Bilocation and the at-at theory of time

I was telling my teenage children about the at-at theory of motion: an object moves if and only if it is in one location at one time and in another location at another time. And then my son asked me a really cool question: How does this fit with the possibility of being multiply located at one time?

The answer is it doesn’t. Imagine that Alice is bilocated between disjoint locations A and B, and does not move at either location between times t₁ and t₂. Nonetheless, by the at-at theory, Alice counts as moving: for at t₁ she is in location A while at t₂ she is in location B.

My response to my son was that this was the best argument I heard against the at-at theory. My son responded that the argument doesn’t work if multilocation is impossible. That’s true. But there is good reason to think bilocation is possible. First, the real presence of Christ in the Eucharist appears to require multilocation. Second, God is present everywhere, but never moves. Third, there is testimonial evidence to saints bilocating. Fourth, the argument only needs the logical possibility of bilocation. Fifth, time-travel would make it possible to stand beside oneself.

(The time-travel case is probably the least compelling, though, as an argument against the at-at theory. For the at-at theorist could say that the times in the definition of motion are internal times rather than external ones, and time travel only allows one to be in two places at one external time.)

I’ve been inclining to think the at-at theory is inadequate. Now I am pretty much convinced, but I am not sure what alternative to embrace.

One might just try to tweak the at-at theory. Perhaps we say that an object moves if and only if the set of its locations is different between times. But that isn’t right. Suppose Alice is bilocated between locations A and B at t₁, but at t₂ she ceases to bilocate, defaulting to being in location A. Then the set of locations at t₁ is {A, B} while at t₂ it is {A}. But Alice hasn’t moved: cessation of bilocation isn’t motion. Nor will it help to require that the sets of locations at the two times have the same cardinalities. For imagine that Alice is bilocated at locations A and B at t₁, and then she ceases to be located at B, defaulting to A, and walks over to location A′ at t₂. Then Alice has moved, but the sets of locations at t₁ and t₂ have different cardinalities. I don’t know that there is no tweak to the at-at theory that might do the job, but I haven’t found one.

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.

Partial location, quantum mechanics and Bohm

The following seems to be intuitively plausible:

1. If an object is wholly located in a region R but is not wholly located in a subregion S, then it is partially located in R−S.
2. If an object is partially located in a region R, then it has a part that is wholly located there.

The following also seems very plausible:

3. If the integral of the modulus squared of the normalized wavefunction for a particle over a region R is 1, then the particle is wholly located in the closure of R.
4. If the integral of the modulus squared of the normalized wavefunction for a particle over a region R is strictly less than one, then the particle is not wholly located in the interior of R.

But now we have a problem. Consider a fundamental point particle, Patty, and suppose that Patty's wavefunction is continuous and the integral of the modulus squared of the wavefunction over the closed unit cube is 1 while over the bottom half of the cube it is 1/2. Then by (3), Patty is wholly contained in the cube, and by (4), Patty is not wholly contained in the interior bottom half of the cube. By (1), Patty is partially located in the closed upper half cube. By (2), Patty has a part wholly located there. But Patty, being a fundamental particle, has only one part: Patty itself. So, Patty is wholly located in the closed upper half cube. But the integral of the modulus squared of the wavefunction over the closed upper half cube is 1−1/2=1/2, and so (4) is violated.

Given that scenarios like the Patty one are physically possible, we need to reject one of (1)-(4). I think (3) is integral to quantum mechanics, and (1) seems central to the concept of partial location. That leaves a choice between (2) and (4).

If we insist on (2) but drop (4), then we can actually generalize the argument to conclude that there is a point at which Patty is wholly located. Either there is exactly one such point--and that's the Bohmian interpretation--or else Patty is wholly multilocated, and probably the best reading of that scenario is that Patty is wholly multilocated at least throughout the interior of any region where the modulus squared of the normalized wavefunction has integral one.

So, all in all, we have three options:

• Bohm
• massive multilocation
• partial location without whole location of parts (denial of (2)).

This means that either we can argue from the denial of Bohm to a controversial metaphysical thesis: massive multilocation or partial location without whole location of parts, or we can argue from fairly plausible metaphysical theses, namely the denial of massive multilocation and the insistence that partial location is whole location of parts, to Bohm. It's interesting that this argument for Bohmian mechanics has nothing to do with the issues about determinism that have dominated the discussion of Bohm. (Indeed, this argument for Bohmian mechanics is compatible with deviant Bohmian accounts on which the dynamics is indeterministic. I am fond of those.)

