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Comment by phkahler | Hacker News Reader

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?

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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>.

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…

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.

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..

Free electrons also emit blackbody radiation. Photon quantization has nothing to do with the energy levels of atoms. It is a very fundamental property of the universe - you can tell because the only relevant constant is planck's constant.

I didn't say that light isn't quantized, I just said that for me blackbody radiation of ordinary matter isn't a convincing proof that light is quantized.. As for free electrons emitting blackbody radiations can you give me a reference? If you're talking about a wiggle (https://en.wikipedia.org/wiki/Wiggler_(synchrotron)) I don't see how this is related to the issue.

The sun is full of free charged particles.

So there are two things we should identify separately that go under the name "quantization".

One is that when we send out a single photon worth of energy, even in an isotropic or other multi-path emission pattern, we only ever seem to detect a single photon. You can prove this with a half mirror and two detectors. This is kind of weird - not clear how this works, and the core question driving different quantum interpretations.

Second is that the energy level of a single photon detection is based on the constraint that Et=h (t is period), also written E=hf. This is not related to atomic properties of the detector. Why? Equation does not depend on any variable of the detector.

Same goes for Px=h. (Momentum/wavelength relationship.) You can prove that one with two razor blades close together. Same relationship, but along space axes instead of time.

Also for uncertainty principle - reinterpret EPtx as the variance of their distributions.

All of this is some sort of fundamental constraint of the universe around quantities of dimension force*time*distance (action). Not related to atoms per se. Not that I understand why this constraint exists. I know some ways to think about it mathematically, but not intuitively.

Hmmm… citation needed?

I know it's not exactly Nature, but I like this one for the approachable and intuitive explanation: https://www.youtube.com/watch?v=SDtAh9IwG-I

Fundamentally, you have to believe in nonsense such as kilometer-sized particles that are also infinitesimal if your point of view is that photons are particles.

Many of the "mysteries" of Quantum Mechanics vanish if keep in the back of your mind that QM is just a bag of mathematical tricks that "work" in specific scenarios to solve practical problems, but aren't a model of "what actually occurs".

Practically all of it is restricted to statistical predictions related to matter. Remove matter or remove statistics and then the models are no longer valid.

I'm not making this up. Go dig up any of the original QM papers and these assumptions and/or constraints are called out in the first few paragraphs.

The existence of objects is merely a model of sense data.

> Many of the "mysteries" of Quantum Mechanics vanish if keep in the back of your mind that QM is just a bag of mathematical tricks that "work" in specific scenarios to solve practical problems, but aren't a model of "what actually occurs".

They also tend to start vanishing once you get to studying Quantum Field Theory, which based on your comments I'm not convinced you have. Field Quanta very much "exist" (not getting into a debate on platonism here), and the debate between paticle/wave doesn't.

> I'm not making this up.

No, somebody else is, and you're buying it.

Yes, some crazy YouTube video logger named Schrödinger.

I wouldn't trust him either. What does he know?

You're basically arguing that the assumptions of QM theory that is in many of the older papers for some reason stopped mattering. That they've been invalidated... somehow, instead of simply being dropped for brevity in newer papers, assuming that everyone knows them already.

All I'm saying is that if you go back to the original version of the theory, they were quite clear about the scope: a statistical theory that applies to ensembles of atoms.

In theoretical Physics, especially when dealing with phenomena outside of normal human experience, you have to be very careful with your assumptions otherwise you can make yourself believe all sorts of nonsense.

Such as wave function collapse. Or spooky action at a distance. Both of which stem from a misunderstanding of the limits of the scope of a statistical theory that only applies to some types of physical objects.

This point of view is interesting. It's also inconsistent with the actual physics. (I'm calling the mainstream view of basic QM 'actual physics' because it has been checked obsessively against experiment for 100 years, not because mainstreamy ideas are automagically correct.)

> 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

That is not true, unless I completely misunderstood what you are trying to say. Photons exist in contexts without any atom involved, e.g. particle/antiparticle pair creation or annihilation. The gamma photons emitted during radioactive decay have nothing to do with orbitals either.

Besides, single photons have been observed. It was an important part of the Young slits experiment. Alain Aspect’s experiments also depend on the existence of single photon pairs. How would they work if photons were not a thing?

The double-slit experiment is no proof for the existence of single photons. It can be fully explained (at any light intensity) by assuming a classical electric field. The "clicks" of a "single-photon detector", such as a photomultiplier, only require the existence of quantized charges (electrons).

No, not really. If the amplitude of the EM wave is enough to make an electron jump, all the electrons with an available energy level would jump across the whole detector. To have a single detection in a given region of the detector, you need a localised energy transfer. At this point we’re just reinventing photons.

>> At this point we’re just reinventing photons.

Or is it another case of "wave function collapse"? Remember that diffraction grating work too, and they rely on the wave nature of light. It is sometimes said (and can be modeled) that light takes every path from source to destination and the diffraction grating causes many paths to cancel out so it appears to go specific ways. Mirror reflection can also be modeled this way, and making periodic scratches will turn it into a diffraction grating. In the end, a photon detection is like a "measurement" that collapses the reflected wave function.

Photons are never observed "in flight" they are only detected when they hit something. In between they may as well BE a wave function.

This is not how quantum mechanics works. One standard textbook example is to calculate the dynamics of a two-level atom that is perturbed by an oscillating (classical) electric field. The result is that there is a certain probability per unit time for the atom to transition to the excited state. In a solid, the two levels are replaced by a band structure, and the excitation leads to the generation of an electron-hole pair. The calculation then immediately gives quantized absorption events that follow a Poisson distribution. No photons required.

One could equivalently say that the thing that really exists is the photon, that is, light is a particle and not a wave, and the weird stuff comes from how photons move through space. That was the view of Feynman at least, espoused in his pop science book "QED: The Strange Theory of Light and Matter"

Actually I just checked, on page 15:

> I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves.

Anyway I say "equivalently" because this is just another way to conceptualize the same theory (and make the same predictions), it's not new physics

You're suggesting that the EM field doesn't need to be quantized if everything it interacts with is quantized, so that the non-quantized parts can never be brought to bear and observed?

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.)

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).

> single photons can be polarized by these things, and AFAIK their wavelength is not changed so their energy is unchanged as well.

This is correct. The linked article doesn't explain how the single-photon behaves. It's the same issue that the two-slit experiment has if you try to ignore quantum behavior: You still can't say which slit a single photon passes through.

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.

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.

I think using the term "re-emit" is a mistake on my part. I fully agree that the photons are not absorbed and re-emitted.

However, their speed of propagation in the material, and their properties can be altered by the interaction with dipole moments of the material. Therefore, photons do indeed "go trough without collision" but not without interaction. The dipoles in a dielectric material such as glass need to take the energy to oscillate from the field, and then they give it back to the field by radiating it out again.

I agree that this is not "re-emission" in the absorption/emission diagram sense, and I understand that the field in a material cannot be separated into the material portion and the vacuum portion, since it is both at any given time, but what I have tried to outline in the previous comment is still a useful representation of the role of the material which the original article chose to leave out. I should have used "radiate" and "dipole" instead of "emit" and "electron" to make it a bit clearer.

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)

Easier to think about the polarizer's effect on a non-quantum EM wave, and then understand that our probability of observing a photon is based on the magnitude of the resultant EM wave.

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