Expanding spacetime destroys energy, which violates conservation. Exploitable?

[Twark_Main]

Time-reversal suggests that if we can find (or far more optimistically & long-term, create) a patch of contracting spacetime, we can create kinetic energy.

While in the universe as a whole spacetime is expanding, within galaxies there are local non-uniformities and stellar phenomena that might be exploited in this way.

This is less a warp drive (no FTL), but it could help inform you where to send your Von Neumann probes, so rather than just random walking they can set up shop near a convenient stellar accelerator. Or maybe it just helps give you a tailwind as you "surf" the (smaller) intra-galactic density variations.


(note that I'm not just talking about the usual nova surfing and black hole Oberth cannons, but about the part where you exploit this effect for added kinetic energy)

[CoolScience]

Time-reversal suggests that if we can find (or far more optimistically & long-term, create) a patch of contracting spacetime, we can create kinetic energy.

Not true. Time reversal symmetry does not exist within the laws of physics (entropy is the common one, but I find the weak force assymmetry more fundamental). The video explains in detail how time translation symmetry is broken in GR (basically because dark energy, and the expansion of the universe.) If you can't have symmetry of even a time translation, then thinking about it, this seems another way to disallow time reversal symmetry.

While in the universe as a whole spacetime is expanding, within galaxies there are local non-uniformities and stellar phenomena that might be exploited in this way.

This is less a warp drive (no FTL), but it could help inform you where to send your Von Neumann probes, so rather than just random walking they can set up shop near a convenient stellar accelerator. Or maybe it just helps give you a tailwind as you "surf" the (smaller) intra-galactic density variations.

No, and as the video shows there is a continuity equation that covers this with terms for the gravitational energy, it is in a different way than the global conservation laws we are used to, but it is not magic get out of jail free card for conservation laws.

The rock in the video slows down because the universe is expanding, that expansion does not reverse any more than you can reverse entropy. (This statement is arguable, but to be meaningfully argued, you would need to first come up with an explanation for dark energy.)

(note that I'm not just talking about the usual nova surfing and black hole Oberth cannons, but about the part where you exploit this effect for added kinetic energy)

Those are all you can get, the rest of what you want is based on false assumptions and misinterpretations. There are some interesting tidbits in the details, like how you can swim through space in a high enough gradient gravitational field. However, these basically amount to tiny corrections on the Oberth effect. There is literally nothing in the video that suggests this being useful in the way you are claiming.

[sanman]

Hi, I hate to bust in on the high-fallutin' discussion going on, but it seems quite clear that the universe is not homogenous, and is lumpier in some places than in others (ie. the galaxies example just given)

In which case, if the idea of creating more space is feasible (and I don't know enough to say it is) then would it conceivably be easier to do so out in the farther reaches of space, farther away from any matter lumps? Is it possible that the farther out you go, the easier it might get (even if only minutely)?

How could this be tested? Using good old interferometry, perhaps? Maybe even the atomic kind?

[laszlo]

Sounds like Hoyle's steady-state hypothesis.

[Robotbeat]

This is all just a reminder that we don’t actually fully understand the universe yet. Dark Energy or whatever you call the thing driving expansion, is basically just a fitting parameter. I don’t think it’s actually known yet whether this applies within galaxies or not.

The error bars on astrophysics stuff are actually enormous. There’s lots of room there for truly New Physics, so hope that energy might not be conserved (and therefore that we can get around some of these perceived hard limits from thermodynamics somehow) isn’t completely lost. But I consider it to be a very long term thing. Which is fine, as the constraints on humanity at the Heat Death level are themselves extremely long term.

[CoolScience]

Time-reversal suggests that if we can find (or far more optimistically & long-term, create) a patch of contracting spacetime, we can create kinetic energy.

Not true. Time reversal symmetry does not exist within the laws of physics... The video explains in detail how time translation symmetry is broken in GR (basically because dark energy, and the expansion of the universe.)

The phenomenon in the video does not rely on "the expansion of the universe," it only relies on the expansion of the space between the two objects.

Obviously this is not the case within galaxies, which are dominated by local gravitational effects and not effected by the expansion of the universe.

The video is in fact talking about the expansion of the universe. The expansion of space between 2 objects is the expansion of the universe. Two objects moving apart because of different velocities is just 2 objects moving apart and not the fundamental thing being discussed. The video is talking about the expansion of the universe, either you are contradicting yourself in tour statement here, or your are not understanding what the video says.

Please also consider the rest of what I wrote and the issues with your idea of time reversal.

Again your understanding is wrong. See above.

(this is the "galaxies eventually get ripped apart by expansion" misconception)

I expressed and have no such misconception. I am really tired of in every thread, having to say "No, I am not saying that thing you made up."

