#### JesseM

Science Advisor

- 8,492

- 12

At one point when I was thinking about how the Everett interpretation could explain the results of the EPR experiment in a local realist way, I came up with the following analogy to show howvanesch said:The problem is that then, every physical theory is realist (the word looses its meaning). After all, no matter what theory, its formalism will be "real" in the Platonic sense and is the "machinery that produces the numbers I observe", so the "real description" of the universe.

I saw "realism" as a more constrained way, in that "what is observed is really there, locally encoded in the thing we're observing". So if I see an electron with a spin-up, because that's what my detector says, then I say that there really is an electron with a spin up, and it is spin up even if I wouldn't have observed it. And that's of course NOT possible in QM. The only thing that QM says is that the part of my state that corresponds to my conscient observation that I experience is entangled with the part of the electron's state that has spin up, but there can (or cannot) be other parts of the electron's state that are different (say, spin down), but of which I will never hear again. So the "realism" has shifted from "it is really there" to "a relationship between me, my observations, and part of what's there".

cheers,

Patrick.

*in principle*an Everett-like interpretation could do this:

Now, I realize that the various Everett interpretations are not so straightforward--in my computer simulation above, probability has a clear frequentist meaning, while the problem of getting a notion of "probability" out of any version of the Everett interpretation is notoriously difficult, and perhaps it can't work at all without tacking on extra assumptions. Still, I got the impression that this was the general type of explanation that Mark Rubin was aiming for in his papers, where each observation creates a local splitting of the observer, but the observations of spatially separated observers are only mapped to each other once a signal has had the chance to pass between them.say Bob and Alice are each recieving one of an entangled pair of photons, and their decisions about which spin axis to measure are totally deterministic, so the only "splitting" necessary is in the different possible results of their measurements. Label the three spin axes a, b, and c. If they always find opposite spins when they both measure their photons along the same axis, a local hidden-variables theory would say that if they choose different axes, the probability they get opposite spins must be at least 5/9 (assuming there's no correlation between their choice of which axes to measure and the states of the photons before they make the measurement). I forgot what the actual probability of opposite spins along different axes ends up being in this type of experiment, but all that's important is that it's less than 5/9, so for the sake of the argument let's say it's 1/3.

So suppose Bob's decision will be to measure along axis a, and Alice's decision will be to measure along axis c. When they do this, suppose each splits into 6 parallel versions, 3 measuring spin + and 3 measuring spin -. Label the 6 Bobs like this:

Bob 1: a+

Bob 2: a+

Bob 3: a+

Bob 4: a-

Bob 5: a-

Bob 6: a-

Similarly, label the 6 Alices like this:

Alice 1: c+

Alice 2: c+

Alice 3: c+

Alice 4: c-

Alice 5: c-

Alice 6: c-

Note that the decision of how they split is based only on the assumption that each has a 50% chance of getting + and a 50% chance of getting - on whatever axis they choose, no knowledge about what the other one was doing was needed. And again, only when a signal travelling at the speed of light or slower passes from one to the other does the universe need to decide which Alice shares the same world with which Bob...when that happens, they can be matched up like this:

Alice 1 (c+) <--> Bob 1 (a+)

Alice 2 (c+) <--> Bob 2 (a+)

Alice 3 (c+) <--> Bob 4 (a-)

Alice 4 (c-) <--> Bob 3 (a+)

Alice 5 (c-) <--> Bob 5 (a-)

Alice 6 (c-) <--> Bob 6 (a-)

This insures that each one has a 2/3 chance of finding out the other got the same spin, and a 1/3 chance that the other got the opposite spin. If Bob and Alice were two A.I.'s running on classical computers in realtime, you could simulate Bob on one computer and Alice on another, make copies of each according to purely local rules whenever each measured a quantum particle, and then use this type of matching rule to decide which of the signals from the various copies of Alice will be passed on to which copy of Bob, and you wouldn't have to make that decision until the information from the computer simulating Alice was actually transmitted to the computer simulating Bob. So using purely local rules you could insure that, after many trials like this, a randomly-selected copy of A.I. Bob or A.I. Alice would record the same type of statistics that's seen in the Aspect experiment, including the violation of Bell's inequality.

Note that you wouldn't have to simulate any hidden variables in this case--you only have to decide what the spin was along the axes each one measured, you never have to decide what the spin along the other 2 unmeasured axes of each photon was.