8 ms·

Third-polarizing-filter experiment demystified (2004)

131 points by rahimnathwani 4y ago

rahimnathwani 4y ago
I came across this on Twitter. Someone had posted an image of the same experiment, and said they used it to teach their kid about quantum effects. Several replies explained how the effect can be explained without quantum mechanics. This article (linked in one of those comments) is so clear, and I'm amazed I haven't seen it on HN before.
- bowsamic 4y ago
  That thread was a huge mess of confusion and misinformation. I hope I managed to dispel some confusions there
- gnicholas 4y ago
  It's interesting to see the lack of HN consensus on what's going on, and even lack of satisfaction with explanations on normally-lauded YT channels such as 3B1B.
  - rahimnathwani 4y ago
    If you think this is polarizing (haha!) you should check out Veritasium and Electroboom on whether and how electricity/electrons flow in/around wires.
    - cratermoon 4y ago
      I just watch Electroboom for the booms.
adgjlsfhk1 4y ago
This only works if you forget that light is quantized. The place the weirdness really comes in is that if you shoot single photons at a time, you observe the same effects.
- avodonosov 4y ago
  Does the sibling comment by phkahler cresolve your doubt? (https://news.ycombinator.com/item?id=32514107 https://news.ycombinator.com/item?id=32514107)
- yarg 4y ago
  It's not that weird. You can consider a wave passing through a filter as a sum of two orthogonal waves, rSin(θ) + rCos(θ), θ being the angle between the light-wave and the filtered angle, r being the amplitude of the wave. One wave gets eliminated, and whatever exits exits at the only angle it can, the angle orthogonal to the filtered angle.
  - yarg 4y ago
    I wasn't particularly clear here; but I'm not actually wrong. The reason, for example, that the 45 degree filter leaves 1/(2^.5) ~= .7 It's just the trigonometric break down of the right-angle isosceles.
- aidenn0 4y ago
  It's only weird if you think of light as just being particles. Once you realize it's a wave too, then the math all works out just fine.
phkahler 4y ago
The part that still needs explaining is how the magnitude can be reduced. IIRC single photons can be polarized by these things, and AFAIK their wavelength is not changed so their energy is unchanged as well. I have always thought (how I got there I don't know) that the polarizer did something weird like rotate the photon to the correct phase angle AND passed it through with probability based on the angle / or didn't let it pass. This would give a similar reduction in intensity for a desktop experiment while having similar but different details when looking at the photon level. Is this correct?
- Sharlin 4y ago
  The polarization of a single photon is a quantum property, so it's essentially a probability distribution. Passing a photon through a polarizer modifies the probability distribution such that "more perpendicular" polarizations are now less likely and "more parallel" ones more likely. (Polarization is a superposition (ie. a linear combination) of two orthogonal basis vectors, and a polarizer projects a polarization vector onto one of the basis vectors.)
- abdullahkhalids 4y ago
  The explanation given is at the level of classical electromagnetism, and is sufficient to explain and predict experiments with regular light. If you want an explanation and prediction of what happens at the level of single photons, you need more structure from the theory of quantum optics. But briefly the filter at angle T does a measurement on the photon in the basis {T, T+pi/2}, and you end up seeing the photon on the other side of the filter only with whatever probability the photon has for being in state |T>, as opposed to state |T+pi/2>. So, filters are inherently destructive and fewer and fewer photons pass through each subsequent filter. And a photon that makes it through a filter at angle T, now has a new state |T>.
  - jiggawatts 4y ago
    The thing is that there is no actual evidence that “single photons” exist in the electromagnetic field. That last bit is important: photons are a mathematical shorthand for dealing with emission and absorption by atomic orbitals. They’re not an explanation for continuous waves in between, but the mathematics largely works anyway because photons are very similar to how one would do a Monte Carlo numerical simulations of continuous wave phenomena. This has resulted in an unbelievable amount of confusion…
klodolph 4y ago
> These results can be verified by performing the experiment with an actual light meter — the meter should show about twice as strong a reading in the Figure 1 arrangement as it does in the Figure 3 arrangement. Quantum mechanics predicts that the difference is a factor of 4, not a factor of 2.
- analog31 4y ago
  Watch out for the difference between the wave amplitude and the power. Power is proportional to the square of the amplitude. I don't know if that's what you're doing here, but it's a familiar pitfall.
roesel 4y ago
While this explanation is very nice, it still does not actually explain what is happening on a material level. The light does not "pass" through the middle filter, but it excites oscillations in the material, which effectively re-emits the light with different properties. The incoming light polarized at 0° induces oscillations in electrons which are "bound to a rail" in the material, which allows them to only oscillate in the direction of 45° (and all oscillations in the direction of -45° are absorbed). Therefore, a portion of the incoming field essentially gets re-emitted by the middle filter linearly polarized at 45°. This representation is much less helpful if you think of the light in terms of individual photons rather than fields of course, but it is not worse than the article in this regard either.
