This is easily Sabine Hoffstader’s fringiest video. Please don’t get the wrong idea that this is a point of controversy within physics! As a physicist I’d estimate 99.9% of her colleagues would disagree and I would too. She has no real argument, just vaguely gestures at “maybe the comoving frame makes relativity invalid” and then vaguely gestures at “quantum gravity spoiling everything we know so far about physics”.
As to the second point, quantum gravity is a thing but almost certainly will not upend the special relativistic account of the large scale causal ordering of events. This is for the same reason that it almost certainly won’t change the calculation of the amount of time it takes an apple to drop from a tree —- within the domain of validity of the old theory, its predictions will agree.
Well then again theoretical physics was dominated by "string theory" for decades despite a lack of any physical or experimental evidence, so we should take consensus about speculative physics with a grain of salt. This sort of speculation is precisely what physicists should be doing, within measure. At least Sabine appears to recognize her "out there" aspect, unlike say the string theorists that stifled physics for decades. At worse maybe she'll annoy another theoretician to analytically disprove her.
That said of course whatever quantum gravity would bring it'll need to reduce to match known physics. However at the fringes of known physics what will happen as you get nearer to the singularities in special relativity? Generally speaking in the past singularities in physics have been due to incomplete knowledge of the physics in these regions.
Yes, at the fringes of known physics, as you say. My point is that this question is not at the fringes known physics. Earth and Alpha Centauri are not near the singularity of a black hole and special relativity is perfectly adequate to predict that events now on Earth will take some time to influence events on Alpha Centauri.
Since Sabine Hoffstader labeled the whole string theory (and some particle accelerator physics) as unscientific she shouldn't engage in the very fringes she appalls.
Both String Theory and Brane Theory have been shown to yield equivalent results to the Standard Model (QFT). Nowadays physicists choose the model yielding the simplest computation for the problem at hand. No contradictions have been found after years of doing this.
So you're saying they're the same theory disguised as different, or another facet of the same solid, only useful as a computational shortcut. Maybe it's time to look for contradictions?
I'm not a physicist, just a knowledgable enthusiast. To your point, people are trying to look for contradictions. So far, none have been found. Physicists therefore use the model that simplifies the computation. If a contradiction is ever found then that will provide information as to which model is correct.
I think Dr. Hoffstader enjoys making these controversial, click-bait videos. They probably go a long way toward paying the bills! She is well-aware that until we reconcile GR and QFT then we don't have the final word on anything. Even so, we've yet to observe anything traveling faster than the vacuum speed of light - and we've been looking!
Yes, I am sure that 99% of your colleagues would agree with you. But modern astrophysics seems to be stuck in a rut, wasting decades on string theory, and now steadfastly determined that no advancements beyond relativity quantum mechanics can ever be accomplished. Even discussing such is met with disdain, dismissiveness, and ridicule.
And absolute confidence that this is the correct take.
Now, I understand that the experimental evidence underpinning modern astrophysics is very, very sound. But if that is truly the end, and no further discoveries of any note are ever to be had beyond Solvay 1927… well, perhaps the modern astrophysics community should just collectively hang it up and start a landscaping business.
Wasting decades on String Theory? We've learned String Theory and Brane Theory both yield the same results as the Standard Model (QFT). Nowadays physicists use the model that makes the computations easiest for the problem at hand.
My own thoughts as a well-learned armchair physicist is GR is the wrong model and Einstein's field equations aren't the right model allowing us to include gravity in QFT. This is what most actual physicists believe, and for good reason. Of course it's entirely possible that QFT is also the wrong model and we need a completely different model from everything we have today in order to form a theory of everything. That's what String Theory and Brane Theory attempted to do. The problem being is thus far the new models don't predict any different results than does the existing model - they all appear to be equivalent. But you can't know that unless you tried, so it's not a waste of time.
They could always switch fields to where we need help. People are still dying from cancer and Covid won’t be the last pandemic the human race encounters. We also need plumbers and air conditioner repairmen.
I enjoyed her talk. She pointed out the subtle flaws in the commonly held view that the speed of light is an absolute limit.
At no point does she say this means it is not a limit, only that this conclusion is not fully warranted from the theory we have now, which we also know to be incomplete.
Some of the best science ever done is through that lens though. Cantor's work on cardinality, relativity, bioelectric networks (more recently). Sometimes it's important to revisit the foundational priors your structures are relying on and make sure they're as sturdy as you remember/your peers are telling you.
That isn't to say that Hossenfelder's bold prediction is necessarily true. But that by double-checking the rudiments themselves, we might find _better_ rudiments.
Consider the axiom of choice. We know it can't be constructed from ZF alone, and so we just throw it in sometimes. But - ZF's axioms are constructed for practical purposes as well. While they are taken as basic principles, they have been carefully studied and analyzed to avoid inconsistencies and paradoxes. The axioms provide a stable foundation for set theory that has effectively served as the groundwork for modern mathematics. But that is a construct made from necessity for rigor - not necessity for the "true" model.
That last bit about "true" models delves into philosophy territory. But the point isn't that Hossenfelder is right or wrong. It's that studying these rudiments and finding holes in them can sometimes be the correct place to go looking for new theory.
And also, I think it's abundantly clear from her tone and choice of words that her bold "prediction" is designed to get you thinking about this stuff in that frame of mind and not something she would actually bet millions on. If I had to guess, she probably feels more akin to Knuth does on P vs NP - that if P does equal NP, it will still have no bearing in practice for other quite interesting reasons.
Yeah, it's possible to modify physical theory in a way that is compatible with our current bounded observations, that allows FTL with higher energies then ever controlled or perhaps ever practical to create.
One of SH's hobbyhorses is that the Universe has no obligation to conform to the most elegant symmetrical theory. (See C/P/T symmetries.)
it is reasonable to "rule out" (read: effectively treat as not true, see Russel's Teapot) any theory which lacks sufficient evidence to treat it with any greater weight
There's a sense in which worm holes allow information to travel faster than light. However, since the worm hole itself has a reference frame, you can't use it to create a grandfather paradox.
