Light traveling through a vacuum moves at 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 and in shorthand as "c," or the speed of light.
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, that 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 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 figure out how to travel at such speed. But that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.
What is a light-year?
A light-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," said NASA's Glenn Research Center. "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, the objects appear as they existed at the time that light left them.
Related: Why the universe is all history
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 appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.
How did we learn the speed of light?
As early as the 5th century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of light speed. Empedocles thought 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, is instantaneous. Aristotle was wrong, of course, but it would take hundreds of years for anyone to prove it.
In the mid 1600s, recounts PBS NOVA, 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 varied 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 — a rough version of the Doppler effect or redshift. In a leap of intuition, he 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 the stars due 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, 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 themselves 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.
According to the theory, 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.
That means if we base our understanding of physics on special relativity, 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, according to an explainer article from the Khan Academy.
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.
Why we still love the idea of faster-than-light travel
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.
- Explore the speed of light with NASA.
- Get familiar with the universal constants that define standard systems of measurement around the world on this NIST website.
- Read more history in the book "Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light" (Oxford, 2019) by John C. H. Spence.
Some research for this article by Space.com contributor Nola Taylor Redd.