- moralestapia 4y ago
  Is it photons in -> (new) photons out? Or the same ones reoriented?
  - NotYourLawyer 4y ago
    It’s new photons being emitted.
    - amluto 4y ago
      I disagree. Photons don’t have identity - you can’t distinguish old from new. This is true of all bosons, and it’s quite important to how they behave.
      - moralestapia 4y ago
        (Interesting) Could you elaborate?
      - NotYourLawyer 4y ago
      - mensetmanusman 4y ago
avodonosov 4y ago
The concern that the article presents - that the middle filter influences the light and thus allows it to pass through the third filter - is actually addressed in popular quantum mechanics explanations that use the 3 filter experiment. They say that if we use two entangled photons and let them fly far apart, then pass one of them through two filters, and the second photon through the middle filter, the first photon will be affected - it will get a chance to pass though the pair of filters. That they say is "spooky action at distance" - the second photon will influence behaviour of the first photon at the remote site of the experiment and the "influence" is faster then the speed of light. Example here by MinutePhysics and 3Blue1Brown: https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs Explanation about entanglement starts at around 8:50. But even with that addressed, to me personally this video is not satisfying. If the spooky action at distance can be observed so trivially - choosing a filter at one site site affects what happens at the remote site - we don't need a mathematical inequality (the Bell's inequality), it's already so obviously spooky. There are also serious problems with clarity of their explanation, as I commented in https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N5hwXVR4AaABAg.9E0Ji-B15oK9cUCejB5owm https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N... and https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N5hwXVR4AaABAg.9E0Ji-B15oK9cVOYU37sI6 https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N... I am not sure the real Bell experiments are really done using 3 polarizing filters and will the effect really be observed in experiment with two remote sites. My conclusion, it's problematic to rely on "pupular science" explanations, even by good channels like MinutePhysics and 3Blue1Brown.
- abbeyj 4y ago
  > They say that if we use two entangled photons and let them fly far apart, then pass one of them through two filters, and the second photon through the middle filter, the first photon will be affected - it will get a chance to pass though the pair of filters. I don't think that can be correct. This would allow FTL communication of information. I could put the filter in the path of the second photon when I wanted to send a 1 and leave it out of the path of the second photon when I wanted to send a 0. In the 1 case the other side would sometimes see the photon pass both filters. In the 0 case the other side would never see the photon pass both filters. Combine this with a little error correction and you have a channel for transmitting arbitrary information faster than light which would violate the no-communication theorem. https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem The MinutePhysics video you link describes a different setup with only one filter in the path of each photon. This gives you some information about the probability that other photon got past its filter but it doesn't let you transmit anything.
nh23423fefe 4y ago
Isn't this backward? Usually polarization is the analogy used to explain stern-gerlach. I dont get the desire to cast light as something non quantum...
- fsh 4y ago
  The quantum nature of light is extremely difficult to observe. Almost all laboratory experiments can be explained using Maxwell's equations and the quantization of the electric charge (this explains why photodetectors "click"). Photons usually only show up when higher-order correlation functions are analyzed.
- bowsamic 4y ago
  > I dont get the desire to cast light as something non quantum. Because you can describe it entirely using classical physics in this situation
  - kergonath 4y ago
    Except for the whole quantisation thing, and what happens in single-photon experiments. This thread is very weird, there are a lot of people saying nonsense about photons not existing and absorption and re-emission (things that are detailed in any quantum physics textbook in existence). As if the fact that we can also explain the movement of planets around the sun somehow makes general relativity unnecessary or somewhat invalid. Quite disturbing.
    - bowsamic 4y ago
      I have a PhD in physics and I am on my first postdoc in the field of quantum optics so I'm well aware of the quantisation of the EM field. The point is that a lot of people think this is a demonstration of a purely quantum effect, but it is not, it is a classical effect. Sure, quantum mechanics is the full theory for describing it, but that's like saying that the increase in pressure when you heat up a gas is a "quantum effect". It's basically misinformation, especially in the twitter thread that others mentioned. People are coming out thinking their mind has been blown with some crazy quantum effect, but actually this is described by boring classical EM.
yuan43 4y ago
