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Would you really age more slowly on a spaceship at close to light speed?

• Neel V. Patel archive page

Every week, the readers of our space newsletter, The Airlock , send in their questions for space reporter Neel V. Patel to answer. This week: time dilation during space travel. 

I heard that time dilation affects high-speed space travel and I am wondering the magnitude of that affect. If we were to launch a round-trip flight to a nearby exoplanet—let's say 10 or 50 light-years away––how would that affect time for humans on the spaceship versus humans on Earth? When the space travelers came back, will they be much younger or older relative to people who stayed on Earth? —Serge

Time dilation is a concept that pops up in lots of sci-fi, including Orson Scott Card’s Ender’s Game , where one character ages only eight years in space while 50 years pass on Earth. This is precisely the scenario outlined in the famous thought experiment the Twin Paradox : an astronaut with an identical twin at mission control makes a journey into space on a high-speed rocket and returns home to find that the twin has aged faster.

Time dilation goes back to Einstein’s theory of special relativity, which teaches us that motion through space actually creates alterations in the flow of time. The faster you move through the three dimensions that define physical space, the more slowly you’re moving through the fourth dimension, time––at least relative to another object. Time is measured differently for the twin who moved through space and the twin who stayed on Earth. The clock in motion will tick more slowly than the clocks we’re watching on Earth. If you’re able to travel near the speed of light, the effects are much more pronounced. 

Unlike the Twin Paradox, time dilation isn’t a thought experiment or a hypothetical concept––it’s real. The 1971 Hafele-Keating experiments proved as much, when two atomic clocks were flown on planes traveling in opposite directions. The relative motion actually had a measurable impact and created a time difference between the two clocks. This has also been confirmed in other physics experiments (e.g., fast-moving muon particles take longer to decay ). 

So in your question, an astronaut returning from a space journey at “relativistic speeds” (where the effects of relativity start to manifest—generally at least one-tenth the speed of light ) would, upon return, be younger than same-age friends and family who stayed on Earth. Exactly how much younger depends on exactly how fast the spacecraft had been moving and accelerating, so it’s not something we can readily answer. But if you’re trying to reach an exoplanet 10 to 50 light-years away and still make it home before you yourself die of old age, you’d have to be moving at close to light speed. 

There’s another wrinkle here worth mentioning: time dilation as a result of gravitational effects. You might have seen Christopher Nolan’s movie Interstellar , where the close proximity of a black hole causes time on another planet to slow down tremendously (one hour on that planet is seven Earth years).

This form of time dilation is also real, and it’s because in Einstein’s theory of general relativity, gravity can bend spacetime, and therefore time itself. The closer the clock is to the source of gravitation, the slower time passes; the farther away the clock is from gravity, the faster time will pass. (We can save the details of that explanation for a future Airlock.)

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Why does time change when traveling close to the speed of light? A physicist explains

Assistant Professor of Physics and Astronomy, Rochester Institute of Technology

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Michael Lam does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Rochester Institute of Technology provides funding as a member of The Conversation US.

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Why does time change when traveling close to the speed of light? – Timothy, age 11, Shoreview, Minnesota

Imagine you’re in a car driving across the country watching the landscape. A tree in the distance gets closer to your car, passes right by you, then moves off again in the distance behind you.

Of course, you know that tree isn’t actually getting up and walking toward or away from you. It’s you in the car who’s moving toward the tree. The tree is moving only in comparison, or relative, to you – that’s what we physicists call relativity . If you had a friend standing by the tree, they would see you moving toward them at the same speed that you see them moving toward you.

In his 1632 book “ Dialogue Concerning the Two Chief World Systems ,” the astronomer Galileo Galilei first described the principle of relativity – the idea that the universe should behave the same way at all times, even if two people experience an event differently because one is moving in respect to the other.

If you are in a car and toss a ball up in the air, the physical laws acting on it, such as the force of gravity, should be the same as the ones acting on an observer watching from the side of the road. However, while you see the ball as moving up and back down, someone on the side of the road will see it moving toward or away from them as well as up and down.

Special relativity and the speed of light

Albert Einstein much later proposed the idea of what’s now known as special relativity to explain some confusing observations that didn’t have an intuitive explanation at the time. Einstein used the work of many physicists and astronomers in the late 1800s to put together his theory in 1905, starting with two key ingredients: the principle of relativity and the strange observation that the speed of light is the same for every observer and nothing can move faster. Everyone measuring the speed of light will get the same result, no matter where they are or how fast they are moving.

Let’s say you’re in the car driving at 60 miles per hour and your friend is standing by the tree. When they throw a ball toward you at a speed of what they perceive to be 60 miles per hour, you might logically think that you would observe your friend and the tree moving toward you at 60 miles per hour and the ball moving toward you at 120 miles per hour. While that’s really close to the correct value, it’s actually slightly wrong.

This discrepancy between what you might expect by adding the two numbers and the true answer grows as one or both of you move closer to the speed of light. If you were traveling in a rocket moving at 75% of the speed of light and your friend throws the ball at the same speed, you would not see the ball moving toward you at 150% of the speed of light. This is because nothing can move faster than light – the ball would still appear to be moving toward you at less than the speed of light. While this all may seem very strange, there is lots of experimental evidence to back up these observations.

Time dilation and the twin paradox

Speed is not the only factor that changes relative to who is making the observation. Another consequence of relativity is the concept of time dilation , whereby people measure different amounts of time passing depending on how fast they move relative to one another.

Each person experiences time normally relative to themselves. But the person moving faster experiences less time passing for them than the person moving slower. It’s only when they reconnect and compare their watches that they realize that one watch says less time has passed while the other says more.

This leads to one of the strangest results of relativity – the twin paradox , which says that if one of a pair of twins makes a trip into space on a high-speed rocket, they will return to Earth to find their twin has aged faster than they have. It’s important to note that time behaves “normally” as perceived by each twin (exactly as you are experiencing time now), even if their measurements disagree.

You might be wondering: If each twin sees themselves as stationary and the other as moving toward them, wouldn’t they each measure the other as aging faster? The answer is no, because they can’t both be older relative to the other twin.

