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I've once heard a statement that polarization can only have a quantum explanation. The effect is that when you have two polarizers at right angles, no light get

by sriku 10y ago

I've once heard a statement that polarization can only have a quantum explanation. The effect is that when you have two polarizers at right angles, no light gets through, but when you insert another polarizer at, say, 45 degrees to the others, some light gets through all the polarizers. The thing is, classical EM wave theory predicts the exact same result.

tedsanders 10y ago
Indeed. The development of quantum mechanics added almost nothing to classical E&M because classical E&M already has wave mechanics baked in.
selimthegrim 10y ago
What they ought to have taught you was this: https://en.wikipedia.org/wiki/Hardy%27s_paradox https://en.wikipedia.org/wiki/Hardy%27s_paradox
ivan_ah 10y ago
You raise an interesting point, and you're right that quantum treatment is not necessary in the regime where many light particles pass through the circuit, in which case we can talk about light as a continuous thing, that can be infinitely subdivided. A quantum treatment of polarization is only required in the single-photon regime. Specifically consider an experiment where a light beam passes through three polarizing filters: first a horizontally-polarizing lens H, then a diagonally polarizing lens D, and finally a vertically polarizing lens V. photons ------> H ----> D --> V -> The result of this "triple filtering" for a beam of light (consisting of gazillions of photons, modelled as a continuous wave) can be explained as the wave amplitude being "projected" along the polarization axis of each lens. At each polarization step the projection angle is 45 degrees, so the wave intensity is reduced by cos(45)=1/sqrt(2) which is equivalent to a reduction in optical power of 1/2. The power of the beam that passes through all three lenses is 1/4 of the power leaving the first lens. No need for quantum, since we model the beam of light as a continuous quantity that can be infinitely subdivided. If we repeat the same experiment sending one photon at a time however, we can't say "the photon divides" since by definition a photon is the smallest possible quantum of light. There is not such thing as 1/sqrt(2) of a photon. Basically, when a H-polarized photon reaches a D-polarizing lens, a classical physicists is forced to pick whether the photon goes through, or is reflected—the option "partially goes through" is not allowed. It's only in this regime that the "photon is a wave" explanation fails. The quantum explanation uses a probabilistic approach and explains the probability of a H-polarized photon going through the D-polarizing lens is 1/2 and subsequently the (now D-polarized photon) going through the third V-polarizing lens is also 1/2 so the overall probability of passing through all three lenses is 1/4.
- sriku 10y ago
  When considering a single photon the classical explanation certainly fails. However, this is different from the claim that an effect perceived at macro levels cannot be explained by classical theory and only by quantum mechanics. The paper's result sounds to be along similar lines - claiming that a certain number "could only have come from quantum theory" while classical physics perfectly well yields the same number. By virtue of being a super-theory of classical mechanics, all phenomena are quantum phenomena with quantum explanations, so that's not what is being talked about here.
adrianratnapala 10y ago
Well the single-photon quantum theory is just classical EM. So in a fairly stupid sense, it really is true that the only explanation of the polarisation is the quantum theory.

by sriku 10y ago

tedsanders 10y ago

Indeed. The development of quantum mechanics added almost nothing to classical E&M because classical E&M already has wave mechanics baked in.

selimthegrim 10y ago

What they ought to have taught you was this: https://en.wikipedia.org/wiki/Hardy%27s_paradox https://en.wikipedia.org/wiki/Hardy%27s_paradox

ivan_ah 10y ago

You raise an interesting point, and you're right that quantum treatment is not necessary in the regime where many light particles pass through the circuit, in which case we can talk about light as a continuous thing, that can be infinitely subdivided. A quantum treatment of polarization is only required in the single-photon regime. Specifically consider an experiment where a light beam passes through three polarizing filters: first a horizontally-polarizing lens H, then a diagonally polarizing lens D, and finally a vertically polarizing lens V. photons ------> H ----> D --> V -> The result of this "triple filtering" for a beam of light (consisting of gazillions of photons, modelled as a continuous wave) can be explained as the wave amplitude being "projected" along the polarization axis of each lens. At each polarization step the projection angle is 45 degrees, so the wave intensity is reduced by cos(45)=1/sqrt(2) which is equivalent to a reduction in optical power of 1/2. The power of the beam that passes through all three lenses is 1/4 of the power leaving the first lens. No need for quantum, since we model the beam of light as a continuous quantity that can be infinitely subdivided. If we repeat the same experiment sending one photon at a time however, we can't say "the photon divides" since by definition a photon is the smallest possible quantum of light. There is not such thing as 1/sqrt(2) of a photon. Basically, when a H-polarized photon reaches a D-polarizing lens, a classical physicists is forced to pick whether the photon goes through, or is reflected—the option "partially goes through" is not allowed. It's only in this regime that the "photon is a wave" explanation fails. The quantum explanation uses a probabilistic approach and explains the probability of a H-polarized photon going through the D-polarizing lens is 1/2 and subsequently the (now D-polarized photon) going through the third V-polarizing lens is also 1/2 so the overall probability of passing through all three lenses is 1/4.

sriku 10y ago
When considering a single photon the classical explanation certainly fails. However, this is different from the claim that an effect perceived at macro levels cannot be explained by classical theory and only by quantum mechanics. The paper's result sounds to be along similar lines - claiming that a certain number "could only have come from quantum theory" while classical physics perfectly well yields the same number. By virtue of being a super-theory of classical mechanics, all phenomena are quantum phenomena with quantum explanations, so that's not what is being talked about here.

adrianratnapala 10y ago

Well the single-photon quantum theory is just classical EM. So in a fairly stupid sense, it really is true that the only explanation of the polarisation is the quantum theory.