Claim: Faster than light travel (FTL) can not be discarded with arguments from physics

Itsme

Senior Member
In a very recent Youtube video, Sabine Hossenfelder explains why she thinks faster than light travel is possible. The video mainly explains why arguments from physics that are used to discard faster than light travel are inconclusive, which is why we shouldn't throw out the possibility too easily.

An interesting video that adds some new arguments to the FTL discussion we had in the thread about ufos and skepticism (which is closed).

00:00 Intro
01:51 The Speed of Light as Limit
06:12 The Speed of Light as Barrier
12:44 Time Travel Paradoxes
20:47 Quantum Gravity and Summary

 
In a very recent Youtube video, Sabine Hossenfelder explains why she thinks faster than light travel is possible. The video mainly explains why arguments from physics that are used to discard faster than light travel are inconclusive, which is why we shouldn't throw out the possibility too easily.
The supporting argument is that the maths that models the world admits solutions that would include FTL travel. Which does not imply that the world admits FTL. Our mathematical models are simply less wrong than the ones that came before, there's no reason to think that they're never wrong, and plenty of reasons to believe that they are wrong. Heck, we don't even know if gravity is a force or not yet. Don't be fooled by the unreasonable effectiveness of mathematics in the natural sciences.
 
Did you watch the video or are you just commenting on its title?
Is this a question made in good faith or is it loaded? I'll take it as made in good faith and answer: yes I did watch the video, that's why I can say it's just a collection of speculations. I didn't need it to know that if faster than light travel is possible, then faster than light travel is possible. Time wasted.

(*) I'm using 'travel' as a shorthand for 'transmission of information'.
 
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Is this a question made in good faith or is it loaded? I'll take it as made in good faith and answer: yes I did watch the video, that's why I can say it's just a collection of speculations. I didn't need it to know that if faster than light travel is possible, then faster than light travel is possible. Time wasted.

(*) I'm using 'travel' as a shorthand for 'transmission of information'.
It is a good faith question. You were basically comparing her to old scientists who go off the deep end bordering on crack pottery. I have watched many of her videos and I find her to be very reasonable. She takes on some of the philosophical aspects of physics with ample care and consideration of thought, more than many physicists I know.

So I was simply surprised by your statement coming as a result of thoroughly watching her examine this issue.
 
It is a good faith question.
Accepted!

You were basically comparing her to old scientists who go off the deep end bordering on crack pottery.
Well, this thread started (in my opinion at least) with a big appeal to authority and the reference to Nobelitis was meant as a counterpoint to that. But I admit it was unnecessarily snarky and indeed I regretted it, just it was too late to change.

I have watched many of her videos and I find her to be very reasonable. She takes on some of the philosophical aspects of physics with ample care and consideration of thought, more than many physicists I know.

So I was simply surprised by your statement coming as a result of thoroughly watching her examine this issue.
First time I see a video from her, so I only had that one to judge upon. It's a boring video, filled with puns which are probably meant to be salacious but turn out, for me at least, to be annoying. The general outline of the video is:
  • Present some well-known and reputable arguments against the possibility of FTL
  • Then proceed to invent ad hoc hypothesis to handwave them away (or non-sequiturs, ie. when she says "scientist usually consider infinites to be unphysical so they should too in this case", talking about the infinite energy required to accelerate a mass to light speed).
This is not what I would expect by a scientist, nor by a philospher, and it adds nothing substantial to a discussion on FTL. I see it as a product of wishful thinking (noone likes to have limits).
 
Accepted!


Well, this thread started (in my opinion at least) with a big appeal to authority and the reference to Nobelitis was meant as a counterpoint to that. But I admit it was unnecessarily snarky and indeed I regretted it, just it was too late to change.


