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,
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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?
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.
. 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.)
