FTL and Time Travel question

[Xeriar]

So, I'm confused. Many physicists equate FTL with time travel.

Basic premise can be described as follows:
Two ships flying apart at .866 of c, firing a message at instantanious speed to eachother at t=15, and signalling a reply the instant they receive it.

Because there is a time dilation of 2 between them, according to physics, apparently, they each receive eachother's message at t = 7.5. Receiving eachother's reply at t = 3.725.

My question is - why isn't it t=30 and t=60, respectively? Changes in accelleration get the accounting, and firing a signal is no different.

If the ships instead fire particles that fly at .866 c towards eachother at t=15, they each receive eachothers' particles at t = 60, according to the solution to the Twin Paradox.

Either that, or I've got something wrong here? :confused:

[69dodge]

What's t? Each ship has its own clock, and the two clocks don't agree.

[Xeriar]

Originally posted by 69dodge
What's t? Each ship has its own clock, and the two clocks don't agree.

t is from each ships' individual perspective.

Redefining it better, call ships A and B. ta for ship a, tb for ship b. I'll only have one ship send the signal.

In the instantanious example, B fires its message at A at tb =15. From B's perspective, ta is 7.5, but ta's reckoning says that B fired the signal at ta = 30.

So why do physicists say A gets the message at ta = 7.5 instead of ta 30? With the response arriving at B at tb = 3.75 instead of tb = 60?

If I read the explanation of the Twin Paradox correctly, if B instead fired a signal of some sort that approached A at .866 of the speed of light, at tb = 15, A would assume the signal was fired at ta = 30, and the signal would arrive at A at ta = 60.

[rppa]

Originally posted by Xeriar
So, I'm confused. Many physicists equate FTL with time travel.

I don't think so. Superman comic books make that connection. Physicists equate FTL with causality paradoxes, which is not quite the same thing.


Basic premise can be described as follows:
Two ships flying apart at .866 of c, firing a message at instantanious speed to eachother at t=15, and signalling a reply the instant they receive it.

Because there is a time dilation of 2 between them, according to physics, apparently, they each receive eachother's message at t = 7.5. Receiving eachother's reply at t = 3.725.

My question is - why isn't it t=30 and t=60, respectively? Changes in accelleration get the accounting, and firing a signal is no different.

Um, no. It doesn't work that way. Relativity tells you what happens to intervals, independent of the origin. That is, the effects don't depend on when you set your clock to 0. If your clock said "T=10 days" since you last reset it, physics doesn't say a time dilation of 2 predicts the signal arrives 5 days ago.

If you had instantaneous transmission, then if the signal was sent when both clocks say t=15, it's received when both clocks still say t=15. The time of travel is 0.

[Xeriar]

Originally posted by rppa
Um, no. It doesn't work that way. Relativity tells you what happens to intervals, independent of the origin. That is, the effects don't depend on when you set your clock to 0. If your clock said "T=10 days" since you last reset it, physics doesn't say a time dilation of 2 predicts the signal arrives 5 days ago.

If you had instantaneous transmission, then if the signal was sent when both clocks say t=15, it's received when both clocks still say t=15. The time of travel is 0.

Setting clocks to 0 is making the maths easier. Nothing about what I said matters if they synchronized their clocks at ta=tb=0, -1 trillion or +1 trillion.

The signal can't possibly be sent when both clocks say t=15. One is either t=30, t=7.5, or t=whatever depending on your frame of reference.

In your case we only have to add another ship, C, crossing A at .866 of c at its tc = 7.5

Ship A sends a signal at the speed of light (because, accordingly, if it was sent instantaniously it would not arrive until several seconds later from either perspective) to ship C when ship A receives the signal from B. Ship C, at tc = 7.5, sends an instantanious signal to ship B, who receives the roundabout reply to their message 7.5 seconds before they sent it.

If you can send a message, you can transmit an object's information.

[RussDill]

An instantaneous signal is not just impossible because there is a limit on FTL. It is impossible because it is non-sensical. Realitivity redefines what time and space are, and how they interrelate.

