[HN Gopher] Third-polarizing-filter experiment demystified (2004)
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Third-polarizing-filter experiment demystified (2004)
Author : rahimnathwani
Score : 62 points
Date : 2022-08-18 19:10 UTC (3 hours ago)
(HTM) web link (alienryderflex.com)
(TXT) w3m dump (alienryderflex.com)
| adgjlsfhk1 wrote:
| 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 wrote:
| Does the sibling comment by phkahler cresolve your doubt?
| (https://news.ycombinator.com/item?id=32514107)
| yarg wrote:
| It's not that weird.
|
| You can consider a wave passing through a filter as a sum of
| two orthogonal waves, rSin(th) + rCos(th), th 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.
| phkahler wrote:
| 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?
| gorkish wrote:
| At the single photon level, the photon that goes into the
| polarizer and the photon that goes out of the polarizer are not
| the same photon, so it's not right to say that a photon is
| changed or transformed.
|
| For a photon coming in at 45 degrees to the polarization angle,
| the probability that another photon will be emitted is
| sin(45deg) =~ 70% and the probability that it will be absorbed
| is 1/sin(45deg) =~ 30%.
|
| (This is also a simplification; polarization angle is similarly
| quantum in nature, and I have assumed it to be collapsed here)
| abdullahkhalids wrote:
| 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 wrote:
| 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...
| oh_my_goodness wrote:
| There is lots of evidence for photons in the EM field,
| beginning with Planck's invention of photons. http://hermes
| .ffn.ub.es/luisnavarro/nuevo_maletin/Planck%20(...
|
| If you make the energy in a mode of the EM field
| continuous, you get the famous 'ultraviolet catastrophe.'
| This holds whether the matter involved has a continuous
| emission spectrum or not.
| renox wrote:
| I'm not so sure: 'black body' are made of atoms, atoms
| emits lights only on certain frequency due to the way
| electrons orbitals are structured, with discrete energy
| levels, but I don't see how this is related to EM fields
| themselves..
| gnramires wrote:
| I think that although light is a quantum phenomenon and we can
| detect single photons, many people overlook that photons still
| behave similarly to Maxwell's (wave) equations. In particular,
| the average behavior is that of the electromagnetic wave (up to
| some extremes like extremely high energies). The 'bullet' model
| people think of when the word photon (particle) is mentioned is
| inadequate. This becomes clear in field theory (QED and QFT),
| where there's a more complete description based of this
| phenomena solely based on field (wavelike) behavior. It's
| believe any (small) system follows QFT exactly (there's still
| uncertainty around gravity).
|
| The exact nature of the relationship of quantumness and fields
| (i.e. how the single-particle behavior arises from QFT) is
| still unclear, which is why there are many competing
| interpretations of quantum mechanics. In the Copenhagen
| interpretation, which is the most "easy" one, the behavior of
| photons is just (almost) that of Maxwell's equations, on
| average, s.t. a single photon will be measured with probability
| equal to the average light intensity anywhere (they are said to
| "collapse" at the moment of measurement, which is surely a
| simplification of a more complete underlying theory).
| roesel wrote:
| The magnitue drop is reasonably simple to understand in terms
| of fields. The oscillating optical field might be less
| effective at exciting material oscillations in the middle
| filter due to a mismatch in polarization, but it _still does so
| at the same frequency_. You can think of it as multiple photons
| (incoming field) collectively exciting the same electron on the
| same frequency but with reduced efficiency. The electron then
| re-emits fewer photons (outgoing field) of the same wavelength,
| leading to a lower light intensity detected after the filter.
| Arwill wrote:
| I don't think photons are absorbed and re-emitted by
| electrons. At least that argument does not hold when
| discussing light slowing down in glass or water. Light is
| affected by the electromagnetic field of the material it is
| going trough, is slowed down, or absorbed based on some of
| its property, but photons that go trough are going trough
| without collision. Photons that get absorbed and re-emitted
| are scattered in all directions, and are mostly lost. You
| would not see a consistent image trough a polarising
| sunglass, if the photons you were seeing were re-emitted
| photons.
| Sharlin wrote:
| 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.)
| nh23423fefe wrote:
| 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 wrote:
| 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 wrote:
| > I dont get the desire to cast light as something non quantum.
|
| Because you can describe it entirely using classical physics in
| this situation
| yuan43 wrote:
| 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.
| klodolph wrote:
| > 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.
| rahimnathwani wrote:
| 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 wrote:
| That thread was a huge mess of confusion and misinformation. I
| hope I managed to dispel some confusions there
| roesel wrote:
| 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 0deg induces oscillations in electrons which are "bound to a
| rail" in the material, which allows them to only oscillate in the
| direction of 45deg (and all oscillations in the direction of
| -45deg are absorbed). Therefore, a portion of the incoming field
| essentially gets re-emitted _by_ the middle filter linearly
| polarized at 45deg.
|
| 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.
| function_seven wrote:
| If the material is being excited into oscillations that then
| re-emit "new" light, how is the color and direction preserved?
| Polarization filters tend to pass the full spectrum (or nearly
| so) of visible light, but my understanding of photon absorption
| and emittance is that the wavelengths are dependent on the
| electron energy levels. (I'm thinking of the same mechanism
| that produces lines on a spectrometer, indicating which
| elements are present in a sample.)
|
| I guarantee I've misused a term or two above. Hopefully you get
| what I'm asking.
|
| Taking a stab at my own question, the "rails" are field lines
| within the material, and not electrons themselves that
| interact. Is that close?
| amluto wrote:
| It's because the "re-emission" is coherent in the sense that
| it's in the same phase as the incoming light. As a decent
| analogy: when you sing a pure note, it "excites" (vibrates)
| air molecules as it travels, and those air molecules in turn
| bump into other molecules, all at random, but still all in
| phase so that whoever is listening hears the original note.
| Similarly, when light goes through ordinary glass, it wiggles
| the electrons in the glass, which in turn change the way the
| light propagates, refracting it while still preserving an
| image.
|
| Any textbook on electricity and magnetism will cover this in
| a section called something like "Maxwell's equations in
| materials".
| moralestapia wrote:
| Is it photons in -> (new) photons out? Or the same ones
| reoriented?
| NotYourLawyer wrote:
| It's new photons being emitted.
| amluto wrote:
| 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 wrote:
| (Interesting) Could you elaborate?
| avodonosov wrote:
| 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 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_i1N...
| and
| 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.
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