Arcadian Functor

occasional meanderings in physics' brave new world

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Marni D. Sheppeard

Saturday, January 19, 2008

Monthly Misquote

Either the source was not a coalescing binary or there is some exotic situation where the gravitational waves disappear into another dimension
said Jim Hough of Glasgow University on the latest non-observation of gravitational waves by LIGO (report from Physics World).


Blogger CarlBrannen said...

The dog that didn't bark.

January 19, 2008 2:25 PM  
Blogger Matti Pitkanen said...

Large values of gravitational Planck constant might explain the problem. The continual steady flow of gravitational radiation would be relaced with pulses with much higher temporary power. These kind of pulses would be probably eliminated by the detection procedure as a perturbation having nothing to do with gravitons in GRT sense.

January 19, 2008 6:05 PM  
Blogger Kea said...

Actually, it seems they're still only talking about GRB070201, but the story gets more interesting with time.

January 19, 2008 6:15 PM  
Blogger Philip said...

As quoted, Jim's "either/or" comment is incorrect in the sense that there are other far more likely explanations for the non-observation, the most obvious being the source was just too far away to be seen at LIGO's current sensitivity. If that's what he really said, then it's a strange things to say. So far as I'm aware, GRB events are isotropically distributed in the sky and for that (and other reasons) are supposed to be occurring at cosmological distances, not the 10 or MPc radius that LIGO can see at the moment. If I remember correctly, at current sensitivity and using current stellar population models, the best LIGO could hope for is one NS-NS inspiral event per 10 years. With advanced LIGO it could be 1 to 10 events per year. While it's worth looking, I'd be very surprised if any GW's were detected from a GRB at this stage of LIGO (or indeed, any GWs at all - the project has been very careful to make clear that while they HOPE to detect something with LIGO-I, it's very unlikely).

January 20, 2008 7:29 PM  
Blogger nige said...

If the events supposedly creating the gravitational waves are at cosmological (massive) distances, then the reason why the gravitational waves will be much smaller than mainstream predictions suggest is down to quantum gravity:

1. Quantum gravity works by the exchange of gravitons between masses (and fields possessing energy).

2. If the masses or fields the detector is exchanging gravitons with is located at cosmological distances, then the exchanged gravitons are passing through an expanding distance due to cosmological recession. This will cause redshift of graviton energy.

3. By Planck's law E = hf, a shift of frequency of any field quanta is accompanied by a loss of energy of the field quanta. [This is why the light we see from the big bang is redshifted to microwaves, instead of 3000 K blackbody (mainly infrared) radiation scorching us to a cinder, which would occur in the absence of redshift!]

4. The redshift of exchanged gravitons over cosmological (massive) distances reduces the energy of the received gravitons and thus reduces the effective value of the gravitational coupling constant, G, for the interaction.

5. As a result, the gravitational waves from events at cosmological distances appear far, far smaller than predicted using a constant gravitational coupling constant G. The faulty prediction can be corrected by working out the relative graviton redshift energy depletion effect for the cosmological distance of interest, and simply scaling the value of G by the same factor. I.e., if the redshift is such that quanta suffer a doubling of wavelength and a halving of frequency, their energy will fall by a factor of two, and the correct scaled value of G to use in gravity wave predictions will be G/2. It's as easy as that to compensate for graviton redshift (the amount of graviton redshift will be identical to the amount of visible light redshift at a given distance, and that is well known from the Hubble law).

January 21, 2008 1:51 AM  
Blogger Philip said...

Hi Nige, there's no reason to think that the GWs are smaller than expected based on the the current non-detection. If some GRBs really are the end result of a NS-NS inspiral, and they are further than about 10 MPc away (which seems very likely), then standard GR says that the GW's from them are too faint to be detected with LIGO-I.

January 21, 2008 11:32 AM  
Blogger Philip said...

This comment has been removed by the author.

January 21, 2008 11:32 AM  
Blogger L. Riofrio said...

Fascinating subject! I would refer everyone to a paper by Christos Tsagas, "Magnetic Tension and the Geometry of the Universe" in Physical Review Letters v. 86: 5421-5424. In short, magnetic field lines are capable of flattening any curvature in spacetime. This interpretation rules out the extreme curvature of "inflation" models. It would also flatten out any gravitational waves long before they reached our instruments.

The celebrated result from the Hulce-Taylor binary pulsar did not measure gravitational waves directly, just the energy loss that was expected from them. Gravitational waves may be yet another scientific wild goose chase.

January 25, 2008 7:42 PM  

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