Post B8blxqzXaGM6JPbLbk by sjb@mstdn.io
(DIR) More posts by sjb@mstdn.io
(DIR) Post #B8ZPSTgotD9nxa4uNE by wolf480pl@mstdn.io
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Do I know any quantum physicists?Cause it looks like in the "double-slit experiment", the part about "interference pattern disappears when you observe which way the electron went" was never confirmed experimentally, and I'm starting to suspect it's a misunderstanding.EDIT: and by "disappears" I mean "turns from a double-slit pattern into a single-slit pattern"
(DIR) Post #B8ZUDSGyNIUAzbe0jg by jbowtie@cloudisland.nz
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@wolf480pl https://journals.aps.org/prl/abstract/10.1103/zwhd-1k2t
(DIR) Post #B8ZUDSV9WZpPhaHKq0 by jbowtie@cloudisland.nz
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@wolf480pl Having linked that article, the key thing to understand is that you can't detect photons without absorbing the photon - so it's only been the last decade or two we've actually had the technology to do experimental verification of the interference pattern disappearing. See also https://arxiv.org/abs/1710.02216 for example
(DIR) Post #B8ZUDShYmRkkK45FB2 by wolf480pl@mstdn.io
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@jbowtie I love the jumps between expected familiarity levels of the audience."DId you know electrons are particles but sometimes behave as waves? Also, obviously everyone knows what a tungsten nano-tip is."
(DIR) Post #B8ZXhDNYc78FLmbegy by jbowtie@cloudisland.nz
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@wolf480pl Sorry, maybe going straight to primary sources was a mistake.The Wikipedia page on the double-slit experiment is pretty good as a starting point, the history section covers key experiments - https://en.wikipedia.org/wiki/Double-slit_experimentEDIT: Both of the experiments I linked are referenced in the "Other variations" section of that page.
(DIR) Post #B8ZXhDhlOzIMMS3nBg by wolf480pl@mstdn.io
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@jbowtie nah, I'm managing to slowly follow along, got to page 8 which seems to be where the interesting part starts.
(DIR) Post #B8ZY2lhcC90JssGXRI by wolf480pl@mstdn.io
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@p tbh. I've never seen a description of such an experiment where it was explained how to detect the particles crossing a slit without absorbing it.Well, other than the quantum eraser one, that "clones" the photons using a BBO crystal. But in that one the interference pattern only appears when you use the information from the detectors to select the right subset of the events from the screen.
(DIR) Post #B8ZZRr79JKQ7rWjfJA by wolf480pl@mstdn.io
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@jbowtie ok, got to page 10, where they stop talking about experiment and start talking about simulations.AFAIU, what they showed is:- they can use a laser to control which parts of a 100nm thick tip of a tungsten needle emit electrons- if they make two sites next to each other emit electrons at the same time, there's also a third beam in between resulting from interferenceso basically they're covering one or the other slit, which obviously breaks the interference.1/
(DIR) Post #B8ZZkAywBMqKh1d9E0 by wolf480pl@mstdn.io
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@jbowtie That's not the same as observing which slit a electron (or photon) passed through.AFAIU, there's no spooky 'it only happens when you don't look" in the second experiment you linked.2/
(DIR) Post #B8ZaROKsPPCuYrUzrM by wolf480pl@mstdn.io
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@jbowtie Regarding your earlier posts - yes, I know detecting a photon destroys it. Which is what got me wondering how the "which-way" experiment was done.3/
(DIR) Post #B8Zae4u9xoSS3u3Gym by wolf480pl@mstdn.io
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@jbowtie Also, I did look at the wikipedia article you linked before asking on fedi, but it's jumbles together what to me is three completely different experiments, proding at different questions:A. two strong beams, showing wave natureB. one particle at a time, showing that a particle can interfere with itselfC. which-way experiments, showing how observation affects the outcome, and the motivation behind the "wave function collapse" interpretationI have no problem with A and B.4/
(DIR) Post #B8Zaw9zZmtrx3KCvYW by wolf480pl@mstdn.io
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@jbowtie But the wikipedia article makes it quite difficult to figure out which of the experiments it describes are "which-way" ones, and for some if them - like Wheeler's delayed-choice - even if you follow the link, it doesn't say how the experiment is supposed to work.Anyway, back to trying to undestand "Coherent and incoherent light scattering by single-atom wavepackets"
(DIR) Post #B8Zl3XexBrss1Ziho8 by sjb@mstdn.io
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@wolf480pl @p --[I've never seen a description of such an experiment where it was explained how to detect the particles crossing a slit without absorbing it]--That's because YOU CAN'T. Quantum information is conserved, so if you observe the electron you change its state.Observation means some photon, electron or other particle interacted with the one in the experiment, which delivers information to the experimenter but also changes the original particle's wavefunction.
(DIR) Post #B8Zo0obe1hvtZqZ0jI by wolf480pl@mstdn.io
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@sjb @p ok, so basically there's no "spooky action at a distance" here, I can just treat it as "every interaction that leaves a trace of where the particle went also messes up the particle"?
(DIR) Post #B8Zo5Z7N2U04zNfUzw by sjb@mstdn.io
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@wolf480pl @p No information can be transmitted faster than light, but superpositions are still a thing.
