Tuesday, April 18, 2017

A modified consciousness-causes-collapse interpretation of quantum mechanics

Here are two technical problems with consciousness causes collapse (ccc) interpretations of quantum mechanics. In both, suppose a quantum experiment with two possible outcomes, A and B, of equal probability 1/2.

1. The sleeping experimenter: The experimenter is dreamlessly asleep in the lab and the experiment is rigged to wake her up on measuring A by ringing a bell. If conscious observation causes collapse, then when A is measured, the experimenter is woken up, and collapse occurs. Presumably, this happens half the time. But what happens the other half the time? No conscious observation occurs, so no collapse occurs, so the system remains in a superposition of A and B states. But that means that when the experimenter naturally wakes up several hours later, and then collapse will happen. However, when collapse happens then, it has both A and B outcome options at equal chances. But that means that overall, there is a 75% chance of an A outcome, which is wrong.

2. Order of explanation: The experimenter is awake. On outcome A, a bell rings. On B, a red light goes on. In fact, A is observed. What caused the collapse? It wasn’t the observer’s hearing the bell, because the bell’s occurrence is explanatorily posterior to the collapse. But we said that it is conscious observation that causes the collapse. Which conscious observation was that, if it wasn’t the hearing of the bell? Note that the observer need not have been conscious prior to hearing the bell or seeing the light—the experiment can be rigged so that either the bell or the light wakes up the observer. Perhaps the cause of the collapse was the state of being about to hear a bell or see a red light, or maybe it was the disjunctive state of hearing a bell or seeing a red light. But the former is a strange kind of cause, and the second would be a weird case where the disjunction is prior to its true disjunct.

The first problem strikes me as more serious than the second—the second is a matter of strangeness, while the first yields incorrect predictions.

I’ve been thinking about a curious ccc interpretation that escapes both problems. On this interpretation, the universe branches like in Everett-style multiverse explanations, but a conscious observation in any branch causes collapse. Collapse is the termination of a bunch of branches, including perhaps the termination of the branch in which the collapse-causing observation occurred. The latter isn’t some sort of weird retroactive thing—it’s just that the branch terminates right after the observation.

In case 2, the universe branches into an A-universe and a B-universe (or into pluralities of universes of both sorts). In the A-universe a bell is heard by the observer. In the B-universe a red light is seen by her. When this happens, collapse occurs, and there is no future to the observer after the observation of the red light, because in fact (or so case 2 was set up) it is the observation of A that won out. Or at least this is how it is when the two observations would be simultaneous. Suppose next that the bell observation would be made slightly earlier. Then as soon as the bell observation is made, the B-branch is terminated, and the red light observation is never made. On the other hand, if the light observation is timed to come first, then as soon as the light observation is made in the B-branch, this observation terminates the B-branch, and shortly afterwards the bell is heard in the remaining branch, the A-branch.

Case 1, then, works as follows. The universe branches into an A-universe, with a bell, and a silent B-universe. As soon as the bell is heard in the A-universe, the observation causes collapse, and one of the branches is terminated. If it’s the A-branch that’s terminated, then the observer heard the bell, but the future of that observation is annihilated. Instead, a couple of hours later the observer wakes up in the B-branch, and deduces that B must have been measured. If it’s the B-branch that’s terminated, on the otehr hand, then the observer’s observing of the bell has a future.

Prior to collapse, on this interpretation, we are located in multiple branches. And then our multilocation is wholly or partly resolved by collapse in favor of location in a proper subset of the branches where we were previously located. What happened to us in the other branches really did happen to us, but we never remember it, because it’s not recorded to memory.

On this interpretation, various things are observed by us which we never remember, because they have no future. This is a bit disquieting. Suppose that instead of the red light in case 2, the experimenter is poked with a red hot poker. Then if she hears the bell ring, she is relieved to have escaped the pain. But she didn’t: for if the poking is timed at or before the ringing, then the poking really did happen to her, albeit in another branch and not recorded to memory.

Fortunately for us, the futureless unremembered bad things were very brief: they only lasted for as short a period of time as was needed to establish them as phenomenologically different from the other possible outcome. So in the poked-with-a-poker branch, one only feels the pain for the briefest moment. And that’s not a big deal.

I worry a bit about quantum Zeno issues with this interpretation.

2 comments:

Gorod said...

Sorry, this is unrelated to the current post, but I just wanted to tell you I'm reading your "One Body" book (and loving it) and I found a mistaken footnote. On Chapter 5, footnote 19, it references "Augustine, The Trinity, XII.2" and I went looking for that text, since I am most interested in the idea mentioned, but I couldn't find that text there.

From what I read in this page http://www.newadvent.org/fathers/130112.htm you probably meant Chapter 6, paragraph 8 (I don't know the correct way to quote this, but I'm sure you do).

I just wanted to let you know in case you weren't aware of this yet.

Alexander R Pruss said...

The reference should have been XII.5-6, I guess. Thanks.