Does SR actually forbid FTL travel?

Austin0

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

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 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

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

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

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

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

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

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

DrSnarl

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

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

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

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

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

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

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"?