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The photon is “not just a single particle but is actually an (uncountable) infinity of identical or ‘fungible’ particles”? Reviewer unconvinced.

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The Beginning of Infinity. by David Deutsch (Allen Lane Science)

From “The flawed multiverse,” Alastair I M Rae’s Physicsworld (Sep 22, 2011) review of David Deutsch’s The Beginning of Infinity: Explanations that Transform the World:

According to the quantum-information theorist David Deutsch, our modern understanding of how the world works has provided us with “good explanations” that open up essentially infinite possibilities for future progress. One of these explanations is the idea of the quantum multiverse, which Deutsch discussed in the May issue of Physics World (pp34–38, print version only) and to which he devotes a chapter in his book The Beginning of Infinity.

I believe the many-worlds theory is open to criticism for reasons other than extravagence. One of these concerns probabilities in a situation where both outcomes occur in parallel. If both options are happening, how can it be meaningful to say that one is more probable than the other – as is experimentally the case if the reflector is not exactly 50/50?

As he described in his Physics World article, Deutsch’s response is to propose that before the measurement, the photon is not just a single particle but is actually an (uncountable) infinity of identical or “fungible” particles. After interacting with the reflector, an infinite number of fungible photons exist in both output channels, but the ratio of these numbers is finite, so that each has a “measure” proportional to the squared modulus of the wavefunction. Even though an observer knows they are going to evolve into two copies of themself, they can apparently assign relative probabilities to which copy they expect to become. These probabilities are given by the Born rule. [Registration required.]

Rae isn’t convinced that this Deutsch fix – or others – resolve the problems, and is put off by the book’s dogmatic tone. He comments,

Deutsch willingly accepts that much of his inspiration comes from the work of Karl Popper, whose mantra “we have a duty to be optimistic” clearly underlies his thinking. However, he would have done well to remember that Popper was often dogmatic, to the point where some wags said that his book The Open Society and its Enemies should have been called “The Open Society by one of its Enemies”!

2 Replies to “The photon is “not just a single particle but is actually an (uncountable) infinity of identical or ‘fungible’ particles”? Reviewer unconvinced.

  1. 1
    mike1962 says:

    “Even though an observer knows they are going to evolve into two copies of themself, they can apparently assign relative probabilities to which copy they expect to become. These probabilities are given by the Born rule.”

    Which is what I would expect in a computed universe.

  2. 2

    It was a fascinating book review. I would have to say that there is a peculiar analogy here on the theories of QM. Everett vs Bohr:
    1 principal: many universes:: many principals: one universe.

    That is, Bohr has to add things like “the collapse of the wavefunction” which happens every time something is measured, to the probabilities of QM in order to get a measurement. Everett never collapsed anything, but he had to add many universes to handle all these uncollapsed wavefunctions.

    Now lets jump to a recent paper by Lee Smolin in which he argues that the conservation laws of special relativity require us to be computing in space time. For example, why do we have this principle that the speed of light, measured in meters/second, is a constant? Suppose we take the Fourier transform of our world, where speeds become frequencies, and space becomes k-numbers. (I know its all physicspeak, but humor me please.) Then in this mathematical realm that seems so strange, the speed of light is no longer a number–its a spread-out sort of blob, just like those atoms become in QM. So in a sense, this mathematical world is the QM world, where waves replace particles, and frequencies replace velocities.

    Well, said Smolin, if we live in this equivalent mathematical world, what it to say that conservation laws don’t exist there too? I mean, what makes space and time special if we can convert them into waves and frequencies? So what becomes of special relativity when we make equally arbitrary conservation laws in the QM world?

    Smolin worked on this and concluded that it “fuzzies up” our space-time world. Things we were so sure about in space and time–like the time you dropped the sugar bowl and got whipped for it–could have happened next door instead. We are no longer quite sure when and where things happened.

    Back to Everett and Bohr.

    So maybe the answer to these two interpretations of QM is that they are both right.

    Everett Smolin Bohr
    1:many :: 2 : 2 :: many: 1

    We live in two worlds, and we have two principles.

    Two worlds? Have we heard that before?

    How about the physical and the spiritual?

    And there you have it. Scientific proof for religion.

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