First time I see a video from her, so I only had that one to judge upon. It's a boring video, filled with puns which are probably meant to be salacious but turn out, for me at least, to be annoying. The general outline of the video is:
  • Present some well-known and reputable arguments against the possibility of FTL
  • Then proceed to invent ad hoc hypothesis to handwave them away (or non-sequiturs, ie. when she says "scientist usually consider infinites to be unphysical so they should too in this case", talking about the infinite energy required to accelerate a mass to light speed).
This is not what I would expect by a scientist, nor by a philospher, and it adds nothing substantial to a discussion on FTL. I see it as a product of wishful thinking (noone likes to have limits).
I would simply recommend watching more of her videos to get a better feel of her overall approach to science. Though, considering you didn’t like this one you may not find incentive to do so. She sometimes errs on the side of sensationalism (she does after all have a YouTube channel she is trying to monetize) but overall she has a very sober approach to physics concepts that are usually not treated well online. As a trained, professional physicist myself, I find her descriptions of subjects I am not well versed in to be extremely well presented, when compared to any other channel I have seen. It is rare to find discussions that are at the right level for an educated physicist who isn’t an expert in a particular field. So I greatly appreciate her channel. Your mileage may vary.
 
Whenever I see people dismiss NHI-on-Earth because of the constraints of special relativity, I do a :rolleyes:. That's basically just an argument from incredulity, because in the case that faster-than-light travel is possible in this universe, we presently don't know how it's possible. Clearly our understanding of physics is not wrapped up, complete, and exhaustive, and Hossenfelder's video does a good job at looking at some of the loopholes in current theory that might allow it. (That said of course, there's still no good evidence for NHI-on-Earth, regardless.)
 
Just for reference, not in response to any prompting, I find Sabine utterly fantastic, she's a breath of fresh air in science. Rather than papering over the cracks, she sharpens her nails and dives right in, ripping open the weaknesses and flaws. But it's not out of disrespect for the field, it's the field that pays her wages, I'd almost say it was the opposite. She's shining a light on where new interesting stuff can be done. I'd want her heading a funding committee, definitely.
 
Just for reference, not in response to any prompting, I find Sabine utterly fantastic, she's a breath of fresh air in science. Rather than papering over the cracks, she sharpens her nails and dives right in, ripping open the weaknesses and flaws. But it's not out of disrespect for the field, it's the field that pays her wages, I'd almost say it was the opposite. She's shining a light on where new interesting stuff can be done. I'd want her heading a funding committee, definitely.
Her videos and Sean Carroll's The Biggest Ideas in the Universe (written for a general audience but with all of the equations) are two of the best things that have happened in popular-science communication in a very long time.
 
The general outline of the video is:
  • Present some well-known and reputable arguments against the possibility of FTL
  • Then proceed to invent ad hoc hypothesis to handwave them away (or non-sequiturs, ie. when she says "scientist usually consider infinites to be unphysical so they should too in this case", talking about the infinite energy required to accelerate a mass to light speed).
This is not what I would expect by a scientist, nor by a philospher, and it adds nothing substantial to a discussion on FTL. I see it as a product of wishful thinking (noone likes to have limits).
In her videos she generally addresses arguments that are "well-known and reputable" in the field of physics, but not backed up very well by experimental evidence (or even not falsifiable by experiments at all). The ad-hoc hypothesis and other counter-arguments are used to demonstrate we are dealing with beliefs and opinions here, not established scientific facts. You could call her a debunker in that sense.
 
I love her videos (just like the ones from the Fermi Lab).
She says important things in the conclusions: we don't have a good grasp of quantum gravity yet, and as in the QM physics things go a bit weird, this means that any argument for FTL would probably not survive in the quantum gravity theory.
 
