The speed of light was determined 340 years ago today, and Google has marked the occasion with a Doodle.
Who figured out the speed of light?
Light speed was discovered in 1676 by a Danish astonomer called Ole Roemer.
But Roemer wasn't the first person to try and measure light's speed. Galileo set about looking for it in 1638.
Galileo's method involved two people standing on a hill top miles apart with a lamp each. The premise was that one of the pair would flash their lamp at the other, who would respond by flashing back immediately.
No matter how far apart the two lamps were Galileo couldn't measure a time difference, leading him to determine that light was too fast to measure like this.
Although he was unable to establish an exact number or if the speed of light was infinite, he did find that light travels 10 times faster than sound.
Finding the speed of light required a distance far greater than a few miles. Roemer began his research decades later in 1673 when he noticed the time elapse between the eclipses of a Jupiter moon called Io, which Galilei had discovered in 1610.
By monitoring the time difference, Roemer estimated "light seems to take about 10 to 11 minutes [to cross] a distance equal to the half-diametre of the terrestrial orbit", or Earth's orbit around the Sun. This means that light travels at about 200,000,000 metres per second, which is around 26 per cent below the established speed.
What is the speed of light?
The speed of light is exactly 299,792,458 metres per second. James Bradley came closer to this figure in 1728 with the discovery of what is called the "aberration" of starlight and a speed of 295,000,000 metres per second for light.
But the exact number wasn't decided upon until 1975, after decades of becoming increasingly specific, when the General Conference on Weights and Measures recommended an official figure.
The symbol for the speed of light is c and is known for being a "universal physical constant", which means that it is an exact quantifiable amount that doesn't change.
Why is it important?
Albert Einstein's theory of relativity, which underpins modern concepts of space and time, is built on the premise that the speed of light in a vacuum is always the same.
The Theory of Relativity
Is the speed of light real?
Is the speed of light real?
The speed of light as a fundamental concept of nature that is unchanging was called into question last week when a group of physicists proposed that at one point in time it might have actually been faster than it is now.
The scientists at Imperial College London, after decades of research, revealed that light may have travelled faster at the beginning of the universe than it does now. If the speed of light has slowed down it could have a knock on effect about the way we understand physics.
But the researchers said they spent a lot of time working on a theory that wouldn't destabilise our understanding of physics.
"The whole of physics is predicated on the constancy of the speed of light," Joao Magueijo told Motherboard. "So we had to find ways to change the speed of light without wrecking the whole thing too much."
Is it possible to go faster than the speed of light?
In 2011 scientists thought they had recorded particles travelling faster than light - a finding that could have overturned one of Einstein's fundamental laws of the universe.
Antonio Ereditato, spokesman for the international group of researchers, said that measurements taken over three years showed neutrinos pumped from CERN near Geneva to Gran Sasso in Italy had arrived 60 nanoseconds quicker than light would have done.
They were forced to admit that this was all a mistake, however, blaming a faulty wire connection.
For now at least, faster than light speed remains the stuff of science fiction...