The twin on the spaceship is not only moving at a particular speed where the frame of references stay the same but also accelerating compared with the twin on Earth. Unlike speeds that are relative to the observer, accelerations are absolute. If you step on a scale, the weight you are measuring is actually your acceleration due to gravity. This measurement stays the same regardless of the speed at which the Earth is moving through the solar system, or the solar system is moving through the galaxy or the galaxy through the universe.

Neither twin experiences any strangeness with their watches as one moves closer to the speed of light – they both experience time as normally as you or I do. It’s only when they meet up and compare their observations that they will see a difference – one that is perfectly defined by the mathematics of relativity.

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Do Frequent Fliers Age More Slowly?

According to einstein's theory of relativity, air travel and time travel are intertwined., valerie ross • october 20, 2010.

You’re squeezed into a middle seat, two rows from the back of the plane. It’s barely two hours into your cross-country flight, though you’d swear it’s been longer. Does it just seem like the minutes of your trip are crawling by — or does time actually pass more slowly for people who are mid-flight than for people on the ground?

Many of us have heard the idea that time doesn’t pass at the same rate for everyone. It’s a common narrative in science fiction, one that has its roots in Einstein’s theory of relativity . The story starts, let’s say, with two twins, one of whom stays on Earth while the other clambers aboard a rocket that’s making a round-trip journey, at a substantial fraction of the speed of light, to a planet in a not-too-distant solar system. When the traveling twin returns to earth, he’s aged more slowly, and now he’s younger than the twin who stayed behind.

This familiar — and paradoxical — plotline comes from a particular tenet of relativity theory known as time dilation. It predicts that a fast-moving clock will tick at a slower rate than a stationary one — or, a man on an interstellar voyage will age more slowly than his twin back on Earth. But time dilation also says that velocity isn’t the only thing that affects the rate at which clocks tick, or people age; gravity does, too. A clock in a stronger gravitational field (the Earth’s surface, let’s say) will have a slower tick rate than a clock subject to weaker gravity (such as a few miles up into the atmosphere).

Scientists have shown that time dilation doesn’t just happen on near-speed-of-light journeys. Physicist Chin-Wen Chou and his colleagues at the National Institute of Standards and Technology lab in Boulder, Colorado, have used super-accurate optical clocks to show that tick rates change at speeds as slow as 25 miles per hour and height differences as small as a foot.

So if time dilation occurs under these everyday conditions, is the slowed-down aging experienced by the space-faring twin also experienced — in a much subtler way — by that more familiar airborne traveler, the frequent flier? A cross-country flight is a slow-moving, brief trip compared to the odyssey of flying off to another planet, sure, but you’re still going a lot faster than someone who’s not traveling at all.

People on commercial flights are subject to both predictions of time dilation, Chou points out. They’re going fast, at speeds of around 500 miles an hour, and because they’re about six miles from the ground, they’re also feeling a weaker gravitational pull. So do airline passengers age more slowly, since they’re traveling at high speeds? Or do they age more quickly, since they’re subject to less gravity?

Chou did the math, and it turns out that frequent fliers actually age the tiniest bit more quickly than those of us with both feet on the ground. Planes travel at high enough altitudes that the weak gravitational field speeds up the tick rate of a clock on board more than the high speeds slow it down.

The difference is so small, however, that even the most tireless jet setters don’t have to worry about extra wrinkles. Consider an extreme case of the commercial air passenger: Ryan Bingham, the constantly traveling businessman played by George Clooney in “Up in the Air.” By the time Bingham racked up those 10 million frequent flier miles, Chou calculated, he’d aged only 59 microseconds more than his colleagues back in Omaha.

About the Author

Valerie Ross

Valerie Ross studied cognitive neuroscience and creative writing at Stanford University. While it was her fascination with understanding and explaining the mind and brain that first got her interested in science writing, Valerie has now written about everything from the neuroscience of memory to drug-resistant bacteria to general relativity. She has interned with Scientific American Mind , Discover , and Popular Mechanics .

How does the gravitational pull change for those inside the plane while in a controlled environment and the plane’s resisting gravity as it’s flying?

It seems intuitive that something totally external to your environment shouldn’t affect you, but General Relativity is not particularly intuitive ;)

The fact that the plane is generating lift to counteract gravity doesn’t affect the passage of time. General relativity only cares about the presence of a gravitational field (curved space time) and how strong (curved) it is. That’s what alters the passage of time. When you’re farther away from the earth, the gravitational field is slightly weaker (although that’s _not_ why astronauts are weightless in space).

Do people skip dimensions, after a few secs or minutes of time delay. Since the clocks should be slowly falling behind in stronger gravitational zones

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What would happen if the speed of light were much lower?

If light traveled very slowly, strange things would happen.

Light is the fastest-moving thing in the universe. So what would happen if the speed of light were much, much slower? 

In a vacuum, the speed of light is about 186,000 miles per second (300,000 kilometers per second). If it were orders of magnitude slower, humans would immediately take notice. 

Any gamer can experience this hypothetical scenario in a computer game that Gerd Kortemeyer, director of educational development and technology at ETH Zurich, a science, technology, engineering and mathematics university in Switzerland, and his colleagues created. In the game, you can see the bizarre effects of changing colors and brightness, and even alterations in the perceived lengths of objects, that would result from a much slower speed of light.

Related: What if Earth had rings?

Human's sluggish speeds

Even at our fastest speeds, humans are slow compared with light.

"The fastest any human has traveled is about 0.0037% the speed of light, and you need to be in some kind of space vehicle to reach those speeds," Philip Tan, research scientist at the MIT Game Lab, told Live Science. 

But by doing thought experiments, physicists have determined that unusual things would happen if humans could travel at near light speed,  said Kortemeyer, who is also an associate professor of physics at Michigan State University. According to Albert Einstein's theory of special relativity — which explains how speed affects mass, time and space — time would slow down, we would measure objects as being shorter as we whizzed past them and the Doppler effect would become visible for light, among other changes. 

