[HN Gopher] Double-slit experiment holds up when stripped to its...
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Double-slit experiment holds up when stripped to its quantum
essentials
Author : ColinWright
Score : 48 points
Date : 2025-07-31 08:00 UTC (2 days ago)
(HTM) web link (news.mit.edu)
(TXT) w3m dump (news.mit.edu)
| habibur wrote:
| I am more interested in its explanation, now that the theory has
| been proven correct again and again.
|
| Especially interested in "delayed choice quantum erasure
| experiment", where you decide to determine the "which path" after
| the photon has passed through the slits and hit the detector. And
| depending on your later decision the photon seems to rewrite
| history going back in time.
| layer8 wrote:
| See https://en.wikipedia.org/wiki/Delayed-
| choice_quantum_eraser#....
| justonceokay wrote:
| My understanding is that this "temporal fuckery" (I'm not a
| physicist) exists even in the basic math of light diffraction.
| When light passes from air to water, it somehow "knows" the
| right angle to diffract at to reach its destination as fast as
| possible, even though from a classical viewpoint the
| destination is not known until after the light has passed
| through the medium.
|
| The short story "Story of your life" (that the movie Arrival is
| based on) uses this as a pseudo-argument for how the aliens
| could have a non-temporal understanding of reality.
| Strilanc wrote:
| The standard explanation for light "knowing" the angle of
| diffraction is that actually light just propagates in every
| direction and then constructive interference is stronger for
| paths near the shortest path because its length is more
| consistent when the path is perturbed (meaning the phases of
| the perturbed paths tend to agree more so they add up instead
| of cancelling). I don't think you even need quantum mechanics
| for this; it occurs in classical wave optics.
|
| You can see Feynman explaining mirrors this way in recorded
| lectures [1]. There's also a recent Veritaseum video
| explaining why the shortest paths dominate [2].
|
| 1: https://youtu.be/SsMYBWpsQu0?si=o1eAEvESwjroTke3&t=2251
|
| 2: https://www.youtube.com/watch?v=Q10_srZ-pbs
| tsimionescu wrote:
| That only works with quantum mechanics - it's a consequence
| of the "path integral" idea of QM. In classical optics this
| wouldn't work, because you'd be able to detect light on the
| other paths if it really did take all paths.
| pdonis wrote:
| Classical optics is just the limiting case of quantum
| optics when the path length is much longer than the
| wavelength. In such a case quantum optics predicts
| basically zero probability to detect light on any path
| other than the classical path--which is classical optics.
| So classical optics doesn't say anything that's actually
| contradictory to quantum optics. It's just a special
| case.
| Strilanc wrote:
| I think you're confusing the distinction between
| classical ray optics and classical wave optics with the
| distinction between classical wave optics and quantum
| mechanics. Quantum mechanics and classical wave optics
| agree on the explanation for diffraction as a path
| interference effect. In classical optics, the reason you
| don't see light coming from angles away from the shortest
| path is because of destructive interference between the
| other paths.
|
| For example, note that the Huygens principle predates
| quantum mechanics by over 200 years [1]. As another
| example, diffraction gratings (which manifestly require
| interference between different paths) were being made in
| the mid 1800s [2] but in physics documentaries you never
| hear of people being confused about how to explain their
| behavior. Because they are explained by classical wave
| optics. Also see this lecture which talks about
| diffraction in the context of ray optics [3].
|
| Where wave optics disagrees from quantum mechanics is in
| the dim-light limit, when you start resolving individual
| photons.
|
| [1]: https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresne
| l_princi...
|
| [2]: https://en.wikipedia.org/wiki/Diffraction_grating
|
| [3]: https://www.youtube.com/watch?v=5tKPLfZ9JVQ&list=PLB
| 1A0BF14E...
| naasking wrote:
| I don't think this "standard explanation" is as standard as
| it is sometimes portrayed:
|
| https://youtu.be/XcY3ZtgYis0?si=9TyD5-7B00WTLzOH
| renox wrote:
| > I am more interested in its explanation, now that the theory
| has been proven correct again and again.
