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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.

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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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Scientists Believe Light Speed Travel Is Possible. Here’s How.

A functioning warp drive would allow humans to reach the far ends of the cosmos in the blink of an eye.

White and his team in LSI’s Houston laboratory were conducting research for the Defense Advanced Research Projects Agency, or DARPA, and had set up these particular experiments to study the energy densities within Casimir cavities, the mysterious spaces between microscopic metal plates in a vacuum. The data plot indicated areas of diminished energy between the plates, which caused them to push toward each other as if trying to fill the void. This is known as negative vacuum energy density, a phenomenon in quantum mechanics called, appropriately enough, the Casimir effect . It’s something that’s helping scientists understand the soupy physics of microscale structures, which some researchers hope can be applied to energy applications that are more practical, such as circuits and electromechanical systems.

But White noticed that the pattern of negative vacuum energy between the plates and around tiny cylindrical columns that they’d inserted in the space looked familiar. It precisely echoed the energy pattern generated by a type of exotic matter that some physicists believe could unlock high-speed interstellar travel. “We then looked, mathematically, at what happens if we placed a one-micron sphere inside of a four-micron cylinder under the same conditions, and found that this kind of structure could generate a little nanoscale warp bubble encapsulating that central region,” White explains.

That’s right—a warp bubble. The essential component of a heretofore fictional warp drive that has for decades been the obsession of physicists, engineers, and sci-fi fans. Warp drive, of course, is the stuff of Star Trek legend, a device enclosed within a spacecraft that gives the mortals aboard the ability to rip around the cosmos at superhuman speed. To the lay sci-fi fan, it’s a “black box”—a convenient, completely made-up workaround to avoid the harsh realities of interstellar travel. However, after decades of speculation, research, and experimentation, scientists believe a warp drive could actually work.

To emphasize: White didn’t actually make a warp bubble. But the data from his study led to an aha moment: For the first time, a buildable warp bubble showed promise of success.

Warp technology’s core science is surprisingly sound. Though the specific mechanics of an actual device haven’t been fully unpacked, the math points toward feasibility. In short, a real-life warp drive would use massive amounts of energy, which can come in the form of mass, to create enough gravitational pull to distort spacetime in a controlled fashion, allowing a ship to speed along inside a self-generated bubble that itself is able to travel at essentially any speed. Warp drives popped up in fiction intermittently for several decades before Star Trek creator Gene Roddenberry plugged one into the USS Enterprise in 1966. But Miguel Alcubierre, PhD, a Mexican theoretical physicist and professed Star Trek enthusiast, gave the idea real-world legs when he released a paper in 1994 speculating that such a drive was mathematically possible. It was the first serious treatment of a warp drive’s feasibility, and it made headlines around the world. His breakthrough inspired more scientists to nudge the theoretical aspects of warp drive toward concrete, practical applications.

“I proposed a ‘geometry’ for space that would allow faster-than-light travel as seen from far away, essentially expanding space behind the object we want to move and contracting it in front,” Alcubierre says. “This forms a ‘bubble’ of distorted space, inside of which an object—a spaceship, say—could reside.”

Physicists tend to speak in relative terms. By injecting the sly qualifier “as seen from far away,” Alcubierre might sound like he’s describing the galactic equivalent of an optical illusion —an effect perhaps similar to driving past a truck going the opposite direction on the highway when you’re both going 60 miles an hour. Sure feels like a buck-twenty, doesn’t it? But the A-to-B speed is real; the warp effect simply shortens the literal distance between two points. You’re not, strictly speaking, moving faster than light. Inside the bubble, all appears relatively normal, and light moves faster than you are, as it should. Outside the bubble, however, you’re haulin’ the mail.

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Alcubierre’s proposal had solved one of the initial hurdles to achieving warp speeds: The very idea clashes with Einstein’s long-accepted theory of general relativity, which states that nothing can travel faster than the speed of light, but it doesn’t preclude space itself from traveling faster than that. In fact, scientists speculate that the same principles explain the rapid expansion of the universe after the Big Bang .

