Who Killed Bob?


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?


Posted 2020-01-02T12:06:42.187

Reputation: 17

Question was closed 2020-01-03T13:58:48.120

1As 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 – 2020-01-02T12:52:23.260

@GeoffreyThomas done! – Juzzy – 2020-01-02T13:42:02.677

1the better physicist ? – None – 2020-01-02T15:02:07.930

1There are lots of classical scenarios in which responsibility seems to be distributed unevenly over multiple actors. What makes the quantum question here different? – senderle – 2020-01-02T19:06:14.253

@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 – 2020-01-02T22:06:59.117

@another_name Or the better comedian perhaps? – Juzzy – 2020-01-02T22:12:51.933

@Juzzy to me that seems easy enough to replicate classically. – senderle – 2020-01-02T22:26:27.347

Was Alice aware that Bob's death is the likely outcome? If so, what difference does it make what she did or did not ask Charlie, or that she did not "send" the bullet? If one places a bomb under someone's car they do not set it off either, the victim does. This is like one of those puzzles stuffed with irrelevant information, except what is relevant is not provided at all. If she planned it, she did, if not, nobody did, unexpected changes kill people all the time. – Conifold – 2020-01-03T22:37:32.307

@Conifold The relevant part of the question (it's not really a puzzle) is that Alice can seemingly control what the bullet does but, unlike your bomb analogy, no information ever passes from Alice to the bullet or Charlie. So it's not an unexpected change, it appears determined but Alice didn't tell Charlie or the bullet to do anything. – Juzzy – 2020-01-04T00:52:21.497

The only relevant issues are whether her "seeming control" actually alters statistics of the outcomes, and whether she is aware of this. Is she? If she is she is guilty even if she did it by jedi magic from a galaxy far far away, as long as the magic reliably works. Why do you drag Charlie into this again? – Conifold – 2020-01-04T01:00:17.863

@Conifold Whether Alice knows or not doesn't matter. It's the fact that Alice can make the bullet behave in a different way without interacting with it. The reason I brought Charlie back into it is because information could have passed from Alice to Charlie causing him to do something to the bullet, which it clearly doesn't. Alice can't possibly have any control over what the bullet does....can she? – Juzzy – 2020-01-04T01:25:58.150

It's a fact that a 2-year old can fire a gun at her sibling and kill him, she will not, however, be deemed responsible. On the other hand, whether Alice "interacts" with the bullet or not is moot, as long as she can reliably change its behavior. So you have it exactly the wrong way around: what you think is relevant isn't, and vice versa. "Alice can't possibly have any control"??? On your description she obviously does. "Information could have passed from Alice to Charlie" is not something you should be wondering about, it must be specified in your setup to make it meaningful. – Conifold – 2020-01-04T01:33:41.003

@Conifold I just noticed the thread was closed so I'll just say that the question was deliberately framed that way because of it's deep philosophical implications and I was curious as to what philosophy had to say about it...I guess I got my answer. All the best! – Juzzy – 2020-01-04T01:48:01.363



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?

Lee Daniel Crocker

Posted 2020-01-02T12:06:42.187

Reputation: 1 565

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 – 2020-01-02T23:52:02.470

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. – Lee Daniel Crocker – 2020-01-03T00:00:14.543

@senderle Simulate? Yes! Replicate, I doubt it. – Juzzy – 2020-01-03T00:18:54.267

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 – 2020-01-03T00:31:39.770