A new model for faster-than-light travel that uses conventional physics
popularmechanics.com
popularmechanics.com
https://www.popularmechanics.com/science/a35788050/dyson-sph...
Anyone else remember TNG's New Ground? That was the episode where the Enterprise helps test a soliton wave-based warp mechanism. If I remember right, Geordi is enthusiastic, while Data and Worf are skeptical.
Also, couldn't that have used this tech to send superweapons towards Borg space in the Delta Quadrant? Picard has no problem straight up murdering assimilatees in First Contact who are much less far gone than Locutus was in Best of Both Worlds, so I don't think there's much of an ethical issue at stake. Or they could have just used Data's adversarial image virus to wipe out the collective through Hugh.
And then Q, uninvolved but likely bored with whatever else he was doing, bullying some stardust or something, and omniscient enough to hear Picard shouting, would dutifully appear. He'd turn the whole impossible situation into an unfair but winnable game, gleefully taking credit for the whole thing, and having so much _fun_.
I bet Picard could've pulled that about a million times!
The additional issue is whether something going faster than the speed of light violates causality, like a warp drive. I've never understood how to reconcile causality with the fact of so called 'spooky interaction at a distance'. If it's faster than the speed of light, and you can communicate even the info of a bit, isn't that going to violate causality? Recent experiment showed that it was communicating faster than the speed of light at the least. https://www.extremetech.com/extreme/150207-chinese-physicist...
> requires roughly 100 times the energy contained in the mass of Jupiter, said Lentz. That’s about 30 orders of magnitude higher than the power of modern nuclear reactors.
Quantum 'spooky action at a distance' is something else though, as you can't communicate info with it (see 'no communication theorem').
(Assuming the same assumptions as the Bell tests)
(And assuming things like "measurement" and "macroscopic" have well defined meanings, which Bell had problems with too)
To the constantly accelerating interstellar traveler distances get really small after a year or so. You’d be able to make it to the edge of the universe and back in a lifetime. Well, ‘back’ is probably not going to have a lot in common with this place, so maybe stick to distances in our local group or something.
https://en.m.wikipedia.org/wiki/Space_travel_using_constant_...
I'm surprised that so far nobody here has complained that they got the Star Trek trivia wrong! I don't remember the details but there's plenty of exotic matter and energy involved in the handwaving that's supposed to explain how warp bubbles form in Star Trek.
But still, fair point: that does suggest "warp core" is a bit of a misnomer. Then again, we all know the difficulty of naming things properly ;)
Indoctrination taken to religious levels, in a confined and controlled environment, would be more than enough to propagate programming for hundreds if not thousands of years.
It does raise a moral argument though for only using robots.
1) A psychological screening program to select those suited for long life, or
2) A whole society that manages to perpetuate itself long enough to reach the goal?
(Not that these are our only two options in a slow travel scenario)
The big problem being fuel and how to carry it [1].
[1] https://en.wikipedia.org/wiki/Interstellar_travel#Required_e...
That's the twin paradox btw.
I’m as skeptical about this as the next person, but the universe doesn’t make its rules based on my intuition.
Is moving faster than speed of light a limitation that can be overcome in a higher dimension? Does it really require a lot of energy or is that only so under certain circumstances?
There is a lot we still don't know
This is just an analogy anyway. My main point is, or what I hope science can figure out is whether the speed of light is a true limitation of reality or just a limitation of our own perception/understanding
https://en.wikipedia.org/wiki/The_Boy_Who_Reversed_Himself
This book covers all sorts of situations about objects in fewer and additional dimensions.
Why would being able to move through higher dimensions help you travel faster through the 3 we're familiar with?
There is a 3D/4D equivalent, but it can't be visualized, and it's unknown whether it has any possible physical meaning.
This would be the coordinate system on a crumpled piece of paper analogy, yes? I can't help but feel this is highly unlikely.
Do we have any reason to believe this would be the case? Wouldn't we experience this crumpled space or is momentum crumpled as well?
Why would we expect new dimensions to be bent through our dimensions when the 3 we experience do not exhibit this behavior?
Those are all words, and that's about all there is to say about your comment.
In both Einstein's theory of special and general relativity, causality means that an effect cannot occur from a cause that is not in the back (past) light cone of that event. Similarly, a cause cannot have an effect outside its front (future) light cone.
---- end wiki
I guess FTL would violate that definition of casuality, but I assume if discovered, we'd just change the definition of casuality and prolly go with a more intuitive, 'cause must preceded effect'(removing the information speed limit); I assume that's the confusion here.
cause must precede effect _for all observers_. Relativity is involved now, and these observers will not agree on how much time has passed, or even if 2 events were simultaneous or not.
FTL travel of matter or information inevitably gives rise to frames of reference where this 'cause must preceded effect' is not so.
