One concept which exists in Science, is that certain theories are Falsifiable. This means that a given hypothesis will predict some sort of experimental outcome, which other theories would not predict, and then an experiment can be performed to test whether this outcome is according to the theory. If it is not, then this test will break the theory, and will thus falsify it.
Quantum Mechanics is often Falsifiable. If the reader thinks it is not, then maybe the reader is confusing Quantum Mechanics with String Theory, which is supposedly not falsifiable? And thinking that String Theory is just the same thing as Quantum-Mechanics, is a bit like thinking that Cosmology is just the same thing as Astronomy.
(Edit 02/03/2018 :
There is an aspect to a theory being Falsifiable, which I did not spell out above, assuming that the reader could infer it. But certain conversations I’ve had with people I personally know, suggest that those people do not understand this concept.
The result of a physical experiment can easily be, that the outcome is according to the theory. Just as much as the inverse situation would falsify the theory, such an outcome can eventually confirm the theory, and without confirming the theory, there is no real way in which Scientists can know, whether a new theory is in fact valid.
There is no specific imperative to prove a theory wrong, in the theory being Falsifiable. )
(Edit 02/15/2018 :
One aspect to how this posting should be read, which some readers might infer, but which other readers might not infer, is that it begins by stating a hypothesis. At first, I declared this hypothesis as distinct from several other theoretical explanations of light.
But it would break the flow of a blog-posting, if every paragraph which I wrote after that, began with a redeclaration, stating that the truth of the paragraph depends on the initial hypothesis.
This dependency should be assumed, and belongs to my intended meaning. )
According to Quantum Mechanics, light can be polarized, just as it can according to the classical, wave-based theory of light. Only, because according to Quantum-Mechanics light is driven by particles – by photons – its explanation of polarization is quite different from polarized light, according to the classical, electrodynamic explanation.
According to wave-based light, plane-polarized light is the primary phenomenon, and circular-polarized light is secondary. Circular-polarized light would follow, when waves of light are polarized in two planes at right-angles to each other, but when these waves also have a 90⁰ phase-shift.
(Edit 02/20/2018: A Hypothesis which I’ve just disproved, but which this whole posting’s validity depends on.)
According to Quantum-Mechanics, the photon is in itself a circular-polarized quantum of light, of which there can trivially be left- and right-handed examples. According to Quantum-Mechanics, plane-polarized light forms, when left- and right-handed photons pair up, so that their electrostatic components form constructive interference in one plane, while canceling at right-angles to that plane.
From a thermodynamic point of view, there is little reason to doubt that photons could do this, since the particles which make up matter are always agitated, and since the photons in an original light-source also have some random basis. So a conventional plane-polarizing filter, of the kind that we used to attach to our film-cameras, would not be so hard to explain. It would just need to phase-shift the present left-handed photons in one way, while phase-shifting the present right-handed ones oppositely, until they line up.
But there exists one area in which the predictions of Quantum-Mechanics do not match those of classical wave-mechanics. If we are given a digital camera that accepts lens-attachments, we will want to attach circular polarizing filters, instead of plane-polarizing filters. And the classical explanation of what a circular polarizer does, is first to act as a plane-polarizer, which thereby selects a plane of polarization which we want our camera to be sensitive to, but the output of which is next circularly-polarized, so that light reaches the autofocus mechanism of the camera, which is still not plane-polarized. Apparently, fully plane-polarized light will cause the autofocus to fail.
This behavior of a polarizer is easily explained according to Quantum-Mechanics. The plane-polarized light which is at first admitted by our filter, already possesses left- and right-handed photons. After that, we could visualize sorting out the photons that are circular-polarized in the wrong direction.
But the opposite behavior of a filter would
not be predicted by Quantum-Mechanics. According to that, if we first pass randomly-polarized light through a circular polarizer, and if we then pass the resulting beam into a plane-polarizer, we should not be able to obtain plane-polarized output from the last polarizer.
According to the classical explanation of light, this should still be an easy thing to do. Our circularly-polarized light is supposed to have two components at right-angles, and our plane-polarizer should only allow vibration in one plane. But according to
Quantum-Mechanics, if the incident beam is already circularly-polarized, it should only consist of either left-handed or right-handed photons, and then a simple filter should not be able to conjure photons that are not present in the original beam. And so our circularly-polarized light should not be convertible into plane-polarized light.
Now, there is a tiny way in which Quantum-Mechanics makes an allowance for such situations, in which left-handed particles can be flipped into right-handed particles and vice-versa. But as far as I can see, this phenomenon may be possible in theory, but simply will not take place in simple filters. (It would be accompanied by some loss of energy.)
