Re: MWI experiment proposal

From: Hal Finney <>
Date: Mon, 17 Jun 2002 11:17:53 -0700

Fred Chen writes:
> Here is a paper by R. Plaga on experimental verification of many-worlds
> hypothesis. It is also referred to in the recent paper by Cirkovic.
> Proposal for an experimental test of the many-worlds interpretation of
> quantum mechanics
> Author: R.Plaga
> The many-worlds interpretation of quantum mechanics predicts the
> formation of distinct parallel worlds as a result of a quantum
> mechanical measurement. Communication among these parallel worlds would
> experimentally rule out alternatives to this interpretation. A procedure
> for ``interworld'' exchange of information and energy, using only state
> of the art quantum optical equipment, is described.

I would be extremely skeptical that this experiment will work. It is
generally accepted that the MWI produces the same predictions as
other interpretations, except where the other methods invoke "magic"
(i.e. nonphysical interactions with consciousness, etc.). I found
one comment on this article, from 1995, at

> This Week's Finds in Mathematical Physics (Week 68)
> John Baez
> ...
> 5) R. Plaga, Proposal for an experimental test of the many-worlds
> interpretation of quantum mechanics, preprint available as
> quant-ph/9510007.
> John Gribbin brought this one to my attention and asked me what I
> thought about it. Basically, the idea here is to isolate an ion from
> its environment in an "ion trap", and then perform a measurement on
> with two possible outcomes on another quantum system, and to excite the
> ion only if the first outcome occurs, before the ion has had time to
> "decohere" or get "entangled" with the environment. Then one checks
> to see if the ion is excited. The idea is that even if we didn't see
> the outcome that made us excite the ion, we might see the ion excited,
> because it was excited in the other "world" or "branch" --- the one in
> which we *did* see the outcome that made us excite the ion. The author
> gets fairly excited himself, suggesting that "outside physics, interworld
> communication would lead to truly mind-boggling possibilities".
> Does this idea really make sense? First of all, I don't think of this
> sort of thing as a test of the many-worlds interpretation; I think that
> all sufficiently sensible interpretations of quantum mechanics (not
> necessarily *very* sensible, either!) give the same concrete predictions
> for all experiments, when it comes to what we actually observe. They may
> make us tell very different stories about what is happening behind the
> scenes, but not of any testable sort. As soon as one comes up with
> something that makes different predictions, I think it is (more or less
> by definition) not a new "interpretation" of quantum theory but an actual
> new theory. And I don't think the many-worlds interpretation is that.
> So the question as I see it is simply, will this experiment work?
> Will we sometimes see the ion excited even when we didn't excite it?
> It seems hard; usually the decoherence between the two "branches" prevents
> this kind of "inter-world communication" (not that I'm particularly fond
> of this way of talking about it). What exactly is supposed to make
> this case different? The problem is that the paper is quite sketchy
> at the crucial point... just when the rabbit being pulled from the hat,
> as it were. I haven't put much time into analyzing it, but some people
> interested in this sort of thing might enjoy having a go at it.

If this kind of inter-world communication were possible, quantum
computers would be much more powerful. You wouldn't have to rely on
tricks like the Shor and Grover algorithms which manage to make the
results available in every branch. You could just do straightforward
N-way parallelism and whichever branch got the result, they could tell
the others. This would allow quantum computers to solve NP complete
problems, which I believe they have been proven not to be able to do.
If this is correct then that would prove that this experiment can't work.

I think it might also be possible to measure a photon's quantum state with
more accuracy than is theoretically possible, by sending it into a bunch
of universes and letting each one do a slightly different measurement,
then using Plaga's technology to broadcast their results to the other
universes. This would violate more quantum theorems, and allow for
faster than light communication to boot.

I'm no physicist so I don't know for sure that these implications would
follow, but I am very doubtful that interworld communication is consistent
with the basics of quantum mechanics. The fact that this paper has not
been published in peer reviewed journals in 7 years indicates that it
probably doesn't work.

Received on Mon Jun 17 2002 - 11:32:08 PDT

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