Events can only happen simultaneously in realitivity in reference to a particular observer. If you claimed that ship a sent a message to ship b instantaneously, a "stationary observer", ie, one who is sitting between either ship as they travel away in opposite directions, would not see it that way. They would see the message received by b before a sent it.

[Xeriar]

Err, that's pretty much what I'm talking about above. I'm not using equations the equations for standard FTL because they throw imaginary numbers all over the place (possibly an ironic summary of FTL, but anyway)

My argument is is that the math that supports your comments, RussDill, does not seem to jive with the math that solves the Twin Paradox.

When I do said math, the stationary observer sees B send said instantanious message before A receives it, when adjusting for the speed of light.

I know that physicists say the exact opposite, but I see that statement and their solution to the Twin Paradox as a contradiction. If my math is wrong, I want to know where and why.

[RussDill]

Originally posted by Xeriar
Err, that's pretty much what I'm talking about above. I'm not using equations the equations for standard FTL because they throw imaginary numbers all over the place (possibly an ironic summary of FTL, but anyway)


I'm not aware of any math for FTL. I'm going by special relativity.


My argument is is that the math that supports your comments, RussDill, does not seem to jive with the math that solves the Twin Paradox.


eh?


When I do said math, the stationary observer sees B send said instantanious message before A receives it, when adjusting for the speed of light.


The idea of an istantaneous reply is ever more counterintuitive. What is the difference between an instantateous message, and an instantaneous reply. If they were instataneous in the classical sense, it would all happen at the same time, no point in a reply. And again, if you are asking about it in the classical sense, it is a non-sensical question.


I know that physicists say the exact opposite, but I see that statement and their solution to the Twin Paradox as a contradiction. If my math is wrong, I want to know where and why.

I'm not sure what math can be used to represent a "simultaneus" message. You could pick a moment from a particular frame of reference, and claim that a message was sent, however, no message would actually be sent, and from any other observer, it would not be simultaneous. Perhaps you can clue me in to what math you are talking about.

[RussDill]

If the question of why FTL would be equated with time travel, it is because of one of the possible interpretations when you move v beyond c. However, none of the equations end up being sensible anyway. velocity is not a scalable component like matter, approaching c is like approaching infinity.

[rppa]

[QUOTE]Originally posted by Xeriar
Setting clocks to 0 is making the maths easier. Nothing about what I said matters if they synchronized their clocks at ta=tb=0, -1 trillion or +1 trillion.

The signal can't possibly be sent when both clocks say t=15.

Not in the real world. But you said "instantaneous", which means 0 delay. If somebody sends me a a signal when my clock reads t=15, and it takes no time AFTER that to make the trip, then it arrives when my clock says t=15.


One is either t=30, t=7.5, or t=whatever depending on your frame of reference.

Let me try to convince you how absurd it is to think time dilation works this way.

Suppose we are stationary relative to each other. Not moving. Time dilation factor = 1.0 exactly. Our clocks can be synchronized.

Now, let's say we are 1 light second apart, i.e. light takes one second to go from you to me.

You send me a signal when your clock (and mine) say t=15. Do you that because time dilation = 1, the signal arrives when my clock says t=15?

Let's go back to your example, where the dilation = 2. Suppose there's somebody else in the same spaceship, but he didn't reset his clock at the same time. Instead, his clock reads one second off of yours, so when you say t=15, h says t=16. Suppose he transmits a signal at the same time.

Now you say the relativity predicts the signal arrives at the destination at either t=30 or t=7.5 NO MATTER HOW FAR IT HAS TO TRAVEL. Do you also think that the other signal, sent from the same spaceship at the same time (but from a clock that was reading t=16), has to arrive at the same destination at either t=32 or t=8?

[rppa]

Originally posted by Xeriar
If my math is wrong, I want to know where and why.

Your math is wrong. I hope my new explanation is clearer as to where and why.

First figure out what time a signal should arrive if there is no time dilation at all, i.e. time dilation factor = 1 instead of 2.

[Xeriar]

Originally posted by rppa
Let's go back to your example, where the dilation = 2. Suppose there's somebody else in the same spaceship, but he didn't reset his clock at the same time. Instead, his clock reads one second off of yours, so when you say t=15, h says t=16. Suppose he transmits a signal at the same time.