This is because in order for a photon to be left- or right-handed, it must not only have momentum, but is actually theorized to have angular momentum as well. And this property has been confirmed experimentally. In order for a left-handed photon to be flipped to being right-handed, for example, something needs to take place which conserves both energy and momentum. So in principle, a crystal would need to carry away the momentum, that changed in the photon.
The way crystals do this in Quantum-Mechanics, is by carrying away an actual phonon – a quantum of sound. The problem here is that this quasi-particle – the phonon – will also have energy. Thus, in the process of being flipped, a photon should be losing energy to a phonon. And what this would also mean, is that the color of light should get red-shifted, because of this loss of energy – from one photon.
This loss of energy can be reduced – but not entirely avoided – by making the crystal harder. But in practical polarizing filters, we do not see the behavior, that monochromatic green light turns red, etc.. We can see the behavior by which photons of different wavelengths will get sorted, or by which they could be absorbed with probabilities depending on several arbitrary variables, or by which they are phase-shifted, but not usually the sort of phenomena, by which the wavelengths of individual photons is changed or shifted.
(Edit 02/20/2018 :
Actually, the possibility remains that this angular momentum is so minute, that if in fact it was not conserved, its effect on the wavelengths of the light, might not even be measurable by practical means. )
And so it would seem that Quantum-Mechanics is easily falsifiable. And yet the body of Science available today embraces it, and does not reject it.
P.S. I suppose that one question which an astute reader might ask would be: ‘If the conservation of momentum is important, why does a beam of light bend, as it enters a refractive substance?’ The answer which Quantum Mechanics offers, is much messier than the answer classical electrodynamic models would have offered: When a photon is passing through a medium, it is not exactly the same particle, as a photon traveling through a vacuum.
The way QM explains refraction is a fascinating world unto itself. Apparently, a photon needs to couple with another particle, which can either be a Plasmon, a Phonon, or an Exciton, in order for both to propagate through a medium. If the coupling succeeds, the result can be one of
- A photon-plasmon (surface) polariton,
- A photon-phonon polariton, or
- A photon-exciton polariton.
The conservation of both momentum and energy is still basic, except that once inside the medium, the photon is now attached to a particle which was not there before. The combined momentum and energy of the resulting polariton need to equal those of the original photons.
One factor that makes Quantum Mechanics unaesthetic, is its over-reliance on quasi-particles.
And a philosophical question which follows but still has no answer, would be ‘What signals the event, by which the photon has crossed the boundary into a refractive substance, thus requiring it form a polariton, if all that matters finally is particles?’
While the QM explanation for refraction is ugly, it explains certain facts, such as that some metals – copper and gold in particular – have specific surface-coloration, which is referred to as ‘pink’. According to classical theories, either metal should be equally capable of reflecting light, the same way chicken-wire reflects radar echos. There do exist holes in the chicken-wire, but the wavelengths of the radars in question, are longer that units of the mesh.
Well according to that, the atoms of copper and gold should also form a mesh smaller than the visible wavelengths of light, and should just reflect it. But according to QM, they only reflect wavelengths much longer, than their inter-atomic distances. Which we can see in their not having a typical, neutrally-colored luster.
But this relates back to plane-polarized light. There are essentially two types of filters which produce it:
- A polarizing mirror, or
- A gel block.
Unless I am mistaken, Excitons are not circularly-polarized but rather form dipoles.
In case (2), the medium has been treated by old-fashioned means, so that repeated polarization along one axis loses energy – maybe because the gel has been made ohmic along that axis – while repeated polarization at right angles to that does not lose energy. But circularly-polarized photons still need to be coupled with the Excitons, to form Polaritons.
If there are only left-handed photons and no right-handed photons present, then regardless of along which axis the medium is lossy, the corresponding photon should constantly be polarized the wrong way, and should eventually be absorbed.
(Edit 04/26/2017 : )
I could engage in some speculative thinking.
When one photon couples with one exciton, the resulting polariton may remember in what direction the photon was polarized. Next, the medium could manipulate the polariton based on the orientation of its exciton. I.e., the medium could absorb most of the polaritons whose excitons were oriented along the disfavored axis.
(Edit 02/20/2018 : )
The rest of this posting essentially tries to resolve the logic issues which would follow from this premise. But this premise is easily bypassed, just by suggesting that if the Exciton is linear – such as in the case of birefringence – the coupled photon has also become linear, just due to a phase-shift in its two, virtual, plane-polarized components, so that they are in-phase again.
Yet, an issue might still exist somewhere, in the question of how, then, the lenses of a camera transmit circularly-polarized light as such, and yet exhibit predictable refractive indexes.
The original text of this posting has been removed – by me.