[WalterBright]

One issue about warp drives that I never see mentioned is one has to be careful one is not creating a perpetual motion device.

Consider a warp drive that transports you from, say, earth's orbit to pluto's orbit. Now, you fall back towards earth's orbit. This means that the warp drive, in order to not violate conservation of energy, must require at least as much energy as the potential energy difference between the two positions.

[yk]

Conservation of energy may or may not be valid in general relativity, at least in a naive understanding of conservation of energy. The technical explanation for this is, that conserved quantities are related to geometrical symmetries via Noethers theorem [1], and on a curved background these symmetries may or may not be there. ( A rather handwaving explanation would be, that there is no straightforward way to define the energy density of a gravitational field.)

In fact it is possible to build a perpetuum mobile by just filling some region of space with dark energy. This region of space will then expand at constant energy density, therefore creating energy.

[1] https://en.wikipedia.org/wiki/Noether%27s_theorem

[maxerickson]

"In fact it is possible" is a pretty strong way to talk about moving something that might not exist.

[yk]

Agreed, the wording is rather strong. But at least it is shorter than: "According to current understanding of GR and the contents of the universe it should be possible."

[maxerickson]

It seems if you had the power to violate causality and worked hard enough at it, that future event would have propagated into the present day past by now.

Maybe that's really what killed the dinosaurs.

[egypturnash]

I think you're describing a feature, not a bug. Especially if you can figure out a way to derive energy from the fall.