Making light work of gravity gradients
Aug 7, 1998
Physicists in the US have used an atom interferometer to measure the gradient of the Earth's gravity field. Such measurements could provide more accurate measurements of the gravitational constant, G, and better tests of general relativity. Other potential applications include covert navigation, oil-well logging and the detection of underground structures.
The interferometer was built by Mark Kasevich and colleagues at Yale University in the US and used individual atoms, rather than macroscopic man-made objects, to measure the gradient. By observing the phase and frequency shifts of laser light passed through two cooled ensembles of atoms, they believe that the interferometer has the potential to be at least thirty times more accurate than any instrument currently available (M J Snadden et al. 1998 Phys. Rev. Lett. 81 971).
The apparatus consists of two magneto-optical traps at different heights - the first trap about one metre above the second - in which clouds of caesium atoms are cooled to 3 microKelvin. Various laser beams are used to excite the caesium atoms, and the probability of finding the atoms in an excited state after a sequence of laser pulses is related to the value of gravity at that point. Therefore differences in gravity at the different heights will be reflected in the different levels of excitation of the atoms, which can also be measured with lasers.
Unlike other devices which measure gravity gradients, this new instrument automatically cancels out background noise caused by vibrations or sudden knocks on the equipment, making it more suitable for field work.