No argument? She has shown that within widely accepted theory FTL could be possible. You need to poke a hole in that to say that she has no real argument. I think many physicist may dismiss it as, well, yes, given certain assumptions.. but it's a really good argument.
Personally my biggest question would be why didn't we observe anything FTL in the Universe yet. But it could be, just like with many scientific discoveries, what has been a great data point has been dismissed as some measurement problem.
The form of argument "maybe X invalidates theory Y, so we shouldn't trust prediction of Y" isn't terribly useful since it could be applied to any prediction of science; "Maybe quantum gravity so let's not worry about conservation of energy." "Maybe comoving frame, so maybe we can build antigravity devices", etc. Of course, it could be true. Most anything could be true. It's just not a useful basis for adjusting one's priors.
Actually it is. There is no good reason scientists should all follow the same course of action. In fact, per your own appeal to statistical reasoning, such reasoning is only meaningful if you have an actualization of the samples. If no scientists ever try the unlikely scenarios then you will always reach local optimum.
What is at stake is "Some scientists should consider FTL travel possible, and give it a go." for such a statement, the argument definitely holds water.
All of the arguments for it being possible are essentially equivalent to "it could be that we are all just a simulation program, and within that program its possible to pause/change properties/unpause and make something appear as FTL travel within the simulation".
This could be the case in reality. But its utterly useless information, because it doesn't really give you a direction in which you can even begin to investigate it. Saying that something is possible in this regard is saying that our knowledge of our reality has holes, and "anything" can be in those holes, nothing about the specifics of what could be in those holes.
> Personally my biggest question would be why didn't we observe anything FTL in the Universe yet.
If something was FTL, and we need light to see things, how could anything FTL be seen, as light could not move fast enough to reach it then reach our eyes. Further, assuming we could travel FTL, while traveling we'd be nearly blind, and anything we did see wouldn't make any sense.
I don't know much about physics, but cameras take pictures of things faster than their shutter speed all the time. What occurs is blurring and bright spots.
Just because something is moving faster than the speed of light doesn't mean the light reflected from it never reaches our eyes.
I think another way to perceive it is as if you have a buffer that is beginning to overflow because you can't empty it faster than the speed of light. That overflow is how I'd imagine you'd detect FTL objects. It's like the visual equivalent of a sonic boom. Again idk what I'm talking about, but it was nice to think about :)
I just saw a fun YouTube video on this: https://youtu.be/vFNgd3pitAI (at 13m). I don't know how sound their reasons are, but it seems pretty well put together.
Basically you'd see the same thing from multiple locations, as the light emitted closest to you will hit you at the same time light emitted in the past from further away reaches you. It would construct as two objects going in opposite directions.
Imagine a triangle from points A B C. If I'm at C, and I know distance between A and B, then if something is at A then at B and I measure time between these events in my frame of reference..
> Personally my biggest question would be why didn't we observe anything FTL in the Universe yet.
We have observed at least one FTL phenomenon -- sort of. We know entangled particles communicate with each other FTL because of the Bell inequality. We just don't have any influence on what message they communicate (and we probably never will).
Isn't it just due to specific interpretations of QM that particles communicate? The Many-Worlds interpretation (also pilot-wave theories) wouldn't require any FTL communication between particles.
Hard to say, but other videos I've seen are in areas where things are genuinely murkier; for instance, the status of fine-tuning arguments and the interpretation of quantum mechanics. Not so for this video, which is settled territory (and presents no extraordinary counter evidence). Generally, I do enjoy her off-kilter takes.
> Please don’t get the wrong idea that this is a point of controversy within physics! As a physicist I’d estimate 99.9% of her colleagues would disagree and I would too.
I am pretty sure 99.9% of her colleagues won't state that "this is impossible" when presented a consistent model. Why would they? Why do you do that?
Also please note, that either relativity or quantum (or both) is total wrong.
None of the laws and equations hold quantitatively, also the basic objects do not exist, and the basic axioms don't hold either. All of the qualitative results, conservation laws, negative laws that forbid things are total useless.
There is a chance that relativity is wrong, there is no spacetime, there are no frames, E, m and c are useless numbers, and albeit they are definable and measurable, E=mc^2 never ever holds. (< or > holds instead)
Or the same can happen with quantum, there are no states, you can't add states together, Dirac equation never ever holds quantitatively, etc etc.
It’s quite rare for a theory to be dismantled so thoroughly that we look back on it and say “oops, all of that was just coincidence”. Rather, the existing theory is a special case of the one that replaces it.
For instance, Newtonian physics wasn’t demolished by Einstein et al. It was refined, such that Newtonian physics applies very well in scenarios and scales that we would consider ordinary.
But you are completely right. When the theories are unified then people could very well be saying “you can’t just add these states together”, just like I’d often say that when talking about adding speeds together (it doesn’t convince the traffic officer). What I do think, though, is that a perfectly good response to that will be “ah, yes, that isn’t a problem though because this isn’t in the presence of a magnetic, rotating, supermassive black hole with a funny hat on, so the flopookle correction we learned about in physics 201 is negligible”
"Please don’t get the wrong idea that this is a point of controversy within physics!"
I was a little surprised by her comment but perhaps generating controversy is her actual point. It seems to me that with the long-standing and seemingly insurmountable gulf between QM and GR there needs to be more controversy—not less.
I've always considered >c discussions and arguments speculative and hypothetical and my view hasn't changed but with the QM/GR elephant in the room perhaps it's time to remind ourselves of a similar controversy. When Galileo proposed his controversial theory that likely >99.9℅ of the establishment was against him or thought him crazy.
I don't have anything to add to the topic, but I wanted to ask you about your usage of this character: ℅
Obviously, you meant to use the percent sign (%), but instead have used the "care of" sign (℅). This is the second time in a month I've seen this error, and I'm curious how it can occur.