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Those same changes would occur if, instead of humans speeding up, light slowed down. In both cases, we'd be moving at near light speed.

A slower speed of light

While Kortemeyer was working as a visiting professor at MIT, he, Tan and colleagues at the MIT Game Lab created a computer game to illustrate what the world would be like if the speed of light were slow enough that special relativity were noticeable in everyday life. In the game, released in 2012 and called " A Slower Speed of Light ," the player controls a character who collects beach ball-like orbs. Every time the character collects one of the 100 orbs, the speed of light slows. 

In reality, the speed of light would not slow down the way it does in the game. The speed of light in a vacuum never changes and is constant for every observer. However, the speed of light does change depending on the materials it's passing through, but that doesn't change the effects of special relativity, or how we perceive them, Kortemeyer said.

If we could witness special relativity, however, we would notice changes in colors, time, distance and brightness, and the team incorporated those effects into the game.

Color changes

When the speed of human motion approaches the speed of light, something called the relativistic Doppler effect becomes perceptible. To understand this, remember that light acts as both a particle and a wave . As a wave, it's characterized by its wavelength, or the distance from crest to crest, which determines its color, and its frequency, or how many crests pass in a given time. 

Related: What if there were no gravity?

Similar to the way that, according to the Doppler effect, approaching a sound source makes its frequency, or pitch, seem to increase as the wave crests reach your ear faster and faster, moving toward a light source makes its wavelength seem shorter, shifting the apparent color of the light toward the blue and violet end of the color spectrum, Kortemeyer said. Moving away from an object, on the other hand, shifts its apparent color toward the red end of the spectrum. In sum, "the thing coming toward you looks bluer, or the thing moving away from you looks redder," Kortemeyer said. 

Changes to time and distance

Perhaps one of the most famous effects of special relativity is that for a human moving near the speed of light, time slows down. In this scenario, a person moving at near light speed would age more slowly. This effect is called time dilation . 

In the game, "technically, you are experiencing time dilation; but without having something to compare it against, it doesn't really mean anything," Tan said. Time dilation may not be noticeable during the game, but at the end, players see a screen informing them that less time has passed for them than passed for a stationary clock, Tan said. Time dilation, like the other effects of special relativity, happens during the game because the game's character is moving close to the speed of light.

Another effect of special relativity is that the lengths of objects moving near the speed of light — or stationary objects as you whiz past them at near light speed — shorten. This is called length contraction. But the effect is complicated, Kortemeyer said. Objects zooming at close to the speed of light might experience length contraction and might be shorter, according to measurements by a stationary observer, but they would actually appear longer to that person's eyes due to another effect of special relativity called the runtime effect, Kortemeyer said. 

For example, say a bicycle is coming toward you. The light from the front of the bike has a shorter distance to travel to your eyes than light from the back of the bike. As a result, you see the front of the bike as it was more recently and the back of the bike as it was further in the past, when the bike was farther away. "That overall makes the bicycle appear longer," Kortemeyer said. Sometimes, this same effect can make objects appear warped. 

In other words, if the speed of light were much slower, objects moving near that speed might appear longer and/or warped to stationary observers.

Related: What if Earth shared its orbit with another planet?

Changes to brightness

When you walk in the rain, you might notice that you get wetter in front than in back. As you walk into the rain, you encounter more raindrops than you would standing still, but the front of you protects the back of you from those extra raindrops. Something similar would happen if you were moving at near the speed of light, Kortemeyer said. 

That's because light sometimes behaves like a collection of particles, called photons, which are like little droplets of light. As you move toward an object in the computer game, it appears brighter than it does when you're standing still, because you're walking into its photons. This is called the searchlight effect.

Mr. Tompkins in Wonderland

Kortemeyer and Tan were not the first to imagine a world with a slower speed of light. In 1939, physicist George Gamow published a picture book, called "Mr. Tompkins in Wonderland," in which the title character rides a bike through a city with a slowed speed of light and experiences relativistic effects. Einstein "really liked that little booklet," Kortemeyer said. 

— What if Earth started spinning backward?

— What if Earth were twice as big?

— What if the moon disappeared tomorrow?

What might the great physicist think of "A Slower Speed of Light"? "Curiosity might have made him play in the first place, since if historians are to be believed, already at the age of 16 he asked what you would see if you were riding a beam of light — which, of course, you can't, but in the game, you can reach almost the speed of light," Kortemeyer said. "But then I think he would have just played the video game until he got hopelessly motion sick — most physicists remain playful."

Originally published on Live Science.

Ashley P. Taylor is a writer based in Brooklyn, New York. As a science writer, she focuses on molecular biology and health, though she enjoys learning about experiments of all kinds. Ashley's work has appeared in Live Science, The New York Times blogs, The Scientist, Yale Medicine and PopularMechanics.com. Ashley studied biology at Oberlin College, worked in several labs and earned a master's degree in science journalism from New York University's Science, Health and Environmental Reporting Program. 

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Here's What Actually Happens When You Travel at the Speed of Light, According to NASA

NASA created a fun video to answer all of our burning questions about near-light-speed travel.

Ever wish you could travel at the speed of light to your favorite destinations ? Once you see the reality of that speed, you may rethink everything.

"There are some important things you should probably know about approaching the speed of light," NASA's video, Guide to Near-light-speed Travel , explains. "First, a lot of weird things can happen, like time and space getting all bent out of shape."

According to the video, if you're traveling at nearly the speed of light, the clock inside your rocket would show it takes less time to travel to your destination than it would on Earth. But, since the clocks at home would be moving at a standard rate you'd return home to everyone else being quite a bit older.

"Also, because you're going so fast, what would otherwise be just a few hydrogen atoms that you'd run into quickly becomes a lot of dangerous particles. So you should probably have shields that keep them from frying your ship and also you."

Finally, the video tackles the fact that even if you were moving at the speed of light, the "universe is also a very big place, so you might be in for some surprises." For example, your rocket's clock will say it takes about nine months to get from Earth to the edge of the solar system. An Earth clock would say it took about a year and a half. Fortunately, NASA astronauts have a slew of tips for avoiding jet lag along the way.