While concluding that warp speed was indeed possible, Alcubierre also found that it would require an enormous amount of energy to sustain the warp bubble. He theorized that negative energy—the stuff hinted at by White’s experimentation with Casimir cavities—could be a solution. The only problem is that no one has yet proved that negative energy is real. It’s the unobtanium of our spacefaring imaginations, something researchers only believe to exist. In theory, however, this unknown matter may be sufficiently powerful that future warp drive designers could channel it to contract spacetime around it. In conceptual drawings of warp-capable spacecraft , enormous material rings containing this energy source surround a central fuselage. When activated, it warps spacetime around the entire ship. The more intense the warping, the faster the warp travel is achieved.

Of course, it’s not that simple. Physicist José Natário, PhD, a professor at the Instituto Superior Técnico in Lisbon, wrote his own influential paper about the mathematical feasibility of warp drives in 2001. However, he is concerned about practical conundrums, like the amount of energy required. “You need to be able to curve spacetime quite a lot in order to do this,” he says. “We’re talking about something that would be much, much more powerful than the sun.”

Alcubierre is similarly skeptical that his theoretical ideas might ever be used to develop a working warp drive. “In order to have a bubble about 100 meters wide traveling at precisely the speed of light, you would need about 100 times the mass of the planet Jupiter converted into negative energy, which of course sounds absurd,” he says. By that standard, he concludes, a warp drive is very unlikely.

Physicists love a challenge, though. In the 29 years since Alcubierre published his paper, other scientists have wrestled with the implications of the work, providing alternative approaches to generating the energy using more accessible power sources, finding oblique entry points to the problem, and batting ideas back and forth in response to one another’s papers. They use analogies involving trampolines , tablecloths, bowling balls, balloons, conveyor belts, and music to explain the physics.

They even have their own vocabulary. It’s not faster-than-light travel; it’s superluminal travel, thank you. Then there’s nonphysical and physical—a.k.a. the critical distinction between theoretical speculation and something that can actually be engineered. (Pro tip: We’re aiming for physical here, folks.) They do mention Star Trek a lot, but never Star Wars . Even the scruffiest-looking nerf herder knows that the ships in Star Wars use hyperdrive, which consumes fuel, rather than warp drives, which don’t use propulsive technology but instead rely on, well, warping. They’re also vague about details like what passengers would experience, what gravity is like on board since you’re carrying around boatloads of energy, and what would happen if someone, say, jumped out of the ship while warping. (A speculative guess: Nothing good.)

Such research isn’t typically funded by academic institutions or the DARPAs and NASAs of the world, so much of this work occurs in the scientists’ spare time. One such scientist and Star Trek enthusiast is physicist Erik Lentz, PhD. Now a researcher at Pacific Northwest National Laboratory in Richland, Washington, Lentz was doing postdoctoral work at Göttingen University in Germany when, amid the early, isolated days of the pandemic, he mulled the idea of faster-than-light travel. He published a paper in 2021 arguing that warp drives could be generated using positive energy sources instead of the negative energy that Alcubierre’s warp drive seemed to require.

“There are a number of barriers to entry to actually being able to build a warp drive,” Lentz says. “The negative energy was the most obvious, so I tried to break that barrier down.”

He explored a new class of solutions in Einstein’s general relativity while focusing on something called the weak-energy condition, which, he explains, tracks the positivity of energy in spacetime. He hit upon a “soliton solution”—a wave that maintains its shape and moves at a constant velocity—that could both satisfy the energy-level challenge and travel faster than light. Such a warp bubble could travel along using known energy sources, though harnessing those at the levels needed are still far beyond our capabilities. The next step, he notes, may be bringing the energy requirements for a warp drive to within the range of a nuclear fusion reactor.

A fusion-powered device could theoretically travel to and from Proxima Centauri , Earth’s nearest star, in years instead of decades or millennia, and then go faster and faster as power sources improve. Current conventional rocket technology, on the other hand, would take 50,000 years just for a one-way trip—assuming, of course, there was an unlimited fuel supply for those engines.

