Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Better understanding of causality

  1. Aug 18, 2009 #1
    An important part of SR is explaining which events may cause other events (e.g., a signal cannot get from A to B when they are space-like separated).

    Moreover, I vaguely remember that at least some minor arguments in relativity actually rely on the assumption that causality must not be violated (I might be wrong on this, though).

    Can someone explain to me why we are so worried about preservation of causality?

    Let me clarify my question, referring to either the classical relativity theory, or the quantum relativity with only unitary equations (it's just too confusing for me to discuss wave function collapse). In either case, the world is completely deterministic: what will happen in the future is fixed and unchangeable just as much as what happened in the past.

    In this situation, it makes no sense to talk about an information signal from A to B, since what happens at both events is known in advance. An information signal would only be meaningful if an observer at A could change something there, and then notify an observer at B about that change.

    It is as if the whole world's past and future in written down in a special book. Would it be meaningful to ask whether a signal from page 100 of the book can reach its page 200? No, because the book's pages aren't going to change, and so the signal can carry no information.

  2. jcsd
  3. Aug 18, 2009 #2
    Assume my superluminal punch.if I punch you, you will be hurt. if this event is space like for any one of us(or any other observer), he sees you getting hurt even before i punch. Thus i do not need to punch for getting you hurt. Since this is against conservation--pt.(1), you cd use that energy to punch me even without claiming to have hit me. Due to point 1 and since it is not good for the both of us, causality has to be preserved
  4. Aug 18, 2009 #3


    Staff: Mentor

    The problem with causality violations is simply that it is a pretty easy source of paradoxes. Killing your own father, sending a signal which stops you from sending the signal, etc.
  5. Aug 18, 2009 #4
    It would seem that in the terms of the question that: Both your FTL punch and the consequent reaction would ,both, already be fait accompli. You could no more, NOT spend the energy to enact the intention than the recipient could avoid the consequence.
    No loss of conservation.
    Not that I have anything in particular against causality , I am all in favor.
    But I also dont see any particular logical reason to assume the universe must operate for the convenience and comfort of its inhabitants.
  6. Aug 18, 2009 #5
    Hows this for a resolution to the grandfather paradox?

    Fred goes back and kills gramps. Fred pops out of existance. Gramps proceeds merrily along and in due time Fred is born and later proceeds back in time etc etc etc.
    The end result is Fred has created a closed time loop for himself. Deja vu all over again, and again.......... The rest of the universe rattles right along , with perhaps a few wondering where Fred got off to , but basically unaware that anything untoward has occured at all.

  7. Aug 18, 2009 #6
    I don't think either of these is a problem if laws are deterministic. The critical point is that deterministic laws forbid any free will.

    Whether or not you punch / kill / send a signal is not something you can decide on the spur of the moment. These actions are fully predetermined by the full solution to the universe's initial conditions. Of course such solution by definition satisfies all laws of physics. Hence whatever is going happen will look perfectly reasonable at each point (energy is conserved, dead people don't give birth, etc.).
    Last edited: Aug 18, 2009
  8. Aug 18, 2009 #7
  9. Aug 18, 2009 #8


    Staff: Mentor

    The point is that the laws of physics as currently understood and formulated don't seem to have this "edit out paradoxes" property. For example, we can build a transmitter that will send a 1-bit signal. We can build a receiver that will receive a 1-bit signal. We can build a processor that triggers the transmitter to send a 0 if it receives a 1 and a 1 if it receives a 0. Clearly none of the laws of physics forbid any of that. And because of causality we do not have to worry about paradoxes.

    So, now assume that we want to propose a new set of laws of physics but we want to relax the requirement of causality, but at the same time we want to change as little as possible about the laws that we have studied. Now, if we take the current set of laws and try to add only non-causal transmission of information then we get a paradox. It received a 0 so it transmitted a 1 which means that instead of receiving the 0 it actually received a 1 so it transmitted a 0 so ...

    Thus if we were to simply take the current laws and do a minimal tweak to allow violations of causality, then we would get paradoxes. It is conceivable that there are some self-consistent laws of physics that would forbid such paradoxes, but they would not look very familiar in the "non-causal limit" and yet they would have to reduce to the familiar laws in the "causal limit". That means one of three things:
    1) Either the real laws of physics are causal
    2) Or the real laws of physics are very strange
    3) Or the universe is fundamentally illogical

    Personally, I am fine with either 1) or 2), but 3) would really bother me.
  10. Aug 19, 2009 #9
    I think you have come up with a relevant and interesting perception and question.
    And you are right about SR using the argument that causality must not be violated.

