Does SR actually forbid FTL travel?

[PAllen]

Ah, so I misunderstood you. Now it is clear that t0 is the earliest moment of time at which the text of the book exists. So, I think your paradox is - who is the true author of the book? I would say, and I think you would agree, that the answer is: Nobody - it just appeared spontaneously at t0. Shakespeare is only the first guy who discovered it, but it existed even before his discovery.

Exactly.

 

[Whovian]

Ah, so I misunderstood you. Now it is clear that t0 is the earliest moment of time at which the text of the book exists. So, I think your paradox is - who is the true author of the book? I would say, and I think you would agree, that the answer is: Nobody - it just appeared spontaneously at t0. Shakespeare is only the first guy who discovered it, but it existed even before his discovery.

This raises another question. Say someone's capable of time travel. They have a device which they give to their friend to free themselves in their own past. Their friend then frees the first person and gives the device to the first person ... and the first person accidentally gives the one their friend received from themselves to their friend, instead of the one they had all along which they were planning to give to their friend. How old is that device? Shouldn't it have crumbled to dust by now?

 

[DrSnarl]

This raises another question. Say someone's capable of time travel. They have a device which they give to their friend to free themselves in their own past. Their friend then frees the first person and gives the device to the first person ... and the first person accidentally gives the one their friend received from themselves to their friend, instead of the one they had all along which they were planning to give to their friend. How old is that device? Shouldn't it have crumbled to dust by now?

I think this and other questions of time travel do not have to be paradoxes. It is reasonable to state that any time loop will result in multiple, overlapping spaces that are related only in that they share the same time. There is no reason why such a loop could not be infinite, resulting in an infinite number of overlapping spaces.

Causality is thus preserved. In your scenario, the device will eventually crumble, and there will be no more time loops (at least not at that point in space-time).

In the Shakespeare play scenario, Shakespeare wrote the play. However, in one set of space coordinates, he essentially received the play from the overlapping version of himself (the one located in the overlapping space).

I suppose you could call these spaces "multiple dimensions", but that is kind of misleading, as they do not have to be global. There is also no reason (philosophically speaking, at least) why these overlapping spaces could not re-merge in the future. In other words, there is no issue with the Shakespeare who received the play via time travel meeting up with the Shakespeare who wrote it.

 

[DrSnarl]

One technique for limiting the impact of paradoxes is to propose, e.g. the Novikov Consistency conjecture (see, for example http://en.wikipedia.org/wiki/Novikov_self-consistency_principle ).

Novikov Self-Consistency seems like a bit of a stretch to me. What would happen in the Shakespeare scenario? Would it be like the movie "Final Destination", where "fate" just seems to have your number, foiling you at every turn as you try to send the play back in time? It may be logically consistent, but it seems pretty fanciful.

 

[Whovian]

Yea, I don't like Novikov. I've thought of sort of the general concept of what could become a theory that sort of brings Novikov and the multiple universe theory together, or, at least, gives a reason for Novikov, but if all my previous modifiers haven't notified you, it's highly speculative and actually yields no testable predictions that aren't yielded by Novikov.

 

[DaveC426913]

In the Shakespeare play scenario, Shakespeare wrote the play. However, in one set of space coordinates, he essentially received the play from the overlapping version of himself (the one located in the overlapping space).

Yes, I've heard of this hypothesis. Essentially, traveling into the past is not traveling into your own past, it is traveling into a past that was identical to yours, but that no longer will be. I guess that's tantamount to splitting off a whole universe when you time travel into the past.

 

[PAllen]

In the context of SR with tachyons (and my author-less Hamlet scenario can readily be constructed this way), the problem with universe splitting as a solution (rather than Novikov + you must accept causeless information), is that every event of tachyon creation or absorption must split the universe, because any such event represents time travel for some class of inertial observers. Also, there is no model of universe splitting in either SR or GR, so you must add the phenomenology of this. So, pick your poison if you want FTL in SR or GR. To me, the more parsimonious theory is clearly Novikov + acceptance of causeless entitities. However, my honest opinion is that I don't accept author-less plays, and therefore I believe:

- tachyons will never be observed
- a successor to GR will avoid CTC's, singularities (note: I don't believe GR as a physical theory, even its current form, really allows wormholes or alcubierre drive because of the negative energy requirement; but CTC's and singularities occur without negative energy).
- no form of FTL (=time travel) will ever exist.

 

[DrSnarl]

In the context of SR with tachyons (and my author-less Hamlet scenario can readily be constructed this way), the problem with universe splitting as a solution (rather than Novikov + you must accept causeless information), is that every event of tachyon creation or absorption must split the universe, because any such event represents time travel for some class of inertial observers. Also, there is no model of universe splitting in either SR or GR, so you must add the phenomenology of this. So, pick your poison if you want FTL in SR or GR. To me, the more parsimonious theory is clearly Novikov + acceptance of causeless entitities. However, my honest opinion is that I don't accept author-less plays, and therefore I believe:

- tachyons will never be observed
- a successor to GR will avoid CTC's, singularities (note: I don't believe GR as a physical theory, even its current form, really allows wormholes or alcubierre drive because of the negative energy requirement; but CTC's and singularities occur without negative energy).
- no form of FTL (=time travel) will ever exist.

