# E = mc^2 and gravitational potential

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1. May 5, 2015

### CaptainSlog

If two masses that are separated by a distance are created from pure energy using the equation E=mc^2, where did the gravitational potential energy between them come from? Does this mean the speed of light isn't really constant, and must be changed very slightly to accomodate it? Could it be that gravity itself is necessary to balance energy?

2. May 5, 2015

### Orodruin

Staff Emeritus
You are mixing two concepts that do not really go together, namely gravitational potential, which is a concept from classical mechanics, and mass energy equivalence, which is a concept from relativity.

In order to make sense of this, you need to apply general relativity. In GR, mass is not the source of gravitation. Instead, the energy-momentum tensor is and any type of energy is therefore a gravitational source. It is not that you can simply create a new gravitational source out of nothing.

3. May 5, 2015

### CaptainSlog

OK, thanks. I had always assumed gravity was due to mass, but if mass and energy are equivalent then that makes sense.