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Hypothetical FTL communication black-boxes and causality

  1. Mar 8, 2015 #1
    I found the thread "SR, LET, FTL & Causality Violation", looked through through all of it and read large parts. I believe I found my answer there, but to make sure, I'm asking the question here:

    Suppose we had some "magical" black-boxes that allow us to communicate information in both directions between them faster than light or even instantaneous.

    This could lead to violating special relativity causality, but not to violating causality in lorentz ether theory, is that correct ?
  2. jcsd
  3. Mar 8, 2015 #2


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    Staff Emeritus
    Science Advisor

    In special relativity, there is no single absolute notion of "simultaneous", because simultaneity in SR depends on the observer. If you'd like a reference, google for Einstein's train, or ask some more questions about the details. So you'll have to define what you mean by simultaneous in more detail, the SR notion of simultaneous would be ambiguous in this context.

    I would expect that the easiest and possibly the only way to completely specify what you mean by "simultaneous" would be to pick one particular preferred frame or coordinate system - and use the existing , traditional SR notion of simultaneity in that particular "special" frame or coordinate system. This would definitely violate the theory and spirit of special relativity, which states that there are no preferred frames.

    The details of what happens depends on how you set up this new, hypothetical theory - it's not something we can answer according to the rules of SR, because you've already broken them.
  4. Mar 8, 2015 #3
    According to what I read "lorentz ether theory" or LET is a real historical theory, that makes the same predictions like STR. (But has other axioms. It assumes absolute simultaneity, for example.)

    Isn't that true ?
  5. Mar 8, 2015 #4


    Staff: Mentor

    There is something called LET that makes the same predictions as SR, yes. And because it makes the same predictions as SR, it can't be used to answer your question in the OP, because any theory that makes the same predictions as SR cannot account for violations of relativistic causality, and your OP assumes that there are violations of relativistic causality.

    If there is some other theory that is also called "LET" but which makes different predictions from SR in certain situations, such as allowing violations of relativistic causality, then you could use that theory to analyze the scenario in your OP. But I'm not aware of any mainstream theory that does that--and if one did exist, it would most likely have been falsified by experiment, since experiments have confirmed the predictions of SR to very high accuracy, including its predictions about causality.
  6. Mar 8, 2015 #5
    I think you’re doing injustice to my question, or alternatively I didn’t express myself accurately, so I’ll try to do better:

    Would an experimental result demonstrating FTL transmission of information disprove the STR ?
    Would the same experiment disprove the LET ?

    I know STR is extensively experimentally verified. Note also, that I’m not attacking STR! Consider the following instead:

    FTL communication would refute STR.
    FTL communication would not refute LET.

    Above 2 sentences are true ? Well, then STR seems, in principle, more falsifiable than LET. But this is actually an argument pro accepting STR and contra accepting LET, because more results that could falsify STR means STR is more accurate. Right ?
  7. Mar 8, 2015 #6


    Staff: Mentor


    It depends on which version of "LET" you are referring to. The version that makes exactly the same experimental predictions as SR would be disproved. A version (if there is one) that makes different predictions from SR about FTL transmission of information might not be disproved. But, as I said, I'm not aware of any such version of "LET"; the only version I'm aware of is the one that makes exactly the same predictions as SR.
  8. Mar 8, 2015 #7
    If there is a single preferred reference frame in which FTL turns out to be possible, does this turn out to be "a violation of SR" ?
  9. Mar 8, 2015 #8


    Staff: Mentor

    Yes, since this would imply that there is some law of physics that is not the same in all reference frames.
  10. Mar 9, 2015 #9
    Thanks for the insight, PeterDonis. So my primary question is answered, because I only care about the LET version(s) that make the same predictions like STR.

    Still I’m puzzled how this could be.
    LET assumes absolute simultaneity, even if we still couldn’t exactly determine it. In STR, in contrast, simultaneity is conventional. And yet FTL transmission of information disproves LET, just like with STR! Hmmmm...

    I did read on on the topic on how FTL communication is incompatible with STR and made some sense of it. But in case anyone wants to give an example on how ftl transmission of information could be used to make a contradiction in LET, it would give me something to seriously ponder about.
    (Even if it means I’ll have to find a way to improve my math beyond what I learned in highschool to completely understand the example :D )

    Alternatively, answers to these questions would help me as well: In LET theres absolute simultaneity(<--This makes me think I can understand STR better by understanding LET first. Even more so now that I know they are falsifiable exactly like one another, thanks to the previous answer) .. Then with that equivalent LET theory, is the mere knowledge of things that could be known by FTL communication contradictory ? Or is it that this knowledge could be used to do something impossible ?
  11. Mar 9, 2015 #10


    Staff: Mentor

    LET has an undetectable absolute simultaneity. So a detectable absolute simultaneity violates LET.
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