any interest in space opera knows two things: the first is that it is absolutely de rigueur to have your heroes buzz around in FTL ships of some sort, and the second is that it takes an infinite amount of energy to accelerate a mass to light speed because said object increases in mass with additional velocity until that mass itself becomes infinite.

This has led space opera authors to devise all sorts of clever side-steps to travel faster than light without accelerating mass to light speed. An early solution was E.E. “Doc” Smith’s inertialess drives, an idea later reused by well-known writers such as Robert A. Heinlein, Larry Niven, and Alastair Reynolds as well as in Rick Priestley’s 40K miniature game. It’s an idea not without issues. A photon may have a theoretical mass of zero at rest but it surely has momentum when it gets underway—all of which means that an inertialess space ship would bounce back like a super-ball if it collided with a photon. As there are quite a few photons knocking around in space, a trip in an inertialess starship would be a bare-knuckle ride to beat everything else in the cosmic fairground.

Other science fiction universes, such that found in the Star Wars movies, have a gizmo called a “hyperdrive,” or something similar, and leave it at that. Press the big red button and off you go. A variant is the hyper-jump where the spacecraft instantly jumps from point A to point B without actually moving. Battlestar Galactica employs something along these lines. Arguably the cleverest, and certainly the most amusing, hyper jump variant is the Infinite Improbability Drive in Douglas Adam’s The Hitchhiker’s Guide to the Galaxy .

Star Trek used warp drives, where a ship compresses space in front and expands it behind the vessel. Not too far from this concept are starships which “bend” space to “reduce” the distance between two points. People can even do it with their minds by “tesseracting” in Madeleine L’Engle’s A Wrinkle in Time . And then there are our old friends the worm holes, whether natural or synthetic. Down the rabbit hole you go, and where you end up is . . . a long way away. A variant on this are star gates as used as plot devices as in, well, the Stargate series and movies. Arthur C. Clarke probably invented the term “star gate” but A. E. van Vogt employed a similar plot device, the cyclopex, in the early fifties.

The last major group of FTL travel methods involve stepping outside the universe into somewhere else where the laws of physics are subtly different. David Drake’s RCN series books provide a highly imaginative example. In his stories normal Newtonian spaceships drop out of space to sail their way through bubble universes like Napoleonic men o’ war.

However, ingenious those these ideas be, they fail to address the key problem that makes FTL impossible, in so far as we currently understand the universe: an FTL drive is also a time machine.

We experience a Newtonian universe in our daily lives. When we travel across a fixed distance our watches inflexibly tick away the seconds at one second per second. If we want to get somewhere quicker we have to move faster: the rate of time is fixed, we can’t alter it, but speed is variable. In like manner, the same seconds are ticking away at the same rate on the watch faces of the people around us, even if they are travelling in different directions. But our experience is illusory or, to be more accurate, a special case of large objects moving slowly.

In the universe described by relativity, the speed of light is always constant irrespective of whether the object from which the light derives is stationary or moving. The laws of physics are constant throughout the universe, i.e. the speed of light is the same everywhere. It is time that is variable. Hence, the rate of time slows down as velocity climbs towards light speed.

One other key difference between the relativistic universe and our common perception of it has serious implications. We tend to see the Earth around us as stationary and to judge velocity according to that frame of reference. If I ride a bike at ten miles an hour relative to the Earth and throw a ball in front of me at ten miles an hour, the ball has an initial speed of twenty miles an hour, not ten. Fighter pilots trying to attack a distant target will accelerate their plane as much as possible before firing a missile to increase its range.

Fig 1. A confusion of starships.

When we start considering space opera warships, or any object, moving fast and firing laser cannons or similar at each other, then completely different rules apply. Take three warships moving in convoy equidistant apart in a line and let’s see the universe from Ship 2’s frame of reference. From its viewpoint, it is stationary and, because Ships 1 and 3 have the same velocity they are also stationary in 2’s frame of reference. Now let’s assume the ships have faster than light “sub-space” radios that broadcast a constant signal.

There is a mutiny on Ship 2, but the mutineers can’t just take off on a new heading because they will be seen by Ships 1 and 3 who, when they fail to get a satisfactory answer on the subspace radio, will open fire. Fortunately, one of the mutineers understands relativity and comes up with a cunning plan. He advises the mutineers to open fire simultaneously on Ships 1 and 3. Because Ship 2 is stationary in its frame of reference, both targets are at the same fixed range, and as the speed of light is also fixed, then both 1 and 3 will be destroyed simultaneously. Neither will be able to get off a warning on their FTL transmitters so that the other can take evasive action.

