Gravity and the first law of thermodynamics.

[Fearghus]

(Not sure if anyone has asked this before)
This idea might be a little vague and I'm sure there is a simple solution but its been annoying me since I thought of it and i'd like to know the answer.

If you convert energy into matter then surely matter (with mass) exerts a gravitational pull on other matter (with mass) around it. So if energy does not exert a gravitational pull - and I assume it doesn't - then surely by changing energy to matter you create gravitational potential energy. But you can't create energy from nothing says the first law of thermodynamics, so how does this work?

Help appreciated.

 

[LumenPlacidum]

Note that you said that you were converting energy into mass. Energy is being used in some way. Then, at the end, you say that energy is now present because of the presence of the mass. I'm not entirely sure how the numbers work out, but I would assume that a chunk of the energy initially put into the mass is going to the gravitational potential energy. So, it's not that the energy is coming from nowhere, but rather it's coming from the energy you initially are using to "create" matter.

 

[A.T.]

So if energy does not exert a gravitational pull - and I assume it doesn't

Well, it does.
http://en.wikipedia.org/wiki/Stress-energy_tensor