Breaking the warp barrier: solitons in Einstein–Maxwell-plasma theory (2021)
iopscience.iop.org
iopscience.iop.org
The fact that certain galaxies are receding from us faster than the speed of light due to the expansion of the universe (again just changes in the metric) provides a clear example of this kind of 'FTL' travel that is permitted in general relativity.
Every HN thread about FTL travel includes multiple comments warning us not to treat present-day knowledge as final.
Doesn't it take considerable effort/energy to make that happen? And is spacetime elastic, so it would spring back?
Abstract from https://arxiv.org/pdf/2105.03079.pdf from Feb. 2022:
> Three recent articles have claimed that it is possible to, at least in theory, either set up positive energy warp drives satisfying the weak energy condition (WEC), or at the very least, to minimize the WEC violations. These claims are at best incomplete, since the arguments as presented only assert but do not prove the existence of one set of timelike observers, the co-moving Eulerian observers, who see relatively “nice” physics. While these particular observers might arguably see a positive energy density, the WEC requires all timelike observers to see positive energy density. Therefore, one should carefully revisit this issue. A more careful analysis shows that the situation is actually much grimmer than advertised — within the framework adopted by those three papers all physically reasonable warp drives will certainly violate the WEC, and both the strong and dominant energy conditions. Under plausible subsidiary conditions the null energy condition is also violated. While warp drives are certainly interesting examples of speculative physics, the violation of the energy conditions, at least within the framework of standard general relativity, is unavoidable. Even in modified gravity, physically reasonable warp drives will still violate the purely geometrical null convergence condition and the timelike convergence condition which, in turn, will place very strong constraints on any modified-gravity warp drive.
From https://arxiv.org/pdf/2205.15950.pdf from May 2022:
> Specifically, this last year has seen multiple misguided claims [22–24, 28–33] as to the asserted possible avoidance/amelioration of the known energy condition violations in warp drive spacetimes; violations that were first firmly established over 20 years ago.
This paper is in those "multiple misguided claims".
One speculation he threw out was that due to causality issues if you flew FTL you might actually be going to parallel universes without realizing it until you noticed hundreds of years later that your light speed radio emissions never made it home. They wouldn’t arrive because you sent them from another universe. Would also solve the Fermi paradox via a combinatorial explosion of universe X FTL trajectory combinations. Your warp ships never meet anyone else because that would violate causality. One warp ship arrival per universe only.
Edit: thought of an even weirder Fermi paradox answer based on the above. Since intelligence eventually figures out FTL, only one intelligence exists per slice of the multiverse and they will never meet along any FTL hyper-trajectory. Causality protection conjecture gone wild.
Also wouldn’t it suck if your multiverse trip was impossible in reverse? Go to the Centauri system, shoot some cool selfies, and come back to a barren lifeless Venus-like Earth. Every warp drops you in a universe where you are the only sentient life.
If an FTl spaceship went to a different universe then presumably a ship in orbit of Earth aiming at mars would simply vanish and never appear in mars orbit, telescopes on earth wouldn’t see it 8, 16, 30 minutes or even years later
• https://physics.stackexchange.com/questions/574395/why-would... • https://selfawarepatterns.com/2019/02/03/why-faster-than-lig...
There are a lot of YT videos that discusses it, too.
But none of this is likely to convince a skeptic if you don't already have a mathematical understanding of SR. So if you're serious, you should really start by learning that.
FLRW has a preferred velocity at all points, right? Couldn't you use that to construct a global time coordinate and require FTL to experience that time coordinate as increasing? (This would be dependent on the metric; I realize that GR has solutions that have closed timelike curves inherently.)
SR is a subset of general relatively. So SR can be used perfectly well in situations that obey the constraints under which it was designed to be used.
As for your second paragraph, I'm not qualified to answer your question. GR is an extremely rigorous subject that requires a very high level of math to understand.
But something about your tone makes me wonder if you might be engaged in some kind of motivated reasoning... That you desire the viability of FTL travel because it's exciting or romantic?
If you think that describes your feelings on the subject, then the answer is almost certainly going to be "No".
Motivated reasoning, really? Yes, a universe that censors CTCs but not FLT is more interesting than one where all FLT results in CTCs. I can't understand how that makes it less likely; the universe is in general extremely interesting, not dull.