"If you want to get to farther out vacation spots," the video explains, "you'll probably need more than a few extra snacks. A trip to the Andromeda Galaxy, our nearest large neighbor galaxy, can take over one million years. And a trip to the farthest known galaxy where it currently sits might take over 15 billion years, which is more vacation time than I think I'll ever have."

The video doesn't explain how your rocket will travel at the speed of light. Our technology just isn't there yet, but maybe the aliens will share that tech with us soon. Until then, you can track the first crew launch of Artemis II , a rocket that will fly around the moon in 2024 before making its first lunar landing in 2025.

Does space travel make people age more slowly?

• 2 min. read ▪ Published February 22, 2021
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Scientists have recently observed for the first time that, on an epigenetic level, astronauts age more slowly during long-term simulated space travel than they would have if their feet had been planted on Planet Earth.

“Many of us assume that being exposed to radiation or other harm in space would be reflected by increased aging. But there’s also been a lot of research that has shown the opposite,” said Jamaji C. Nawanaji-Enwerem, Berkeley Public Health postdoctoral fellow and first author of a study published in Cell Reports in November 2020. The study reviewed data from the six participants of the Mars-500 mission, a simulated space travel and residence experiment launched by the European Space Agency in 2010.

In space, people usually experience environmental stressors like microgravity, cosmic radiation, and social isolation, which can all impact aging. Studies on long-term space travel often measure aging biomarkers such as telomere length and heartbeat rates, not epigenetic aging. To fill in the gap, Nawanaji-Enwerem and his team members took the novel step to look at epigenetic biomarkers such as DNAmPhenoAge, a robust marker of disease risk, and DNAmGrimAGE, a predictor of mortality risk.

The findings show that space mission duration will lead to a slower aging process, which looks like a good thing. “But if the mission goes on for longer, it can actually be a bad thing for you,” said Nawanaji-Enwerem.

“It also informs future research in terms of what biomarkers of aging are important to measure,” said Andres Cardenas, study co-author and assistant professor of Environmental Health Sciences at Berkeley Public Health.

During the Mars-500 experiment, six astronaut crews stayed in an isolated space and lived as if they were on Mars for 520 days. Cosmic radiation and microgravity were not replicated in the experiment, so the slower aging process found by scientists is caused by social isolation and other relative effects.

Although it’s not clear why space travel would lead to slower epigenetic aging, the findings will be valuable for understanding the health implications for future space travel.

“It’s not if, but when, we’re going to transition to space living,” said Cardenas.

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Three ways to travel at (nearly) the speed of light.

Katy Mersmann

1) electromagnetic fields, 2) magnetic explosions, 3) wave-particle interactions.

One hundred years ago today, on May 29, 1919, measurements of a solar eclipse offered verification for Einstein’s theory of general relativity. Even before that, Einstein had developed the theory of special relativity, which revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space — a key area of research to keep spacecraft and astronauts safe from radiation.

The theory of special relativity showed that particles of light, photons, travel through a vacuum at a constant pace of 670,616,629 miles per hour — a speed that’s immensely difficult to achieve and impossible to surpass in that environment. Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light.

One of NASA’s jobs is to better understand how these particles are accelerated. Studying these superfast, or relativistic, particles can ultimately help protect missions exploring the solar system, traveling to the Moon, and they can teach us more about our galactic neighborhood: A well-aimed near-light-speed particle can trip onboard electronics and too many at once could have negative radiation effects on space-faring astronauts as they travel to the Moon — or beyond.

Here are three ways that acceleration happens.

Most of the processes that accelerate particles to relativistic speeds work with electromagnetic fields — the same force that keeps magnets on your fridge. The two components, electric and magnetic fields, like two sides of the same coin, work together to whisk particles at relativistic speeds throughout the universe.

In essence, electromagnetic fields accelerate charged particles because the particles feel a force in an electromagnetic field that pushes them along, similar to how gravity pulls at objects with mass. In the right conditions, electromagnetic fields can accelerate particles at near-light-speed.

On Earth, electric fields are often specifically harnessed on smaller scales to speed up particles in laboratories. Particle accelerators, like the Large Hadron Collider and Fermilab, use pulsed electromagnetic fields to accelerate charged particles up to 99.99999896% the speed of light. At these speeds, the particles can be smashed together to produce collisions with immense amounts of energy. This allows scientists to look for elementary particles and understand what the universe was like in the very first fractions of a second after the Big Bang. 

Download related video from NASA Goddard’s Scientific Visualization Studio

Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.

When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. The rapid change in a region’s magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.

Those speedy particles also create a variety of side-effects near planets.  Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.

NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.

Particles can be accelerated by interactions with electromagnetic waves, called wave-particle interactions. When electromagnetic waves collide, their fields can become compressed. Charged particles bouncing back and forth between the waves can gain energy similar to a ball bouncing between two merging walls.

These types of interactions are constantly occurring in near-Earth space and are responsible for accelerating particles to speeds that can damage electronics on spacecraft and satellites in space. NASA missions, like the Van Allen Probes , help scientists understand wave-particle interactions.

Wave-particle interactions are also thought to be responsible for accelerating some cosmic rays that originate outside our solar system. After a supernova explosion, a hot, dense shell of compressed gas called a blast wave is ejected away from the stellar core. Filled with magnetic fields and charged particles, wave-particle interactions in these bubbles can launch high-energy cosmic rays at 99.6% the speed of light. Wave-particle interactions may also be partially responsible for accelerating the solar wind and cosmic rays from the Sun.

Download this and related videos in HD formats from NASA Goddard’s Scientific Visualization Studio

By Mara Johnson-Groh NASA’s Goddard Space Flight Center , Greenbelt, Md.

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Astro for kids: Why does time change when traveling close to the speed of light?

Why does time change when traveling close to the speed of light?

Timothy, age 11 Shoreview, Minnesota

Imagine you’re in a car driving across the country watching the landscape. A tree in the distance gets closer to your car, passes right by you, then moves off again in the distance behind you.

