Does SR actually forbid FTL travel?

[DrSnarl]

Hi No I missed that one but find it fascinating. it seems so much more direct and unequivocal than the statistical studies derived from detector results. if you cn remember where you read it I would be grateful. Thanks

It looks like I remembered the details incorrectly, but the gist is the same. Here is a writeup I found about it at John Cramer's Alternate View site (a great site, by the way):

http://www.analogsf.com/0612/altview.shtml

 

[DrSnarl]

What you can say is that using conventional Lorentz simultaneity, accelerating towards a distant object causes it to 'move' towards your past. You're the one changing simultaneity as you accelerate.

That's just a matter of perspective. It moving to your past is the same as you moving to its future. Either way, when you accelerate towards it, you will think that its clocks are all advanced from where they were before you accelerated, no?

 

[Austin0]

It looks like I remembered the details incorrectly, but the gist is the same. Here is a writeup I found about it at John Cramer's Alternate View site (a great site, by the way):

http://www.analogsf.com/0612/altview.shtml

Thanks for the link.You definitely captured the gist as I read it.
Will be interesting to see future results ,

On the acceleration: According to local inertial clocks next to you (the accelerating observer),
compared to the inertial clocks at the location of the distant observer you are moving towards, you are in that observers past. SO as you continue moving toward that observer
you are moving from its past towards its present (according to clock readings). Which is of course where you will be when you are actually colocated and simultaneous for that moment ;-)

 

[PAllen]

I have researched the experiments but only found superficial accounts lacking in relevant details of parameters and methods. As I understand it paired photons from a down conversion crystal are identically polarized on emission. That a change of polarization is applied to one photon stream and a comparable change is detected in the other.
Otherwise, if it is a case of simply measuring at separate locations why wouldn't correlation be expected if they started out being identical??

The difference from classical correlation is that your results are influenced not only by the set up of the state, but also by decisions made by the other experimenter. What prevents signalling, according to current understanding, is that you cannot actually deduce the other experimenter's decisions from your results. You can only find out that they mattered later by comparing results. Why you can't specify a plausible causality hypothesis is that it is frame dependent which measurement occurred first (if they are done far enough apart).

 

[PAllen]

Wouldn't it perhaps be more correct to say it puts that observer in your past moving toward your present? ;-)

It depends on what event you are talking about. My phrasing was in terms of the world line of the distant observer - this world line shifts in your past direction. So some events on this world line move from far in the future to less far in the future; others move from future to past. All move in the direction of your past (as you now label it using Lorentz simultaneity of an instantly comoving frame). I guess I could have been more explicit.

 

[PAllen]

That's just a matter of perspective. It moving to your past is the same as you moving to its future. Either way, when you accelerate towards it, you will think that its clocks are all advanced from where they were before you accelerated, no?

No, because it is asymmetric. You've changed your interpretation of simultaneity, the distant observer has not (assuming they haven't changed their motion). According to them, you've moved a tiny amount, your clock has advanced a tiny amount, and that is all. It is only you that has changed which events you choose to consider in your past versus your future (by virtue of your change in motion, and following conventional Lorentz simultaneity).

 

[PAllen]

I will make one, hopefully final, comment on this thread.

In modern majority SR view, the issue of distant simultaneity might be described as follows:

- There is no unique or even preferred definition. It is purely a matter of convention how you divide events between your future light cone and past light cone into past, present, and future. A particular convention may have advantages for a particular analysis, but doesn't make it physically preferred. Any convention, properly used, will lead to identical predictions about what can be observed. Three conventions I have seen used with advantage for different analyses are:

1) Lorentz frame of instantaneously co-moving inertial observer
2) Radar simultaneity.
3) Lorentz frame of inertial observer that would reach your current position from some chosen trip starting point.


What is interesting for this topic is that FTL in general, and teleportation, in particular, make simultaneity no longer a matter of convention. Each of the above conventions would lead to different physical predictions about what would happen if you teleport, accelerate, teleport back to your starting world line. Thus, the existence of FTL would change the content of SR to make simultaneity no longer a matter of convention.

