While preparing for the job of US Secretary of Energy in the incoming Obama administration (and being director of one the top labs in the US and Nobel Prize winner to boot), Steven Chu has somehow found time to post the results of his latest experiment on the arXiv. And it’s an impressive piece of work too.
Chu, who is director of the Lawrence Berkeley National Laboratory, and his colleagues have built an atom interferometer with a sensitivity that is dramatically higher than previous models. To prove its worth, they’ve measured the fine structure constant to an accuracy of 3.4 parts per billion, which is within an order of magnitude of the best measurements.
But the real benefit of the new device is that, among other things, it will allow a new generation of tests of the equivalence principle. That is, it will test whether the m in F=ma and the m’s in F = Gm1.m2/r^2 refer to the same thing.
In physics-speak, the question is whether gravitational and inertial mass are the same. It’s something we always assume but have never proven and there are a number of ongoing programs to study the question.
Here’s how Chu’s work will change the game…
Atom interferometers work by zapping a beam of atoms with photons, causing it to split in two . The two beams then travel different paths, perhaps under the influence of gravity, before being zapped again by photos that push them together again where they interfere. Any difference in their paths then shows up in the interference pattern they produce.
The trouble is that the photons have to be identical, otherwise they kick the atoms in different ways, introducing extra changes in the interference pattern that swamp the signal you’re trying to see.
This problem significantly limits the size of interferometers and hence the sensitivity of their measurements.
What Chu and colleagues have done is develop a method for cancelling any differences in the photons being used.
That makes it possible to build much larger interferometers. In fact, in their first set of experiments, Chu and co have increased the area enclosed by the arms of the interferometer by a factor of 2500. That’s a massive increase.
It also allows atoms of different types to be used in the same interferometer, opening the way to amazingly sensitive tests of the equivalence principle.
Not bad for a busy chap. Let’s look forward the breakthroughs Chu can come up with while in office.
Ref: http://arxiv.org/abs/0901.1819: Noise-Immune Conjugate Large-Area Atom Interferometers