Re: Proper Frequency


Mike Helland wrote:
> I know there is such a thing as proper length and proper time for an
> object like a rod, but for an electromagnetic wave is there such a
> thing as proper frequency and proper wavelength?
>
> Obviouslly, a red-shifted light wave is going to have a frequency that
> appears lower because the growing amount of space in between, but is
> there a "proper" frequency for the light, that represents its frequency
> indepedent of the doppler shift?

Since the proper length and proper time are defined as the length of an
object and the amount of time that passes in a comoving frame, we can't
really define a proper frequency for an electromagnetic wave. If we
talk about an observer comoving with the wavefront, his proper time is
zero and we can't define a frequency (not to mention the fact that
frequency doesn't really mean much when you're travelling with a wave
front, and the wave would carry zero energy from a comoving frame).

Another way of looking at it is by defining "poper energy" in the same
way as for any object, that is to say, the length of the energy
momentum four vector - which is the mass energy. Obviously, EM waves
(or photons if you prefer) have no mass, so E = pc, and the length of
the four vector is zero, so in a comoving frame (if such a thing could
meaningfully exist) the light would have no energy or frequency.

.

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