“IF YOU COLLIDE WITH SOMETHING ON YOUR PATH, IT WOULD ALMOST CERTAINLY BE CATASTROPHIC.”

Like Alcubierre’s original thesis, Lentz’s paper had a seismic impact on the warp drive community, prompting yet another group of scientists to dig into the challenge. Physicist Alexey Bobrick and technology entrepreneur Gianni Martire have been particularly prolific. In 2021, they released a paper theorizing that a class of subliminal warp drives, traveling at just a fraction of light speed, could be developed from current scientific understanding. While that paper essentially argued that it’s perfectly acceptable to walk before you can run, they followed it up with another theory earlier this year that describes how a simulated black hole , created using sound waves and glycerin and tested with a laser beam, could be used to evaluate the levels of gravitational force needed to warp spacetime. The duo coded that breakthrough into a public app that they hope will help more quickly push theoretical ideas to practical ones. Though the team is waiting for the technology to clear a peer review stage before releasing details, the app is essentially a simulator that allows scientists to enter their warp-speed equations to validate whether they’re practical.

“When somebody publishes a warp metric for the first time, people say, ‘Okay, is your metric physical?’” Martire says. The answer to that question—whether the metric has practical potential or is strictly theoretical—is hard to establish given the challenges of testing these hypotheses. That determination could take six to eight months. “Now we can tell you within seconds, and it shows you visually how off you are or how close you are,” he says.

While useful, the app will speed up the preliminary math only for future researchers. Galaxy-sized challenges remain before we ever experience turbocharged interstellar travel. Alcubierre worries in particular about what may happen near the walls of the warp bubble. The distortion of space is so violent there, he notes, that it would destroy anything that gets close. “If you collide with something on your path, it would almost certainly be catastrophic,” he says.

Natário mulls even more practical issues, like steering and stopping. “It’s a bubble of space, that you’re pushing through space,” he says. “So, you’d have to tell space ... to curve in front of your spaceship.” But therein lies the problem: You can’t signal to the space in front of you to behave the way you want it to.

His opinion? Superluminal travel is impossible. “You need these huge deformations that we have no idea how to accomplish,” Natário says. “So yes, there has been a lot of effort toward this and studying these weird solutions, but this is all still completely theoretical, abstract, and very, very, very, very far from getting anywhere near a practical warp drive.” That’s “very” to the power of four, mind you—each crushing blow pushing us exponentially, excruciatingly further and further away from our yearned-for superluminal lives.

Ultimately, the pursuit of viable high-speed interstellar transportation also points to a more pressing terrestrial challenge: how the scientific community tackles ultra-long-term challenges in the first place. Most of the research so far has come from self-starters without direct funding, or by serendipitous discoveries made while exploring often unrelated research, such as Dr. White’s work on Casimir cavities.

Many scientists argue that we’re in a multi-decade period of stagnation in physics research, and warp drive—despite its epic time horizons before initial research leads to galaxy-spanning adventures—is somewhat emblematic of that stagnation. Sabine Hossenfelder, a research fellow at the Frankfurt Institute for Advanced Studies and creator of the YouTube channel Science Without the Gobbledygook , noted in a 2020 blog post that physics research has drifted away from frequent, persistent physical experimentation to exorbitant infusions of cash into relatively few devices. She writes that with fewer experiments, serendipitous discoveries become increasingly unlikely. Without those discoveries, the technological progress needed to keep experiments economically viable never materializes.

When asked whether this applied equally to warp drive, Hossenfelder sees a faint but plausible connection. “Warp drives are an idea that is not going to lead to applications in the next 1,000 years or so,” she says. “So they don’t play a big role in that one way to another. But when it comes to the funding, you see some overlap in the problems.”

So, despite all the advances, the horizon for a warp drive remains achingly remote. That hasn’t fazed the scientists involved, though. A few years ago, while teaching in France, White visited the Strasbourg Cathedral with his wife. While admiring its 466-foot-tall spire, he was struck by the fact that construction began in 1015 but didn’t wrap up until 1439—a span of 424 years. Those who built the basement had no chance of ever seeing the finished product, but they knew they had to do their part to aid future generations. “I don’t have a crystal ball,” White says. “I don’t know what the future holds. But I know what I need to be doing right now.”

