American Scientist

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."

Sylvan Rubin
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.