Boy, Was I Wrong! How the Delayed Choice Quantum Eraser Really works
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- Опубліковано 15 тра 2024
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REFERENCES
Original 2019 video on DCQE: • Delayed Choice Quantum...
Quantum Decoherence (Wave collapse) explained: • How Quantum Mechanics ...
Original 1998 paper by Kim et al: arxiv.org/pdf/quant-ph/990304...
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. - Наука та технологія
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.
Exactly! This explanation doesn’t get rid of the retrocausality at all!
The photon pattern on D1 is created before the the beam splitter either preserves or erases the path information. It doesn’t matter that it requires correlating the photons from the detectors afterwards. You could imagine a more sophisticated version of this experiment where instead of a probabilistic beam splitter you had an actual person making a choice.
There was a reply here, which sadly was deleted. I was vested in the follow up, please anon repost. Thank you, and thank you @antimatterhorn for your insight!
I also agree that this doesn't debunk the retrocausality. The fact the the patterns at 4 and 5 combine to apparently give no pattern is because there is interference (which is the important fact) but it's out of phase when you combine them.
@CraigGidney has a video called "The non-quantum delayed choice eraser". He kinda whizzes by going back from the classical analogy to the quantum system (as he admits in a comment), but to me it helped a lot in getting a taste of what to look for. He has a follow-up video setting up a simple simulation of the quantum system as well.
What D1 records is always a superposition of two interference patterns, regardless of what happens later at any of the other detectors. It's just that D4 and D5 give you the information needed to separate those patterns, whereas D2 and D3 don't.
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! 🌌🌠
Agree!
Problem is that society kinda punishes those who admit a mistake (sign of weakness/incompetence) and hold people with overinflated egos in high regard.
Luckily the scientific community is different.
@@daveogfans413I don't think the scientific community is any different. It would be necessary to see what made him change his mind or by who or who he was convinced. Because honestly, if one reviews the video's claims seriously, they don't hold up.
Nah, he's a crackpot.
@@Razor-pw1xn So... What is the point? What is disputed? What doesn't convince u?
It's unclear what disagreement u have.
Kudos to whoever is animating these experiments 👏
Kudos to the sound designer too!
You’re welcome
@@rickring1396I imagine 8-10 minute laser table sounds was a fun day.
@@rickring1396are you the one who animated these?
@@nag0074 No, I was just being polite
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: preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser
Yes it is already collapsed because of another detector is at D0 which is not mentioned in these discussions. The link you posted is then arguing the pattern interference is now separated by left and right spins into horizontal and vertical subsets. If I was to ask the person who wrote that article, to write their name on a piece of paper and then turn it around in front of a mirror, would they read the paper upside-down or back the front? It would depend on which whey they turned the paper, in a horizontal direction or a vertical one. So I question whether the slits were side by side or one on top of each other would yield a vertical pattern or a horizontal pattern, and is valid proof of that two different subsets are due to the spin of a collapsed waveform, but more an example of the orientation of the slits. The shift between D4 and D5 could be because they are a mirror versions of D1 and could be because D4 and D5 waveform interferes with D1.
The only explanation I can fathom is that waveform and photons are not the same. If anything, the original two slits experiment is proof that a particle does not need to be tied to its wave function to move from point A B C and D and the photons are slowed by encountering contamination as the detector only appears to collapse the wave function, and are disassociated from the wave function, and this still agrees there is no retro-causality.
So the erasers maybe able to recombine a much-faster-than-photon wave function to D1 from D5 and D5 due to entanglement. Being a laser, the photons are in resonance with the waveform from any time. And is why the wave function can be recombined at D4 and D5 and show an interference pattern after D1 showed its interference pattern. This agrees with causality, and entanglement is with wave function in resonance, and not associated with individual photons.
Photons might only oscillate within the waveform. The waveform is not collapsed. Photons may be only be disassociated from the waveform peaks and pulled backwards down into the waveforms 0 point by the detectors electromagnetism that the photons induct..
@Arvin Ash The truth is, I quote: "So, where the entangled pairs of the photons at D1 end up is not totally random(???). It corresponds to where they had landed on D1. So if anything, their position at D1 affects their position at any of the other detectors, not the other way around."
In this version of the experiment, unfortunately, we will not see two complementary interference images (after the coincidence of entangled photons D1-D4 and D1-D5). Mere ignorance of the roads is not enough for interference to occur! Two paths (one from both BBOs) must be directed symmetrically to the beam splitter.
It is not correct to interpret that: "But you have to remember that the wavefunctions of the photons at D1 have already been collapsed at the BBO crystal."
Both photons would follow SINGLE paths, and interference would be impossible.
Very cool of you to correct yourself. Most of us would have never known 😂 but we try and you def help!
The most trustworthy and worthwhile science communicators do this!
Thank you for providing an explanation of the optics involved in this experiment. It's good to learn that we don't have to contend with evidence that physical things in the present can affect their past and therefore violate the laws of logic (throwing all of our reasoning about reality into uncertainty).🙂
JPB
What if the measurement sets the quanta into a polarized state where it increases the focus/higher center chances?
He is really really LATE. This was debunked quite awhile ago.
@@kayakMike1000Nothing was debunked. The discussion continues as always. The proof is that many people continue to think that the interpretation is not satisfactory. Not only because of the comments here, or on Wikipedia where it mentions that there are two groups of opinion, but because in reality there is no serious explanation or scientific paper where the main interpretation of the experiment is refuted. One cannot make a video and say that it already refutes a scientific interpretation. I'm not saying that's the case with Arvin, because he doesn't mention the word debunking. He simply believes he has corrected how the experiment actually works. However, this correction is based on "someone" who has told you that this is how it works. It doesn't seem fair to me then not to explain why he believes that it works like that. The discussion continues more alive than ever. And it is a legitimate discussion. Can you imagine debunking Aspect's entanglement experiment in a video in that way and with those arguments? Try to do it.
Man, this video is great. The user correcting you and then you explaining it to us is Internet at is best.
The text from the original paper hilighted in the video simply says that the authors have shown that which path information (represented by idler quanta) for quanta passing through a two slit apparatus can be erased after those quanta’s signal quanta have been recorded. This is in fact what the experiment shows. The paper does not say anything about retro-causality. I think that part is embellishment added later by others.
Things I googled during that video:
Retrocausality is the idea that future events can influence past events, contrary to our usual understanding of causality and time.
Decoherence refers to the process by which a quantum system's behavior becomes classical and loses its quantum properties due to interactions with its environment.
Honestly this is the first UA-cam video I've seen this explained so succinctly and accessibly. I've been on the quantum mechanics physics forums for years trying to figure this out and it took a long time until I really understood it. You absolutely nailed it.