Trying to make sure I understand this. According to the article, the "spookiness" comes from a misunderstanding of what a polarizer does. It doesn't "block" all light polarized on axes different from the polarizer. We know this is true because otherwise sunglasses would transmit much less light than they do. Imagine sunglasses could block any photon within +/- 1 degree of the polarization plane. That means that just 1/180th of the light would get through. But the observed transmission is much higher. Instead, the polarizer does two things. First, it emits light polarized parallel to its axis. But, and this is the key, all incident light gets effectively passed. Along the way the intensity (amplitude, or "magnitude" in the article) is attenuated based on deviation from the polarizer's plane. The attenuation is 0% for light polarized in parallel and 100% for light polarized perpendicularly. Now we can understand the experiment with a new mental model. Three filters are placed in series (A, B, and C). However, we can disregard A for the most part and treat this as a two-filter system (B, C), where the light exiting B is attenuated relative to the light entering A and polarized along B's axis. This model explains all of the observations.
- ithkuil 4y ago
  And if you add more and more polarizers angled the right way, more and more light passes through, until in the limit you reach 1/2 the initial intensity, see https://aip.scitation.org/doi/pdf/10.1063/1.4943751 https://aip.scitation.org/doi/pdf/10.1063/1.4943751
subroutine 4y ago
I thought this effect was so cool I went and bought a few filters to try myself at home. Here's a short vid: https://youtu.be/0yCtJAgO5gc https://youtu.be/0yCtJAgO5gc
aaaaaaaaaaab 4y ago
This could be demonstrated at the macro scale with a rope passing through slits of different alignments. You start shaking the rope in a circle, and observe what happens after it passes through each slit (the “polarizers”).
donatj 4y ago
This is roughly the logical explanation I tried to propose on a YouTube video and was told I was wrong and unqualified to comment.
WheatM 4y ago
cratermoon 4y ago
I'm a bit confused here. This link reads to me like "if we ignore any observations that can only be explained by quantum behaviors, then we don't need quantum behaviors to explain the observed behaviors". What am I missing?
dschuetz 4y ago
The article grossly simplifies what is happening in the experiment and completely ignores the physical properties of EM-waves and polarizing materials. And there seems to be a misconception of how the polarizing filters relate to each other. Don't bother reading, just open a physics book from college or something.
stared 4y ago
If you want to play with that interactively, https://lab.quantumflytrap.com/lab/three-polarizer-paradox?mode=laser https://lab.quantumflytrap.com/lab/three-polarizer-paradox?m.... (Full disclosure - I am one of its authors.)
- Strilanc 4y ago
  I really like quantum flytrap. Happy to see you added a bunch of tools to do basic classical processing of the events, so I don't have to hack around that anymore by like making the bombs conditionally transparent. One feature request: the setups are small enough that you could fit a description of them into the URL. This would allow people to much more easily share setups on forums and twitter and etc. This is what I did in Quirk and it's been one of the more useful features https://algassert.com/quirk#circuit=%7B%22cols%22%3A%5B%5B%22H%22%5D%2C%5B%22%E2%80%A2%22%2C%22X%22%5D%2C%5B%22Chance2%22%5D%5D%7D https://algassert.com/quirk#circuit=%7B%22cols%22%3A%5B%5B%2...
  - stared 4y ago
    And I do like Quirk (we mention it in the intro to our recent paper https://doi.org/10.1117/1.OE.61.8.081808 https://doi.org/10.1117/1.OE.61.8.081808). And in the (not yet released) interactive element description for CNOT (along with Quantum Odyssey). When it comes to links - a good point. I do like the StackOverflow style. I will discuss it with the team.
Strilanc 4y ago
Fun fact: if you use more and more polarizing filters adjusted by finer and finer algorithms, and assume they are ideal (no loss of correctly oriented photons), that block of filters acts like a polarizing filter that filters to one angle but outputs at an angle rotated by 90 degrees. This is an aspect of the quantum zeno effect. Normally it means to measure quickly while rotating an object to pin the object against the rotation. But here you instead measure a static object quickly while rotating the measuring device to force the object to rotate.
- roesel 4y ago
  And this would be a great way to explain how optically active materials [1] work. [1] https://en.wikipedia.org/wiki/Optical_rotation https://en.wikipedia.org/wiki/Optical_rotation
ot 4y ago
Incidentally, this is how LCDs work: two orthogonal polarizers (so no light should pass through), and sandwiched between them liquid crystals which rotate the light so it can pass through. When a current is applied, the crystals align, effectively turning off the rotation and thus the light. It's also a nice exercise to see the polarizer as a linear transformation on the horizontal and vertical direction of the light: if the polarizer is oriented at direction θ, it's equivalent to rotating the light by -θ, keeping only the horizontal component, and rotating back. If you work out the matrixes for rotations 0, π/4, and π/2, you can see that multiplying 0 and π/2 gives the zero matrix, while multiplying 0, π/4, and π/2 keeps a nonzero component.
l33tman 4y ago
The article claims to explain this without reference to the "spookyness" of QM, but in fact it uses an analogous projection postulate as the QM version. After the polarizer, the photon state is projected onto the "measurement" of the polarizer, and in the article's figures the author does the same which leads to the same conclusion (kind of obviously). What QM brings to the table is the explanation for what happens particle-by-particle.

