occasional meanderings in physics' brave new world

Name:
Location: New Zealand

Marni D. Sheppeard

## Friday, March 21, 2008

### GRB 080319B

AAVSO alert. Stolen from the ccd astronomy blog: an image of this week's record bright GRB 080319B. Thanks to Tommaso Dorigo for the alert. Distance indicated by a significant redshift of 0.94. No prizes for guessing the next question ... has anyone phoned LIGO?

Update: NASA news and images.

L. Riofrio said...

Thanks to both you and Tommaso. Here is a place many people agree that we could find a Black Hole.

Have you read about Avraham Trahtman solving the Road Colouring Problem? He worked in many menial jobs before succeeding as a mathematician. http://arxiv.org/abs/0709.0099

March 21, 2008 6:22 PM
Kea said...

Thanks for the link! I will check it out.

March 21, 2008 6:23 PM
nige said...

Can anyone theoretically predict the exact shape of the gravitational shock-wave transmitted via the spacetime fabric (gravitons, i.e. gravitational field quanta) from a gamma ray burster?

How much of the energy of a gamma ray burster is supposed to get converted into gravitational waves, anyway?

Before any statistical sense can be made of measurements, you need to understand theoretically what is going on.

I think that the simplest and best way to understand gravitational waves is by analogy to electromagnetic radiation due to the acceleration of electric charges.

If you accelerate an electron, it emits radio waves.

Of course, if you accelerate a hydrogen atom, you don't get any net radiation output because both the accelerating electron and the accelerating proton in the hydrogen atom are both emitting radio waves exactly out of phase with one another, so the two radiative waves perfectly interfere, "cancelling out" completely as seen from a distance which is large compared to the size of the atom (i.e., a distance that's large, as compared to the distance between electron and proton). What happens is that both of the opposite accelerating charges in the atom exchange electromagnetic radiation with one another, which allows them to accelerate without losing energy.

In the case of gravitational waves, gravitational charge consists of mass-energy so the acceleration of any mass should cause the emission of gravitational waves in a way similar to the emission of radio waves by accelerating single charges.

However, the coupling constant for gravitation for single charges is 10^{-40} times that of electromagnetism, so the power of emission of gravitational waves are correspondingly weaker than radio waves.

If gamma ray bursters are stars collapsing into black holes, then this physical mechanism (acceleration of gravitational charge effect, by analogy to electromagnetism) suggests the power of gravitational waves emitted by a gamma ray burster will on the order of be 10^{-40} of the energy of the observed gamma ray burst.

Since gamma ray bursters emit 10^44 J in the form of gamma rays, it follows that they emit only 10,000 Joules as gravitational waves.

That's the amount of energy released by 2.4 grams of TNT.

Sorry, GIGO isn't going to measure that gravitational wave over a cosmological distance. It's going to be swamped with too much noise from natural earth tremors.

In order to unequivocally detect the gravitational waves from a gamma ray burster, it would have to occur so close that we'll all get a lethal dose of gamma rays!
Gravitational waves are just too weak to detect by comparison.

March 22, 2008 1:46 AM
CarlBrannen said...

With a red shift of 0.9, it's kind of hard to believe that there will be any detectable gravitational radiation even if the stuff travels at speed c.

March 22, 2008 11:57 AM
Philip said...

It's in excess of 2 GPc. LIGO will not see it. If it's an inspiral it's too far away by a couple orders of magnitude.

Nige: it seems plausible that at least some types of GRBs really are the endpoint of a neutron-star binary inspiral. The gravitational wave signal from the inspiralling system prior to merger has been theoretically modelled (under some simplifying assumptions like velocities of the bodies being small compared to c) and the energy balance is consistent with observations of known binary NS systems and with numerical relativity calculations. The inspirals are in LIGO's sensitive band for 10s of seconds and this is what is hoped to be detected with a signal model. At current sensitivity can see a NS inspiral out to around 20 MPc.

As far as I'm aware there aren't good models for the fully relativistic inspiral just prior to merger, or for the merger itself (which is very complicated) where the gamma rays are produced, however various people have put estimates on the amount of gravitational energy produced in the merger. It is not small. Typical estimates I've seen are around 1% of the mass of the system, but because there's no model for the signal the mergers are harder to detect. A number of ways have been proposed that all rely on seeing a statistically unlikely 'burst' in the data, eliminating all possible local effects, and seeing if the same burst occurred in the other detectors. This is why it's nice to have two identical widely separated detectors (Hanford WA and Livingston LA) and other detectors elsewhere (VIRGO in Italy and GEO in Germany).

March 22, 2008 1:03 PM
Kea said...

Thanks, Philip. I thought it was too far, but being so bright I wasn't sure.

March 22, 2008 6:48 PM
CarlBrannen said...

Kea, it's somewhat outside my area of specialty, but I was under the impression that the explanation is that E&M radiation is dipole while gravitational radiation is quadropole and this causes their signals to drop off at different powers of distance.

March 23, 2008 2:09 AM
Kea said...

LOL. Yes, Carl, this was just an immediate reaction to the amazing brightness of the thing, without thinking about the numbers at all. Anyway, you know I don't believe in gravitational waves.

March 23, 2008 6:23 AM
CarlBrannen said...

Kea, I do believe in them. I just think they're, uh, fast. I don't know how far the uber-Webers are to detecting gravity waves at the expected random amplitudes but it has to be close.

By the way, I tacked on a reference to circulant matrices and MUBs at the end of the cubic matrix post.

March 23, 2008 7:40 AM
Philip said...

This comment has been removed by the author.

March 23, 2008 3:37 PM
Philip said...

Sorry, submitted the above before it was finished...

What are your reasons for not believing in gravitational waves? If and until they're observed I'd grant you there's a chance they don't exist, but I think it's almost certain they do exist.

So far GR is consistent with experiment, so it appears to be at least approximately correct. The derivation of gravitational waves from linearised gravity is not difficult, and I believe there are exact solutions of the full Einstein equations with wavelike properties as well. I don't have it handy but certainly Schutz's book discusses this.

I'm sure you're aware there is good indirect evidence for gravitational waves as well. This
plot shows the observed decay in the orbit of the Hulse-Taylor binary pulsar compared with GR predictions over a 30-year period, assuming the energy is being carried away in the form of gravitational waves - they're remarkably close. By the way, Hulse-Taylor is the one everyone quotes in regard to GW's because it's the most relativistic binary system known, but there are a few others around which also show good agreement with GR but I didn't manage to find any plots.

I don't know of any plausible alternatives for how the binary pulsar orbit could decay in such close agreement to GR.

March 23, 2008 4:23 PM
Kea said...

I agree that the binary pulsar is extremely good evidence that GR describes the local system well, but this is very different to the observation of GWs propogated over cosmic scales, where I believe quantum gravity is essential to an understanding of localised observables. Of course, I might be wrong. The way I see categorical M theory developing, GR has a wide range of applicability with regard to continua but NOT wrt point particles.

March 23, 2008 6:36 PM
Philip said...

I think we can agree that on the quantum scale, all bets are off :)
On the cosmological scale the propagation of GWs seems like something that can only be settled by experiment, and hopefully this will happen within the next few years. Advanced LIGO should be running by 2015. A result either way would be pretty interesting.

March 24, 2008 1:52 AM
Kea said...

Yeah, I agree a result either way would be interesting! It's not for theory to say whether they're or not, in the end.

March 24, 2008 6:15 AM
Kea said...

Nigel, I have deleted your comment. Please do not abuse my hospitality by posting very lengthy comments, which you are free to do on your own blog. I don't care how interesting they are, or how correct they are.

March 26, 2008 11:51 AM