DrCaleb DrCaleb:
I think it will come down to if that matter to energy conversion is an ongoing process, or a one time. If creating 'antigravity' is a single event, or if it needs to keep happening. After all , gravity isn't always being created, is it? The Earth doesn't lose mass in it's distorting of space time due to create its force of gravity?
I don't know, but I hope to be alive when we find out.

Maybe we are using it up, but the distortion in space is also distorting time, so we will never actually find out? Lol.
Jokes aside, the Earth does lose mass every year (I believe approx 50,000 tons), and our core is indeed loosing energy. Both of which are happening at rates that will take trillions of years to exhaust...unless? I would hate to find out, after the fact, that we have accelerated that by using gravity as an energy source.
I guess I would like them to figure out where it comes from, and how it is created before we figure out how to tap it. History tells me that we will tap it first.