Reply to post: Re: So here's a thought:

Years after we detected two neutron stars crashing into each other, we're still picking up X-rays. We don't know why

Anonymous Coward
Boffin

Re: So here's a thought:

You understand entanglement, or at least you're correct that gravity (as we understand it) is a classical – non-quantum – thing. Obviously a correct theory of gravity will be quantum, but we don't have one: we only have the classical limit theory, which is GR.

The thing you describe in your last paragraph is the Alcubierre drive. The big significant thing about it is that it requires negative energy density, which really means negative mass. I think people are pretty confident that negative mass isn't possible, and really the Alcubierre drive is something which makes us more confident of this. You can use an Alcubierre drive to create what are called 'closed timelike curves', which is the GR terminology for being able to get into your own past, aka building a time machine aka violating causality.

So this is getting closer to being a nice thing: in particular if you could show that in order to violate causality you required negative mass, then you've reduced the problem of showing that causality is not violated to that of showing that negative mass can't happen.

A lot of this stuff is like that: there are various absurd things (negative energy, causality violation, singularities) which everyone kind of knows are implausible but we can't yet say why. So if we can come up with results that show that thing a either implies thing b, or is equivalent to thing b, then we know that if we can show that thing a can't happen we've solved the thing b problem too (or alternatively if we can show that thing a can happen we know we're in bad trouble, which would be even more exciting – in this case if it turns out that negative energy is possible we know that causality can be violated and time machines are possible).

Of course people then glom onto the Alcubierre drive because it says 'drive' (and sometimes even 'warp') and start saying that this means that we've discovered FTL travel, because they desperately need FTL travel to be true so all their SF can be true, or at least less silly. I don't really understand this: I read SF, including SF with FTL travel, but I understand that the 'F' stands for 'fiction'.

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