Time Warp: Delayed-choice quantum erasure

https://drgblackwell.substack.com/p/time-warp-delayed-choice-quantum

by Gnarl • 1 year ago

92 72 1 year ago

drgblackwell.substack.com

TaurenHunter• 1 year ago

It would be nice if photons from the past could be captured to take pictures of millennia ago.

short_sells_poo• 1 year ago

I hope this is a sarcastic post about telescopes :)

sunrunner• 1 year ago

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• 1 year ago

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/MerrilEdTheYearsBestSF05/Merril_...

user3939382• 1 year ago

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• 1 year ago

50 million light years would not get you light from 65+ million years ago.

rapnie2• 1 year ago

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.

NikkiA• 1 year ago

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.

itishappy6• 1 year ago

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.

furstenheim• 1 year ago

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• 1 year ago

Just imagine of all the plants that have absorbed photons…the memories would be corrupted…though photons do dispersed isotropically…

empath75• 1 year ago

All photons you see are "from the past". Photons moving backwards in time would let you see _the future_.

wasabi9910113• 1 year ago

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.

Strilanc2• 1 year ago

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).

cwillu• 1 year ago

So basically the monte hall problem :)

latenightcoding• 1 year ago

To be fair, almost everything in quantum is poorly named. That's how they attract funding.

airstrike• 1 year ago

Except for "quantum supremacy", which is the best name in the entire multiverse.

LPisGood• 1 year ago

I am I crazy or do Feynman diagrams not explicitly show interactions of particles moving back in time?

yubblegum• 1 year ago

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.

nyeah3• 1 year ago

No it isn't. Please stop.

Wololooo• 1 year ago

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.

wasabi9910113• 1 year ago

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.

itishappy6• 1 year ago

Photons are their own anti-particle, so the distinction isn't particularly useful, but the representation is equally valid.

AnotherGoodName2• 1 year ago

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• 1 year ago

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.

itishappy6• 1 year ago

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• 1 year ago

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.

EGreg3• 1 year ago

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.

wasabi9910113• 1 year ago

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.

EGreg3• 1 year ago

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• 1 year ago

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.

FollowingTheDao3• 1 year ago

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• 1 year ago

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

meindnoch2• 1 year ago

I've watched the videos, but they were just ramblings without any physical or mathematical substance.

FollowingTheDao3• 1 year ago

This is a depressing comment to me. Solving this problem is not just about "Math" and he explained it in the videos. The current math is based on a flawed premise, that time continues when there is no movement or change.

ziofill2• 1 year ago

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• 1 year ago

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...

ziofill2• 1 year ago

Sure sure, you can ignore that wording. The point is that the first measurement determines the state of the particle undergoing the second measurement.

Strilanc2• 1 year ago

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.

nyeah3• 1 year ago

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• 1 year ago

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...

anotherpaulg2• 1 year ago

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.

DesiLurker• 1 year ago

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?

sdwr• 1 year ago

I dug up this photo that says you can create a bell inequality with a single source of light

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?

wnmurphy• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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.

anotherpaulg2• 1 year ago

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

Time Warp: Delayed-choice quantum erasure

Related Stories