Her videos and Sean Carroll's The Biggest Ideas in the Universe (written for a general audience but with all of the equations) are two of the best things that have happened in popular-science communication in a very long time.
They happen to be adjacent tabs in one of my always-open browser windows:

Sabine's blog: http://backreaction.blogspot.com/
Sean Carroll's Biggest Ideas: https://invidious.kavin.rocks/playl...WcrxH3jyjUUrJlnoyzX&disable_polymer=1&ucbcb=1
Sean Carroll's Mindscape Podcast: https://invidious.kavin.rocks/playl...RExpDXr87tzRbPCaA5x&disable_polymer=1&ucbcb=1

(those are youtube channels, but invidious instances are more usable for me)
 
They happen to be adjacent tabs in one of my always-open browser windows:

Sabine's blog: http://backreaction.blogspot.com/
Sean Carroll's Biggest Ideas: https://invidious.kavin.rocks/playl...WcrxH3jyjUUrJlnoyzX&disable_polymer=1&ucbcb=1
Sean Carroll's Mindscape Podcast: https://invidious.kavin.rocks/playl...RExpDXr87tzRbPCaA5x&disable_polymer=1&ucbcb=1

(those are youtube channels, but invidious instances are more usable for me)
So, I just started to take a look at Sean Carrol’s channel and the fact that he starts with conservation of momentum already tells me I’m likely going to really enjoy his videos.
 
I love her videos (just like the ones from the Fermi Lab).
She says important things in the conclusions: we don't have a good grasp of quantum gravity yet, and as in the QM physics things go a bit weird, this means that any argument for FTL would probably not survive in the quantum gravity theory.
She actually says: "any argument about FTL", not for FTL...
 
Sabine Hossenfelder is a strange one. She's not always consistently wrong, but she's wrong often enough that her content is easy to disrecommend for any non-expert. Unlike the usual crank, however, which is typically earnest in their idiosyncratic beliefs and motivated by clout or hubris, I find her motivation to be much more mundane: clicks.

I'll give just one example and then I'll move on because it's not the topic. On the topic of dark matter, the bullet cluster is considered a "smoking gun" of sorts of its existence, as opposed to the competing explanation that gravity itself ought to be modified. She makes the flimsy argument that it's actually the opposite, that the bullet cluster is an argument in favor of modified gravity and against dark matter. When someone with expertise in the relevant literature pointed out all the mistakes and misrepresentations made in her article, she replied with,
Yes, the whole purpose of this post was to make a one-sided claim, as one-sided as the claims that the Bullet Cluster is evidence for particle dark matter. Infuriating, if someone cherry picks their evidence, isn't it?
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I find this to be a consistent pattern. Make controversial claim, enjoy the clicks, profit. She presents herself as a maverick of sorts fighting against an ossified establishment, and somehow this works for her, even if it's a very common trope. As always with science, particularly with physics, which is the most mature and developed of all, when a clear consensus develops there's usually a good reason.

Given the above, and the fact that the her previous video on the topic of FTL contained approximately zero insight (mostly repeating claims, even the wrong ones, found on a recent paper by Bobrick and Martire (which also contained no novel insight)), I trust you'll forgive me for having only skimmed this one. There's some wrong claims made about the Higgs field etc. but the only argument I'll address in detail is the issue of causality, which is the most persuasive argument against FTL travel/communication. She claims it is not a "good reason" to use the rules of special relativity when making the argument because general relativity is the more fundamental theory.

That is completely wrong.

Special relativity is a limiting case to general relativity. This means that there are important circumstances where the conclusions of the latter are virtually identical to those of the former, most obviously when dealing with small patches of spacetime (analogously to how special relativity recapitulates Newtonian mechanics for slow enough objects), but also for large enough regions of spacetime: as long as you're ok ignoring the detailed geometry near massive bodies, and as long as empty space itself doesn't have significant curvature (this is the case in our universe), special relativity is once more a good description.

For example, say you want to describe the trajectory of a spacecraft around a black hole. Looking from up close, something like the Schwarzschild metric describing stationary blackholes has intense spacetime curvature and important structures such as the event horizon, the singularity, etc. But if you're taking a macro view, and you want to describe what happens by assuming the black hole is a point particle, you can. You just set up a typical elastic collision using special relativistic kinematics and everything will work as it should. From sufficiently far away, the Schwarzschild metric is basically the same as empty space.