 

[Saw]

... FTL in general, and teleportation, in particular, make simultaneity no longer a matter of convention. Each of the above conventions would lead to different physical predictions about what would happen if you teleport, accelerate, teleport back to your starting world line.

Well, another possible interpretation is that all those conventions are still valid but their different outputs must be interpreted differently so that they all lead to the same predictions... or rather to the acknowledgment that you will not be able to make any predictions, because that is precisely the point: if you measure with sub-luminal means, you cannot guess what happens if you introduce in the play supra-luminal actors.

But anyhow I am not familiar with the differentiation between the three conventions. Can you elaborate? Does any of them differ from what I understand by the standard convention, i.e. "simultaneity is what is measured by clocks synched through the Einstein-Poincaré convention"?

 

[PAllen]

But anyhow I am not familiar with the differentiation between the three conventions. Can you elaborate? Does any of them differ from what I understand by the standard convention, i.e. "simultaneity is what is measured by clocks synched through the Einstein-Poincaré convention"?

They're all based on this convention, and all give the same standard result for inertial frames. They differ in how they deal with a non-inertial observer. Note that in SR, Einstein (so far as I know) never introduced any idea of non-inertial frames. He readily dealt with non-inertial motion in some chosen inertial frame (and that is the only sane thing to do in SR IMO). However, as a stepping stone to GR and to answer philosophy questions like 'distant simultaneity' for non-inertial motion (philosophy because you can't observe distant simultaneity - that is, without something like teleportation), you can invent many ways of constructing non-inertial coordinates. In fact the three simultaneity conventions I described are all parameter points in general parametric schemes for analyzing simultaneity conventions.

I will describe the three schemes in relation to the simple case of inertial motion away from some 'home world line', followed by instant turnaround back. I will focus on the mathematical instant after turnaround: your direction of motion has changed, but zero seconds have elapsed since turnaround, zero distance covered, and zero time since the instant before turnaround. Both speeds relative to the (inertial) home world line are identical.

First, note that an inertial frame for your away motion centered on the turnaround, there is some event e1 on the home world line that you consider simultaneous. For the inertial frame centered on the turnaround for your return motion, there is some event e2 considered simultaneous. e2 is later than e1 on the home world line.

1) The instant comoving convention: you say the turnaround instantly changed home event you consider simultaneous from e1 to e2. This convention can be generalized to Fermi-Normal coordinates in GR.

2) The radar convention: You must know your future motion to determine simultaneity for your current moment. You determine simultaneity for the turnaround based on a signal sent in your past that reaches the home world line, then reflects back to your future world line, such that the turnaround is the midpoint in proper time along your world line between the sending event and the (future) receiving event. Note that this convention starts smoothly differing from (1) before the turnaround. At the turnaround (for the simple case I have described), home world line event simultaneous to the moment after turnaround is mid way between e1 and e2. This convention generalizes to radar coordinates in GR.

3) The third convention says the turnaround has no effect on your simultaneity until you begin to travel some way in the new direction. In particular, you use the inertial frame corresponding to an inertial path from your trip starting point (you must make some choice for this) to your current position. Your simultaneity effectively reflects your total divergence from inertial motion. For my symmetric turnaround scenario, the moment after turnaround, the simultaneous event remains e1. (I've never seen anyone build a GR coordinate system on this basis, but it could be done, with some uniqueness issues if you try to make the coordinate patch too large in strongly curved regions).

The total parameter space for such conventions applied at turnaround runs from an event before e1 (the 'first' event beyond your past light cone on the home world line), to an event past e2 (the 'last' event before your future light cone on the home world line). Note, the instant turnaround has no effect whatsoever on your past and future light cones at that moment.

So, for teleportation, (1) says you would teleport to e2 after turnaround, (2) says midway between e1 and e2, while (3) says you would teleport to e1. Note that (1) says only your current 4-velocity determines where you teleport to. (2) says both your past and future (if you hadn't teleported) affect where you end up. (3) says your past but not your future influences where you end up. (3) is also dependent on an arbitrary choice - your world line starting point.