Now you say the relativity predicts the signal arrives at the destination at either t=30 or t=7.5 NO MATTER HOW FAR IT HAS TO TRAVEL. Do you also think that the other signal, sent from the same spaceship at the same time (but from a clock that was reading t=16), has to arrive at the same destination at either t=32 or t=8?

Err no, that's not what I'm saying at all.

There is also a distance in time. A and B begin at rest to eachother with respect to time - that is, they are moving at the exact same velocity. They begin to diverge, at .866 of c. They spend 15 seconds doing this, their notion of 'now' is different.

From B's perspective, at tb = 15, ta = 7.5. From A's perspective, at ta = 15, tb = 7.5.

[69dodge]

Originally posted by Xeriar
Redefining it better, call ships A and B. ta for ship a, tb for ship b. I'll only have one ship send the signal.

In the instantanious example, B fires its message at A at tb =15. From B's perspective, ta is 7.5, but ta's reckoning says that B fired the signal at ta = 30.

So why do physicists say A gets the message at ta = 7.5 instead of ta 30? With the response arriving at B at tb = 3.75 instead of tb = 60?When you say "instantaneous," you have to say which reference frame you're referring to, because of the relativity of simultaneity at a distance. The "physicists" are supposing that the message from B to A is instantaneous according to B, and the message from A back to B is instantaneous according to A. You're supposing the opposite, that the message from B to A is instantaneous according to A, and the message from A back to B is instantaneous according to B.

You can see the problem with the idea of instantaneous communication. If A thinks that his message to B is instantaneous, B thinks that he got it before it was sent.

[rppa]

Originally posted by Xeriar
Err no, that's not what I'm saying at all.

There is also a distance in time. A and B begin at rest to eachother with respect to time - that is, they are moving at the exact same velocity. They begin to diverge, at .866 of c. They spend 15 seconds doing this, their notion of 'now' is different.

I'm having a little trouble even understanding what the setup is.

There is a version of the Twin Paradox where you dispense with the acceleration and make it completely symmetric. You have them exchange messages while moving at constant velocity with respect to each other. That sort of sounds like what you're doing.

But then you say they start out at rest with respect to each other and "diverge at .866c". That doesn't make sense. 0.866c is a velocity. If their distance is growing at a rate of .866c, then they aren't moving at "the exact same velocity". Forget relativity, just look at two cars. Imagine you're driving along the road at 30 mph, and another car passes you at 60 mph. There is a moment when it is right next to you, 0 distance. Then it appears to recede at 30 mph. From your point of view, that other car is moving at 30 mph. There was never a point at which you appeared to be moving at the exact same velocity.

So please help me understand the setup. Do you wish to say that at t=0 they are right next to each other and after that B appears to be separating at .866c from the point of view of A?

From B's perspective, at tb = 15, ta = 7.5. From A's perspective, at ta = 15, tb = 7.5. [/B][/QUOTE]

[Xeriar]

Before they start moving, they are moving at the exact same velocity, not once they start diverging.

I wish to say that, at t = 0, they are stopped with respect to eachother. No relative motion at all. Two cars neck and neck on the road at 60 mph, or whatever. Then, one or both pull away, at a total of .866 of c relative to eachother.

Can you show me the version of the Twin Paradox where they both accellerate? It might help me describe it better.

[RussDill]

Originally posted by Xeriar
Before they start moving, they are moving at the exact same velocity, not once they start diverging.

I wish to say that, at t = 0, they are stopped with respect to eachother. No relative motion at all. Two cars neck and neck on the road at 60 mph, or whatever. Then, one or both pull away, at a total of .866 of c relative to eachother.

Can you show me the version of the Twin Paradox where they both accellerate? It might help me describe it better.

And what equation do you want me to use to describe "instananeous communication" and from what observers viewpoint? I get the impression you are attempting to apply classical physics to this problem.

[rppa]

Originally posted by Xeriar
Before they start moving, they are moving at the exact same velocity, not once they start diverging.

I wish to say that, at t = 0, they are stopped with respect to eachother. No relative motion at all. Two cars neck and neck on the road at 60 mph, or whatever. Then, one or both pull away, at a total of .866 of c relative to eachother.