So how is that relevant? Well, if you're hoping to engineer some means of FTL travel, and you don't have enough energy to change the entire spacetime structure of the universe, you'll have to live with the fact that from far enough away the whole trip will be well-described by special relativity. For example, say I have a wormhole going from the moon to somewhere near Alpha Centauri. The detailed geometry of the wormhole will be quite complicated and you'll need GR (or a replacement thereof) to fully describe it. But if I don't care about describing the wormhole itself, all I care about is describing the trip, I can remove the entire region of spacetime containing the wormholes from consideration, begin and end my trip far away from the wormhole, use conventional means (i.e. rockets) to move to and from it, and now the whole thing is well-described by special relativity again. I may neglect some of the gravitational time dilation due to the wormhole itself by doing this, but the subjective experience of the observer going through the wormhole is not important to the FTL=time travel argument.

Put another way, the way FTL round-trips easily turn into causal time loops is a topological feature of Minkowski spacetime, which makes it extremely robust to small deformations.

She then makes the claim that because it's general relativity you could have faster than light travel provided it's only allowed forward in time in the frame comoving with the rest of the matter in the universe. That's also wrong. In general relativity, there's nothing special about the comoving frame apart from the fact that stuff is (on average) at rest in that frame. Provided there's not a net curvature, which is true for our universe, Lorentz invariance is still a good symmetry, locally and globally. The exact same argument would apply. The only way it wouldn't apply is if there actually is something special about that frame, which is to say, if special relativity is wrong and Lorentz invariance is not a good symmetry after all.

That's the crux of the whole argument. Physics is not saying "faster than light travel is impossible". Physics is saying: faster than light travel, relativity, causality: pick at most two. This section is merely saying "what if relativity is wrong though" which is not adding anything new.

Lastly, she says that "causality and locality become really screwed up in quantum mechanics" and therefore it would be "extremely implausible" that any argument about FTL would survive in quantum gravity. This, too, is wrong, for two reasons: first, causality is not at all screwed up in quantum mechanics. In fact, it's one of the cornerstones that led to the development of quantum field theory and the clearest reason we must talk of "fields" instead of "particles" in relativistic theory. This is something you find in the first chapters of any quantum field theory textbook in the guise of the "equal time commutation relations" which dictate the causal structure obeyed by the fields; a particularly clear exposition can be found in chapters 1 and 2 of Sidney Coleman's lecture notes. As for locality, the extent to which it's "screwed up" is perhaps a matter of taste, but it's definitively not screwed up enough to allow for superluminal signaling and causality violations.

Secondly, we do have a theory of quantum gravity: string theory. We may not know if it's the right theory, but it's a quantum mechanical theory that describes gravity, which fits the bill as far as this discussion is concerned. One of the key things about string theory is that it preserves Lorentz invariance as an exact symmetry, and therefore the round-trip argument carries through exactly as it did in special relativity. In this light, saying it's "extremely implausible" that arguments about FTL would survive in a theory of quantum gravity when they easily survive in the only known example is at best disingenuous.

The title of the video is "I Think Faster Than Light Travel is Possible. Here's Why." In the video itself, however, we see no argument for why one might believe faster than light travel is possible, rather, we see holes being poked, with varying relevance and accuracy, in the usual arguments for why it seems impossible. That's not really enough to establish the claim promised in the title. Even taking the title as pure clickbait and examining the video purely as an exploration of the supposed flaws in the arguments against faster the light travel, it misses the point: the point is not to declare how nature ought to behave, but rather to examine what must be logically true in order for FTL travel/communication to be possible. That is to say, you must abandon either causality or relativity -- arguably the most well-tested theory in the history of science. This video makes no dent in that argument.
 
