Whence the redshift?

Apr 06, 2011 Posted Under: physics

A few weeks back my son asked me to explain the redshift, and I discovered I could not!

Having started with the usual comparison to the Doppler effect in the sound waves of a train or siren, it suddenly occurred to me there was no correspondent to the collection of air molecules the sound wave compresses. Raw space doesn’t correspond, having no mass.

So if there is nothing for light to “wave”, what does its frequency represent? Is the photon a whirling energy dervish, its energy measured by how fast it spins? I struggle with images like these on the advice of Einstein, who said you need a good image to get to a reasonable understanding of things. And while the quantum wave mathematics is useful for computing many things, it doesn’t yet help me with an image.

After a couple of weeks thinking about it, I got sort of ok with the notion of gravitational redshift, the photon losing energy climbing out of a gravitational “well”.

But I’m still not getting the redshift arising from the relative speed of the photon originator separating from tts detector. It seems to me there was only needed one acceleration of the transmitter (and receiver — equal and opposite impulses) to achieve the speed, which would then last a very long time in deep space, there being very little to friction against. Is the redshift only related to the acceleration needed to achieve that speed of separation?

Other questions arise:

  • Is the photon’s frequency changed by its traversal of the various dark matter gravitational structures encountered in its maeanderings of the space between us and it?
  • How exactly does the speed-of-light relationship between space and time constrain the “travel” of the photon? Does it have to touch down on each planck length on its journey?
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