LIGO: The set of arms that feels space-time waves

( A follow-up to my last story about what makes things fall, composed using the Up-Goer Five text editor)

What sort of thing can we build to see the waves? It has to be able to feel those tiny pulls and pushes. So we make a thing with two long arms. Imagine putting one arm up over your head, and one arm straight out to your side, and then lie down on the ground. That's how the arms need to be. When a wave coming from the sky pulls you taller, the arm above your head would become longer than the one out to your side. Then, as the wave makes you wider, the arm out to your side gets longer than the one over your head. We need to know about how the arms get longer or shorter than each other.

The longer the arms are, the bigger the change the waves make by pushing or pulling on them. So we make very long arms, arms that would go across half a hundred city blocks. And still the arm gets longer or shorter by a very tiny bit: a bit that is as much smaller than a human hair is wide as the human hair is smaller than  sun is wide. That is crazy small!

This is why we don't feel any of these waves going through us.

So how do we know if the arms got longer or shorter?

We use the fact that light is also made of waves, and light waves will go up and down many many times as they go along the arms. We start the waves going down the arms with their ups and downs matched. Then they hit a mirror at the end of the arm and come back. We line the mirrors up so, when there are no space-time waves, the ups and downs in the light from one arm come back in such a way that they end up going down and up instead when they reach the middle. In the middle we add the light from the two arms together. We add down with up and up with down, and end up with nothing: no light gets out.

But if a wave goes through, one of the waves has farther to go because one arm is longer. That means it gets through a little bit more of its up and down on the way. So now not-quite-down-anymore adds with up and not-quite-up-anymore adds with down and a little bit of light can escape the middle. This little light tells us a wave is going past.

There are a lot more parts to our set of arms to make it feel space-time waves better.  Like, the light gets turned around to go through the arms lots of times before it comes back to the middle, and we carefully make sure all the parts are very still, and we're also starting to use some other weird things that light can do. But adding the light from two arms is the biggest idea.

It's still very hard to do this, so we make more than one of these sets of arms. Then we can check the light from the two sets against each other. In case something that's not a space-time wave lets light through one of them, the other one will say "No! That's not a real wave. If it was, I would have felt it too." We also have friends around the world with their own sets of arms and we share what we know about the waves going past.

We also think a lot about what kind of waves will come from the sky. If we know what kind of wave we expect to see, we can look more carefully for exactly that type of wave and ignore some of the other things going on.

We haven't seen any space-time waves yet, but people are hard at work making our sets of arms even better. If we're right about what's going on in space, we should be able to notice some of the space-time waves before five more years have passed!