The first successful measurement of the speed of light took place in 1676. Danish astronomer Ole Rømer was trying to measure the orbit of Io, Jupiter's third largest moon, by watching how long it took to pass around the planet. Watching Io over many years, Rømer made a surprising discovery, says the American Museum of Natural History:
The time interval between successive eclipses became steadily shorter as the Earth in its orbit moved toward Jupiter and became steadily longer as the Earth moved away from Jupiter. These differences accumulated. From his data, Roemer estimated that when the Earth was nearest to Jupiter..., eclipses of Io would occur about eleven minutes earlier than predicted based on the average orbital period over many years. And 6.5 months later, when the Earth was farthest from Jupiter..., the eclipses would occur about eleven minutes later than predicted.
Roemer knew that the true orbital period of Io could have nothing to do with the relative positions of the Earth and Jupiter. In a brilliant insight, he realized that the time difference must be due to the finite speed of light.
Before Rømer, scientists were unsure if light had a limited speed or if its speedometer was permanently stuck at “infinite.”
A few hundred years later, techniques for measuring the speed of light have grown astoundingly more precise and, in some cases, more complex. But in the video above, the folks at the At Bristol science center show off a relatively simple way to calculate the speed of light that doesn't involve years of looking through a telescope eyepiece. In fact, their approach uses nothing but simple kitchen equipment—and chocolate.
In the video, hosts Ross Exton and Nerys Shah use little more than a microwave oven and a chocolate bar to show how to calculate the speed of light. The video doesn't make it perfectly clear how measuring the melted bits on a chocolate bar relates to the speed of light. But breaking it down a little more just requires taking a look at some of the units used in their measurements.
Hertz is the physics stand-in for “cycles per second.” The microwave used in the video produced light waves with a frequency of 2,450,000,000 Hertz, or that many cycles per second. Going from peak to peak in a wave—in this case the distance between the first and third melted bit of chocolate—is one cycle. Exton and Shah measured that distance as 0.12 meters, or 0.12 meters per cycle. Multiplying something measured in “meters per cycle” by something in “cycles per second” will give a measurement in “meters per second.” That's the wave's velocity—the speed of light.
The trick that makes the At Bristol team's approach work is that we in the modern era already know a few important things about light: that it has a finite speed, and that that speed is largely constant. We also have the benefit of physicists having already teased out the relationship between wave length, frequency and velocity.
When Ole Rømer looked to Jupiter and first deduced the speed of light he came up with 214,000,000 meters per second. “This measurement, considering its antiquity, method of measurement, and 17th century uncertainty in exactly how far Jupiter was from the Earth, is surprisingly close to the modern value of [299 792 458] meters per second,” says Dave Kornreich for Cornell.
Using a microwave and a chocolate bar Exton and Shah got 294,000,000 meters per second—not bad for a little bit of kitchen science.