How to "read" a gravitational wave

Hans
Hans
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Topic 210524

During the publication of the latest discoveries, we have heard about gravitational waves emitted by both black hole mergers and neutron star mergers. My layman's question would be, how do you extract all that information from a wave that only shifts spacetime by the order of the diameter of a proton? It must be like looking at a wave in the water of a lake and determining the kind of ship that created it. If someone has a understandable explanation for this, I would be very interested, thanks.

Michael

Logforme
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In my layman understanding it

In my layman understanding it consists of:

1. A detector (LIGO) collection lots of information that look like noise.

2. A computer using an algorithm (FFT) to find signals hidden in the noise.

The scientists at Einstein@Home know about a lot of different "boats" and they guess what kind of wave pattern they would generate. They design an application to search for those patterns and we run that search against a small part of the LIGO information on our computers.

Hans
Hans
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Logforme schrieb:and they

Logforme wrote:
and they guess what kind of wave pattern they would generate.

If the pattern of a GW can only be guessed, then I would think that once a GW is actually detected, it's source could probably also only be guessed.

Mike Hewson
Mike Hewson
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There was tremendous relief

There was tremendous relief with the first detection. Prior to that it was like listening for the sound of a song bird in a noisy forest, only one didn't know what the bird sounded like having never heard it before. But there were as it turned out good guesses as to the song. There was a theory that could predict the songs.

As for the order of magnitude of the effect to be detected, the so called "stiffness" of spacetime goes like c4/G where c is the speed of light and G is Newton's constant* from his theory of gravitation. With c ~ 3 x 108  and G ~ 7 x 10-11 in MKS units that gives this coupling value around 1045 . A solar mass is about 1030 kg. The idea here is to get any wiggles at all you have to jump up and down alot, so even several Sun's worth of disturbance only achieves a 1030/1045 = 10-15 wiggle. But that is at the source of the event. Out here in the boondocks this is diminished with distance and thus we look for 10-24 stuff etc. 

{ Without relativity the stiffness is infinite because the speed of light is deemed as infinite ie. no gravitational waves, everything is transmitted instantaneously and one can think of the Universe as having a single clock. Thus time doesn't depend on local circumstances in the classical physics viewpoint. }

The interferometers pick up the tiny amount of disturbance of the waves as they continue onwards past us. They only diminish with distance and not by intervening material like a planet, or a star or even a galaxy. Compare that with light which only tells you about the last thing it bounced off : a cloud, a planet, a tree, a supernova bubble, a shampoo bottle. Electromagnetic astronomers always have to account for effects during transmission of photons from source to their detectors.

Cheers, Mike.

* General Relativity retains that constant as is it closely approximates the 1/r2 behaviour of Newton's law when the field strengths are low.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

mmonnin
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The great thing about the

The great thing about the neutron star collision was that as soon as the detection occurred it was able to observed by telescopes around the world. Since 3 detectors were operational the location was able to be pinpointed to a small patch of sky.

The peak of the waves might tell how massive the objects were. A neutron star just wasn't big enough of a star before super nova for the mass to be dense enough for it to be contained within its own Schwarzschild radius and become a black hole.

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