Why would FTL automatically invert causality? Assuming we chuck out, the speed of light limit, like I originally said in my previous comment, which if FTL was possible, would end up being the case. Has the expansion of space ( present day -or- the inflation event, both involving FTL which is allowed under relativity ) already inverted causality?
Mostly I was just trying to point out that it appeared commenters were arguing from two different point of views, one with the intuitive cause and effect definition, and the one from relativity which makes certain assumptions about the universe ( well tested assumptions, but still assumptions, that could be proven false one day ).
And yet it does, that is just the mathematics of it. Another commenter linked this worked example: https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone
> well tested assumptions, but still assumptions, that could be proven false one day
But not in the way that you seem to be implying. They may be proven _incomplete_, but existing measurements confirm that these assumptions are mostly correct (to high degree of accuracy, and over a lot of measurements), and so won't be superseded by something completely different that contradicts those measurements.
The best essay on this subject is "The Relativity of Wrong", Asimov, 1989
https://chem.tufts.edu/answersinscience/relativityofwrong.ht...
I think you have hit on something there. Human intuition is a product of human evolution, and for the landscape in which our ancestors evolved, it works very well. I'm talking about moderate speeds and moderate sizes, basically what's covered by classic Newtonian mechanics.
Now the "Schrodinger's cat thought experiment" as a pop culture meme serves to re-enforce that the quantum realm of the very small is counter-intuitive - and it is very much so, so we don't expect intuition to be a good guide to quantum physics.
Unfortunately there's no light-speed equivalent of Schrodinger's cat; or perhaps the innate assumption is it's a "small" thing if the "very small" realm is blurry, but not so much with the very large and fast. Whatever the reason, people continually arrive with assumption that their intuition is a decent guide to relativity, with tragicomic results seen upthread. Intuitive arguments or analogies about thunderstorms just aren't good guides here.
One possible solution to the problem is time expansion in the bubble: taking a 200ly trip would age the universe by 200 years outside the bubble (from the frame of reference of those inside).
There are galaxies moving faster than c away from us right now.
In my mind the "cascade of simplicity" occurred when I subconsciously realized that bullet-time mechanics could really occur is a multiplayer game. The player in bullet-time would experience higher tickrate that anyone else, meaning that anyone else would see them speeding away. There's more to make it possible in a simulation, but that's unimportant: you can reason about these scenarios. My reasoning maybe incorrect, but it does afford me the ability to approach these mind-blowing subjects.
Either way, it may be impossible for a subluminal observer to identify a superluminal apart from a warp traveller (aside from the former being outright impossible). The observation is identical, the cause is completely different.
Pretty much every explanation I've seen as to why this would ostensibly be so would - when applied consistently - consider the average thunderstorm to be full of causality violations because you can see the lightning before you can hear it.
It shouldn't matter how long it takes me to go from Point A to Point B. As long as I arrive at Point B after I departed from Point A, then causality should be preserved. Doesn't matter if n=1s or 100s or 10000s or 0.0000000000000000000000001s. As long as n is positive, then where, pray tell, was causality violated?
And it shouldn't matter if someone else sees me arrive before one sees me depart, for the same reason why it shouldn't matter if the light from a flash of lightning arrives before the sound does. That is: just because you perceive something out-of-order does not mean it was actually out-of-order.
And no, just because some arbitrary frame of reference is slow to receive information doesn't mean the events creating that information have scrambled causality. Just like how just because I heard lightning strike B before lightning strike A doesn't mean B struck before A.
You are more convinced by a metaphor than the actual theory of relativity. That can't be helped.
That example has multiple assumptions that seem to make said example only relevant to a very specific case, rather than to superluminal communication in general (least of all to approaches specifically designed to avoid those assumptions):
1. Alice and Bob are themselves moving at relativistic speeds relative to each other while communicating. That doesn't seem relevant for cases where they are not doing so - in particular when they are communicating across a region of spacetime crafted specifically such that they are stationary relative to each other.
2. Alice's signal to Bob and Bob's reply to Alice are both somehow themselves moving through normal spacetime at a speed greater than c - i.e. the messages are tachyonic (hence: tachyonic antitelephone). Again, when discussing mechanisms of warping spacetime such that no >c movement within any local frame of reference is actually necessary, bringing up tachyons doesn't seem especially relevant.
> You are more convinced by a metaphor than the actual theory of relativity. That can't be helped.
That can be helped, specifically by explaining why the metaphor is wrong, and where it actually contradicts general or special relativity.
In particular it does not matter whether you use a warp drive or not. The violation occurs relative to reference frames outside of the warp bubble where special relativity fully applies. Here, explicitly looks like a worked out example: http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev...
You can create instances of FTL travel or communication that don't violate causality. But the ability to perform such FTL tricks implies the possibility of constructing a closed timelike curve so long as you're talking about a localized phenomenon like a warp drive or tachyon where special relativity holds outside of a negligible area.
