[HN Gopher] Time Warp: Delayed-choice quantum erasure
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Time Warp: Delayed-choice quantum erasure
Author : Gnarl
Score : 79 points
Date : 2025-02-27 14:00 UTC (9 hours ago)
(HTM) web link (drgblackwell.substack.com)
(TXT) w3m dump (drgblackwell.substack.com)
| TaurenHunter wrote:
| It would be nice if photons from the past could be captured to
| take pictures of millennia ago.
| short_sells_poo wrote:
| I hope this is a sarcastic post about telescopes :)
| sunrunner wrote:
| 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.
| MassPikeMike wrote:
| If you can't get photons from the future, just get a hold of
| some thiotimoline, a compound invented by SF master Isaac
| Asimov that dissolves slightly before it is added to water.
| After first describing it in a spoof chemistry paper, he
| returned to it several times, exploring its different
| applications and the new scientific field it created,
| "chronochemistry".
|
| The best IMHO is "Thiotimoline and the Space Age" from 1960.
| You can read it on archive.org: https://archive.org/details/M
| errilEdTheYearsBestSF05/Merril_...
| user3939382 wrote:
| 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.
| some_furry wrote:
| 50 million light years would not get you light from 65+
| million years ago.
| elliotec wrote:
| Imagine seeing the exact point in time the asteroid hit!
| rapnie wrote:
| There is this sci-fi book about this, but I forgot
| name/author. In the book a wormhole end can be positioned
| anywhere in space and time history. The tech starts as an
| ITER-like project and then becomes available in mainstream
| products, and all the dirty secrets of mankind come to light.
| bsenftner wrote:
| That is Arthur C Clark's last novel "The Light of Other
| Days", and it even has an Elon Musk like no ethics
| billionaire as one of the plot driving main characters.
| rapnie wrote:
| Thank you! Yes, that is the one. I actually found it on
| my bookshelf just now, but in searching for "that book
| with the wormholes" several times before I never thought
| to check this one. Great sci-fi story.
| gpderetta wrote:
| Asimov has written a somewhat similar novel called "The
| Dead Past". In fact from the description I thought parent
| was referring to this story.
| NikkiA wrote:
| The photons would be so spread out that you'd need a
| telescope with a primary optic bigger than anything else in
| the universe. And even then, you'd struggle to capture enough
| to have any meaningful information.
| itishappy wrote:
| You don't even need a wormhole. Light can "reflect" (ok more
| like slingshot) around a blackhole 25 million light years
| away, giving us a direct visual path to our own planet 50
| millions years ago.
|
| Of course our telescopes don't have anywhere near the
| resolution for this right now.
| thrance wrote:
| Will they ever? Surely at some point there just wasn't
| enough light reflected off of dinosaurs skin to reconstruct
| anything more than scattered dots of light.
| delecti wrote:
| Theoretically you'd just need an _enormous_ telescope to
| collect enough light to resolve an image. Though based on
| the features that modern telescopes can resolve on the
| moon ~1 light second away, it 'd probably need a
| telescope on the scale of a galaxy
| bee_rider wrote:
| At some point there might be it be enough photons,
| though, right?
| itishappy wrote:
| Nope! We can detect individual photons (and measure their
| properties) and a mirror can (nearly) perfectly focus and
| collect every photon that hits it. The frequency of
| detection events would change with the distance, but the
| frequency doesn't hugely matter, one photon per week is
| enough for science!
| bee_rider wrote:
| It is definitely possible that I'm missing something
| here.
|
| 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?
| itishappy wrote:
| Scattered dots of light can tell us a lot! If you find a
| photon of a frequency that's only produced by, say, a CO2
| molecule, you can prove that CO2 existed 50 million years
| ago. Ok wow, probably not too useful. Collect a bunch of
| photons, count how many indicate CO2 and how many
| indicate O2, and you can figure out the composition of
| Earth's atmosphere. All of this using single photon
| detections!
| wizzwizz4 wrote:
| This would be a _very_ expensive way to get a very noisy
| copy of data that we can more easily extract from the
| geological record.
|
| In other words: it's awesome, and we should totally do
| it.
| furstenheim wrote:
| 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
| slicktux wrote:
| Just imagine of all the plants that have absorbed photons...the
| memories would be corrupted...though photons do dispersed
| isotropically...
| empath75 wrote:
| All photons you see are "from the past". Photons moving
| backwards in time would let you see _the future_.
