Does Gravitational Radiation Exist?

[wolram]

http://arxiv.org/PS_cache/arxiv/pdf/0711/0711.3041v1.pdf

Just a guess but i bet they do not find any.

 

[jimgraber]

The question is when. Now: 10% chance or less.
Three years after Advanced LIGO is complete, i.e. 2014ish,
No gravitational waves seen would be a big surprise.
Actually, even one year of Advanced LIGO is expected to yield
some real detections.
Best
Jim Graber

 

[Chronos]

The evidence favoring gravitational radiation is already overwhelming - e.g., binary neutron stars. I would be astonished if it is not confirmed by LIGO.

 

[wolram]

I think some people will be astonished.

 

[pervect]

While people can (and frequently do) think anything they want to based on absolutely no evidence whatsoever, we like to encourage people here at PF to think about the experimental evidence, and also what has been published in the professsional literature.

The 1993 nobel prize was won for an indirect observation of gravitational waves. The waves themselves were not observed, but a binary pulsar was observed to lose angular momentum and energy at "just the right rate".

It is therefore generally expected that when our instruments (such as LIGO) become sensitive enough, that we will confirm this highly suggestive indirect detection with direct detection.

 

[Wallace]

I hope LIGO doesn't find anything, just because things will be more interesting that way! (just like if the LHC doesn't find the Higgs). But I have to agree that the evidence for gravity waves is pretty strong so it will be very surprising if they turn out not to exist.

 

[Chris Hillman]

To the contrary, astronomy should become very interesting if the instrument does work. Did you miss the point about "new window on the universe"? If we could directly detect the gravitational radiation emanating from some of the most violent events in the universe, we would learn a lot more about them. Gravitational radiation is fundamentally different from EM radiation (according to gtr and many related gravitation theories), and in many scenarios it carries fundamentally different kinds of information about distant events. The expectation is that once gravitational wave astronomy takes off, it will be routinely used with EM astronomy (infrared, optical, UV, X-ray bands) to make powerful deductions about far-off places and long-ago times.

As pervect already noted, the existence of gravitational radiation has already been indirectly confirmed from the orbital decay of binary pulsars, and to judge from how closely this conforms to precise gtr predictions, gtr most likely accurate models the properties of the gravitational radiation.

Some points to remember:

• LIGO is in essence a Michelson interfermeter, but of a very novel design incorporating several ingenious innovations. It is the most sensitive and ambitious scientific instrument ever constructed; it aims to detect variations in length over a several kilometer course of less than a thousandth the "diameter" of a proton!
• LIGO has already produced reams of data which may well contain signals; the problem is to extract the signals from the noise. It was known from the outset that this would be very difficult, and so far, as I understand things, this has proven to be even more difficult than was expected a decade ago.
• LIGO is expected to continue to improve its sensitivity, and this should greatly improve the chances of a signal which can be extracted and unambiguously be interpreted as the effects of a passing gravitational wave (as much as anything is ever unambiguous in science, that is).
• Hopefully forthcoming space-based observatories (e.g. LISA) are designed to be sensitive to a different band of UV radiation, a prospect which should further excite young astronomers.

I would caution against rushing to assume that LIGO is disproving anything; I counsel continued patience as we await the long expected solid results.

 

[Wallace]

Sure, there's a bunch of new observations you could do, which would be fun for observers. But to really shake things up requires a measurement to say something completely unexpected. Then the theorists get to have the fun!

 

[Chris Hillman]

But Wallace, did you not take the point made years ago by Kip Thorne? He pointed out that as we extended "electromagnetic" astronomy from optical to infrared and to UV and X-ray bands, we made completely unexpected discoveries such as quasars and pulsars, but LIGO is designed to yield observations using an entirely different kind of radiation, so we should expect, he argued, that gravitational wave astronomy will indeed reveal completely unexpected phenomena. I think that is when theorists will once again be able to have fun with astronomy!

 

[Wallace]

Sure, I'm obviously being flippant. Either way the future of cosmology is interesting, for observers and theorists alike.

Actually I read a paper the other day about 'gravity wave sirens' which are hypothetical 'standard candles' of gravitational radiation. Not that we know that any such things exist, but the paper was estimating what we might be able to learn if such things were discovered in the future, depending on how 'standard' they are, how much power they put out and how sensitive our measurements are.

 

[Chris Hillman]

Harwit's argument against yet undiscovered "radically new" phenomena

I am guessing you know the textbook by Martin Harwit, Astrophysical Concepts, 4th edition, Springer, 2006, but have you read his book Cosmic Discovery: The Search, Scope, And Heritage Of Astronomy? One of the interesting "information theoretic" arguments he gave applies a statistical analysis which had previously been applied to guess the remaining oil reserves from the changing rate of discovery of new oil fields to conclude that the number of completely unexpected phenomena such as quasars is in fact finite and has, Harwit asserted, probably been exhausted, a prediction which has been borne out in the quarter century since the book was published. (I forget whether he took gravitational radiation into account, but this would only postpone the inevitable, according to this line of argument.) This claim set off a firestorm of invective from fellow astronomers, curiously unaccompanied by mathematical argument

If we dare, perhaps in another thread we can revisit the issue of whether or not it is reasonable to expect further "heroic ages" in any currently known field of science. The argument is rather general, a bit like counting spheres packed into a keyspace, and it would be interesting to try to abstract away the elements of quantum theory, if possible.

 

[Chronos]

Personally, I'm looking forward to a neutrino telescope with decent resolution. That will be the backbone of astrophysics in this century, IMO.

 

[Chris Hillman]

Evidently we fear being denounced on the floor of the U.S. Senate

Color us cowards, then

 

[Chronos]

Agreed, Chris. And this is why scientists must be so cautious, ask all the right [and wrong] questions, and draw conclusions circumspectfully. There is enormous competition for the $$ allocated to science and our "enemies' would not hesitate to politicize any indiscretions to siphon funds from 'frivolous' projects, IMO.