If you're messing with the global geometry of spacetime then you can make very weird things happen and I would not be surprised if there were such a geometry that allows FTL communication in a limited way that doesnt allow CTCs. But that would not like our universe or a warp drive, a specifically localized situation.
...which is why I hate to say that I still ain't getting it, lol
> Along the path actually taken need not be the case, we are reasoning about the endpoints of the communication independent of the means.
So the means doesn't matter at all? It doesn't matter that as far as the endpoints are concerned they're at a fixed distance from one another? That seems hard to believe - not to mention contrary with the math presented in that antitelephone article, which seems to make a big and explicit deal about the relative movement between the two endpoints.
Even when factoring outside observers...
> The violation occurs relative to reference frames outside of the warp bubble where special relativity fully applies.
...how would this apply if the information in question is constrained to that warp bubble between its transmission and receipt? What would there be to observe? If we're talking about side effects of that communication (say, our Alcubierre messenger pigeon drops a feather back into "normal" space somehow), would the propagation of those side effects not just revert back to being subluminal? That is: from every local perspective, the pigeon is traveling subluminally, so this should still apply in the event the pigeon or any signal from it escapes its bubble and reenters "normal" spacetime, no?
> Here, explicitly looks like a worked out example: http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev...
Thanks, this is useful.
...unfortunately, I don't know if it's really agreeing with you. Or me, for that matter.
Namely, it seems to admit that the metric as Alcubierre describes[1] would not produce a closed timelike curve; the paper instead describes ways to produce custom metrics separate from Alcubierre's which introduce the possibility of CTCs, in which case it seems like the answer would simply be to... just not do that, right? Indeed, the paper speculates that there might be other mechanisms that would prevent such a metric from being constructed (specifically naming Hawking's chronology protection conjecture).
That is:
> But the ability to perform such FTL tricks implies the possibility of constructing a closed timelike curve so long as you're talking about a localized phenomenon like a warp drive or tachyon where special relativity holds outside of a negligible area.
Only, from what I can gather, if these equations are indeed the only ones at play, which even that paper admits might not be the case.
----
But if I can make two warp bubbles in two different frames, I can make a round trip that arrives before it starts.
So you need to violate special relativity in a global way to avoid these issues. You have to have a spacetime where only certain warp geometries are possible.
I apologize for being a bit impatient before, these issues are subtle. The only real way to get it is to bear with the math onesself.
No worries, and thanks again :)
> But if I can make two warp bubbles in two different frames, I can make a round trip that arrives before it starts.
Even for the metric as Alcubierre describes? Or for one that's modified per Everett? Now that I'm rereading the Everett paper, I'm not really sure where he's getting his "Lorentz boost"; if there's technically no actual "motion" (because everything's locally at rest and the warp bubble is outright expanding/contracting space around it), then I'm having a hard time figuring out what there would be to "boost", since the relevant Lorentz transformations should be no-ops if everything's locally at rest. Is Everett moving the ship itself at relativistic speeds within the bubble? Is an observer moving at relativistic speeds outside the bubble?
> So you need to violate special relativity in a global way to avoid these issues.
Which the universe kinda already does, no? The mechanism here (from what I understand) is the same as the one driving universal expansion (the difference being that there's no corresponding contraction in the universal case - right?). If that expansion were to be reversed somehow, would that, too, result in causality violations?
And further, doesn't special relativity already only hold in cases with low gravitational potential - so i.e. not in a gravitational field?
Sorry if these are kinda dumb questions.
Now, if special relativity holds in the large then there is no reason why they can't also construct a warp bubble. Relative to you and your bubble that bubble is in motion. Since this is allowed, we can construct a closed timelike curve with two judiciously chosen bubbles.
> Which the universe kinda already does, no?
The expansion of the universe does violate special relativity but not in a way that protects warp geometry from creating paradoxes. The kind of violation you would need would distinguish between frames of reference that SR says are equivalent.
> And further, doesn't special relativity already only hold in cases with low gravitational potential - so i.e. not in a gravitational field?
Special relativity holds well enough in most situations that the same argument applies even if you accounted for general relativity except near extreme situations like black holes
Warp drives are not permitted by any deviation from special relativity. You need specific contrived geometries of the entire universe that don't match up with what we know about it.
I am curious as to how?
If, we take as axiomatic that there is no such thing as "an object in motion", only "an object in motion with respect to another object", i.e. motion is not a property of an object, but a relation between two objects.
And that since it's 4.3 light-years to Alpha Centauri, therefor a cause (e.g. a radio wave) sent from Earth today cannot have an effect (e.g. an Astronomer writes a paper about it) at Alpha Centauri for 4.3 years, and vice versa. There is 4.3 years of "Absolute elsewhere" to get through first where there is no possible cause-and-effect relation between events. At the other side of our galaxy it's around 100 000 years. And further out, objects are not just far, but receding from us (or us from them, equivalently)
Assuming an unbounded and expanding universe, for very distant parts of the universe receding from us at lightspeed (or speeds faster than light?) and us from them, equivalently. So lightspeed signals from there never reach us, by definition? That part is is utterly unobservable, permanently Absolute elsewhere. The universe's observable edge is a slowed-down red-shift that trails off into the unobservable. In other words, the at no time in the future will those signals have an effect on Earth, or us on them. So, they can't have an effect in our future, let alone our past. No impact to causality at all?