Let me be very clear here, I never said anything like the misconception you just assigned to me. If you think otherwise, you can PM me where you think my statement was insufficiently precise, all that is relevant in public is that you acknowledge I didn't say that and we can move on.

You conveniently deleted the part from your reply where I clarified that our best data and models say that dark energy is uniform everywhere, and thus expansion is happening everywhere, but that there is uncertainty (see Hubble Tension). So you can "easily" argue against my point by suggesting an alternate model of the universe's expansion. (Lots of scientists are trying to do exactly that.) Assigning misconceptions to me that I don't hold is not a counterargument.

Obviously other effects dominate within a galaxy, but that is just the thing, those are other effects.

Those are all you can get, the rest of what you want is based on false assumptions and misinterpretations.

Your invariant pessimism is very reliable. Unfortunately, this means "but CoolScience says it fails" doesn't hold any real evidentiary weight. 

Telling you that what you are claiming is not supported by the video you linked is no pessimism, it is a fact. What you have done here is an ad hominem, please just apologize so we can move on and not have to bring in the mods.

There are some interesting tidbits in the details, like how you can swim through space in a high enough gradient gravitational field. However, these basically amount to tiny corrections on the Oberth effect. There is literally nothing in the video that suggests this being useful in the way you are claiming.

Yep, this was my general expectation. Probably so small that it's in the margins, for expected intra-galactic gradients.

This suggests that to get any real "juice" we'd necessarily be looking to energetic stellar phenomenon, not just run-of-the-mill galactic density variations. 

If you are in fact, agreeing with my final conclusion, why did you bother attacking me, and misrepresenting what I had to say? You are also rather contradicting your previous statement of:

(note that I'm not just talking about the usual nova surfing and black hole Oberth cannons, but about the part where you exploit this effect for added kinetic energy)

Just because the Oberth effect may be better/more interesting near a black hole doesn't make your claims in the first post helpful. Please clarify which of your 2 statements you mean, the original claim that this isn't about some miniscule effect only relevant near massive gravity sources, or your new claim that it was your " general expectation. Probably so small that it's in the margins,"

For more information on how energy is conserved (or not, depending on how you handle gravitational energy as the video discussed) see:
https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html
A relevant statement:
"The Schwarzschild metric is both static and asymptotically flat, and energy conservation holds without major pitfalls."
The fact that the universe is expanding and not "static" is the unambiguous thing that breaks this for our universe, making the other complications of defining energy in the video/this link necessary to consider.

[CoolScience]

The video is in fact talking about the expansion of the universe. The expansion of space between 2 objects is the expansion of the universe.

Of course I know that.   The point is it doesn't have to be the expansion of the universe. Anything that causes that space to expand will suffice.

I'll just assume the use of the word "not" in your earlier post was some sort of typo, since it implies the exact opposite of what you just said now.

There is exactly 1 thing causing space to expand, dark energy, as I have said you are free and more than welcome to propose new alternatives, but I don't really see you doing that, and the video certainly doesn't.

Your "issue" with time reversal doesn't apply. Nothing about the phenomenon in the video relies on entropy, just GR which is fully time-reversible. Yes I know about the expansion of the universe (and how you think it ruins time-reversal here), but remember we can use any source of expansion/contraction.

My issue does not rely on entropy, and entropy applies separate from which fundamental forces you are dealing with anyway. An argument cannot be based on "time reversal" and also expansion that invalidates time reversal.

Obviously other effects dominate within a galaxy, but that is just the thing, those are other effects.

Glad we agree!

It is exactly those "other effects" I am referring to.

Those other effects are not the expansion of space the video is referring to, and are standard things like Oberth effect (with minor corrections.) There still is a type of energy conservation that exists in GR as the video says

Just clarifying that I'm not talking about light/particle sails or the Oberth effect.

You don't seem to be talking about anything, you want not the general expansion from dark energy, but you propose nothing else, and the things that are left are things like the Oberth effect.

I don't think we'd really mind if we're "stealing" energy or creating it, as long as it gains our ship energy.

It is called dark energy for a reason, space itself has energy in quantum, and that is our best guess at what dark energy is. So the expanding space actually creates (unusable by definiton) energy even if it robs objects of kinetic energy. Either way there is no free lunch happening,

So we're looking for phenomenon involving contracting space-time. Obvious candidates are things like forming black holes, but the contracting space (of course) starts out inside the collapsing star, so I don't know how useful that can actually be for a practical system. Seems like these would be difficult environments to design a spaceship for.

There are none. The video does not support this being a thing. You want to disagree, it is simple provide a direct example, and not something handwaved, but something where you precisely handle the definition of contracting. A black hole isn't it,  A collapsing black hole stretches to an infinitely deep gravity well. And releases a lot of what in standard gravity we refer to as gravitational potential energy. Same as a hydroelectric power plant.  It is not the effect talked about in the video, and as that quote I provided shows, energy is fundamentally conserved in a Schwarzschild metric.

