Tuesday, December 19, 2006


Mathematical physicist John C.Baez has developed something of a cult following since he began writing This Week's Finds in Mathematical Physics in 1993. His last couple of posts are full of some wonderful stuff. In Week241 he visits the Laser Interferometry Gravitational-Wave Observatory (LIGO) at Livingston, Louisiana, http://math.ucr.edu/home/baez/week241.html, (an interferometer is a device which measures interference between light which is out of phase). According to general relativity, a gravitational wave will stretch space in one direction, and squash it along a perpendicular direction. Hence, the two LIGO facilities which have been built in the US consist of a pair of tubes, each 4km in length, arranged in an L-shape, along which a laser beam is bounced back and forth. Baez describes the growing pains of LIGO in Week189, http://math.ucr.edu/home/baez/week189.html:

In a series of "engineering runs", both facilities identified and tried minimize all sources of noise. For example: microseismic noise, caused mainly by ocean waves hitting distant shores. Thermal noise of various sorts, minimized by cooling things to 2 kelvin, hanging mirrors attached to fused quartz test masses on steel wires... and many other clever tricks! Shot noise, meaning the uncertainty in the laser beam phase due to quantum mechanics. Radiation pressure noise, from the lasers pushing on the mirrors! Noise from residual gas in the evacuated tubes. And so on.

The battle against noise and other sources of error led in some strange directions. The Livingston facility had to remove a cattle guard at the entrance because of the microseismic noise produced whenever a car rolled over it. More annoyingly, it turned out that commercial logging near this facility caused real trouble every time a tree fell. And at the Hanford facility, wind-blown tumbleweeds piling up along the pipe would sometimes throw the beam out of alignment, thanks to their gravitational pull.

In this week's post, Week142, Baez berates the US government for their plans to set-up a base on the Moon, and the peril in which this apparently places a number of fascinating projects, such as the Laser Interferometer Space Antenna (LISA):


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