| wasabi991011 wrote:
| 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.
| Strilanc wrote:
| 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).
| latenightcoding wrote:
| To be fair, almost everything in quantum is poorly named.
| That's how they attract funding.
| airstrike wrote:
| Except for "quantum supremacy", which is the best name in the
| entire multiverse.
| LPisGood wrote:
| I am I crazy or do Feynman diagrams not explicitly show
| interactions of particles moving back in time?
| yubblegum wrote:
| 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.
| nyeah wrote:
| No it isn't. Please stop.
| DennisP wrote:
| I don't know if this is what GP was talking about but
| Cramer's transactional interpretation of quantum mechanics
| has wave functions moving both forward and backward in
| time.
|
| https://en.wikipedia.org/wiki/Transactional_interpretation
| Wololooo wrote:
| 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.
| wasabi991011 wrote:
| They do, but the interpretation is that those represent anti-
| particles moving forwards in time.
|
| Either way, the article does just fine elucidating the delayed-
| choice quantum eraser without quantum field theory.
| itishappy wrote:
| Photons are their own anti-particle, so the distinction isn't
| particularly useful, but the representation is equally valid.
| AnotherGoodName wrote:
| 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.
| xenadu02 wrote:
| Not sure how we could ever prove something like that but it's
| certainly an amusing and symmetrical view of the universe:
| big bangs create two universes, moving in opposite time
| directions from each other. Each seeing the other's particles
| as anti-particles.
|
| 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.
| wizzwizz4 wrote:
| In the early days, the universe was opaque.
| AnotherGoodName wrote:
| I'm not at all invested in the above except as a vague
| thought experiment but imho...
|
| 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.
| itishappy wrote:
| 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.
| GlibMonkeyDeath wrote:
| 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.
| EGreg wrote:
| I literally posted about this topic a few days ago! But for some
| reason it was flagged as a "Dupe" and buried by someone:
|
| 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 wrote:
| 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.
| EGreg wrote:
| You're totally right that I am not a scientific expert on
| this by any means. But often, breakthroughs in science come
| not from orthodoxy or experts, but exactly the people who
| don't believe in the orthodoxy.
| https://en.wikipedia.org/wiki/Planck%27s_principle
|
| 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.
| emaro wrote:
| 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.
| FollowingTheDao wrote:
| No, and Yes, because there is no "back in time", nor "forward in
| time". Time is just a useful illusion we create to navigate
| space.
|
| 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 wrote:
| This seems like the most likely answer to the Fermi paradox.
| Our assumptions about time and space are wrong.
|
| If we understood them we wouldn't be looking this way
| meindnoch wrote:
| I've watched the videos, but they were just ramblings without
| any physical or mathematical substance.
| ziofill wrote:
| 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.
| Filligree wrote:
| > 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...
| ziofill wrote:
| Sure sure, you can ignore that wording. The point is that the
| first measurement determines the state of the particle
| undergoing the second measurement.
| Strilanc wrote:
| 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.
| nyeah wrote:
| 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.
| jfengel wrote:
| Yes, that's the main way we produce entangled photons. You
| put in a photon; you get out two photons, each with half the
| energy. To follow conservation laws, they must have
| complementary values of things like polarization: if one is
| up-down polarized, the other must be left-right polarized.
|
| 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...
| nyeah wrote:
| Thanks. Somehow I thought the author was trying to explain
| a simple linear two-slit experiment using two photons, each
| with half the energy. I think it was a reading problem on
| my end.
| anotherpaulg wrote:
| Good point!
|
| 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.
| wnmurphy wrote:
| 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.
| goatlover wrote:
| 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.
| zitterbewegung wrote:
| 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.
| WhitneyLand wrote:
| Apparently the author writes picture poetry books and has no
| degree in physics.
|
| I like to give people the benefit of the doubt, can anyone speak
| to his credibility on this topic?
| nayuki wrote:
| A highly visual explanation of the same topic:
|
| Physics Videos by Eugene Khutoryansky: Delayed Choice Quantum
| Eraser - Quantum Physics
| https://www.youtube.com/watch?v=SzAQ36b9dzs (26m31s) [2015-07-16]
| calny wrote:
| 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.
| anotherpaulg wrote:
| 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.
|
| [0] https://www.preposterousuniverse.com/blog/2019/09/21/the-
| not...
|
| [1] https://www.youtube.com/watch?v=RQv5CVELG3U
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