Everything you said makes sense. I agree completely regarding tachyons (along with the prediction that they will never be observed). However, your third assertion (that any form of FTL = time travel in some inertial frame) brings us full circle.

I still do not understand how SR equates teleportation to time travel. Set aside for the moment that there is no known mechanism for teleportation; I am trying to figure out why SR says that (unless you allow for time travel) such a mechanism CANNOT exist.

Just so we are discussing the same thing, by "teleportation", I mean an instantaneous coordinate shift in space while preserving a sub-c frame of reference. This is an important distinction. I completely understand why tachyons would result in time travel.

It seems to me that SR does not allow for teleportation, not because it violates causality, but because the result would create the unresolvable paradox described in the thought experiment with which I started this thread. Another way of looking at it is that SR does not tell us anything about what would happen if we teleported.

Personally, I think the idea of teleportation is far fetched. However, if a science fiction author were to postulate undiscovered physical laws that allowed for teleportation, would those laws conflict with SR by very definition, regardless of what they were? I am picking on teleportation, but the same argument applies to any FTL coordinate shift while in a sub-c reference frame.

 

[PAllen]

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?

However, teleportation is really much more absurd than tachyons. There is no plausible theoretical framework for it that I've seen for it (while there are, up to a point, consistent theoretical frameworks for tachyons and GR time travel options). Note that so called quantum teleportation does not involve transmission of matter, nor does it even allow sending FTL messages.

In any case, my position is that neither SR per se, and certainly not GR, preclude certain fanciful FTL/time travel(or message) scenarios. What they do say is that FTL + relativity principle => time travel, with the concomitant choices (and I don't distinguish between time travel by messages versus matter; the choices for dealing with the results are the same).

I don't see any unresolvable paradoxes with teleportation=FTL=time travel - just choices I find implausible.

 

[DrSnarl]

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?

I think that thought experiment does not appear to create a paradox because we are only considering one frame of reference. Add a rocket at home that is also traveling at .9c, and now you have a problem. Consider the following additions to your experiment:

1) You have three clocks. One you keep with you (your clock). One you leave at home (home stationary clock). One you put on your home rocket (home rocket clock - not the distant rocket that we will teleport to).
2) Synchronize all these clocks so that they read 0.
3) Accelerate the home rocket to 0.9c (towards the away rocket). All clocks still read 0.

4) Teleport to the away rocket. Let's go ahead and accelerate the away rocket after you get there so that you don't instantly accelerate to 0.9c by splattering on the back of the spaceship. (I know, all of our accelerations are instantaneous, so why should this bother us now?)
- After teleporting (but before accelerating), here is what the clocks will read:
From "home stationary perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = 0
From "home rocket perspective":
----home stationary clock=0
----home rocket clock=0
----your clock=FUTURE
From "your perspective":
----home stationary clock=0
----home rocket clock=FUTURE
----your clock=0

5) Now, accelerate in the "away rocket" to 0.9c away from home. Here are the new clocks:
From "home stationary perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = PAST
From "home rocket perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = 0
From "your perspective":
----home stationary clock = PAST
----home rocket clock = 0
----your clock = 0

6) Finally, teleport back home. Here are the new clocks:
From "home stationary perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = PAST
From "home rocket perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = 0
From "your perspective":
----home stationary clock = PAST
----home rocket clock = 0
----your clock = 0

If you only look at the "home stationary clock", then it would appear that you traveled back into the past. However, the home clock also thinks that it traveled into your past. Additionally, the home rocket thinks that nobody traveled into anybody's past.

According to SR, once you are back home, everyone should agree on the value of everyone else's clocks (they don't have to be the same values, but they should agree on what those values are).

Which clock values are incorrect?

 

[PAllen]

I think that thought experiment does not appear to create a paradox because we are only considering one frame of reference. Add a rocket at home that is also traveling at .9c, and now you have a problem. Consider the following additions to your experiment:

1) You have three clocks. One you keep with you (your clock). One you leave at home (home stationary clock). One you put on your home rocket (home rocket clock - not the distant rocket that we will teleport to).
2) Synchronize all these clocks so that they read 0.
3) Accelerate the home rocket to 0.9c (towards the away rocket). All clocks still read 0.