Of course, you know that tree isn’t actually getting up and walking toward or away from you. It’s you in the car who’s moving toward the tree. The tree is moving only in comparison, or relative, to you — that’s what  we physicists  call  relativity . If you had a friend standing by the tree, they would see you moving toward them at the same speed that you see them moving toward you.

In his 1632 book “ Dialogue Concerning the Two Chief World Systems ,” the astronomer Galileo Galilei first described the  principle of relativity  – the idea that the universe should behave the same way at all times, even if two people experience an event differently because one is moving in respect to the other.

If you are in a car and toss a ball up in the air, the physical laws acting on it, such as the force of gravity, should be the same as the ones acting on an observer watching from the side of the road. However, while you see the ball as moving up and back down, someone on the side of the road will see it moving toward or away from them as well as up and down.

Special relativity and the speed of light

Albert Einstein much later proposed the idea of what’s now known as  special relativity  to explain some confusing observations that didn’t have an intuitive explanation at the time. Einstein used the work of many physicists and astronomers in the late 1800s to put together his theory in 1905, starting with two key ingredients: the principle of relativity and the strange observation that the speed of light is the same for every observer and nothing can move faster. Everyone measuring the speed of light will get the same result, no matter where they are or how fast they are moving.

Let’s say you’re in the car driving at 60 miles per hour and your friend is standing by the tree. When they throw a ball toward you at a speed of what they perceive to be 60 miles per hour, you might logically think that you would observe your friend and the tree moving toward you at 60 miles per hour and the ball moving toward you at 120 miles per hour. While that’s really close to the correct value, it’s actually slightly wrong.

The experience of time is dependent on motion.

This discrepancy between what you might expect by adding the two numbers and the true answer grows as one or both of you move closer to the speed of light. If you were traveling in a rocket moving at 75% of the speed of light and your friend throws the ball at the same speed, you would not see the ball moving toward you at 150% of the speed of light. This is because nothing can move faster than light – the ball would still appear to be moving toward you at less than the speed of light. While this all may seem very strange, there is  lots of experimental evidence  to back up these observations.

Time dilation and the twin paradox

Speed is not the only factor that changes relative to who is making the observation. Another consequence of relativity is the concept of  time dilation , whereby people measure different amounts of time passing depending on how fast they move relative to one another.

Each person experiences time normally relative to themselves. But the person moving faster experiences less time passing for them than the person moving slower. It’s only when they reconnect and compare their watches that they realize that one watch says less time has passed while the other says more.

This leads to one of the strangest results of relativity – the  twin paradox , which says that if one of a pair of twins makes a trip into space on a high-speed rocket, they will return to Earth to find their twin has aged faster than they have. It’s important to note that time behaves “normally” as perceived by each twin (exactly as you are experiencing time now), even if their measurements disagree.

The twin paradox isn’t actually a paradox.

You might be wondering: If each twin sees themselves as stationary and the other as moving toward them, wouldn’t they each measure the other as aging faster? The answer is no, because they can’t both be older relative to the other twin.

The twin on the spaceship is not only moving at a particular speed where the frame of references stay the same but also accelerating compared with the twin on Earth. Unlike speeds that are relative to the observer, accelerations are absolute. If you step on a scale, the weight you are measuring is actually your acceleration due to gravity. This measurement stays the same regardless of the speed at which the Earth is moving through the solar system, or the solar system is moving through the galaxy or the galaxy through the universe.

Neither twin experiences any strangeness with their watches as one moves closer to the speed of light — they both experience time as normally as you or I do. It’s only when they meet up and compare their observations that they will see a difference – one that is perfectly defined by the mathematics of relativity.

Michael Lam , Assistant Professor of Physics and Astronomy,  Rochester Institute of Technology

This article is republished from  The Conversation  under a Creative Commons license. Read the  original article .

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to  [email protected] . Please tell us your name, age and the city where you live.

And since curiosity has no age limit — adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

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Science News

Speed of light not so constant after all.

Pulse structure can slow photons, even in a vacuum

SHIFTING SPEEDS  Even in vacuum conditions, light can move slower than its maximum speed depending on the structure of its pulses. The finding could be important for physicists studying extremely short light pulses.

Jeff Keyzer/Flickr ( CC BY-SA 2.0 )

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By Andrew Grant

January 17, 2015 at 6:00 pm

Light doesn’t always travel at the speed of light. A new experiment reveals that focusing or manipulating the structure of light pulses reduces their speed, even in vacuum conditions.

A paper reporting the research, posted online at arXiv.org and accepted for publication, describes hard experimental evidence that the speed of light, one of the most important constants in physics, should be thought of as a limit rather than an invariable rate for light zipping through a vacuum.

“It’s very impressive work,” says Robert Boyd, an optical physicist at the University of Rochester in New York. “It’s the sort of thing that’s so obvious, you wonder why you didn’t think of it first.”

Researchers led by optical physicist Miles Padgett at the University of Glasgow demonstrated the effect by racing photons that were identical except for their structure. The structured light consistently arrived a tad late. Though the effect is not recognizable in everyday life and in most technological applications, the new research highlights a fundamental and previously unappreciated subtlety in the behavior of light.

The speed of light in a vacuum, usually denoted c, is a fundamental constant central to much of physics, particularly Einstein’s theory of relativity. While measuring c was once considered an important experimental problem, it is now simply specified to be 299,792,458 meters per second, as the meter itself is defined in terms of light’s vacuum speed. Generally if light is not traveling at c it is because it is moving through a material. For example, light slows down when passing through glass or water.

Padgett and his team wondered if there were fundamental factors that could change the speed of light in a vacuum. Previous studies had hinted that the structure of light could play a role. Physics textbooks idealize light as plane waves, in which the fronts of each wave move in parallel, much like ocean waves approaching a straight shoreline. But while light can usually be approximated as plane waves, its structure is actually more complicated. For instance, light can converge upon a point after passing through a lens. Lasers can shape light into concentrated or even bull’s-eye–shaped beams.