 

[sam_bell]

Commenting on the original post:

Teleporting is really too much. Two teleports could then get you to ANY point in spacetime. You could be everywhere, anytime. A key point in your argument is that it is unclear what time you arrive at when you return. But I don't think you're being steadfast to the philosophy that the laws of physics are invariant in all frames (i.e. similar teleport possible in all frames).

Perhaps faster than light travel can exist in 1D if you can never slow down less than the speed of light (and so never turn around). Similar to how you can't travel back in time. .. And maybe in higher dimensions too, but I don't know how to frame the constraint there.

 

[DrSnarl]

(4) All clocks read zero. This is frame independent. According to home rocket,'your clock' is now in its PAST, but still reads zero. According to "your perspective", all clocks are still zero and in the PRESENT - "your clock" is still in same frame as home stationary clock, just teleported.

(5) All clocks still read zero for everyone (we are assuming effectively instant accelerations and effectively zero time betweein (4) and (5)). "your clock" is in past compared to "home rocket", and "home clock" is now in future compared to "away clock" (and also according to "your clock"). But they all stil read zero. Note, home rocket thinks teleport was into past from its 'present', away rocket (now that it is going .9c) thinks teleport was to its present from its future.

(6) Teleport back, in away rocket's frame, to away rocket's now simultaneous point on home world line. This will bring you to, say, -1 on this world line. What everyone agrees: your orginal world line ended at its reading of zero. Your 0 time self is now located at -1 on the home world line. Your past self clock reads -1, your coincident present self clock reads 0. All agree on this. Home rocket and away rocket both think the second teleport was a simultaneous teleport (in (5), they both considered the first teleport to be in the past direction). Home stationary observer thinks the second teleport was into the past.

You are right.

In the first thought experiment, I was not treating the time shift due to acceleration as being frame invariant (as you and others pointed out, but I now understand what you meant). In the second thought experiment, I for some reason decided that accelerating causes remote observers to think you traveled in time, which is incorrect.

So it seems that unless you invoke some kind of science fiction type sub-space (that has the property of normalizing time everywhere when you enter it), then SR does indeed forbid FTL travel or information transfer.

Thank you everyone who responded in this thread (especially you PAllen - you're awesome!).

 

[DrSnarl]

...then SR does indeed forbid FTL travel or information transfer.

Or more accurately, SR does indeed dictate that FTL travel or information transfer IS time travel, as we have been discussing.

 

[parthchavo]

First of all you cannot travel at the speed of light or faster than it is because the equations of relativity itself forbid it. When you travel with a velocity v with respect to something, mass increases by underoot(1-v^2/c^2), time speeds up by underoot(1-v^2/c^2) and length contracts by underoot(1-v^2/c^2). So if we add c or something greater in the equation, all the equations of mass, time and length will fall apart. This will mean the collapse of everything around you.

 

[parthchavo]

I would also like to point out that teleportation is not forbaded by relativity. It is actually quantum mechanics which allows it because of the connection and pairing of particles. If somehow we could feed all the information about the quantum state of particles in your body in a computer, then transfer that information to another computer which would rebuild your body with the particles paired to the particles of your body, then you would have actually been teleported. Amazingly a physicist in Hawaii actually teleported a photon from one island to another.

 

[DaveC426913]

I would also like to point out that teleportation is not forbaded by relativity. It is actually quantum mechanics which allows it because of the connection and pairing of particles. If somehow we could feed all the information about the quantum state of particles in your body in a computer, then transfer that information to another computer which would rebuild your body with the particles paired to the particles of your body, then you would have actually been teleported.

There's been a bit of confusion. When we've been talking about teleportation, we've been talking about it as instant - or at least faster than light. The observer is here, then the observer is there, instantly.

What you are talking about is collecting data about something and then sending it by conventional slower-than-light communications to another place and then reassembling it.

 

[parthchavo]

Oh I get it now, no I don't think that is possible. It could have been possible if we could travel faster or at the speed of light, since in that case time would completely stop with respect to us. But we know we can't, so there's no point in talking about it.

 

[DrSnarl]

(4) All clocks read zero. This is frame independent. According to home rocket,'your clock' is now in its PAST, but still reads zero. According to "your perspective", all clocks are still zero and in the PRESENT - "your clock" is still in same frame as home stationary clock, just teleported.

