How do you measure the speed of light? I was thinking about this today, but can’t think of a good method. Obvious ones like bouncing light from a distant object and measuring the time sound OK, but how do you accuratly measure the distance in the first place?
Oh, and apologies if this has been answered before, but I couldn’t find it in the search engine. Is there some trick of getting a decent search out of the thing? It didn’t throw up many results for my search at all.
Here’s how I did it in freshman physics lab. Not terribly precise, but it’s pretty easy to do.
Hook an LED up to a square wave generator so that it’s blinking on and off rapidly, and point it at a mirror across the room. Set up a photomultiplier tube to detect the reflected light, and view both the photomultiplier output and the square wave driving the LED on an oscilloscope. Just measure the time delay between the rising edge of the square wave and the rising edge of the photomultiplier output, measure the distance from the LED to the mirror and from the mirror to the photomultiplier, divide, and Bob’s your uncle. The time delay you get from having a ~6 meter light path is within the range of a high-end oscilloscope.
Thanks Bobert, Phobos. I was really wondering how they managed to measure it so accurately, rather than at all, since the only real way I could see to measure the distance with any real accuracy was using light itself.
From Chronos’s site, I discovered how they got around this. Being practical science sorts, they cheated.
When the accuracy was known to within 1 m/s “It became more practical to fix the value of c in the definition of the metre and use atomic clocks and lasers to measure accurate distances instead.”
I guess that’s the smart way to do it 
You can also use a rapidly spinning mirror, and measure how much it rotates between the light hitting it two times (a lot easier to explain with a diagram), or independently measure epsilon[sub]0[/sub] and mu[sub]0[/sub], physical constants related to the strengths of the electric and magnetic forces respectively, and derive it from them (c = 1/sqrt(epsilon[sub]0[/sub]*mu[sub]0[/sub]) ).
This is a particularly low-cost method- everything required can be found in a hardware store.
Arjuna34
Oops- forgot to mention that although using mu[sub]0[/sub] and epsilon[sub]0[/sub] to compute c is perfectly valid, it’s not DIRECTLY measuring the speed of light- if you suspect a giant conspiracy hiding the true value of c, you probably wouldn’t trust the formula c = 1/sqrt(epsilon[sub]0[/sub]*mu[sub]0[/sub] either, and only a direct measurement would convice you
Arjuna34