Absolute gravity measurements have applications in many diverse fields such
as standards and metrology, geophysics, oil-exploration, water reservoir
monitoring, and even homeland security. One of the best methods for
measuring gravity involves tracking a freely falling mirror in a vacuum
using a laser interferometer. The method is akin to satellite laser-ranging
with the difference that our satellites can be stored in a room-temperature
can in the laboratory. It is no small technical challenge to launch a mirror
into free-fall and watch it fall with a precision and accuracy of less than
1Å (~ 1 atom) but it is possible due to the use of laser interferometry. It
is also possible to measure small gravity gradients by launching two
different satellites at the same time and measuring the difference in their
free-fall accelerations. This technique is a modern day version of Galileo's
famous experiment from the leaning tower of Pisa. Gradiometers can be useful
for tunnel detection and even searching for diamonds!
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