Arvin is the best on UA-cam- hands done "Explaining complicated things simply"
One cannot understand quantum mechanics, he can only get used to it. Only fools believe, that a wave function represents a statistical ensemble👿
There is a nice book "Through two doors at once ", this might appear helpful so that it can give you (at least) a different perspective of the entanglement photons , or either of bigger particles. Also you can find the author of the book on UA-cam.
The book if I remember correctly disagrees with Arvin in that there is retrocausality occurring
@@johnbach3144 I'm not sure which part of the book you're referring to,I'm not sure if you remember Anton Zeilinger's experiment (conducted in Gran Canaria and Tenerife), he describes the concept of delayed choice, where the cause precedes the effect. Both he and Alain Aspect were awarded the Nobel Prize last year.Anyway , I found it interesting and it was more clear to me since it was a real experiment.furthermore , In my opinion, I think that every creator discussing these topics should, at the very least, address them in their work.
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.
OK let's test how good of an "explanation" it was.
Why is the subset of photons on D1 detector showing interference pattern? Photons in that subset only LATER ON landed on D4 or D5. So why is there interference pattern on this subset?
@@srb00There is no retro causality only retro correlation. Instead of changing the past what is happening is that the photons in D1 "knows" what will happen to their partner entangled particle in the future. So the photons in D1 behaves exactly correlated to their entangled partner so they sort of know beforehand where the partner particle will land in D2, D3, D4, or D5. So the quantum eraser is actually a quantum Oracle.
@@srb00 All the photons have a wave function whether or not you know which slit they passed through. The photons whose which-way was measured still have a wave function & show a one-slit diffraction pattern. Nothing is being "erased". The "restored" interference pattern comes from just selectively disregarding the particles from one of the recombining beam splitters & not from mysterious retrocausality. All you ever see on the screen is the "fuzzy collection of dots" Arvin mentions at 7:28, from which they select the interference pattern. It's really that simple. The question is why did they choose to invoke retrocausality as an explanation? Maybe they were just looking for attention.
@@duprie37 Unfortunately your understanding of the experiment does not exceed that of Ash. D1 detector shows interference pattern only for subsets of photons that landed on D4 and D5 that have erased which-way information. D1 detector does not show an interference pattern for photons that landed on D2 or D3 that carry which-way information. Why is that? Photons first land on D1 and only later on their entangled pairs land on other detectors. So how do the photons from a subset that shows interference pattern on D1 know that they later on landed on D4 or D5?
@@kazedcat I like your term 'retro correlation', maybe that is a better word for this than 'retro causality'. Either way, Arvin isn't addressing this non-local affect which is critical to the experiment, he seems to be ignoring it.
Your channel has become so invaluable to expanding my ability to conceptualize complex ideas of which I previously haven't been able to find comprehensive explanations for. Thank you for brilliantly articulating so many elements of science that have previously eluded my full understanding!
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..
wondering the same. ignoring the BBO lens issue, how does d1's result look and how does manual seletion on d1 work? seems to me the video says d1's pattern remain unchanged and the researcher picked out the fraction of photons entangled to those reaching d4d5 by data post-processing?? or by some quantum effect?? if it is the latter case, it is still retro-causality. And the former case sounds completely pointless to do.
1. If the photons really exist in the first place, and we presume our interpretation is "quantum mechanics is secretly wrong" as so many physicists have, then retro causality is implied.
2. In Copenhagen, photons and electrons and all that aren't actually real. Only final measurements. The final measurements will always be found to magically agree with prediction, and we shouldn't worry our tiny little brains about what "really" happened.
3. In pilot-wave theory, the pilot wave is already a wibbly-wobbly timey-wimey sort of thing so it sees no surprises here.
4. Many Worlds can't explain this result at all (as with so many of the more recent experiments) and is probably just incorrect.
5. The quantum decoherence model would not say that the detector 1 "measurement" happened first at all. Instead, quantum states don't collapse, but rather "decohere" as everything finally comes to a final state. So the final measurement of detector 1 doesn't really exist until all the other measurements finish the decoherence process.
@JDTradesFutures In this version of the experiment, we will not obtain two interference patterns in (the D1-D4 and D1-D5 coincidence)! Interference is not caused by lack of knowledge about the photon's path. For this to happen, the photon must move in a superposition of both paths (i.e., as if moving "both at once") and both components interfere on BS. The interference depends on the amplitudes from both photon paths and the phase difference of their optical paths.
Yep I'm wondering the same too
@wiesawnykiel1348 but the interference pattern on d1 related to the entangled photons from d4 and d5 happens before the entangled photons pass through the beam splitter that randomly assigns them to d4 or d5. You cannot parse the information from d4 and d5 to get an interference pattern on d1 with a linear time explanation. At least I can't understand how..
Arvin has a real talent for making explanations of the complex and complicated so readily accessible.
He also has no idea what he's talking about. He completely missed the point in this "explanation" of his.
How so or is this just a drive by comment?@@srb00
@@srb00 What is the point you claim that he completely missed. I'm quite sure he is not the one who has no idea what he is talking about.
@@talleyhoe846 How do the photons from the subset of D1 know they LATER ON landed on D4 or D5? They need to know that in order for the subset to generate an interference pattern.
@@srb00Suggest you rewatch the video (from around 9 minutes on) where he provides a detailed explanation of how the relationship and interdependence of the D1/D4/D5 outputs are formed and constituted. He further explains that the confusion arises from treating an interpretation of the experiment founded on an arbitrary and incomplete sampling of outputs as reflecting the comprehensive sampling of outputs. The alleged retro-causation manifested in D1 vanishes once the result of the full suite of interactions is taken into account. If you consider his explanation is in error, then explain what specifically is the error, where does the error occur, what is the correct explanation and what is the science-based justification for this explanation.
Sabine Hossenfelder was the first on youtube to explain this a while ago. She even explained the part where combining the two interference patterns gives you the same non-interference spread out pattern.
Prof. Sean Carroll of Caltech did it prior.
Matt from PBS Spacetime almost got it too, but missed the mark in his main video. The answer to his accompanying challenge question for that series basically brushed up against it, but didn't quite spell out the whole picture. But the problem is that most people who watch his channel don't understand everything fully, and having the answer at the end of an already long video guarantees that some people will miss it. Matt's response comment in Sabine's video was a pretty straightforward "you are are right."
Yes I remember that 😊
That’s a true teacher. Admitting where he was wrong and showing why and explaining what is now currently known. Yup new subscriber now.
@anolakes Imagine being so bitter you can find issue with another thinking it's good to admit when you are wrong.