OK, I now remember why I thought that accelerating causes a remote observer to think you've traveled through time. I thought that the resulting time shift depending on distance was what created the perception of length contraction.

So after step 4), wouldn't "home rocket" be in the future from your perspective (with CLOCK=FUTURE) and not in the PRESENT? It seems like this would be necessary or else you would not perceive "home rocket" as being length contracted. By the same reasoning, after you have teleported, it should appear that home rocket is no longer home, but much closer to you.

 

[PAllen]

OK, I now remember why I thought that accelerating causes a remote observer to think you've traveled through time. I thought that the resulting time shift depending on distance was what created the perception of length contraction.

So after step 4), wouldn't "home rocket" be in the future from your perspective (with CLOCK=FUTURE) and not in the PRESENT? It seems like this would be necessary or else you would not perceive "home rocket" as being length contracted. By the same reasoning, after you have teleported, it should appear that home rocket is no longer home, but much closer to you.

Your accelerating will never cause someone else to conclude anything about you other than:

- your clock rate has changed (not theirs)
- your rocket/rulers etc. have changed in length (not theirs).

SR is strange at first exposure, but not nearly as strange as you propose. Nobody else's clocks, rulers, or division of past and future change by virtue of what you do.

As for 'your perception' of home rocket in step (4) of your scenario, I am using your definition of teleporting as a pure shift in in position in your rest frame, with no other effects. So, in (4) you have teleported, but are still in the same rest frame. That the home rocket has started accelerating has no consequence for you at all - except for effects like I noted above.

The accelerating rocket does experience home rest frame distances as changing, and (using conventional SR simultaneity) a shift in what home rest frame events are in its future versus its past. It is the accelerating observer that experiences the changes, not any inertial observer.

You seem to have a number of things about SR backwards.

 

[Austin0]

The difference from classical correlation is that your results are influenced not only by the set up of the state, but also by decisions made by the other experimenter. What prevents signalling, according to current understanding, is that you cannot actually deduce the other experimenter's decisions from your results. You can only find out that they mattered later by comparing results. Why you can't specify a plausible causality hypothesis is that it is frame dependent which measurement occurred first (if they are done far enough apart).

Hi Somhow I missed your reply until now.

I understand regarding the necessity of transferring information after the fact to establish the results.
And of course generally, correlation dosn't necessarily imply causation.
But in this instance, if their is repeatable correlation between the actions of the experimanters and the results, I don't see why establishing temporal order is a necessary requirement for an inference of a causal connection.
In this thread you are positing FTL translation violating temporal order, but do you think this implies there is no causality happening?

 

[DrSnarl]

Your accelerating will never cause someone else to conclude anything about you other than:

- your clock rate has changed (not theirs)
- your rocket/rulers etc. have changed in length (not theirs).

If that is true, then consider what would happen when a rocket on the far side of a distant wall accelerates away from you. Due to Lorentz contraction, it would appear to you as though the rocket contracted backwards THROUGH the wall.

If, on the other hand, Lorentz contraction was just an illusion caused by relativity of simultaneity, you would instead see the rocket accelerate gradually (ie. slower than it really was accelerating) as it shifted through time from your perspective, with the back of the rocket seeming to accelerate slightly faster than the front. While this was happening, you would also see their clock moving much more slowly than yours - slower than could be accounted for with time dilation (ie. it would be moving into your future). Once they stopped accelerating from your perspective, their clock would speed up to the normal dilated rate, and their length would not contract any further.

In this second scenario, you would not see the rocket come back through the wall, even if the acceleration was instantaneous. (That would be bizarre - you would actually see the rocket disappear, then reappear in your future fully length contracted and without having moved before flying away.)

 

[PAllen]

Hi Somhow I missed your reply until now.

I understand regarding the necessity of transferring information after the fact to establish the results.
And of course generally, correlation dosn't necessarily imply causation.
But in this instance, if their is repeatable correlation between the actions of the experimanters and the results, I don't see why establishing temporal order is a necessary requirement for an inference of a causal connection.
In this thread you are positing FTL translation violating temporal order, but do you think this implies there is no causality happening?