Special relativity describes a flat lorentzian manifold. The uniform expansion of the universe implies that the curvature of spacetime is slightly negative.
Special relativity is not simply a theory of causality but specifically of the geometry of spacetime. From this theory you can derive predictions about specific phenomena, like causality.
But that doesn't mean other spacetime geometries don't share properties with flat minkowski spacetime. Just not all properties. For example there is no cosmological horizon in minkowksi spacetime but there is for our universe.
https://en.wikipedia.org/wiki/Cosmological_horizon
Oh, that's the term that I was groping for. Thanks!
- There is no real cause and effect.
- Cause and effect are real, but effects can come before causes.
- The laws of physics are not the same in all frames of reference.
Our current understanding/belief is that some things really do cause other things, that causes always precede effects, and that the laws of physics are the same in all frames of reference.
How? Sure, it's possible that the light from an event happening at T2 arrives before the light from an event happening at T1, but that says nothing about when those events actually happened - just like how if we hear one lightning strike before another, that doesn't mean the one we heard first actually happened first.
That is: this notion that all superluminal travel somehow magically necessitates the possibility of backwards time travel doesn't seem to be logically sound, and seems to be one of these things that people keep repeating without bothering to actually explain why that is. Specifically, this "FTL necessitates backwards time travel" argument seems to depend on some notion that light is infinitely fast (because the only way to go back in time is to have a speed greater than infinity - i.e. one where the t component in d/t is negative), which is empirically false (we can measure light's speed, and it's clearly finite).
> this notion that all superluminal travel somehow magically necessitates the possibility of backwards time travel doesn't seem to be logically sound.
There is literally decades worth of physics that answers your how, and explains the soundness of it, I would urge you to study the subject in well, any depth at all before naively dismissing it as "unsound". I mean, who's more likely wrong: Physics since Einstein's 1905 papers, or guy on the internet who's been thinking about it for 10 minutes, and has questions with basic misconceptions in them.
Asking for answers in the comments is a very poor way to approach a subject like this. All you'll get are quick responses and displays of ignorance.
And as I've already stated, none of it has been even remotely adequate. If it was, then I wouldn't be asking stupid questions with "basic misconceptions" in them. Maybe I'm too stupid to understand it.
> or guy on the internet who's been thinking about it for 10 minutes, and has questions with basic misconceptions in them.
Care to actually articulate those misconceptions?
Like, I'm under no illusion that I'm correct here. I'm literally a community college dropout. I'm only attempting to find answers to questions I've been asking for years and for which I've yet to find an answer.
Instead of an answer, it seems like today I'll be getting downvotes and ad hominem dismissals. Oh well.
> Asking for answers in the comments is a very poor way to approach a subject like this.
It's called having a discussion, which is quite literally the point of this website. If you don't want to engage in that discussion, then nobody is forcing you to.
> because the only way to go back in time is to have a speed greater than infinity
> Instead of an answer, it seems like today I'll be getting downvotes and ad hominem dismissals. Oh well.
Probably because your approach is very confrontational for someone who claims to be "just asking questions" e.g. "unsound", "not remotely adequate". And then you're dismissive - get directed to a wikipedia page where it's answered, and the mechanics of it are done in mathematics, and: "That doesn't answer the question. ... by what mechanisms?"
OK, maybe that approach isn't for you you. Pop science books are a thing, try those instead?
I'll add one thing: I'm a software engineer, but if you gave me 1000 lines of Rust to read, all I could say is "I have a very superficial reading of this code based of function and struct names, because I don't really know Rust at all."
There's nothing wrong with that: specialised topic is specialised. It's ok to admit you don't know the details. What I would not say is "this program is not an answer, because I don't understand it." That would just be an egocentric misconception.
> nobody is forcing you to
You're right, if someone is being obnoxious and pointlessly confrontational, they won't get the best from the people who interact with them. No-one is forcing them to do it! But combining that with "oh why am I getting downvoted and dismissed" wailing seems disingenuous though.
And how is that a misconception? That seems to be exactly the thing the "answers" describe: the time it takes to travel from point A to point B being negative, somehow. Even when discussing problems where we've defined that duration to be positive.
> Probably because your approach is very confrontational
Just because you perceive it to be so doesn't mean it is. Just like, relevantly, how two signals arriving at a reference frame doesn't necessarily indicate the causality of whatever generated those signals.
