The real double-slit quantum eraser they don't want you to know about!
The real double-slit quantum eraser they don't want you to know about!
The recent post made me fear that a lot of you are taking this "monkey looks at double-slits" meme, which was only ever supposed to be a funny monkey meme, actually seriously. Honorable mention goes to @[email protected], whose 12 posts on the topic, insisting that the quantum eraser experiment (but not the delayed-choice quantum eraser!) proves that the double slit is somehow bizarre, forced me to make my own meme. This meme explains the (non-delayed choice) quantum eraser paper from arXiv:quant-ph/0106078 and the figures are numbered to reference the paper.
First of all, looking at the photons, you the conscious intelligent monkey, MAKES NO DIFFERENCE. You can't actually "see" the photons going through a slit the way you could see say a bowling ball. The only way to detect a photon is to absorb or reflect it, and if the photon is getting absorbed by your eye that means it's not going through the slit or hitting the screen. The interference pattern stays visible on the screen WHETHER OR NOT YOU LOOK AT IT.
They've lied to you when they said the pattern changes when you "look" at which slit it the photon goes through. What the physicists actually do to measure the "which path" information is they put these circular polarizer filters in front of the slits, one clockwise one counterclockwise. Then the pattern disappears and you get this one single blob of density (Not even double! Figure 3). This is because light polarized in opposite directions cannot interfere with itself - wikipedia calls this the "Fresnel–Arago laws". In principle you could have put a polarization detector in place of the screen and record which way the light hitting it is polarized, which would tell you which slit the photon must have went through. The physicists DON'T EVEN BOTHER DOING IT. The fact alone that the light is polarized when it hits the screen is sufficient to destroy the interference pattern.
Well, NO SHIT. You put these giant 3D glasses in front of the slits and you still expect to see interference? This is very much a "mechanical interaction", not some "non-obtrusive conscious observation". Everything that destroys coherence will ruin your quantum experiment! Mystery solved!
So what about the quantum eraser, @kromem will ask? Popular science has created this myth that you can look at the screen and you can make the interference pattern literally shimmer in and out of existence by just flipping a switch, connected to second detector positioned elsewhere, turning it off and on. An action at a distant place (the detector POL1 observing "twinned" entangled photons created by this fancy nonlinear barium crystal before the slits, Figure 1) changes whether light over here behaves as a particle or a wave, right in front of your eyes. Spooky action at a distance, right?
THIS FUCKING DOESN'T HAPPEN. The monkey will see the single blob from Figure 3 and only single blob, no matter whether it turns the second detector on or off! The interference pattern will NEVER shimmer back into existence. The light never switches between behaving like a wave and behaving like a particle. It always behaves the same way, all the time, everywhere in the universe - like fucking light!
So what do the physicists actually fucking mean when they say the interference pattern is "restored"? If you observe the photons hitting the screen one at a time and you correlate them with simultaneous detections at detector POL1, you can mark those events as either "yes coincidence" category A or "no coincidence" category B. If you look at just all the category A events (Figure 4) you will see an interference pattern, and just category B you will see another (Figure 5). You cannot see these patterns by eye on the screen! You have to use a computer to record the events individually and separate them, you will only ever see a single blob by eye. The two interference patterns are subsets of that blob. They were always part of it, their hills and valleys mesh together into a single continuum. NO ONE EVER FUCKING EXPLAINED THIS.
The detector POL1 has a linear polarizer filter in front of it, so straight out the gate it will not see 50% of the twinned photons at all, because they will get stuck in the filter. Your category A can never match more than 50% of events. It gets worse, since the non-linear crystal in reality has very low efficiency and most photons going through are not twinned, so you cannot measure category B directly. In the experiment they do it by rotating the filter 90°, which changes the correlation to category B. In the meme I show them as if the crystal was 100% efficient.
The delayed-choice quantum eraser works similarly - you only ever see a single blob and can never see the interference pattern shimmer in and out of existence. You need the correlation data from the second detector to split the blob into two intermeshed interference patterns using a computer. The Sabine video was the first one I've ever seen that explains this correctly. Every other popular science video up to that point has lied to me!
