Do photons carry more than energy, momentum and angular momentum?

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  • #1
jarekd
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Noether theorem says that with symmetries come conservation laws, and so because of time, translation and rotation symmetry, EM field itself guards energy, momentum and angular momentum conservation.
While atom deexcitation there clearly appears energy and angular momentum difference, so there should be created EM field configuration/wave "carrying this difference".
Photon's angular momentum is usually imagined as only some abstract quantum property, but in fact it is a very real "mechanical" torque.
For example Richard Beth in 1936 has measured the tiny reaction torque due to the change in polarization of light on passage through a quartz wave plate: http://prola.aps.org/abstract/PR/v50/i2/p115_1
Here is nice video of rotating macroscopic object using circularly polarized light - at about 20 second the polarization was switched to opposite one:

Is optical photon something more than just EM wave carrying energy, momentum and angular momentum?
If not - what more? Other than EM interactions? Some electric/magnetic moments?
Is it just a "twist-like wave"? - like behind marine propeller, but this time in viscosity-free environment - and so does not dissipate: can travel undeformed for years (is soliton) from a concrete deexciting atom to be absorbed by anther one ...
We say that because of spin conservation, it has also spin 1 - e.g. due to electron changing spin from -1/2 (down) to +1/2 (up) ... but isn't it just 180deg rotation - twist again? Especially that in opposite to other particles with spin, photon doesn't have magnetic dipole moment...
Another question: why it has momentum? Is that just required for this kind of waves or maybe there is some momentum change required while atom deexcitation?
 
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  • #2
UltrafastPED
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Classical electromagnetic field theory (Maxwell's equations) tell us that light has momentum; it follows from Poynting's theorem.

There are two kinds of angular momentum: from light polarization (circular, elliptical) and beam structure (orbital angular momentum). You can read about the sources for angular momentum here:

http://en.wikipedia.org/wiki/Angular_momentum_of_light


But a photon is not an electromagnetic wave, though we often think of it that way: little packets that make up the light. Instead it is the quantization of the electromagnetic field modes, and its state function gives the probability amplitude.
 
  • #3
jarekd
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Indeed photon's angular momentum can change both spin of electron in atom, or its angular momentum, like in this summary table for selection rules.

If by "photon is not an electromagnetic wave" you have meant plane wave, I completely agree - given photon was produced by concrete atom in concrete moment and this energy doesn't dissipate while traveling through light years - photon is localized collective excitation of EM field (is soliton).
It has time length of order of single period (lambda/c), what can be seen by introducing delay in one arm of Mach-Zehnder interferometer: we loose interference if delay is larger than about lambda/c.
 
  • #4
UltrafastPED
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Sorry, but this is incorrect. What you are describing is perhaps an ultrafast laser pulse.

I'm not interested in discussing your private theory of how things work.
 
  • #5
jarekd
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What is incorrect? selection rules? photons being EM waves? please elaborate
I cannot find it now, but I have read somewhere that if you introduce delay (glass plate) in one arm of M-Z interferometer for single optical photons, you loose interference if delay is larger than about lambda/c ... do you disagree?
 
  • #6
mikeph
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Classical electromagnetic field theory (Maxwell's equations) tell us that light has momentum; it follows from Poynting's theorem.

Really?

How does that follow?
 
  • #7
jarekd
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Indeed, Poynting theorem says only about energy transfer ... and you are using there that photons are EM waves (not necessarily plane waves).
 
  • #8
mikeph
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I'm not sure the Poynting theorem is even valid in non-plane cases. A standing wave won't transfer any energy but you wouldn't put your faith in ExH being zero everywhere.
 
  • #9
Cthugha
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given photon was produced by concrete atom in concrete moment and this energy doesn't dissipate while traveling through light years - photon is localized collective excitation of EM field (is soliton).
It has time length of order of single period (lambda/c), what can be seen by introducing delay in one arm of Mach-Zehnder interferometer: we loose interference if delay is larger than about lambda/c.

None of this is correct. Whether you see interference or not depends solely on the coherence time of the photon, which may be short or long. A photon is not necessarily localized. It is just a quantized excitation. You cannot even localize it much better than half a wavelength, otherwise strange effects occur like the maximum of the energy density occuring at a different position than the maximum of the probability density for photon detection for polychromatic light fields (see e.g. the Mandel/Wolf, which has a short chapter on the problem of localizing photons).

