# Photons have no Mass **URL:** \ **Category:** Factual Questions\ **Created:** [May 17, 2008, 3:52am UTC](https://boards.straightdope.com/t/photons-have-no-mass/449635 "2008-05-17T03:52:03Z")\ **Posts on this page:** 2\ **Page:** 2
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 21, 2008, 2:51am UTC](https://boards.straightdope.com/t/photons-have-no-mass/449635/21 "2008-05-21T02:51:13Z")
[QUOTE=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- [/QUOTE] 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. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 21, 2008, 3:01am UTC](https://boards.straightdope.com/t/photons-have-no-mass/449635/22 "2008-05-21T03:01:21Z")
[QUOTE=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. [/QUOTE] 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 [Previous page](https://boards.straightdope.com/t/photons-have-no-mass/449635.md?page=1)