Boy, Was I Wrong! How the Delayed Choice Quantum Eraser Really works

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REFERENCES

CHAPTERS
0:00 The original paper implied retrocausality
1:23 Really cool metal posters: Displates!
2:37 A classical interpretation would show retrocausality
3:49 How the double slit experiment works
6:25 Debunking the clean double line pattern
7:49 The Delayed Choice Quantum Eraser set up explained
11:54 How the Scientis hand-selected the outcome of the Delayed Choice experiment

SUMMARY
The original paper by the authors who first performed the Delayed Choice Quantum Eraser implied retro causality. But retro causality is true only if you assume a classical way of thinking. But that's not the way quantum mechanics works, and I was wrong for interpreting it that way in my original 2019 video. When viewed with the standard interpretation of quantum mechanics where a particle is always a wave until the moment it is measured, there is no retro causality.

How the double slit experiment works: If you send photons one at a time through the slits, at first you will see what looks like a random distribution of dots. But after a while, you will see that those dots create an interference pattern.

If you then put detectors on the slits to measure which slit the photon passes through, you see a pattern like you would if you were sending individual particles through the slits. The act of measuring seems to affect the results. But the change is due to the nature of quantum mechanics. All quantum objects like photons and electrons are really waves. But if they interact with anything, that is, if an irreversible energy exchange takes place, their waves become localized like a particle. This is called “wave collapse.” Wave collapse also occurs when the photon interacts with the screen in the back. And we this as a dot on the screen.

The Delayed Choice Quantum Eraser is like the double slit experiment on steroids. First, I want to point out that if you have a detector that measures the path, you don’t really get two clean lines of photons like it's usually illustrated. You get a single spread out distribution of photons.

How does the delayed choice experiment work?
It starts with the double slit, but first the photons go through a special optical device called a Barium Borate crystal. It splits a single photon into a pair of entangled photons with half the energy each of the original. Note that the process of creating entangled photons effectively results in a measurement. In other words, the wave function of the photon collapses so that it is now a particle. And since the path from the top slit to detector 1 is slightly different than the path from detector 2, the which way information of the photon is known. Thus the pattern that will show up at detector 1 will always be a spread out pattern, not an interference pattern. It doesn’t matter what happens at any of the other detectors.

So why is it illustrated as changing depending on what happens at the other detectors? This is the center of the confusion, and where the idea of retro causality comes in.

Well the confusion is from the way this experiment is presented - as D1 changing its pattern to match the interference pattern at D4 or D5 when the photons end up there, but showing a different pattern, a spread out pattern, if the photons end up at D2 or D3.

So this implies that what happens at D2, D3, D4 or D5 influences what happens at D1. But since the path to D1 is shorter than the path to any of the other detectors, the photons reach D1 BEFORE they reach D2, D3, D4 or D5. So the implication is that the pattern at D1 which would be in the past, is being affected by what happens in the future at D2, D3, D4 or D5. So people have naturally been led to think that this means retro causality. This is wrong.

The quantum eraser has no effect on the original screen. What’s really happening is that the changing patterns are due to the scientists, conducting this experiment, selecting subsets of the photons in D1 to show the same patterns as at each of the other detectors. This can be done because the particles hitting the screen at D1 and the particles going to the other detectors are entangled.
#delayedchoicequantumeraser
#quantumphysics
So in the presentations that you see, including the one I originally made, the interference pattern you see get at D1 is nothing but a hand-selected subset of the actual original spread out pattern at D1, corresponding to photons that ended up at D4 or D5. This is done post-experiment by hand! The patterns do not change on their own. The future does not affect the past.

[nadersaeed]

The explanation clarified that there's no retro-causality involved. However, the explanation may suggest another unusual and counterintuitive idea: that the photons going to Detector 1 somehow "knew" which detector their entangled photons would land at, i.e. either (Detector 2 or 3) or (Detector 4 or 5) despite the random selector being present.

1. When the photon takes the shorter path to Detector 1, it registers on "Screen 1" first, regardless of where its entangled pair ends up (Detector 2 or 3) or (Detector 4 or 5).

2. If you then compare the subset of photons registered on "Screen 1" with the patterns on "Screen 4" or "Screen 5" and get an interference pattern, it implies that the photons heading to "Screen 1" "knew" whether to show "Interference pattern" or "No Interference pattern" in that subset of photons, before their entangled pairs even reached the random beam splitter!

