One of the more laborious scientific enterprises of the last few decades has been the development of gravity wave detectors. These detectors, which are supposed to pick up the tiny ripples in space-time due to events involving massive objects are, themselves, massive. The biggest hang-up is that, despite their enormous size, these things need to be incredibly sensitive, making building one rather challenging.

In an intriguing talk, called "Underground and airborne matter wave inertial sensors: towards fundamental tests of gravitation," Phillipe Bouyer of CNRS d'Optique presented some interesting results and a grand vision for a new type of gravity wave sensor at the conference Physics of Quantum Electronics.

What Bouyer and his co-workers can do is make Bose Einstein condensates (BEC) on a very small and portable scale. The nice thing about BECs is that they are a special type of matter that can exhibit a phenomena called interference. It's possible to take a BEC, split it, and recombine it. As the two clouds of atoms pass through each other, there will be stripes, called fringes, through the cloud where there are no atoms at all, while at other locations there will be more atoms than you would expect. Where and when these fringes appear depends on how far the two clouds have travelled while separated.

If a gravity wave should pass through while a BEC is separated, then one of the clouds will travel further than the other thanks to the stretching of space. That will be detectable as a shift in the locations of the fringes. The actual implementation of such a scheme is far more subtle than what I have described, but the essence of the experiment remains the same.

So you can imagine that there is a lot of interest in testing general relativity at very small scales. For instance, a BEC may allow us to see whether gravity and acceleration really are indistinguishable—something called the universal equivalence principle. This basically involves taking BECs, putting them in free fall, and seeing if BECs made from different atomic species fall at different rates.

Bouyer and his team have chosen to do this in the vomit comet, a plane that follows a flight trajectory that provides periods of time where everything in the plane experience free fall (including the passengers). Those of you who have flown recently, however, will remember that planes don't exactly provide an ideal environment for sensitive physics. Aside from the vibrations and electronic noise from the plane itself, there are also changes to the gravitational forces due to turbulence changing the plane's trajectory.

What followed was a demonstration of technical excellence, as Bouyer showed the lengths that they had gone to to overcome each these problems. In the end, they have an accelerometer that is pretty sensitive and, most importantly, free from drift. They make excellent seismometers and could also be used to image underground structures, much as geophysicists do today with gravitometers.

But what caught my attention is an idea that Bouyer and colleagues have proposed to the EU: why not build an underground network of these sensors, all linked by a laser. A laser is used to make the measurements anyway and, by using just one laser, you get the differences from each BEC cloud, eliminating sources of interference that are common to all clouds.

One cool thing about this is that you get a gravitational wave detector. But, at the same time, this apparatus could provide an accurate picture of the density, movement and strain on the continental plate as it moved around. Underground water flow could be mapped. All sorts of possibilities are right there, just waiting to be explored.

At present, all that the EU has been asked to fund is a design feasibility study, while some pilot experiments are underway. Even so, it is a grand vision and I hope he gets the funding.

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