LETTERS TO THE EDITORS
To the Editors:
I had hoped that Peter S. Shawhan's article "Gravitational
Waves and the Effort to Detect Them" (July-August), would have
cleared up a question about light waves. Shawhan states "Any
object encountered by a gravity wave is stretched and shrunk along
with the space in which it lives.…" Does this distortion
include the wavelength of any light propagating in that space?
The interferometric comparison of the relative lengths of the arms
of a gravity wave detector essentially measures the number of
wavelengths of laser light contained within each arm. If the arm
length is changed by the incidence of a gravity wave, and the light
wave moving parallel to the arm is changed by the same amount, then
the number of wavelengths in each arm will be unchanged, so there
will be no interferometric evidence of the gravity wave.
I can't see any way that the interferometric detectors can work if
the light wave is also "stretched."
Los Altos, California
Dr. Shawhan responds:
The wavelength of light is not a physical length, but a measure of
the distance over which its electric and magnetic fields complete
one cycle of oscillation. In relativity theory, length and distance
are effectively defined in terms of the time required for light to
travel from one point to another.
When a gravitational wave stretches the effective length of one arm
of an interferometer, the laser light takes a slightly longer time
to travel from the beam splitter to the end mirror and back to the
beam splitter, so the phase of the light evolves through an extra
fraction of a cycle. That is, the arm contains a slightly greater
number of wavelengths of laser light.