I myself have independent motivations for embracing the denial of (2): I believe in extended simples.

Of balloons and transubstantiation

Our three-dimensional space is curved, say, like the surface of a balloon--except that the surface of a balloon is two-dimensional while space is three-dimensional.

Now imagine you have an inflated balloon. Draw two circles, an inch in diameter, on opposite sides, one red and one blue. Put your left thumb in the middle of the red circle and your right thumb in the middle of the blue circle. Press the thumbs towards each other, until they meet, with two layers of rubber between them. The balloon now looks kind of like a donut, but with no hole all the way through. Imagine now that you press so hard that the two layers of rubber between your thumbs coalesce into a single layer of rubber.

Now the single layer of rubber between your thumbs is at the center of the red circle and at the center of the blue circle. We can think of each circle as defining a place, and the coalesced rubber inside it is found in both of these places.

Replace the red circle with a drawing of a church and the blue circle with a drawing of heaven. The same coalesced layer of rubber is both inside (a drawing of) a church and inside (a drawing of) heaven. Suppose now that the rubber is infinitely thin, and that there is a space that coincides with this rubber, and little two-dimensional people, animals, plants and other objects inhabiting this space, much as in Abbott’s novel Flatland . Suppose that the pictures of the church and heaven are replaced with two-dimensional realities. Then the space of the church and the space of heaven literally overlap, so that there is a place that is located in both. An object found in that place will be literally and physically located both in the church and in heaven. In one sense, that object is physically located in two places at once. In another sense, it is located in a single place, but that single place is simultaneously located both in heaven and in the church.

There is no serious additional conceptual difficulty in three-dimensional space curving in on itself similarly.

(This is largely taken from a forthcoming piece by Beckwith and Pruss.)

Why so few kinds for so many particles?

There are something like 10⁸⁰ individual particles and only something like 10² kinds of particles. It seems an incredible coincidence; so many particles, all drawn from so few kinds, even though surely the space of metaphysical possibility contains infinitely many kinds. It's like a country all of whose citizens have names that start with A, B or M.

But perhaps one could explain this by the massively multilocated particle hypothesis (MMPH), namely that to each kind there corresponds only one individual, but highly multilocated, particle (Feynman proposed something like this)? It isn't surprising, after all, if all the names of the villagers in a village start with A, B or M when there are only three villagers.

Still, MMPH does bring in a new mystery: Why are there so very few particles? But perhaps that is a less pressing question?

Multilocation

A sufficient condition for multilocation is being wholly present, at the same time, in two or more disjoint locations. This condition is not necessary, however. Two bosons, unlike two fermions, can share exactly the same location. Suppose that tomorrow I will travel back in time, and then very gently touch the shoulder of my blog-posting self, in such a way that a boson from my time-traveling self is co-located with a boson from my blog-posting self. In that case, surely, I am multilocated, but I am not wholly present at two disjoint locations, but at two slightly overlapping locations.

Fortunately, by making use of the notion of being present at a location (where I count as present wherever any of me is present) in addition to the notion of being wholly present at a location we can easily account for multilocation:

• An object x is multilocated at time t if and only if that there are two disjoint locations, L₁ and L₂, such that at t, x is wholly present in L₁ and present in L₂.

A counterexample to the Private Language Argument

Wittgenstein's Private Language Argument contends that it is impossible for one to form a language by oneself. Here's a counterexample. You live on an island that has no people other than yourself. You live for forty years there. Then you step in a time machine that was left there by someone else. You go back forty years. You do this 999 times. (Let's assume that the time machine also fixes up your body so you can live for a subjective length of 40000 years.) To an outsider, the island looks populated by a thousand people of remarkably similar appearance. There is a community there. But that community in fact includes only one person, you. So, it's possible to have a community with only one person. But there is no reason why such a community couldn't develop a language, since it functions just like all other communities do.

A fun question: Suppose Marcy and George join you on the island, but they don't do any time travel, and so there you are located in 1000 places on the island, and then there is Marcy and there is George. In elections, should you get 1000 votes, with each of them getting only one—there does, after all, seem to be a sense in which you have a lot more interests—or should each of the three people on the island get exactly one vote?

By the way, the clip below illustrates the wrong way of imagining the scenario. In my scenario, it is a mistake to think of first the island having you in one place, then of it having you in two places, and so on. Over the 40 year period in my scenario, you always are in 1000 places.