131 points by rahimnathwani 4y ago

I came across this on Twitter. Someone had posted an image of the same experiment, and said they used it to teach their kid about quantum effects. Several replies explained how the effect can be explained without quantum mechanics. This article (linked in one of those comments) is so clear, and I'm amazed I haven't seen it on HN before.

bowsamic 4y ago
That thread was a huge mess of confusion and misinformation. I hope I managed to dispel some confusions there
gnicholas 4y ago
It's interesting to see the lack of HN consensus on what's going on, and even lack of satisfaction with explanations on normally-lauded YT channels such as 3B1B.
- rahimnathwani 4y ago
  If you think this is polarizing (haha!) you should check out Veritasium and Electroboom on whether and how electricity/electrons flow in/around wires.
  - cratermoon 4y ago
    I just watch Electroboom for the booms.

adgjlsfhk1 4y ago

This only works if you forget that light is quantized. The place the weirdness really comes in is that if you shoot single photons at a time, you observe the same effects.

avodonosov 4y ago
Does the sibling comment by phkahler cresolve your doubt? (https://news.ycombinator.com/item?id=32514107 https://news.ycombinator.com/item?id=32514107)
yarg 4y ago
It's not that weird. You can consider a wave passing through a filter as a sum of two orthogonal waves, rSin(θ) + rCos(θ), θ being the angle between the light-wave and the filtered angle, r being the amplitude of the wave. One wave gets eliminated, and whatever exits exits at the only angle it can, the angle orthogonal to the filtered angle.
- yarg 4y ago
  I wasn't particularly clear here; but I'm not actually wrong. The reason, for example, that the 45 degree filter leaves 1/(2^.5) ~= .7 It's just the trigonometric break down of the right-angle isosceles.
aidenn0 4y ago
It's only weird if you think of light as just being particles. Once you realize it's a wave too, then the math all works out just fine.

phkahler 4y ago

The part that still needs explaining is how the magnitude can be reduced. IIRC single photons can be polarized by these things, and AFAIK their wavelength is not changed so their energy is unchanged as well. I have always thought (how I got there I don't know) that the polarizer did something weird like rotate the photon to the correct phase angle AND passed it through with probability based on the angle / or didn't let it pass. This would give a similar reduction in intensity for a desktop experiment while having similar but different details when looking at the photon level. Is this correct?

