Breaking the warp barrier: solitons in Einstein–Maxwell-plasma theory (2021)

https://iopscience.iop.org/article/10.1088/1361-6382/abe692

by danboarder • 4 years ago

60 64 4 years ago

iopscience.iop.org

eesmith2• 4 years ago

From 2021? Stick it into Google Scholar. 26 citations.

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".

danboarder• 4 years ago

Thanks, I updated the title to reflect publication in (2021).

api• 4 years ago

A physics professor told me once that pretty much all of current cosmology rules out FTL, but then he said he doesn’t 100% dismiss it due to how weird things in QM keep turning out to be. He said that if FTL were to turn out to be possible it would mean the universe is yet another order of magnitude stranger than we thought due to all the insanely weird implications it would have.

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.

midasuni• 4 years ago

Imagine a wormhole which dumped you from New York to LA instantly. How would that provide time travel? Ok you beat your message by a few milliseconds, HFTs love it, but that’s now different to the ping routing via Sydney. how do you get the message back to the location it left before it left?

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

jleahy• 4 years ago

What if the other end of the wormhole is travelling close to the speed of light? What if one end of the wormhole is accelerating?

SkittyDog3• 4 years ago

So just to start with... Every legitimate physicist accepts that time travel and other major causality paradoxes are an inevitable result of ANY mechanism that provides a meaningful version of FTL travel or communication. This is provable from the math of special relativity. You need some algebra to show it, but the proof isn't super difficult if you already understand the math of special relativity.

• 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.

AnotherGoodName• 4 years ago

It comes down to special relativity not allowing an absolute frame of reference. Without any absolute frame of reference there is no instantaneous.

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.

grishka2• 4 years ago

pretty much all of current cosmology rules out FTL

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.

plutonorm• 4 years ago

I completely agree and often get told I'm an idiot for it. But some of it just doesn't ring true for me.

andrewflnr2• 4 years ago

Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence commonly cited, including but not limited to galaxy rotation curves.

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.

layer8• 4 years ago

It’s not just redshift, it’s also measurements of the CMB, and other effects. The evidence for metric expansion is considered overwhelming today: https://en.m.wikipedia.org/wiki/Expansion_of_the_universe#Ob...

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...

Tade0• 4 years ago

That last paragraph would make an interesting setting for a book.

toastedwedge• 4 years ago

Or a scifi short on Dust

bitcharmer• 4 years ago

I always struggled with Fermi paradox. We've been floating through insanely vast universe for laughably short time and to me its just bizarre to assert that if we couldn't find evidence of other civilizations so far, that for sure means we're the only intelligent species there is.

As much as I appreciate Fermi's work, this is something I can't at all agree with.

ClumsyPilot• 4 years ago

1 - we think that a highly advanced alien civilisation would build massive startships that move at increadible speeds, Radio beacons that signal to alien civilisations, dyson spheres, etc. These things would be massive, easy to detect, and stick out like a sore thumb.

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/

alasdair_2• 4 years ago

One obvious answer is that any civilization that is advanced enough to be at the dyson sphere level probably has no problem hiding their existence from humanity if they want to.

nootropicat• 4 years ago

Ftl means there's one global reference frame most likely the cmbr one.

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.

mcswell2• 4 years ago

"Every warp drops you in a universe where you are the only sentient life." Well that explains the Fermi Paradox.

rob_c• 4 years ago

Yey, except the inevitable paywall from a journal... I'll let someone download the pdf, strip it of tracking and metadata and leak it.

eesmith2• 4 years ago

Preprint at https://arxiv.org/abs/2006.07125 .

at_a_remove4• 4 years ago

FTL always implies closed timelike curves, and that means paradoxes. Until whatever mechanism people come up to travel FTL with has a reasonable way of preventing paradoxes right there also in the paper, it isn't even worth looking at.