(DIR) Post #B8ZtiYz1yMFZdWBN6e by loke@functional.cafe
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@wolf480pl @sjb @p there might be. What you imply in your question is whether hidden variables can be used to explain observations (i.e. the photon took a specific path, but the path cannot be observed).Now, we have to add another restriction, which is that the hidden variables should be local, but worth that assumption the Bell Test rules this out. https://en.wikipedia.org/wiki/Bell_testBut there are nonlocal theories where particles do take a specific path, with Pilot Wave Theory being the main one. It is, however, a nonlocal theory which means that it depends on all other particles which you could call spooky action if you like?
(DIR) Post #B8aJXkNdr9DSbeCHVw by wolf480pl@mstdn.io
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@lokethere's also MWI which males the most sense to me, I just didn't understand how this experiment works in MWI@sjb @p
(DIR) Post #B8aTKNpU58LOExy1E8 by taylan@fedi.feministwiki.org
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@wolf480pl Sabine Hossenfelder released a video about this just a day or two ago. Can recommend her content on this stuff.
(DIR) Post #B8aTKO3fEPgcwwbLKS by wolf480pl@mstdn.io
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@taylanyeah, I watched it, this post is a follow-up to it
(DIR) Post #B8aVTGU4xbgvySJlDs by wolf480pl@mstdn.io
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@lokeactually, let's step back for a second:is a which-way double-slit experiment already unexplainable with local hidden variables, or do you need a different experiment to show that local hidden variables are insufficient?@sjb @p
(DIR) Post #B8aZAkkKccik93Ldtg by wolf480pl@mstdn.io
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@lokehmm ok after thinking about it more, the only local hidden variables I can come up with end up looking like either MWI or Pilot Wave (which IIRC is just MWI with one world line designated as "real")@sjb @p
(DIR) Post #B8acHNqeSF67199pLs by loke@functional.cafe
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@wolf480pl @sjb @p in the Copenhagen interpretation, the measurement happens at the wall where there photos are registered. The spooky action in question would be the instant communication over the entire target area to ensure that you don't have more than one registration of a photon. This communication would have to be instantaneous.
(DIR) Post #B8adcafM5qRuo4Y9Ue by wolf480pl@mstdn.io
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@lokeYeah but "measurement" is even more spooky than action at a distance.@sjb @p
(DIR) Post #B8aeU2nQEk1TOloOAK by loke@functional.cafe
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@wolf480pl @sjb @p yes, and it's not even well defined. It's all very handwavy.It's where the popular description of the double slit experiment comes from though. If you want to detect if a particle passed a certain slit you have to perform a measurement there, so you'll collapse the wavefunction.
(DIR) Post #B8blxqzXaGM6JPbLbk by sjb@mstdn.io
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@wolf480pl @loke @p I think you end up with two observers superimposed, one for each outcome of the measurement. In each one's world, the particle went that way. The interference apparently disappears because the observer can no longer access other parts of the wavefunction.Basically |Obs>(|A>+|B>) turns into|Obs=A>|A> + |Obs=B>|B>and those added terms evolve independently because of the linearity of quantum mechanics.
(DIR) Post #B8bmbjDVuQLrmb9PaS by sjb@mstdn.io
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@wolf480pl @loke @p Physicists now seem to prefer something called "einselection" but to be honest I don't understand it. So this may be wrong.
(DIR) Post #B8bmbjQH8yYmQB7bTk by wolf480pl@mstdn.io
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@sjb @loke @p ok so let's consider a spot on the screen that is dark in the two-slit interference pattern but bright in a single-slit patternthere are timelines in which the particle lands in that spot, but for every such timeline where it went through slit A, there's a timeline where it went through slit B, and that timeline is identical except 180° out of phase, so these timelines cancel out, i.e. they end at that point and nobody is there to remember them as one's past?1/
(DIR) Post #B8bmjpxyhNnZ24kbho by wolf480pl@mstdn.io
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@sjb @loke @p but when the which-way information changes some other part of the world in one timeline in a pair differently than in the other, then they're no longer identical-except-negated, they're different in other ways, which means they don't cancel out?
(DIR) Post #B8bn1L5yDPsaV3poIa by sjb@mstdn.io
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@wolf480pl @loke @p The "180 degrees out of phase" just means you're subtracting something from the wavefunction instead of adding it. If cancellation is perfect it just makes a zero probability of observing something. Now remember the *screen* and the *observation before the screen* are different and time-ordered, so one overrides the other.
(DIR) Post #B8bnEDn8gKZHc2AYNM by wolf480pl@mstdn.io
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@sjb @loke @p > just subtractingyeah I know how e^iφ workswhat do you mean one overrides the other?
(DIR) Post #B8bneUkeFRUzk2X49g by sjb@mstdn.io
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@wolf480pl @loke @p When you haven't observed the particle, its wavefunction is just psi(x) where x is its position, multiplied by the rest of the universe of course, but not ENTANGLED with the rest of the universe.As soon as you observe it going through a slit, the wavefunction is F(x,observer) not f(x)g(observer). So then the situation isn't so clean, there is no longer a perfect cancellation, or at least it depends on what observer's state is.
(DIR) Post #B8bnl98f0BVejow3ma by wolf480pl@mstdn.io
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@sjb @loke @p isn't that the same thing that I said?
(DIR) Post #B8bntpn0JNS1KjwsMK by sjb@mstdn.io
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@wolf480pl @loke @p Dunno, but you worded it as a question, so I gave my explanation.
(DIR) Post #B8bnuPogMcBT4N0iyO by wolf480pl@mstdn.io
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@sjb @loke @p I worded it as a question hoping you'd agree or disagree xD