Which action caused which consequence? Causality, as I understand it, has a direction. If you want to define some idea 'causal linkage', without specification of what caused what, then I might well agree with it, but would not call it causality.

As for FTL, I have tried to show that (if SR remains true), there is inherently the possibility of causal paradoxes despite Novikov (complex information without cause). Therefore, I personally don't believe it will ever happen.

 

[DrSnarl]

(That would be bizarre - you would actually see the rocket disappear, then reappear in your future fully length contracted and without having moved before flying away.)

Actually, the entire rocket would not reappear at once; you would see the back appear before the front.

 

[PAllen]

If that is true, then consider what would happen when a rocket on the far side of a distant wall accelerates away from you. Due to Lorentz contraction, it would appear to you as though the rocket contracted backwards THROUGH the wall.

No, this is just plain false. If the wall is in your frame, the distance between you and the back of the rocket is always increasing, the distance to the wall never changes, and the distance between the wall and the back of the rocket always increases. The only thing length contraction predicts is that the (if the rocket maintains same length in its own frame), the front will accelerate less than the back, such the rocket is getting shorter. What, exactly, have you studied on SR? These are the fundamentals you are misunderstanding. It really seems you should read one of the basic books on SR (or Einstein's paper on line) before making pronouncements.

[Edit: several of your other statements in the post I replied to were also wrong, but this is the starting point, indicating that you need to study/review the fundamentals.]

 

[DrSnarl]

No, this is just plain false. If the wall is in your frame, the distance between you and the back of the rocket is always increasing, the distance to the wall never changes, and the distance between the wall and the back of the rocket always increases. The only thing length contraction predicts is that the (if the rocket maintains same length in its own frame), the front will accelerate less than the back, such the rocket is getting shorter. What, exactly, have you studied on SR? These are the fundamentals you are misunderstanding. It really seems you should read one of the basic books on SR (or Einstein's paper on line) before making pronouncements.

[Edit: several of your other statements in the post I replied to were also wrong, but this is the starting point, indicating that you need to study/review the fundamentals.]

Let's place the rocket and the wall at distance "x". After acceleration has finished, the perceived distance of the rocket should be:

x' = γ(1-v/c)x​

x' will be less than x - in other words, the rocket will have moved backwards through the wall in your perception.

This is flat out wrong, as you said, but it seems to be what you get if you simply treat Lorentz contraction as physically real and not just a symptom of relativity of simultaneity.

Regarding my background, since you brought it up, I am obviously not a physicist (I am an engineer). I was exposed to relativity at a cursory level in school (ie. here are the equations for the transformations, etc.), but I have recently begun to try to understand the resolution of the various paradoxes which I have come across. This has led me to read up on various explanations of the theory - some better than others. I posted here because I was finding that available literature was not addressing my questions, and one cannot engage in dialogue with a book.

 

[Austin0]

Teleportation trivially leads to time travel in SR as follows (again, also assuming principle of relativity - same laws in all inertial frame):

1) Teleport to a rocket traveling at .9c. Pure coordinate shift in the home frame.2) Teleport back home from the rocket. Pure coordinate shift in the rocket frame.

You arrive home before you left. Teleportation is pure FTL. How can it be different from tachyons in the phenomena it allows?

Hi
Certainly after the fact , when you suddenly appear on the rocket, an observer in your initial frame can establish an event. Pure coordinate shift wrt your frame.

But before the fact: You have decided on a destination within the rocket frame but how do you determine where and when that point is at the instant you depart??

Right off the bat you have two contradictory locations depending on the frame.

Is there some characteristic of teleportation that would indicate the preferred frame on which to base your arrival time and place ?
Which is the actually simultaneous location??

Or in fact any reason to suppose that either location is absolutely simultaneous with your location at that instant?

If you choose the initial frame for calculations , do you believe that conventionally synchronized clocks are actually or absolutely simultaneous??

That if you teleport to a distant location in this same frame you will arrive there with the same local proper time reading as your departure point?