Eric Adams is a writer and photographer who focuses on technology, transportation, science, travel, and other subjects for a wide range of outlets, including Wired, The Drive, Gear Patrol, Men's Health, Popular Science, Forbes, and others.

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Brace for impact —

Just how dangerous is it to travel at 20% the speed of light, breakthrough starshot has examined the impact of stray atoms, cosmic dust..

John Timmer - Aug 23, 2016 7:12 pm UTC

Breakthrough Starshot is one of the more exciting scientific ideas that has popped up in the past decade, with its promise to deliver hardware to the nearest star in time for many people currently alive to see it. While the idea would work on paper as an extrapolation of existing technology, there are a lot of details that need to be thoroughly checked out, because it's possible that one of them could present a show-stopper.

Further Reading

There's a bit of good news there: Breakthrough Starshot is apparently funding the needed research to give its concept a thorough vetting. A recent posting to the arXiv describes a careful look at the odds of a spacecraft surviving an extended journey at the speeds planned for the trip. Overall, things look good, but a bit of shielding will be needed, and there's the potential for a catastrophic collision with a speck of dust.

The work, done by a team of four astronomers, focuses on one of the most basic issues: spacecraft survival. The goal of Breakthrough Starshot is to accelerate its craft to about 20 percent the speed of light. At that speed, even individual atoms can damage the vehicle, and a collision with a bit of dust could be catastrophic. So the team set out to quantify just how risky these collisions could be.

The space between here and the nearest star isn't completely empty. Earlier generations of stars have left a sparse collection of tiny dust particles, and various processes have left individual atoms scattered through the interstellar space. These particles pose very distinct risks, and the team worked through each separately.

The main problem with running into an atom isn't the physical collision but rather the energy deposited in the spacecraft by the impact, which can cause local heating that damages the craft in two ways. When high enough, the heating causes the material the craft is made of to evaporate into space. Below that threshold, it can cause local melting, leading to a change in material properties as the area resolidifies.

Using information on the bombardment of quartz (silicon dioxide) and measurements of the interstellar gas concentration, the authors performed some calculations to describe the damage that will occur during flight. They found that while hydrogen and helium are the most common atoms the spacecraft will encounter, heavier atoms—specifically oxygen, magnesium, and iron—will do the vast majority of the damage.

Dust presents a somewhat different problem. Small dust particles will essentially act like a simultaneous bombardment by a lot of gas atoms. That's because the energy binding things together in a dust particle is tiny compared to the energy of the collision itself, and the dust is largely composed of heavier atoms. But a sufficiently large dust particle will create a collision energetic enough to destroy a craft. And "sufficiently large" isn't very big; the authors estimate that it only has to be 15 micrometers across to kill off the craft. Fortunately, dust particles this size are rare, and the authors calculate the odds of running into one at 10 50 to one against.

Overall, the authors find the effect of gas to be minor and only likely to cause damage down to a depth of 0.1 millimeters. Dust, however, is a different story. It will evaporate about 1.5 millimeters off the surface of the spacecraft, and melting will happen at depths of up to 10 millimeters. When every gram counts, this could be significant.

That said, the authors suggest a few potential ways to lower the impact of this problem. The simplest is to just limit the cross-section of the craft in the line of travel, which would involve folding the solar sail it relies on for its initial acceleration—and possibly placing it behind a shield. The main body of the craft could also be shaped a bit like a bullet in order to limit the area that presents a dust target. As the solar sail is used for communications with Earth in some potential designs, however, this may not be possible.

Since heating poses the biggest problem, the team also suggests adding a layer of graphite to the front of the craft, as this will diffuse the heat more efficiently. Comparison of graphite to quartz indicates that the damage caused by impacts is much smaller in this material.

The one problem that these calculations don't tackle, however, is the change in momentum caused by impacts. Each dust particle will create a small jet of evaporated material that pushes against the craft itself, redirecting it slightly. Over time, these jets are likely to roughly average out, but there will be changes to the flight path based on how rough that averaging is and how long it takes to happen.