We would like to do FTL, but the universe isn't obliged to humor us.

ben_w4• 4 years ago

One thing I’ve been wondering about that, but I can’t reliably keep track of the frames and their transformations and I know I need to do that to get the answer myself:

The Alcubierre metric was designed to not have any time dilation inside the bubble. Does that change anything about the paradoxes?

at_a_remove4• 4 years ago

No. There's a good physics stackexchange on it.

sterlind2• 4 years ago

I thought the "closed" part of closed timelike curves takes care of that. Like in a Feynman diagram where a photon splits and recombines into an electron/positron pair, isn't that equivalent to an electron looping through time and annihilating itself? You could argue it's just a formalism, but mathematically it still seems like (trivial) time travel.

Why can't you extend that from a single particle to a system of particles? Where's the math break down?

at_a_remove4• 4 years ago

No, in a closed timeline curve, the closed portion means "You can arrive where you started, eventually, if you turn around or whatever." The timelike means that you have, effectively, gone into the past.

Wherein you can shoot yourself before you step into your FTL ship.

Communitivity• 4 years ago

Let's say we have three broad possibilities: time is point-like or doesn't exist, time is linear, time is a branching tree where a different universe exists for every possible outcome (this is a bastardization of the Copenhagen theorem, but is discussed).

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...

monocasa2• 4 years ago

The point of an Alcubierre drive is that there aren't any reference frames that are moving FTL. There's cases where spacetime itself is stretching FTL but we know that's possible because we can already see that in the expansion of the universe. Pretty much all of the galaxies outside of the local group are going to slowly fade away from our perspective because of the stretching of space crossing FTL.

at_a_remove4• 4 years ago

This is wrong. The method of travel is irrelevant.

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.

mcswell2• 4 years ago

If God had intended for us to travel faster than the speed of light in a vacuum, He would have slowed down light.

SkyMarshal• 4 years ago

Are we sure he didn't already do that?

upwardbound• 4 years ago

Paradoxes aren't necessarily a deal-breaker. It's possible that reality accepts paradoxes and has some reconciliation method where e.g. new changes overwrite older ones. The best deep dive I've read on how this would work is the twist ending of Isaac Asimov's excellent story The End of Eternity. https://www.amazon.com/The-End-of-Eternity/dp/B08CFWPQW9/

hansvm• 4 years ago

Closed timelike curves are only a problem if you can act differently to change the outcome. The whole loop could just as easily be deterministic, playing the same scene forever.

yarvos• 4 years ago

I am honestly shocked at the number of comments strongly asserting the impossibility of FTL based on simultaneity arguments from special relativity. Yes, faster than light travel through space is prohibited in both special and general relativity by simultaneity. But not so for sub light speed travel (or even being stationary) on a space which itself has the property where distances between the passenger and destination are decreasing faster than the speed of light, which is precisely the class of metric solutions that the paper is investigating.

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.

323• 4 years ago

Not to mention that everybody seems certain that the current theory, Einstein GR, is the final theory and that no new radical theory is possible that could upend everything in terms of possibilities.

analog31• 4 years ago

I've found that that belief in future violations of present-day theory is actually quite commonplace. Judging from conversations with colleagues at the lunch table, not only are violations of theory considered to be possible, but specific violations such as time travel, and warp drive, are considered to be as good as certain.

Every HN thread about FTL travel includes multiple comments warning us not to treat present-day knowledge as final.

phkahler• 4 years ago

> on a space which itself has the property where distances between the passenger and destination are decreasing faster than the speed of light

Doesn't it take considerable effort/energy to make that happen? And is spacetime elastic, so it would spring back?

xwolfi• 4 years ago

Their image is receding faster. It's likely they are not.

ncmncm• 4 years ago

If their image were receding faster, we could not see it. So, no image. We just happen to know the space they are in is going that fast now, based on how far away and thus how far in the past, the image we see is.

russdill• 4 years ago

The galaxies receding faster is a bad example as the motion will never "outrun" a ray of light so no causality issues come up.

galaxyLogic• 4 years ago

If it was possible for me to travel back in time say just 1 hour, where would I end up in space?

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?

Breaking the warp barrier: solitons in Einstein–Maxwell-plasma theory (2021)

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