A few months ago, we reported on a theoretical paper that discussed the potential advantages of a gravity wave detector based on a torsion bar, which the creators called TOBA. In the intervening time, the team has not been idle, as they have a small-scale test bar up and running. Deep in the night of August 15, 2009, they performed a test run to look for gravity waves—not gravity induced pressure fluctuations in Earth's atmospheric pressure, but stretching space-time.

The good thing about the TOBA experiment is that it fills an important spectral gap in the current generation of gravity wave detectors. Cosmological and astronomical observations can be used to look for extremely low frequency (10-6Hz) gravitational waves, while the laser interferometer detector (LIGO), and other Earth-bound instruments are used to look for gravity waves with frequencies in the 100Hz plus range. TOBA is designed to operate in the 0.1-1Hz range.

This is important because of something called the cosmic gravitational wave background. You may have heard of cosmic background radiation. This radiation is the oldest in the Universe. It originates in that time interval when the hot dense Universe went through a phase transition from a plasma (consisting of unbound electrons and ions) to neutral atomic species. Before this time, radiation was scattered an awful lot, meaning that its history was rapidly lost. After the Universe became neutral, the scattering was very much reduced, allowing this light to retain its history. The remnant that we measure is the unscattered light from that transition but, optically, that moment in time is like a wall that we simply cannot see through.

So, all the physics of the Universe from times before the transition must be tested rather indirectly, based on what the physics implies about the phase transition. That doesn't mean it's all guesswork, but more direct observational data would always be welcome.

This is where gravitational waves can play an important role. Gravity waves don't care about electromagnetic charge, so this phase transition is unimportant as far as they are concerned. Furthermore, the early Universe is thought to have rung like a bell, with space-time stretching and contracting rhythmically. The frequency and directionality of these waves will tell us about the symmetry properties and, as a result, the physics of the early Universe. These waves are spread across the low frequency end of the gravity wave spectrum, including the range covered by TOBA.

Now that we know why it's exciting, let's take a look at TOBA itself. The little TOBA is a torsion bar with some cleverness attached to it. It's not suspended by a wire; instead, the Japanese research group used a property of a particular class of superconductor to hold the torsion bar suspended in vacuum. When a magnetic field penetrates a superconductor, it gets pinned to a specific location. By attaching a magnet to the center point of the torsion bar and hanging the torsion bar below a superconductor, the magnetic field lines get held in place, creating a suspension system that has very little resistance.

The result is that, when the torsion bar is set in motion, it stays in motion for a long time. Another consequence of this low resistance suspension is that the resonant frequency of the torsion bar is very low—around 5mHz in this case—making it very sensitive to low frequency disturbances in the gravitational field.

As with all things, I suspect the experiment has been operational for some time now. But all the early work would have been making sure that the performance was as expected. After all the additional sources of noise had been expunged, the research group took 12 hours of preexisting data, selected the least noisy sections, and analyzed them for the presence of gravity waves. End result: they didn't find any. I suspect they would have been shocked if they had.

This paper isn't really about the gravity wave data; instead, it is about an instrument that, in its first stage of development, is performing as expected. What is next? The researchers plan to move the experiment to the bottom of a mine, so that the environment is a bit quieter. They plan to improve the vacuum conditions, so that the intrinsic instrumental noise is reduced. They also plan to add a second torsion bar at right angles to the first. This second bar will allow the researchers to eliminate many local sources of vibrational noise, increasing their sensitivity further.

Finally, should all of that work as expected, they get to build the big version of the experiment. Clearly there are many more years of work and several excellent research projects waiting for students here. And I look forward to seeing them report their progress.

Physical Review Letters, 2011, DOI:10.1103/PhysRevLett.106.161101