atomic_fungus (atomic_fungus) wrote,

#1686: This has always bothered me.

LIGO has not detected any gravity waves in 2 years of operation.

A gravity wave detector operates by (supposedly) detecting changes to the shape of space:
LIGO is composed of a series of L-shaped detectors. At each detector, a laser beam is split in half, with each half routed through one of the arms of the L, and then the two are reunited. If the two beams have travelled the same distance in their separate arms, when they are merged again they will overlap perfectly.

Gravitational waves are thought to stretch and compress dimensions of space-time perpendicular to the plane of the wave.

If a strong enough gravitational wave passes through the area of the detectors, it will stretch one of the arms of the L and shrink the other, causing the first laser beam to be offset from the second. Researchers would notice this as a slight lessening of power in the resulting laser beam.
There is one major problem with this: the light is in the space as it changes shape.

Of course this thing is not detecting anything: the photons in the laser beams "see" the same distance regardless of what space is doing.

It's like this. Take a piece of cloth and put two dots on it a certain distance apart. If you take the corners of the cloth and pull on them, the apparent distance--to an outside observer--between the two points seems to change. But if you're on the surface of the cloth and you walk from one point to the other, it'll be the same distance.

So when gravity waves compress space, there is no way to tell.

This bothers me because of the Michelson-Morely experiment, the one designed to detect the ether--it used a similar interferometic arrangement, and the expected result was to see a change in the interference pattern. It failed, and that result helped lead Einstein to his theories of relativity.

The problem was that as the Earth moves around the sun, it's moving at some 3,000 miles per second, about 2% of the speed of light. If ether existed, it was expected that there would have to be some fringing in the interference pattern as the light beams--propagating in the luminiferous ether--were displaced.

They weren't; the luminiferous ether theory was finally retired and Einstein concluded that light goes lightspeed, all the time, in every reference frame.

The legs of the M-M interferometer were subject to the Lorentz-Fitzgerald contraction: if a leg of the thing was aligned with Earth's motion around the sun it was moving at 2% of the speed of light, and it therefore was foreshortened (very slightly) and light nonetheless took the same amount of time to propagate.

Very simply put, space appeared to "warp" so that the light traveling along that leg wasn't going 102% of the speed of light, but 100%. And there was no way to tell.

If gravity compresses space-time--as we know it does--we cannot observe the compression from within; it's like trying to measure the voltage on a car battery with one connection.

The idea of making a really big gravity wave detector is to maximize propagation delay: theoretically one leg will shorten before the other does. But again, as the leg is shortened due to the change in the shape of space, whatever is in that space is also shortened. And it doesn't notice the change.

Take a piece of paper and draw something on it. A square. Now fold the paper as small as you can. What happened to the square? Has it turned into some other shape? NO--it is still a square, and if you unfold the paper you will see your square on the paper, the same way you left it. The square cannot tell that anything happened to the space it exists in.

So draw an "L" shape down two sides of the page. Imagine a laser at the junction of the "L" firing beams down both legs. They follow the surface of the paper to the ends of the legs, bounce off mirrors there, and return to the detector at the junction.

Now fold the paper up again. The laser at the junction fires another set of beams down the legs of the "L". They don't hop over the folds; they follow the surface of the paper.

The shape of space has been changed: you folded the hell out of it. But the laser beams don't know that.

This is the fundamental flaw with the entire idea of the "gravity wave detector": they are trying to measure changes in the shape of space using something which moves along paths which follow the fabric of space-time regardless of its N-dimensional configuration.

..or maybe I don't know what the hell I'm talking about. But it's always bothered me; no one has ever been able to explain the thing to me in such a way that my concerns were dealt with. I simply do not see how this thing can do what they expect it to.

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