The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber. The other photon went through a pair of devices that manipulated the structure of the light and then switched it back. Had structure not mattered, the two photons would have arrived at the same time. But that didn’t happen. Measurements revealed that the structured light consistently arrived several micrometers late per meter of distance traveled.

“I’m not surprised the effect exists,” Boyd says. “But it’s surprising that the effect is so large and robust.”

Greg Gbur, an optical physicist at the University of North Carolina at Charlotte, says the findings won’t change the way physicists look at the aura emanating from a lamp or flashlight. But he says the speed corrections could be important for physicists studying extremely short light pulses.

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What would happen if we travelled at the speed of light?

My youngest child is seven; he is a boy of many questions. Lately he has turned his attention to speed, specifically the speed of light, and what would happen if you travelled that fast.

The first question came at bed time (why is it always bed time??). He wanted to know what would happen if he travelled at the speed of light and would it change time. I answered as best I could (while trying to back out the door and turn off the light) and left it at that but the question has resurfaced and I know this little guy will not let it rest until he is sure he has full understanding of the answer. So, to satisfy my own son’s curiosity, and in case anyone else out there wanted to know… here is a quick low down on high speed.

Let’s start with the basics

Firstly, the speed of light is a staggering 299,792,458 metres per second (or approximately 299 792 kilometres per second). Albert Einstein may not have calculated this, but he was the one that recognised it as the fasted thing in our Universe, a cosmic speed limit.

This is the speed of light in a vacuum and is commonly denoted as c . Light travelling at different speeds depending on what it is travelling through, so for light to travel through anything other than a vacuum, it will travel a little slower. For example, light travels about 90,000 m/s slower in air (that’s about 0.03% slower).

In water light travels at 75% the speed it would in a vacuum.

It’s all relative

Einstein’s work on this cosmic speed limit led him to develop a little theory , calling it the Theory of Relativity .

Einstein’s Theory of Relativity… E = mc 2

E stands for energy, m is the mass of the object and c is the speed of light. But it still looks pretty confusing, right? Keeping it simple, this equation says two interesting things…

• it ties mass and energy together
• it says that nothing with mass can travel as fast as, or faster than the speed of light

You might like a refresher on what mass is… mass is basically a measure of how much matter (atoms) something is made up of, or how densely packed those atoms are. We usually talk about mass in terms of weight (kilograms) but when we do so, we are typically saying how much it weighs here on Earth.

Close, but not close enough

Light is made up things called photons and they have no mass. Everything else we can think of in our everyday lives does have mass.

Applying Einstein’s Theory of Relativity, the closer an object (with mass) gets to the speed of light, the more energy is required to keep it moving, until eventually the object would have an infinite mass and require and infinite amount of energy to move it… and that’s just not possible.

So nothing with mass, including us, or a big rocket, can move faster than the speed of light.

The fastest speed of a manned spacecraft to date was achieved by the Apollo 10 lunar module , on May 26, 1969 when it reached speeds of 39,897 km/h (about 11 km/s) before re-entering the Earth’s atmosphere.

Take your time

Where does time come into all this? Well, you might remember that the c in E=mc 2 is a unit with distance and time in it, so time is part of the equation too.

What happens to time when we start to travel at close to the speed of light? The answer to that depends on where you are standing, in other words, it depends on where you are observing from.

Let’s take an example, and remember, this is all hypothetical … you are in a rocket travelling through space and you manage to travel at speeds approaching the speed of light. So for you, time slows down and you reach your destination in a relatively short space of time. You arrive, do whatever it is you went there to do and then head back to Earth (again at speeds close to the speed of light).

The main thing you would notice when you get back home is how old everyone is! People who were the same age as you when you left would be a lot older than you when you come back. Remember, as Einstein said, it’s all relative! It depends on where you are observing from; if you are on Earth then time continues as normal. But if you head off into space and travel at speeds that slow down time, then a little time for you will equal a lot of time back on Earth.

Scientists like to call this the twin paradox ; if you took a set of identical twins and sent one travelling off in space at speeds close to the speed of light and left the other here on Earth, when the first twin returned from his cosmic travels he would be younger than his twin who remained on Earth.

In summary… we can’t actually travel at the speed of light, but if we could travel close to the speed of light then yes, time would slow down (for us anyway) but by the time we got back to Earth, everyone else would have aged more than us!

What did my son think of my explanation? I read this post to him last night and broke some of the theories down into seven year-old sized chunks of information and he was happy enough with the answer, he especially liked the twin paradox 🙂

Then he added some theories of his own… I’m not sure what Einstein would make of these but this guy certainly has some interesting ideas; Have a listen to a seven year-old’s theories on what else would happen if you travelled close to the speed of sound! 

Science blogger and writer; Owner of Dr. How's Science Wows; Mother of three junior scientists who have taught me that to be a great scientist you need to look at life through the eyes of a child!

3 thoughts on “ What would happen if we travelled at the speed of light? ”

Pingback: Do fish ever get thirsty? - Dr. How's Science Wows

Awesome! I just love those explanations! Keep up the great content 🙂

Thanks Diane… I may even be able to hand over the content generation to the 7yo soon 😉

Comments are closed.

What is the speed of light?

The speed of light is the speed limit of the universe. Or is it?

What is a light-year?

• Speed of light FAQs
• Special relativity
• Faster than light
• Slowing down light
• Faster-than-light travel

Bibliography

The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed. 

According to physicist Albert Einstein 's theory of special relativity , on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter's mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe . The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology , it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin .

But despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend time contemplating faster-than-light travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.

Related: Special relativity holds up to a high-energy test

A l ight-year is the distance that light can travel in one year — about 6 trillion miles (10 trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe.

Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight takes about 8 minutes to reach our eyes, so the sun is about 8 light minutes away. Light from Alpha Centauri , which is the nearest star system to our own, requires roughly 4.3 years to get here, so Alpha Centauri is 4.3 light-years away.

"To obtain an idea of the size of a light-year, take the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end," NASA's Glenn Research Center says on its website . "The resulting distance is almost 6 trillion (6,000,000,000,000) miles!"

Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. And everything astronomers "see" in the distant universe is literally history. When astronomers study objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them. 

This principle allows astronomers to see the universe as it looked after the Big Bang , which took place about 13.8 billion years ago. Objects that are 10 billion light-years away from us appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.

Related: Why the universe is all history

Speed of light FAQs answered by an expert

We asked Rob Zellem, exoplanet-hunter and staff scientist at NASA's Jet Propulsion Lab, a few frequently asked questions about the speed of light. 

Dr. Rob Zellem is a staff scientist at NASA's Jet Propulsion Laboratory, a federally funded research and development center operated by the California Institute of Technology. Rob is the project lead for Exoplanet Watch, a citizen science project to observe exoplanets, planets outside of our own solar system, with small telescopes. He is also the Science Calibration lead for the Nancy Grace Roman Space Telescope's Coronagraph Instrument, which will directly image exoplanets. 

What is faster than the speed of light?

Nothing! Light is a "universal speed limit" and, according to Einstein's theory of relativity, is the fastest speed in the universe: 300,000 kilometers per second (186,000 miles per second). 

Is the speed of light constant?

The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light *can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second). 

Who discovered the speed of light?

One of the first measurements of the speed of light was by Rømer in 1676 by observing the moons of Jupiter . The speed of light was first measured to high precision in 1879 by the Michelson-Morley Experiment. 

How do we know the speed of light?

Rømer was able to measure the speed of light by observing eclipses of Jupiter's moon Io. When Jupiter was closer to Earth, Rømer noted that eclipses of Io occurred slightly earlier than when Jupiter was farther away. Rømer attributed this effect due the time it takes for light to travel over the longer distance when Jupiter was farther from the Earth. 

How did we learn the speed of light?

As early as the 5th century BC, Greek philosophers like Empedocles and Aristotle disagreed on the nature of light speed. Empedocles proposed that light, whatever it was made of, must travel and therefore, must have a rate of travel. Aristotle wrote a rebuttal of Empedocles' view in his own treatise, On Sense and the Sensible , arguing that light, unlike sound and smell, must be instantaneous. Aristotle was wrong, of course, but it would take hundreds of years for anyone to prove it. 

In the mid 1600s, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart. Each person held a shielded lantern. One uncovered his lantern; when the other person saw the flash, he uncovered his too. But Galileo's experimental distance wasn't far enough for his participants to record the speed of light. He could only conclude that light traveled at least 10 times faster than sound.

In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA , accidentally came up with a new best estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter's moon , Io, from Earth . Over time, Rømer observed that Io's eclipses often differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from one another, showed up ahead of time when the planets were approaching and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know as the Doppler effect, the change in frequency of light or sound emitted by a moving object that in the astronomical world manifests as the so-called redshift , the shift towards "redder", longer wavelengths in objects speeding away from us. In a leap of intuition, Rømer determined that light was taking measurable time to travel from Io to Earth. 

Rømer used his observations to estimate the speed of light. Since the size of the solar system and Earth's orbit wasn't yet accurately known, argued a 1998 paper in the American Journal of Physics , he was a bit off. But at last, scientists had a number to work with. Rømer's calculation put the speed of light at about 124,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley based a new set of calculations on the change in the apparent position of stars caused by Earth's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — accurate to within about 1% of the real value, according to the American Physical Society .

Two new attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly rotating toothed wheel, with a mirror set up 5 miles (8 km) away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel to perform essentially the same experiment. The two independent methods each came within about 1,000 miles per second (1,609 km/s) of the speed of light.

Another scientist who tackled the speed of light mystery was Poland-born Albert A. Michelson, who grew up in California during the state's gold rush period, and honed his interest in physics while attending the U.S. Naval Academy, according to the University of Virginia . In 1879, he attempted to replicate Foucault's method of determining the speed of light, but Michelson increased the distance between mirrors and used extremely high-quality mirrors and lenses. Michelson's result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, until Michelson re-measured it himself. In his second round of experiments, Michelson flashed lights between two mountain tops with carefully measured distances to get a more precise estimate. And in his third attempt just before his death in 1931, according to the Smithsonian's Air and Space magazine, he built a mile-long depressurized tube of corrugated steel pipe. The pipe simulated a near-vacuum that would remove any effect of air on light speed for an even finer measurement, which in the end was just slightly lower than the accepted value of the speed of light today. 

Michelson also studied the nature of light itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang . The best minds in physics at the time of Michelson's experiments were divided: Was light a wave or a particle? 

Michelson, along with his colleague Edward Morley, worked under the assumption that light moved as a wave, just like sound. And just as sound needs particles to move, Michelson and Morley and other physicists of the time reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (also known as "ether"). 

Though Michelson and Morley built a sophisticated interferometer (a very basic version of the instrument used today in LIGO facilities), Michelson could not find evidence of any kind of luminiferous aether whatsoever. Light, he determined, can and does travel through a vacuum.

"The experiment — and Michelson's body of work — was so revolutionary that he became the only person in history to have won a Nobel Prize for a very precise non-discovery of anything," Siegal wrote. "The experiment itself may have been a complete failure, but what we learned from it was a greater boon to humanity and our understanding of the universe than any success would have been!"

Special relativity and the speed of light

Einstein's theory of special relativity unified energy, matter and the speed of light in a famous equation: E = mc^2. The equation describes the relationship between mass and energy — small amounts of mass (m) contain, or are made up of, an inherently enormous amount of energy (E). (That's what makes nuclear bombs so powerful: They're converting mass into blasts of energy.) Because energy is equal to mass times the speed of light squared, the speed of light serves as a conversion factor, explaining exactly how much energy must be within matter. And because the speed of light is such a huge number, even small amounts of mass must equate to vast quantities of energy.

In order to accurately describe the universe, Einstein's elegant equation requires the speed of light to be an immutable constant. Einstein asserted that light moved through a vacuum, not any kind of luminiferous aether, and in such a way that it moved at the same speed no matter the speed of the observer. 

