Editor:
A common question relating to the LIGO story by Barry Evans (Field Notes, Oct. 26) is “how can light beams detect gravity waves when light and matter are equally stretched and shrunk by passing gravity waves?”
The answer lies in the constancy of the speed of light, so that a stretched light wave has a lower frequency. The two light beams no longer cancel each other after traveling different lengths at the same speed (even though the lengths differ by way less than the diameter of a proton!). I urge Barry to follow his story with LIGO’s fantastic detection of merging neutron stars.
Don Garlick, Fieldbrook
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This article appears in Play It Forward.
Thanks for this, Don. If both the wavelength and the space between the mirrors are equally stretched, how is it possible to detect anything? Heres my understanding. Its true that the laser beams actually in the LIGO tubes at the moment the gravity wave starts coming through the apparatus see nothing unusual. But gravity waves vibrate about 100 times per second, much slower than the laser light, so fresh light coming into the already-stretched tubes has to travel fartherand its that fresh light that detects the passing gravity wave.