This implications are even more unsettling, you can't win against Quantum Mechanics.

[brikorn]

I'm a janitor. The toilets in the first floor restroom where I work need to be flushed twice. So I used the principles of retro-causality to make sure it happens. It's effect before cause. I put a sticker that reads "Thank you for flushing twice." Stating what hasn't happened yet as though it has, thus causing it to happen.

[carlosmartinezbadia]

So many basic errors here...🤦‍♀️
To start with, what causes the photon to behave like a particle is NOT its interaction with a detector, as if the energy exchange between them broke the photon's wave 'form' : the wave also collapses a) without the photon itself interacting at all with a detector but when its entangled photon interacts with a detector, as happens in this experiment. B) it is also possible to make the wave function collapse and then reconstruct it precisely by manipulating the photon: first change the polarisation of the photon thus making its path distinguishable from the other and the wave collapses; then change the polarisation again down the line, the two paths become indistinguishable again and the wave reappears. That is, the wave is not "broken" by the interaction with a macroscopic element, otherwise you would't be able to reconstruct it with further interaction.

Second: by being entangled, the two photons do not just start behaving like particles "because they have become localised". Each entangled photon, as the experiment shows, keeps splitting and recombining and creating interference patterns. The different distances between the slits and detector 1 and between the slits and detectors 2 and 3 have nothing to do at all with their not showing interference patterns. This just happens because D2 and D3 give away which way their twins in D1 went. The fact that the distance from the slits to D2 and D3 is longer than from slits to D1 just seems to prove that this giving away happens AFTER their twins have already hit D1without interfering with each other, and therefore, apparently, they have retroactively forced their twins to choose passing through one slit rather than passing through both. When, on the other hand their which-way info about the slits is not later leaked by their twin photons at D2 and D3 but erased by the latter's recombination before detection at D4 and D5, the photons at D1 behavED like waves at the slits and interferED at D1. That is, the giving away or hiding the which-way information of what happenED at the slits is apparently BOTH the result and the cause of that happened there!
And I say there really ARE interference and non-interference patterns at D1 because there's nothing arbitrary in discriminating the pairs of detections that correspond to each other: each pair reveals a different story that really happened at a different time, of two entangled photons, one of which determines (apparently a posteriori) the past trajectory of the other. The interference patters at detectors D4 and D5 is not made up by the experimenters and correspond to detections of their twin photons at D1 at the same time (at our scale, for D1 should light up infinitesimally earlier than the other detectors, as it's closer to the slits) .Then the different pairs of detections are put in 4 sets, each corresponding to the 4 different trajectories of the entangled pairs: all the D1+ D2, all the D1 + D3, all the D1 + D4 and all the D1+ D5. Thus the interference or non-interferece patterns are revealed: revealed, not made up by cherry-picking, for, again, these four sets of detections, do correspond to four different pairs of trajectories that did take place independently from the others and need to be considered separately. Of course if you put them all together you get just a blob: for the same reason that all cats are grey in the dark.

I'm not saying that this experiment proves retrocausality, but if not, something equally weird must happen. Dimissing it as the banal result of the confusion of the scientists that created it (and the ones who peer-revewied it) just reveals misunderstanding it.

[QuantenMagier]

I think you totally missed the point here. The important part, is that the chosen subsets on detector 1 corresponding to detector 2 or 3 do not show interference, while the subsets corresponding to detector 4 or 5 do show interference on detector 1. This means the selected photons on detector 1 show interference depending on which path they took in the future.

[ArvinAsh]

Let me try to clarify a question several of you asked. I apologize that I failed to address this in my video, as I did not realize that this would be a source of confusion. There is no retro causality. There in only a look-back at the positions of the subset of photons post-experiment. The positions of the D1 photons correspond to the D4 (or D5) photons because the photons are entangled, therefore correlated. So the fact that the patterns match should not be surprising. It’s to be expected due to the fact that we are looking at positions of entangled photons. They are expected to have complimentary positions.

If you want a mathematical and more detailed explanation, physicist Sean Carroll, and friend of this channel, does a great job here:

[kevinsayes]

Very cool of you to correct yourself. Most of us would have never known 😂 but we try and you def help!