4) Teleport to the away rocket. Let's go ahead and accelerate the away rocket after you get there so that you don't instantly accelerate to 0.9c by splattering on the back of the spaceship. (I know, all of our accelerations are instantaneous, so why should this bother us now?)
- After teleporting (but before accelerating), here is what the clocks will read:
From "home stationary perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = 0
From "home rocket perspective":
----home stationary clock=0
----home rocket clock=0
----your clock=FUTURE
From "your perspective":
----home stationary clock=0
----home rocket clock=FUTURE
----your clock=0

5) Now, accelerate in the "away rocket" to 0.9c away from home. Here are the new clocks:
From "home stationary perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = PAST
From "home rocket perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = 0
From "your perspective":
----home stationary clock = PAST
----home rocket clock = 0
----your clock = 0

6) Finally, teleport back home. Here are the new clocks:
From "home stationary perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = PAST
From "home rocket perspective":
----home stationary clock = 0
----home rocket clock = 0
----your clock = 0
From "your perspective":
----home stationary clock = PAST
----home rocket clock = 0
----your clock = 0

If you only look at the "home stationary clock", then it would appear that you traveled back into the past. However, the home clock also thinks that it traveled into your past. Additionally, the home rocket thinks that nobody traveled into anybody's past.

According to SR, once you are back home, everyone should agree on the value of everyone else's clocks (they don't have to be the same values, but they should agree on what those values are).

Which clock values are incorrect?

I am not going analyze all your verbiage above. It's all unnecessary distraction. This repeats stuff others have commented on. I don't need to deal in clocks at all. The space time path for the the teleportation round trip connects one event on a timelike world line to an earlier event on a timelike world line. Different frames will have different views on which teleportation went back in time and by how much, but all will agree on the order of events on a timelike worldline, and all will agree that your two teleportations have you arrive at an event your world line before you left. All will agree with the amount you have gone back along your initial timelike world line.

[edit: taking a quick look at your write, the thing that is just plain wrong is that there is never frame dependence on what any actual clock reads, however overly complex you make your scenario. There is disagreement only on the order teleportation end point events and coordinate time differences between these endpoints. By construction, no clock time passes for a teleported clock. I would have to correct most of what you wrote, and I am not willing to bother.]

 

[DrSnarl]

I am not going analyze all your verbiage above. It's all unnecessary distraction. This repeats stuff others have commented on. I don't need to deal in clocks at all. The space time path for the the teleportation round trip connects one event on a timelike world line to an earlier event on a timelike world line. Different frames will have different views on which teleportation went back in time and by how much, but all will agree on the order of events on a timelike worldline, and all will agree that your two teleportations have you arrive at an event your world line before you left. All will agree with the amount you have gone back along your initial timelike world line.

I certainly don't fault you for not wanting to analyze my "verbiage"; in fact, at admire your willingness to continue in this thread at all.

I'll try to be as concise as possible: observers in all frames must agree not only on the order of events, but also exactly on how far back in time you went, if all observers are co-located with you. Your thought experiment shows that they do not. I merely took your thought experiment and listed out clock values to illustrate this.

Perhaps the extra rocket was a distraction. Here is the basic problem: when you teleport home, you travel into your home's past, but your home also travels into your past. That is obviously bogus.

 

[PAllen]

I certainly don't fault you for not wanting to analyze my "verbiage"; in fact, at admire your willingness to continue in this thread at all.

I'll try to be as concise as possible: observers in all frames must agree not only on the order of events, but also exactly on how far back in time you went, if all observers are co-located with you. Your thought experiment shows that they do not. I merely took your thought experiment and listed out clock values to illustrate this.

Perhaps the extra rocket was a distraction. Here is the basic problem: when you teleport home, you travel into your home's past, but your home also travels into your past. That is obviously bogus.

1) All observers need not and do not agree on the order of events with spacelike separation (e.g. the endpoints of a teleportation). Observers need not agree on the amount of coordinate time difference between these spacelike separated events (could be 0, -5, + 10 depending on observer). All of this is fine and normal for SR.

2) All observes do agree on order and amount of proper time between events on a timelike world line. Thus all agree (despite different interpretations of what each teleport represented), that e.g. you left with your clock reading 5:00 pm, and arrived at a point on your world line where your past copy has 4:00 pm. You and your past self are next to each other. You have 5:00, they have 4:00. All frames agree on this.

 

[Austin0]

On the last paragraph, nonsense. No one knowledgeable says entanglement can't transfer information FTL because it would violate SR. Instead they say entanglement can't transfer information because the actual mathematics of entanglement says it can't. Period. Note, especially, that you can't even verify that you have successfully produced entanglement without transferring information from one place to another by some other means. Then, (long) after the fact, you can verify you successfully entangled interactions.

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?.

 

[PAllen]

Ok, I had some time to fool with this. Taking your scenario, clarifying that rocket movement is in +x direction and teleportation is in +x direction. I will use 'your' exactly where you did. Then my disagreement starts at your (4): (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.

There are no discrepancies about what clocks read, or about sequence and time differences on the home, timelike world line. There are only SR typical differences on the interpretation of events with spacelike relation.

 

[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.