[HN Gopher] Breaking the warp barrier: solitons in Einstein-Maxw...
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Breaking the warp barrier: solitons in Einstein-Maxwell-plasma
theory (2021)
Author : danboarder
Score : 31 points
Date : 2022-07-10 16:06 UTC (6 hours ago)
(HTM) web link (iopscience.iop.org)
(TXT) w3m dump (iopscience.iop.org)
| rob_c wrote:
| Yey, except the inevitable paywall from a journal... I'll let
| someone download the pdf, strip it of tracking and metadata and
| leak it.
| eesmith wrote:
| Preprint at https://arxiv.org/abs/2006.07125 .
| at_a_remove wrote:
| 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.
| Communitivity wrote:
| 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...
| monocasa wrote:
| 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.
| sterlind wrote:
| 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?
| ben_w wrote:
| 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?
| yarvos wrote:
| 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.
| eesmith wrote:
| 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 wrote:
| Thanks, I updated the title to reflect publication in (2021).
| api wrote:
| 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.
| Tade0 wrote:
| That last paragraph would make an interesting setting for a
| book.
| midasuni wrote:
| 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 wrote:
| 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?
| SkittyDog wrote:
| 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.
| zasdffaa wrote:
| > This is provable from the math of special relativity ...
| you already understand the math of special relativity
|
| 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.
| scatters wrote:
| But our universe doesn't obey special relativity, since it
| has gravity; you have to use general relativity.
|
| 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.)
| SkittyDog wrote:
| I don't know where you got the idea that our universe
| doesn't obey special relativity? That's an odd thing to
| have learned...
|
| 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".
| AnotherGoodName wrote:
| 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.
| Arwill wrote:
| The geometric description of the universe is a high-level
| description of the universe, but its just a description,
| its not what fundamentally is. The large-scale universe
| emerges from the low-level behaviours of its smallest
| parts, the particles and the vacuum.
|
| Causality is not a product of the universe, but the other
| way around.
| bitcharmer wrote:
| 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 wrote:
| 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/
| grishka wrote:
| > 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.
| andrewflnr wrote:
| 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.
| grishka wrote:
| > but what the heck else are you expecting to pop up to
| explain redshift
|
| 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?
| ben_w wrote:
| > some kind of phenomenon that happens to photons but
| only when they travel great distances. Or that maybe the
| space(-time) is not uniform
|
| 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.
| scatters wrote:
| Redshift also occurs to light emitted from within a gravity
| well. Obviously that isn't what's happening at a cosmic
| scale (our light isn't blueshifted to other observers) but
| it's at least conceivable that some third phenomenon could
| be the cause.
| layer8 wrote:
| 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/wik
| i/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#Obs
| ervational_ev...
| plutonorm wrote:
| I completely agree and often get told I'm an idiot for it.
| But some of it just doesn't ring true for me.
| whatshisface wrote:
| If you're not convinced, you should look in to the evidence
| and the deductions that lead to what is now presented as
| fact. Very few popular presentations cover the reasons
| behind the wacky stuff scientists say, which is to blame
| for you evaluating that stuff on the basis of whether it
| rings true - because you've been given no other way to
| evaluate it.
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