No Gravity From Photons

Rev. Green | 23:12 Tue 08th Nov 2022 | Science

47 Answers

A photon has energy. Why does it not generate a gravitational field? Energy = mass.

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jim360

Best Answer

"Energy = mass" is sadly misleading. Photons have energy, but an individual photon has no mass. This isn't a contradiction. A more complete statement is something closer to "energy = mass + momentum" (although even this isn't strictly correct, but I don't see a need to be more precise in this conceptual discussion). Gravity is sourced really from energy,...

09:39 Wed 09th Nov 2022

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jim360

Really, the whole points of my posts is to explain that thinking of momentum as mass times velocity is unhelpful -- and, indeed, wrong -- for photons, so I'd encourage you not to do that.

ZebuSanctuary

^^^^^ I second that! ^^^^

This is what was holding back OG. Eventually he saw the light (pardon the pun).

ZebuSanctuary

// At the speed of light the momentum of a massless photon becomes zero //

This seems to suggest the photon moves at different speeds?

ScienceNoob

I just want to say that this is fascinating stuff! How do you guys know so much about this?

Rev. Green

Question Author

Are there any massive particles for which momentum is not mass times velocity?

Zacs-Master

It is in Newtonian mechanics. Changing the definition of momentum isn't possible as force, mass, acceleration, and velocity are all part of the same equations. Change one, and you change them all.

Zacs-Master

Sorry, that should read, it is m x v in Newtonian mechanics.

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