I did just notice that it's possible to type it on an Android keyboard by long pressing the percent sign. That seems challenging to do by accident. I'm curious if there's another way to easily type it by accident.
Well, I've looked again and I'm not much further ahead. OpenBoard's help docs are essentially nonexistent, it assumes a standard AOSP Android keyboard so help shouldn't be necessary but there are differences compared with Google's GBoard. Incidentally, my reason for using OpenBoard is that it doesn't spy on one like
Google's GBoard.
The only way I can see to get the "care of" character, '℅', in OpenBoard is to use the number toggle and then select the alternate list of characters. As I said this is different to GBoard which allows one the choice between '‰' and '℅' in its long-press of the percent key.
In summary I can't see how I could have entered it from the keyboard without toggling the complete change of characters for the keyboard layout I was using and I don't think I did that—but then I must have. Either that or it's the result of spurious characters arising from hitting multiple keys at once that I mentioned in my last comment.
I hope that helps but I'm still not satisfied with what I've found for the reason that I often want to use Unicode characters that are not on the default keys and I'd to extend that list but it doesn't seem possible. At present, my only solution is to use a separate Unicode app to drop the character in and that's a bit of a pain.
Thank you for pointing that mistake out, I just didn't notice it (the phone is set to small font).
The message was sent from the same Android phone I'm using now. Moreover, I'm still not sure how it happened because the KB app I'm using is OpenBoard and its long-press is '‰'.
I'm curious too because OpenBoard only has the one long-press option (as above) unlike GBoard which has two—the first being '‰' and the second the "care of" sign. Trouble is that GBoard is normally disabled [not just deselected] on this phone and I had to re-enable it to check.
I've noticed occasionally that I've had 'strange' characters appear that do not seem to be long-presses when I've been typing in a hurry so there must be some option for selecting them that I'm unaware of.
It's late here now so I'll investigate later and get back to you when or if I find the answer.
I've seen this video a couple of days ago, and basically it can be summarized as:
1. Special relativity disallows FTL because it requires moving faster than light.
2. Which is impossible, because you'd need an infinite amount of energy to do that.
3. However, from the past experience with physics, we know that mathematical infinities or singularities are often just an evidence of the theory breaking down. So perhaps it's the case here, and a more complete theory will allow to work around the infinity somehow.
4. The other problem is that moving faster than light will result in causality paradoxes in the Special Relativity.
5. However, if there exists a privileged reference frame, then this can be worked around.
I somewhat agree with the arguments given in the first part of the video. We do know that the relativity theory is incompatible with QM, so it's not impossible that there is a way to work around this.
I'm much more skeptical of her example of a privileged frame solving the time-travel paradox. It would require some pretty un-physical behavior, like not being able to move in certain directions if you receive an FTL message.
I would also add point 3.5 which explains that there has been a historic transition (Electroweak Symmetry Breaking) where particles which were massless and travelling at the speed of light before, slowed down and attained mass due to the Higgs field condensing. As the energy that was released back then due to this transition had to be finite, the Lorentz factor argument about infinite energy can't be entirely accurate: if the transition works in one way without involving infinite energy, it should surely work in the other way too (otherwise energy conservation would be violated).
No, the second argument was "there exists a reference frame such that the paradox is resolved" not " this is the only situation in which the paradox is resolved. "
She just wanted a counter example to the claim "faster than light travel produces a paradox no matter what" by showing " well, not in _this_ case" so there might be other cases. Or even all cases, who knows.
However, the existence of the privileged frame by itself won't allow the paradoxes to be resolved. It would also require some kind of a "time police" as well, some kind of a process that would force the event ordering to be causally consistent.
I'm trying to visualize this, and it would look like a cone that would block any FTL messages in the direction of the receiver. As the time passes, the cone will get slimmer, until it disappears once the causality catches up.
> It would also require some kind of a "time police" as well, some kind of a process that would force the event ordering to be causally consistent.
If I understood the video correctly, there does not exist a time police at all, let alone one that forces things to be out of order (i.e., causality loop), vs in order or visa-versa. If it's ambiguous, then it's not fair to assume that it would cause a paradox any more than it's fair to assume it wouldn't -- and the argument goes that is not evidence for a paradox at all. Again, hammering down on the argument that "here is a just-as-plausible situation in which there would be no paradox -- however specific it might be, therefore we cannot assume there is a paradox necessarily".
No, that kinda is right - under special relativity, as your apparent velocity from a fixed reference frame increases, it takes more and more energy to accelerate closer to light speed, so _moving_ faster than light is disallowed.
Another way of putting this is: we know because of special relativity that you can't just strap a lot of rocket boosters on a spaceship and expect to go faster than light. That won't work; if you want to _travel_ faster than light, you have to do it another way.
For a particle travelling faster than light (a tachyon) it takes more and more energy to decelerate the closer to light speed you are, and at zero energy velocity would be infinite. At least via a naïve look at what SR says.
> 3. However, from the past experience with physics, we know that mathematical infinities or singularities are often just an evidence of the theory breaking down. So perhaps it's the case here, and a more complete theory will allow to work around the infinity somehow.
Then it won't take infinite amount of energy, just (infinity minus one). Also, how do you stop? need another near-infinite energy?
But also also, due to time dilation and length contraction, wouldn't you end up in a 2d universe where time doesn't pass for you?
She absolutely mentions it. It's basically her entire point.
I was waiting and waiting for her to make her point and explain why FTL would not violate causality. Then she hand waves about the comoving frame in two sentences, insinuating that it might be a preferred frame of SR, and that's it.