|
| What do you call an explanation? An interpretation of QM? There
| are dozens but none are especially satisfying..
|
| As for the 'delayed choices' IMHO it is a poor interpretation
| of the data: see https://www.youtube.com/watch?v=RQv5CVELG3U
| for example.
| Uehreka wrote:
| I don't have a source to hand at the moment, but when I looked
| into the famous Delayed Choice Quantum Erasure experiment the
| consensus seemed to be:
|
| - The double slit experiment's conclusions still hold, but:
|
| - The particularly exciting and stark results of the Quantum
| Erasure experiment may have been misinterpreted or
| miscommunicated to the public, in particular:
|
| - The presenter of PBS SpaceTime has said that he regrets
| certain things about how he worded his video on the Quantum
| Erasure experiment, and I think may have left a comment on the
| video to that effect.
|
| Every time I look into QM, I keep coming back to the same
| fundamental axiom: "Quantum Mechanics' weirdnesses can make
| otherwise straightforward things frustrating, but will never
| make interesting inventions possible." Like how entanglement is
| able to break locality (which is frustrating) but without
| breaking causality (which would be interesting). If you hear
| about a quantum principle and think "Wow, I could use that to
| build X," then it's more likely that you're not fully
| understanding the principle (not "you" specifically, I've
| fallen for this myself countless times).
|
| The only exception seems to be Quantum Computing, but even that
| only arises out of a deep deep mathematical analysis (you can't
| get to QC on your own from the things in popular science books)
| and is only applicable to really niche applications.
| naasking wrote:
| Entanglement doesn't violate locality, it's _measurement_
| that does that. And that 's because we don't have a rigourous
| handle on what measurement actually is, and why we call it
| "the measurement problem"!
| whoknowsidont wrote:
| >about a quantum principle and think "Wow, I could use that
| to build X,"
|
| We use quantum principles to build things all the time. What
| are you talking about?
|
| https://en.wikipedia.org/wiki/Quantum_sensor#Research_and_ap.
| .. is just a few examples.
| Uehreka wrote:
| I'm talking about building sexy things like ansibles or FTL
| engines. The kinds of transcendent ambitions that Quantum
| Mechanics often inspire in laypeople like me.
| scoopdewoop wrote:
| https://youtu.be/fbzHNBT0nl0
|
| This video blew my mind wide open about the double slit
| experiment by showing the simpler case, the single slit
| experiment, and I think it clears up a LOT! Sadly, I can't do
| the explanation any justice
| kgwgk wrote:
| https://philarchive.org/archive/ELLWDC
|
| Why Delayed Choice Experiments do NOT imply Retrocausality
|
| David Ellerman
|
| University of California/Riverside
|
| October 16, 2014
|
| There is a fallacy that is often involved in the interpretation
| of quantum experiments involving a certain type of separation
| such as the: double-slit experiments, which-way interferometer
| experiments, polarization analyzer experiments, Stern-Gerlach
| experiments, and quantum eraser experiments. The fallacy leads
| not only to aawed textbook accounts of these experiments but to
| flawed inferences about retrocausality in the context of
| delayed choice versions of separation experiments.
| briffid wrote:
| I don't get the point. The article says that if you "somewhat"
| measure, then you lose "somewhat" from the wavelike nature. So
| the photon is a wave by X%, and a particle by 100-X%?
| 12_throw_away wrote:
| A quantum object is its own thing - it has both wavelike and
| particle-like properties.
|
| Measurement here might be better understood to "filter out" any
| parts of the wave that don't agree with the measurement. So a
| precise measurement will project out a lot of the wave, giving
| you something more localized and particle-like. A fuzzy
| measurement will project out only a bit of the wave, giving you
| something that's still spread out and quantum and wave-like.
| moktonar wrote:
| The simulation rolls back to match the constraints, easy
| mhb wrote:
| Quantum Interference 1: A Simple Example:
|
| https://profmattstrassler.com/2025/03/18/quantum-interferenc...
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