One problem with your setup is that gamma (the dilation factor) changes as they accelerate, so it doesn't make sense to use any constant value. I'm not sure why you need the acceleration anyway. I'll assume we're starting at t=15, but the acceleration is done.

Can you show me the version of the Twin Paradox where they both accellerate? It might help me describe it better.

I'm not aware of one of those. However, here's a link with the version where neither one accelerates, but we're at the point where both are moving at .866c (relative to some intermediate frame).

http://sheol.org/throopw/sr-twin-01.html

I have to digest this a little myself. I'm not an expert in relativity.

Some other good discussion:
http://mentock.home.mindspring.com/twinrdux.htm
http://www.incentre.net/tcantine/TP.html

If nobody else jumps in with a clear explanation, I promise I'll give it a go over the weekend.

[RussDill]

Any time you are talking about an acceleration and specifically want to know about how a situation is different when acceleration is involved (Ie, instead of the twins passing eachother at somepoint and calling that t=0), you need to invoke general relativity, the equations are a tad more complicated.

Again though, the questions he keeps asking relate to what clocks say when a simultaneous event happens, and that depends *entirely* on what reference frame the event is simultaneous in reference to.

[epepke]

Having just finished Final Fantasy X, I love your icon.

However, in spite of having been a research scientist in a mostly physics research program for 13 years, I can't understand your question.

What's this about a message at "instantaneous speed"? I don't understand what you mean. In any event, seen from an observer for whom the spaceships are going at the same speed, I don't see a paradox.

As for FTL implying time travel, there's one caveat. Time travel is implied if one can go FTL in all inertial reference frames. But if one can go FTL in only some of them, then that violates relativity, because it suggests that some reference frames are more privileged than others.

[Art Vandelay]

Originally posted by Xeriar
Two ships flying apart at .866 of c, firing a message at instantanious speed to eachother at t=15,
Instantaneous in what reference frame?

Because there is a time dilation of 2 between them, according to physics, apparently, they each receive eachother's message at t = 7.5. Receiving eachother's reply at t = 3.725.

No, one of them receives the message at t=7.5, the other at 30. They each receive the reply at the same tame they sent the message, t=15.

I wish to say that, at t = 0, they are stopped with respect to eachother. No relative motion at all. Two cars neck and neck on the road at 60 mph, or whatever. Then, one or both pull away, at a total of .866 of c relative to eachother.
So one of them instantaneously goes from rest to .866c? Acceleration just adds more confusion to the issue.

[Xeriar]

Originally posted by epepke
Having just finished Final Fantasy X, I love your icon.


w00t :-)


What's this about a message at "instantaneous speed"? I don't understand what you mean. In any event, seen from an observer for whom the spaceships are going at the same speed, I don't see a paradox.

http://www.google.com/search?q=FTL+%22Time+Travel%22

I've scoured most of the sites listed here, as well as an article on Everything2, and each time I get a different result than the page author. Searching for evidence to the contrary gives me nothing.

Take the picture from here, for example:
http://www.orionsarm.com/intro/ftl-paradoxes.html

If you rotate the picture so that 'Bob' and 'Carol' are verticle, their 'now' line is not that purple line - no implication of time travel there.

The first link in the search expresses the kind of argument that confuses me
http://sheol.org/throopw/tachyon-pistols.html


As for FTL implying time travel, there's one caveat. Time travel is implied if one can go FTL in all inertial reference frames. But if one can go FTL in only some of them, then that violates relativity, because it suggests that some reference frames are more privileged than others.

Something has to happen with the reference frame of whatever is going FTL - that's a granted, but that doesn't necessarily imply a causality violation that I can figure.

A few statements I've read abuse the Lorentz transformation.

As much as the Instantanious thing is a hangup, I'll need to construct something that is 'merely' FTL it seems, to try and better illustrate what my problem is.

[RussDill]

Originally posted by Xeriar

As much as the Instantanious thing is a hangup, I'll need to construct something that is 'merely' FTL it seems, to try and better illustrate what my problem is.


So....you are confused about why, when you try to throw completely random equations that describe FTL into relativity, why there is confusion and contridiction? There is a reason why trying to add FTL to relativity doesn't work.