It's not like this is some huge oversight that she found there. After all, we define the age of the universe to be in the CMBR rest frame, because it will be less in any other frame. So the CMBR rest frame is unique, yes, but it is not fundamental in the framework of SR. See this, for instance: https://www.astro.ubc.ca/people/scott/faq_basic.html
So, in the abstract an absolute reference frame eliminates paradoxes, sure, but isn't it the case that in the real universe, every version of the experiment in your blog that has been carried out has found no evidence of such a frame? Because if we're making claims about the real universe as Hossenfelder is doing, that means you can't actually lean on an absolute reference frame. The CMBR might kind of look like one if you squint, but the CMBR comes from matter that is in the end subject to the same relativity we are. Unless some version of the Mach principle turns out be real, with teeth.
It's really not. I have no idea from the video what the actual consequences of privileging the co-moving frame are on an attempt at creating a paradox except "maybe not a paradox! :)"
They mistook a diagram in the video showing flow of time for velocity. You can go there and back again in space at faster than light, but your local time bubble runs forward in each case. No traveling back in time. An external observer sees you traveling back in time the way we see gravitational lensing but your personal frame of time marches forward.
>Which is impossible, because you'd need an infinite amount of energy to do that.
To date I’ve never seen any experimental evidence that the energy required for
A -> B = DV1
X -> Y = DV2
Where X >>> A and DV1 == DV2, was any different. I’ve read countless people claiming it is so under existing theory, but again I’ve seen no concrete experimental evidence.
Imagine two people standing on the “nose” of two mostly identical but different asteroids in vacuum, where one is traveling at 1000mph relative to galactic center and another at 95% the speed of light.
If both jump with equal force I believe they’d accelerate the same, though one should in theory require far more energy to do so if acceleration towards C requires exponentially increasing energy.
I’m aware of the existing theory hack around “reference frames” and that the rate of time is different in each location due to the dV but I don’t actually believe that any of that occurs for macroscopic masses.
There’s better evidence velocity messes with our clocks than velocity messes with time.
The blog doesn't really provide any additional information, and it obscures that you'll have to watch the video to hear her arguments (and I've gathered that many people are only here for text content & abstain from videos).
The blog literally just embeds the video in its post with a paragraph of intro. Additionally it's the blog of the same Youtuber you have so much contempt for.
I would probably have titled it "I think FTL travel might not be impossible". She does not propose a mechanism for FTL travel, just addresses claims that it's impossible.
She seems to have good credentials and mostly sticks to claims that seem to have some evidence...but her penchant for clickbait titles and other YouTube algorithm hacking commentary makes it hard to take her seriously.
She also seems to really enjoy taking contrarian views, which resonates with a lot of people who distrust "the scientific establishment" or who "want to believe".
This is the thing that bothers me about her videos, though. Certainly established science should be challenged when there is the potential that something isn't quite right. (Science itself may be "perfect", but it is performed and chronicled by flawed, biased humans.)
But I feel like she takes a contrarian view just for the sake of being contrarian, so it's hard to tell which of her ideas actually have merit, and which simply exist to challenge the status quo, even when the status quo is conclusively proven.
Edit: that said, I really do want to believe this! The speed of light being an absolute limit to how fast we can travel (or even communicate), means most of the universe (even most of the galaxy) will be forever out of reach to us. And that's kinda depressing. Unfortunately, physics doesn't care if I'm sad or happy about it.
Edit2: on the other hand (I wrote the above after only having watched have the video, shame on me), I do find compelling her idea that some mathematical constructs (like special relativity) inherently cannot fully describe reality (in SR's case, SR doesn't include gravity). So saying "special relativity says FTL travel causes time paradoxes and thus cannot be possible" may be true, but since SR can't fully describe reality (hell, neither can GR), maybe it just doesn't matter. That doesn't prove that FTL travel/communication is actually possible, but at least suggests that we shouldn't be so adamant about it being impossible. But who knows, IANAPhysicist, etc.
Most of her arguments seem to end up along “everything currently funded should be canceled and what I am doing should be funded instead”, which I find extremely hard to take seriously.
That, or yes - clickbait contrarian. This seems to get a lot of loud followers though.
>The speed of light being an absolute limit to how fast we can travel (or even communicate), means most of the universe (even most of the galaxy) will be forever out of reach to us. And that's kinda depressing.
One-way space travel would still be possible using constant acceleration:
>A spaceship using significant constant acceleration will approach the speed of light over interstellar distances, so special relativity effects including time dilation (the difference in time flow between ship time and local time) become important.
>At a constant acceleration of 1 g, a rocket could travel the diameter of our galaxy in about 12 years ship time, and about 113,000 years planetary time.
It’s really not upsetting that “people exist who don’t agree with the prevailing narrative” especially in science, because the “prevailing narrative” has extremely good reason to exist, and more importantly, especially where physics is concerned, the prevailing narrative almost certainly worked, and gave correct answers to the questions that were asked.
The problem with the “prevailing narrative” in physics is never that it’s “wrong”. It’s that it’s either not deep enough or not wide enough an explanation.
Even Einstein’s upending Newtonian physics abs basically proving it as “wrong” as any prevailing narrative can probably be proven has made little difference in the usage of Newtonian physics, which still continues to be employed for nearly everything it would have been if relativity was never discovered.
It just means that we now have a theory that can answer questions that Newtonian physics could not. And we have a clearer and broader picture of what reality is. It means that we have become aware of unknown assumptions and approximations in our earlier understanding, that we didn’t even know about.
She's a professional YouTuber (among other work), those sites basically require you to do that sort of thing to get and maintain traction over the long term.
This one isn't particularily bad and the content is fine.
That’s a bit unfair toward her. She’s not just a professional YouTuber. She is also a bonafide[1] physicist, who has spent years acting as a contrarian to the mainstream ideas.
Yeah but calling her a “professional YouTuber (among other work)” is a very blatant misrepresentation of who she actually is, considering the opinion most people, especially on this site, would have of a YouTuber. An HNer who does not know of her would almost certainly come away with the idea that much like most YTers, she doesn’t know much of what she is talking about.
