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Asking out of curiousity. All answers appreciated.

You have a machine that generates micro black holes on the outside of it which evaporate quickly and then are immediately generated again upon evaporation.

You make sure a part of the machine is next to where the event horizon of these micro black holes would be. Each black hole evaporates before the machine falls into the event horizon.

From repeatedly being pulled toward each micro black hole, never falling into the event horizon, could this generate movement of the machine?

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    $\begingroup$ This sounds like an energy-demanding method of travel 🙂 $\endgroup$ – Steeven Oct 16 '19 at 22:13
  • $\begingroup$ Each black hole evaporates before the machine falls into the event horizon. It’s hard to fall into something that is smaller than a proton. $\endgroup$ – G. Smith Oct 16 '19 at 22:48
  • $\begingroup$ Gravity pulls the machine toward the hole. The pressure of the Hawking radiation pushes the machine away from the hole. $\endgroup$ – G. Smith Oct 16 '19 at 23:08
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    $\begingroup$ and then are immediately generated again upon evaporation Huh? What does this mean? Each black hole evaporates before the machine falls into the event horizon. No idea what you're talking about here. $\endgroup$ – user4552 Oct 17 '19 at 2:10
  • $\begingroup$ sounds like a scenario for scifi.stackexchange.com $\endgroup$ – anna v Oct 17 '19 at 5:27
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I think that what is implicitly assumed here is that the machine somehow magically projects a black hole a certain distance away and that it will now start exerting gravity on the machine. But that assumes the mass comes from nothing.

Mass-energy and momentum are locally conserved in general relativity. So your machine needs to contain the mass-energy to make the black holes before making them. So the process of making the holes is just equivalent to putting some of the initial mass on one side. That will not give you any momentum, unless you also give the mass momentum (and then you have a normal rocket).

In fact, the evaporation of the black holes would give you some momentum because presumably part of the radiation emitted would bounce off your machine and push it with radiation pressure. However, this is a fairly inefficient way of moving.

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