# Passive and Active Gravitational Mass of Photons

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## Main Question or Discussion Point

As I understand a photon has zero rest mass (as far as we can tell) but it does have a passive gravitational mass in order for it to be able to respond to gravity.

But I've been shown that passive gravitational mass should be equal to active gravitational mass, and if this is true and photons have active gravitational mass then is it the case that a beam of light will have its own gravitational field? So would two beams of parallel light in an empty Universe (i.e. no gravity from other objects) be drawn to each other?

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Orodruin
Staff Emeritus
Homework Helper
Gold Member
You have a too classical view of how gravity works. It isunclear what you think gravitational mass means for objects travelling at or close to the speed of light. You cannot apply Newtonian gravity, you need to apply general relativity.

phyzguy
Yes. An EM wave has a non-zero stress-energy tensor, so it will distort spacetime and have its own gravitational field. Note that the effect would be extraordinarily weak.

Applying GR should give the same conclusion though shouldn't it? If photons have passive mass and therefore have active mass they are able to curve space.

Dale
Mentor
To further what @Orodruin and @phyzguy said, in GR mass is not the source of the gravitational field, the stress energy tensor is. Light has energy and momentum so it gravitates, regardless of the fact that it is massless.

The gravitational field of light is called a pp wave spacetime.

Jonathan Scott
Gold Member
I've heard that parallel light beams do not interact with one another gravitationally, but antiparallel beams would attract each other. I think it's possible to guess that anyway from the fact that the mutual acceleration between massive particles travelling at the same speed at very nearly the speed of light in the same direction is time dilated to near zero.

pervect
Staff Emeritus
As I understand a photon has zero rest mass (as far as we can tell) but it does have a passive gravitational mass in order for it to be able to respond to gravity.

But I've been shown that passive gravitational mass should be equal to active gravitational mass, and if this is true and photons have active gravitational mass then is it the case that a beam of light will have its own gravitational field? So would two beams of parallel light in an empty Universe (i.e. no gravity from other objects) be drawn to each other?
I'd be curious to see where you were shown that passive gravitational mass was equal to active gravitational mass. Do you have a reference? Can it be tracked down to a peer-reviewed paper?

To answer your question, as others have pointed out, parallel beams of light won't attract each other, while anti-parallel beams will attract.

PeterDonis
Mentor
2019 Award
I've heard that parallel light beams do not interact with one another gravitationally, but antiparallel beams would attract each other.
Yes, and this result can be derived from the stress-energy tensor of the light beams. MTW has a brief discussion of this, and (IIRC) refers to a paper by Tolman that was the first publication of the result.