I had a brief correspondence with Deutsch about this, and this was his response:
An analogue of your suggestion about universes is that photons cease to exist after they have passed the Earth, so that we can never catch up with them, heading in directions in which they will never strike anything.
I think it is actually arguable that such photons indeed do not exist. John Cramer's transactional interpretation of quantum mechanics (TQM) actually argues exactly that, and relies on the big bang singularity as the "absorber of last resort" to insure that it is not possible for any real photon to travel "in a direction where [it] will never strike anything." But the validity of TQM is far from certain. It's considered pretty unfashionable nowadays.
I think I've found another way out of this conundrum, but like anything having to do with QM it's a little weird. So in the spirit of such things I invite you to consider the following claim:
Darth Vader is Luke Skywalker's father.
Is it true? Of course it depends on your point of view. If you suspend disbelief and look at it from the counterfactual perspective of the Star Wars universe, then yes, it is true. But if you do not suspend such disbelief and look at it from the point of view of the "real" universe, the answer is no because neither Darth Vader nor Luke Skywalker actually exist.
Trick is, what we so glibly refer to as the "real" universe doesn't "actually" exist either. It "emerges" from quantum mechanics through decoherence, but classical reality is only an approximation to the "real" underlying truth. But here's the rub: we ourselves inhabit this approximation and I don't mean that in a metaphorical way. We humans are classical entities. The computer that you are reading this essay on is a classical entity. The information that comprises this essay and the patterns of activation in your neurons that reading this essay gives rise to are all classical entities. When we talk about "reality" we have no choice but to suspend disbelief at least to certain extent, because if we really peel back the curtain all the way, we vanish.
And it's not just the quantum curtain that we have to leave in place in order to function. The reality we perceive is actually several layers removed even from fundamental classical reality. For example, we perceive ourselves to have (more or less) solid bodies. But the "real" truth is that the atoms in our bodies are mostly empty space, and what we perceive as solidity is "really" clouds of electrons jealously guarding their territory by means of their mutual repulsion. Then there are cognitive disconnects with "reality" that manifest themselves as optical illusions, delusions, sensory blind spots of various sorts, etc. etc. etc. Then there are subjective experiences that are only accessible to you. Does chocolate "really" taste good? How about sushi? Are you "really" in pain, or is it all just "in your head"?
Note that I am not arguing for metaphysical relativism here. It is not true, as some imagine, that you can remake "reality" (whatever that means) simply by visualizing a better future or some such new-age claptrap. What I am arguing for is that there are different kinds of reality. Reality is not one monolithic thing. Reality is layered, with one layer emerging from the one below. Classical reality emerges from quantum reality. Chemistry emerges from physics. Life emerges from chemistry. Brains and other data processing devices emerge from life. Memes emerge from brain. Mega-memes emerge from collections of brains.
Whether a proposition is true, or even meaningful, depends on which of these layers you choose to operate on. In the meme layer, Darth Vader is Luke Skywalker's father. In the classical reality layer, Darth Vader and Luke Skywalker do not exist (and hence the question of paternity is meaningless).
Many philosophical conundrums can be resolved by being explicit about which layer of reality you're operating on. At the quantum layer, shadow photons "exist". (I put "exist" in scare quotes here because photons, shadow or otherwise, don't "really exist" at the quantum layer.) At the classical layer, they don't. At the subjective layer, we have free will. At the chemical layer, we don't.
There. I have just singlehandedly solved mankind's most intractable problems. Seems like a good day's work. I'm going to go get some lunch.
I take it that you're not a fan of Multiple Worlds. The idea that quantum superposition is all there is, there never is a collapse, and in the double-slit experiment both detectors register a photon -- it's just that there are different copies of the human experimenter, each of whom sees only a single detector go off.
Is it just intuitively unappealing to you, or do you have some other problem with it?
I am indeed not a "fan" of multiple-worlds. But I do believe that superposition is "all there is" -- at the quantum level. There is indeed never a collapse. But it is plainly not the case that "both detectors register a photon", and how to account for this is what all the fuss is about.
Personally I like the QIT interpretation: there is no photon, and there are no detectors. Photons and detectors are classical entities. They only emerge at the classical level, they do not exist at the quantum level.
Multiple-worlds is not just intuitively unappealing, it is plainly at odds with experiment. The experimental evidence that there is only one of me (and only one of you) is overwhelming. In fact, the whole notion of an "experiment" (or even a "result") is a classical notion. The only reason that any of this is at all problematic is that people try to conflate classical reality and quantum reality. Trying to decide whether both detectors "really" register a photon is *exactly* like trying to decide whether Darth Vader is "really" Luke Skywalker's father, with the exact same attendant difficulties.
Put this another way: at the classical level there are only two tenable theories: either there are zero classical universes (the QIT story), or an infinite number of them (multiple worlds). Either choice is equally valid. Personally, I find zero to be the parsimonious choice. YMMV.
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