This is a very interesting, novel take on explaining the delayed choice quantum eraser. I think I can summarize it as follows:
The signal photon hits the screen, which is a measurement. The entangled idler's wave function is thereby constrained by that measurement, influencing the probabilities of later detecting it at each of the D1-4 detectors. It's not that the fate/erasing of the idler changes the already committed path(s) of the signal. It's that the measurement of the signal constrains the subsequent detection of the idler.
Two other high quality discussions of eraser experiments are by Sean Caroll [0] and Sabine Hossenfelder [1]. Like the OP, both Sabine and Sean demystify/debunk these experiments.
These three discussions all use different language to explain the outcome, which is clearly predicted by the QM math. Sean's article includes the gist of the math.
I appreciate this article, as I agree with the author that the delayed-choice quantum eraser is a misnomer due to ignoring what we now know of quantum states. It's really frustrating learning modern quantum mechanics but then reading about the delayed-choice quantum eraser making conclusions from an older understanding.
However, I still haven't seen anyone do the math about it. It shouldn't be too hard to keep track of a photon's state through Kim et al.'s experiment, and I think it would be clearer than relying on words alone (as done by the author here). I have attempted this myself, but I am particularly terrible at quantum optics. If anyone has seen such a derivation before please let me know.
I don't think it has anything to do with what we know "now". It's just paying attention to the fact that the signal photon hitting the screen causes a collapse that affects the state of the idler photon. Which then explains the data via the collapsed state depending on the position of the hit, and one of the possible idler measurements being in a basis perpendicular to those variations. All quantum interpretations give the right answer for this experiment, and very few of them invoke retrocausation, therefore the experiment clearly doesn't require retrocausation.
I don't even think the delayed choice eraser is a "quantum" paradox. It involves quantum particles, but they're really just there for flair. They're not crucial. You can apply the same confusion to a classical experiment. Set up some basic correlation between A and B, with A revealed first and then a choice to reveal B or an unrelated C. Then describe the situation so badly that it sounds like choosing to measure B vs C is changing the probability distribution of A backwards in time (since if you condition on B you'll see the correlation vs A, but conditioning on C shows no correlation).
Our understanding of the world is overfit to the macro level, where we project concepts onto experience to create the illusion of discrete objects, which is evolutionally beneficial.
However, at the quantum level, identity is not bound to space or time. When you split a photon into an entangled pair, those "two" photons are still identical. It's a bit like slicing a flatworm into two parts, which then yields (we think) two separate new flatworms... but they're actually still the same flatworm.
Experiments like this are surprising precisely because they break our assumption that identity is bound to a discrete object, which is located at a single space, at a single time.
Depends on your interpretation of quantum mechanics. In Bohmian Mechanics, there is a discrete particle guided by a wave described by the wave function. Also, macro discrete objects are not illusions, they're the result of decoherence. The superposition is suppressed from view, assuming the wave function isn't collapsed or just a mathematical prediction tool.
Quantum physicist here. My PhD back in the day was about the entanglement between downconverted photons. I've thought about this more than I like to admit.
While I appreciate the blog post, it seems a bit disingenuous. I hope everyone understand that if you take two entangled photons A and B and detect A before B, then the outcome of the measurement of B must depend on the outcome of the earlier measurement of A, because measuring A causes the collapse of the joint state and determines the wavefunction of B undergoing the later measurement.
The MAGIC about delayed choice measurements is that they work even when the temporal order is UNDETERMINED. By this I mean that the two measurements of A and B can be set up to occur so close in time to each other that there is no time for a signal travelling at the speed of light to travel between the two events. Under this condition, you can witness both orderings (A measured before B and B measured before A) just by changing your reference frame. Under these conditions, the delayed choice experiment STILL WORKS!
In this case, there cannot be any argument like "but the idler was measured first", because "first" does not make any sense.
I was fascinated by this experiment when I first learned about it. At one point I thought of a modified version where you let the 'B' go across a event horizon of a Black-hole. in this case there would be a clear before and after right? will DCQE still work across singularity boundary?
This is easy to picture if you imagine widely spreading out the equipment used for the eraser experiment. If the signal hitting the screen and idler hitting one of the detectors are space-like separated events... the OP's explanation no longer seems to apply.
The delayed choice experiment doesn't contain a bell inequality, so spacelike seperation doesn't really mean much here. You can reproduce the results with local classical models.
> I hope everyone understand that if you take two entangled photons A and B and detect A before B, then the outcome of the measurement of B must depend on the outcome of the earlier measurement of A, because measuring A causes the collapse of the joint state and determines the wavefunction of B undergoing the later measurement.
This bakes in an assumption that collapse happens, which I don't believe everyone agrees with...
The author talks about "two downconverted photons" each at half the energy, in that simple linear experiment. Is that mainstream physics? If so ... big ask, but do you happen to have a butt-simple reference at the undergrad QM level? It feels like I need more equipment than a fixed pair of slits to downconvert frequencies.
A time reversed photon is still a photon, and as such photons in Feynman diagrams aren't given a direction. They're equally valid to view traveling forward or backward in time.
This is as opposed to an electron, which is given a direction, because reversing it in time produces an anti-electron.
Not in the science-fiction sense. It's just a convenient way to express anti particles in the diagrams but the travelling back in time should not be taken literally.
They do. In fact i always thought this was the answer to the matter/antimatter imbalance.
Feynmann diagrams literally show anti matter as the same particle as a matter particle, just travelling back in time (see election/positron interactions).
So what happens when matter and antimatter are created in a big boom? Well the antimatter is in the past, we're here in the future.
Yes they do and that is the secret to the wave-particle duality manifested in the double-slit experiment. Information from future completely illuminates (pi) the paradox.
PBS Spacetime did an interesting video on DCQE, but it tripped me up trying to fully understand what was happening: https://www.youtube.com/watch?v=8ORLN_KwAgs&t=601s ... Later Sabine Hossenfelder did a video debunking the proposition that DCQE somehow showed that the past was being rewritten. https://www.youtube.com/watch?v=RQv5CVELG3U And Matt from PBS Spacetime acknowledged she was right in this respectful comment:
> Sabine, this is amazing. You are, as usual, 100% right. The delayed choice quantum eraser is a prime example of over-mystification of quantum mechanics, even WITHIN the field of quantum mechanics! I (Matt) was guilty of embracing the quantum woo in that episode 5 years ago. Since then I've obsessed over this family of experiments and my thinking shifted quite a bit.
I did some actual research in making a delayed choice quantum erasure system and the TLDR of why any of these "time travel" results is that what you have to keep in mind is that it is that the entanglement you create you have to still use a classical channel of information to transmit the information as a result which is still bound the speed of light which is what the quantum erasure systems must also show that it doesn't occur.
I always enjoy reading about quantum optics, and the concept of entanglement is mind-blowing (I mean after all, no less than Albert Einstein went to his grave thinking we must be missing something in the theory.)
However, the quantum erasure experiments are really just a variation on other Bell Violation experiments (which also appear to violate causality at first glance.) At their heart the wave functions appear to say that particles are in multiple locations at the same time (so "touching" one particle must affect the state of the other particle instantly.) This information, though, isn't useful until all the results are brought back to one location (i.e., all the strangeness is buried in _some combination_ of the lists of results in the different locations, which can only be combined in a way that obeys causality.) Each list of results by itself looks random. So in this sense, "when" exactly you get your result won't tell you anything.
So no, you can't retake that picture you really wanted but flubbed on your last vacation.
If you could use a wormhole to travel 50 million light years away from earth instantly, then look back at earth with a sufficiently powerful microscope, I believe you could observe the dinosaurs.
It "can" be done. Some photons leaving the earth a thousand years ago. Bouncing off a mirror in some distant planet and allowing us to see two thousand years into the past
I'd personally appreciate it very much if photons from the future could be captured to take pictures from week and years ahead. Could be used to help with those awkward tasks like choosing the correct lottery numbers, choosing the correct stocks, choosing the horse that's definitely going to win, etc.
I will summarize by saying that I think our current understanding of Faster-than-Light communication is wrong, and the no-go theorem about no information transmission faster than light will be debunked (in very specific but slight ways I describe in the link) soon as quantum error-correction gets better. Before you say it’s preposterous, skim the above chat, maybe looking at my side of it for instance. This is an interesting format I often share ideas in these days.
I'm sorry but the chat format is a really poor way to share ideas.
It's long, meandering, and contains many instances of wrong and not-even-wrong assertions by you (expected, you don't claim to be an expert, but hurts your credibility) and by chatGPT (expected, hallucinations, but makes it impossible to believe any assertion without already knowing what makes sense).
If you believe you came up with something worth sharing, please take the 30 minutes to edit it into something coherent.
I tried to understand as much as I could from the conversation, but there was some much where I had to pause and consider if it makes sense or not that it's almost as much effort as if I had to come up with the idea myself in the first place, and that's too much effort for someone else's idea.
I'm sorry for the negativity, but I believe I am giving constructive criticism. You're clearly very curious and creative, but that's sadly not enough.
Ah yes, ChatGPT, the tool famous for it's rigor and accuracy which can never be brought to agree with something untrue, no matter how hard you try. /s
Honestly, I think this is a horrible format to share ideas. It's a scratch pad with very elaborate but not necessarily true explanations why your ideas could be true. Either submit your ideas directly, so it's easier to point out mental leaps/errors and visible how hard you thought about it or do the fact-checking yourself and publish a blog post with sources.
Edit: 100% agree with wasabis reply and they criticized more constructive so take my comment with a grain of salt.
anotherpaulg|1 year ago
The signal photon hits the screen, which is a measurement. The entangled idler's wave function is thereby constrained by that measurement, influencing the probabilities of later detecting it at each of the D1-4 detectors. It's not that the fate/erasing of the idler changes the already committed path(s) of the signal. It's that the measurement of the signal constrains the subsequent detection of the idler.
Two other high quality discussions of eraser experiments are by Sean Caroll [0] and Sabine Hossenfelder [1]. Like the OP, both Sabine and Sean demystify/debunk these experiments.
These three discussions all use different language to explain the outcome, which is clearly predicted by the QM math. Sean's article includes the gist of the math.
[0] https://www.preposterousuniverse.com/blog/2019/09/21/the-not...
[1] https://www.youtube.com/watch?v=RQv5CVELG3U
wasabi991011|1 year ago
However, I still haven't seen anyone do the math about it. It shouldn't be too hard to keep track of a photon's state through Kim et al.'s experiment, and I think it would be clearer than relying on words alone (as done by the author here). I have attempted this myself, but I am particularly terrible at quantum optics. If anyone has seen such a derivation before please let me know.
Strilanc|1 year ago
I don't even think the delayed choice eraser is a "quantum" paradox. It involves quantum particles, but they're really just there for flair. They're not crucial. You can apply the same confusion to a classical experiment. Set up some basic correlation between A and B, with A revealed first and then a choice to reveal B or an unrelated C. Then describe the situation so badly that it sounds like choosing to measure B vs C is changing the probability distribution of A backwards in time (since if you condition on B you'll see the correlation vs A, but conditioning on C shows no correlation).
latenightcoding|1 year ago
wnmurphy|1 year ago
However, at the quantum level, identity is not bound to space or time. When you split a photon into an entangled pair, those "two" photons are still identical. It's a bit like slicing a flatworm into two parts, which then yields (we think) two separate new flatworms... but they're actually still the same flatworm.
Experiments like this are surprising precisely because they break our assumption that identity is bound to a discrete object, which is located at a single space, at a single time.
goatlover|1 year ago
ziofill|1 year ago
While I appreciate the blog post, it seems a bit disingenuous. I hope everyone understand that if you take two entangled photons A and B and detect A before B, then the outcome of the measurement of B must depend on the outcome of the earlier measurement of A, because measuring A causes the collapse of the joint state and determines the wavefunction of B undergoing the later measurement.
The MAGIC about delayed choice measurements is that they work even when the temporal order is UNDETERMINED. By this I mean that the two measurements of A and B can be set up to occur so close in time to each other that there is no time for a signal travelling at the speed of light to travel between the two events. Under this condition, you can witness both orderings (A measured before B and B measured before A) just by changing your reference frame. Under these conditions, the delayed choice experiment STILL WORKS!
In this case, there cannot be any argument like "but the idler was measured first", because "first" does not make any sense.
sdwr|1 year ago
https://physics.stackexchange.com/questions/318967/can-bells...
Does that mean quantum calculations are just a fancy way of describing correlated probabilities, and have nothing to do with spooky action?
DesiLurker|1 year ago
anotherpaulg|1 year ago
This is easy to picture if you imagine widely spreading out the equipment used for the eraser experiment. If the signal hitting the screen and idler hitting one of the detectors are space-like separated events... the OP's explanation no longer seems to apply.
Strilanc|1 year ago
Filligree|1 year ago
This bakes in an assumption that collapse happens, which I don't believe everyone agrees with...
nyeah|1 year ago
nayuki|1 year ago
Physics Videos by Eugene Khutoryansky: Delayed Choice Quantum Eraser - Quantum Physics https://www.youtube.com/watch?v=SzAQ36b9dzs (26m31s) [2015-07-16]
LPisGood|1 year ago
itishappy|1 year ago
This is as opposed to an electron, which is given a direction, because reversing it in time produces an anti-electron.
Wololooo|1 year ago
AnotherGoodName|1 year ago
Feynmann diagrams literally show anti matter as the same particle as a matter particle, just travelling back in time (see election/positron interactions).
So what happens when matter and antimatter are created in a big boom? Well the antimatter is in the past, we're here in the future.
wasabi991011|1 year ago
Either way, the article does just fine elucidating the delayed-choice quantum eraser without quantum field theory.
yubblegum|1 year ago
calny|1 year ago
> Sabine, this is amazing. You are, as usual, 100% right. The delayed choice quantum eraser is a prime example of over-mystification of quantum mechanics, even WITHIN the field of quantum mechanics! I (Matt) was guilty of embracing the quantum woo in that episode 5 years ago. Since then I've obsessed over this family of experiments and my thinking shifted quite a bit.
WhitneyLand|1 year ago
I like to give people the benefit of the doubt, can anyone speak to his credibility on this topic?
zitterbewegung|1 year ago
GlibMonkeyDeath|1 year ago
However, the quantum erasure experiments are really just a variation on other Bell Violation experiments (which also appear to violate causality at first glance.) At their heart the wave functions appear to say that particles are in multiple locations at the same time (so "touching" one particle must affect the state of the other particle instantly.) This information, though, isn't useful until all the results are brought back to one location (i.e., all the strangeness is buried in _some combination_ of the lists of results in the different locations, which can only be combined in a way that obeys causality.) Each list of results by itself looks random. So in this sense, "when" exactly you get your result won't tell you anything.
So no, you can't retake that picture you really wanted but flubbed on your last vacation.
TaurenHunter|1 year ago
user3939382|1 year ago
short_sells_poo|1 year ago
furstenheim|1 year ago
slicktux|1 year ago
sunrunner|1 year ago
empath75|1 year ago
FollowingTheDao|1 year ago
This idea is set up on a false premise.
But I was extremely happy to read; "There’s no such thing as wave-particle duality" "Light only ever travels as a wave".
Everything is only fundamentally a wave.
Please take a look at "The end of time : the next revolution in physics" by Julian Barbour. Or here are some YouTube videos:
https://www.youtube.com/watch?v=K49rmobsPcY
https://www.youtube.com/watch?v=GoTeGW2csPk
https://www.youtube.com/watch?v=1ogiQ2E6n0U
gitfan86|1 year ago
If we understood them we wouldn't be looking this way
meindnoch|1 year ago
Gnarl|1 year ago
[deleted]
1oooqooq|1 year ago
[deleted]
mog_dev|1 year ago
hencoappel|1 year ago
unknown|1 year ago
[deleted]
Ridius|1 year ago
EGreg|1 year ago
https://news.ycombinator.com/item?id=43173195
It links here:
https://chatgpt.com/share/67bde29f-a56c-800a-8e26-44a5a3ad23...
I will summarize by saying that I think our current understanding of Faster-than-Light communication is wrong, and the no-go theorem about no information transmission faster than light will be debunked (in very specific but slight ways I describe in the link) soon as quantum error-correction gets better. Before you say it’s preposterous, skim the above chat, maybe looking at my side of it for instance. This is an interesting format I often share ideas in these days.
wasabi991011|1 year ago
It's long, meandering, and contains many instances of wrong and not-even-wrong assertions by you (expected, you don't claim to be an expert, but hurts your credibility) and by chatGPT (expected, hallucinations, but makes it impossible to believe any assertion without already knowing what makes sense).
If you believe you came up with something worth sharing, please take the 30 minutes to edit it into something coherent.
I tried to understand as much as I could from the conversation, but there was some much where I had to pause and consider if it makes sense or not that it's almost as much effort as if I had to come up with the idea myself in the first place, and that's too much effort for someone else's idea.
I'm sorry for the negativity, but I believe I am giving constructive criticism. You're clearly very curious and creative, but that's sadly not enough.
emaro|1 year ago
Honestly, I think this is a horrible format to share ideas. It's a scratch pad with very elaborate but not necessarily true explanations why your ideas could be true. Either submit your ideas directly, so it's easier to point out mental leaps/errors and visible how hard you thought about it or do the fact-checking yourself and publish a blog post with sources.
Edit: 100% agree with wasabis reply and they criticized more constructive so take my comment with a grain of salt.