[Boot Hill]

Buckhead asks:   "Are there any forces in interstellar space that would cause a decrease in the momentum and hence frequency of photons...?"

Yes! Here is a list...

1. Since I first posted my speculation to you about the possibilities of so-called twisted light (light with "orbital angular momentum"), I have found that it has already been demonstrated that twisted light does indeed have a "red shift" which is a consequence of a loss in the linear momentum of the photon caused by a transfer of linear momentum to (orbiting) angular momentum.

   CITATION:
   "MECHANICAL EFFECTS OF SPIN AND ORBITAL ANGULAR MOMENTUM: SPINNING AND ORBITING and ROTATIONAL DOPPLER EFFECT"

2. Light colliding with interstellar gas, resulting in a loss of momentum and thereby a loss in frequency has been shown to cause a red shift.

3. As "PatrickHenry" correctly pointed out in #53, there is also a gravitational caused red shift. Again, as with all the others causes noted, the red shift is due to a loss of momentum.

4. And of course there's our old perennial favorite, the Doppler or velocity induced red shift.

One final word:   When I speak of a "loss" of momentum, I don't want to mislead you. Momentum must be conserved. So to be precise, it is not a actually a "loss", but rather a transfer of momentum from the linear motion of the photon to other things. Sort of like taxes!

--Boot Hill

[PatrickHenry]

I donno. The mass of the photon (whatever that is) won't change, nor it's speed (in vacuuo). So I don't see any changes in momentum. Only changes in wavelength seem likely (and spin and twist -- whatever that amounts to). Anyway, thanks for a truly momentous post.