So the collisions may make it hard to get the craft as close to any planets as we would like. And given that weight and space considerations will likely limit the quality of the sensors on board, getting the craft close to planets will probably be critical.

The arXiv. Abstract number: 1608.05284  ( About the arXiv ).

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Have we made an object that could travel 1% the speed of light?

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Have we made an object that could travel at at least 1% the speed of light? – Anadi, age 14, Jammu and Kashmir, India

Light is fast . In fact, it is the fastest thing that exists, and a law of the universe is that nothing can move faster than light. Light travels at 186,000 miles per second (300,000 kilometers per second) and can go from the Earth to the Moon in just over a second. Light can streak from Los Angeles to New York in less than the blink of an eye.

While 1% of anything doesn’t sound like much, with light, that’s still really fast – close to 7 million miles per hour! At 1% the speed of light, it would take a little over a second to get from Los Angeles to New York. This is more than 10,000 times faster than a commercial jet.

The fastest things ever made

Bullets can go 2,600 mph (4,200 kmh), more than three times the speed of sound. The fastest aircraft is NASA’s X3 jet plane , with a top speed of 7,000 mph (11,200 kph). That sounds impressive, but it’s still only 0.001% the speed of light.

The fastest human-made objects are spacecraft. They use rockets to break free of the Earth’s gravity, which takes a speed of 25,000 mph (40,000 kmh). The spacecraft that is traveling the fastest is NASA’s Parker Solar Probe . After it launched from Earth in 2018, it skimmed the Sun’s scorching atmosphere and used the Sun’s gravity to reach 330,000 mph (535,000 kmh). That’s blindingly fast – yet only 0.05% of the speed of light.

Why even 1% of light speed is hard

What’s holding humanity back from reaching 1% of the speed of light? In a word, energy. Any object that’s moving has energy due to its motion. Physicists call this kinetic energy. To go faster, you need to increase kinetic energy. The problem is that it takes a lot of kinetic energy to increase speed. To make something go twice as fast takes four times the energy. Making something go three times as fast requires nine times the energy, and so on.

For example, to get a teenager who weighs 110 pounds (50 kilograms) to 1% of the speed of light would cost 200 trillion Joules (a measurement of energy). That’s roughly the same amount of energy that 2 million people in the U.S. use in a day.

How fast can we go?

It’s possible to get something to 1% the speed of light, but it would just take an enormous amount of energy. Could humans make something go even faster?

Yes! But engineers need to figure out new ways to make things move in space. All rockets, even the sleek new rockets used by SpaceX and Blue Origins, burn rocket fuel that isn’t very different from gasoline in a car. The problem is that burning fuel is very inefficient.

Other methods for pushing a spacecraft involve using electric or magnetic forces . Nuclear fusion , the process that powers the Sun, is also much more efficient than chemical fuel.

Scientists are researching many other ways to go fast – even warp drives , the faster-than-light travel popularized by Star Trek.

One promising way to get something moving very fast is to use a solar sail. These are large, thin sheets of plastic attached to a spacecraft and designed so that sunlight can push on them, like wind in a normal sail. A few spacecraft have used solar sails to show that they work, and scientists think that a solar sail could propel spacecraft to 10% of the speed of light .

One day, when humanity is not limited to a tiny fraction of the speed of light, we might travel to the stars .

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How to travel at (nearly) the speed of light.

One hundred years ago, on May 29, 1919, measurements of a solar eclipse offered proof 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.

Scientists suspect magnetic reconnection is one way that particles are accelerated to nearly light speed. This illustration depicts the magnetic fields around Earth, which snap and realign, causing charged particles to be flung away at high speeds. Find out all three ways that this acceleration happens .

Image Credit: NASA

Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

In 1676, by studying the motion of Jupiter's moon Io, Danish astronomer Ole Rømer calculated that light travels at a finite speed. Two years later, building on data gathered by Rømer, Dutch mathematician and scientist Christiaan Huygens became the first person to attempt to determine the actual speed of light, according to the American Museum of Natural History in New York City. Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum" of empty space is about 186,282 miles per second (299,792 km per second) — but his assessment showcased that light travels at an incredible speed.

