Question about gravity

Actually, not particularly I think. Gravitational waves wouldn't be sourced by a spherically symmetric event (from what I recall, could be wrong on that one), and although you'd not expect a supernova to be entirely spherically symmetric it would be pretty symmetric, so the waves wouldn't be terribly strong. Much better off with merging neutron stars and black holes, as the last phase of the merger involves some extremely rapid orbiting of the two objects.
 
edd said:
Actually, not particularly I think. Gravitational waves wouldn't be sourced by a spherically symmetric event (from what I recall, could be wrong on that one), and although you'd not expect a supernova to be entirely spherically symmetric it would be pretty symmetric, so the waves wouldn't be terribly strong. Much better off with merging neutron stars and black holes, as the last phase of the merger involves some extremely rapid orbiting of the two objects.
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If memory serves they require a quadrapole moment instead of a dipole moment
 
sol invictus said:
It's a good analogy, but as was pointed out earlier it's a bit of a misleading scenario. It violates the laws of physics for mass to disappear - mass a gauge charge, like electric charge, and its disappearance would violate Gauss' law.
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I know that... it was just a "for instance" type thought experiment to make the point. Thanks for the clarification though.
 
robinson said:
Thanks. A Super Nova would certainly produce profound gravitational waves, correct?
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I believe the kind of events that experiments such as LIGO are analyzing are the collision of two pulsars (mutually orbiting neutron stars). I'm guessing that your "standard" supernova probably isn't powerful enough to yield a gravity wave signal strong enough to be detected by LIGO, but I don't know the details. Does anyone here know?

Incidentally, you can download software onto your computer that will analyze LIGO data as a screensaver setting. The program is called "Einstein @ Home" in honor of the original such public data-analysis program via the Internet, SETI @ Home.

Here's a link: http://einstein.phys.uwm.edu/

They have a great link at that site about the science behind gravity wave detection and LIGO. Very fascinating stuff!

Cheers - Mattus
 
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MattusMaximus said:
I believe the kind of events that experiments such as LIGO are analyzing are the collision of two pulsars (mutually orbiting neutron stars). I'm guessing that your "standard" supernova probably isn't powerful enough to yield a gravity wave signal strong enough to be detected by LIGO, but I don't know the details. Does anyone here know?
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The biggest gravity wave signal in the frequency range LIGO is sensitive to (which is 200Hz or so) is expected to come from collapsing binary systems (pairs of neutron stars, black holes, or possibly a star and a BH).

Supernovae do not necessarily generate gravity waves because they may be roughly spherically symmetric.
 
sol invictus said:
Supernovae do not necessarily generate gravity waves because they may be roughly spherically symmetric.
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Would a cylindrically symmetric event produce jets of gravitational waves? Angular momentum tends to make most events cylindrical rather than symmetrical, with a disc around the equator and jets at the poles. Presumably supernovae should be the same, so it could be a question of catching one that happens to be pointing at us?
 
Cuddles said:
Would a cylindrically symmetric event produce jets of gravitational waves?
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Yes. The configuration needs to have a time-varying quadrupole moment, which that would.

Angular momentum tends to make most events cylindrical rather than symmetrical, with a disc around the equator and jets at the poles. Presumably supernovae should be the same, so it could be a question of catching one that happens to be pointing at us?
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Yes - for example GRBs would be a source. I don't know how tightly beamed the gravity waves are though.
 
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sol invictus said:
Yes - for example GRBs would be a source. I don't know how tightly beamed the gravity waves are though.
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I'm guessing not very. I'd also guess that since the source isn't oscillating per se that the gravitational "wave" is just going to be a single wave front (from the initial explosion), though possibly with a very long tail. So even if pointing at us, it's probably still going to be harder to detect than a decaying binary system since there wouldn't be much to see if you miss that wave front or can't pick it out from noise.
 
Ziggurat said:
I'm guessing not very. I'd also guess that since the source isn't oscillating per se that the gravitational "wave" is just going to be a single wave front (from the initial explosion), though possibly with a very long tail. So even if pointing at us, it's probably still going to be harder to detect than a decaying binary system since there wouldn't be much to see if you miss that wave front or can't pick it out from noise.
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I think that's true all else being equal, but with GRBs you have the advantage of being able to see them. So one thing LIGO tries to do is correlate signals with observed GRBs. Nothing so far.
 
sol invictus said:
I think that's true all else being equal, but with GRBs you have the advantage of being able to see them. So one thing LIGO tries to do is correlate signals with observed GRBs.
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Good point. That could help with the signal to noise issue quite a bit.
 
sol invictus said:
Yes - for example GRBs would be a source. I don't know how tightly beamed the gravity waves are though.
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Do gravity waves follow the inverse square law? I am remembering that they reduce in intensity faster than that but am not confident in my memory.

So as they are so very far away would they be a good source for detection?
 
ponderingturtle said:
Do gravity waves follow the inverse square law? I am remembering that they reduce in intensity faster than that but am not confident in my memory.
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The energy in any spherical wave in flat space must always fall off as 1/distance squared. IIRC the "strain" Ligo measures - which is the fractional change in length of its arms - falls off only as 1/distance.

So as they are so very far away would they be a good source for detection?
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It needs to be unbelievably sensitive. Here's a fact - LIGO can measure a change of less than 10^-17 meters in the average distance between two mirrors several kilometers apart. Try to wrap your head around that - and remember, atoms around 10^-10 meters across, and atomic nuclei about 10^-15.
 
sol invictus said:
It needs to be unbelievably sensitive. Here's a fact - LIGO can measure a change of less than 10^-17 meters in the average distance between two mirrors several kilometers apart. Try to wrap your head around that - and remember, atoms around 10^-10 meters across, and atomic nuclei about 10^-15.
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Arrgh - my head hurts :drool:

Taking into account the conversion between meters and kilometers, that's a sensitivity to one part in 10^20, at least!

Or, for visual effect, one part in 100,000,000,000,000,000,000
 
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Advanced LIGO

They are upgrading the LIGO interferometers in Livingston, LA and Hanford, WA to enable detection of the "standard" neutron star pair inspirals out to about the distance of the Virgo supercluster.

Hope they find some waves, or we may need to look at a different gravity theory..................
 
Ziggurat said:
Approximately, yes.
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But Ligo doesn't measure the energy in the wave. It measures the "strain", or differential change in the length of its arms. That's a field strength rather than an energy density, and it falls off like 1/distance, not 1/distance squared. So LIGO can actually see very, very far.
 
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