Question: What has been your favorite experiment involving light?

Matthew Malek answered on 11 Mar 2014:

My favourite experiment with light is LIGO, the Laser Interferometer Gravitational wave Observatory.

Gravitational waves are small ripples in the fabric of spacetime. According to Einstein’s general relativity, spacetime tells matter how to curve and spacetime tells matter how to move. Moving matter doesn’t immediately change spacetime everywhere, though. The changes flow outwards at the speed of light. So anytime that you or I move, we are making tiny changes in the shape of spacetime.

Although general relativity predicts gravitational waves, they have yet to be detected. That’s because they are very very small. Big events, like colliding black holes, may make gravitational waves big enough for us to detect. That’s where the laser light comes in.

As you may already know, we can treat light as a wave in certain conditions. It has a wavelength, and if you add two beams of light, you combine the heights (or “amplitudes”) of the waves at any given point. If you have two waves of light that are identical EXCEPT for being exactly out of phase, then their amplitudes are always exactly opposite and adding them together gives a big round zero — nothing detected. This is called “destructive interference”.

Anyway, gravitational wave detectors like LIGO take a beam of laser light and split it into two parts. Each part goes down a long (4 km) tunnel, hits a mirror, and comes back. The two tunnels are perpendicular to each other (this will be important later). On return, the two halves are recombined and go into a detector. The catch, as you may have guessed, is that the experiment is designed so that they return out of phase and, due to the destructive interference, nothing is detected.

Now let’s say that a colliding pair of black holes has caused a ripple in spacetime via graviational waves. In that case, the path of one laser beam will be a tiny bit shorter (or longer) as the wave passes through. The difference is astounding tiny — smaller than even a proton! On its own, we could never detect such a distance… but it changes the phase of the light so that when the beams are recombined, they are no longer exactly out of phase. Instead of complete destructive interference, a bit of laser light reaches the detector.

This hasn’t happened yet, but it should be possible. Gravitational waves are not new physics; they are predicted by general relativity. We just haven’t seen them yet. Once we can detect them, then the fun begins! We have learned so much about the universe by studying the light from the cosmic microwave background. But that only goes back to when the universe was 380,000 years old. Before then, the universe was not transparent to light. Gravitational waves have no such limitation; if we can find a cosmic background of gravitational waves, we could learn so much more about the early universe!

So that’s my favourite experiment with light: Using lasers in underground tunnels to look for ripples in spacetime — each much smaller than an atom — caused by colliding black holes. Pretty cool, huh?