While the larger universe does depart from SR's constraints, it does not depart from the fundamental facts that disprove the possibility of FTL. The math is more complex in GR, but the conclusion is the same: FTL isn't compatible with the reality we observe... You have to invoke some kind of hidden, unobservable, untestable dicta in order to arrive at FTL.
As for excitement... If we decide to take that approach, what's wrong with me believing in Faeries and Santa Claus, because it makes the universe more interesting?
I'm not saying anything is possible or not possible, but maths is a model, no more. Asserting the conclusions of the maths is to demonstrate nothing until you also make a convincing case that the maths is an accurate representation of reality, which you haven't done. I suppose the shorter version is, the map isn't the territory. I guess.
Without the math you can't get a deep understanding or reason about it too much but you can see the issue.
You may think of instantaneous as being 90 degrees away on a spacetime diagram. But someone else's frame of reference will observe your instantaneous at a different angle than 90 degrees on their own spacetime diagram, perhaps they'll even see it as going backwards in time. Their frame of reference is valid too. So if they sent a message back at a 90 degree angle on their own spacetime diagram using the same mechanism they'd be sending a message to us back in time in our frame of reference. It gets really funky when you do this with 3 different observers all observing each other communicating backwards in time to the point where they can cycle a message between the three of them that goes further and further back in time.
You'd have to first throw away frames of reference to make such travel possible but that's the entire basis of relativity and good luck finding a better explanation as to why light always appears to go at the same speed for any observer in the universe.
Causality is not a product of the universe, but the other way around.
Except current cosmology is full of strange things that feel to me like they are only there as placeholders for our gaps in the understanding of the universe. The expansion of space only exists because we need something to explain redshift. Dark matter only exists because we need something to make the galaxy rotation speed match our formulas. I do have a feeling that some of these ideas may turn out wildly wrong in the future and we'll be looking at "the universe is expanding" the same way we look at "combustion releases phlogiston out of whatever you're burning" today.
Honestly skepticism of dark matter is understandable, but what the heck else are you expecting to pop up to explain redshift, a phenomenon that happens always and only when an object is moving away from you? What's the mystery there? You might be confusing universe expansion with dark energy (which IMO might well disappear, being vulnerable to improvements in the cosmic distance ladder), or cosmic inflation (apparently well substantiated but quite ad hoc, but if so you should really learn your terminology before you try to make predictions about future discoveries.
For example, some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform and we're in an area that has properties slightly different from the surrounding space.
What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?
I think people have explored the photons thing with regard to “solving” dark matter, and found the evidence lacking. As for spacetime… trivially yes, but I don’t think this is what you meant? I mean, spacetime being non-uniformly curved just means there’s stuff in it, and that is literally one of the things we can directly observe, and we see that in e.g. the Bullet Cluster the dark matter kept moving when the visible matter collided and slowed down.
> What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?
If, after careful work to exclude all other possibilities, then a party and a Nobel prize.
Tried, found wanting. Repeatedly. https://en.wikipedia.org/wiki/Tired_light
Anisotropy of space is (a) again adding complications and (b) unlikely to produce such a spherically symmetric redshift. The CMB is infamous for how smooth it is. The different region of space would have to be perfectly spherical with us in the dead center. May as well revive geocentrism.
Look, if we did find evidence like that from JWST or whatever, we'd re-evaluate what best meets Occam's razor for the totality of the evidence. But that doesn't change the fact that with our current evidence, that explanation is expansion of space. Other stuff is sci-fi at best. Fun to think about, but no reason to actually believe it.
The story for dark matter is more complicated, but similar in broad outline. The more the alternative theories try to fit all the evidence, the closer they get to reinventing dark matter. You have something that walks like matter and quacks like matter, so the smart money says it's matter.
Similarly, for dark matter it’s not just galaxy rotation, it’s also observations of gravitational lensing and again the CMB, formation and evolution of galaxies, mass location during galaxy collisions, and movement of galaxies within galaxy clusters. Again, the observational evidence today is overwhelming: https://en.m.wikipedia.org/wiki/Dark_matter#Observational_ev...