> get directed to a wikipedia page where it's answered
It's not answered in said page. If it was then I'd be discussing that answer, rather than continuing to ask for that answer. (Not to mention that just linking some Wikipedia page without any attempt at providing context is itself pretty dismissive, but whatever)
Specifically, the "answer" in that page only seems to describe the situation where the signal in its own reference frame is actually travelling faster than c, whereas the very premise of things like warp drives is to, you know, warp spacetime such that the distance covered in that signal's own reference frame is substantially shorter than normal.
That is: a tachyonic antitelephone assumes that, you know, the signal exists as tachyons. Citing the concept of a tachyonic antitelephone in the context of a discussion around warping spacetime to shorten the distance entirely (i.e. remove the reliance on tachyons entirely) suggests - to me, at least, with my dumb dropout monkey brain - some implication that any situation where a signal from Point A arrives at Point B (where A and B are, say, 1 lightsecond away) any sooner than 1 second after being sent from Point A necessitates that signal to be treated as tachyonic, no matter what the signal "perceives" in its own reference frame. Thus, my question: why?
And if you can't answer that question, then that's perfectly fine! I can't answer that question, either, so it ain't like I'd hold it against you. But if you're going to take the time to dismiss that question as being stupid or misguided or what have you, the least you could do is attempt to explain why it's a stupid question - or, better yet, just answer the question.
> Pop science books are a thing, try those instead?
Would you care to recommend one?
> But combining that with "oh why am I getting downvoted and dismissed" wailing seems disingenuous though.
If you actually believed me to be the disingenuous one in this conversation, then why bother continuing to comment? If you don't have anything to contribute, and don't believe I have anything to contribute, then, you could always, you know, leave me to my ignorant wailing :)
Yes, I see that you're capable of being confrontational all day, and unwilling to follow up via clicking links on your own. Why don't you find someone else to troll?
Asking why tachyonic antitelephones are relevant to a discussion about forms of superluminal communication that do not involve tachyons is not trolling. Like, I'm really not trying to be confrontational here (well, at least I wasn't, until you decided I was and immediately decided to be a jerk about it). These are honest questions I'm asking, and I wouldn't be asking them if the answers were really so obvious that no explanation (beyond "here's a tangentially related Wikipedia link, now bug off") is worthwhile.
If it's not trolling it's completely missing the entirely of the point. I mean, really? There are no tachyons, it's just a way to talk about literally any forms of superluminal communication. The maths is the same.
Are you familiar by "proof by contradiction"? (if not, google it and don't bother me) It's a very basic thing, that proceeds along the lines of "assume not x ... steps ... therefor 1=2" and it proves x, because "not x" cannot be true.
Now "tachyon" above is merely "assume something faster than light". and it gives rise to contradiction. I can talk about it for you, but I can't understand it for you. Are you familiar with "thought experiment"?
> I immediately decided to be a jerk
Jerk is as jerk does. We're at the point where it's impossible to say if your misunderstanding are deliberate or not, since you admit to jerkiness.
Here, can we reset a bit? Obviously we didn't start this conversation on the best note, so I guess it shouldn't be surprising that it's gone sour, but that doesn't mean it has to stay that way, right? I'm sorry for my own jerkiness. Let's start over:
Howdy! I'm YellowApple, and I have some questions about the implications of superluminal communication...
> There are no tachyons, it's just a way to talk about literally any forms of superluminal communication.
So I think this the core of my question. If we can agree that these signals are not themselves tachyonic, then why are we subjecting them to mathematics that do assume them to be tachyonic?
That is: the math on that Wikipedia page seems to specifically describe signals that are moving faster than c within their own reference frames, whereas we're discussing spacetime topologies where that shouldn't be necessary - i.e. where from the signal's perspective it's moving slower than the speed of light. So if from the signal's point of view it ain't moving faster than c, how would it still result in a negative t?
> Are you familiar by "proof by contradiction"
I am, yes.
Unless you're thinking of "loopholes", e.g. "the Rocinante travels through the stargate to the Illus system, and since it travels through the gate at 100 km/h with respect to the gate, it's clearly not superluminal". The answer is that light takes 104 years to travel from our solar system to the Illus system, so yes it very much is superluminal, there's no way around that. Same applies to "the Enterprise travels in warp bubble where it's at rest". If it's 8 light years to Wolf 359 and you get there before lunchtime, it's superluminal regardless of warp bubbles. Debating how your signals or spaceship carrying a message travel is pointless - the only question is, did it get there faster than light would or not?
You can't have your cake and eat it too here.
BTW: I already said off of that:
> Literally any method of getting from A to B faster than light would, has a causality paradox. it does not matter if you cover the miles between FTL, or pop out of a hypothetical wormhole gate: If you end up outside of your own light cone, no matter how, there's a causality paradox.
Okay, but that's the thing: from one perspective (the signal's) the signal is indeed subluminal, but from another (an observer's, e.g. the sender or recipient) it's not. So which math applies?