According to Albert Einstein 's theory of special relativity , light travels so fast that, in a vacuum, nothing in the universe is capable of moving faster. 

"We cannot move through the vacuum of space faster than the speed of light," confirmed Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama in Huntsville.

Question answered, right? Maybe not. When light is not in a vacuum, does the rule still apply?

Related: How many atoms are in the observable universe?

"Technically, the statement 'nothing can travel faster than the speed of light' isn't quite correct by itself," at least in a non-vacuum setting, Claudia de Rham, a theoretical physicist at Imperial College London, told Live Science in an email. But there are certain caveats to consider, she said. Light exhibits both particle-like and wave-like characteristics, and can therefore be regarded as both a particle (a photon ) and a wave. This is known as wave-particle duality.

If we look at light as a wave, then there are "multiple reasons" why certain waves can travel faster than white (or colorless) light in a medium, de Rham said. One such reason, she said, is that "as light travels through a medium — for instance, glass or water droplets — the different frequencies or colors of light travel at different speeds." The most obvious visual example of this occurs in rainbows, which typically have the long, faster red wavelengths at the top and the short, slower violet wavelengths at the bottom, according to a post by the University of Wisconsin-Madison . 

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When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second). This means both radio waves and gamma rays have the same speed," Rhett Allain, a physics professor at Southeastern Louisiana University, told Live Science in an email.

So, according to de Rham, the only thing capable of traveling faster than the speed of light is, somewhat paradoxically, light itself, though only when not in the vacuum of space. Of note, regardless of the medium, light will never exceed its maximum speed of 186,282 miles per second.

Universal look

According to Cassibry, however, there is something else to consider when discussing things moving faster than the speed of light.

"There are parts of the universe that are expanding away from us faster than the speed of light, because space-time is expanding," he said. For example, the Hubble Space Telescope recently spotted 12.9 billion year-old light from a distant star known as Earendel. But, because the universe is expanding at every point, Earendel is moving away from Earth and has been since its formation, so the galaxy is now 28 billion light years away from Earth.

In this case, space-time is expanding, but the material in space-time is still traveling within the bounds of light speed.

Related: Why is space a vacuum?

So, it's clear that nothing travels faster than light that we know of, but is there any situation where it might be possible? Einstein's theory of special relativity, and his subsequent theory of general relativity, is "built under the principle that the notions of space and time are relative," de Rham said. But what does this mean? "If someone [were] able to travel faster than light and carry information with them, their notion of time would be twisted as compared to ours," de Rham said. "There could be situations where the future could affect our past, and then the whole structure of reality would stop making sense."

This would indicate that it would probably not be desirable to make a human travel faster than the speed of light. But could it ever be possible? Will there ever be a time when we are capable of creating craft that could propel materials — and ultimately humans — through space at a pace that outstrips light speed? "Theorists have proposed various types of warp bubbles that could enable faster-than-light travel," Cassibry said.

But is de Rham convinced?

"We can imagine being able to communicate at the speed of light with systems outside our solar system ," de Rham said. "But sending actual physical humans at the speed of light is simply impossible, because we cannot accelerate ourselves to such speed.

"Even in a very idealistic situation where we imagine we could keep accelerating ourselves at a constant rate — ignoring how we could even reach a technology that could keep accelerating us continuously — we would never actually reach the speed of light," she added. "We could get close, but never quite reach it."

Related: How long is a galactic year?

This is a point confirmed by Cassibry. "Neglecting relativity, if you were to accelerate with a rate of 1G [Earth gravity], it would take you a year to reach the speed of light. However, you would never really reach that velocity because as you start to approach lightspeed, your mass energy increases, approaching infinite. "One of the few known possible 'cheat codes' for this limitation is to expand and contract spacetime, thereby pulling your destination closer to you. There seems to be no fundamental limit on the rate at which spacetime can expand or contract, meaning we might be able to get around this velocity limit someday."

