I think faster than light travel is possible [video]
backreaction.blogspot.com
backreaction.blogspot.com
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.
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.
- We have not found any evidence for it.
- Some predictions have been made from it that weren't found (e.g. no supersymmetric particles in the LHC).
- It can be tuned to fit almost fit any universe and any observation, which doesn't really make it scientific.
- The reasons for inventing it might have been misguided or flawed. (universe must be beautiful / simple / etc).
The lack of results are turning a growing part of the scientific community off, while there is still a core amount of scientists holding on to it.
What would be your estimate of disagreement with her other takes?
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.
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.
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.
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.
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 :)
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).
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.
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.)
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.
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.
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.
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.
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?
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.
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”
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.
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.
Screenshot: https://ibb.co/YdGKwZd
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.
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.
I don't.
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.
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.
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".
Special relativity disallows faster than light travel because it requires moving faster than light.
I can't gain a lot of insight from that point.
It should be: "Special relativity disallows faster than light travel because you need to cross the lightspeed barrier".
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.
I don't think that is necessary, what do you have in mind?
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.
At relativistic speeds, the mass goes up (significantly).
So the same amount of force results in lower acceleration.
What evidence is there of this actually occurring with any human-scale masses?
There's this, however: https://www.scientificamerican.com/article/stars-that-race-t...
How could it not if it does for smaller masses? What is the theory for the difference?
Don't know if she mentioned it, but there is at least a unique reference frame, which is the CMBR rest frame.
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
Anyway, no experiment to-date has detected an absolute reference frame. But that is not to say that one doesn't exist.
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?
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).
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!
I am very happy about this article!
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.
Actually doing the thing may be impossible for other reasons.
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.
This one isn't particularily bad and the content is fine.
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.
Nothing wrong with making a living being popular on YouTube for making physics videos. Even if you have to occasionally bend titles to the algorithm.
She’s deconstructing absolutist nonsense into something less absolutist. It’s almost anti-clickbait.
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.
That, or yes - clickbait contrarian. This seems to get a lot of loud followers though.
Which I guess shouldn't be surprising. Loud followings are usually created by saying things that are controversial, or offensive.
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.
https://en.wikipedia.org/wiki/Space_travel_under_constant_ac...
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.
1. This isn't a two valued system, you can think it may be possible without believing it's possible.
2. Rhetorically these are very different statements, and I believe GP was referring to the rhetorical difference between them.
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".
> I would probably have titled it "I think FTL travel might not be impossible".
Again, what does “not impossible” mean? The only meaning, logically, is “not impossible = possible.”
The fact that you’ve used the word “not” implies a two state system. LEM applies.
The original title made sense. Your proposed change to the title is either: equivalent to the original title or logically inconsistent.
- known possible
- not known to be possible or impossible
- known impossible
"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.
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?
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.
> I was trying to explain why the arguments saying it's impossible are inconclusive and we shouldn't throw out the possibility too quickly
Specifically the point is that the author believes that impossibility has not been adequately proven. Not that possibility has been proven.
(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).
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.
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.
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.
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.
Still should require a ridiculous amount of energy though, if at all possible.
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 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.
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.
The best explanation for why FTL breaks causality that I have seen is the tachyonic antitelephone: https://en.wikipedia.org/wiki/Tachyonic_antitelephone
Drawing a bunch of spacetime diagrams might help as well.
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.
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.
If you can travel backwards in time, you can kill a younger instance of yourself.
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.
My intuition:
Teleport to Mars.
Wait 3 minutes to watch myself go poof on Earth.
Teleport to Earth.
But say instead of waiting you just keep going poof.
I'm the one that introduced the 'start the trip' condition, and they used it in their comment.
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.
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.
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.
The implication there is actually that you can't go faster than light.
Enjoy your Sunday.
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.
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.
Nope, we're talking about moving through space
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.
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.
This animation of a tachyon goes well with your explanation:
https://commons.m.wikimedia.org/wiki/File:Tachyon04s.gif#mw-...
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.
FTL comes with causality issues/paradoxes, the easiest to comprehend video on that topic that I've seen is https://www.youtube.com/watch?v=an0M-wcHw5A
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.
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.
What distinction are you drawing?
What is the difference between "actual" and "what [every] observer observes?
Part of the concept of relativity is that there is no preferred clock. That's why it is called "relativity".
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.
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.
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.
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.
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.
How does sendding information FTL not violate causality?
What is the speed of light in the reference frame of the traveling information? Or does the information teleport?
What is the speed of light in the reference frame of light traveling at the speed of light?
The time coordinate increases with causality. (Same as in Newtonian physics.)
If you want SR to hold, then yes, information teleports.
If you want to travel FTL, then SR can't hold obviously.
"Why Going Faster-Than-Light Leads to Time Paradoxes" from the Cool Worlds channel
We don't KNOW for a fact that light travels the same speed in both directions when we measure it, but it seems likely.
Regular photons in quantum mechanics can move faster than light (*)!
* as long as they are virtual.
It is based on the theory that FTL travel is extremely unlikely.
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1.) The current framework fits the data so well (muons, gps)
2.) the current framework (Lorentz) makes ftl travel impossible
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.
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.
I'm not sure where you're going with that?
That's constant acceleration, which doesn't add anything to the situation besides more things to keep track of and thus be confused by.
There are a whole slew of particles in which to experiment with.
I welcome the Big Bang theory being displaced by a Mother of All Blackhole spewing galaxies (or star matters).
Like Galileo, individual scientists do not matter.
https://bigthink.com/hard-science/black-holes-prove-the-univ...