Now let’s introduce Ship 4 scudding along at velocity Y. In 4’s frame of reference the ship is stationary, so the convoy is moving at Y on a reciprocal heading. Therefore 2’s laser shot will have farther to go to hit Ship 1 than Ship 3 because 3 is moving towards the laser burst but 1 is moving away. The fact that 2 was moving when it fired is irrelevant. Both laser bursts cross space at the speed of light.

Ship 3 will therefore be destroyed first in Ship 4’s frame of reference. Cessation of its FTL transmission warns Ship 1 that something is wrong, so it takes evasive action. The laser misses and Ship 1 survives.

Now introduce another ship moving in the same direction as the convoy but faster so it is overtaking at speed X. Ship 5 is also stationary in its frame of reference so the convoy is moving backward, at speed X on a reciprocal course to Ship 5. The laser shot now hits Ship 1 before Ship 3 so Ship 3 can take evasive action and survive.

Frames of Reference

Ship 2: Ships 1 & 3 are destroyed but it can get on its FTL transmitter and talk to Ships 4 & 5.

Ship 4: Can talk to Ships 4, 2 & 1 but not 3 because it’s destroyed.

Ship 5: Can talk to Ships 2, 3 & 4 but not 1 because it’s destroyed.

Should be an interesting conversation!

Now this is a simplified and rather contrived tongue-in-cheek example and much better explanations written by physical scientists (I’m a biologist) can be found on the web.

For example:

https://en.wikibooks.org/wiki/Special_Relativity/Faster_than_light_signals,_causality_and_Special_Relativity

And a more detailed explanation here:

http://www.theculture.org/rich/sharpblue/archives/000089.html

The example employed here used FTL radios. Once you introduce FTL drives, then it is possible to come up with examples where a starship can return home from a journey before it sets off.

Just think, a crewman could meet himself before he left, and warn himself not to go because the captain will turn out to be a tyrant and the food terrible. But if he didn’t go then he wouldn’t warn himself not to travel so he would sign aboard.

The fact that everything can have a different frame of reference doesn’t matter so much if nothing moves faster than light. Once anything does, then the universe hits the buffers of: FTL = Time Travel = Potential Causality Paradox.

As Matthew Buckley points out, it’s as if the universe is saying to us that when it comes down to Relativity, Causality and FTL, pick two—because you can’t have all three.

Note that it is the act of sending information (including large objects as information) FTL that causes the problem, not the method by which the information is sent.

So could relativity be wrong? Well, I’m a biologist not a physicist so I have no idea ultimately, but the physical scientists seem struck on the idea and it has been repeatedly tested and not found wanting. Then what about causality? Is our idea of causality erroneous?

Causality is one of those primary building blocks of science (and reason). Although there can be some subtle effects when considering quantum mechanics, it is difficult to imagine a universe where events precede causes. And that’s before we get into the classic time paradoxes of shooting one’s grandfather.

In the final analysis, if we are not prepared to give up on causality, and relativity also remains as one of our scientific core theories, is there any way we can twist the universe to allow us to have FTL?

The key point here is to prevent a time paradox occurring. One suggestion is that the universe might act in some blind fashion to ensure causality always “works,” and so prevent paradox or, to put it another way, an immutable law ensures that the past can’t be changed. What has happened has happened and can’t be altered. So you can’t go back in time and shoot your grandfather. You may try to go back in time with that intention, but the time machine or gun will malfunction, or you will shoot your grandfather only to find out that your grandmother was “no better than she should be,” and you came from different stock.

Another related possibility is that the universe might allow a time paradox to occur but then realign itself to wipe the paradox from history as if it never occurred. If you shoot your grandfather you cease to exist, which may mean he ceases to exist to tidy things up, or your family ceases to exist or your nation, species, planet, solar system, right down to the universe itself, ceases to exist.

The third logical solution is that a paradox splits the universe into two, so each event can happen in some way without breaking causality.

Doctor Who has taken the first option: horrible monsters fly out of the Time Vortex and seal off the paradox, trying to destroy everything involved to repair the fabric of time (“Father’s Day” episode). Of course, Doctor Who is one of the few popular SF franchises that has tried to seriously tackle issues of time paradox.

A TARDIS is a highly complex sentient device and one can bet that not many were made. Also a “Time Lord” is not a normal Gallifreyan. They are superhuman (supergallifreyan?) in many ways, physically different having “looked into the abyss” with “non-linear perception of time.” So presumably Time Lords can be trusted to travel in space and time FTL without carelessly causing time paradoxes.

There are “fixed points” in the Doctor Who universe which can’t be altered. These are events that have happened to the Doctor in his past so they can’t be changed without risking a causality paradox. For example, he can’t bring a dead assistant back to life when he witnessed the death because “it’s already happened”—already happened in his frame of reference, that is.

