Why the Einstein-Rosen-Podolsky Paradox is no Mystery under the Many Worlds Interpretation

To understand why, please imagine the following. On Earth two particles are produced and sent flying in opposite directions. It is known that when they are finally measured, they possess identical orientations. Thus under the unchanging agreement or convention of what is horizontal and what is vertical (with respect, say, to the sun's axis of rotation), the two particles are always found to have the same polarization (or spin).

This does not mean that the particles have the same orientation before they are measured. Evidently that's a classic beginning error in quantum mechanics. See, for example, Penrose's "The Emperor's New Mind", or "Shadows of the Mind"---they're equally good.

Suppose that we know that in this way countless experiments have confirmed that when one particle registers H so does the other particle, and that when one registers V, so does the other. We learn from quantum mechanics that until a particle hits at least one measuring device, it's improper to suppose that it already has any orientation, but instead exists in a superposition. Here is the fact that troubled Einstein, Rosen, and Podolsky: the ordinary account given by quantum mechanics appears to imply that faster-than-light communication between the particles takes place, or, worse, that there is something to the Copenhagen blather about no phenomenon actually existing until some apparatus detects it for human consumption. (Choke, gag.)

Here's how an account, based roughly on the Copenhagen interpretation, (with its "collapse of the wave function") fiendishly explains what happens. Annoyingly, this account fits the facts. Say that one of the two particles travels one-and-a-half seconds to the left where it encounters a particle detector or polarizer on the Moon. Meanwhile, the other particle heads to the right towards Mars. The particle is "first" measured on the Moon, because of the comparativly short distance between the Earth and the Moon, and is found in this particular case to have, say, an H orientation. Instantly the scientists on the Moon compose a prophetic, condescending telegram and send it to Mars: "Dear Mars Colleagues, you will find that this telegram arrives on Mars just a few seconds after the arrival of the second photon which is headed your way. Yet already we predict with confidence what its polarization will be! You will find that it will be an H particle! So you see, we were the ones to collapse the wave function, not you. Somehow when we here on the Moon pricked the bubble, the wave function collapsed instantaneously, and the photon speeding toward you was overtaken and had its polarization determined to be H. Thus you must admit that there is a wave function collapse (or something that acts just like one), and that we were the ones that caused it. Considering that light takes at least four minutes to go from the Moon to Mars, you must admit that indeed it was us, here, who were first in the know. Sincerely yours, Moon Scientists."

(Actually there is a flaw in that reasoning doubtless apparent to experts in special relativity theory. Hint: how would an observer half way between the Earth and Mars have told the story, given that (a) he is traveling at a velocity close to c and headed towards Mars also, and (b) he has decreed that Mars scientists are also supposed to send prophetic telegrams at the conclusions of their experiments, just as Moon scientists are wont to do?)

Anyway, how does the Many World Interpretation explain what has happened, and why do theorists find EPR unproblematical in the MWI framework? Well, according to MWI, what has happened is this: one particle went to the left and one particle went to the right and each encountered a polarizer. Usually, there would be two so-called splittings (really "distinguishments") of the universe causing it to "break up" into four universes: one would contain HH, one HV, one VH, and one VV. However, this time there are only two universes: HH and VV. The reason is that when you perform the quantum mechanical "sum" of these entangled states, you obtain only two terms, not four. (See pp. 287-292 of Penrose's "Shadows of the Mind" for a wonderful explanation of the easy QM calculation involved.)

So you see, only two possible universes result. Observers on the Moon, who behold H will later on discover that they're in the same universe as observers on Mars who beheld H, and the observers who are on the Moon but who saw V will likewise find themselves inhabiting the same universe as the observers on Mars who saw V. We need entertain no ideas about a "collapse" of the wave function, and---also thanks to the Many Worlds Interpretation of Quantum Mechanics---no unanswered mystery concerning EPR.


Thanks to John Gribbin, in "In Search of Schroedinger's Cat", who wrote

"For anyone who has studied the EPR thought experiments, and the various tests of Bell's inequality, the attraction of the Everett interpretation is much greater [than that of the Copenhagen interpretation]. In the Everett interpretation, it is not that our choice of which spin component to measure forces the spin component of another particle, far away across the universe, to magically take up a complementary state, but rather that by choosing which spin component to measure we are choosing which branch of reality we are living in. In that branch of superspace, the spin of the other particle always is complementary to the one we measure. It is choice that decides which of the quantum worlds we measure in our experiments, and therefore which one we inhabit, not chance. Where all possible outcomes of an experiment actually do occur, and each possible outcome is observed by its own set of observers, it is no surprise to find that what we observe is one of the possible outcomes of the experiment."

...even though I think that my explanation is clearer.