> The answer is that light takes 104 years to travel from our solar system to the Illus system, so yes it very much is superluminal, there's no way around that.
Except there is a way around that, no? Specifically that from the signal's point of view it was not actually superluminal, and therefore not subject to the infinite energy requirements of locally-superluminal travel?
If A sends a message at tA=1, and it takes one second for the message to reach B (so, tB=2), then how does that result in tA > tB? No matter which frame of reference we're using (A's, B's, the signal's), I'm failing to see where causality would invert.
That is, going back to the start of this conversation:
> If you end up outside of your own light cone, no matter how, there's a causality paradox.
From which frame of reference is that light cone authoritative? If I'm B, and A sends me two copies of a message - one by some wormhole or warp pigeon or whatever, and one by radio - and the W message takes 1 second while the R message takes 10, then where does that leave room for the W message to reach me before it was sent?
The math behind a tachyonic antitelephone - as far as I can tell - only answers this for cases where the signal is itself traveling faster than c within its own frame of reference, but that wouldn't be the case in this situation.
All observers are valid. The maths answers differ due to different inputs but they apply the same formula. All should get self-consistent results, though not necessarily the same results: the idea that two events are "simultaneous" is observer-dependant.
Details here https://en.wikipedia.org/wiki/Relativity_of_simultaneity
If _any_ observer, including one not on the fast spaceship, can communicate FTL, that will lead to paradoxes.
> Except there is a way around that, no?
No.
> Specifically that from the signal's point of view it was not actually superluminal,
Irrelevant. Another observer - for whom the signal is superluminal - can leverage that.
> and therefore not subject to the infinite energy requirements of locally-superluminal travel?
I didn't mention infinite energy, and it's a problem for FTL, but not the problem that we were talking about.
> If A sends a message at tA=1, and it takes one second for the message to reach B (so, tB=2), then how does that result in tA > tB? No matter which frame of reference we're using (A's, B's, the signal's), I'm failing to see where causality would invert.
If a signal can travel at lightspeed or lower from A to B, causality of A -> B cannot invert. if it _cannot travel_ , then all bets are off, which is pretty much the point. Basically by stretching "Relativity of simultaneity" to breaking point.
> From which frame of reference is that light cone authoritative?
All frames of reference are equally valid.
> where does that leave room for the W message to reach me before it was sent?
An observer moving with respect to A and B can "hack" this to produce effects before cause in their frame of reference. It's no co-incidence that Alice and Bob in the Tachyonic antitelephone are moving with respect to each other, therefor with different frames of reference.
> the signal is itself traveling faster than c within its own frame of reference,
That's not a thing. The tachyonic antitelephone is a dialog between two observers with different frames of reference. The "Tachyons" could be miniature starship enterprises in "stationary" warp bubbles, flying back and forth; and it would work the same.
So in line with "this has been said before" it's what is here: https://news.ycombinator.com/item?id=26617487 Note "I can find a frame of reference" - all frames of reference are valid, and should be free of paradox.
Again, though: how? Specifically: how would a third observer have any impact at all? He's an observer. Judging by that new Wikipedia article, the answer seems to be "well the observer will perceive events at different times from when they actually happened, and therefore two simultaneous lightning strikes weren't actually simultaneous because the observer didn't see them simultaneously", but I'm failing to see why we should be treating the observer's interpretation as fact: just because the observer perceived something at various times doesn't mean that's how they actually played out.
Further:
> It's no co-incidence that Alice and Bob in the Tachyonic antitelephone are moving with respect to each other, therefor with different frames of reference.
Are they, though? This seems to assume that their movement around and perception of each other is only happening through "normal" space, but the very premise of this discussion states otherwise: that they are communicating via a region of spacetime where their distance is fixed (or at least changing much more slowly than the relativistic speeds given in those Wikipedia examples). If Alice and Bob are talking to each other through a wormhole, for example, then through that wormhole they are (approximately) stationary, and therefore there'd be no significant time dilation as far as that communication is concerned.
Same deal if they're communicating via "miniature starship enterprises", as you put it. The very premise of a warp drive is to get an object from A to B without any actual movement from A to B - specifically by warping spacetime such that the distance from A grows and the distance from B shrinks. This therefore creates a pathway between A and B where they are effectively stationary relative to one another, specifically because the message is stationary relative to each of them. Therefore, messages conveyed on this pathway don't seem to leave any possibility of time dilation, and therefore no possibility of inverted causality.
Now, obviously I'm missing something here, but from what I can tell, the math checks out fine. And a lot of physicists who are not community college dropouts like I am seem to agree (or else they wouldn't be putting forward hypotheses involving warp drives and wormholes and such as a means to achieve superluminal communication without time-dilation-related effects).
