# What is the Speed of Light and How Did They Discover It?

The speed of light is a fundamental constant of the universe, meaning that no matter where an observer is or how fast they’re traveling, the speed of light remains the same. Although it’s called the speed of light, the measurement is actually the speed of any particle that has no mass when traveling through a vacuum. As a constant, the speed of light is an important part of a lot of scientific work, especially for physicists. It’s no surprise, then, that scientists spent centuries calculating exactly how fast light travels.

## What is the Speed of Light?

At its most simple, the speed of light is the speed at which light travels through space. The exact speed of light is 299,792.458 kilometers per second (kps) or 186282.397 miles per second (MPs). This is often rounded up to 300,000 kps or 186,000 miles per second. The speed of light is commonly denoted by a c in equations.

## The Speed of Light in Ancient Times

As the speed of light often turns up when discussing space travel, it may seem like a modern question, but scientists and philosophers have been pondering the issue since Ancient Greece. Empedocles believed that it took some time for light to travel from the sun to the earth, but Aristotle disagreed, believing that light traveled instantaneously. Other early thinkers on the topic included Euclid, Ptolemy, and Heron of Alexandria who argued that the speed of light is infinite, as objects appear immediately upon a person’s eyes opening.

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## The Work of Galileo

The next major step in science’s knowledge of the speed of light came when scientists tried to take measurements. In 1629, Isaac Beeckmann used gunpowder and mirrors in an attempt to come up with an answer, asking observers about the delay between seeing the flash and hearing the bang. Galileo’s experiment involved people with covered lanterns standing at a known distance from one another uncovering their lanterns. He couldn’t measure any time lag but did conclude that light is at least ten times faster than sound.

## The Moons of Jupiter

The next experiments came in 1676 and measured light coming from space, rather than on Earth. Astronomer Ole Romer was studying the moon Io, which is regularly eclipsed by Jupiter. Romer found that the eclipses were often later than expected, eventually predicting correctly that an eclipse would be 10 minutes late. The eclipses lagged the most when the planets’ orbits took them the maximum distance from each other, as the light had further to travel.

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## Early Measurements of the Speed of Light

Measuring the speed of anything relies on knowing the distance between two points and scientists in Romer’s time were still figuring out the distance between the Earth and the sun. This led to estimates of the time it takes for light to travel from the sun to Earth ranging from 7 to 22 minutes. Romer eventually came up with a number of 125,000 miles per second. Physicist James Bradley used the change in the position of the stars for his calculation and got a figure of 185,000 miles per second, just 1 percent off the correct figure.

## Fizeau and Foucault

Up until the mid-1800s, figures had been based on observations of the changes of planets. French rivals Fizeau and Foucault came up with similar experiments to directly measure the speed of light. The machines they created used mirrors that reflected light onto a rotating apparatus and the scientists measured the amount of rotation that occurred before the light bounced back to determine the speed of light. This gave them a figure very close to the modern estimate.

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## Michelson and Luminiferous Aether

Until the 1870s, scientists believed that a medium called luminiferous aether moved light throughout the universe. In an attempt to detect luminiferous aether, physics lecturer Albert Michelson recreated Foucault’s experiment but with distances of 610 meters rather than 18 meters. This increased the accuracy of the experiment and Michelson’s figure was the standard for 40 years. However, it also proved that the aether didn’t exist, and Michelson became the only person to win a Nobel prize for a very precise non-discovery.

## The Work of Einstein

Albert Einstein did some of the most important work on the speed of light. Building on the suggestion that light was an electromagnetic wave, Einstein showed that the speed of light is a constant, traveling at the same speed no matter how fast the observer is moving. This is known as the theory of special relativity. Einstein gave the scientific world the value of c, which is now a fundamental constant.

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## Why is the Speed of Light Important?

Knowing the measurement of the speed of light allows scientists to make calculations and develop theories about the universe, which helps further scientific advances and gives the world new ideas and technologies. The figure is fundamental in the study of physics and is also used in other areas, including astronomy, computer technology, and optical fiber technology.

## Questions for the Future

The speed of light is essentially the speed limit of the universe. Einstein’s theory showed that any object with mass can’t travel faster than the speed of light because doing so would require an infinite amount of energy. However, scientists are always striving to conquer new frontiers, even if it means breaking the laws of physics. Traveling faster than the speed of light brings to mind time travel and intergalactic travel. While these are still in the realm of science fiction, science offers infinite possibilities.

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