Monday, July 6, 2009

Some crazy LIGO

Everyone is asking me, "Hey mr. miller, what crazy things are you doing this summer?" Well, as I've already let slip, I'm working on LIGO, the Laser Interferometer Gravitational Wave Observatory. We're looking for gravitational waves. Specifically, I'm in the group which looks for gravitational waves which come from compact binary coalescences (CBCs). That basically means when two black holes smash together.

Have we found any gravitational waves yet? Well, the other day, I was at Chandler Cafe, and I found one in my noodles. It looked sort of like this:
Graphic made using a Mathematica Demonstration

Unfortunately, I don't really own a camera, and I was hungry. So I slurped it up, and it made a sound like this: voooooooooooooooooooouP (also available as mp3, from "Gravitational Wave Sounds"). I guess we'll just have to find another one now, huh?

A slightly more serious answer: I couldn't tell you even if we had seen anything. It is "privileged" information. Exciting! But let me say this: We have a fairly good idea of the density of binary neutron stars and black holes, and how "loud" they would be when they merge. So we can calculate the expected rate of detection. By one estimate (see arxiv), the expected detection rate of neutron star mergers is once per two hundred years of observation (probably even smaller for black hole mergers). Basically, we don't expect to see anything. The real excitement will occur when "advanced LIGO" starts in 2013, increasing the observation rate to about 20 per year.

Of course, there are other sources of gravitational waves--Gravitational Wave Pulsars, Big Explodey Things, etc.--so maybe we'll see some of those. I think these other sources aren't as well understood, so we don't have such precise estimates on their expected detection rates. So who knows, we may be lucky.

And if it turns out we're lucky, you probably wouldn't notice right away. LIGO data is littered with what we call "non-Gaussian noise", meaning that every so often, there's a data glitch, causing the measurement to jump up by some really high number. These glitches look like gravitational waves; the computer has trouble telling the difference. And there are so many of them. We toss the glitches through every statistical filter we can think of, and we're still overflooded with them.

But we still have some tricks up our sleeves. I'm working on one of those tricks. What I do, is give the computer a bunch of false signals and a bunch of "real" signals (which are inserted artificially). Then the computer uses these to learn the difference between the two. It's basically Skynet, except it's not even remotely like Skynet.

Instead, I would analogize it to a tree. You throw a bunch of apples and oranges at the tree, and then the tree tries to tell a supercomputer what the difference is between a fruit and a black hole. (I am joking! Don't take my analogy too seriously. It's not really like a tree at all; if anything, it's a forest.)

So basically, if you want to know what I'm doing this summer, you can visualize me tossing a bunch of black holes at trees in hopes of finding delicious spaghetti. That's more or less the right idea.

1 comment:

The Barefoot Bum said...

Miller, you are a seriously weird person. I say that as the highest sort of compliment.