LIGO - A Tale of Great And Small
By now everyone has heard of the recent detection of gravitational waves produced by two black holes colliding a billion miles away. By the time those waves reached us here on Earth they moved us (you, me, everything, the entire planet) a tiny fraction of the diameter of a proton. A. Proton. Not a fraction of an atom. A tiny, tiny fraction of a proton.
For a mere fraction of a second, we were able to detect that motion. As miniscule an effect as it was, and over so brief a time, it was still enough to cause me to get off my butt and drive about five hours to look at the machine built for the specifc purpose of detecting gravity waves.
I propose to describe my visit and some of my impressions here.
I'll begin with: What is a gravity wave? Here's the way I think of it. First, everything that has matter or energy has gravity. Which means everything attracts everything else. And when something moves, it turns out that it gives off waves. It's a bit like waves in water. Throw a rock in the water: waves come off. A boat moves through the water: waves come off.
But we're speaking of gravity waves. So, the moon moves around the Earth: waves are given off. With a water wave, it's pretty easy to see the water move. What exactly is moving when the moon gives off gravity waves? Space itself is moving. Ripples traveling through space. And everything they pass through ripples too.
The moon, even though it's very close, gives off gravity waves much to small for us to detect. Turns out, only really, really big events will even have a chance for us to detect. That's where black holes come in. Turns out, they can generate gravity waves big enough for us to detect, even though they are so very far away.
OK, at this point I suggest you go visit this website where the physicist Brian Green explains gravity waves to Stephen Colbert:
It's a fine summary and I highly recommend it. And here's the video clip from that episode of The Late Show with Stephen Colbert (some great visuals here):
So I'd seen all this, just as you have, and I thought I'd look up the LIGO facility. Here's their website:
Here is their page on public events (they only have tours a couple times a month):
I noticed they had a tour scheduled for Friday February 26, 2016 at 3pm. Well. Had to go see that, didn't I?
I've posted a few photos from my visit on Facebook.
And I can't seem to figure out how to embed it into this website, you can see a 360 degree view of the LIGO-Hanford facility by clicking here.
OK, here's where my mind just gets bent. Start with this schematic of the interferometer:
Laser light hits the Beam Splitter, sending half the laser beam down one 4 km leg and the other half down the other 4km leg. Those two beams bounce back, again pass through the beam splitter where they are recombined and land in the Photodetector. As the two return laser beams recombine, they interfere with each other and from the interference pattern you can tell if the path length along one path has gotten longer or shorter than the other.
All this thing is, really, is a ruler. One that's really, really good at small changes. How good is it? They can tell if the path length of one leg has changed compared to the other by as little as about 10-19 m. Ten to the minus 19 meters.
How small is that? First, for you metricophobes one meter is about yard. This number is about 10,000 times smaller than a proton. It's a million billion times smaller than the distance you take in one step.
So this machine is able to measure movements that small, over a distance of 4,000 m (2.5 miles). That's like being able to measure the distance to the nearest star (alpha centauri, 4.3 light years away) to within about the thickness of a a human hair. Well, technically, it's more like measuring a change in the distance to that star to within a hair. But still.
On September 14, 2015 LIGO detected some motion in their machine, which can be processed and played as a sound, which you can hear here:
It only lasts a couple tenths of a second and sounds like a little 'chirp'. Anticlimatic? Well, you're listening with just your ears. Allow your mind to hear, and think about what it means.
Now, how do they know this chirp was generated by two black holes, each about 30 times the mass of our sun, as they collapsed into each other forming one even bigger black hole? And all this happened a billion years ago, a billion light years away, just in time to show up on Earth on Sept. 14 last year? How do they know all that?
First, they have a second, identical detector in Louisiana which recorded the identical signal, but about 7 milliseconds later. That's too fast for anything like an earthquake vibration to have reached the other detector. But it's exactly the same intensity, and it's a physical vibration so it's not anything to do with light. So it really looks like it's a gravity wave.
They scientists on this project have been working for years to calculate events that might be big enough to produce gravity waves that we might detect here on Earth. By now, they have quite a catalog of different sorts of events. But of all the things they've imagined, only two black holes colliding would look just like they say. And that calculation looks exactly like what they recorded.
It really does look like they've detected gravity waves. The effort took scientists, starting with Einstein 100 years ago, working on these really complex equations, and a team of hundreds of scientists and engineers, working for decades, to build these detectors, and to be there to record and analyze the results.
It is truly a tour de force of human achievement. And it's true that one's appreciation may grow the more one understands of the maths involved, or the engineering to make these machines so exquisitely sensitive. But I like to think one can appreciate it even without being able to solve Einstein's relativity equations. Or understanding everything about lasers, optics, mechanical control theory and computer science needed to make the machines work.
I can appreciate Ode to Joy without understanding German, and without being able to play a note of music myself. And the detection of gravity waves is an intellectual and esthetic human achievement up there with any you care to name.
Lastly, if you're still interested in more details, poke around the LIGO website. They have everything there, from press releases and background stories, to the full detailed scientifc publications. In particular, for the geeks of the world I highly recommend their Physical Review Letters paper on this first detected event here. Sure, there's some techno babble. But it's actually pretty accessible. And you don't need to speak Japanese to understand the beauty of The Mikado.