He has more to lose as far as damage to his reputation and people second guessing past and future presentations if he does not correct his mistake ASAP. The comments below his original video include a challenge to his conclusion from someone with a Ph.D. in quantum optics. What is he going to do when he realizes that he made a mistake, just ignore it? Doesn't work that way in most of science. It is much more embarrassing to hang on to your mistake. He did not have to be so thorough in explaining his error, but he is a teacher with integrity.
@anolakes Wherever it is you live, certainly there are politicians. Politicians do not only deny mistakes they know they have made, they create them intentionally. And yes, like actors and celebrities, they tend to be very insecure. Their jobs and livelihood depend on how they are perceived. You can pretend your circle only includes honest people, but the dishonest ones, especially politicians, affect your life whether you include them or not.
@anolakes I also have an academic background with a Ph.D., now retired. Not sure why you are including me with your use of the phrase "you guys". In my comment above the one directed to you, I stated: "What is he going to do when he realizes that he made a mistake, just ignore it? Doesn't work that way in most of science. It is much more embarrassing to hang on to your mistake." I did not read your original comment carefully enough. But academics also emphasizes clear communication. To say you are sorry for "the world" someone lives in is a pretty broad category and despite your last sentence, it was not previously clear that you meant "one's circles". Also, your reply to the OP was flip, arrogant, and demeaning as far as assuming his comment was a reflection on "the world (he) lives in" i.e., "his circle" (a very vague term, especially considering it was intended to clarify the original).
With ALL science.. I don't believe it was a right/wrong situation. Science is ever flowing and nothing is right until it's proven wrong (yes, I meant to be wrong) we thought the earth was flat.. and until we showed it to be globular, it was for all intents and purposes - flat. The original scientists made a claim, devised an experiment that backed it up.. along comes many decades and improvements in measurements and voila.. its updated
But it's still "mysterious" to me how each photon from the relevant entangled pair "knows" how (i.e. in what pattern) it must hit the detector screen at D1. Because its entangled brother is at that point still en route to D2/3/4/5 and the photon at D1 strikes the screen before D2/3/4/5 is hit.
Sure, you have to manually separate the patterns at D1 to make them match the patterns at D2/3/4/5, but the fact that they match is at all is weird enough, no?
I always thought it would make this topic easier to grasp if you could explain how the “detector” works.
Or how you shoot one electron/photon. Or how do you know a wave pattern is not being created from polarization or ricochet? Or how you get a layer of gold one atom thin? Am I poking god again?
Wikipedia exists.
I guess the detector that only detects and doesn't interfere does not exist. I've read they used polarisation filters which are quite different than the animations would suggest and obviously interact.
@@jamesflames6987
Thanks for self identifying as “that guy” in this video’s comments.
@@gregmorris2022 The concept of quantum erasure is hard to grasp. Making the video hours long by explaining lots of incidental details which are not hard to grasp with a single Google search would not make it easier to understand.
Very cool. The overlapping of the patterns to show how the subsets combine did it for me. Until then, I still hadn't grasped an understanding. But after the merge, it was so much more intuitive. Well done.
The overlaping pattern doesn't explain anything. This is also present in the original paper of the experiment. It is one of the effects in creating the photon pair in which the phase is shifted by 1/2. For this reason there is interference patter and interference "anti-patern" or if you prefer, 2 well-differentiated interference patterns, which when analyzing the total sum of all the impacts appear as if there were no interference pattern.
Razor is right. Go read Ash's link "Original 1998 paper by Kim et al:" Scroll to the bottom to see the results.
Yeah, this doesn't help me understand the why. Yes, if you add up the energies across the detectors then you'll get the same values, that's at least plausible. But not why those particular patterns show up, from an infinite set of possibilities
@@jamesyoungquist6923 Why is always tricky with quantum physics. Ash is right that this experiment doesn't prove retrocausality. But if a detector shows which slit the photon went through the pattern on the D1 screen is that of a single slit. If not the pattern shows a wave went through both slits.
The important thing to understand, which is what the purpose of the video is, is that the pattern at D1 (particles or waves) does not magically change depending on which other detector you choose to look at. The pattern at D1 due to ALL of the photons is ALWAYS a spread out pattern. Because the which way information is always known by D1. And this is where the importance of understanding entaglement and superposition comes in. Encoded whithin all the photons reaching D1 is all the other possibilities about which slit the photon went through. And because of entanglement, one can determine which set of photons within ALL the photons reaching D1 correlate with those reaching one of the other detectors. The screen at D1 doesn't magically change. They used the screen at D1 to show the SUBSET of photons that correlate with one of the other detectors. That subset will show as an interference pattern if it correlates with detectors D4 or D5, because in those photons, the which way info is lost due to recombination. Conversely, the subset of photons displayed on the D1 screen will show a spread out pattern (wave function collapsed) for those photons correlated with detectors D2 and D3, because in those detectors, the which way info is still known. What I think this experiment really shows is the amazing phenomenon of entanglement, and due to it, you can instantly deduce information about an entagled partner, even though those two particles are separated by ANY distance.
That summary in the notes is full of interesting and useful info. Nicely done, thanks.
I was confused seeing your 2019 video after Sabine's debunk video.
Thanks for the update, it's now exactly as what Sabine said in her debunk video!
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.
If you are referring to detector1, then sometimes they can use polarizing filters as means for detection, or they use photomultiplier's which is devices that converts incident photons into electrical signals. It's often more complicated then what the illustrations shows.
BBO crystal?
Sabine's video from a year ago made the same points, but it's good to see more videos like this were science youtubers question and clarify the "standard" explanations for quantum weirdness.
I watched that one, couldn't understand it, had an inkling of what was really happening.
Now here is my pat on the back for a right guess, but no gold star since I've not got any working out.
I believe what Sabine said was that its like a left and right hand glove or sock, which is what Einstein also said. Meaning that the pairs’ outcome was set and determined at the very beginning. But that is incorrect.
She came to the correct conclusion, but how she got there was incorrect, is my understanding.
Her channel should be "Sabine ruins...." She has a talent for contrariness and I love her for it lol
Oh quantum is still super weird don’t get too comfortable just because it doesn’t time travel.
She did refer to the quantum bomb experiment as more interesting
That is the most down-to-earth explanation of delayed choice as well as the original double slit experiment I have ever seen on UA-cam. I wish I could give more than one like!
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.
This is a much cleaer explanation of all the parts of the experiment than I have seen anywhere else. Thanks for taking the time to explain and illustrate this.
This is a better explanation of how the actual experiment was performed and what the results show, than the explanation in many other videos. However, this explanation, doesnt change anything about retrocausality. Retrocausality can still be the case here, if there is no other "better interpretation". Science asylum had an explanation how retrocausality cant be the case here. In his interpretation, it is very likely that the way that the first photons interact with the D1 screen, can influence the possibility that their counterpart will pass or not from the eraser, and all this leading to the interference pattern for those that pass the eraser. In that case, the past (photons hitting D1) is effecting the future (counterpart hitting the eraser) and not the opposite. And this makes much more sense, as the entaglement should be lost when the first photons hitting the D1. How the entaglement was lost, should then effect the counterparts photons path to either go through the eraser or not. It doesnt make much sense for the entaglement to still exist after first photon hitting D1.