Think of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative movement to themselves as zero. But observers moving nearly the speed of light would still perceive light as moving away from them at more than 670 million mph. (That's because moving really, really fast is one of the only confirmed methods of time travel — time actually slows down for those observers, who will age slower and perceive fewer moments than an observer moving slowly.)

In other words, Einstein proposed that the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 

Therefore, objects with mass cannot ever reach the speed of light. If an object ever did reach the speed of light, its mass would become infinite. And as a result, the energy required to move the object would also become infinite: an impossibility.

That means if we base our understanding of physics on special relativity (which most modern physicists do), the speed of light is the immutable speed limit of our universe — the fastest that anything can travel. 

What goes faster than the speed of light?

Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. The universe expands at a little more than 42 miles (68 kilometers) per second for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous article for Space.com . (A megaparsec is 3.26 million light-years — a really long way.) 

In other words, a galaxy 1 megaparsec away appears to be traveling away from the Milky Way at a speed of 42 miles per second (68 km/s), while a galaxy two megaparsecs away recedes at nearly 86 miles per second (136 km/s), and so on. 

"At some point, at some obscene distance, the speed tips over the scales and exceeds the speed of light, all from the natural, regular expansion of space," Sutter explained. "It seems like it should be illegal, doesn't it?"

Special relativity provides an absolute speed limit within the universe, according to Sutter, but Einstein's 1915 theory regarding general relativity allows different behavior when the physics you're examining are no longer "local."

"A galaxy on the far side of the universe? That's the domain of general relativity, and general relativity says: Who cares! That galaxy can have any speed it wants, as long as it stays way far away, and not up next to your face," Sutter wrote. "Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."

Does light ever slow down?

Light in a vacuum is generally held to travel at an absolute speed, but light traveling through any material can be slowed down. The amount that a material slows down light is called its refractive index. Light bends when coming into contact with particles, which results in a decrease in speed.

For example, light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, slowing down by just three ten-thousandths of the speed of light. But light passing through a diamond slows to less than half its typical speed, PBS NOVA reported. Even so, it travels through the gem at over 277 million mph (almost 124,000 km/s) — enough to make a difference, but still incredibly fast.

Light can be trapped — and even stopped — inside ultra-cold clouds of atoms, according to a 2001 study published in the journal Nature . More recently, a 2018 study published in the journal Physical Review Letters proposed a new way to stop light in its tracks at "exceptional points," or places where two separate light emissions intersect and merge into one.

Researchers have also tried to slow down light even when it's traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, even as it moved through a vacuum, as described in their 2015 study published in the journal Science . In their measurements, the difference between the slowed photon and a "regular" photon was just a few millionths of a meter, but it demonstrated that light in a vacuum can be slower than the official speed of light. 

Can we travel faster than light?

— Spaceship could fly faster than light

— Here's what the speed of light looks like in slow motion

— Why is the speed of light the way it is?

Science fiction loves the idea of "warp speed." Faster-than-light travel makes countless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop back and forth between star systems with ease. 

But while faster-than-light travel isn't guaranteed impossible, we'd need to harness some pretty exotic physics to make it work. Luckily for sci-fi enthusiasts and theoretical physicists alike, there are lots of avenues to explore.

All we have to do is figure out how to not move ourselves — since special relativity would ensure we'd be long destroyed before we reached high enough speed — but instead, move the space around us. Easy, right? 

One proposed idea involves a spaceship that could fold a space-time bubble around itself. Sounds great, both in theory and in fiction.

"If Captain Kirk were constrained to move at the speed of our fastest rockets, it would take him a hundred thousand years just to get to the next star system," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in a 2010 interview with Space.com's sister site LiveScience . "So science fiction has long postulated a way to beat the speed of light barrier so the story can move a little more quickly."

Without faster-than-light travel, any "Star Trek" (or "Star War," for that matter) would be impossible. If humanity is ever to reach the farthest — and constantly expanding — corners of our universe, it will be up to future physicists to boldly go where no one has gone before.

Additional resources

For more on the speed of light, check out this fun tool from Academo that lets you visualize how fast light can travel from any place on Earth to any other. If you’re more interested in other important numbers, get familiar with the universal constants that define standard systems of measurement around the world with the National Institute of Standards and Technology . And if you’d like more on the history of the speed of light, check out the book " Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light " (Oxford, 2019) by John C. H. Spence.

Aristotle. “On Sense and the Sensible.” The Internet Classics Archive, 350AD. http://classics.mit.edu/Aristotle/sense.2.2.html .

D’Alto, Nick. “The Pipeline That Measured the Speed of Light.” Smithsonian Magazine, January 2017. https://www.smithsonianmag.com/air-space-magazine/18_fm2017-oo-180961669/ .

Fowler, Michael. “Speed of Light.” Modern Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson .

Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, and Miles J. Padgett. “Spatially Structured Photons That Travel in Free Space Slower than the Speed of Light.” Science, February 20, 2015. https://www.science.org/doi/abs/10.1126/science.aaa3035 .

Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. “Light Stops at Exceptional Points.” Physical Review Letters 120, no. 1 (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901 . 

Hazen, Robert. “What Makes Diamond Sparkle?” PBS NOVA, January 31, 2000. https://www.pbs.org/wgbh/nova/article/diamond-science/ . 

“How Long Is a Light-Year?” Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm . 

American Physical Society News. “July 1849: Fizeau Publishes Results of Speed of Light Experiment,” July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm . 

Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses.” Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017 . 

NIST. “Meet the Constants.” October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants . 

Ouellette, Jennifer. “A Brief History of the Speed of Light.” PBS NOVA, February 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/ . 

Shea, James H. “Ole Ro/Mer, the Speed of Light, the Apparent Period of Io, the Doppler Effect, and the Dynamics of Earth and Jupiter.” American Journal of Physics 66, no. 7 (July 1, 1998): 561–69. https://doi.org/10.1119/1.19020 . 

Siegel, Ethan. “The Failed Experiment That Changed The World.” Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/ . 

Stern, David. “Rømer and the Speed of Light,” October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm . 

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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