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from the will-this-help-me-win-the-lottery? dept.
A team in Australia turned thought experiment into lab reality by using lasers. Their subject matter was not a photon but a helium atom. The lasers they used served as a pair of grates, one before the other, with the second grate randomly dropped in.
What they found is weirder than anything seen to date: Every time the two grates were in place, the helium atom passed through, on many paths in many forms, just like a wave. But whenever the second grate was not present, the atom invariably passed through the first grate like a particle. The fascinating part was, the second grate's very existence in the path was random. And what's more, it hadn't happened yet.
In other words, it was as if the helium particle "knew" whether there would be a second grate at the time it passed through the first.
Also covered at: phys.org. An abstract is available; full report is pay-walled. The original news article is at Australian National University
Original Submission
(Score: 4, Interesting) by gman003 on Thursday June 11 2015, @02:29PM
Almost all of the "absurd" results in quantum physics, like this one, come from most quantum physicists having no idea how their own field actually works. Partially because they don't accept the obvious correct interpretation, Many-Worlds, as being correct, but also because even the ones who do follow MWI don't fully understand how it works at an intuitive level.
So let's look at this experiment from a MWI perspective:
Experiment setup: 1 device to launch single helium atom on straight path. Two conditional junctions, A and B, where lasers may or may not alter the atom's trajectory, as determined by a QRNG. A is always present, B is sometimes added (also determined by QRNG) at some point after the atom leaves A. Two unconditional reflectors, c and d, that use lasers to alter the atom's path. Two particle detectors, 1 and 2. And finally, and observer. Setup is such that these are the only valid paths:
B not present, A does not trigger: AcN2
B not present, A triggers: AdN1
No junctions trigger: AcB2
Junction A triggers: AdB1
Junction B triggers: AcB1
Both junctions trigger: AdB2
Atom of helium reaches first junction. QRNG randomly triggers a laser pulse at junction A.
We now have two worlds, one where the helium is going to reflector c, and another where it's going to reflector d. I'll label them Ac and Ad for now. The observer remains unentangled at this point, so they see a superposition of Ac and Ad, which I'll call A*.
A QRNG now determines whether to move junction B into place. If it does, we get two worlds AcB and AdB; if it does not, we get two more worlds AcN and AdN.
The atom of helium continues on to the reflector, then reaches junction B. Here the four worlds split into six:
AcN results in AcN2
AdN results in AdN1
AcB splits into AcB1 (if B triggers) and AcB2 (if it does not).
AdB splits into AdB2 (if B triggers) and AdB1 (if it does not).
However, from what I can see of their experimental setup, the quantum state of B "knows" which way the atom was coming from. So the superposition of states seen by the final particle detector is:
A*N*
AcB*
AdB*
Further, at least in photon-based versions of this experiment, path AdB2 and AcB2 both end at the same detector, but the photons end up 180 degrees out of phase. So detector 2 in an A*B* superposition sees A*B2 as always zero, as the two paths cancel each other out. That is what they consider "acting as a wave". However, when you have distinct configurations AcB* and AdB*, detector 2 does not see a superposition, but separate paths that do not cancel out, and it is "acting as a particle".
Once the atom hits the detector, the observer becomes entangled with the system. So we get A*N*O, AcB*O, and AdB*O.
Because the presence of the second junction is part of the system's quantum state, the observer does not see it acting as a wave when junction B is in position. The observer only sees wave action in A*N* worlds, and only sees particle action in AcB* and AdB* worlds.
Note how no information traveled in any direction through time except forward. The mysterious effect was the observer learning which world they ended up in, not the particle being told which way it was going to go.
Hopefully that made sense, I had to guess on some of their setup. But at least to me, this makes a hell of a lot more sense than "the future affects the past".
(Score: 1, Insightful) by Anonymous Coward on Thursday June 11 2015, @05:02PM
Lol what a jackass.
"the obvious correct interpretation, Many Worlds"
Thanks for the laugh. I will leave you to go back to solving every other trivial matter for those of us with a brain smaller than Jupiter.
(Score: 4, Touché) by threedigits on Friday June 12 2015, @08:54AM
The mysterious effect was the observer learning which world they ended up in
This explains it very nicely for some of the observers, the ones that ended on the "right" worlds that we observed. Can you please explain how observers on the rest of worlds saw it? And more importantly, why we didn't end in one of those worlds?