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Post by kristian on Jan 11, 2018 21:15:47 GMT
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Post by pellius on Jan 11, 2018 21:34:55 GMT
This has been pretty well established for quantum phenomena for quite some time. I'm not sure why the linked article has a photo of the Large Hadron Collider. It's not in Australia, and not needed to perform the so-called delayed choice quantum eraser experiment. en.m.wikipedia.org/wiki/Delayed_choice_quantum_eraserDefinitely counterintuitive. I think this stuff is absolutely fascinating. It makes me wish I was a lot better at math. Then I might have been able to study physics in college.
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Zen_Hydra
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Post by Zen_Hydra on Jan 11, 2018 22:37:21 GMT
This only exists at the quantum level, and does not scale up.
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Post by Timo Nieminen on Jan 11, 2018 23:05:59 GMT
The gist is that cause and effect was reversed. The short answer is that no, time didn't go backwards. The "causality violation" stuff in delayed choice experiments is due to bad explanations, and an over-literal interpretation of wave-particle duality. To be fair, wave-particle duality doesn't just live in coffee-table book physics and pop science, but is also deeply entrenched in teaching about quantum mechanics. But, fundamentally, there is no wave-particle duality. First, there are no particles. We have quanta, which are often analogised as similar to classical particles but are not (the "particle" behaviour we see is mostly due to the wavefunction, which gives particle-like trajectories in the short wavelength limit just like wave optics gives rays of light in the short wavelength limit. Second, there is no duality: we have both quanta and wavefunctions at the same time. If you get away from thinking of photons (and other quanta) as little classical-like marbles of light, then the causality "problem" goes away. Over-classical particle-picture of quanta have been misleading people for a long time (e.g., the controversy over the Hanbury Brown-Twiss intensity interferometer en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect when it was first done with visible light). For a fun paper on problems with over-literal reading of "particle" in quantum mechanics, see Willis Lamb, "Anti-photon" (famous enough so you should be able to find a copy).
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Post by MOK on Jan 11, 2018 23:09:17 GMT
"I think I can safely say that nobody understands quantum mechanics." (Richard Feynman, The Character of Physical Law)
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Post by Timo Nieminen on Jan 11, 2018 23:34:06 GMT
Enough problems arise from sloppy analogies when people use them to "understand" quantum mechanics that Feynman's favourite "shut up and calculate" interpretation of quantum mechanics is useful.
For sure, standard causality-non-violating theory and maths predicts the results of delayed choice experiments. Any "time going backwards" is only introduced by humans trying to "understand" it.
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Post by Jordan Williams on Jan 12, 2018 4:29:15 GMT
They asked me if I had a degree in theoretical physics; I told them I had a theoretical degree in the subject, and was given the job. - Mr. Fantastic, NCR Scientist.
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Post by AndiTheBarvarian on Jan 12, 2018 5:24:34 GMT
This stuff always makes me feel uncertain. Particle-wave duality can be experienced after boozing!
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Uhlan
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Post by Uhlan on Jan 12, 2018 5:40:07 GMT
Check out Lubos Motl: motls.blogspot.nl/https://motls.blogspot.nl/He is one of only a very few in this world who really functions on this very high level. A super Sheldon Cooper if you will. He absolutely makes short shrift with the likes of Sabine Hosenfelder and other, what he calls, idiots. Lee Smolin is an other one of those ideological driven leftards. When he rips them to shreds its very funny to read. Don't worry. Nothing is gonna happen. Cheers.
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LeMal
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Post by LeMal on Jan 12, 2018 6:30:30 GMT
This only exists at the quantum level, and does not scale up. Pretty much captures it. And as pellius pointed out, nothing that hasn't been known for quite some time. Or rather, there's a combination here of two features that have been known for some time: "retrocausation" because of its time-reversible symmetry, and entanglement and non-locality. I guess what has everyone excited is that this is an experiment combining those features. Still, if long seen separately in quantum experiments, why wouldn't they combine?
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christain
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Post by christain on Jan 12, 2018 11:05:51 GMT
SCH-WIIING...Right over my head.
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Post by AndiTheBarvarian on Jan 12, 2018 11:52:24 GMT
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Post by AndiTheBarvarian on Jan 12, 2018 11:53:16 GMT
It's a bit like this!
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Post by MOK on Jan 12, 2018 12:04:34 GMT
Yes, it is.
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Post by Cosmoline on Jan 12, 2018 18:20:21 GMT
Isn't it a mistake to talk about "time" at all in this context? It's just causality, right? Time is a human concept based on perception. It isn't necessary for atomic interaction.
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christain
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Post by christain on Jan 12, 2018 20:31:43 GMT
Isn't it a mistake to talk about "time" at all in this context? It's just causality, right? Time is a human concept based on perception. It isn't necessary for atomic interaction. Exactly. It's the same as 'a tree falling in the woods'. If no one or nothing with ears is around to hear it ....
