The Economist has an article this week entitled To catch a gravitational wave. It’s about the proposed LISA satellite experiment designed to measure gravitational waves, with a much greater sensitivity than LIGO. According to the article, what would you guess is one of the main goals of the LISA experiment? Exactly, like most other ambitious experiments, it will solve the problem of how to test string theory:
[LISA] could allow scientists to examine the validity of string theory, which says that there are more than four dimensions to space-time and that the extra dimensions are hidden. String theory has come under fire because its predictions have so far proved untestable. The normal version has it that these dimensions are curled up in strings that are smaller than the known elementary particles. However, in some versions strings form very long “superstrings” that stretch across the universe. These superstrings form loops and vibrate, radiating gravitational waves; they can also crack like whips, sending bursts of gravitational waves towards Earth. “Seeing direct evidence of strings would be as important as discovering that the world is made of atoms,” claims Craig Hogan, an astronomer at the University of Washington, who is a member of the international science team for LISA.
The writer appears to be a bit confused about what a superstring is, guessing that it is a really big string (a cosmic string). This is presumably all based on the idea promoted by Joe Polchinski that it is in principle possible to come up with superstring theory models with cosmic scale superstrings, whose effects would be visible through gravitational lensing and gravitational waves. As far as I can tell, this is just another case of the phenomenon that one can get pretty much anything one wants out of string theory, and there’s no reason at all to expect cosmic strings with just the right properties to have been invisible so far, but visible through gravitational wave effects measurable by LIGO or LISA.
Two years ago there was a press release about this from UCSB quoting Polchinski as saying
the gravitational signatures from cosmic strings are remarkable because they are potentially visible even from the early stages of LIGO! That means ‘potentially visible’ over the next year or two.
LIGO hasn’t seen anything, so time was up for this nearly two weeks ago but I haven’t noticed any UCSB press releases reporting that things haven’t worked out.
There was some excitement a year or so ago when a group claimed that an astronomical object might be a single galaxy lensed by a cosmic string. Turned out to just be a pair of nearby galaxies.
LISA is tentatively scheduled for launch nearly ten years from now, so it will be a while before this particular “test of string theory” brings in any results. This past week the 6th Annual International LISA Symposium was held in Maryland.
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