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

— What if the speed of sound were as fast as the speed of light?

— How does the rubber pencil illusion work?

Allain is similarly confident that going faster than light is far from likely, but, like Cassibry, noted that if humans want to explore distant planets, it may not actually be necessary to reach such speeds. "The only way we could understand going faster than light would be to use some type of wormhole in space," Allain said. "This wouldn't actually make us go faster than light, but instead give us a shortcut to some other location in space."

Cassibry, however, is unsure if wormholes will ever be a realistic option.

"Wormholes are theorized to be possible based on a special solution to Einstein's field equations," he said. "Basically, wormholes, if possible, would give you a shortcut from one destination to another. I have no idea if it's possible to construct one, or how we would even go about doing it." Originally published on Live Science.

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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3 Ways Fundamental Particles Travel at (Nearly) the Speed of Light

Light-speed travel is a staple of science fiction in space. No "Star Wars" movie seems complete until the Millennium Falcon (or a rival ship) uses its hyperdrive. And many "Star Trek" fans enjoy talking about the relative star-system-jumping speeds of the USS Enterprise, against the speeds of other Federation ships.

But in real life, physics gets in the way. Einstein's theory of special relativity essentially puts a speed limit on cosmic travel; as far as we can tell, nothing goes faster than the speed of light. Worse, any object that has mass tends to get more and more massive — dragging down the object's velocity — as it approaches light speed. So as far as we know, only small particles can get anywhere near the speed of light.

One hundred years ago, on May 29, 1919, scientists performed measurements of a solar eclipse that confirmed Einstein's work. To celebrate, NASA offered three ways that particles can accelerate to amazing speed in a new statement .

Related: Why Don't We Have a 'Star Wars' Hyperdrive Yet?  

Electromagnetic fields

The sun is a wacky environment to study physics, because it is so extreme compared to Earth. It's also a real-life laboratory showing how nuclear reactions happen. It also is an example of an environment with electromagnetic fields — which, as NASA points out, is the same force that stops magnets from falling off your fridge.

Magnetic fields and electric fields work together to accelerate particles with an electric charge. This charge allows electromagnetic fields to push particles along — sometimes at speeds approaching the speed of light.

We can even simulate this process on Earth. Huge particle accelerators (like at the Department of Energy's Fermi National Accelerator Laboratory, or at the European Organization for Nuclear Research's Large Hadron Collider ) create pulsed electromagnetic fields. These fields accelerate charged particles close to the speed of light. Next, scientists often crash these particles together to see what particles and energy are released. 

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In fractions of a second after these collisions, we can quickly observe elementary particles that were around in the first few seconds after the universe was formed. (That event, called the Big Bang , happened about 13.8 billion years ago.)

Magnetic explosions

The sun is also host to phenomena called solar flares . Dancing above the sun's surface is a tangle of magnetic fields. At times, these fields intersect and snap, sending plumes of solar material off the surface — and, sometimes, charged particles along with it.

"When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection," NASA officials said in the statement. "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."

Particles streaming off the sun may accelerate close to the speed of light, thrown from the sun thanks to magnetic reconnection. One example of such objects is the solar wind , the constant stream of charged particles the sun emits into the solar system. (There may be other factors speeding these particles as well, such as wave-particle interactions — which is explained in the next section of this article.) 

Magnetic reconnection also likely happens at large planets, such as Jupiter and Saturn. Closer to home, NASA studies magnetic reconnection near Earth using the Magnetospheric Multiscale mission , which measures our planet's magnetic field using four spacecraft. The results may be useful to better understand how particles accelerate all over the universe, NASA officials said.

Wave-particle interactions

Particles can also careen at high speeds when electromagnetic waves collide; that phenomenon is more technically 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," NASA officials said.

These interactions take place all over the universe. Near Earth, NASA missions such as the Van Allen probes are watching wave-particle interactions to better predict particle movements — and protect electronics on satellites. That's because high-speed particles can damage these delicate spacecraft parts.

