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shminux comments on Open Thread, May 11 - May 17, 2015 - Less Wrong
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To any physicists out there:
This idea came to me while I was replaying the game Portal. Basically, suppose humanity one day developed the ability to create wormholes. Would one be able to generate an infinite amount of energy by placing one end of a wormhole directly below the other before dropping an object into the lower portal (thus periodically resetting said object's gravitational potential energy while leaving its kinetic energy unaffected)? This seems like a blatant violation of the first law of thermodynamics, so I'm guessing it would fail due to some reason or other (my guess goes to weird behavior of the gravitational field near the wormhole, which interferes with the larger field of the Earth), but since I'm nowhere close to being a physicist, I thought I'd ask about it on LessWrong.
So? Any ideas as to what goes wrong in the above example?
I have a PhD in Physics and my thesis was, in part, related to wormholes, so here it goes. (Squark covered most of your question already, though.)
If something falls into a black hole, it increases the black hole mass. If something escapes a black hole (such as Hawking radiation), it decreases the black hole mass. Same with white holes. A wormhole is basically two black/white holes connected by a throat. One pass through the portal would increase the mass of the entrance and decrease the mass of the exit by the mass of the passing object.
A portal with two ends having opposite masses would behave rather strangely: they sort of repel (the equivalent of Newton's law of gravity), but the gravitational force acting on the negative-mass end propels it toward the positive-mass end. As a result, the portal as a whole will tend to accelerate toward the positive end (entrance) and fly away, albeit rather slowly.
In addition, due to momentum and angular momentum conservation, the portal will start spinning to counteract the motion of the passing object.