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LeMal
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Post by LeMal on Jan 12, 2018 20:35:35 GMT
Isn't it a mistake to talk about "time" at all in this context? It's just causality, right? Time is a human concept based on perception. It isn't necessary for atomic interaction. Well, not exactly. Perhaps in some ways more the reverse. "Time" indeed has specificity in physics, particularly of course (dimensionally) in relativity. The problem, especially regarding our perception, is perceiving Time as having a directional "arrow." When in neither relativity, nor in quantum theory, there's a preferred direction. This is what in turn mucks with our perception--and questions of causality. To us, Time's arrow is inextricably linked with the "past" CAUSING the "present/future." The question then becomes: if that's not the case, why does it sure as hell seem so, from our experience, and on the macro-scale? en.wikipedia.org/wiki/Entropy_(arrow_of_time)Personally, I lean toward most if not all of the explanation being the thermodynamic arrow of time. Simply put, "retrocausation" (or, for those who object to the term, non-locality that appears retro-casual) indeed being not only possible but very common--at the very small scale, in tiny, individual quantum systems. But "time travel" not being something that can scale up because of the mathematical limitations. The odds of pulling it off (just like the odds of me phasing through a "solid" wall just because it's in principle possible) just being too prohibitive. Still, the fact that it may be practically impossible to scale it all up hugely doesn't mean it's not interesting. And who knows what groundbreaking technology we might come up with if we can either scale it up just a smidgen, and/or creatively use it at the scales we've easily discovered? All we have to do is look at what's happening with quantum computers (or even simple electronics before that) to see that. It's not just that we only can get the wildest sci fi scenarios or nothing. "Small moves."
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Post by Timo Nieminen on Jan 13, 2018 0:15:37 GMT
The problem, especially regarding our perception, is perceiving Time as having a directional "arrow." When in neither relativity, nor in quantum theory, there's a preferred direction. Same for classical (i.e., Newtonian) mechanics, classical electromagnetic theory (as far as the basic theory and maths goes; see below), optics, and more. The only major branch of theory in physics with an in-built directionality of time is thermodynamics. Entropy: in classical thermodynamics, it's phenomenological rather than fundamental (which can be said about most stuff in classical thermodynamics), and in statistical mechanics, it's a statistical result. Classical electromagnetism is interesting because the theory is time symmetric, but we have an arrow of time from observed phenomena: we see outgoing radiation, but not incoming radiation. This can be explained as a statistical result. None of these have anything to do with quantum eraser experiments. Personally, I lean toward most if not all of the explanation being the thermodynamic arrow of time. Simply put, "retrocausation" (or, for those who object to the term, non-locality that appears retro-casual) indeed being not only possible but very common--at the very small scale, in tiny, individual quantum systems. But "time travel" not being something that can scale up because of the mathematical limitations. The odds of pulling it off (just like the odds of me phasing through a "solid" wall just because it's in principle possible) just being too prohibitive. Simple quantum eraser experiments can be done at large scales. Laser, beamsplitter, and a couple of mirrors: en.wikipedia.org/wiki/File:Beam_Split_and_fuse.svg as described in en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#A_simple_quantum_eraser_experimentFor experiments using multi-photon states (entangled states) and things like helium atoms, there are technical issues like brightness and coherence of sources, but these experiments don't have the issues one gets with trying to scale up quantum computers (which is scaling up to many gates, not scaling up to large physical sizes, and has noise and loss issues that quantum eraser experiments don't care about). The short story on which the movie Arrival was based made a big deal of something related: Fermat's principle of least time. How does the photon know what will be the quickest path from A to B (which is indeed the path it follows from A to B) without knowing that it's going to B? When asked in this backwards way, this has exactly the same mysteries as retrocausation in delayed choice quantum eraser experiments. If you ask in the forward way, there is no time travel, no foreknowledge of the future by photons, and no retrocausation: Q: Why does the light arrive at B? A: Because that's where it went. If I point a laser pointer at a screen, a dot of light appears where I point it. It appears there because I pointed the laser pointer there. Why would I ask how the photons knew that I was pointing the laser pointer there so as to know how to find their way there? If I point a water-pistol at the same screen, why would I ask how the water knows to get to the spot I squirted it at? en.wikipedia.org/wiki/Story_of_Your_Life
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LeMal
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Post by LeMal on Jan 13, 2018 4:16:41 GMT
Well, exactly. The operative "scaling" problem is simply the nature of the application, how much coherence is needed and how long or well it needs to be maintained. My point being is that when people get over-excited about matters like retrocausation/time travel, they fail to make the distinction between applications where that's easy (which are either usually small, or in the scaled up cases like the laser, too simple to do much with) and those involving, probably in most imaginations, a human body.
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stormmaster
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Post by stormmaster on Jan 13, 2018 4:26:41 GMT
u guys watch that show 12 monkeys?
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