Photons have no Mass

[Ring]

@Frylock

Does light exert gravitational force?

My guess would be no. But it would just be a guess.

Another way to ask the question is: is it the rest mass of a body that that determines gravity, or is it just its energy?

-FrL-

Yes light can exert a gravitational force but the light system must have a zero momentum frame.

In other words (and simplified) a photon or photons all moving in the same direction have no mass, but a system of two photons moving in opposite directions does have mass.

[Ring]

@Ring

Yes light can exert a gravitational force but the light system must have a zero momentum frame.

In other words (and simplified) a photon or photons all moving in the same direction have no mass, but a system of two photons moving in opposite directions does have mass.

Here’s an interesting example:

If a sufficiently energetic sphere of photons were heading towards and centered on the Earth we could be inside a black hole right now and not even know it.

ETA assuming the sphere of light was inside it’s Swartzchild’s radius