> get an object from A to B without any actual movement from A to B
This sentence is pure gibberish: You got from A to B in time T ... but you also didn't get from A to B in time T? Ok, whatever. You're splitting hairs about "actual movement" and again in the long and utterly incoherent text about "Alice and Bob in the Tachyonic antitelephone" moving but also not, which I can't be bothered to untangle because I'm not paid to teach people who think they already know a lot more than they really do.
Buy a book on the subject, those authors do get paid for this. This kind of posting doesn't help you learn anything.
> but from what I can tell, the math checks out fine.
I'm going to draw a line here, and I don't mind being a Jerk now, so: What the actual fuck are you talking about? In Physics, "the math" isn't a metaphor, it's math.
The math checks out? What math? There's no math here, just waffle.
The math is the real thing, as observed here https://www.youtube.com/watch?v=NbzWYjVrvpI&t=55s
> Can recommend again that you turn to books on this subject
Got any recommendations? In fact, I asked you this already, the last time you dickishly suggested I go read books (as if I haven't done that already).
> This sentence is pure gibberish
Then go read Alcubierre's paper[1]. Expanding spacetime on one side of an object and contracting it on another is exactly what's being described. Maybe "without acceleration in the local frame of reference" rather than "without movement" makes more sense to you?
> The math checks out? What math?
[0]: https://arxiv.org/abs/gr-qc/0009013
Oh, or this one from NASA, which even includes pretty slides that your galaxy brain can understand: https://ntrs.nasa.gov/api/citations/20110015936/downloads/20...
Now, I tried to be civil, and even went so far as to recognize when I stopped being civil, apologized for it, and tried again. Yet you still insist on being an asshole about it. Well guess what:
> There's no math here, just waffle.
Yep, "just waffle" is all you've contributed to this discussion, and by the looks of it all you intended to contribute. Gotta flex your intellectual superiority, right? Too bad you're - like me - just another guy on the Internet, so any such superiority you think you might have is illusory, at best.
> and I don't mind being a Jerk now
You clearly didn't mind being a jerk from the get-go. Probably because that's all you can contribute, but here I was hoping that maybe you'd contribute something else.
Oh well. Have a good one, I guess. Maybe next time we run into each other it'll be on more civil terms.
https://news.ycombinator.com/item?id=26614286
and later ones https://news.ycombinator.com/item?id=26619504
Boring circular drivel, and when you're directed somewhere more fruitful, you think it's "dickish". - I'm sorry that I don't have a scientifically current recommendation and I wouldn't presume to offer an outdated one. If you want recommendations list, google or amazon can help you. Whichever way you approach it, if you are really interested in understanding, you're going to have to do some of the work yourself, and you can't expect to be spoon-fed. That would be dickish.
That said, I don't think yellowapple is trolling, I this he's saying "that article isn't accessible at my level".
The issue isn't when the light arrived. The issue is that I can find a frame of reference where T2 < T1, that is, the actual events occurred in the other order. It's not when the light reached me. The problem is the time of the events themselves.
Except that your notion of T1+t1 is already going to be different from any other (non-stationary) observer's, yes?
> I do a little math to calculate how long it took the light to get there (t1)
Doesn't this assume that light has a fixed speed? We know already that this ain't necessarily the case, even in a vacuum; there are plenty of things - like sufficiently strong gravity wells - that can slow down light considerably. And in the context of situations involving warp drives and wormholes and other things bending spacetime willy-nilly, that further throws a wrench in attempting to figure out how long it took light to reach you.
> and now I know what T1 is
More like you have a vague idea of what T1 should be, assuming a flat spacetime. My point is that spacetime curvature can already throw this out the window, and that it therefore doesn't seem like we should be taking that estimated departure time as fact.
That is: if how an observer is perceiving something is indeed what actually dictates reality, then I don't see how causality can actually exist in the first place, because different observers can perceive the same events in different sequences based on their relative movement. If causality is to exist at all, even under normal conditions, then it seems like it needs to be possible for observers to be wrong, no?
So maybe I need to ask this even more specifically:
> The issue is that I can find a frame of reference where T2 < T1
Okay, which frame of reference would that be? Under what conditions? By what mechanisms that would apply to literally all cases of superluminal communication (even ones explicitly formulated to avoid things like relativistic relative speeds upon which thought experiments like the tachyonic antitelephone depend)?
> Except that your notion of T1+t1 is already going to be different from any other (non-stationary) observer's, yes?
Yes, and so is my distance from my current position to X, which I use to calculate my value of t1, which I use to calculate my value of T1. All the numbers are different for all the observers. Special relativity tells us how to convert the numbers between observers.
> > I do a little math to calculate how long it took the light to get there (t1)
> Doesn't this assume that light has a fixed speed? We know already that this ain't necessarily the case, even in a vacuum; there are plenty of things - like sufficiently strong gravity wells - that can slow down light considerably. And in the context of situations involving warp drives and wormholes and other things bending spacetime willy-nilly, that further throws a wrench in attempting to figure out how long it took light to reach you.
