> 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
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.
Here's the problem: Event A at location X at time T1 causes event B at location Y at time T2 > T1. If I can travel FTL, I can find a frame of reference where T2 < T1 - that is, where the effect comes before the cause. This leaves a few possibilities, and they're all bad:
- 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.
> If I can travel FTL, I can find a frame of reference where T2 < T1
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).
> How?
> 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.
> There is literally decades worth of physics that answers your how
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.
Misconception:
> 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.
It's not a question of which light arrived first. I see light arrive from X at some time T1+t1, I look at how far away I am from X, I do a little math to calculate how long it took the light to get there (t1), I subtract, and now I know what T1 is. Same for T2.
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.
> I see light arrive from X at some time T1+t1
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)?
> because the only way to go back in time is to have a speed greater than infinity
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 :)
> Just because you perceive it to be so doesn't mean it is.
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?
Right, because daring to ask "why" is trolling. Apparently it's entirely inconceivable that I did already click the one link provided, read through it yet again (as if it's the only time someone's thrown a Wikipedia link at me), and - finding that it still does not answer my question - indicated such.
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.
> Asking why tachyonic antitelephones are relevant to a discussion about forms of superluminal communication that do not involve tachyons is not trolling.
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.
sigh
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.
> > I see light arrive from X at some time T1+t1
> 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.
> An observer moving with respect to A and B can "hack" this to produce effects before cause in their frame of reference.
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).