[HutcH]

I think this was the single best explanation of the original experiment contained without getting to why there was a misinterpretation.
Which was quite clear and convincing.

[VictorDiaz]

Arvin is awesome! He admits his mistakes because he genuinely wants to learn and genuinely wants to teach as well. Some people let their egos get in the way.

Thanks for the clarification, Arvin! 🌌🌠

[JDTradesFutures]

Hi Arvin, thanks for the updated explanation. I am still wondering about the retro causality question though. If the D1 photons can be parsed out from the record of their entanglement with D2, D3, D4 & D5, and the difference between D2/D3 and D4/D5 patterns is caused by the erasure of the "which way" information which happens after the first entangled photons have already reached D1, doesn't that imply retro-causality? To make my point, for any particular photon that has just landed on D1, it's entangled particle will later land on D2-D5. If it lands of D2 or D3, it will be a "blob" pattern on D1 after parsing but if it lands on D4 or D5, it can be parsed into a wave pattern, caused by a beam splitter that RANDOMLY lets particles through or reflects them. This random act happened after recording the photon at D1 but it's influence can be seen at D1. What am I missing that says this isn't retro-causality? Also, if I understand you correctly. you said that the BBO crystal causes the which way information of the photon to be known, so why are we still seeing wave patterns at D4 and D5? Does it return to a wave once the which way information is destroyed? That still doesn't explain the ability to parse the entangled D1 photons into waves without some retro-causality. Genuinely interest if you or anyone else can enlighten me..

[shechshire]

I really like your explanation. Very well explained. Most people can’t explain this because they’re not good teachers. The interference pattern & slit pattern pictures at the end really puts everything together.

[ernestuz]

Man, this video is great. The user correcting you and then you explaining it to us is Internet at is best.

[cosmic_gate]

Kudos to whoever is animating these experiments 👏

[antimatterhorn]

there's still some mystery in that selecting a subset of particles in D1 that went into D4 or D5 recovers an interference pattern, but selecting D2 or D3 particles from D1 does not. /that/ is the delayed choice, that particles that will eventually end up in D4 or D5 but haven't yet hit the recombiner still nevertheless make an interference pattern at D1 (if you select them out afterward). like most things having to do with entanglement, there's no useful data to glean at the time of the experiment, but looking back at the data from the future shows that the data was nevertheless encoded with something, albeit unreadable in the present. that is still in conflict with the copenhagen interpretation, but then so is everything because that interpretation hand-waves "probability collapse" in a way that violates every conservation law, simply to preserve some irreducible level of indeterminacy and to preserve free will, and it isn't fair to say "how this happens isn't well understood" because "how this happens" has no explanation whatsoever in the copenhagen interpretation.

[benmcreynolds]

Im so glad this is being talked about. I was getting so tired of people talking about "just observing it effects it nonsense" I've always felt it was more like you described but so many people ran with this universal conscious observer effect.. i love science but sometimes certain things just get out of hand. I'm glad that we can adapt and improve on our concepts and ideas. That's true growth.

[mategido]

This is actually spooky, not only does it know when it's detected, it also knows when the uncertainty is restored

[joshprior]

How is this not retro causality? The position of the D1 photon reflects the fact that it's pair had an interaction with an apparatus that caused it to either have interference or not. The interaction that the pair photon experienced occurred AFTER the D1 photon was detected. If there were no retro causality, then shouldn't the D1 photon land in a different position than it's pair since it was detected before the pair photon experienced interference?

[turbotong]

Can anyone help me out with this? What exactly is the "detector" or "measuring device" in the middle that causes the wave to become a particle? How does it work? This detail is critical but no one ever explains it and everyone just vaguely concludes that some black box "detector/measuring device" causes the wave to collapse.

[pedrosuarez]

The definition of causality that we accept and our mathematical ability to recover the vacuum solution are the two fundamental requirements to be able to distinguish physical from non-physical solutions in Einstein's equations. It is as easy to talk about superdeterminism as it is about retrocausality, what it does not make is any mathematical sense.

[shurmurray]

Thumb up for the attempt, but i think this explanation still flawed. Not inline with some of descriptions of original quantum eraser experiment setup.

[fritt_wastaken]

There's no confusion, it still implies retrocausality. Your correction doesn't change the essence of the experiment