Sharlin 4y ago
The polarization of a single photon is a quantum property, so it's essentially a probability distribution. Passing a photon through a polarizer modifies the probability distribution such that "more perpendicular" polarizations are now less likely and "more parallel" ones more likely. (Polarization is a superposition (ie. a linear combination) of two orthogonal basis vectors, and a polarizer projects a polarization vector onto one of the basis vectors.)
abdullahkhalids 4y ago
The explanation given is at the level of classical electromagnetism, and is sufficient to explain and predict experiments with regular light. If you want an explanation and prediction of what happens at the level of single photons, you need more structure from the theory of quantum optics. But briefly the filter at angle T does a measurement on the photon in the basis {T, T+pi/2}, and you end up seeing the photon on the other side of the filter only with whatever probability the photon has for being in state |T>, as opposed to state |T+pi/2>. So, filters are inherently destructive and fewer and fewer photons pass through each subsequent filter. And a photon that makes it through a filter at angle T, now has a new state |T>.
- jiggawatts 4y ago
  The thing is that there is no actual evidence that “single photons” exist in the electromagnetic field. That last bit is important: photons are a mathematical shorthand for dealing with emission and absorption by atomic orbitals. They’re not an explanation for continuous waves in between, but the mathematics largely works anyway because photons are very similar to how one would do a Monte Carlo numerical simulations of continuous wave phenomena. This has resulted in an unbelievable amount of confusion…

klodolph 4y ago

> These results can be verified by performing the experiment with an actual light meter — the meter should show about twice as strong a reading in the Figure 1 arrangement as it does in the Figure 3 arrangement. Quantum mechanics predicts that the difference is a factor of 4, not a factor of 2.

analog31 4y ago
Watch out for the difference between the wave amplitude and the power. Power is proportional to the square of the amplitude. I don't know if that's what you're doing here, but it's a familiar pitfall.

roesel 4y ago

While this explanation is very nice, it still does not actually explain what is happening on a material level. The light does not "pass" through the middle filter, but it excites oscillations in the material, which effectively re-emits the light with different properties. The incoming light polarized at 0° induces oscillations in electrons which are "bound to a rail" in the material, which allows them to only oscillate in the direction of 45° (and all oscillations in the direction of -45° are absorbed). Therefore, a portion of the incoming field essentially gets re-emitted by the middle filter linearly polarized at 45°. This representation is much less helpful if you think of the light in terms of individual photons rather than fields of course, but it is not worse than the article in this regard either.

moralestapia 4y ago
Is it photons in -> (new) photons out? Or the same ones reoriented?
- NotYourLawyer 4y ago
  It’s new photons being emitted.
  - amluto 4y ago
    I disagree. Photons don’t have identity - you can’t distinguish old from new. This is true of all bosons, and it’s quite important to how they behave.
    - moralestapia 4y ago
      (Interesting) Could you elaborate?
    - NotYourLawyer 4y ago
    - mensetmanusman 4y ago

avodonosov 4y ago

The concern that the article presents - that the middle filter influences the light and thus allows it to pass through the third filter - is actually addressed in popular quantum mechanics explanations that use the 3 filter experiment. They say that if we use two entangled photons and let them fly far apart, then pass one of them through two filters, and the second photon through the middle filter, the first photon will be affected - it will get a chance to pass though the pair of filters. That they say is "spooky action at distance" - the second photon will influence behaviour of the first photon at the remote site of the experiment and the "influence" is faster then the speed of light. Example here by MinutePhysics and 3Blue1Brown: https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs Explanation about entanglement starts at around 8:50. But even with that addressed, to me personally this video is not satisfying. If the spooky action at distance can be observed so trivially - choosing a filter at one site site affects what happens at the remote site - we don't need a mathematical inequality (the Bell's inequality), it's already so obviously spooky. There are also serious problems with clarity of their explanation, as I commented in https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N5hwXVR4AaABAg.9E0Ji-B15oK9cUCejB5owm https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N... and https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N5hwXVR4AaABAg.9E0Ji-B15oK9cVOYU37sI6 https://www.youtube.com/watch?v=zcqZHYo7ONs&lc=Ugz3tzpDP_i1N... I am not sure the real Bell experiments are really done using 3 polarizing filters and will the effect really be observed in experiment with two remote sites. My conclusion, it's problematic to rely on "pupular science" explanations, even by good channels like MinutePhysics and 3Blue1Brown.

