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.
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.
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.
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
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 flawed textbook accounts of these experiments but
to flawed inferences about retrocausality in the context of delayed choice versions of separation
experiments.
My layman's guess is that our interpretation of the fundamental explanation for quantum mechanics is wrong. That wave-particle duality is wrong. There is an old alternative explanation which has been gaining some attention lately: pilot wave theory. The TL;DR is that there is both a wave and particle. The particle generates the wave, but is also influenced by interacting with it. Veritassiun has a great video on it which is compelling.
But again, I am not a physicist. Just an enthusiastic outside observer.
Quantum physicist here. I can only say that reality down there at the quantum level is really really weird. You can get used to it, but forget making sense of it.
A delayed choice setup is not too dissimilar than a Bell inequality violation experiment. The weirdness there is that you can set things up such that no signal can travel between the systems being measured, and yet the outcomes are more correlated than any classical joint state can be.
So the conclusion is that either locality fails (i.e. it’s not true that outcomes on one side are independent of how you measure the other side) or realism fails (i.e. you can’t assign values to properties before the measurement, or in other words a measurement doesn’t merely “reveal” a pre-existing value: the values pop into existence in a coordinated fashion). Both of these options are crazy, and yet at least one of them must be true.
My understanding is that a lot of the weirdness is that there may be no such thing as a photon, that is, what we call a photon is the em field interacting with matter(electrons really), in transit there is no such thing as a "single photon" That is, the em field is not quantized( or at least not quantized at the level of a photon) It is very possible to have your em field at "sub photon level" and the electrons at the far end will accumulate energy until a photon level is reached and presto a single photon experiment. What it is actually measuring I am not sure.
My understanding of the delayed choice experiment is that causality is preserved only because you can't actually determine the result of a single event without comparing the two results. It's spooky action at a distance, except it's temporal instead of spatial distance.
Which honestly I find to be pretty flimsy reasoning. It's almost ontological: causality is preserved because we can't prove that it isn't.
I know it's more complex than that, but it still feels like papering over a hole in our theories.
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%?
Maybe someone in the field can speak up -- I'm not sure what is new about this study. It seems to be about an analysis of the double-slit experiment using individual atoms, and the press release implies that this is novel, but that experiment was first done over 30 years ago [0]. Is there anything to this study that is actually new?
It seems that photons propagate as massless non-local waves, but become localized when they bounce off an already local, massy particle. In other words, electromagnetic radiation is a fundamentally continuous phenomenon in a continuous field, but emission, absorption and other interactions can only happen as discrete events (quanta).
17 comments
[ 3.3 ms ] story [ 38.1 ms ] threadEspecially 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.
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.
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.
- 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.
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
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 flawed textbook accounts of these experiments but to flawed inferences about retrocausality in the context of delayed choice versions of separation experiments.
But again, I am not a physicist. Just an enthusiastic outside observer.
A delayed choice setup is not too dissimilar than a Bell inequality violation experiment. The weirdness there is that you can set things up such that no signal can travel between the systems being measured, and yet the outcomes are more correlated than any classical joint state can be.
So the conclusion is that either locality fails (i.e. it’s not true that outcomes on one side are independent of how you measure the other side) or realism fails (i.e. you can’t assign values to properties before the measurement, or in other words a measurement doesn’t merely “reveal” a pre-existing value: the values pop into existence in a coordinated fashion). Both of these options are crazy, and yet at least one of them must be true.
https://www.youtube.com/watch?v=SDtAh9IwG-I (Huygens Optics -How big is a visible photon?)
Which honestly I find to be pretty flimsy reasoning. It's almost ontological: causality is preserved because we can't prove that it isn't.
I know it's more complex than that, but it still feels like papering over a hole in our theories.
Because you are definitely not rewriting history, and absolutely nothing is traveling back in time whatsoever.
https://profmattstrassler.com/2025/03/18/quantum-interferenc...
0: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.70...
I feel like the type of detector would have a significant impact on the outcome of the experiment, but explanations for layman always leave it out.