Einstein's theory of special relativity sets of the speed of light, 186,000 miles per second (300 million meters per second). But some scientists are exploring the possibility that this cosmic speed limit changes.

The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second.

Early scientists, unable to perceive light's motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise. Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: light speed — a constant called "c" — is thought to be not achievable by anything with mass, for reasons explained below. That doesn't stop sci-fi writers, and even some very serious scientists, from imagining alternative theories that would allow for some awfully fast trips around the universe.

Speed of light: History of the theory

The first known discourse on the speed of light comes from the ancient Greek philosopher Aristotle, who penned his disagreement with another Greek scientist, Empedocles. Empedocles argued that because light moved, it must take time to travel. Aristotle, believing light to travel instantaneously, disagreed.

In 1667, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart, each holding a shielded lantern. One uncovered his lantern; when the second saw the flash, he uncovered his, as well. By observing how long it took for the light to be seen by the first lantern-holder (and factoring out reaction times), he thought he could calculate the speed of light. Unfortunately, Galileo's experimental distance of less than a mile was too small to see a difference, so he could only determine that light traveled at least 10 times faster than sound.

In the 1670s, Danish astronomer Ole Römer used eclipses of Jupiter's moon, Io, as a chronometer for the speed of light when he made the first real measurement of the velocity. Over the course of several months, as Io passed behind the giant gas planet, Römer found that the eclipses came later than calculations anticipated, although over the course of several months, they drew closer to the predictions. He determined that light took time to travel from Io to Earth. The eclipses lagged the most when Jupiter and Earth were farthest apart, and were on schedule as they were closer.

According to NASA, "that gave Römer convincing evidence that light spread in space with a certain velocity."

He concluded that light took 10 to 11 minutes to travel from the sun to Earth, an overestimate since it in fact takes eight minutes and 19 seconds. But at last scientists had a number to work with — his calculation presented a speed of 125,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley based his calculations on the change in the apparent position of the stars due Earth's travels around the sun. He put the speed of light at 185,000 miles per second (301,000 km/s), accurate to within about 1 percent.

Two 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 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. The two independent methods each came within about 1,000 miles per second of the speed of light measured today.

Prussian-born Albert Michelson, who grew up in the United States, attempted to replicate Foucault's method in 1879, but used a longer distance, as well as extremely high-quality mirrors and lenses. His 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, when Michelson re-measured it.

An interesting footnote to Michelson's experiment was that he was trying to detect the medium that light traveled through, referred to as luminiferous aether. Instead, his experiment revealed the aether didn't exist.

"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," wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang. "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!"

Einstein and special relativity

In 1905, Albert Einstein wrote his first paper on special relativity. In it, he established that light travels at the same speed no matter how fast the observer moves. Even using the most precise measurements possible, the speed of light remains the same for an observer standing still on the face of the Earth as it does for one traveling in a supersonic jet above its surface. Similarly, even though Earth is orbiting the sun, which is itself moving around the Milky Way, which is a galaxy traveling through space, the measured speed of light coming from our sun would be the same whether one stood inside or outside of the galaxy to calculate it. Einstein calculated that the speed of light doesn't vary with time or place.

Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. According to astrophysicist Paul Sutter, the universe expands at roughly 68 kilometers per second per megaparsec, where a megaparsec is 3.26 million light-years (more on that later). So a galaxy 1 megaparsec away appears to be traveling away from the Milky Way at a speed of 68 km/s, while a galaxy two megaparsecs away recedes at 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 wrote.

He went on to explain that, while special relativity provides an absolute speed limit, general relativity allows for broader distances.

"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," he wrote.

"Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."

What is a light-year?

The distance light travels in the course of a year is called a light-year. A light-year is a measure of both time and distance. It is not as hard to understand as it seems. Think of it this way: 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 the nearest star system, Alpha Centauri, is requires roughly 4.3 years to get here, so that star system is said to be 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 writes 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. Thus, when astronomers study objects that lie a light-year away or more, they are seeing it as existed at the time that light left it, not as it would appear if they stood near its surface today. In this sense, everything we see in the distant universe is, literally, history.

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

Is the speed of light really constant?

Light travels in waves, and, like sound, can be slowed depending on what it is traveling through. Nothing can outpace light in a vacuum. However, if a region contains any matter, even dust, light can bend when it comes in contact with the particles, which results in a decrease in speed.

Light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, while light passing through a diamond is slowed to less than half that speed. Still, it travels through the gem at over 277 million mph (almost 124,000 km/s) — not a speed to scoff at.

Can we travel faster than light?

Science fiction loves to speculate about this, because "warp speed," as faster-than-light travel is popularly known, would allow us to travel between stars in time frames otherwise impossibly long. And while it has not been proven to be impossible, the practicality of traveling faster than light renders the idea pretty farfetched.

According to Einstein's general theory of relativity, as an object moves faster, its mass increases, while its length contracts. At the speed of light, such an object has an infinite mass, while its length is 0 — an impossibility. Thus, no object can reach the speed of light, the theory goes.

That doesn't stop theorists from proposing creative and competing theories. The idea of warp speed is not impossible, some say, and perhaps in future generations people will hop between stars the way we travel between cities nowadays. One proposal would involve a spaceship that could fold a space-time bubble around itself in order to exceed the speed of light. Sounds great, in theory.

"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, Calif., 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."

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