abbeyj 4y ago
> They say that if we use two entangled photons and let them fly far apart, then pass one of them through two filters, and the second photon through the middle filter, the first photon will be affected - it will get a chance to pass though the pair of filters. I don't think that can be correct. This would allow FTL communication of information. I could put the filter in the path of the second photon when I wanted to send a 1 and leave it out of the path of the second photon when I wanted to send a 0. In the 1 case the other side would sometimes see the photon pass both filters. In the 0 case the other side would never see the photon pass both filters. Combine this with a little error correction and you have a channel for transmitting arbitrary information faster than light which would violate the no-communication theorem. https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem The MinutePhysics video you link describes a different setup with only one filter in the path of each photon. This gives you some information about the probability that other photon got past its filter but it doesn't let you transmit anything.

nh23423fefe 4y ago

Isn't this backward? Usually polarization is the analogy used to explain stern-gerlach. I dont get the desire to cast light as something non quantum...

fsh 4y ago
The quantum nature of light is extremely difficult to observe. Almost all laboratory experiments can be explained using Maxwell's equations and the quantization of the electric charge (this explains why photodetectors "click"). Photons usually only show up when higher-order correlation functions are analyzed.
bowsamic 4y ago
> I dont get the desire to cast light as something non quantum. Because you can describe it entirely using classical physics in this situation
- kergonath 4y ago
  Except for the whole quantisation thing, and what happens in single-photon experiments. This thread is very weird, there are a lot of people saying nonsense about photons not existing and absorption and re-emission (things that are detailed in any quantum physics textbook in existence). As if the fact that we can also explain the movement of planets around the sun somehow makes general relativity unnecessary or somewhat invalid. Quite disturbing.
  - bowsamic 4y ago
    I have a PhD in physics and I am on my first postdoc in the field of quantum optics so I'm well aware of the quantisation of the EM field. The point is that a lot of people think this is a demonstration of a purely quantum effect, but it is not, it is a classical effect. Sure, quantum mechanics is the full theory for describing it, but that's like saying that the increase in pressure when you heat up a gas is a "quantum effect". It's basically misinformation, especially in the twitter thread that others mentioned. People are coming out thinking their mind has been blown with some crazy quantum effect, but actually this is described by boring classical EM.

yuan43 4y ago

Trying to make sure I understand this. According to the article, the "spookiness" comes from a misunderstanding of what a polarizer does. It doesn't "block" all light polarized on axes different from the polarizer. We know this is true because otherwise sunglasses would transmit much less light than they do. Imagine sunglasses could block any photon within +/- 1 degree of the polarization plane. That means that just 1/180th of the light would get through. But the observed transmission is much higher. Instead, the polarizer does two things. First, it emits light polarized parallel to its axis. But, and this is the key, all incident light gets effectively passed. Along the way the intensity (amplitude, or "magnitude" in the article) is attenuated based on deviation from the polarizer's plane. The attenuation is 0% for light polarized in parallel and 100% for light polarized perpendicularly. Now we can understand the experiment with a new mental model. Three filters are placed in series (A, B, and C). However, we can disregard A for the most part and treat this as a two-filter system (B, C), where the light exiting B is attenuated relative to the light entering A and polarized along B's axis. This model explains all of the observations.

ithkuil 4y ago
And if you add more and more polarizers angled the right way, more and more light passes through, until in the limit you reach 1/2 the initial intensity, see https://aip.scitation.org/doi/pdf/10.1063/1.4943751 https://aip.scitation.org/doi/pdf/10.1063/1.4943751

subroutine 4y ago

I thought this effect was so cool I went and bought a few filters to try myself at home. Here's a short vid: https://youtu.be/0yCtJAgO5gc https://youtu.be/0yCtJAgO5gc

aaaaaaaaaaab 4y ago

This could be demonstrated at the macro scale with a rope passing through slits of different alignments. You start shaking the rope in a circle, and observe what happens after it passes through each slit (the “polarizers”).

donatj 4y ago

This is roughly the logical explanation I tried to propose on a YouTube video and was told I was wrong and unqualified to comment.

