4 ms·

I thought photons also contributed to the curvature of spacetime because they have momentum (and thus energy). I would expect it to be a pretty miniscule contri

I thought photons also contributed to the curvature of spacetime because they have momentum (and thus energy). I would expect it to be a pretty miniscule contribution, though.

jacquesm 3y ago
Hm... just an interested layperson here but would that not require some kind of mechanism by which the photon sheds some of that momentum? Which would seem to be pretty hard if it really is an 'elementary' particle. Unless 'photon === graviton' and you find photons shedding other photons!
- someplaceguy 3y ago
  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.

I thought photons also contributed to the curvature of spacetime because they have momentum (and thus energy). I would expect it to be a pretty miniscule contribution, though.

jacquesm 3y ago

Hm... just an interested layperson here but would that not require some kind of mechanism by which the photon sheds some of that momentum? Which would seem to be pretty hard if it really is an 'elementary' particle. Unless 'photon === graviton' and you find photons shedding other photons!

someplaceguy 3y ago
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.