The speed of light in a vacuum is 299,792,458 metres per second, a figure scientists finally agreed on in 1975 – but why settle on that figure? And why does it matter?
Answering those questions takes us on an amazing journey through space, time, physics and measurement, and the tale hasn't quite been told yet. Modern-day studies are calling into question the speed of light for the first time in centuries.
To start at the start though, some history: at the beginning of the 17th century, the general consensus was that light didn't have a speed, that it just appeared instantaneously, either present or not.
During the 1600s this idea was seriously challenged. First, by Dutch scientist Isaac Beeckman in 1629, who set up a series of mirrors around a gunpowder explosions to see if observers noticed any difference in the when the flashes of light appeared.
Unfortunately for Beeckman and the progress of science, the results were inconclusive, but then in 1676 Danish astronomer Ole Rømer noticed strange variations in the eclipse times of one of Jupiter's moons over the course of a year.
Could this be because light took a longer time to travel from Jupiter when Earth was further away? Rømer thought so, and his rough calculations put the speed of light at about 220,000 kilometres per second – not a bad estimate at all, especially considering the data he would have had on planet sizes wasn't all that accurate.
Further experiments with beams of light on our own planet edged scientists closer to the right number, and then in the mid-1800s physicist James Clerk Maxwell introduced his Maxwell's equations – ways of measuring electric and magnetic fields in a vacuum.
Maxwell's equations fixed the electric and magnetic properties of empty space, and after noting that the speed of a massless electromagnetic radiation wave was very close to the supposed speed of light, Maxwell suggested they might match exactly.
It turns out Maxwell was right, and for the first time we could measure the speed of light based on other constants in the Universe.
At the same time, Maxwell's work strongly suggested that light was itself an electromagnetic wave, and after this idea was confirmed, it got picked up by Albert Einstein in 1905 as part of his theory of special relativity.
Today the speed of light, or c as it's commonly known, is considered the cornerstone of special relativity – unlike space and time, the speed of light is constant, independent of the observer.
What's more, this constant underpins much of what we understand about the Universe. It matches the speed of a gravitational wave, and yes, it's the same c that's in the famous equation E=mc2.
We don't just have the word of Maxwell and Einstein for what the speed of light is, though. Scientists have measured it by bouncing lasers back from objects and watching the way gravity acts on planets, and all these experiments come up with the same figure.
However, the story doesn't quite end there, thanks to quantum theory, that branch of physics hinting that the Universe might not be quite as constant as we think.
Quantum field theory says that a vacuum is never really empty: it's filled with elementary particles, rapidly popping in and out of existence. These particles create electromagnetic ripples along the way, the hypothesis goes, and could potentially cause variations in the speed of light.
Studies into these ideas are ongoing, and we don't know for sure one way or the other yet. For now, the speed of light remains the same as it has for centuries, constant and fixed... but watch this space.