First, light does have a fixed speed, locally (that is, at each point within the gravitational well or whatever). Second, gravitational wells are pretty well understood in terms of their effects on light, and I'm pretty sure we could calculate the effect of the warp drive's gravitational distortion on it as well. "The effect exists" does not equal "therefore we can't do the calculation".
> That is: if how an observer is perceiving something is indeed what actually dictates reality
Nobody said that - at least, not until you drag quantum mechanics into the picture, and even then, there are debates about that.
> If causality is to exist at all, even under normal conditions, then it seems like it needs to be possible for observers to be wrong, no?
I have no idea where you got that. If causality is to exist at all, then 1) causal signals can't travel faster than the speed of light, and 2) neither can an observer. If those two hold, then we can have real causality with no observers being wrong, because all observers always observe cause-and-effect happening in the same order.
> > The issue is that I can find a frame of reference where T2 < T1
> Okay, which frame of reference would that be? Under what conditions? By what mechanisms that would apply to literally all cases of superluminal communication (even ones explicitly formulated to avoid things like relativistic relative speeds upon which thought experiments like the tachyonic antitelephone depend)?
Not me. I'm not going to walk you through all the equations. I'll tell you where to start, and then I'm done. Start by learning the Lorentz Transform - not just reading the equations, but understanding what they mean and how they work. That will clear up most of your misconceptions. Any basic book on Special Relativity should get you there. The Wikipedia article might, if you take the time to really work through it carefully.
Which specifically describes coordinate transformations between two reference frames in motion relative to one another, right? My whole point is that in this context the reference frames are not (significantly) moving relative to one another - that relative movement is exactly what things like wormholes and warp drives are designed specifically to avoid.
So no, that doesn't clear up much at all, unfortunately.
To expand on that, because I still think we're talking past each other here:
> If causality is to exist at all, then 1) causal signals can't travel faster than the speed of light, and 2) neither can an observer.
Okay, which wormholes, warp drives, etc. seem to satisfy:
1. The causal signals - from the reference frames of the signal, sender, and recipient - are all moving such that v is less than c, because spacetime has been warped accordingly
2. In order to observe these signals, the observer would, too, have to be part of that system of warped spacetime (i.e. inside the wormhole, or in the path of our Alcubierre pigeon, or what have you). Otherwise, how would the observer actually see anything?
In any case, thanks for bearing with me here.
Now, the warp does affect things. We're now playing with general relativity, not just special. You're locally moving at less than light speed. Does that warping also fix causality? I'd want to see that proven, not just assume that it magically does so.
> Does that warping also fix causality? I'd want to see that proven, not just assume that it magically does so.
Fair. My impression is that it does fix causality, since the warping eliminates the need to (locally) move at relativistic speeds (and therefore reduces any time dilation to negligibility, and thus the opportunities for retrocausality).
So I guess I'm more coming at it from the opposite direction: that if any superluminal communication violates causality (which seems to be a common assertion, both in this thread and elsewhere), then I'd want to see that proven, rather than assume it does so.
At most, what I can gather is that it's mathematically possible to construct a warp bubble capable of backwards time travel, just like how it's mathematically possible to construct a wormhole that can do so; when you're mangling spacetime, that obviously opens the door for all sorts of wacky shenanigans. Possibility != certainty, though, and various online discussions (including this one) feature folks who seem to assert that all FTL travel/communication must violate causality, which leaves me wondering why - especially when, to your point, plugging the relevant numbers into the Lorentz transformation doesn't transform anything (no local movement, so v = 0, so the Lorentz factor becomes 1, and vx/c^2 = 0, therefore no time dilation - is that math right?).
There's also Hawking's chronology protection conjecture. Yeah, just a conjecture, so it can obviously be disproven at some point, but from what I understand string theory seems to support it (reading the relevant papers is on my TODO list).
In any case, thanks again. I don't feel like I'm closer to a definitive answer, but at least I'm left with more to think about.
you're a
you've been given FTL travel and you’re viewed as a FTL entity.
If people confirm those things enable FTL, I think we will see a lot of people trying to reevaluate the General Relativity.
In fact it’s very unlikely. On Earth we have a planet full of the results of 4bn years of competitive evolution. Put that against one single species resulting from evolution. The chances are at least one species on Earth will eat its lunch. Furthermore Earth life is customs tuned to the Earth environment, but the space bug is most likely optimised for a different environment.
That means Earth is moving away or towards the sun at 1 million km per month. That's 33,000 km per day. So 1300 km/h or so? Basically unless you're really fast, climbing the stairs is overwhelmed by other effects.
Haven't taken the Earth's rotation into account but it should be less of an effect.
(Sigh) this is why I'm poorly suited for Twitter...
July 4 is aphelion day this year, so celebrate by doing an entirely useless physics problem.
But look on the bright side, with scrith we could build a Ringworld as well.
edit: aside from being completely theoretical at this point
This is a rather large assertion to make.