[TillEulenspiegel]

I'm not quite sure what exactly the case is, but it sounds like Xeriar has put himself in a preferred frame of reference separate from the two ships. As we all know there is no such frame , if there were the information exchange between the two ships would APPEAR to b instantaneous. Could be wrong in my understanding the case tho.

[Vim Razz]

Originally posted by Xeriar
So, I'm confused. Many physicists equate FTL with time travel.

Basic premise can be described as follows:
Two ships flying apart at .866 of c, firing a message at instantanious speed to eachother at t=15, and signalling a reply the instant they receive it.

Because there is a time dilation of 2 between them, according to physics, apparently, they each receive eachother's message at t = 7.5. Receiving eachother's reply at t = 3.725.

My question is - why isn't it t=30 and t=60, respectively? Changes in accelleration get the accounting, and firing a signal is no different.

If the ships instead fire particles that fly at .866 c towards eachother at t=15, they each receive eachothers' particles at t = 60, according to the solution to the Twin Paradox.

Either that, or I've got something wrong here? :confused: It looks like you're confused because you're looking at a faulty argument, but you're not sure where the error is so you're trying give it the benefit of the doubt. :p The basic problem is here:... firing a message at instantanious speed to eachother ...Relativity is structuraly inconsistant with the notion of FTL travel/comunication. This causes problems when you try to "plug it in" -- I'll get to that more in a minute, but first I want to go back and review the original setup.


Looking at the situation from ship A's perspective:

- Ships A and B take off in opposite directions at Ta = Tb = 0

- Ship A's clock hits 15 (Ta = 15) and he fires off his signal. (The signal travels at light speed.) While he's doing this he glances over his shoulder and it appears to him that ship B's clock still only reads 7.5 (Tb = 7.5)

- Ship A gets the signal from B, fires off his reply, looks at his clock: Ta = 30, and glances over his shoulder again at ship B -- ship B's clock now apears to read 15 (Tb = 15). This is the same information he would have gotten by glancing at the "timestamp" on ship B's signal because the two sets of information traveled at the same rate from ship B to A.

- Ship A recieves the reply to his original signal at Ta = 60. Glancing over his shoulder again, Tb=30.


Comparing this situation with a "similar" one that you linked -- the tachyon pistol (http://sheol.org/throopw/tachyon-pistols.html) page by Wayne Throop -- there are a few things which need to be considered right off the bat:

1) Tachyons are purely hypothetical particles. To the best of my knowlege, they only "exist" to the extent that if they are real, then they allow certain problems in quantum mechanics to be solved in a convenient fasion. While a number of particles have been predicted this way (neutrinos were hypothisised by W. Pauli long before they were found, positrons by P. Dirac, and many others) it cannot be taken for granted that tachyons will follow the same patern. Without coroberation, they are merely an artifact of certain speculative variations of QM theory.

2) Throop is atempting to combine Relativity and QM in his example. This is a notoriously difficult thing to do, as the two fields sometimes make rather contrary predictions -- if it were easy it would have been figured out already. In this case, He's trying to drop a hypothetical faster-than-light particle into a system which is simply not equiped to handle the notion of FTL travel.

As a result, the math breaks down and he gets a nonsensical answer. This situation is analgous to the classic mathmatical "proofs" that 1=2 by using clever algebra to hide the fact that you've multiplied or divided both sides by zero.


If the notion of FTL travel ever does prove to bear out in the real world, (and some interpretations of the work on entanglement sugest that it will, although I dont know enough about it to comment) then it will involve departures from current relativistic mechanics as drastic as the departure of relativity from conventional Newtonian mechanics. Atempting to use relativity to asses such situations simply doesn't work.

Heading back to the original question:My question is - why isn't it t=30 and t=60, respectively? My answer is -- because Throop's math is badly flawed. He's using a system where 1 = 2. ;) :p

[Vim Razz]

Originally posted by RussDill
So....you are confused about why, when you try to throw completely random equations that describe FTL into relativity, why there is confusion and contridiction? There is a reason why trying to add FTL to relativity doesn't work. Yeah, what he said. :p

(I should have read that sooner)