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Comment by AnimalMuppet | Hacker News Reader

I think this is a misunderstanding of what's going on in quantum mechanics. A photon is emitted as a probability wave function. (Note well: This is not the same as "sometimes a photon acts like a wave!) It travels as a probability wave function. It gets to the detector as a probability wave function. The form of the detector determines how the wave function collapses into an observable state. That's when the "choice" gets made. There is no projection of the choice onto the past; there is only the projection of the choice onto the probability wave function in the present. So this "retrocausality" stuff is actually a fundamental mis-understanding of quantum mechanics.

What the article got right: Relativity and quantum mechanics are talking about two fundamentally different things when they use the word "time". Reconciling those two ideas is going to be critical to a fundamental understanding of the universe.

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That is true, but, (if you haven't already), I would recommend looking into the delayed choice quantum eraser experiment. I think it is probably more true to the essences of what the article was trying to communicate.

I don't think I can do the idea justice, but the basic idea is such: if you 'tag' the photons in the double-slit experiment by which path they went through, then you necessarily break superposition and won't see an interference pattern.

However, if you 'throw' away this information completely, the interference pattern re-emerges. The delayed-choice part of it is just a way to through away the 'tag' information after the photon has moved through the slits. It seems to suggest that you can uncollapse a wavefunction if you just erase any knowledge you have of the measurement you made. (I've probably butchered it, read here for more[1])

Although, honestly, you don't need retrocausality, and personally I think it has much more interesting applications to information theory (how does one destroy information? How does the wavefunction know about our records? What even is information, man? etc)

[1] http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

Related, worth watching (IMO):

http://youtu.be/dEaecUuEqfc

The issue with this circumvention is that it assumes wave function collapse, which is equally mysterious. If you do away with collapse, you are left with a many-worlds type interpretation, in which the wave function simply evolves unitarily forever. It is in this sense that the current state "dictates" the previous states---the histories consistent with one measurement or another separated based on the kind of measurement made. If you do the interference experiment, the histories consistent with outcomes of that experiment are ones in which the photon takes no definite path. If you do the path-determining experiment, you no longer get interference patterns but also the consistent histories are ones in which the photon takes a definite path---either one slit or the other.

Since you get to decide which kind of experiment to do after the photon passes the slitted screen, your decision seems to force the histories to become either the interference kind or the definite-path kind after you might expect that to have already been determined. That's the sense in which the future seems to control the past.

I agree that this was not explained clearly in the article. It's a very slippery subject.

Let's try to fool a wave function: Let's take one, run it through a beam splitter, take the two halves, and put one in a lab that is standing still and has a normal clock and the other half in a moving lab, a space ship, with a slow clock.

But, we want the wave function to be of a particle that also has a clock. So, let the wave function be that of an atom of a radioactive element with a short half life.

We get in our space ship and travel at, say, 90% of the speed of light, let the particle go, let it encounter the beam splitter, let one half of the wave function continue to bounce around inside our space ship with a slow clock and let the other half exit the space ship through a window, enter the lab that is standing still, and bounce around there.

Then the question is, what is the half life of the particle? Is the expected time, in the clock of the normal lab, to decay different for decays in the space ship than that for decays in the lab that is not moving?

I'm sure this is all wet in some sense, but I'm trying yet another students efforts to trip up quantum mechanics with special relativity!

The setup you describe doesn't bring QM and SR into conflict because the two split halves of the wavefunction never do anything interestingly "quantum". The radioactive decay process acts as a measurement of "which branch of the beam-splitter did the particle go down?" and so the experiment can be analyzed as if the particle went down one branch of the other in its entirety and was never split.

To get at the quantum effects you'd have to consider a case where the two parts of the wavefunction are eventually brought back into interference with each other, but any sort of decay process is going to make them mutually incoherent, so you won't see any interference fringes. You might get something interesting by having a moving mirror in an interferometer, which would redshift the reflected wave in that arm by a bit, and that would result in moving interference fringes due to the resulting frequency difference (you'd need to gate the interference pattern on the position/velocity of the mirror to see this.)

But even in that case, nothing very interesting would happen. In any experiment where you can detect which arm a particle goes down it will behave as if it went down that arm, even if your choice is delayed. In any interference experiment it will be impossible to tell which arm it went down. Nature is really good at doing the book-keeping to maintain the quantum veil.

Thanks! I suspected I was all wet!

I was guessing that there might be tricking some EPR thing due to the two paths being different in SR. Or I was trying to mess up the "bookkeeping"!

You seem to want to get the situation back to the Michelson-Morley interferometer! Okay! Or, sure, just Young's double slit.

I know; it's not nice and not easy to fool Mother Nature!

When I get done with software and business, I want to take a careful pass through SR, GR, and QM looking for just something wrong with the bookkeeping. Wheeler was, so I'll try too!

Thanks. I needed that!

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