Alcubierre Drive and Time Travel

[Lorizael]

Negative energy in physics is almost always an accounting thing and not necessarily a physically real thing we should believe is out there.

[Dauphin]

The Auditors of Reality are villains for a reason.

[BeBMan]

In Expanse they had the Epstein drive

[Geronimo]

Negative energy in physics is almost always an accounting thing and not necessarily a physically real thing we should believe is out there.

when you say "almost always" do you think of it as "always. for all practical purposes." or do you recognize some important exceptions that stop you from claiming "always"?

[Lorizael]

I'm hedging my bets in case there's something I'm failing to consider. But for the physics I'm familiar with, you're almost always (implicitly) dealing with differences in energy rather than absolute amounts of energy. Which means if you've called some energy level 0, you'll end up with both positive and negative energy, especially when conservation of energy is relevant. But what you call 0 ends up being arbitrary because there's usually other bits of energy that are either not calculable or not relevant, but still definitely there.

[Dauphin]

Given energy is conventionally seen as a conserved property due to time invariance, if negative energy were a thing, it would presumably have some interesting impact on understanding of time.

[Geronimo]

Given energy is conventionally seen as a conserved property due to time invariance, if negative energy were a thing, it would presumably have some interesting impact on understanding of time.

are you thinking the way hawking radiation evaporates black holes or the casmir effect or some other specific situation here? If there's CTCs associated with these or really any other paradoxical real phenomena that would certainly be cool to know more about

[Dauphin]

My recollection is that the Casimir effect is an outside pressure pushing on a lack or inside pressure. Not seeing how negative energy comes into it regardless of it being described that way. It’s a lower than usual vacuum energy and associated pressure(?)

For Hawking radiation, it is certainly interesting to me that by adding a (virtual) particle to a black hole you can reduce its mass.

What I was actually thinking was a scenario where you can test the non-conservation of energy. For example, If space is being created and it contains a vacuum energy, what is the conserved quantity? Is it balanced in some way through a new symmetry and negative energy? Is there a resulting time arrow if you break the time invariance? An unseen second time dimension to account for this quantity? Etc. No idea what it may lead to but would be intriguing.

[Lorizael]

My recollection is that the Casimir effect is an outside pressure pushing on a lack or inside pressure. Not seeing how negative energy comes into it regardless of it being described that way. It’s a lower than usual vacuum energy and associated pressure(?)

Yeah, you're artificially creating a space with a lower vacuum energy density than the zero-point energy density outside the space, but I still that that zero-point is more or less arbitrary.

For Hawking radiation, it is certainly interesting to me that by adding a (virtual) particle to a black hole you can reduce its mass.

The virtual pair thing is an analogy Hawking used in a pop sci book that--my understanding is--a lot of physicists think is not useful and deeply misleading. So I wouldn't take it as an example of negative energy.

What I was actually thinking was a scenario where you can test the non-conservation of energy. For example, If space is being created and it contains a vacuum energy, what is the conserved quantity? Is it balanced in some way through a new symmetry and negative energy? Is there a resulting time arrow if you break the time invariance? An unseen second time dimension to account for this quantity? Etc. No idea what it may lead to but would be intriguing.

I think probably yeah the truth is just that energy is not conserved globally.

[Geronimo]

can you explain hawking radiation in a less misleading way? At least show how we can know which math to use to describe it and how we empirically know that's the right model for the job? admittedly that's a tall order I'm sure.

[Lorizael]

Probably the easiest thing to do is just point you here: https://www.youtube.com/watch?v=qPKj0YnKANw

[Geronimo]

I hate videos but that was succinct and helpful.

[Jon Miller]

I think probably yeah the truth is just that energy is not conserved globally.

Why do you think this? Most physicists I know will drop everything else first.

JM

[Lorizael]

I mean you're a real physicist and I'm just a guy with an astronomy degree, but my understanding is that there's no good reason to expect energy to be conserved in the absence of time-translation symmetry.

[Jon Miller]

My comment was based on my experience in the field, ie at conferences and in discussions.

I think that physical laws being the same today as in the past (and future) is a key assumption that scientists (and physicists) don't question. But that means that they don't really think about it.

JM
(I left academia 6 years ago, although I have done a bit of work since then. I work on ML in the semiconductor industry and am still involved in physics.)

[Lorizael]

Ah. Energy maybe not being conserved on a cosmological scale is something I've picked up from reading cosmologists/physicists online. At conferences I mostly talk to people who want you to know about the planetary science mission they're a Co-I on.