[tofof]

Not quite. Yes, they show that superluminal (faster than light) warp drives still require negative energy, though dramatically less than previously calculated, which is what your quote refers to.

However, TFA featured on HN focuses on a different finding in the same paper - that positive-energy warp drive solutions exist, that still warp spacetime, but are necessarily sub-lightspeed. This is still useful, because it's still a potential avenue on how to actually approach accelerating something to some large fraction of c. These also solve a lot of the immediate objections to prior art, such as being delicately between event horizons of a black/white (ish) hole pair, the accompanying hawking radiation, and causality paradoxes.

[d1sxeyes]

I am not sure that I understand this. As far as my understanding goes, all massive objects warp spacetime.

What's the theoretical difference between a modern rocket engine and one of these warp drives, or is it just one of degree?

[ars]

Are they talking about a reactionless drive? Or one of those drives where you move in the universe without actually changing velocity?

[JumpCrisscross]

> Are they talking about a reactionless drive?

No.

The “warp shell allows one to modify the state of spacetime inside it…the time in the inner region may go faster or slower than it would go without the shell” [1]. Being “internally moving shells of normal or exotic matter,” these warp shells “do not have any natural way of changing their velocities. They are just like any other types of inertially moving objects. Similarly, just like for any other massive object, achieving a certain velocity for a warp drive requires an externally applied force or, more practically, some form of propulsion.”

Subliminal warp drives let a timelike passenger compress the subjective experience of the journey. It doesn’t change the journey’s propulsion requirements. Think: cryosleep benefits from science fiction, not Star Trek-esque warp.

[1] https://arxiv.org/pdf/2102.06824 §§ 5.1, 5.2

[api]

It would seem reactionless but physics wise you are actually warping space-time, so you are kinda just out of Newton’s domain entirely.

[api]

Would a sub light warp drive be much different performance wise from, say, a fusion torch engine with insane iSP like the fictional Epstein drive from The Expanse?

Of course if we figured out how to build a sub light warp before figuring out how to build a fusion torch wouldn’t that be interesting?

So many more questions. Would you experience time dilation? When you kicked off the drive what would your momentum be relative to your destination? Would speeds like 99.99999% the speed of light be possible with enough energy? Of course you would probably be energy limited just like with crazy high iSP drives.

It’s possible that sub light warp drives are possible but FTL still just isn’t, or would require ludicrous “convert the mass of Jupiter into raw energy” levels of power.

Also if anything like this is possible it makes the Fermi paradox even more “real.” Right now the crazy difficulty of interstellar flight with reaction drives provides a solid chunk of a default explanation. If you could hit even say 20% light speed (effective velocity) you could hop between stars in practical lengths of time.

If someone builds any kind of working warp drive I’m going with either we are alone in at least our galaxy or UFOs are aliens, not much middle ground.

[JumpCrisscross]

> Would a sub light warp drive be much different performance wise from, say, a fusion torch engine with insane iSP

Yes.

The “warp shell allows one to modify the state of spacetime inside it…the time in the inner region may go faster or slower than it would go without the shell” [1]. It is not a system of propulsion.

That said, the paper does hint at forming “a region within a subliminal warp drive which is similar to ergospheres of spinning black holes” for use “as an efficient energy storage” and “wherein black hole-like regions of the spacetime may be used to produce accretion power” for “both a source of energy and a source of propulsion.”

> Would you experience time dilation?

Theoretically, not sure. Practically, yes: you would appear to an outside observer to be travelling subliminally.

> When you kicked off the drive what would your momentum be relative to your destination?

Whatever it was immediately before.

> Would speeds like 99.99999% the speed of light be possible with enough energy?

Yes. Same as it already is.

[1] https://arxiv.org/pdf/2102.06824 § 5.1