# What is the speed of light?

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.

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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.

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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.

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“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.

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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.

## 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 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).

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### 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.

As early as the 5th century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of speed of light. 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.

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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.

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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.