Spooky action nonetheless. “Retro-causation” at the photon to photon scale makes more sense imo. Would the photon at d1 then be randomly like a wave or practical? “Retro-causation” is an explanation that begins with the axiom that photon-wave-particle-double-slit-measurement is nonrandom, determined by measurement, whether explicitly the physical instrument or otherwise.
So if that interpretation of the double slit experiment is valid, I don’t see how the photon at d1 could determine the later registry. “Can’t eat the cake and have it too”
been finding for delayed double slit simple explaination. this is really perfect one.. thanks arvin❤
Yours is the best explanation of the quantum wave/particle experiment I have ever seen. Bravo to you and your commenter with the quantum optics degree!
I was kind of obsessed with this when PBS Spacetime first showed it. They part they leave out is that there is no interference pattern on the screen, there is only an interference pattern when correlating entangled pairs back to the screen.
at least hey commented on Sabine's video admitting all the fault :)
Matt did say that though... it was the answer to the challenge question for that series (if you could use this to communicate to your past self) he just didn't tie it together to say that there is no retrocasuality.
I knew retro-causality was wrong before I even knew it!
I felt like I knew this video was coming, and I've been excited for it! Something told me only Arvin was going to be able to break this down in a way I could understand it.
It’s lunchtime, I’m sitting in my car eating while working on a Sunday, and I really needed an Arvin Ash video. I’m kind of glad I missed it the day it came out
Yes it still means that the particles going to D1 that are entangled with D4 and D5 know to be 'interference' vs particle
Wow, that really cleared up a lot. Thanks Dr. Ash, excellent video!
Brother, your videos are awesome; keep up the great work.
Thank you, Arvin.
Excellent talk.
This is the most lucid illustration of the DCQE that I have seen (I’ve seen some pretty bad ones). However, it still is a fact that the pattern recorded at Detector 1 is recorded BEFORE any of the subsequent goings-on even occur. You might say the Universe doesn’t have a clue what is going to happen later when the “blob” pattern at Detector 1 is recorded. Since which path information is available for ALL photons when the pattern at Detector 1 is recorded, we - according to the explanation of the normal two slit experiment when observations at the slits are made - expect that there is no interference pattern formed either “hidden” within the blob or not. It’s still difficult then to explain how the blob can be made to yield up interference patterns by manipulations that erase which path information later.
I think it's important to understand that this experiment collects a lot more information from the detectors than the usual double slit experiment, which typically just has a screen or photographic film that munges all the photon detection events together. If you do that, then of course you can't extract any other patterns from it.
But here, each individual photon landing on D1 is recorded separately, together with the position at which it was detected, allowing the possibility of correlating these events with other events happening elsewhere and elsewhen.
I think it also means that the usual way of talking about the ordinary double-slit setup, that detecting which slit the photon went through "destroys" the interference, is overly simplistic. The interference is still there, it's just obscured in a way that makes it impossible to observe using a single detector.
@@gcewing If the interference patterns are there at D1 ready to be recovered from the get-go even when which path information exists for all photons passing through the two slit apparatus, why did the investigators go to the trouble of later erasing that which path information for the photons used to reveal the interference patterns? In fact, there is no way to recover the interference patterns without erasing the which path information. The experiment is really a lot easier to understand conceptually than many people make it out to be. Erase the which path information, see the interference patterns.
What more strange is that these things exist and happen in the first place
Thanks for making this follow up video!
This was very enlightening! Thank you very much!
Wow, this is a fantastic video. I’ve never seen this explain so clearly and succinctly. I love the de-mystification of quantum mechanics. It’s cool and interesting enough, no need to make it sound like magic. Thank you Arvin.
One of the worst offenders has imo been using the word "measure" when explaining to non scientist. I mean sure we measure the particle, but maybe we should go with something like "disturb", "collide", or even "poke". Then it wouldn't invoke the classical meaning of the word.
A huge amount of woo comes from misunderstanding the word.
Wrong !!!
@@cmddcd What a thoughtful response, I'll have to chew on that for a while.
@@VikingTeddy yeah that’s a great point.
Video gets real at 14:10
Is the double slit experiment done in a vaccum? Why doesn't every air molecule along the way that feels the photon act as a detector that collapses the wave?
The photon isn't reacting with the air molecules. The quantum nature means the air molecule would have to absorb the entire photon, at which point nothing gets to the detectors and the sample is thrown out. (Or it doesn't get to one of the two detectors, and thus the one that did is thrown out.)
Really well made video! Keep up the good work
Great video Arvin, thanks for clearing that up. We cant measure something without "interfering" with it.
One thing is worth pointing, if the distance between d1 and d2-d4 was, lets say, mars to earth, d1 pattern gets drawn minutes ahead of the other 4. If somehow, we could at d2-d4 choose the final path of the photon and d1 still gets drawn ahead of time, this would mean faster than light communication.
The d1 pattern gets drawn first, but before d2-d5 patterns also get drawn, there's no way for you to pick their subsets from d1.
@@meyes1098 dont have to, If I can force all photons at d2-d5 to go one specific detector, D1 will change behavior according to what I choose and then, I can assign wave/particle behavior the 0/1 values.
Either I just found a way to do faster than light communication, which I doubt, or the explanation is missing why I can't control d2-d5 behavior.
@@Arag0n
You can't control d2-d5 because of the random beam splitters.
@@meyes1098 didn't say you can, I'm just saying that it would be worth explaining more about how and why the photons choose their final hitting location
@@Arag0n
Arvin could probably answer this if he sees the question :D
this still does not remove retrocausality from functioning within a multiverse setup where the data doesn't come from the future but the present of an alternate timeline that is a possible version of our future. but speculative, I know. But I never thought retrocauslity functioned on a local level like the quantum eraser suggests, You have to factor in many worlds theory.
More woowoo? There's no evidence for a multiverse. Many Worlds Interpretation is just a thought experiment. Stay away from those god awful marvel movies.
closed thinking won't get us anywhere either@@Light-ji4fo
This is a very good explanation. I didn't understand the experiment before because the the exact misconceptions you pointed out.
Hi Arvin,
I have a question concerning the collapse of the wave function resulting the blob pattern instead of the interference patter when not observed.
Does the wave function reconstruct itself if the distance between the slits and observation is given enough distunce and if so what will be the distance, any relationship between light frequency and distance if the wave function constructs and/or will the interference pattern reemerge if you provide enough distance between the two slits and the final target?