Also the emission event does not necessarily happen in some distinct moment. It has uncertainty associated with it. You can get the emission time with an accuracy roughly equal to the coherence time of the light. (lambda/c) amounts to roughly 1.5 fs for blue light. Due to time-energy uncertainty this would require a very broad spectral distribution. In most cases, the coherence time is rather given by the spectral width of the transition of the atom which is usually rather narrow. Even in semiconductor settings, where coherence times are usually pretty short, single photon coherence times of tens of nanoseconds have been realized.
 
  • #10
jarekd
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Cthugha, but if you have a single photon source which is in some distance from the detector - when photon is detected, with high precision (up to uncertainty) you can tell where this photon was in given moment before (between source and detector) - doesn't it mean that this photon was quite well localized?

Problem appears when we add interference - e.g. Mach-Zehnder interferometer between them. Then you have two different positions this photon could be in given moment before detection.
However it doesn't mean that it objectively hasn't been (localized) in just a single one of them - in dBB interpretation corpuscle travels through single trajectory, while its conjugated wave travels through all of them - "piloting" to interference, like in interference for Couder's walking droplets.

ps. About interference depending only on the coherence time, so you say that no matter what delay you would put in one arm of M-Z interferometer for single optical photons, you can still get interference?
If not, what is this maximal time such that, in orthodox language, "single photon can interact with itself" to get interference?
Isn't this time time length of single photon?
 
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  • #11
Cthugha
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Cthugha, but if you have a single photon source which is in some distance from the detector - when photon is detected, with high precision (up to uncertainty) you can tell where this photon was in given moment before (between source and detector) - doesn't it mean that this photon was quite well localized?

You can tell that the detection event was localized. Thinking this tells you something about what happened in transit is jumping to conclusions. You took out one quantum of energy from the field locally. That does not mean that a bullet of energy traveled ballistically to this position or that the energy was localized in the field at the same position.

edit:
About interference depending only on the coherence time, so you say that no matter what delay you would put in one arm of M-Z interferometer for single optical photons, you can still get interference?

Well, the delay still needs to be shorter than the coherence time is. People have done that. See for example Matthiesen et al., "Subnatural Linewidth Single Photons from a Quantum Dot", Phys. Rev. Lett. 108, 093602 (2012). Also available on the Arxiv: http://arxiv.org/abs/1109.3412. The delay where you still see interference corresponds to a distance of several meters.
 
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  • #12
jtbell
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Classical electromagnetic field theory (Maxwell's equations) tell us that light has momentum; it follows from Poynting's theorem.

Really?

How does that follow?

See here for example (page 3 onwards):

http://www.physics.usu.edu/Wheeler/EM/EMenergy.pdf‎

[Oops, that server doesn't allow access to that page from the link above, but you can access it through Google. Do a Google search for "electromagnetic momentum density." For me, it's the third hit, titled "Electromagnetic energy and momentum".]

It doesn't use Poynting's theorem, but the development is similar to the one for electromagnetic energy density. You can probably find something similar in any decent E&M textbook: Griffiths, Jackson, etc.

Added: I think the first sentence of the section on conservation of momentum has a typo. "Conservation of momentum has led us to energy flux." should surely read "Conservation of energy has led us to energy flux."
 
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  • #13
jarekd
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Cthugha, If this energy did not traveled through trajectories, so how it has traveled for a single photon in just an empty space (vacuum)?

Even for double-slit interference they can now (weakly) measure average trajectory of photons ( http://www.sciencemag.org/content/332/6034/1170.abstract ) - aren't they trajectories of energy transfer?
 
  • #14
mikeph
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See here for example (page 3 onwards):

http://www.physics.usu.edu/Wheeler/EM/EMenergy.pdf‎

It doesn't use Poynting's theorem, but the development is similar to the one for electromagnetic energy density. You can probably find something similar in any decent E&M textbook: Griffiths, Jackson, etc.

Thanks for the help, but I'm getting access denied.

Is this momentum definitely a classical feature, or is it due to the introduction of relativistic corrections? I know Griffiths and Jackson both go into this territory.
 
  • #15
jtbell
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Thanks for the help, but I'm getting access denied.

I just realized it myself. Do a Google search as described in my edit.

It's definitely purely classical. As I recall, Griffiths does it before he starts on relativity.
 
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  • #16
Cthugha
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Cthugha, If this energy did not traveled through trajectories, so how it has traveled for a single photon in just an empty space (vacuum)?

Pick your favorite interpretation. Most of them (including de Broglie-Bohm variants) introduce some non-locality somewhere.

Even for double-slit interference they can now (weakly) measure average trajectory of photons ( http://www.sciencemag.org/content/332/6034/1170.abstract ) - aren't they trajectories of energy transfer?

