# What is a gravity wave detector measuring?

1. Oct 22, 2015

### Grinkle

I am having trouble understanding how a gravity wave detector can be de-coupled from the wave it is intended to measure.

For the sake of conceptual discussion, if I am eyeballing a ticking clock some distance away and a wavefront hits the clock and say 10 or so waves pass through the clock before the wavefront reaches me, and my eyeball is the detector, what kind of behavior am I on the lookout for from the clock? Since the wavefront reaches my eyeball at the same time as information from the clock that was affected by that same wavefront, (I think so anyway) can I de-couple the wavefront effects on my eyeball from what I am hoping to measure about the clock?

2. Oct 22, 2015

### Staff: Mentor

The distance to the clock changes, you could see this as oscillations in the observed time difference between your clock nearby and the distant clock. Those differences are too small for actual clocks, so interferometry is used.
Ideally, waves come in orthogonal to the baseline(s) of the experiment for maximal length changes.

3. Oct 22, 2015

### Staff: Mentor

A detector that is decoupled from whatever it is trying to measure won't detect anything. In order for any detector to work, it has to be coupled in some way to what it is trying to detect. So I think you might need to reformulate your question.

4. Oct 22, 2015

### Grinkle

Agreed. I'll attempt an analogy.

If I think of a laser floating on a pond surface and a photon detector floating some distance away watching the laser, then my problem is exampled like so -

The pond waves (gravity waves in my analogy) travel at the same speed as the laser photons so when the laser photons reach the detector, the detector is lifted by the same wavefront that lifted the laser, so the detector can't tell the laser was moved by the wave because it was also moved by exactly the same wavefront in exactly the same manner when its detecting the laser's photons. I am seeing common mode rejection, I think, anytime I try to figure out how gravity wave detection might work.

I don't see how the photon detector can be placed out of the way of the gravity-wave wavefront.

What am I missing? Is there some gravity wave that is planar rather than traveling spherically in all three space dimensions so a detector can be "out of the way" of the wave? Or maybe the wave is asymmetric and this asymmetry leaves some non-common signal?

5. Oct 22, 2015

### Staff: Mentor

This just means you are detecting the waves the wrong way; you're trying to use a detector that's oriented along the wave direction, instead of perpendicular to it. See below.

The detector shouldn't be trying to tell whether the laser was moved by the wave. It should be trying to tell whether the detector itself, and objects near it, were moved by the wave.

Try it this way: we have a bunch of floats that each have a laser and a detector mounted on them. As the wave passes by, the floats move up and down. But if we've been smart about how we place them, they won't all move in sync, so each one will be able to detect changes in the distance and direction of the others, relative to it. These changes will be evidence of the passage of the wave.

6. Oct 23, 2015

I get it.

Thanks!