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.
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?
Yep I'm wondering the same too
The answer is very simple. One half of the diffraction pattern in D1 belongs to D4, the other half to D5. When the original photon - on a BBO crystal - splits into two pairs of enragled photons, the properties of the photon pairs determine each other. If the photon going towards D1 is incident at x1 location on D1 such that it belongs to the D4 case, then its pair is in full destructive interference towards D5 and a total constructive interference towards D4, so it can only incident on D4 (or D2/D3). If a photon going towards D1 is incident at a different x2 location than D1, then its pair will be in destructive interference with itself towards D4 and can only arrive towards D5 (or D2/D3).
The entanglement - and of course the right experimental setup - guarantees that HOW the photon arrives at D1 will determine whether the other photon can end up towards D4 or D5 (where there will be destructive / strengthening interference).
@@janosmadar8580 I thin most misunderstood what restrocausality as future changing the past.
But actually retro causality is same as reversibility of time in quantum interactions.
Which means that causality works both forward and backward in time.
The only issue with time symmetric causality is that decoherence break this retro causality and produce a loss of information that raise entropy.
@@janosmadar8580 Everything is fine, but in THIS version of the experiment in D1 we will not see interference with D4 and D5. For this it is necessary - as in Kim's original version - for both paths of the passive photon to interfere on the BS.
@@zazugee You understood correctly - retrocausality is the influence of future processes on past ones. However, there is no need - when making an interpretation - to use some vague concept of a "retroactive cause". The fact that QM equations are reversible does not mean that physical processes are also reversible. If you think otherwise, give an example of how you imagine backward causation in this experiment.
This was great. Need to watch it a couple more times.
Well explained. Good job is taking the time to view the interpretation of the results and selecting the correct interpretation. It is also refreshing to see admission of being incorrect on previous interpretation.
Thank you. "The Delayed Choice Quantum Eraser, Debunked" video by Sabine Hossenfelderis also good.
It is equally dumb and is not explaining the experiment either.
Arvin, thank you very much for this clarification. As a non scientist, I realize that simplifications have to be made for communication purposes and appreciate clarifications like this when people are clearly not getting the big picture items from the simplified explanation. However, I am left with this question : after the clarification, what Is the quantum eraser experiment telling us?
Although you have asked Arvin, I allow myself to doubt that the experiment is misinterpreted. The only scientific way to disprove this experiment is by using hidden variables or pilot waves, which lead to super-deterministic conclusions.
@@Razor-pw1xnAre you familar with the mathematics?
Yes... like what was the purpose of the experiment ? After this clearance, does it imply that this experiment was basically of no use ?
I think this is a reasonable question. What do the results mean? It does seem like it doesn’t tell us anything.
@@SKguy23 Quite the contrary, Arvin's interpretation seems incorrect to me. I don't think the wave function in the crystal collapses.
This still didn't clear my confusion. The individual entangled photons at separate detectors matching the detector pattern at D1, when selected as such, WAS the point of the retro-causality claim. Simply merging them together and claiming there is no discrepancy with the standard model does not explain it. The fact that when the concerned photons are identified individually, and compared to shorter paths, they DO show future states affecting the past. Either that, or our understanding of entanglement is wrong, which I have always suspected to begin with.
Yes, this is why it is called DELAYED CHOICE experiment.
There is no influence of future states on past states. The position in D1 is related to the probability of detecting an entangled photon in D4 or D5 (if it has not been previously detected in D2 or D3). The order does not matter - whether it was D1, D4 or D5
As I understood this explanation, what they are basically saying is...
- We create a pattern of light, we'll call that D1
- Then we process quantum linked light in various ways, so that only part of those photons will still be visible.
- We now unsurprisingly conclude that the remaining visible light can also be found back in the exact same position on the original pattern. And this remains true for all of the split off patterns of course.
- If we put all the various different light back together, like it was in D1, then we get the D1 pattern again... unsurprisingly.
If my understanding of the explanation is correct as such, all one is doing is basically eliminating parts of the light and making the extremely obvious discovery that the remaining light has the same position as it does in the first light spot. Because why would that change if you didn't shift the location?
@@Quickshot0 The point is that in D1 we can obtain different patterns depending on what happens to the entangled photons "on the way" to the detectors D2, D3 or D4, D5. Although - such a small detail - in my opinion, in Ash's version, contrary to what everyone thinks, we will not see interference fringes (D1/D4 and D1/D5)
@@wiesawnykiel1348 But is it really surprising to get the same patterns in D1 when what you're really doing is processing the light in different ways for D2-D5 which effectively block out parts of the light. And then after the fact when you remove the same parts of light in D1 would you not expect to find the exact same pattern?
Of course this is based on my current understand of the explanation in this video. But on that basis I think you'd need an explanation for why the pattern found wouldn't be the same when you after the fact effectively manipulate the light in D1 the same way as in D2-D5. Because that would normally be exactly what you'd expect in such a case.
As a student of physics, eagerly waiting for your every next video.
I'm curious if the detection in detector 1 is before or after the information is erased by the crystals. It makes sense to me if the detection is after, but still seems like retrocausality if the detection is before the erasure, because then the patterns are there before the intervention.
Yes, it seems so to me as well. It's just that looking at detector 1 does not give away the future information of what happens with the entangled particles that go to the other detectors. However, once the measurement is made at the other detectors, you can use that information to check where the entangled partners landed in detector 1 and it does show that those partners in detector 1 landed in a way that depended on what happened in the future to the other entangled particle. But just like the original "spooky action at a distance" entanglement does not allow instant communication, so does this "retroactive" entanglement does not allow communication with the past or future.
Exactly! I posted a similar comment because I couldn't find anyone else mentioning this. I hope Arvin or someone else can clear this up.
The detection at detector 1 happens before the erasure. That's the whole point.
From detector 1's perspective: The erasure of information happens in the future, and changes its present state.
@@beri4138 So it is still spooky then?
@@jherbranson very
That's a nice explanation. It's interesting too, that the "spread out" pattern actually contains information about a hypothetical result, that is, the pattern one might see if interference took place. If a blob like pattern contains information about a hypothetical result, if such information can be extracted from it, the "blob" isn't in any way random, and the quantum eraser experiment might have other implications.
really excellent clear explanation, thank you!
Thank you for this. I still think there is a puzzle here. Yes, the pattern on the first screen is always the ‘blob’ showing which way information. But consider a single pair of entangled photons, where the second photon has had the which way information erased. Then the first photon will be in one of the interference populations, and which one it is in will depend on where the second photon ended up. In particular, how does the first photon ‘know’ to avoid the areas with destructive interference ?
Isn't it because D1 always gets a particle, not wave, so there is no interference?