I'll give just one example and then I'll move on because it's not the topic. On the topic of dark matter, the bullet cluster is considered a "smoking gun" of sorts of its existence, as opposed to the competing explanation that gravity itself ought to be modified. She makes the flimsy argument that it's actually the opposite, that the bullet cluster is an argument in favor of modified gravity and against dark matter. When someone with expertise in the relevant literature pointed out all the mistakes and misrepresentations made in her article, she replied with,
Yes, the whole purpose of this post was to make a one-sided claim, as one-sided as the claims that the Bullet Cluster is evidence for particle dark matter. Infuriating, if someone cherry picks their evidence, isn't it?
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I took a quick look at the link you provided for this, and it does indeed seem to be that she has missed the point about the Bullet Cluster. Perhaps she has a better understanding that she's just not representing, but she in this post she is only addressing the speed and/or likelihood of the collision itself and not the apparent displacement between the mass that is causing the gravitational lensing and that which is emitting in X-rays. As I understand it, it is the latter point that is the so-called 'smoking gun' of dark matter.

When confronted by this by one of the commenters, she just seems to hand wave it away by saying that she has heard from MOND folks that they can fit the bullet cluster, but doesn't give any details (or even citations) to let the reader assess whether that is accurate or not.
 
Sabine Hossenfelder is a strange one. She's not always consistently wrong, but she's wrong often enough that her content is easy to disrecommend for any non-expert. Unlike the usual crank, however, which is typically earnest in their idiosyncratic beliefs and motivated by clout or hubris, I find her motivation to be much more mundane: clicks.

.. (a series of well-presented and sound arguments follow)

Thanks both for the discussion of Dr. Hossenfelder's (non)-arguments and for saving me from watching more of her videos to check if my bad first impression was justified. However, after @markus's post, I have to revise my position about wishful thinking being the source of the ideas expressed in the video, there are indeed more possibilities.
 
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That is to say, you must abandon either causality or relativity -- arguably the most well-tested theory in the history of science. This video makes no dent in that argument.
You don't need to completely abandon relativity. The only condition she needs to prevent time travel paradoxes is at least one inertial frame in which you can only move forward in time at superluminal speeds. She takes the co-moving frame as an example, but it could be any inertial frame.

This does not necessarily have to change anything to special relativity at subluminal speeds. In fact, special relativity only works for subluminal speeds - the Lorentz transformation does not yield any meaningful results at superluminal speeds. The idea that you move backward in time at speeds >c is an assumption and has never been verified experimentally. So all tests that confirmed special relativity can remain valid, the only thing that needs to be invalidated is the assumption that you move backwards in time at speed >c. And this not even for all inertial systems; just one would suffice.
 
And there is something reasonably profound about stating that mathematical singularities are indications of the limitations of a physical theory. Not everybody seems to get this but it has been pivotal historically to the advancement of science. And will likely be so in the future too with singularities that currently exist in our theories.
 
She lost a lot of credibility with me when she endorsed Havana Syndrome as real.

Not relevant to discussion at hand but... Her attempts at dry wit are off putting because she's awful at it.
 
This does not necessarily have to change anything to special relativity at subluminal speeds. In fact, special relativity only works for subluminal speeds - the Lorentz transformation does not yield any meaningful results at superluminal speeds.

The mathematician in the room says "waaat!?!?!?". As a homeomorphism of Minkowski space that preserves the Minkowski metric, vectors that represent superluminal speeds are mapped onto outputs *equally meaningful* as they are as inputs...

The idea that you move backward in time at speeds >c is an assumption and has never been verified experimentally.

... No. It is definitional in the maths. If you accept that they are valid as inputs to the Lorentz transformation, then there exist an infinitude of reference frames within which the hyperbolic rotation of the original superliminal vector yields a vector with a negative time component. The rotation from "forward in time" to "backward in time" is no more strange for this transformation than the rotation from "moving to the left" to "moving to the right" is (achieved by just moving faster to the left than the thing you perceive as moving to the left). This is not an experimental statement, this is mathematical consistency. It's as true as 1+1=2. If you reject that, then you must throw out everything.