Whatever you do, DO NOT watch the DR. QUANTUM video with an open mind! (Not even going to link to it, @manual3204 linked it in the other thread.) It's from a documentary produced by a literal UFO cult to promote their quantum woo woo, only masquerading as a quirky science video. It came out in the early days of youtube, when its production and animation quality were unusually high for its time, so it immediately became youtube's go-to video for double slit experiment. Copies of it remain highly ranked there even to present day. It's total baloney!
That wall of text got me hard as diamonds
Keep going I'm almost there..
I appreciate you and the effort this took.
It basically sounds like he's saying "The experiment is bullshit, we just can't measure shit." but then why isn't Dual Slit widely debunked then?
I wouldn't agree with your paraphrased characterization but I think the reason that the experiment results are widely misunderstood is for the same reason any retraction or updated information can't reach the entire same audience as the original information.
The experiment was popularised by Feynman in the 60's and widely discussed as the basis for quantum mechanic. Feynman generally was a fucking rad dude, but he did have a penchant for the poetic, which is probably why he was so popular. Einstein weighed in on the concept too, so big names with big topics in a lunar-landing sci-fi loving era. And quantum mechanics was a fun new mindfuck development in its own right.
So, when a few decades later, the tech catches up to the theory, in experiments by smaller-fame scientists, and the theory further refined; then you've got a legion of adults who grew up with the 60's romantic understanding published in mainstream media, teaching that to the next generation... and you get this.
I can personally blame Brian Greene's 2005 https://www.penguin.co.uk/books/54483/the-fabric-of-the-cosmos-by-brian-greene/9780141011110. His section on the experiment didn't feel right at the time, but feels aren't reals, so I just went with my very limited understanding of an expert's overview. The refined explanation now feels a lot more sensible, for what it's worth.
monkey can't see this
Hey, I'm an ape you dick!
I'm great one too.
A certain librarian would be very upset.
Magic isn't real
Aw. :(
What's the difference between magic in fantasy worlds and just another force of nature like electricity?
Disproportionate energy expediture. Little effort for magic mans big effort for science mans. I'd love to move earth around me effortlessly and I only occasionally like flinging dirt around with a shovel.
Depends on the setting. Sufficiently understood magic is equivalent to science.
The paper doesn't use an actual screen, they only have the detector D_S that they move up and down to record the coincidences. I simulated what the monkey would see had there been a screen in place for the purpose of the meme. I copied down the datapoints from the graph and simulated 100,000 photons hitting the screen with the probabilities indicated by those points. My javascript pastie is available here: https://html.cafe/xcd2a5ed3?k=19f51bff26c65bcf253ee5257a5257d4f11570d9 Importantly, the monkey can never see images 4 and 5 on the physical screen - those can only be displayed on the computer. The monkey will only ever see image 3, which is the sum of 4 and 5.
So what about the clean double stripes in the meme, when do we see that?
Also if we shoot it through only one slit, would it look like fig 3?
Both this paper and the delayed choice quantum eraser experiment paper (arXiv:quant-ph/9903047) only show a single blob, not the double stripe. If anyone has a paper that clearly shows a photo with the double stripes the way it's shown in the classic monkey meme, I'd like to add it to my collection!
Obviously if the slits are big and wide enough apart you will just get two spotlights, so that doesn't count. It wouldn't even demonstrate wave physics, let alone quantum. It has to be a paper where there is some switch you turn on or some filter you slide in place or whatever that makes the image on the physical screen toggle between two stripes and multiple.
If we cover one slit, it will look like figure 3, shifted to the side and at half intensity.
As someone who enjoys quantum mechanics, but papers are too far over my head; and has struggled with a nagging sense that something about all the explanations
just wasn't making sense, thank you for this beautifully clear explanation of how these conclusions were reached.I still don't get it, basically what I get from this is "The Dual Slit is bullshit, we just can't measure it correctly."
Oh thank fuck, the quackery in the other thread was driving me mad.
We have a winner
Don't tell me what memes to take seriously. I can do my own "research" iykwim (I don't know what I mean).
