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Causality in quantum mechanics and relativity

  1. Feb 28, 2010 #1
    According to Heisenberg uncertainty principle, certain events such as double slit and decay of an atom has no causal history, hence a violation of causality, uncaused events.
    But relativity states faster than light travel violates causality.

    Since quantum mechanics does not respect causality, why should sending information faster than c do so?
  2. jcsd
  3. Feb 28, 2010 #2
    In these two cases the word "causality" is used in different contexts and has different meanings.

    When in QM we say that atomic decay is not causal we mean that there is no identifiable physical reason for the atom to decay at this particular moment.

    In SR we are dealing with two classical events A and B, such that A is definitely a cause of B. The causality postulate demands that A happens earlier than B in all reference frames. If information travels superluminally from A to B, then one can find a reference frame in which the effect (B) occurs earlier than the cause (A). This violates the causality postulate.

  4. Feb 28, 2010 #3
    So if the two events are quantum, A and B, then there causality would not apply?
  5. Feb 28, 2010 #4
    Only if the event is superluminous. It is the act of exceeding light speed where time becomes skewed. Thus if A causes B.. B could happen before A.
  6. Feb 28, 2010 #5


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    The issue is the word "causality". It means different things to different people.

    In QM, things do not happen for known reasons. They instead follow laws of chance. Those statistical laws are very useful. But they do not - at this time - allow for one to say definitely that "cause A leads to effect B". So QM is considered acausal. Some do not accept this, but the experimental results appear to support it. There is no actual evidence that there is any direct cause to the outcome of a quantum event.

    An example would be radioactive decay. Or actually, any decay event.
  7. Feb 28, 2010 #6
    Oh my mistake. I didn't even address the contextual difference.

    This corresponds to the random production and destruction of photons as well right?
  8. Mar 1, 2010 #7
    DrChinese, (I have wanted to ask you about this once. :smile:)

    Can you say the experimental results such as the fine structure(like one between 2P1/2 and 2P3/2) also support the "probabilistic" acausal idea?
    You are probably talking about the acausal Schroedinger equation (S.E.) which shows the probability density.
    But the "probabilistic" S.E. can't explain the relativistic effects (including the spin-orbital interactions).
    So it is "incomplete".

    On the other hand, the Dirac equation (D.E.) which satisfies the (relativistic) causality can explain this relativistic effects.
    Of course, D.E. is not "probabilistic".(= D.E. doesn't show the probability density.)

    As shown in this site, As the atoms become heavier (which means the atomic nucleus charge becomes larger), the experimental results become more different from that of S.E., because the electon's speed becomes faster.
    In S.E., irrespective of the nucleus charge, the electrons of any atoms are all static as electron clouds obeying the probability density, aren't they?

    [To be precise, in D.E., only one of plus or minus energy solutions is not causal. And if we use the Coulomb force, it is not causal. But as an approximation, D.E. is superior to S.E.]

    I think QM always contains the "vague" parts like this (which will continue forever, as long as QM continue).
    Over 80 years have passed since the QM appeared.
    But the discussions like the "interpretation" continue even now.
    (Here, I'm not talking about the interpretation of QM, but talking about the "inconsistency" between the relativistic and nonrelativistic QM.)

    In QM, the relativistic QM is superior to the nonrelativistic QM in the experimental results.
    But, are you saying "acausal" QM is superior ?
    Of course, "Photons" which satisfy the Maxwell's equation are "relativistic" particles, too.
    Last edited: Mar 1, 2010
  9. Mar 1, 2010 #8

    80 years is a blink of the eye in any science.
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