Gravity Wave Blues

August 15, 2008

Back in 1974, a pair of scientists located a pair of neutron stars in the Milky Way galaxy, one of which was a pulsar: that is, it emits regular pulses of radio waves easily detectable on Earth. Joseph Taylor and Russell Hulse used those very precise, regular pulses as a kind of clock, and observed the orbiting neutron stars over two decades. And they found a pronounced shift in the timing of those pulses. That meant that the stars had lost energy, carried away by gravitational waves -- just as Albert Einstein had predicted in his theory of general relativity.

Chalk up another victory for Einstein, and one for Taylor and Hulse: they shared a Nobel Prize in 1993 for their discovery. But direct observation of gravitational waves continues to elude scientists. According to general relativity, mass warps the fabric of space-time, and this curvature accounts for what we observe as gravity. When a large celestial mass moves suddenly -- for instance, if a star explodes as a supernova -- some of that curvature will ripple outwards, just like the ripples in a pool of water if you suddenly dropped a rock into its center.

The same thing can happen with a neutron star, the incredibly dense burned-out core that remains after a star explodes. Two of these incredibly dense objects circling each other stir space-time as they move -- much like a very large kitchen mixer -- and this causes ripples of gravitational energy through the fabric of space-time. These ripples are very, very faint, since the waves weaken as they ripple outward, so by the time they reach Earth, they are very weak indeed. But with sensitive enough instruments, we should, in theory, be able to detect them.

How hard can it be to catch a gravitational wave? Pretty darn hard, it turns out. Back in 1969, a physicist  named Joseph Weber at the University of Maryland set up giant cylindrical bars, thinking that should a gravitational wave pass by, it would cause them to vibrate, or ring like a bell. He claimed he 'd succeeded, but no one else could reproduce his results, so it remains a highly disputed experiment, even though several groups today still listen for the telltale ripples using similar detectors.

The most promising type of detector developed so far is called a laser interferometer, an instrument that precisely measures how long it takes light to travel between suspended mirrors, using laser light. Any ripples in space-time should cause the distance measured to change as the gravitational wave passes by, and this change can be picked up by a photodetector. In essence, it acts like a microphone, converting gravitational waves into electrical signals.

The Laser Interferometer Gravitational Wave Observatory (LIGO) has three laser interferometers: two near Richland, Washington, and a third near Baton Rouge, Louisana. You need at least two instruments separated by a great distance to rule out false signals. To date, LIGO hasn't detected any gravitational waves.

In a bid to further increase sensitivity, LIGO scientists combined their search with a fourth detector, the GEO600 in Germany, all scanning the heavens simultaneously for those telltale ripples. They announced their results in an arXiv paper last week, and the news is not encouraging. They collected data for a full month and concluded that "No candidate gravitational wave signals have been identified." Considering the hundreds of millions of dollars spent to date on LIGO, this doesn't bode well for future projects, such as LISA, which -- if built -- would take the search for gravitational waves into space, thereby increasing the chances of making a direct observation.

The good news is that this need not mean that general relativity is "wrong," since many of its other predictions have been verified repeatedly. But it may be incomplete. Until scientists can definitively make such a conclusion, the search for those elusive ripples in space-time will (funding permitting) continue.

Photo: Illustration of ripples in the fabric of space-time, Kip Thorne (Caltech) and T. Carnahan (NASA/GSFC). Source: NASA/Jet Propulsion Laboratory.

Comments

[Xerxes]

I wouldn't get my hopes up for beyond-GR physics at this stage. The big inspiral events they're looking for occur close enough only a few times per decade. Probably LIGO/GEO didn't see anything because there were no events big/close enough to see. Even if they don't see anything at the higher sensitivities, I think the finger will get pointed at binary-evolution models long before anybody (sane) questions GR.

[LVCgradSlacker]

Xerxes is right to say that the primary reason LIGO/GEO did not see anything is that there was a large enough event close enough to see. Mind you that 'close' means 15 Mega-parsecs (~ 50 Million light years) away from Earth which is only out to the Virgo Supercluster. Future upgrades will extend the range by a factor of 10, which in turn expands the listening volume by a factor of 1000.

The binary inspirals, which are the most likely candidate for a first detection, are modeled in both full numerical relativity as well as other methods. Now if we don't see anything after the upgrades to the detectors are complete, it is likely that something is serious wrong with our understanding of stellar astrophysics, not GR.

[ZZMike]

There's a sorta-related article in the UK Telegraph (instapundit links to it) that talks about the possibility of a "warp drive":

http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/earth/2008/08/15/sciwarpdrive115.xml

"In their scheme, in the Journal of the British Interplanetary Society, a starship could "warp" space so that it shrinks ahead of the vessel and expands behind it."

There's a slight catch:

"All this extraordinary feat requires, says the new study, is for scientists to harness a mysterious and poorly understood cosmic antigravity force, called dark energy."

Well, if that's all that's standing in the way .....

But then things start to get complicated:

"The new warp drive work also draws on string theory....

... it is by changing the size of this 10th spatial dimension in front of the space ship ..."

OK, that's it then. Turn over the plans to thge engineers and let's get going (or as they say over there, Bob's your uncle).

I wonder if not finding gravity waves is about the same as Michelson-Morley not finding the "aether".

On a cultural note, Joseph Weber (one of the first investigators) was married (until his death) to a noted astrophysicist, Virginia Trimble. We had the pleasure of hosting them both at a Sigma Xi meeting in Southern California (a long time ago).

[Rich]

The search for gravity waves seems a bit like the search for the CMB anisotropy prior to the COBE satellite; our theories suggest it ought to exist, but it's a faint signal and so we may need a few generations of instrument to get there. Compare the first COBE detection (statistical only) with the latest WMAP maps - it's hard to imagine how it was ever so hard to get such a strong signal!