When in reality, she’s a bona-fide professional physicist, whose knowledge of physics is why she’s a YT personality. Before she was a YTer she had (and possibly still does) have one of the better physics blogs on the internet as well.
Give her a break. Unfortunately the way search is these days, you need to use clickbait titles even if your content is great. So it's part and parcel with our times and how the Algorithm works, unfortunately.
On the other hand, without the click bait titles and other YouTube algorithm hacking, how many of us would have watched it?
I’m happy having watched it. The argument about particles travelling at light speed and then suddenly having mass and travelling below light speed is one that I haven’t heard before. The note about 0/0 being finite rubbed me the wrong way, but I see the point.
Indeed, two such mechanisms that would largely circumvent the issues around CTCs and invalid Lorentz transformations and such entirely - Alcubierre drives and wormholes - are barely mentioned.
(They don't necessarily circumvent the problems around infinite energies and/or zero/negative masses being apparently necessary, to be clear - I can't imagine the amount of energy needed to create and stabilize a wormhole or warp bubble to be all that trivial - but it's pretty obvious from their very premises that they would not inherently produce causal paradoxen).
If something is "not impossible", then it is possible. To say otherwise is to violate LEM, which is an essential part of all of our logical systems. The vast majority of mathematical proofs, many of which form the foundation of our physical understanding, would not hold without LEM.
Stated another way, if something is "not impossible", then what is it?
> I would probably have titled it "I think FTL travel might not be impossible".
(Emphasis added)
Belief isn't binary, ergo no LEM.
LEM doesn't apply to all logical systems, it only applies to two-valued logic. In practice lots of logic is 3 valued or otherwise non-binary (such as partial equality or when we have a probability distribution over a set of outcomes). Some schools of mathematics (the constructivists and intuitionalists) actually reject the LEM.
I personally, as a PhD in having internet opinions, wouldn't go so far as to reject it, but relying on the LEM or proof by contradiction is a risk. You've shown `~X`, and you're betting that a proof of `X` does not exist. But it could be that `X and ~X` is the paradox that reveals your axioms are flawed. But I digress.
The criticism is that X isn't shown to be true, it's just not shown to be false. That would be sufficient if the only values were true and false, but that's not the case here. What the original commenter was saying was, even if you accept her argument in full, it moves the needle to "maybe".
“Not known to be impossible” is not the same as “not impossible.” I think original suggestion was trying to add some linguistic flair, but generally double negatives are hard to understand. It’s better to just state your point plainly.
"Is possible" suggests case 1, if "is known" is implied (we are talking about science here, so that should be the case). "Is not known to be impossible" covers 2&3.
I think the misunderstanding here is that `X` is different from `X is known,` and `X` doesn't imply `X is known.` Consider if we are analyzing a program which does not halt (`~doesHalt`). The halting problem tells us that we can't actually prove this (`doesHalt is not known`).
Can we flip the question around? Under what scenario would you accept the middle condition of, "we do not know whether X is or is not possible" to exist?
There are plenty of things you can say "we don't know whether or not this exists." But that doesn't somehow invalidate LEM. Whether or not something is possible is a binary state, regardless of whether or not we know the answer.
For instance, we don't know whether God exists or not (just a contrived example). Now consider the statement: "It's not impossible for God to exist." That's equivalent to saying "It's possible for God to exist." The two statements are totally equivalent. Sure, we don't know whether or not God exists, but that doesn't make the statement "It's not impossible for God to exist" fundamentally different from "It's possible for God to exist."
Now consider the headline: "I think faster than light travel is possible." And the suggestion is to change this to "I think faster than light travel is not impossible." Again, the two statements are totally equivalent because whether or not something is possible is a binary state and the law of the excluded middle.
No! This is the most seismic and fundamentally perspective changing news that I have heard from a physicist in my life. It needs this strong title.
My entire life, as I imagine has been the case for many older nerds, has been one of imagining traveling to the stars, but to only have that possibility become more and more clearly shown to be impossible. Impossible because faster than light travel is impossible. Impossible, because the time-dilation makes the travel a one way trip. I canna break the laws of physics cap’n!
If the laws of physics do not forbid it, it changes everything! It provides hope, it means the search is worth the effort. It allows my kids to dream!
Not a physicist, but for what is worth, my understanding is it's fairly realistic we could send probes to Alpha Centauri within a human lifetime or so using an approach like Breakthrough Starshot (regardless of whether that particular project succeeds).
Yes, there is still plenty to explore in the near-medium term.
I’m also very excited about the proposal to use the sun’s gravitational lensing as a method of imaging distant planets. Might even see that one in my lifetime.
There are multiple levels to this. One is her attacks on the current orthodoxy around arguments that invoke causality or Special Relativity (as opposed to General Relativity). The arguments seem sound, and I’m sure it will hurt egos and generate controversy (a specialty of this particular blogger/youtuber).
The second is that it reframes the speed of light limit as a “barrier”, and more clearly defines why accelerating mass to the speed of light is impossible (while explaining what mass actually is).
It doesn’t give the answer of “well you just need to do this”, but by removing artificial constraints “causality says no”, it will hopefully allow more young smart people to get interested in studying more in this area.
I strongly applaud Sabine in her attempts to demystify General Relativity and Quantum Mechanics (across other videos). We should introduce these (particularly Quantum Mechanics) into the later school curriculum. It’s a shame that the majority of people never get to study this, as it’s so fundamental to our current knowledge of physics.
The curve she shows is not a closed timelike curve. There are no CTCs in special relativity. A particle going faster than light moves on a spacelike curve. I'm surprised she'd get that wrong.
FWIW I'm not buying her argument that the comoving frame represents a preferred frame in SR either. She glosses over quite some fundamental questions there. I'd love to see this written up as a proper paper.
No OP is correct, her use of the term CTC is not what is usually meant. The curve she draws is not timelike along the faster than light segments in the sense of having tangent 4-vector with timelike norm with respect to the underlying metric. She would not disagree I think.
