We have a mach zhender type interferometer that can receive quantum bullets. Charlie fires the bullets continuously into the device and (assuming a perfect experiment) the bullets all exit at output A. Bob sits safely at output B. Alice is somewhere along the two paths that the bullets can take and chooses to block them, therefore, with 1/4 chance a bullet exits the interferometer at output B and, assuming it does, kills Bob. So, my question is...Who killed Bob?

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    As the situation is described, no-one killed Bob since you nowhere say that Alice did block the bullets or that Bob is dead; all you tell us is that Alice can block the bullets if she chooses and that if she does choose then there is a 0.25 chance a bullet will exit the interferometer at output B thereby killing Bob. Charlie fired the bullets, that's given: but since we don't know what Alice chose to do, Bob's fate is indeterminate. If he's alive because Alice decided not to block the bullets, no-one killed Bob. Re-wording is needed, I think.
    – Geoffrey Thomas
    Jan 2, 2020 at 12:52
  • @GeoffreyThomas done!
    – Juzzy
    Jan 2, 2020 at 13:42
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    the better physicist ?
    – user38026
    Jan 2, 2020 at 15:02
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    There are lots of classical scenarios in which responsibility seems to be distributed unevenly over multiple actors. What makes the quantum question here different?
    – senderle
    Jan 2, 2020 at 19:06
  • @senderle The difference, as far as I can tell, is that Alice seems to have made the decision that Bob is killed and at what time, However, Alice didn't ask Charlie to kill Bob and neither did she send a bullet towards Bob.
    – Juzzy
    Jan 2, 2020 at 22:06

1 Answer 1


Who made the last conscious choice to act, knowing the likely outcome of his/her action? Does Charlie reasonably believe that his bullets will continue to miss Bob because he doesn't know about Alice? Does he know that someone might possibly block the safe path without him seeing it, and is therefore acting with reckless disregard for Bob's life? Did Alice move to block the safe path knowing what effect it would have on Carlie's bullets? Did Dave mislead Charlie and/or Alice into acting with the intent of killing Bob?

  • Hi Lee, yes, that's a lot of questions!!! Who made the last conscious choice to act? Let's rule out Charlie, he is just randomly firing bullets, so it must be Alice? But, even if we say that Alice deliberately collapsed the wavefunction at no point did she ever tell Charlie or the bullet what to do and so the only thing that could have made the decision was the bullet itself??? Which is obviously crazy.
    – Juzzy
    Jan 2, 2020 at 23:52
  • I don't see any problem here at all. If Alice knew what the likely consequences of her action were, then she's responsible. If Bob knew but Alice didn't, he's responsible. If neither knew, then whoever set up this crazy gadget is responsible. If I hire you to walk into a room and press a button, telling you nothing about what it might do, and it causes someone's death, *I'm" the killer no less than if I fired a gun. In your scenario, someone, at some point, willingly took an action that they knew would result in an extremely likelihood of Bob's death, and that person is responsible. Jan 3, 2020 at 0:00
  • @senderle Simulate? Yes! Replicate, I doubt it.
    – Juzzy
    Jan 3, 2020 at 0:18
  • Lee Daniel Crocker - The question of responsibility is easy enough to solve. Just blame me for dreaming it up in the first place, but the question is 'who killed him?' or more accurately, who, or what, made the decision to kill him? If we flip it around and say Alice didn't block any bullets, then who made the decision that Bob should live. How did the bullet 'know' what Alice wants when Alice didn't interact with Charlie, Bob or the bullets? – Juzzy 10 mins ago
    – Juzzy
    Jan 3, 2020 at 0:31

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