# Who Killed Bob? [closed]

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?

• 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. Jan 2, 2020 at 12:52
• @GeoffreyThomas done! Jan 2, 2020 at 13:42
• the better physicist ?
– user38026
Jan 2, 2020 at 15:02
• There are lots of classical scenarios in which responsibility seems to be distributed unevenly over multiple actors. What makes the quantum question here different? 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. Jan 2, 2020 at 22:06