3 ms·

I'm a layman as well, but I think photons can effectively lose energy in various ways, one of them being the well-known redshift effect. And one of the main cau

I'm a layman as well, but I think photons can effectively lose energy in various ways, one of them being the well-known redshift effect. And one of the main causes of redshifting is photons climbing out of a gravitational well (this is called a gravitational redshift). However, whether that translates to a loss in momentum is a bit more fuzzy and I really can't tell whether it does or doesn't. Although I'm far from being a physicist, so hopefully someone more knowledgeable chimes in to enlighten us... But I'm curious: what is your reasoning for asking whether photons have a mechanism to lose momentum as a consequence of them affecting the curvature of spacetime? It's not at all obvious to me the relationship between these two concepts.

jacquesm 3y ago
If they don't lose momentum then there is no interaction (in order for any kind of interaction you need to lose some energy). Momentum is pretty much all a photon has and it could conceivably toss off much lower energy photons to shed that momentum.
- nyssos 3y ago
  > in order for any kind of interaction you need to lose some energy This is not true, elastic scattering is very common.
  - jacquesm 3y ago
    Elastic scattering between photons and gravitational fields?
    - nyssos 3y ago
      You don't generally talk about "scattering" off a field, but photons and gravitons can scatter off each other, sure.
nyssos 3y ago
> However, whether that translates to a loss in momentum is a bit more fuzzy and I really can't tell whether it does or doesn't. Yes, redshifted photons lose momentum. Momentum (really the stress-energy tensor) is conserved locally, and along trajectories that preserve the metric, but global momentum conservation in GR isn't even well-defined.