If this is so then would it not follow that instantly returning to your initial location you would also arrive there at the same proper time ??

So if we assume that that distant location was proximate to the spaceship then the round trip would still take no proper time wrt home.

If we take the further step of simply moving the destination from right outside the ship to inside it for a momentary appearence , what justification could there be for now assuming that the arrival home would occur before leaving??

That the phenomenon of teleportation would be affected by the synchronicity of the ships clocks . That the destination of its instantaneous translation would now be determined by a different set of absolutely synchronous clocks?

0f course overlooked in this discussion is the factor that teleportation into a ship at 0.9c would be messy for any time over an instantaneous appearence as you would still be going 0.9c relative to the ship , ouch.

If on the othr hand you don't assume the initial frames clocks are absolutely synchronized then what basis do you have for guessing what the local arrival time would be for even an inframe hop let alone one to another frame ?

 

[PAllen]

Let's place the rocket and the wall at distance "x". After acceleration has finished, the perceived distance of the rocket should be:

x' = γ(1-v/c)x​

x' will be less than x - in other words, the rocket will have moved backwards through the wall in your perception.

This is flat out wrong, as you said, but it seems to be what you get if you simply treat Lorentz contraction as physically real and not just a symptom of relativity of simultaneity.

Regarding my background, since you brought it up, I am obviously not a physicist (I am an engineer). I was exposed to relativity at a cursory level in school (ie. here are the equations for the transformations, etc.), but I have recently begun to try to understand the resolution of the various paradoxes which I have come across. This has led me to read up on various explanations of the theory - some better than others. I posted here because I was finding that available literature was not addressing my questions, and one cannot engage in dialogue with a book.

Ok, you don't understand the meaning of the x and x' in these equations. x,t describe events in one frame. Having specified this, you use Lorentz transform to see how these would look in another frame (x',t'). Given some inertial frame seeing some sequence of events, x' is never used to describe what this inertial frame sees. It is used to transform to what would be seen by some other inertial frame. If you want to posit accelerated frames, that is a whole other ball of wax, which I wrote several long posts about in this thread - but you are not really ready for that until you understand inertial frames. (Further, I don't really recommend ever using accelerated frames in special relativity).

To get at the length contraction of an accelerating rocket, the correct approach is to define Born rigidity for object in general. However, an adequate approach is to require that the length of the rocket stays the same in the instantaneously comoving inertial frame of the back of the rocket. Then you would find (as I said before) that (in the inertial frame) the front of the rocket would accelerate less than the back, and length of the rocket (in the inertial frame) would get shorter. However, at all times, (in the inertial frame) the front and back of the rocket are always accelerating, moving away from some starting point. There is no shrinkage of distance to the rocket.

I really suggest, as a starting point, read part I of:

http://www.fourmilab.ch/etexts/einstein/specrel/www/

then come back and ask more questions.

 

[PAllen]

Hi
Certainly after the fact , when you suddenly appear on the rocket, an observer in your initial frame can establish an event. Pure coordinate shift wrt your frame.

But before the fact: You have decided on a destination within the rocket frame but how do you determine where and when that point is at the instant you depart??

Right off the bat you have two contradictory locations depending on the frame.

Is there some characteristic of teleportation that would indicate the preferred frame on which to base your arrival time and place ?
Which is the actually simultaneous location??

Or in fact any reason to suppose that either location is absolutely simultaneous with your location at that instant?

If you choose the initial frame for calculations , do you believe that conventionally synchronized clocks are actually or absolutely simultaneous??

That if you teleport to a distant location in this same frame you will arrive there with the same local proper time reading as your departure point?

If this is so then would it not follow that instantly returning to your initial location you would also arrive there at the same proper time ??

So if we assume that that distant location was proximate to the spaceship then the round trip would still take no proper time wrt home.

If we take the further step of simply moving the destination from right outside the ship to inside it for a momentary appearence , what justification could there be for now assuming that the arrival home would occur before leaving??

That the phenomenon of teleportation would be affected by the synchronicity of the ships clocks . That the destination of its instantaneous translation would now be determined by a different set of absolutely synchronous clocks?