Just because GR does not allow in all cases a clean formula of energy conservation, does not mean that you can just point to anything you want and say it is extra energy. True space compression is not a phenomena that exists. The stuff inside galaxies is just gravitational forces and energy transfers. (By the way to save you some time, I have checked, gravitational waves can be shown to be equally efficient to photons for propulsion in energy/momentum ratio, just impossible to generate in practice)

And just to repeat, my main point is that you need to give something specific of actual supposed "space contraction", or this thread has no point.

[CoolScience]

Nope. I was pointing out that the are non-identical statements.

"The space between you and I expanded."

"The universe expanded."

The first thing can be caused by the second thing, but the first and second thing are not identical claims. And we only need the first one. We don't actually need the second one. It is sufficient but not necessary.

This is simply untrue. Thank you for clarifying that you were think of a set/subset type relationship, but that is simply not the relationship. I already said this, but to be even more explicit:

1. The fabric of spacetime itself expanded between 2 objects

2. 2 objects have different relative velocities, and are moving apart from each other.

The first is perfectly identical with the expansion of the universe, the second is irrelevant to what is discussed in the video, and in this context, describing it as the space between the objects expanded is not accurate. A rocket launching to orbit, or a bowling ball falling off a tower change distances, but that works normally even in GR for energy conservation. Space itself expanding is a separate thing.

Wishful thinking can't create a 3rd option. It seems you keep trying to conflate these things, but they are simply and fundamentally different. And yet again, you are free to define a 3rd option, because for a change from other threads in this section, but you have to define it before making assertions about it makes sense.

For someone who really, really hates it when you think people are putting words in your mouth, you have a nasty habit of casually doing the same thing you accuse others of doing.  I guess that's why you're so sensitive about it, don't want any of your own medicine huh? 

More ad hominem. Stop it. You can PM me where you think I did this, since you didn't specify here, all that really is needed in public is an apology for making this personal attack. And if you can point me (in PM) to where you think I put words in your mouth and it is a remotely reasonable interpretation, I will apologize for the miscommunication.

To clarify what I did say, I corrected words you seem to be misusing. It is not putting words in your mouth, it is explaining how the sentences you wrote mean under more standard terms, to allow for clarification. I gave options based on possible interpretations.

"Dark energy" is (as we both know) a euphemism for "I don't know what it is, and it might not even exist."

Since (as we also know) the spacetime inside galaxies does not expand with the expansion of the universe. clearly there's more going on than "Dark Energy or Bust."

Your statement that spacetime does not expand within galaxies is simply false. To the best of our knowledge the cosmological constant is constant and applies everywhere. Whether we have good data inside a galaxy to measure it I think is no, which is one of the reasons I keep saying you are free to present your model where contracting spacetime exists.
You can go look up the equation on the wiki page for it, it is a simple constant factor.
https://en.wikipedia.org/wiki/Cosmological_constant

Dark energy is the best we have, a better model would be welcome, but you would have to actually describe that model before you can start making assertions about energy conservation in it.

Again I'm not relying on "the expansion of the universe," as I have repeatedly explained (including again, above).

The video you linked relies on it, either you are talking about something entirely different, or the basic premise you are starting from relies on it. You can pick one, I am not forcing your statement, but you need to clarify which is true.
I have repeatedly stated that the video is based on this, yet I don't think you have even acknowledged that fact.

and are standard things like Oberth effect (with minor corrections.)

Minor corrections?? Wait, what happened to "Only Dark Energy Shall Pass?"   

Which is it? Is it "impossible unless you have Dark Energy," or is it "really tiny effect and don't worry about how it works or how you might arrange things to maximally exploit the effect?" 

None of this is even close to what I said. The only model we have for dark energy is uniform expansion of space based on a constant factor, and as the video says, this effectively slowly erases kinetic energy. This effect is tiny on reasonable length scales. GR has other effects in it that are not this, like time dilation and relativistic mass/energy relationships. None of those other effects are what the video talks about or directly cause problems with writing conservation laws. Those other effects are also tiny in any reasonable gravitational field.

See how you put things in quotes that I didn't say? See how I didn't do that, and instead paraphrased not in quote to clarify terms? There is a distinct difference there. Please go back and read this and other posts (Including the repetitions, because clearly me writing the same thing 3 different times with different words is necessary, because even after that you concluded I said something entirely different.) If you can't start your next post by acknowledging that my argument is different than what you somehow read, then there is no point in you posting, because arguing points I didn't make is a waste of time. If you somehow still read my posts as those things you incorrectly claimed were my points, you can ask questions, but don't bother arguing against points I am not making.