You are incorrect on this point. A spacetime interval is defined as:
ds^2 = c * dt^2 - dx^2 - dy^2 - dz^2
If ds^2 > 0 then the interval is timelike.
If ds^2 = 0 then the interval is lightlike.
If ds^2 < 0 then the interval is spacelike.
Let's just focus on an object moving strictly along the x-axis faster than the speed of light, this results in a spacetime interval as follows:
ds^2 = c * dt^2 - dx^2
Since the object is moving faster than light, the dx^2 term would exceed the c * dt^2 term, resulting in a ds^2 < 0 and hence spacelike rather than timelike curve.
I disagree. A curve can be closed that is either timelike or spacelike. The interest in timelike curves is that it hypothetically can result in time travel whereas spacelike curves do not describe the trajectory of physical objects. Closed spacelike curves can be used to describe the boundary between causally connected events.
Within the context of special relativity, an object travelling faster than light would be travelling along a spacelike curve, not a timelike curve. The significance of general relativity is that there are solutions that allow for the trajectory of an object to move along a closed timelike curve without exceeding the speed of light.
Uhh, no, they are closed and timelike. It's in the name.
The whole point of CTCs is that in exotic spacetime geometries, in particular wormhole-like structures, there can be curves that are entirely timelike, meaning that the object never moves faster than light locally, and are closed at the same time, meaning the object ends up at the same point in time from where it started.
In SR, a closed curve requires at least some spacelike sections of the curve on which the object moves faster than light.
It's an interesting idea, and ideas that are not impossible have a tendency to become practical. But there's a really big "but" in this one: Unless you can start traveling faster than light without accelerating your mass (or without having any mass) to the speed of light, it still takes an infinite amount of energy.
The interesting point of Alcubierre’s work was precisely that: it doesn’t accelerate the traveler - it compresses and expands regions of space ahead and behind the traveler so that they remain still within their inertial frame.
Still should require a ridiculous amount of energy though, if at all possible.
IANAP but even if FTL travel is possible wouldn't it require a lot of energy? That seems way more inefficient than having life forms whose state can be digitized and beamed through space to a receiver?
FTL travel may make sense for installing the initial receiver network, but after that's done you just digitize life and beam it through space, reassembling at the target, rather than transporting matter.
Just like we talk about carbon footprint now, in the future I think people will talk about free energy footprint, and how to minimize it. Transporting matter through space seems really inefficient.
The information would travel at the speed of light, sorry if I wasn't clear. I don't think FTL travel is possible, but more to my point it isn't desirable since it would use so much energy. Even traveling at 0.9 c would probably be cost prohibitive. This is why I think space travel will be the realm of information streams, not matter.
The reconstructed lifeform would not be a clone. It would probably be housed within a body adapted for its environment. I'm not imagining somehow copying a human at the quantum level.
Does anybody know of a good intuitive explanation for the claim that FTL breaks causality? By intuitive I mean something more than "the numbers in this formula don't add up without breaking causality".
I've tried to build a foundation for understanding this concept several times and failed each time. It makes total sense that with FTL, it would be possible to OBSERVE events in the wrong order, but just this alone wouldn't break causality. My intuition is not helping me get any further than this.
You can think of the speed of light as the speed at which causes produce effects. In the same way that the speed of sound[1] is the speed at which one air molecule can tell another air molecule that it's been been moved, the speed of light is the speed at which one piece of spacetime can tell another piece of spacetime that something has happened to it. Light just happens to move as fast as it is possible to move, the speed of light might be better described as the speed of causality.
This speed defines the "happens before" relation in spacetime, like a Lamport clock defines the "happens before" relation in a distributed system. Event A "happens before" event B if all the information about B was known when A happened. Otherwise B happens at the same time or later than A.
If you can travel faster than light (faster than causality), there's nothing stopping you from constructing a scenario where A happens before B and B happens before A. And that's what it means to violate causality.
[1] To head off a potential confusion, note that while a jet may be able to move faster than sound, the sound of that jet passing by does not.
To an observer who is only able to sense sound, in his reality nothing should be able to move faster than the speed of sound because otherwise he'd observe event B happening before event A. Of course in our reality moving faster than the speed of sound is possible. And in terms of sound we hear the object actually moving back in time. That is we hear it first when it's at our location and then we hear the sound it made further and further back in time. From the jets point of view though he was not moving backwards in sound-time.
For us, the same would apply to something moving faster than the speed of light. We would observe it moving backwards in time but that does not mean it actually does from its point of view.
Point being we have to be very careful about distinguishing between when we observe something happening and when it actually happens. Observing something going backwards in time is fine while actually traveling backwards in time probably is not.
I think you can observe this effect in the high speed Schlieren footage of a whip in this video https://youtube.com/watch?v=AnaASTBn_K4 (you can skip to around 8 minutes) but I'm not sure, I keep going back and forth on it.
You cannot hear any sound as it approaches. When it is at the cameras location, you hear a boom as a lot of soundwaves hit you at nearly the same time. After that you hear a mix of the soundwaves that the jet produces while traveling away and the other part is the soundwaves arriving from earlier during the approach. Unfortunately we can't really distinguish between those with our ears though a machine could. With light you would see indeed the object suddenly appearing out of nowhere and then splitting into two. One traveling into the direction of time and one seemingly going backwards.
What makes things a bit weird are for example time dilation and length contraction. Our equations for those would result in imaginary numbers (e.g. with "i") and at least to me it's a bit unclear as to what that would mean.
It is also unclear to me if a particle traveling faster than light would still see light traveling at the speed of light from its frame of reference.
From the perspective of a being that can sense only sound the hypersonic traveling jet would be equally confusing - it could assume that there must be things happening in another dimension, one that it cannot sense/observe. And so one could theorize that an object being able to travel faster than light could exist in a dimension outside our 4 spacetime ones - one that we cannot observe directly.
