As long as the particles are heavy enough and moving fast enough, that kind of setup sounds like you should be able to build a time machine starting from a universe where everything was stationary. But what we found is when you added up the deficit angles of all the particles, you always got a number greater than two pi,. i.e. 360 degrees. So in an open universe, you will never have enough energy to make a time machine. It's just general relativity forced you into that conclusion - which I thought was just enormously neat.

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