Re: Quantum Computers [was Read any good books lately?]

John Clark (
Tue, 17 Aug 1999 13:22:07 -0400

Hash: SHA1 <> On Tuesday, August 17, 1999 Wrote:

>This points up an ambiguity in the MWI, which is, when do other universes
>exist? That is, when does the universe split? You can give two answers.
>One is that it splits whenever there is an alternative which is explored
>in the quantum realm. The other is that it splits whenever there is
>a measurement which causes what would conventionally be called wave
>function collapse.

I've never heard of that second version of the MWI before, I always thought it's entire advantage was that it didn't have to explain what a measurement is. Do the two-slit experiment, but instead of using film to stop the photons after they pass the slit, let them head out into infinite space. If Many Worlds is correct then the entire universe splits into 2 when the photon hit's the 2 slits and never recombines. There is nothing special about you the observer, you split just like everything else, you know that the photon went through one and only one slit, but of course you have no way of knowing which one. Now let's do the more usual two-split experiment and put the film back in. The universe splits just as it did before when it passed the two slits, but when the photon hits the film and it no longer exists in either universe then the 2 are identical and fuse back together again. Looking back we find evidence that the photon (or electron) went through both slits and this causes an interference pattern. Again there is nothing special about an observer in this, the same thing would happen if nobody looked at the film, or even if you used a brick wall instead of film, because the important thing is not that the photon makes a record (whatever that is) but simply that it is destroyed. Mind has nothing to do with any of this so I don't need to explain it, or measurement, or representation, or consciousness.

John K Clark


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