There is nothing intuitive about special relativity. If you want to understand SR (or GR or QM), you must throw intuition out the window and strictly follow the rules of the theory.
Just because there's one unintuitive step that doesn't mean everything following that is incomprehensible without math though.
The fact that the speed of light is constant might be unintuitive, but once you accept that, the fact (for example) that you can never reach it is intuitive. As is the fact that it would require infinite energy to do so. It's perfectly fair for someone to seek an intuitive explanation even with an unintuitive assumption. I have no idea whether there is one in this case, but it doesn't seem true that there's nothing intuitive about special relativity.
Thanks, that's what I was afraid of. I guess I've been trying to look for a shortcut, but in the end I'll have to dive much deeper and learn much more to really understand this topic.
I've read the tachyonic antitelephone article several times over the years, but the problem is that (as far as I understand it), this thought experiment exactly boils down to "the numbers don't add up without breaking causality", without really explaining why that is in any intuitive way for me.
Please define "causality", as precisely as you can.
Presumably you'll need a "clock" of some kind, so please describe a clock in a way that it is independent of the speed of light and any particular frame of reference (unless you believe there is a preferred frame of reference. If so, which one is / which ones are?)
Remember that the theory and experiment of special relativity state and show that the speed of light is always measured to be a constant, no matter who measures it and how they are moving with respect to anyone else.
>It makes total sense that with FTL, it would be possible to OBSERVE events in the wrong order, but just this alone wouldn't break causality.
There seems to be a general misunderstanding here. A different order of events can easily be observed without FTL, but only for events outside of our own light-cone. If we could observe events within our own light-cone without causal ordering, they could still affect us and thus would easily create paradoxes. However, if you are limited by the speed of light, this can't happen.
Ok, so, at 14:45 in the video she shows some spacetime diagrams. The "normal" interpretation is that the distortions shown, including the event-ordering flip she shows, represent reality. Relativistic motion is actual time dilation and not merely your perception of someone else's motion. This is backed up by evidence in a lot of extreme cases.
I don't have the skills to know if one is really "allowed" to say that there exists some frame of reference that is preferred or if that breaks something else, but I think that any such frame where time must always go forwards would do what she's saying here.
Possibly.
Unfortunately, I know this is an area where not only does my intuition often fail me, but where I can easily have the wool pulled over my eyes with plausible sounding but erroneous maths.
Alice, Bob, initially at rest, Bob accelerates to relative motion of 0.5c, turns around, returns, decelerates to rest with respect to Alice, both agree Bob experienced less time.
No clock is preferred, but the dilation is real.
That the Alcubierre metric is supposed to avoid dilation made me ask ages ago if it also avoided the usual paradoxes, but everyone I asked insisted it didn't help at all with that.
Say we're in a universe with a +- metric signature. We're looking at tx, ignoring yz.
Rotating within a 45 degree cone (which hides the hyperbolic geometry of +- space) is easy. That's just normal acceleration.
Rotating more than 45 degrees is the hard part. That corresponds to going FTL. It's hard because of the hyperbolic structure of +-.
Once you're going FTL, changing your direction to point backwards in time is trivial; you just turn your engines on and accelerate.
Read Greg Egan's "Clockwork Rocket" and "Dichronauts" books and support material on his website to develop an intuitive understanding of spacetimes with ++++ and ++-- signatures and (by contrast) our own spacetime with a +--- signature.
FTL in SR does not break causality. Some FTL travel is "allowed", at least causality allows it. A natural model for SR + FTL that is consistent with causality, is that SR is an emerging phenomenon / effective theory over a Galilean space time. If you can, say, send information in the Galilean space time with arbitrary velocity, then you can send information FTL, but causality is total cool with it.
It is also possible, that you can move with arbitrary speeds in this Galilean spacetime (but I believe we haven't observed anything in the universe that moves FTL).
However, arbitrary FTL would mean you can send information from anywhere and anywhen to anywhere and anywhen, which breaks causality.
I am just a lay person but I tend to think about FTL travel in terms of traveling faster than the speed of sound. There is a sight dimension in which we can see the object before we hear it. Is there another dimension that we can observe the object traveling FTL?
It's because "simultaneous" is relative in the same way that "in the same place" is relative. You can bounce a ball against the same spot repeatedly, but if you're on an airplane and another observer isn't, the places the ball hits are separated by a large distance in their reference frame.
Most concretely, the FTL-breaks-causality conclusion comes from: If special relativity holds (it does), and you have FTL travel, and there is no universal privileged frame of reference affecting your FTL drive, then it's possible to get back to your starting point before you leave by making multiple FTL hops. You have to be able to make the two hops in different reference frames, that is, you need to do a bit of conventional acceleration between hop 1 and hop 2.
From everything we know about the universe it seems unlikely that there's a special preferred frame out there, but if for example you're developing a hard-science-fiction setting and want FTL without causality violations, you can throw in that background detail to keep the physicists happy.
> it would be possible to OBSERVE events in the wrong order
True, but of course that's possible even in normal earth-bound newtonian physics. All of the discussions about relativity assume that all the observers are taking speed-of-light delays into account when they're observing.
One of the Xeelee Sequence novels deals with this problem specifically: a pilot in an FTL space fighter uses multiple hops to defeat an enemy, and ends up back before he left. It's a Xeelee Sequence book, grimdark of grimdark, so hilarity does not ensue.
> To get to Mars faster than light, you have to be able to travel backwards in time (you could watch yourself start the trip).
I think the issue is that seeing something from the past is not the same as time travel. If I yell, then travel faster than sound, then stop, I'll hear my previous yell, but so what? It's not like I can run back faster than sound and stop myself from yelling.
Yeah, if you use a false analogy it doesn't make any sense. Traveling faster than light isn't the same as (or rather, wouldn't be similar to) traveling faster than sound.
Yes, and that is, specifically, the part that is not apparent. What makes it not the same other than one being light/faster and the other sound/slower?