0f course overlooked in this discussion is the factor that teleportation into a ship at 0.9c would be messy for any time over an instantaneous appearence as you would still be going 0.9c relative to the ship , ouch.

If on the othr hand you don't assume the initial frames clocks are absolutely synchronized then what basis do you have for guessing what the local arrival time would be for even an inframe hop let alone one to another frame ?

I am not going to get involved in answering all these points. Most, have been dealt with, in passing, in this thread. The OP proposed a hypothetical model of teleportation which, it happens, is effectively the zero energy limit of tachyons, for which there is well established theory. That is what I used for analysis. Obviously, in such thought experiments, one ignores issues like teleporting to the inside of a rapidly moving spaceship and then surviving 'instant' acceleration to .9c as you hit the floor/wall of the ship.

No physicist I know of would take teleportation seriously; many, if not most, doubt tachyons. However, the intent of the OP was to inquire whether, and if so, why, SR would equate FTL and/or teleportation to time travel. That question is well defined once you define your flavor of FTL or teleportation - and the OP, at various points, did define their conception enough so it it could be analyzed.

 

[Austin0]

Which action caused which consequence? Causality, as I understand it, has a direction. If you want to define some idea 'causal linkage', without specification of what caused what, then I might well agree with it, but would not call it causality.

As for FTL, I have tried to show that (if SR remains true), there is inherently the possibility of causal paradoxes despite Novikov (complex information without cause). Therefore, I personally don't believe it will ever happen.

Of course I would agree causality as we know and understand it in the real world has direction.
But here we are discussing FTL and EPR so in one case we're out of this world completely and in the other case into a new real world.

In your Shakspeare scenario (great BTW) wouldn't you agree that the arrival of the manuscript back in time was a purely causal consequence of your action?? That Shakespeare
wrote/transcribed it was the cause of your consequent action?
SO what caused what? And if this is indeterminate does this mean there was no causality at all??
In the case of the entangled photons: It is true we cannot say in a frame independent way which occurred first but there are really only two possibilities.
In the first: A precedes B = normal causality is preserved.
In the second: B precedes A and causality is reversed . Demonstrating the very premise of the FTL +SR =time travel hypothesis.
I agree regarding the improbability of FTL or teleportation.

 

[yuiop]

Let's place the rocket and the wall at distance "x". After acceleration has finished, the perceived distance of the rocket should be:

x' = γ(1-v/c)x​

x' will be less than x - in other words, the rocket will have moved backwards through the wall in your perception.

This is flat out wrong, as you said, but it seems to be what you get if you simply treat Lorentz contraction as physically real and not just a symptom of relativity of simultaneity.

This is wrong, but not for the reasons you think. Where did you get the equation? It is not the Lorentz transformation and it is not the length contraction equation.

 

[PAllen]

Of course I would agree causality as we know and understand it in the real world has direction.
But here we are discussing FTL and EPR so in one case we're out of this world completely and in the other case into a new real world.
.

Well, entanglement is very much of the world we live in. Further, I think it tells us something very fundamental about our world.

In your Shakspeare scenario (great BTW) wouldn't you agree that the arrival of the manuscript back in time was a purely causal consequence of your action?? That Shakespeare
wrote/transcribed it was the cause of your consequent action?
SO what caused what? And if this is indeterminate does this mean there was no causality at all??
.

If you use an entropic definition of time ordering, as Demystifier suggested (and I agree), then it is clear that the text of the play appeared at some point in 1600s (without authorship or origin), then Shakespeare transcribed it, then someone in the future sent it back in time). Of course, I didn't invent this scenario, I don't know who first proposed it. Brian Greene has argued, that like it or not (he likes it more than me, I think), such causeless information is a necessary possibility in GR, and Novikov does nothing to relieve this.

In the case of the entangled photons: It is true we cannot say in a frame independent way which occurred first but there are really only two possibilities.
In the first: A precedes B = normal causality is preserved.
In the second: B precedes A and causality is reversed . Demonstrating the very premise of the FTL +SR =time travel hypothesis.
I agree regarding the improbability of FTL or teleportation.

How on Earth do you distinguish that A precedes B is normal causality??!