So it turns out the original meme was kind of right? It just didn't illustrate what it was talking about very well. According to @ada@[email protected]:
The double slit experiment is about observation.
When you fire photons through the double slits, one photon at a time, they cause wave interference patterns with themselves as if each photon travelled through both slits.
Yet if you set something up to measure which slit each photon passed through, they no longer interfere with themselves, and give you the two straight lines pattern, rather than the interference pattern.
In this experiment, they didn't even bother measuring which slit each photon passes through. The 3D glasses don't measure or observe the photons, they merely polarize them (although they do block 50% of light). The detector D_S doesn't measure which-path information either. The researchers could have placed a circular polarizer in front of D_S, and when they get a hit they could have said with confidence "this photon came through the top slit!" but they didn't even bother doing it this time. The fact that the 3D glasses alter the light in a manner which makes the which-path information theoretically measurable (even if not actually measured), alone is sufficient to destroy the interference pattern.
So life isn't just a lazy simulation being rendered as we see things? Shit the bed I'm never going to become superman.
Don't worry, you would only be an NPC in someone else's simulation anyway.
Fucking too right!
Why is this whole post about photons? I always thought that the double slit experiment was interesting because this happened with electrons, even individual electrons would still generate the interference pattern, and I guess I always thought that you could detect which slit the election went through by detecting an induced current or something
My knowledge on the actual thing is minimal
Yes, double slit interference happens with both photons and electrons, and even with C60 buckyballs and organic fluorescent dye molecules (https://arxiv.org/abs/1402.1867)! This post is more so about the quantum eraser, as a counterpoint to the 12 posts about it that @kromem wrote in the other thread. The first experimental quantum eraser paper from 2001 uses photons, so that's the figures I used here. There might be newer papers that use electrons, like this one https://doi.org/10.1126/science.1248459 from 2014, but I don't have access to it. I presume detecting the electron there using induced current or whatever would disturb its wavefunction to the same severity as using the polarizer filter does here.
even with C60 buckyballs and organic fluorescent dye molecules (https://arxiv.org/abs/1402.1867)
Holy smokes! I always thought it just showed that electrons and photons aren't 'really' particles, I had no idea it also worked for atoms and even molecules.
Found this paper from 2019 with open access, where they do double slit electron diffraction and then slide a shutter in to close the second slit.
They talk about how it was never actually possible to do this before, because it requires very fine "electron optics" and manufacturing of components, like slits and shutters, with nanometer precision. So while the thought experiment with electrons itself was proposed by Feynman in 1963 (which is probably what inspired the monkey meme and the like), it was not actually realized until 2019. I'm also now guessing that the electron quantum eraser paper from 2014 doesn't use a double slit but some other electronic quantum circuit that is easier to work with.
The two-stripe photo to match the monkey meme, with electrons and measuring which-way information, probably doesn't exist yet. So that's why!
Too late I read about the poo science of this and now I think its just Schrödinger's light switch whether they do one pattern of slits or the other to know if light is a wave or a photon or something I forget.
Sadly, Magic is once again missing from the world.
Btw, please don't use Anti-Asian phrases created by a literal Pedophile in your debunks (Woo Woo)
All else I have to say is, knowing that amount of things you have to do to even measure this wave in the first place... There's no great mystery here, it's just "The ways we have to measure this thing are insanely shitty."
I will die sadder than I can possibly imagine.
Btw, please don't use Anti-Asian phrases created by a literal Pedophile in your debunks (Woo Woo)
Wut? I'm pretty sure this has been a derogatory term for pseudoscience since at least the early 90s. I dunno what you think the origins of the word are, but the only relation to Asian people or culture I can find are in the form of it being used to mock charlatans peddling vaguely Asian-sounding spiritual beliefs.
This person can’t actually back the claim up, but they will double down on it.
It was made up by James Randi, a crazy man who claimed he debunked meditation and climate change... Sure he's also the reason no one takes Uri Geller seriously, but that's like the ONE good thing he did...
James claimed it was based on music in sci-fi films, but the suspicious similarity to the asian phrase "Wu", and his own contempt for Eastern Religious Practices are noteworthy.