Everything you say makes sense if you already presume that FTL implies breaking causality, but I'm missing the explanation for why FTL implies breaking causality in the first place.
I mean, it's completely logical that if you get to Mars faster then light, you can watch yourself start the trip, but why does this also mean that you are actually going back in time?
Intuitively it would seem that even if you teleport to Mars with no travel time, observe yourself on Earth, then teleport back to Earth, you should arrive the next instant after you left, not before. So time travel does not follow immediately from the fact that you were able to observe yourself in the past.
Causality propagates at the speed of light. “Speed of light” is just a more intuitive way to say the speed of causality. FTL travel is the same as faster than causality travel. So going faster than light exceeds the speed of causality. To me, that means the order of events is no longer predictable so everything about cause and effect breaks down hence breaking causality.
>If I arrive back on Earth just after I went poof, then I'm pretty sure I went backwards in time a couple of minutes.
How?
Teleport in this context is an instant transportation process. but the image of you going poof is not ists a speed of light.
Like If I throw a ball at a target first THEN Fire a gun at the same target, the bullet will get there first but it didn't time travel to do that.
Lets assume that the teleport process is a wormhole or a folding of space process that reduces the distance between the two locations rather than making you go faster.
You could step through onto mars and back again and not see yourself when you arrive back but then go back to mars again and see yourself disappear and re-appear twice. still no time travel. You're just watching a delayed image.
If you wait 3 minutes on Mars, why would an instant teleport back to Earth result in you arriving just after you went poof rather than 3 minutes after you went poof?
I mean, it's completely logical that if you get to Mars faster then light, you can watch yourself start the trip, but why does this also mean that you are actually going back in time?
I'm the one that introduced the 'start the trip' condition, and they used it in their comment.
I didn't say that traveling faster than light was the same as traveling backwards in time and then, for some reason, that that includes being able to kill a younger version of yourself.
The speed of light is a fundamental aspect of space-time. Time isn't separate from space-time, it is bound up with space into space-time. And then if you goof the rules, goofy things happen.
If you look down in the cousin comments to yours, I had already posted a comment making it clear that I don't think light propagates instantly (3 minutes to Mars and all that).
It's certainly possible that someone will come up with a theory that allows us to set aside the idea of space-time, but it's not worth worrying about until it happens.
Think of it this way: photons don't have to exist, they might just be a model for the propagation of information. In the reference frame of a photon, propagation is instantaneous.
To be clear, you are saying if someone can make it to mars in under three minutes, they can kill their younger self because of the laws of physics that say "if you goof the rules, goofy things happen" ?
To get to Mars in less than 3 minutes you have to either convert mass into energy and back or invent entirely new physics. It'd be pretty cool if either of those things happen.
Specifically, in response to a question about why FTL breaks causality, I said "To get to Mars faster than light, you have to be able to travel backwards in time."
The implication there is actually that you can't go faster than light.
If you went to Mars in 1 minutes instead of 3 you would be going faster than light but not going backwards in time. I'm not sure where you are getting that from.
Like if Mars is reachable by travel though space in one minute, that reachable Mars is inextricably in the past of the Earth that you start from.
If you don't travel through space, then you literally have the ability to jump between points in space-time and are sort of obviously traveling through time.
Like if Mars is reachable by travel though space in one minute, that reachable Mars is inextricably in the past of the Earth that you start from.
Nope, you are mixing up philosophy with science and math.
If you don't travel through space, then you literally have the ability to jump between points in space-time and are sort of obviously traveling through time.
I didn't watch the entire video, so SH may have mentioned, but there has long been a theoretical particle called a tachyon which can (could!) move faster than light because it is created already moving at that speed (if it existed).
There is zero reason to believe that tachyons exist. It's just a name we gave to a concept. Dark matter doesn't have to exist either purely because we decided to give an idea this name. (But there are other reasons to believe that dark matter exists, namely observations.)
I definitely agree with her point about the division over zero case not implying impossibility, but I also disagree with the implication that division over zero is necessarily infinity.
When Alice is in the co-moving frame, I'm not sure of what Sabine meant by "And now ASSUME that faster than light travel is only allowed forward in time in this particular frame.". This is an assumption so what can she infer from it other than the obvious? I know that this is just an example of a situation where FTL -/-> paradoxes, but I'm unable to see if this is a factual example or else.
Feels kind of like P = NP. Everyone is like yeah, it could be possible. But basically everything we know so far points in the opposite direction. And we know a lot, so some fundamental rethinking of everything we know would have to happen for this to unfold
On the one hand, good on her for taking a bold stance. I don't agree with it at all, but I appreciate the conviction.
On the other hand, this makes some of her criticisms (e.g. supersymmetry being unphysical) ring a little hollow. But perhaps there's a banger paper on the way. I'll wait until there's something more substantial to chew on.
I heard somewhere that reaching the speed of light was not possible, but that also meant that if you were faster than light, you could not slow down to reach the speed of light. That's where the basis of the tachyon particle came from. I don't know if that's actually physics or science fiction though.
Faster than light travel is not as impossible as adding two plus two and getting three, but it's more impossible than almost everything else a normal person thinks of as being impossible.
Her glazing-over of the causality paradox is also seriously weak. She's playing with language and spaceship directions instead of applying the least-convenient-possible-world principle to her argument. Instead of her two spaceships going two directions, how about a single one going in a circle faster than light? Like I get it, I hate it too, but you can't just dismiss it like that.
It might be that that is what relativity theory implies. Then again, it might not. We don't know and that is what Hossenfelder is pointing out. We just have some formulas that seem to fit our observations mostly on large scales. We know they don't fit on quantum scales.
As to the second point, quantum gravity is a thing but almost certainly will not upend the special relativistic account of the large scale causal ordering of events. This is for the same reason that it almost certainly won’t change the calculation of the amount of time it takes an apple to drop from a tree —- within the domain of validity of the old theory, its predictions will agree.