Sweet! Very clear explanation. We can all make mistakes, real men admit it and correct them. Thanks!
Great job Arvin!
Hi Arvin, great video! I have a question though: why would the subset of photons from D1 that had entangled pairs which hit D4/D5 show an interference pattern at D1?
The way I see it, this subset hits D1 before their entangled partners hit the beam splitters… so how would they “know” to make an interference pattern? I may be misunderstanding, but I’m not sure how the way it’s a “subset” prevents retrocausality… The way the subsets are split up (which photons are in which subsets) seems to still depend on a future event? I’m not sure if youll ever see this, but I would really appreciate some clarification :)
i think we need some explanation of _how_ the subset is selected to "pull out" the data on D1
@@itemushmush I agree... how the subset is selected has been left to interpretation, and so it is confusing. Initially I thought each photon going to d5 will be part of the left interference in D1, and photons in d4 will be part of the d1 right interference. But maybe this is not the case. It is all about the overall picture and not photon by photon.
@@josemarin359 The subsets are formed in the following way: By time, position and detector. The idler photons, regardless of their phase (left interference or right interference), arrive at D4 and D5 randomly. So, when a photon arrives at D1 first, its arrival time and position are recorded in the coincidence counter. 8 nanoseconds later the idler photon arrives at D4 or D5 random. The coincidence counter registers these 2 events and classifies them according to whether the idler photon has reached D4 or D5 (D0-D4, D0-D5). And so on. The results show that both subsets form interference patterns. Which is logical that there are two due to the phase difference of the pair of photons created. However, in D0-D1 and D0-D2, these interference patterns are not drawn. Which demonstrates the postulate "which way information". It is important to know that entanglement does not play any role in the phase of the photons here.
@@Razor-pw1xn this would imply that given enough time to classify photons in D1 by their position we would know ahead of time if their entangle will hit D4 or D5 8 nanoseconds later... and this is suppose to be random, indicating a hidden variable.
@@josemarin359 But hidden variable theories were already refuted with Alan Aspect's experiment when Bell's inequality was violated. Although there are still many who defend them. That is why the rest of us invoke some type of retrocausality or reversibility of states.
This is actually spooky, not only does it know when it's detected, it also knows when the uncertainty is restored
Hi Arvin,
I think am missing something in this explanation, and can use some help understanding where. Isn't the ability to extract those collapsed wave/wave interference patterns from the subsets of D2-5 in detector 1 the entire 'eraser' part of the experiment? The fact that it can draw those conclusions from entangled photons at an earlier interval is the question I am seeking to answer. I don't have a background in physics outside of personal interests, but have been looking for explanations of this experiment for years. The closest I have found to a complete answer is within the theories of Hugh Everett. I would also guess that similar lack of local 'realness' shown in Bell's inequality play a part too, but I don't know that anything directly connects them.
I, too, am open to the idea of being completely wrong here, and would be extremely curious to know what I am missing in this video that my shed some light
Amazing explanation Arvin sir, you have earned a new subscriber today ❤
I will never forget this video.🙏
Amazing and simple to follow explanation!
Excellent work :) It is important to also realise "measurement" or "collapse" or even those slits are nothing special - its all just a wave function spanning some fields interacting with another wave function to make zero, one or two new wave functions. An "experiment" is making sure certain types of wave functions are at certain spots to interact.
An excellent clarification, thanks Arvin
Sabine made a vid on this a couple of years ago (tried to paste the url but YT erased the reply, I guess censorship is a far worse eraser than quantum)
@@jagatiello6900I fear that Arvin is also getting carried away with super determinism, which is the only way to scientifically refute the conclusions of this experiment.
@@Razor-pw1xn A satisfactory explanation of the measurement process may be the key...or may be not, who knows.
My good friend thought retro-causality was a thing. I told him, "If you think time travel is the answer to any question, it is clear you don't understand what you are seeing, or what question you are asking, because your answer is nonsense. There is no past. There is no future. There is only NOW. And yes, in NOW you can experience time at different rates, while still being always in the NOW."
Before I go, let's try to consider the brain-breaking fact about time: We use the speed of light to define time, and... we use time to define the speed of light. If that doesn't make your head hurt, I don't know what possibly could.
Thanks, Arvin, cool story.
Question, was this simplified version of the setup of the article tested or computed?
In the article, the reflections count to D4 (or D5) (using the video notation, D1 and D2 in the article) is different along the paths from A respect to path from B (the two regions of the BBO crystal), which yields the fact that the amplitudes sums with opposite sing for D4 and D5 (ref. to eq. 4), this in turn yields the "pi phase shift" which brings to the results in eq. 10, where one is sin^2 and the other is cos^2 on the "interference" pattern, with the conclusion that putting both together we have the sinc^2 "diffraction blob" that we observe in D1 if we ignore its partition given by the coincidences with D4 and D5.
But in the simplified setup of the video we have always one reflection going in D4 or D5. If the coincidence results with them produce "interference" patterns for the partition of the blob in D1, from where does the shift of their fringes comes, so that they sum to the correct blob?
From the different optical lengths that we have here (9:00) between the red and blue paths into D4 and D5, which is inverted between the detectors?
The article assumes instead equal length interferometer arms.
I think you get close to explaining it but there isn't enough information in your video for me to be convinced. If the position of an individual photon hitting the screen in detector 1 (with shorter distance) is associated with its entangled pair needing to hit detector 4 or 5 (with longer paths) that seems like a form of retro causality. I just don't think this can be explained simply without the math, but thanks for the attempt Arvin. It seems obvious that you get the sum of everything at detector 1 given the random effect of the splitters and you are shooting the photons 1 at a time.
What happens at D1 stays at D1😂
So glad to finally understand this. Thank you.
Brilliant video. Your animation and explanation make it sound so obvious :)
I love your channel Arvin! It takes an adult to admit mistakes. Your doing so is a great example of how it's supposed to be done! Great job, my friend!
Arvin’s channel has only gotten better over the years.
Saying "I was wrong to agree with them before, here's now they are wrong" isn't really the same as saying "I was wrong". Especially since he's wrong in this video.
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.
I am really disappointed in how much the quantum eraser was hyped up by media, non-scientists and even scientists making claims about how the past can be effected by the future and hundreds if not thousands of articles written about it before anyone even began to question the results. It seems like the whole peer review process is falling by the wayside because every research team wants to publish first.
@@michaeljorgensen790The same is happening in medical science, unfortunately
I think the problem is that every other video I've watched has used that exact phrase without expanding much on the meaning. This is the first video I've watched that actually described what causes the change from wave to particle.
Amazing explanation!
Very well done!! I argued that one with a couple of physics grads (I am an engineer) who didnt buy my explanation that there was no quantum eraser. it was a thought experiment that was flawed.
