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B What happens if the observer dies?

  1. Dec 1, 2016 #1
    Basic double slit experiment one observer knows the path of the particle. The back screen is a locked room and has never been seen. The observer dies and I decide to look at the screen. What will I see?
    I'm assuming the wave will still be collapsed.
  2. jcsd
  3. Dec 1, 2016 #2


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    You are making the pop-sci mistake of thinking that a conscious observer is required. If there is a setup that would allow a conscious observer to know the path then it matters not at all whether there actually IS a conscious observer.
  4. Dec 1, 2016 #3


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    If by "the wave collapsed", you mean that there will be no interference pattern at the screen then the answer is "yes, of course". As soon as the detector found the particle, there wasn't going to be an interference pattern; it is completely irrelevant whether anyone looked at the detector output, and even more irrelevant whether that person is still alive.

    The idea that conscious observers have anything to do with quantum mechanics dates back to the early days (almost a hundred years ago) when the theory was less well understood. Back then the idea seemed to solve a serious problem with the theory, but it was pretty much abandoned when we found a more satisfactory solution (David Lindley's book "Where does the weirdness go?" is a good source). Unfortunately, by then the "consciousness causes collapse" idea had leaked into the popular imagination.... so to this day you'll find people telling you that it matters whether someone looks at the measurement results.

    As this thread is based on a misunderstanding, we can close it now.
  5. Dec 1, 2016 #4


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    What's important for the appearance of wave function collapse (I use the word "appearance" because there is no consensus that there is any wave function collapse) is what's called entanglement. More specifically, entanglement and irreversibility work together.

    What this means is that if some microscopic fact (such as which slit a particle went through) triggers an irreversible change in something other than that particle, then there can be no possibility of the interference effects between alternatives that are the hallmark of quantum mechanics. My observing which way the particle went clearly is an irreversible change; my memories are permanently changed by the observation. Even if I die, details about my brain's memory will be different, due to making that observation. Even if you cremate my body and scatter the ashes onto the ocean, there will be slight differences due to my making the observation. As long as there is a persistent effect due to the microscopic event, there will be no possibility of interference with the other alternatives. This lack of interference is what gives the appearance of wave function collapse (regardless of whether that really happens, or not).
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