Gravitational Waves and Lasers and Stuff · 6:01pm Feb 20th, 2016
About that big discovery last week.
This is one which we have been expecting for some time. I first read about the LIGO experiment some twenty years ago when it had just started construction and I was a young student dreaming about doing a summer placement in California—which turned out to be just a dream, but it was fun perusing the lists of student projects on offer at Caltech.
What are gravitational waves? Ripples in the fabric of space—see my post on General Relativity. Or this video.
No one seriously doubted that gravitational waves exist. They are a natural consequence of general relativity. Wherever you get accelerating masses, they squash and stretch space itself sending waves outwards at the speed of light. And as GR is the most exquisitely beautiful theory ever, it just must be true—or so the theorists say.
But actually seeing them is a bit of a challenge, as space is not very wavy. If gravitational waves were big, then the earth would lose all its orbital energy radiating them away and we would have crashed into the sun long ago. As it is, they have so little effect on our little world, that it’s only astronomers that know much about them.
To make a big enough wave to have a chance of seeing it, you need something like two really massive super-dense objects (black holes or neutrons stars) getting really close together, so they spiral around each other, radiating their orbital energy away as gravitational waves until they get so close together that they crash together in a great big catastrophic event.
How often is that likely to happen? Pretty hard to say. Black holes are rather difficult to see, so we don’t really know how many there are out there. Best way to find out is to build a super sensitive gravitational wave telescope.
How to do that? This is the bit I find cool (I like experiments). We want to measure changes in the shape of space—basically measuring a change in the distance between two points to a high precision. This is done by interferometry. Take a laser beam, split it into two beams; send one beam down one tunnel, and the other down another, at ninety degrees to the first. At the ends of the pipes, the beams are reflected back by mirrors, and come back to the base, where they recombine. As light is a wave, we get interference. If the two beams travel an identical distance, then they come back perfectly in phase → constructive interference → bright light. Move one mirror by fraction of a micrometre and the beams are out of phase → destructive interference → dark band. The result is a pattern of circles due to the interference of light rays at slightly different angles. If a gravitational wave comes by, it distorts the distances, and we see the pattern move.
It is possible to build one at home with a cheap laser pointer, mirrors and a beam splitter. But if you want to reach the attometre precision of the LIGO experiment, you need four-kilometre tunnels, with mirrors suspended in vacuum on a multiple-pendula suspension system to isolate them from seismic waves. Even then one is not enough. To be sure a signal is real, it must be seen in two locations a long way apart (say Washington to Louisiana).
But we all start off with simple table top projects. This week I heard a rumour that you could build an interferometer with a beam splitter made from a CD case. I was sceptical, and decided to try it out. Sure enough, when you shine a laser through the clear plastic cover it will let half the beam go through and reflect the other half to one side. But with optical measurements, quality components are important (why else do top photographers spend so much on lenses?). A CD case creates a double spot as the light is reflected both when entering and leaving the plastic. And the surface is not really polished to sub-micron smoothness. So I wasn’t too surprised not to see any interference pattern.
But it gave me a chance to have a go at photographing a laser beam next to a Sci-Twi doll. One day I will learn to do this properly.
(The playing cards are a necessary component to adjust the vertical tilt)
SciTwi looks very happy.
oh god, know I want to write a fic about SciTwi geeking out about gravitational waves to all her friends
Heh. There's just no getting away from the rubber sheet comparison, is there?
In any case, any apparatus description that includes the phrase "The playing cards are a necessary component" meets with my approval.
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I fully approve of this story idea.
Then Pinkie Pie comes along, and points out that if you combine all the corrected noise data you filter out and throw away, you get more than a global holographic seismograph array, but also one that pick up the repetative distortions of relative orbiting masses. At least the moon going round should be a lot of trouble, causing distortions of centimetres instead attometres, isnt that a form of gravitational wave?
I wonder if LIGO can pick up th gravity displacement of the ISS going over, and what fun will be had when LISA and its successor come into the array, giving space based, seismic free, signaling.
We really need three axis space borne LIGOs. then again, gold mines go down 4000 metre, so South Africa could build a three axis unit? Any other deep mines, mountain tunnels that could be used at this time, or how many could be built if all you need is a tube, given displacement oil well and fracking well tunneling?
Maybe we can see dark matter effects by using the dark interference pattern from a single photon non illuminating target design?
Im wondering how to code or design a sensor that uses the multiple department proposed idea that strength of gravity at a point is a measure of the fractal folding, fractal dimention at that point. but if the quanta of folding length is down at Plank, well, it would still look pretty smooth to most anything we could use?
Pinkie draws dots and blobs on a crinkly transparency sheet, then squashes it up into a tight ball and throws it to Twilight. Who, when picking it up to look at it, sees two orthogonal straight lines through the volume and a cube at their intersection.
There you go Twilight, a 2 dimentional sensor arranged in a 3D volume existing in a fractally folded 4 D realm. 
According to Conservapedia, Relativity is just a way of pushing cultural relativism. ¡My country is doomed!