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In fabric of reality interpretation of QM chapter 2 how does David Deutsch conclude that the "shadow photons" in the one-particle-at-a-time double slit experiments are themselves partitioned off into separate universes individually?

If the property of the photons is that the "tangible photons" of a universe only interacts with the "shadow photon" counterpart, then in another universe where that particular "tangible photon" ("shadow photon" as seen from our universe) also interacts with other "shadow photons" as seen from that universe, so how can D. Deutsch come to the conclusion that as viewed from our universe, each "shadow photon" is partitioned off in a separate universe as opposed to the conclusion that all of the "shadow photons" come from the same universe?

Can someone who has read and understood this chapter, help provide a way to understand how the "shadow photons" are partitioned off into parallel universes among themselves in terms of David Deutsch's interpretation of this phenomena?

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    $\begingroup$ Could you quote the exact passage where DD wrote that? $\endgroup$
    – alanf
    Commented Nov 20, 2023 at 16:54

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I'm inclined to say that Deutsch is just wrong.

What he calls parallel universes in all of his writings are what everyone else calls paths or histories (in the context of the Feynman path integral / sum over histories). I don't know of anyone else who uses the term in the way he does. It doesn't match other concepts of parallel universes such as brane worlds or the worlds of the many-worlds picture.

What he calls the tangible photon is one of the terms in the sum over histories, and the shadow photons are the other terms. Since they are all terms on the same footing, it makes sense to say that the shadow photons are all related to each other in the same way that they are related to the tangible photon.

What doesn't make sense is singling out one of the histories as "tangible". There is no theoretical or experimental basis for that distinction. His justification for it seems to be that if you put detectors at each slit, at most one of them clicks per photon, and that shows where the "tangible photon" went. But if you put detectors at each slit, there is no multi-slit interference pattern on the screen. In the experiment without detectors at the slits, the pattern on the screen is different, so the behavior of the photons is evidently different, and it doesn't make sense to assume that what you found at the slits in the other experiment will still apply.

It's simply not true that one photon history is the one that triggers detectors, and the others merely interfere with that one. Whatever one may believe about the reality of individual histories, they're all equally real.

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  • $\begingroup$ Right, I see your point. Not sure how Deutsch holds on to this phenomena, but in his line of thinking, how did he separate out these other histories (in his interpretation being photons from parallel universes), to be from separate universes among themselves ? Yes Deutsch might be wrong, but I am also trying to put my self in his shoes to justify his logical deductions of these shadow parallel universe photons being in separate parallel universes among themselves. Thanks :) $\endgroup$
    – TheSprintingEngineer
    Commented Sep 30, 2021 at 15:19
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In Chapter 2 on p.45 David Deutsch wrote:

Single-particle interference experiments such as I have been describing show us that the multiverse exists and that it contains many counterparts of each particle in the tangible universe. To reach the further conclusion that the multiverse is roughly partitioned into parallel universes, we must consider interference phenomena involving more than one tangible particle. The simplest way of doing this is to ask, by way of a ‘thought experiment’, what must be happening at the microscopic level when shadow photons strike an opaque object. They are stopped, of course: we know that because interference ceases when an opaque barrier is placed in the paths of shadow photons. But why? What stops them? We can rule out the straightforward answer – that they are absorbed, like tangible photons would be, by the tangible atoms in the barrier. For one thing, we know that shadow photons do not interact with tangible atoms. For another, we can verify by measuring the atoms in the barrier (or more precisely, by replacing the barrier by a detector) that they neither absorb energy nor change their state in any way unless they are struck by tangible photons. Shadow photons have no effect.

So the reason why shadow photons are partitioned into separate universes after measurement and decoherence is that otherwise they would have to obey different laws of physics than the tangible photons.

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Per Deutsch "a tangible photon is real, a shadow photon is merely a way in which the real proton could have possibly behaved, but did not" I believe this could offer some some explanation to the existence of a parallel universe. The parallel universe is at a separate time and space and not occupying the same space as the photon itself.

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..i have had dreams of separate parallel universe which is a masked universe made by the photon duality ...a universe that can help ours by harnessing the energy that will cleanse our universe , well at least our world with problems with energy and protection from eroding atmosphere by its invisible energy ...such energy that can be accounted for by the duality of quantum as interpreted by boolean complex defination on the state of being there and not...

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