But doesn’t the Doctor often change time? Well, not in his time line. He may have been the cause of that event that happened in our past all along and it occurs in his future (interesting ramifications from this last point—see below). There is no change in our frame of reference because, say, what the Doctor did in 55 A.D. was what had always happened. We just didn’t know that until the episode was broadcast.

In the early stories, when the Doctor returns to Gallifrey, which is in our past, he arrives a set time after he left. It’s as if the Time Lord/TARDIS combination can maintain its own frame of reference in synchronicity with Gallifrey—and hence all other time travelling Time Lords— irrespective of how he hops in and out of our time line. The Doctor also tends to drop assistants back on Earth further down their own time line from where they took off, thus preventing paradox. The one time The Doctor breaks this rule with Rose Tyler, she prevents her father’s death creating a causality break—Father’s Day—hence the appearance of the Time Vortex monsters acting as a sort of Causality Police.

It’s true that you can find all sorts of anomalies in Doctor Who , given that hundreds of script writers must have worked on the plot lines since 1966. Nevertheless, you have to give the series points for at least trying.

This brings me to think how space opera writers can handle the FTL = Time Travel conundrum. Here are some suggestions.

#1: Ignore the problem

No, really, you are writing fiction in a fantastic setting. Who cares about frames of reference? Not your readers. Not if you offer them a cracking good story populated with fascinating characters. For example, the giant sentient spacecraft in Iain M. Banks’ Culture universe “push against the grid” to go FTL. How this avoids time paradox is never touched upon and no one notices. It just does , okay! Banks and Drake, The Culture and RCN are my all-time favorite space opera book series, and that they never mention causality bothers me not a jot.

#2: Choose Causality and FTL

Abandon the Theory of Relativity as a flawed concept. In your universe the speed of light is not fixed, so unwanted time travel ceases to be an issue. Essentially, this is the approach that Dave Drake and I took in the Citizen series, although we never say so explicitly.

#3: Invent a “Plot Voucher”

Plot voucher is a term created by Nick Lowe for an object used by the protagonist to move the plot along. Q’s gadgets in James Bond films are classic examples. In this case the plot voucher is some way of negating time travel impacts. It could be a Time Standardisation Rectification Medium, better known as TIMSRECTUM, which controls the frame of reference so your heroes can’t land back at base before they take off. Just don’t go into the physics of your magic gubbins too deeply.

#4: Embrace the Issue

Tackle time paradoxes and causality maintenance as a component of your plot. Use the difficulties that can be created as problems for your protagonists to overcome. Make your space opera universe a strange and terrifying environment where the normally accepted rules of life hold no sway.

#5: Don’t Go FTL

It is possible to write space opera with STL (slower than light) spaceships using (i) the time-slowing property of fast moving objects, (ii) “stasis fields” or (iii) multi-generation colony ships. Personally speaking, I don’t find such stories as exciting as FTL based novels, but maybe that’s just me. Throw in immortality if you want the characters in your novel to meet up in the future after riding on different STL ships

#6: Sidestep the Issue

Have your protagonists travel through gates that come out in different universes so causality maintenance is then irrelevant. I really like Philip Farmer’s World of Tiers setting, which has many of the features of space opera of the star gate type but doesn’t involve FTL.

I am sure that there are many other solutions that I haven’t thought of.

So which way should an author jump? Which approach should be selected? Well, a good writer can make something out of anything. They can spin fantastic and wonderful stories out of the most unpromising material. By that token, any of these will work if handled right.

If I am prodded into choosing, I would favor approach number 1—ignore the problem. If it’s good enough for Drake, Weber, and Banks it might be good enough for you. Option 3 is fascinating and, with the right author and the right touch, could make for amazing stories. However, it is probably the most difficult of all the options to plot.

Don’t say I didn’t warn you.

Mind how you go, people, especially if you’re going FTL!

Copyright © 2018 John Lambshead

Dr. John Lambshead is a retired senior research scientist in marine biodiversity at the Natural History Museum, London. He was also the Visiting Chair at Southampton University, Oceanography, and Regent’s Lecturer, University of California. He writes military history and designs computer and fantasy games. Lambshead is the author of swashbuckling fantasy  Lucy’s Blade , contemporary urban fantasy  Wolf in Shadow , and coauthor, with nationally best-selling author David Drake, of science fiction adventures,  Into the Hinterlands and  Into the Maelstrom .

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5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

• Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
• Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

• Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
• Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

• Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
• Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

• Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
• Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

• Books:  Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

• The Solution: time travel is impossible because of the very paradox it creates.
• Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
• The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
• Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

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