An outstanding video. It would have been illustrative (at a later point in the discussion after talking about post separation) to use different colors to show pairs vs paths.
An interesting point about correlating (matching up) “entangled” photon pairs is that there must be some more “Devil in the Details” aspects as depending upon ACTUAL experiment physical layout, the path lengths are purposely of a chosen different path length to provide a pseudo-retrocausality aspect. So it would seem that to correlate entangled photon pairs that arrive at different times (to allow for the post separation selection to occur), the experiment setup must be careful about the rate at which these entangled pairs are generated and have a built in temporal gating window to allow for the necessary data to be available for post separation. Is that what is happening?
Also, since “Measurement” requires “Interaction” which effectively means that MEASUREMENT is INTERACTION (that can and does lead to collapse), this brings up the whole concept of “WEAK MEASUREMENTS” and its implications. AND, are not the BEAM SPLITTERS in an of themselves, in “REALITY” (if one believes in reality) massively interacting with the photons if one ascribes an actual “real”probabilistic mechanism to their operation. OR in other words, could this experiment be fully and accurately described by the mathematics of De Broglie-Bohm theory (considering that this experiment was not using relativistic electrons) OR more modern descendants?
On the first question, Sabine Hossefelder mentioned an interesting bit of information, the BBO crystal generating the entangled couple of photons has a very low efficiency, so much so that the temporal separation of each single run is essentially warranted by the crystal itself.
@@ThePinkus
Thank you, that would definitely put a lot of noise into the experiment and gum up the correlation of “entangled” particles and their associated interference distributions. Interesting!
That all measurements are interactions does not imply that all interactions are measurements. Interaction with a beam splitter is not a measurement, for example, neither are interactions with lenses. Not even the interaction with the BBO crystal, which creates two new photons, constitutes a measurement (contrary to what Arvin Ash says).
@@renedekker9806 I agree.
My suggestion would be that the traditional (von Neuman) notion that measurement (objective, and not subjective observation) is correlating interaction is insufficient to the measurement problem, while the further specification that measurement is decoherence is sufficient. But this can be a long story.
For what is relevant here, if BBO measured the position of the source of the photon to the point of discriminating A from B (the two positions as named in the article), I would consider this to mean that the two positions are decohered, but then the computations would be different.
It seems instead necessary for the computations to yield the desired results to assume that any capacity of the BBO to decohere/measure the position of the source of the entangled photons is negligible in this setup.
At 8:35 there is this statement that the BBO "measures" the position of the source either on A or B, as a justification of the blob in D1 (he refers to collapse, to me collapse is a bad word for conditionalization, i.e., subjective observation, which is not needed at all to make the difference, what does make the objective difference in the results is decoherence).
If that was the case and there was such measurement/decoherence, a photon from A would have a 50% chance to D5, and 25% to D2 and D4, one from B 50% in D5 and 25% in D3 and D4, and in no case the partition of the results in D1 according to coincidences with the others would show interference fringes.
Now, I am not entirely sure why we end up with a blob in D1, aside from the result in eq. 10 of the article, which does sum up to a "diffraction blob" because of the different count of reflections in the two combined paths (which we don't have in this simplified setup, but perhaps here is the difference in the optical lengths of the recombined paths, and which is inverted between D4 and D5, that does the trick?).
@@renedekker9806
Yes you are definitely correct and I was not trying to imply that all interactions are measurements, but was stating that all interactions are interactions (which sounds circular) that in some situations such as a “measurement” usually lead to collapse (or decoherence, depending up preferred theory). The issue is that if you look at the actual mechanism of how a beam splitter works as the photons transit the geometry of the electrical fields of the lattice of atoms comprising the device, the conclusion that I have come to is that there are some significant interactions occurring. One can look at it from a purely probabilistic wave function perspective and overlook the need to understand the details of the beam splitting and just “calculate” a correct final result. OR one could approach the experiment from the perspective that there are massive amounts of interactions happening and point out that a purely orthodox approach purposely chooses what interactions are mathematically relevant to achieving a global result that provides a reasonable estimation of the outcome.
Personally I don’t think that De Broglie-Bohm theory is even remotely correct (and is only relevant in non relativistic examples) but it does bring up a historically generalized separation of orthodox vs “real” approaches to quantum mechanics. A more complete “real” approach to QM would be worrying over the “devil and the old one are in the details” in those very places where orthodox QM usually over simplifies for expediency. It is an issue of “completeness”.
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.
Thank you so much! I FINALLY understand it and it makes sense.
Indeed, I find this very clear, thanks!
This has been my favorite science channel for years! Such a good teacher
I remember Sabine explained exactly this in her old video.
Thank you for explaining this in a simpler way and to correct the previous unintended confusion !!
In order for the Nature of the sub-atomic Particle to be known, all the results have to be obtained. In order for all the results to be obtained, both the sub-atomic particle and the observer has to 'know' what those results are. If the observer is not aware of all the results, how do we know if the nature of that sub-atomic was changed in the past or not ? If the observer is aware of all the results, how do we know the results are or are not from a particle being influenced in the Past ? As John Wheeler has postulated, wether a photon leaving a Star is a particle or a wave, depends on wether an observer from billions of years in the future observes it as a particle or a wave. Thats even though that particular Star still even exists when it is observed in the far far future.
Perfect explanation! 👌🏻
Cannot help leave a comment to thank you for such great video on this topic before I finished it. Thanks!
This video highlights why I find you content so valuable. Those who prioritize self reflection, looking for mistakes, are much more trustworthy than those that ignore error. You seek truth, and I can't think of anything more noble. Great video Arvin!
I can't believe Arvin joined that club! 😮 Lol
Great video! This is the closest I've felt to understanding the delayed choice quantum eraser.
Perhaps I am confused.
- D1 is getting the results from all the other detectors.
- Naturally the data from D1 needs to be sorted to compare the path of each electron against the other detectors.
- D1 is split into d2, d3, d4, and d5.
- There should be no difference between : d2 and D2, d3 and D3, d4 and D4, and d5 and D5.
Hope we are on the same track here.
- Without observation the electrons reach the split, pass the split, and reach the glass. At this point they will have interfered with each other, with the interference scaling off the split distance, and distance to the glass after the split.
- The glass/crystal interacts 'observes' and produces two entangled units. These are now particles, not waves.
Given interactions with lens are negligible as the particles path is 'known'. D1 should show whatever interference occurred before the glass. If observed before the split it would not show interference.
1) Assuming that the entangled pair inherit the 'known' property from the other pair all detectors should match the results in D1 pattern also. [D2=d2,D3=d3,D4=d4,D5=d5]
2) Assuming that the entangled pair interfere with each other after the glass and before the rando-miser, then the detectors should show a modified interference pattern not matching D1 exactly.[D2!=d2,D3!=d3,D4!=d4,D5!=d5]
3) Given that D2 and D3 show a bar/strip pattern, and together where D2 and D3 are not the same. Then D4 and D5 must show dual bar/strip. As should D1.[D2=d2,D3=d3,D4=d4,D5=d5]
4) Given that D2 and D3 show a bar, but are not the same. But D4 and D5 show an interference pattern. Then something has happened that has caused the collapse of the electron wave function before interacting with the split in the first place. Retro-causal. [D2=d2,D3=d3,D4=d4,D5=d5], or [D2!=d2,D3!=d3,D4=d4,D5=d5] if you assume the D1 readings don't change.
5) Given that .....
Are you sure 11:58 is correct? That D5 and D2, and D4 and D3 shouldn't be swapped? Otherwise those randomisers are proficient sorters.
Hmm ... if swapped though then ... the electrons from one slit interfere distinctly differently than the ones from the other slit. Hmm, almost as if they were behaving as particles going through the split, then on the way to the 'random' detector interfered with itself producing an interference pattern. D2 and D3 get interference patterns, the overlap of left/right bars waves causes the cluster result as shown on D4 and D5. Okay. However D1 is odd, having a 'known' path to the detector. [D2=d2,D3=d3,D4=d4,D5=d5] would be very strange. [D2!=d2,D3!=d3,D4!=d4,D5!=d5] and the patterns are similar then maybe 'known' path is a wrong assumption. [D2!=d2,D3!=d3,D4=d4,D5=d5] would imply that interference only occurs when ... not sure, the only electrons entering wave interference after being entangled are the ones the randomiser sends to the split specific detector, therefore retro-causal?
Really great, intuitive explanation. Thanks for debunking the cranks.
Waves interact with particles of air then why doesn't there wavefunction becomes localised?
@@pwinsider007the interaction is coherent
Given that the people who invented the experiment gave the retrocausality argument, I'd be hard-pressed to call them "cranks." Not everyone who is wrong is a crank.
@@darrennew8211 Exactly. “Retro-causation” isn’t written on the D1 screen, but it seems like an accurate interpretation of the effect they studied.
He's not "debunking" anything, just clarifying details he originally got wrong.
Don’t buy the explanantion. The data botón at D1 doesn’t know the entangled one is going to be deviated to D4 or D5 and it still creates an interference pattern
Question: the measuring is interfering with the photon and it's wave function collapses - that happens at the BBO then at detector 4 and 5 we see interference because they are new non-measured photons? Ok, then the mirror didn't measure it when reflected the photon. But then who caused the wave collapse for the photon at detector 2?
Thanks, great explanation.
The punchline boiled down : there are 4 subsets that correspond to D2-5, which are altered by a longer future path. They are chosen because they correspond. This by definition means backwards causality. However, Arvin ignores causality because they are chosen by the scientists. I do not get his logic, because the subsets either correspond, or they do not. If they do not, then, they are not subsets and there are no subsets that correspond. If they are subsets, then we are back to the beginning of the original interpretation of backward causality.
I need a video to understand the tortured non logic at the end of the video, as he simply does not make any sense.
Can someone tell me why a subset is not a subset. If not then why are we talking about an imaginary subset?
You have refuted the post-selection interpretation very well. Otherwise it would be impossible to form interference patterns with the coincidence counter. Once this interpretation is refuted, they usually come up with the Bohmian interpretation, which establishes causality between signal and idler. This interpretation of "pilot wave" and hidden variables, although legitimate, was already refuted in my opinion with the Aspect experiment, therefore it would not be applicable to DCQE. Nor is there any attempt to establish a causal relationship between signal and idler, if one believes in the random operation of beam splitters.
I am also just an enthusiast, but this is what I understood from the video:
The phrase "altered by a longer future path" already contradicts what the video is trying to say. The idea is that, past the BCO crystal, the "future" path is not being altered at all. The entire wavefunction continues splitting and propagating through the mirrors simultaneously.
The photon strike at D1 "seems to" influence the wavefunction, due to entanglement. As the bottom photon continues, the statistics of where and how the photon will arrive at D2-5 has to proceed with certain statistics - such that the detectors' corresponding subsets at D1 appear as shown.
Note how this doesn't require the impacts on D2-5 to "reach backwards" to affect D1. Rather, the nature of the wavefunction affects both the strike at D1 and the strike (and implied path) through D2-5 "simultaneously", through statistical correlations, as described by the wavefunction.
(I think technically, neither detection happens first from the perspective of the photons. Time does not pass for a photon at light speed, so it would seem to all happen simultaneously. But I don't know if this fact is important to avoid the idea of "backwards causality".)
@@AySz88The "seems to" influence the wave function is appreciated. I also agree with your last paragraph. However, there is no way for the photon in D1 to influence the results of the other detectors in the future since it is determined by the randomness of the beam splitters.
Awesome video again Arvin! This only shows your keen appetite for learning, accepting past misinterpretation (prefer this term to mistake), updating knowledge and passing on to us fortunate viewers.. A true Scientific spirit indeed! Keep it up! 👏🏻👏🏻👏🏻👍🏻👍🏻👍🏻
He's holding off the woo warriors. Whatever the data, we can't admit the warriors are correct.
WOW WOW WOW WOW, Thanks so much for clearing that up! I've never liked/understood quantum erasers because of that confusion.
What if you spin (at a consistent or measured rate of speed) the device that projects the Particles and conduct the same experiment? Would very much like a comparison between the two students with this variable.
If they select a subsample, it does not take away retro-causality.
Initially i thought the same, but he says at 8:32 that "the proccess of creating the entagled photons results in a measurement" and thus we will always get the collapsed particle nature of the electrons at the first screen. There's nothing else to say here, the rest of the expirement has no significance i guess.
@@teoval1827 Then how come we get an interference pattern at 4 and 5? Remember that if a photon is detected at 4 or 5, we don't know which slit it went through, which is exactly the condition for the original double slit experiment. And if we correlate the photons at 4 or 5 with the partner (entangled) photons at 1, we also get an interference pattern, whereas for photons hitting 2 or 3, we know which slit they went through and they don't produce interference and neither do their entangled partners at slit 1. Screen 1 will always show no interference if you look at all the photons. That's why you have to break the screen 1 results into subsets.
It's not collapse of the electrons that is disputed. The experiment shows that if the which way information is removed in future, the past regains interference.
@@willemesterhuyse2547 interference can only exist if the photons have not colapsed , 3:28 "once it is measured, that is , once it's path is known, its wave colapses to become a localized particle".
@@teoval1827 No they can collapse at a point on the screen corresponding to where the waves interfered constructively.