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Old Jul 23rd 2017, 12:32 PM   #1
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Two photons and gravity

If there were a universe containing only 2 photons would they attract each other (forgetting for the moment if such a scenario is even possible). That is would they exert a gravitational pull on each other. Or do they simply follow the pre-existing gravitational fields in the space-time continuum (which wouldn't exist in a 2 photon universe) without exerting any of their own?

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Old Jul 23rd 2017, 02:24 PM   #2
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Originally Posted by kiwiheretic View Post
If there were a universe containing only 2 photons would they attract each other (forgetting for the moment if such a scenario is even possible). That is would they exert a gravitational pull on each other. Or do they simply follow the pre-existing gravitational fields in the space-time continuum (which wouldn't exist in a 2 photon universe) without exerting any of their own?
I'll say, yes.

I phrase it in that way because the general theory of relativity, aka gravitation, is a classical theory whereas photons are quantum mechanical entities. So the question mixes two different concepts. Its only when a quantum theory of gravity is created and widely accepted can we depend on the answers given.

Until then I'll say that photons have momentum therefore they have inertial mass. As a result they also have passive and active gravitational mass. As such they can generate a gravitational field and be affected by a gravitational field.

Tolman et al studied the gravitational field generated by various forms of radiation distributions such as a pulse of radiation. If you think of a classical photon as such a pulse then I can definitely say that they create a gravitational field. Since they'd also move on null geodesics they'd also be affected by them. However the results of Tolman are about a particle in a field which implies that the particle is small enough in energy that it won't effect the field in which its moving.

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Old Jul 23rd 2017, 04:58 PM   #3
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what about a high energy photon? Has anyone measured its gravitational potential in the lab ?
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Old Jul 24th 2017, 09:07 AM   #4
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Originally Posted by kiwiheretic View Post
what about a high energy photon? Has anyone measured its gravitational potential in the lab ?
No. It's not possible to measure, at least not with current technology. Its far too week to detect.

I created a webpage to show how a directed beam of light can generate a gravitational field. The original derivation is cited on that page.

The Gravitational Field of a Directed Beam of Light
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Old Jul 24th 2017, 12:18 PM   #5
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I'll have to brush up on my tensor calculus
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