Supernovas, or star explosions, may also play a role in more far-away interactions. Researchers have theorized that after a star explodes, it creates a blast wave — a shell of hot, dense compressed gas — that zooms away from the stellar core at high speed. These bubbles are full of charged particles and magnetic fields, creating a likely environment for wave-particle interactions. This process may eject high-energy cosmic rays — which consist of particles —  at velocities close to the speed of light. 

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Follow Elizabeth Howell on Twitter @howellspace . Follow us on Twitter @Spacedotcom and on Facebook .

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

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, " Why Am I Taller ?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

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What would you see if you could travel at the speed of light?

Einstein's Special Theory of Relativity was born from this very question, and the answer is as weird as you'd expect.

Robert Matthews

Asked by: Pete Groves, London

That's what Einstein asked himself as a schoolboy and it led him to his famous Special Theory of Relativity. Its equations show that objects look increasingly distorted as the speed of travel increases, with the view ahead becoming progressively brighter. Then truly bizarre effects start to kick in, with objects far ahead apparently moving further away, while those behind come into view.

Eventually, at lightspeed there's nothing but a dazzingly bright spot of light surrounded by complete blackness. Weird!

• Does the speed of light ever change?
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How does a photon know to travel at the speed of light?

25 August 2021

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How does a photon “know” to travel at the speed of light?

Elaine Patrick Cyffylliog, Denbighshire, UK

I don’t know, ask Erwin Schrödinger. He was a relative of mine on my mother’s side. He told her mum, but she couldn’t understand it either. We’ve been in the dark ever since.

Yang Guijen Balik Pulau, Penang, Malaysia

The laws of the universe require that all the energy and matter particles occupying its space must abide by its rules – so as to maintain a viable home and playground for all. One of these rules is that if you are a massless particle of electromagnetic origin, and you want to play in vacuum space, then you must move at the speed of light, 299,792,458 metres per second, consistently.

If you are a particle with mass, however, then there are other rules that you can follow.

@kbachmann, via Twitter

Wouldn’t any speed travelled by photons be, by definition, the speed of light?

Ian Glendinning Vienna, Austria

All massless particles always travel at a speed represented by the letter c , whereas massive particles can travel at any speed between zero and c . Since photons are massless, they travel at c , which is called the speed of light because the photon was the first known example of a massless particle.

So the short answer to the question is that a photon knows to travel at the speed of light because it is massless.

Ken Appleby Ledbury, Herefordshire, UK

What we call photons are actually interactions of electromagnetic fields. Between interactions, photons don’t exist. You can’t watch a photon in transit, only detect an excitation of the electromagnetic field when it happens.

Photons don’t exist as particles. There are no particles, just interactions of quantum fields. Maxwell’s equations embody and explain in elegant mathematics the empirical results of Faraday’s experiments into electrostatic and magnetic fields – fields being a novel concept of Faraday’s invention. The equations reveal the existence of electromagnetic waves, which are always observed to travel at the constant speed c , regardless of the motion of emitter, receiver or observer. It was this apparent paradox that Einstein’s special relativity paper resolved, by dispensing with the notions of simultaneity, absolute space and time.

So at root, the answer to your question is just simply that that is reality. That is what we observe. The reasons are illuminated by the equations of electrodynamics, but ultimately it is an empirical observation. At least, so far.

James Bailey Southampton, Hampshire, UK

This question is the wrong way round. A photon is a packet of electromagnetic radiation. A very small part of the spectrum of that radiation (wavelengths of around 400 to 750 nanometres) is detectable with our eyes and we call this light. It is like asking why light takes 1/299,792,458 of a second to travel 1 metre, when in fact we just find it more convenient to define it as that, rather than use the old definition of a metre as a ten-millionth of the distance from the equator to the North Pole.

The really interesting question for me is why does electromagnetic radiation travel at 300,000 kilometres per second, and that brings us back to the question of time that has been raised before. Does light travel through time? If so, what exactly is it that it is travelling through? Or does time itself do the moving and is constantly sweeping past us like the wind while everything else stands still?

Does time actually exist as anything or is it just a convenient invention to allow us to talk about how things are moving?

To answer this question – or ask a new one – email [email protected] .

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