Does SR actually forbid FTL travel?

[PAllen]

Although everything you have said here is true , isn't it also true that:
if the findings developed after the fact, that entangled interactions have occurred, are accurate, then this seems to infer non-local instantaneous transfer of information.
Not at all useful for human communication of information , for the reason you pointed out, but information just the same . On the particle level a transmission of a change of state of some kind from one location to another.
Am I missing or misinterpreting some aspect of the experiments?.

Correlation is not information. You may have a mental model that correlation requires causation or information transfer, but that is not part of the quantum entanglement model. The parsimonious interpretation is that there exists causeless distant correlation. [Edit: better: there exists a single non-local measurement of an entangled state that is classically interpreted as two correlated distant measurements. And now we can ... but I won't... get into the various quantum interpretations.]

 

[Austin0]

Does SR actually imply that FTL travel would allow for violations of causality? Here is some reasoning to suggest that it does not:

Any thoughts?

Here's a different perspective:
IMHO SR has nothing to say about FTL as it is simply outside it's domain of applicability.
FTL values entered in the Lorentz math produce imaginary results.
Entered into the Addition of V's formula they produce nonsensical, inconsistent results and consequences.
The clocks in SR are synchronized using light speed so any readings or predictions with regard to an FTL particle have no real basis. All interpretations of time ordering of disparate events is based on this particular synchronicity. But this is not taken to be actual simultaneity but only operational ordering of events which of course works fine for c and below.
But is it reasonable to think this will automatically apply to imaginary particles or velocities?

Using your example of teleportation: The concept is of instantaneous translation between two points. Absolute simultaneity of occurrence at separate locations. How can SR predict when that instant will occur at a location in another frame?
That would only be possible if the clocks in the initial frame were absolutely synchronous.
But we know that is not the case by the principles of SR itself.
SO for me, expecting SR to meaningfully make predictions regarding FTL is equivalent to thinking Thermodynamics or low temperature physics will produce meaningful predictions for temperatures below 0^o K
So I guess in effect I am agreeing with you and think you can proceed with your teleportation experiments without necessarily violating causality :-)

 

[Austin0]

Correlation is not information. You may have a mental model that correlation requires causation or information transfer, but that is not part of the quantum entanglement model. The parsimonious interpretation is that there exists causeless distant correlation. [Edit: better: there exists a single non-local measurement of an entangled state that is classically interpreted as two correlated distant measurements. And now we can ... but I won't... get into the various quantum interpretations.]

It appears that I may have misunderstood the experiments. I thought that actions at one location , change in polarization for eg. displayed a statistically significant result at a separate location. This seems like cause and effect. It is correlation that explains or is the mechanism for the effect, I get that , but why would it be considered there was no causality involved.
Of am I in error regarding the actual parameters of the experiments?
Thanks

 

[PAllen]

It appears that I may have misunderstood the experiments. I thought that actions at one location , change in polarization for eg. displayed a statistically significant result at a separate location. This seems like cause and effect. It is correlation that explains or is the mechanism for the effect, I get that , but why would it be considered there was no causality involved.
Of am I in error regarding the actual parameters of the experiments?
Thanks

It doesn't matter what order you do the measurements. Further, what order they are done is frame dependent. How do you propose which way causality goes? All that you know is that if you later compare measurements, there was correlation that cannot be explained classically.

 

[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.

So am I wrong then in thinking that when you accelerate towards a distant observer, you move into its future (and when you accelerate away, you move into its past)? If that is false, then I apparently do not understand the resolution to the twin paradox.

 

[DrSnarl]

It appears that I may have misunderstood the experiments. I thought that actions at one location , change in polarization for eg. displayed a statistically significant result at a separate location. This seems like cause and effect. It is correlation that explains or is the mechanism for the effect, I get that , but why would it be considered there was no causality involved.
Of am I in error regarding the actual parameters of the experiments?
Thanks

I read about an experiment (perhaps the same one to which you are referring) when the experimenter used an elaborate setup to cause a stream of photons with unknown polarity to create an interference pattern, similar to the double-slit experiment. Using this with a stream of entangled photons, they inserted a polarizing filter into one of the streams, in effect collapsing the uncertainty in the other stream and causing the interference pattern to vanish.

Granted, they were not able to use this to communicate FTL (or it obviously would have made the news). The reason why is that they were dealing with multiple wavelengths, and they had to filter out the photons of the incorrect wavelength, which required information from both ends. However, the idea definitely seems like it merits further investigation.

 

[PAllen]

So am I wrong then in thinking that when you accelerate towards a distant observer, you move into its future (and when you accelerate away, you move into its past)? If that is false, then I apparently do not understand the resolution to the twin paradox.