WheatM 4y ago

cratermoon 4y ago

I'm a bit confused here. This link reads to me like "if we ignore any observations that can only be explained by quantum behaviors, then we don't need quantum behaviors to explain the observed behaviors". What am I missing?

dschuetz 4y ago

The article grossly simplifies what is happening in the experiment and completely ignores the physical properties of EM-waves and polarizing materials. And there seems to be a misconception of how the polarizing filters relate to each other. Don't bother reading, just open a physics book from college or something.

stared 4y ago

If you want to play with that interactively, https://lab.quantumflytrap.com/lab/three-polarizer-paradox?mode=laser https://lab.quantumflytrap.com/lab/three-polarizer-paradox?m.... (Full disclosure - I am one of its authors.)

Strilanc 4y ago
I really like quantum flytrap. Happy to see you added a bunch of tools to do basic classical processing of the events, so I don't have to hack around that anymore by like making the bombs conditionally transparent. One feature request: the setups are small enough that you could fit a description of them into the URL. This would allow people to much more easily share setups on forums and twitter and etc. This is what I did in Quirk and it's been one of the more useful features https://algassert.com/quirk#circuit=%7B%22cols%22%3A%5B%5B%22H%22%5D%2C%5B%22%E2%80%A2%22%2C%22X%22%5D%2C%5B%22Chance2%22%5D%5D%7D https://algassert.com/quirk#circuit=%7B%22cols%22%3A%5B%5B%2...
- stared 4y ago
  And I do like Quirk (we mention it in the intro to our recent paper https://doi.org/10.1117/1.OE.61.8.081808 https://doi.org/10.1117/1.OE.61.8.081808). And in the (not yet released) interactive element description for CNOT (along with Quantum Odyssey). When it comes to links - a good point. I do like the StackOverflow style. I will discuss it with the team.

Strilanc 4y ago

Fun fact: if you use more and more polarizing filters adjusted by finer and finer algorithms, and assume they are ideal (no loss of correctly oriented photons), that block of filters acts like a polarizing filter that filters to one angle but outputs at an angle rotated by 90 degrees. This is an aspect of the quantum zeno effect. Normally it means to measure quickly while rotating an object to pin the object against the rotation. But here you instead measure a static object quickly while rotating the measuring device to force the object to rotate.

roesel 4y ago
And this would be a great way to explain how optically active materials [1] work. [1] https://en.wikipedia.org/wiki/Optical_rotation https://en.wikipedia.org/wiki/Optical_rotation

ot 4y ago

Incidentally, this is how LCDs work: two orthogonal polarizers (so no light should pass through), and sandwiched between them liquid crystals which rotate the light so it can pass through. When a current is applied, the crystals align, effectively turning off the rotation and thus the light. It's also a nice exercise to see the polarizer as a linear transformation on the horizontal and vertical direction of the light: if the polarizer is oriented at direction θ, it's equivalent to rotating the light by -θ, keeping only the horizontal component, and rotating back. If you work out the matrixes for rotations 0, π/4, and π/2, you can see that multiplying 0 and π/2 gives the zero matrix, while multiplying 0, π/4, and π/2 keeps a nonzero component.

l33tman 4y ago

The article claims to explain this without reference to the "spookyness" of QM, but in fact it uses an analogous projection postulate as the QM version. After the polarizer, the photon state is projected onto the "measurement" of the polarizer, and in the article's figures the author does the same which leads to the same conclusion (kind of obviously). What QM brings to the table is the explanation for what happens particle-by-particle.