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What is a superposition really like?

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http://philipball.blogspot.com/2018/05/w...-like.html

EXCERPT: Here’s a longer version of the news story I just published in *Scientific American*, which includes more context and background. [...] what the experiment does show is that quantum mechanics suggests that superpositions are not always simply a case of a particle seeming to be in two places or states at once. A superposition, liker anything else in quantum mechanics, tells you about the possible outcomes of a measurement. All the rest is contingent interpretation. [...]

[...] Bohr’s argument that quantum mechanics is silent about ‘reality’ beyond what we can measure has long seemed deeply unsatisfactory to many researchers. “We know something fishy is going on in a superposition”, says physicist Avshalom Elitzur [...] “But you’re not allowed to measure it”, he says – because then the superposition collapses. “This is what makes quantum mechanics so diabolical.” [...]

So this thought experiment seems to lift part of the veil off a quantum superposition, and to let us say something definite beyond Bohr’s “Don’t ask” proscription. The TSVF [two-state-vector formalism] opens up the story by considering both the initial and final states, which allows one to reconstruct what was not measured, namely what happens in between. “I like the way this paper frames questions about what is happening in terms of entire histories, rather than instantaneous states”, says physicist Ken Wharton [...] “Taking about ‘states’ is an old pervasive bias, whereas full histories are generally far more rich and interesting.”

And the researchers’ interpretation of that intermediate history before measurement is extraordinary. The apparent vanishing of particles in one place at one time, and their reappearance in other times and places, suggests a new vision of what the underlying processes are that create quantum randomness and nonlocality. Within the TSVF, this flickering, ever-changing existence can be understood as a series of events in which a particle is somehow ‘cancelled’ by its own “counterparticle”, with negative energy and negative mass.

Elitzur compares this to the notion introduced by British physicist Paul Dirac in the 1920s that particles have antiparticles that can annihilate one another – a picture that seemed at first just a manner of speaking, but which soon led to the discovery that such antiparticles are real. The disappearance of quantum particles is not annihilation in this same sense, but it is somewhat analogous.

So while the traditional “two places at once” view of superpositions might seem odd enough, “it’s possible that a superposition is a collection of states that are even crazier”, says Elitzur. “Quantum mechanics just tells you about their average.” Post-selection then allows one to isolate and inspect just some of those states at greater resolution, he suggests. With just a hint of nervousness, he ventures to suggest that as a result, measurements on a quantum particle might be contingent on when you look even if the quantum state itself is unchanging in time. You might not find it here when you look – but had you looked a moment later, it might indeed have been there. Such an interpretation of quantum behaviour would be, Elitzur says, “revolutionary” – because it would entail a hitherto unguessed menagerie of real states underlying counter-intuitive quantum phenomena.

The researchers say [...hope...] the actual experiment [...will...] roll in three to five months. “The experiment is bound to work”, says Wharton – but he adds that it is also “bound to not convince anyone of anything, since the results are predicted by standard quantum mechanics.”

Elitzur agrees that this picture of a particle’s apparent appearance and disappearance at various points along the trajectory could have been noticed in quantum mechanics decades ago. But it never was. “Isn’t that a good indication of the soundness of the TSVF?” he asks. And if someone thinks they can formulate a different picture of “what is really going on” in this experiment using standard quantum mechanics, he says, “well, let them go ahead!”

MORE: http://philipball.blogspot.com/2018/05/w...-like.html
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