The *physical interpretations* of the mathematics are where the troubles lie - you have to be happy with concepts like time being complex space and space being complex time, but you were implicity happy with that as soon as you wrote x^2+(ict)^2 in relativity 101. The superluminal will have three "timelike" dimensions, and one "spacelike" one. There's nothing mathematically weird about that, it's all well defined, Penrose's conformal cyclic cosmology has that property too. We don't have the language to describe such states, that doesn't mean we can't exhibit them. Just "shut up and calculate".

"You can see the rear of something that's in front of you as it comes towards you" is a perfectly valid outcome of such calculations, and that's true even when *nothing* superluminal is involved. "Length contracted objects can[*] appear longer" ditto, for pretty much the same reason. This worries no physicists, they will happily continue to shut up and calculate.

Of course, if you never admit superluminal influences as valid inputs, you never need to worry about the interpretation of the outputs when transforming them, hence my ephasis on "equally meaningful" above.

[* edited to add the word 'can', approaching ones do, receding ones don't]
 
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She lost a lot of credibility with me when she endorsed Havana Syndrome as real.

I don't remember her drawing any particular conlusion about that particular situation. She addressed various different possible causes for what was being reported, and didn't state that any of them were correct. Everything was couched in phrasing like "remains plausible".

As you see, this is a really difficult story and no one presently has a good explanation for what has happened. Most importantly I think we must keep in mind that there could actually be a number of different reasons for why those people fell ill. While it seems unlikely that the first cases in Cuba spread by mass hysteria, the cases in China only began after those in Cuba had made headlines, so that’s an entirely different situation.
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-- http://backreaction.blogspot.com/2021/11/the-3-best-explanations-for-havana.html
(emphasis mine)

Of course, if what you are implying is that you don't believe "Havana Syndrome is real", then you must also believe "UAP reports aren't real", "9/11 conspiracy theories aren't real" and "claims of election rigging aren't real". Effectively "99% of metabunk isn't real". Whether you think there's anything consistent behind them, those medical reports do exist. Sabine defintiely thinks so, but that's the only concept that she endorsed in her video.
 
I don't remember her drawing any particular conlusion about that particular situation. She addressed various different possible causes for what was being reported, and didn't state that any of them were correct. Everything was couched in phrasing like "remains plausible".

Well, from your quote:
While it seems unlikely that the first cases in Cuba spread by mass hysteria...
http://backreaction.blogspot.com/2021/11/the-3-best-explanations-for-havana.html

Thus it's true she couched everything under qualifiers ('seems unlikely') but as the same time it's also true she thinks ('seems likely', reversing her sentence) the Havana cases are due to something different from mass hysteria. This, in my opinion, equals to 'endorsed Havana Syndrome as real' as @Z.W. Wolf said.
 
Well, from your quote:

http://backreaction.blogspot.com/2021/11/the-3-best-explanations-for-havana.html

Thus it's true she couched everything under qualifiers ('seems unlikely') but as the same time it's also true she thinks ('seems likely', reversing her sentence) the Havana cases are due to something different from mass hysteria. This, in my opinion, equals to 'endorsed Havana Syndrome as real' as @Z.W. Wolf said.

If you re-read my final paragraph, you will see that, pretty much by definition, "Havana Syndrome is real". There are the reports, there are classification criteria - it's a syndrome. If you're denying that, then you are saying that the medical records and their histories are faked, which makes you the conspiracy wierdo.

However, if you are giving a more nuanced interpretation to "Havana Syndrome is real", then you need to explain in detail what nuance you are applying. And, given that you are the one who is ascribing opinions to Sabine, you'll need to back that up with actual quotes - not paraphrases - to support that.

I don't see her as endorsing any particular theory. She's simply saying that there remain several possibilities that can't be ruled out, and in that she even includes people's imaginations running wild.
 
If you re-read my final paragraph, you will see that, pretty much by definition, "Havana Syndrome is real". There are the reports, there are classification criteria - it's a syndrome. If you're denying that, then you are saying that the medical records and their histories are faked, which makes you the conspiracy wierdo.