Apparently you do not. The only role acceleration plays in the twin differential aging is to allow a twin to separate from and than meet the other again. Further, your motion never affects a different observer's simultaneity. 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.

 

[Austin0]

It doesn't matter what order you do the measurements. Further, what order they are done is frame dependent. How do you propose which way causality goes? All that you know is that if you later compare measurements, there was correlation that cannot be explained classically.

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??

 

[Austin0]

I read about an experiment (perhaps the same one to which you are referring) when the experimenter used an elaborate setup to cause a stream of photons with unknown polarity to create an interference pattern, similar to the double-slit experiment. Using this with a stream of entangled photons, they inserted a polarizing filter into one of the streams, in effect collapsing the uncertainty in the other stream and causing the interference pattern to vanish.

Granted, they were not able to use this to communicate FTL (or it obviously would have made the news). The reason why is that they were dealing with multiple wavelengths, and they had to filter out the photons of the incorrect wavelength, which required information from both ends. However, the idea definitely seems like it merits further investigation.

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

 

[Austin0]

Apparently you do not. The only role acceleration plays in the twin differential aging is to allow a twin to separate from and than meet the other again. Further, your motion never affects a different observer's simultaneity. 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.

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

 

[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??!

 

[Austin0]

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

I totally agree. I was just suggesting that it is the edge of a new world we are entering.
What started out ,not that long ago really ,as a reductio ad absurdum argument against certain QM concepts has turned out to be a reality that we are just beginning to empiracally explore.

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.

IMHO It is debatable whether or not entropy can be applied so directly in this fashion. Which seems to necessitate an assumption that the flow in a complex open system is going to globally, steadily increase without flux. In the case of the Earth fluctuations could occur over many millenia or longer and 400 years is a hiccup.
You can say the content of the manuscript itself was in some fashion authorless. But the physical object itself did in fact have an origin. It was sent from the future so the complete chain of events as I described applies. You can say it is paradoxical but it is still causal even if it does not follow our normal expectations of temporal ordering. No?

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

I don't distinguish. That is the point. You maintained that no inference of causality could be made because we could not establish temporal order. I.e. because we could not tell if A preceeded B or vice versa. I was simply proposing that it didn't make any difference because there were only two possibilities; normal causality or inverse causality.
So the order I presented was purely arbitrary as it doesn't make any difference which preceeds the other as we can't tell any way.
Does this make it any clearer?

 

[DrSnarl]

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.

Yes, I agree, I was wrong. The source of my error lies in how I was attempting to explain why length contraction occurs.

I think here is the basic thing I do not understand: what happens to the rocket on the other side of the wall when it accelerates? Assume instantaneous acceleration. Immediately after accelerating, from your perspective, where is the back of the rocket and the front of the rocket? If there was a clock at the back and at the front of the rocket, they would not equal each other from your perspective, would they?

Also, the equation I should have used is x'=x(sqrt(1-v^2/c^2)), but even with that, it doesn't fix the problem with my analysis.

 

[PAllen]

Yes, I agree, I was wrong. The source of my error lies in how I was attempting to explain why length contraction occurs.

I think here is the basic thing I do not understand: what happens to the rocket on the other side of the wall when it accelerates? Assume instantaneous acceleration. Immediately after accelerating, from your perspective, where is the back of the rocket and the front of the rocket? If there was a clock at the back and at the front of the rocket, they would not equal each other from your perspective, would they?

Also, the equation I should have used is x'=x(sqrt(1-v^2/c^2)), but even with that, it doesn't fix the problem with my analysis.

Immediately after acceleration, the back of the rocket will have moved closer to the front, according the length contraction formula for the now constant speed of the rocket. Your formula is for length contraction, not coordinate transformation, and is usually written L' = L sqrt(1-v^2/c^), where L is rest length and L' is contracted length. Using x just confuses matters, making it look like the Lorentz transform, which is a different set of formulas (for coordinate transformation).

So, the back of the rocket will have moved away from the wall, while the front will not have moved yet (for this 'instant' acceleration). After this instant acceleration, front and back move from here at new constant speed.

As for clocks, we now have to come back to minimal reality. SR prohibits instant acceleration of a rigid body, just as strongly as it prohibits matter moving FTL. The analysis above shows why - the back of the rocket will have moved a finite distance in zero time. For any length of rocket, you can derive a specific maximum acceleration consistent with (Born) rigid motion - that avoids any part of the rocket moving FTL. If we consider this maximum allowed acceleration (noting also that the front continues accelerating after the back has stopped - to arrive at final rigid motion at constant speed), then for clocks that started in synch on starting inertial frame, both will be end up behind corresponding inertial frame clocks, the back further behind than the front.

