21 January, 2008

Aether Drift

There are plenty of ways to introduce audible modulations into a light source. You can modulate its power supply with a signal, you can use vibrating mirrors, you can burn old floorboards nearby (see an earlier post below), you can flail your arms in front of it - the possibilities are legion.

Alternatively, it can be made to interfere with itself. This works best with laser light as it is coherent (all the waves leave the laser in phase) and is surprisingly easy to do. Try pointing a laser at a photodiode and get the beam to reflect back into the laser aperture, which, if I'm not mistaken, produces an interference pattern through tiny differences in the reflected beam paths. What you should hear is little squeaky noises, which are the interference fringes (light/dark patterns) whizzing about as the muscles in your fingers twitch and the photodiode or laser moves.

A more complicated way is to build an interferometer like this one, which is based on the Michelson-Morley experiment of 1887, except much simplified. Essentially it splits a laser in two using a half silvered mirror, and then recombines the two beams. Nanoscopic changes in the length of the two paths will register as a shift in the interference pattern (there's a nice flash demo in the link above). My fellow traveler Mathew Chadwick came up with the idea that if an interference pattern can be seen to change due to whatever forces are acting upon the apparatus, then maybe there's lots going on which can be revealed through sonification, ie, placing photodiodes into the beam paths so you can hear what's happening. He wasn't wrong.

This is a kind of abuse of an old and important experiment, in so far as the idea normally would be to isolate the thing from all vibration (Michelson's second attempt involved floating the whole thing in a trough of mercury). My engineering skills are somewhat basic, or rather, I don't have that useful personality quirk that many engineers have where everything has to be just right, and therefore the Fisher-Price nature of my interferometer means that it's not as sensitive as it might be. The laser physicists among you will behold my photo above and wince.

The recording here is the hi-lights of about one hour during a group exhibition opening party in a church in Vauxhall, London, UK, 2003. I have plans for a much larger and more sensitive one - in green.