However, if you are giving a more nuanced interpretation to "Havana Syndrome is real", then you need to explain in detail what nuance you are applying. And, given that you are the one who is ascribing opinions to Sabine, you'll need to back that up with actual quotes - not paraphrases - to support that.

Uh? "Havana Syndrome is real" was used by @Z.W. Wolf, not by Dr. Hossenfelder, and it's clear (to me at least) what he meant by that, without need for explaining in detail any nuance.

I don't see her as endorsing any particular theory. She's simply saying that there remain several possibilities that can't be ruled out, and in that she even includes people's imaginations running wild.
Well, she explicitly says mass hysteria "seems unlikely" in the paragraph you quoted:
While it seems unlikely that the first cases in Cuba spread by mass hysteria...
which means she endorses something else than mass hysteria as her preferred, likely explanation. But... all other explanations are bunk, so she endorses bunk, no matter how much she shields behind the "I did not say it was aliens" trope.

She also endorsed bunk in the FTL video, where she also deployed a veritable array of logical fallacies to 'support' her claims about FTL, and, according to @markus (I did not check this), she endorsed bunk also in the Bullet Cluster video. Three time's a charm, for me the case is closed.
 
This is not an experimental statement, this is mathematical consistency. It's as true as 1+1=2. If you reject that, then you must throw out everything.

A mathematical consistency, even in a very well tested physical model, does not have to describe physical reality.

Newton's gravitational law, for instance, mathematically implies that the force of gravity acts instanteneously over arbitrary large distances. This did not turn out to be physical reality, despite all experiments that verified Newton's law. Yet, by rejecting this we don't "throw out everything" - we still use his gravitational law to predict physical reality today, within the limits of its physical applicability.

Time running backwards may be a mathematical construct that can be created with Einstein's laws, but does this sound like a plausible physical reality? If not, you can either assume that Einstein's laws are outside the limits of their physical applicability for v>c, or that FTL is physically impossible. Both assumptions have not been tested yet.
 
A mathematical consistency, even in a very well tested physical model, does not have to describe physical reality.

Time running backwards may be a mathematical construct that can be created with Einstein's laws, but does this sound like a plausible physical reality? If not, you can either assume that Einstein's laws are outside the limits of their physical applicability for v>c, or that FTL is physically impossible. Both assumptions have not been tested yet.
If a singularity in the mathematics of a physical theory indicates a limitation/incompleteness in the theory then an unwarranted extrapolation *through and beyond* the singularity must surely be speculative at its best and likely simply meaningless.
 
Time running backwards may be a mathematical construct that can be created with Einstein's laws, but does this sound like a plausible physical reality?
Trivially. Set up a straight line of alarm clocks a distance of one light-second apart, with a countdown timer such they they will all go off simultaneously in your reference frame. Your clock that goes "one, two, three, ..." to you will go "... three, two, one" to someone on the other side of the straight line. Your forward time goes backwards to another observer, even one in the same reference frame.

Note that "all go off simultaneously" is effectively a superluminal effect. It travels with infinite speed in the frame of the observer who perceives it to be simultaneous. Also note that it is not a superluminal *influence*, there's no causality violation. Compare rotating beacons, or leading edges of spherical bursts, their speed can easily have a speed greater than c too. c isn't the "infinity" of speeds, there's no mystery about values above it, it's simply the *invariant* of lorentzian transform, a very different concept. Check the maths yourself - a superluminal effect with velocity v will be seen as a simultaneous to a colinear observer with subluminal speed c^2/v. (And vice versa, the transform's linear, so reversable.)

No mass-energy needs to be superluminal for this effect to be seen, so there's nothing that needs any new interpretations of physical reality. This is all just Poincare/Lorentz/Minkowski/Einstein SR, nothing new, the maths handles this situation naturally.

TL;DR: If you have problems with the superluminal, you also have problems with simultaneity, as they are the same concept in SR.
 
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