Scientific Method —

Cheap and cheerful gravity wave detector on the horizon

We've been looking for gravity waves predicted by the general theory of …

Gravity waves seem to be the ultimate in hard-to-detect phenomena. Currently, we have a couple of rather large laser interferometers and a giant suspended pendulum looking for them, so far without success. Furthermore, astronomers have been busy observing variations on pulsar frequencies and the like as a way to use astronomical objects as gravity wave detectors. Then there are the next-generation detectors, all in various stages of development, that include space-based observatories, among other approaches.

Now we have another detector to add to the list. A team of researchers from Japan and Germany have proposed a new way to detect gravity waves. The big news is that, compared to the others, it is a lot simpler, probably a lot cheaper to build, and nearly as sensitive.

First questions first: how do we know that gravity waves exist? The general theory of relativity, which is, at heart, a theory of gravity, predicts them. In general relativity, changes in mass at a location cause space and time to stretch and compress. Rather like sound waves, compressing space-time causes stretching in neighboring regions and vise versa. In this way, a moving distortion in space-time is created. We can detect these by measuring the response of a mass to the distortions in space-time.

Now, it's possible that gravity waves don't exist, but for that to be true, much of general relativity would have to be wrong. Or maybe I should say more wrong, or even very, very wrong. So much so that it would be an amazing coincidence that all other tests of general relativity support it.

Nevertheless, the current generation of detectors aren't likely to be sensitive enough to detect gravity waves. And, once we have ones that are sensitive enough to detect gravity waves from relatively frequent astronomical events, they will be insensitive to some of the most interesting: low frequency gravity waves left over from the Big Bang. 

The oldest thing that we can see is the cosmic microwave radiation. And, even though this stuff is pretty old, it comes from the moment when the Universe cooled enough for the plasma of charged particles to condense into atoms. In other words, the oldest light in the universe comes from the moment just after all the interesting stuff occurred. On the other hand, gravity waves were not absorbed by that plasma, so the equivalent—a cosmic gravity wave background—would allow us to see further back in time.

We want to see this stuff, but it is difficult. New detectors will most likely detect gravity waves from recent events, but what we really want are arrays of detectors that are most sensitive at very low gravity wave frequencies (less than one oscillation per second). Enter TOBA, the torsion bar detector, the cheap and cheerful gravity wave detector.

TOBA is like any other gravity wave detector in that it tries to measure distortions in space by observing their effect on a test mass. In this case, the detector consists of a long bar that is gently twisting back about its center. If a gravity wave should impinge upon it, the bar experiences a twisting force, called a torque, that either slows or speeds up the motion. This can be detected by carefully measuring the positions of the ends of the bar as a function of time. 

A pair of these, oriented at right angles to each other and twisting out of phase with each other, provide an excellent detection system, because things like earthquakes effect both in a similar manner, while gravity waves do not, which provides a natural filter against a lot of ground-based noise sources.

Another advantage is that the torsion bar is twisting at its own frequency, and gravity waves are measured as deviations from that frequency. This means that, instead of trying to measure deviations from a stand-still, we are measuring changes to a non-zero frequency. This is important, because the further you move away from the zero-frequency point, the less intrinsic noise there is in the measurement.

The researchers estimated the sensitivity of a 10m long aluminum bar that weighed just 7600kg, finding that it should have a sensitivity of around 10-14-10-191/Hz1/2. In the high frequency range, this is no better than what is expected for the upgraded versions of the LIGO detector, but the low frequency sensitivity is very good. 

In contrast to LIGO, though, a 10m object is small and relatively cheap to build. It could be the key element to the gravity wave detector array. If it were made smaller, it could be sent into space as well. If construction is funded, you can guarantee that you will be hearing about it again.

Physical Review Letters, 2010, DOI: 10.1103/PhysRevLett.105.161101

Listing image by NSF

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