 

[DrSnarl]

Yes, I agree, I was wrong. The source of my error lies in how I was attempting to explain why length contraction occurs.

I do not feel so bad now - it appears that the problem I have with this rocket experiment is illustrated by the Bell spaceship paradox. A quick internet search reveals conflicting explanations that yield conflicting results. Is there a scientific consensus on whether or not the "string would break"?

 

[DrSnarl]

So, the back of the rocket will have moved away from the wall, while the front will not have moved yet (for this 'instant' acceleration). After this instant acceleration, front and back move from here at new constant speed.

The thing that is confusing about that is this: what makes the front of the rocket special? Why would the front of the rocket "not have moved yet"? If you added a nose needle to the rocket, would it now be the front of the nose needle that had not yet moved? It doesn't seem like adding something to the front of the rocket should affect the way the rocket contracts.

I was attempting to reconcile this by thinking that the entire frame of reference contracts towards the observer (hence the wall problem), and I was trying to reconcile that by explaining the contraction as an illusion caused by relative simultaneity. Somewhere in there I got off the rails.

 

[PAllen]

I do not feel so bad now - it appears that the problem I have with this rocket experiment is illustrated by the Bell spaceship paradox. A quick internet search reveals conflicting explanations that yield conflicting results. Is there a scientific consensus on whether or not the "string would break"?

There is no controversy on this. If the front and back maintain the same distance apart seen from the inertial frame, the string breaks. If the string remains taut, but under fixed tension, and no breakage occurs (by properly coordinated acceleration of its constituents), then it gets shorter during acceleration as seen from the inertial frame.

 

[PAllen]

The thing that is confusing about that is this: what makes the front of the rocket special? Why would the front of the rocket "not have moved yet"? If you added a nose needle to the rocket, would it now be the front of the nose needle that had not yet moved? It doesn't seem like adding something to the front of the rocket should affect the way the rocket contracts.

I was attempting to reconcile this by thinking that the entire frame of reference contracts towards the observer (hence the wall problem), and I was trying to reconcile that by explaining the contraction as an illusion caused by relative simultaneity. Somewhere in there I got off the rails.

Instant acceleration is not really possible. Real acceleration is limited by speed of sound in materials (displacement propagates at the speed of sound, and if more than a critical force is applied, the material will deform or break). Born rigid acceleration (such that there are no stresses in the body as it accelerates) requires each piece 'knowing' when and how to accelerate to achieve this. It is this artifice that leads to things like the front knowing when and how to accelerate compared to the back.

 

[DrSnarl]

Instant acceleration is not really possible. Real acceleration is limited by speed of sound in materials (displacement propagates at the speed of sound, and if more than a critical force is applied, the material will deform or break). Born rigid acceleration (such that there are no stresses in the body as it accelerates) requires each piece 'knowing' when and how to accelerate to achieve this. It is this artifice that leads to things like the front knowing when and how to accelerate compared to the back.

So if a rocket had side boosters near the front - so that both the front and the back were accelerating independently but at the same rate - then the rocket would break in half as the front portion and back portion were contracted away from each other?

 

[PAllen]

Here are two recent papers giving an accessible treatment of rigid body motion in SR. There are more elegant, robust, treatments involving e.g. an expansion tensor, but these papers are basically accessible at advanced high school/undergrad level, and cover all the essential ideas.

http://arxiv.org/abs/0906.1919
http://arxiv.org/abs/1105.3899

[EDIT: I should note that I, and many (but not all) knowledgeable participants on this forum, disagree with the emphasis in the first paper on length contraction. It remains very useful for understanding rigid motion, nonetheless.

My disagreements focus on the following:

1) Granting reality only to rest length, and basically claiming contracted length is analogous to 'relativistic mass' which is an increasingly disfavored concept in SR. I think this goes too far, for while relativistic mass is useless (IMO), total energy is not. To deal with the geometry of a moving object, you need a description in the inertial system. This must use some something essentially like length contraction.

2) He briefly mentions Terrell's result for a photograph of a moving sphere. He fails to mention that for non-spherical objects, the conclusions are different - photographs will show a contracted object depending on the relative position of camera and object. In particular, a photograph of a moving rod taken adjacent to its center (in the configuration known as the barn pole paradox) will show the rod completely inside the barn with both doors closed, even though the rod is longer (rest length) than any dimension of the barn (that is, longer even than any diagonal).]

 

[DrSnarl]

Here are two recent papers giving an accessible treatment of rigid body motion in SR. There are more elegant, robust, treatments involving e.g. an expansion tensor, but these papers are basically accessible at advanced high school/undergrad level, and cover all the essential ideas.

http://arxiv.org/abs/0906.1919
http://arxiv.org/abs/1105.3899

Thank you, I will take a look.