As much as I appreciate Fermi's work, this is something I can't at all agree with.
2 - with our instruments, we can observe vast swathes of space, many galaxies, quite precisely. So it appears that no highly advanced alien civilisation is in our galaxy or the nearby galaxies
3 - Even at our technology level, If we wanted to signal out into the void on purpose, we could signal quite far - thousands of lightyears, and if we wanted to target a particular star we could signal precisely with a large laser and that signal would be detectable across most of the galaxy.
So it stands to reason that there is no civilisation similar to ours in the near couple hundred lightyears and that there is no advanced civilisation, the way we envision them, in our or nearby galaxies. It could be that everyone is hiding, or our idea of advanced civilisation is wrong. Or anytthing else really. That;s hwy it's a paradox/
(Inventing cars didn’t stop us from littering.)
At that point, it seems fairly trivial to do direct things like messing with the very small number of telescopes and other methods of detection that we have rather than actually trying to hide by coordinating the entire civ to act a certain way.
The reason relativity predicts weird effects like time travel is because it doesn't allow ftl as an assumption, so the only way to hack ftl into it is negative time. It's baffling how many people seriously argue about consequences of something that completely disproves the theory itself while still using it to generate predictions.
We would like to do FTL, but the universe isn't obliged to humor us.
The Alcubierre metric was designed to not have any time dilation inside the bubble. Does that change anything about the paradoxes?
I don't think it's meant for people like me.
Why can't you extend that from a single particle to a system of particles? Where's the math break down?
Wherein you can shoot yourself before you step into your FTL ship.
though, I still don't see how FTL has to cause paradoxes. What if a wormhole connects you to your destination's past, specifically the 4.3 years ago? There's no causality paradox because both points are in each other's light cones. Earthicans can watch you live by telescope.
...oh. that would imply that you'd see Earth 4.3 years in the future by telescope, while stepping back into the wormhole would either take you into the future (not closed timelike) or to the past (where a Centaurian could shoot themself.) I see.
Closed time-like curves is implied if time is linear in the way most people think of it (B happens after A). If time is a point-like construct, or if time is emergent from observation, or if time is a branching tree, then the problem of closed time-like curves goes away. For example, physicist Carlo Rovelli's theories are based on the concepts that "time itself disappears at the most fundamental level", and "that time is merely a function of our “blurred” human perception".[1]
There are also several ways of time being linear, if that's the case, that are different than current approached. For example, Nicholas Gisin [2] applied intuitionist mathematics and showed that time might be linear but also have an inherent fluidity similar to quantum mechanics.
The above is just the start of a conversation. There's also contractible vs non-contractible closed time-like curves [3], and the Novikov self-consistency principle [4].
The main point is that 'always' in physics is rarely accurate, over sufficient time lengths. The secondary point is that when we start adjusting the metrics of an observer, we start changing what we define as travel (if I stand still and the space around me is deformed such that my observed location is now 10m away, have I travelled?). The tertiary point is that we are still actively theorizing and experimenting about time, with many ongoing competing theories - we just don't know enough yet to say that FTL travel is or is not possible (look at Novikov's experiments proving Bell theorem 'spooky action at a distance').
[1] https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-...
[2] https://en.wikipedia.org/wiki/Nicolas_Gisin
[3] https://en.wikipedia.org/wiki/Closed_timelike_curve#Contract...
[4] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
Visit here https://en.wikipedia.org/wiki/Alcubierre_drive#Causality_vio... and, after that, read further down to the section: "Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[44] where he writes: 'beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)'."
Draw your light cones. Finish the diagram. Any FTL gives CTC, so long as your propulsion method allows for you to steer, or stop and build one that goes in any other direction.
What that slide says is true. But the whole point of the drive is that you aren't traveling FTL from the viewpoint of GR. It's no different than traveling through a wormhole for the purpose of this discussion; it's that you dynamically create a shorter path and don't exceed the speed of light along that shorter path.
Earth is moving through the solar system and solar system through the Milky Way and Milky Way through the local cluster.
So if I went back in time 1 hour I assume I would end up in empty space because one hour ago the location where I am now did not have Earth in it.
Or I might end up inside Earth. Or some other planet or star. How do you time-travelers control where you end up in space?