Comment by SideburnsOfDoom | Hacker News Reader

Comment by SideburnsOfDoom | Hacker News Reader

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SideburnsOfDoom

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.

> 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.

Signals slower than the speed of light (or even at the speed of light, e.g. radio signals in space) are ordinary and have no paradox, so there's nothing to debate.

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.

> Signals slower than the speed of light (or even at the speed of light, e.g. radio signals in space) are ordinary and have no paradox, so there's nothing to debate.

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.

> 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?

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.

> 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).

Can recommend again that you turn to books on this subject; Posting long rants on the internets won't get you anything. You're clearly not learning, and neither are you intellectually impressing anyone.

> 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

Welp, looks like this turned sour again. Whoops! But hey, I tried, I guess.

> 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.

You're _not_ trying. The "Alcubierre's Expanding spacetime" wheeze isn't anything that wasn't covered in literally my first comment

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.

Reply on news.ycombinator.com