    In the reductio ad absurdum logical proof that faster than light travel is impossible because it would result in causality violations.
    SR + FTL = time travel = loss of causality.
    Of course the whole argument is totally meaningless without the assumption of block time, exactly what you are talking about . And as you have pointed out , if you assume block time , predeterminism, then the very concept of acausality becomes moot , rendered meaningless. So in a neat circular paradox the reductio ad absurdum argument provides a reductio ad absurdum refutation.

    I myself dont think that FTL,time travel , acausality or total predeterminism are plausible realities but that just MO
  11. Aug 19, 2009 #10
    Why only one DaleSpam??? We all basically believe the real laws of physics are causal. ANd the universe.

    I cant believe anyone who has ever studied SR or QM didn't at some point think the real laws of physics are very strange.

    As for three; doesn't the fundamental law of conservation of matter and energy, and intrinsic human logic,
    inevitably make the existence of the universe, itself an insoluble paradox and illogical??

    1) Conservation and logic make it impossible to conceive of something (the universe) emerging from nothing.

    2) At the same time the human mind cannot conceive of infinity, of anything existing without a beginning,.

    3) Or perhaps the best resolution lies in acausality. The universe is evolving over unfathomable time to a final evolved state which results in its own birth. The end immediately precedes the begininng.

    I realize that this is not really any more conceivable than the other two but at least it is not mutally exclusive and has a perfect circular symmetry. 0 = [tex]\infty[/tex]
  12. Aug 19, 2009 #11
    Ahh, that's right. Why didn't I think about it? Somehow, I was sure that time travel only presents a paradox if there's free will. The idea of an automatic device that exploits the time travel to create a paradox is so obvious, in retrospect.

    Sounds like the Polchinski paradox is similar to that 0-1 transmitter construction, except much more precisely defined.

    I probably didn't understand the Novikov principle correctly. Apart from being very weird and restrictive (certain boundary conditions are not allowed, at least inside the chronology-violating region of space-time), I'm not even sure how it solves the paradox. What if we fill the universe with robots that look for wormholes and throw bombs into them at all possible angles. How would Novikov principle ensure that no bomb enters a wormhole at a "bad" angle (i.e., at an angle that would lead to the bomb blowing up the robot before it could throw it in)? It's all very good that it's not allowed, but what exactly would stop the robot? (Or worse, suppose the robots can actually calculate the angle that creates the paradox.)
  13. Aug 19, 2009 #12


    Staff: Mentor

    Good point, I should have said "at least one".
    Not really. There is a lot of disagreement and speculation on this topic.

    The big bang was a singularity. A singularity is "everything in the same place" which is vastly different from "nothing". Although I think there may be other similar problems in terms of conservation of phase-space volume for the universe, but I don't really know how it would apply.
  14. Aug 21, 2009 #13
    Yes I was thinking something similar when I read it. One can even make the situation very similar to the original billiard ball one. Just replace the ball with a homing missle. After exiting the wormhole it would change it's path to intercept the incoming one, and any touching would cause an explosion, which would remove any glancing-hit solutions to the problem.

    Has anyone heard of more "advanced" solutions to this paradox than what is written on the wiki page?
  15. Aug 27, 2009 #14
    Re: better understanding of causality


    Originally Posted by Austin0
    Why only one DaleSpam???

    I 'm not sure I had any point,,,I was just having fun

    Originally Posted by Austin0
    We all basically believe the real laws of physics are causal. ANd the universe.

    I wasn't serious there
    Of course in "objective" mode I have no problem with time symetric waves as long as they dont wander off the Q Rancho. Or most any other strange phenomena. I was talking about waking up in the morning confident that our coffee wasn't going to get hotter while we drink it. ANd how much sleep have you ever lost worrying about post-emptive tachyon strikes????

    Originally Posted by Austin0
    As for three; doesn't the fundamental law of conservation of matter and energy, and intrinsic human logic,
    inevitably make the existence of the universe, itself an insoluble paradox and illogical??

    1) Conservation and logic make it impossible to conceive of something (the universe) emerging from nothing.

    My "point" exactly. The singularity concept cleverly but deviously avoids this " something from nothing problem". As do spontaneous fluctuations in an "almost" non existent quatum potential field (before the singularity) and other such valient, but doomed , attempts to handle this basic paradox.
    I myself am firmly convinced that the universe is going to continue expanding at an accelerating rate until everything in it goes superluminal and is tranformed into a cloud of tachyons going backward in time and space to converge and create the singularity.
    No not really Thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook