Time Warp: Delayed-choice quantum erasure(drgblackwell.substack.com) |
Time Warp: Delayed-choice quantum erasure(drgblackwell.substack.com) |
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...
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 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).
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.
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.
Physics Videos by Eugene Khutoryansky: Delayed Choice Quantum Eraser - Quantum Physics https://www.youtube.com/watch?v=SzAQ36b9dzs (26m31s) [2015-07-16]
This is as opposed to an electron, which is given a direction, because reversing it in time produces an anti-electron.
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.
Thinking about it though: photos are their own antiparticle. So I'd expect to see a lot more cosmic microwave background than we should because at least in the early days the antimatter universe would have been visible to us?
Side note: how can photos be their own antiparticle? Same reason they move at the speed of causality. They have no mass thus do not experience the flow of time themselves. So they do not annihilate with themselves. From a photon's POV a trip across the universe is instant.
Either way, the article does just fine elucidating the delayed-choice quantum eraser without quantum field theory.
> 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 like to give people the benefit of the doubt, can anyone speak to his credibility on this topic?
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.
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
If we understood them we wouldn't be looking this way
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.
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?
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.
This bakes in an assumption that collapse happens, which I don't believe everyone agrees with...
Those values are linked: if you measure one, the other will have the opposite. You can tell that it's not just pre-set values by measuring at a 45 degree angle, so you get some up-down and some left-right in each measurement. Take a bunch of those, and you'll see that the expected correlation between your measurements follows what quantum mechanics predicts, and not classical mechanics.
It's usually done with beta barium borate. You can buy it at optical suppliers:
https://eksmaoptics.com/nonlinear-and-laser-crystals/nonline...
Of course our telescopes don't have anywhere near the resolution for this right now.
The best IMHO is "Thiotimoline and the Space Age" from 1960. You can read it on archive.org: https://archive.org/details/MerrilEdTheYearsBestSF05/Merril_...
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.
Examples include:
Einstein's theory of relativity vs luminferous ether orthodoxy
Rutherford's discovery of radioactivity vs the 100 million age of the earth orthodoxy following Lord Kelvin, Feynman noted how scientists kept staying within a certain range until the next generation suddenly felt bold enough to go further https://www.americanscientist.org/article/kelvin-perry-and-t...
Pasteur's discovery of microbes against the orthodoxy that continually ridiculed people like Ignaz Semmelweiss even decades later https://en.wikipedia.org/wiki/Ignaz_Semmelweis ... people still believed in spontaneous generation of living matter
Galileo's insistence on the heliocentric model, etc. https://www.history.com/this-day-in-history/galileo-is-accus...
Ancient greek ideas of phlogiston, the four humours, etc. or the idea that the heart rather than the brain is where thoughts originate, were in place for millennia
So, engage me on the substance. Discuss the actual substance of what I said. I am saying that PWT explains what we observe in quantum mechanics, and preserves realism at the expense of locality. And that FTL communication is not just possible but a lot less weird than Everett's MWI. I am saying that even if we can escape the light cone that doesn't mean necessarily that we can send information back in time (the ways postulated involve a lot of assumptions) and even if we did, it wouldn't violate any major principles.
I'm saying that we have to use classical mechanics to move the entangled parties apart. So we can't escape our existing light cone, or send messages to the past. But going forward, once we do set it up, we can build e.g. security systems that can't be stopped because they "teleport" some information to another location, even if wires are cut and electrical signals are blocked. I'm saying you don't need to classically move things in order to teleport information.
Einstein's objections that you can send information back in time involve exotic constructs and massive assumptions. And even if they were true, this wouldn't cause any paradoxes. Because the effect is tiny, and the probability of it being amplified is tiny, same as the probability of you passing through a wall due to quantum fluctuations.
But, the way it would manifest is that measurements would be probabilistically biased one way or the other. If there is an intelligence on the other side, it can actually act on this information. This FTL doesn't mean sending info back in time. But even if we were able to, then eventually with enough bandwidth we could communicate with the past, through this thing. It would be like a magic 8-ball that tells you some things. With enough bandwidth you might be able to bootstrap a stronger solution, ending up with a Closed Timelike Curve. I'm saying that there are still no paradoxes at that point because if you can believe Everett's MWI of worlds forking all the time everywhere, then you can certainly believe that worlds fork in these extremely rare scenarios of a closed timelike curve. So you have a sort of corkscrew where in some worlds you bootstrapped the thing, and in other world a lot of information is arriving from the future. But all that is very theoretical and not required just for FTL communication.
The way we achieve FTL communication is to improve quantum error-correction, as Microsoft has done. Once we have enough qubits, it may turn out that the randomness was because we are just "were not able to throw the dice in a controlled enough manner". That's what the DeBroglie-Bohm's Pilot Wave Theory says. It postulates local realism (attributing the randomness to our limitations), and accepts FTL information transmission via pilot waves. And by the way the other, wackier, theories don't rule out FTL communication either.
So we will soon enter an era where we can test this. When we overcome the quantum error rates and prevent decoherence, we'll be able to actually INFLUENCE remote measurements at FTL speeds. Not perfectly, but enough that we can send information. That's my prediction. Then we'll know if PWT is actually true.
This idea that all interpretations of quantum theory are exactly the same, in that they produce exactly the same predictions, is only true while the randomness and error rate is high. Once we learn to cancel out the errors, suddenly we'll get a "clearer picture" and be able to throw the dice more accurately.
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.
No theory can match CMB as well as the current theories that add cosmic inflation on an as needed basis in time and space to make CMB observations match perfectly. But cosmic inflation is a completely unknown mechanism (no explanation at the level of particle physics) and it was added specifically to make CMB match perfectly. In some ways i think the current 'we observe CMB to be this so we'll move these free variables to make it match' make CMB non-falsifiable. You could add similar free variables to any theory to make CMB match.
What would change though is the math.
Getting rid of time" in quantum equations would fundamentally change the way quantum mechanics is formulated, potentially requiring a complete rethinking of the theory, as it would involve removing the time variable from the Schrödinger equation.
See: https://www.scientificamerican.com/article/why-some-physicis...
He gave you constructive criticism about the form, specifically the ways in which your form makes it overly difficult to engage with you.
> I'm saying you don't need to classically move things in order to teleport information.
Generally for QM entanglement experiments you can't interpret the result without the measurements from both sides. So while you might be able to demonstrate after the fact that impacts on the state of the system propagated faster than light, you will not be able to make use of that FTL propagation in any directly useful manner.
Modern quantum error correction techniques are dramatically improving:
Decoherence suppression: Extending coherence times of qubits.
Fault-tolerant quantum computing: Reducing errors in quantum state evolution.
Quantum memory storage: Holding quantum states stable for long durations.
So if decoherence is controllable, and if randomness is epistemic (not ontological), then this suggests we can gradually influence quantum measurement probabilities.
Look, here's what we can already do today:
Extend entanglement coherence times (already happening).
Control quantum noise with precision (e.g., superconducting qubits).
Perform weak measurements without full collapse (experimental quantum optics).
I agree that directly biasing entangled measurements nonlocally has not yet demonstrated today, but it could be in the near future, which is what I am predicting can unleash this FTL communication possibility!
Quantum error correction is already proving that decoherence isn’t truly random—it can be controlled. The interaction-free measurement paradox suggests that it’s possible to extract information without collapsing a wavefunction. Bohmian Mechanics & PWT are underexplored experimentally, and this would be a direct way to test them.
> bias the probabilities on one end, before the decoherence happens
Let's start there. What do you mean by that? What would this look like in concrete terms if I went and did it on the bench?
My understanding is that any measurement you take will appear random and uncorrelated until you have the data from the other side. At which point you can say "oh hey look, turns out it was actually correlated" but you can't demonstrate that fact until you have both sets of data in hand.
So in simple, concise, and concrete terms, what violation of the above are you proposing?
In other words: it's awesome, and we should totally do it.
But, for example, if we are trying to get an image of a dinosaur, and we’re only getting one photon per week, how’s that work? The dinosaur should have moved before the second photon is sent off, right?