What is the wavelength of gravitational waves? What is the wavelength of gravitational waves?  I have looked for an answer but so far not identified one.
 A: Like electromagnetic waves, gravitational waves can in principle have any wavelength.  They obey the usual relationship $v = \lambda f$ between the wave's speed $v$, wavelength $\lambda$, and frequency $f$.  Einstein's theory predicts (and recent observations have confirmed) that the speed of gravitational waves is $v = c$, regardless of their frequency.  Thus,  higher-frequency waves have shorter wavelengths, and lower-frequency waves have longer wavelengths, just as with electromagnetic waves.
There are two gravitational wave observatories that have detected gravitational waves to date:  LIGO, a large two-site detector in the USA, and Virgo, a smaller one-site detector in Europe.  These detectors are both optimized to detect gravitational waves with frequencies between a few hundred and a few thousand Hertz, so all of the gravitational waves that have been thus far detected are in this range.  Since $\lambda = c/f$, this means that the wavelengths of these waves was in the range of 100-1000 km (give or take.)
Future detectors have been proposed to detect gravitational waves at even lower frequencies (longer wavelengths).  It is expected that many other sources in the universe will generate gravitational waves with longer wavelength (lower frequency) than the ones that have been thus far detected:

(Image from NASA Goddard Flight Center, via Wikipedia)
