Causality and quantum physics

  • #1
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Let me present what I think is the understanding of a particular situation in quantum mechanics, and ask people to tell me whether I am right or wrong.

To say that everything happens randomly in QM would be misleading at best. We get at least statistical prediction. But discussions such as the following raise an issue in my mind.


http://www.iem-inc.com/information/radioactivity-basics/decay-half-life

https://www.researchgate.net/post/C..._atoms_decay_and_others_dont_before_half_life

http://oregonstate.edu/instruct/ch374/ch418518/Chapter 3.pdf


For example: a bar of uranium lies on a counter. Imagine we can designate a single atom on the north side of the bar as atom A, and another atom on the south side as atom B. At 3:01 p.m., atom A decays, emitting an α particle. At 3:02 p.m., atom B decays. Now we ask: why did atom A decay before atom B? Would not the answer according to the most common interpretation be: “No reason,” or “No cause”?
 

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  • #2
DrChinese
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Would not the answer according to the most common interpretation be: “No reason,” or “No cause”?
Not sure there is a "most common" interpretation regarding this point. Certainly, there is no known cause in a scientific sense. Whether there might exist a cause - or some hypothetical state which would indicate the particle is getting ready to decay - well, that's simply an article of faith. Some interpretations include this in one way or another.
 
  • #3
PeterDonis
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Would not the answer according to the most common interpretation be: “No reason,” or “No cause”?
QM does not predict when individual atoms will decay, no. Nor does it explain why one atom decays before another.
 
  • #4
Nugatory
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The theory of quantum mechanics is completely silent on the question "Why did atom A decay before atom B?". Many people find this silence to be rather unsatisfying, but that's the way it is.

Perhaps there is some deeper theory that explains the "why" in this situation, a theory that given the complete specification of the initial state of the two atoms, including internal properties of which we are not currently aware, accurately predicts when each atom will decay. (This would be analogous to the way, for example, that a detailed knowledge of chemical bonding allows us to explain why the complete combustion of a given volume of hydrogen requires a half-volume of oxygen, while complete combustion of the same volume of methane requires three volumes of oxygen).

However, this "perhaps" is completely idle speculation unless and until someone proposes an actual candidate theory that makes predictions that can be tested experimentally. No such candidate theory has been proposed in any serious peer-reviewed source, so this thread cannot proceed much beyond the answer in the first paragraph of this post without violating the Physics Forums rule against speculation and personal theories.
 

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