Average trajectories of average energy transfer. Yes. But Steinberg's experiment does not say anything about what happens in a single run for a single detection event. They even make clear that this is not the case by saying " It is of course impossible to rigorously discuss the trajectory of an individual particle" near the end of the manuscript.
 
  • #17
jarekd
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Cthugha, the nonlocality in Wikipedia dBB interpretation article is described as: "The de Broglie–Bohm theory is explicitly nonlocal: The velocity of any one particle depends on the value of the guiding equation, which depends on the whole configuration of the universe."
This is exactly like in explanation of tunneling for Couder's droplets - there is some additional field (of quantum phase in QM) - medium for wave propagation, which depends on the whole history of the system - making e.g. going through energy barrier practically unpredictable (we cannot exactly measure this field in QM).

But in both Couder's experiments and dBB interpretation, this field evolves in deterministic, continuous way - energy had to travel through some trajectories.

Having a single photon emitter and detector in vacuum, please explain how this photon could not came thorough a natural trajectory between them?

ps. I have added it previously about delay in M-Z interferometer:
About interference depending only on the coherence time, so you say that no matter what delay you would put in one arm of M-Z interferometer for single optical photons, you can still get interference?
If not, what is this maximal time such that, in orthodox language, "single photon can interact with itself" to get interference?
Isn't this time time length of single photon?
 
  • #18
Dale
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I'm not sure the Poynting theorem is even valid in non-plane cases. A standing wave won't transfer any energy but you wouldn't put your faith in ExH being zero everywhere.
It follows from Maxwell's equations and the Lorentz force law, so it is valid in all cases where those are valid, not just plane waves. A standing wave is not a counter-example.

My favorite pages on energy and momentum conservation on EM are here:
http://farside.ph.utexas.edu/teaching/em/lectures/node87.html
 
  • #19
Cthugha
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Having a single photon emitter and detector in vacuum, please explain how this photon could not came thorough a natural trajectory between them?

This is the same discussion as for entanglement stuff. We know that - in a nutshell - a good theory of quantum optics must either violate locality or realism (some other things like superdeterminism and scenarios without free will are also possible, but usually not considered that important in physics). Why should it come through a trajectory? Maybe it does, but there is no single experiment showing any evidence of what is happening in between emission and detection of a photon. Even the emission process is usually not really clear. The excitation may be delocalized over the whole field and taken from the field in a non-local manner, it may not have a defined value beforehand at all (by the way there is no generally agreed upon position operator for photons which complicates things a bit). We do not know. We just get to see an ensemble average over many repetitions and can speculate what happens in a single run. Pick your interpretation. There is no experimental evidence showing us what is happening.

ps. I have added it previously about delay in M-Z interferometer:
About interference depending only on the coherence time, so you say that no matter what delay you would put in one arm of M-Z interferometer for single optical photons, you can still get interference?
If not, what is this maximal time such that, in orthodox language, "single photon can interact with itself" to get interference?
Isn't this time time length of single photon?

I already gave that answer: The maximal time delay is the coherence time of the photon. This is mostly determined by the line width of the emission process. The concept of a "time length of a single photon" does not exist. There are timescales associated with light like the duration of a cycle or its coherence time, but none of these really tell us much about what happens for single photons in a single emission and detection event.
 
  • #20
jarekd
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About the first answer, you have refereed to the free will ... so there was no quantum mechanics before the first organisms with free will have evolved? Why they even consider Big Bang?
Sure we cannot measure the values of fields, but does it mean that they objectively have no values? And lots of physics work this way - assume some objective state we cannot directly measure (like starting inflation), and find some far predictions we can measure (like hydrogen energy levels).
If there are two observers in light year distance and one of them kills a cat depending on nuclear decay, this information needs a year to propagate to the second observer - so accordingly to his QM, meantime the cat is in quantum superposition ... but does it mean that it objectively isn't dead or alive?

Returning to the single photon in vacuum, if it didn't travel in a line, changing direction means momentum transfer - with what?

About the second answer, so how long the deexcitation process lasts?
Why not exactly this decohence time - delay making photon "misses itself" in interference?

Or what happens while 21as delay in photoemission? ( http://www.sciencemag.org/content/328/5986/1658.abstract ).
 
  • #21
Cthugha
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About the first answer, you have refereed to the free will ... so there was no quantum mechanics before the first organisms with free will have evolved? Why they even consider Big Bang?

What? I have not said anything like that. I said that there are experiments showing that either locality or realism are violated - unless you assume some loopholes like for example free will not existing. You cannot make up something and claim I said it.

Sure we cannot measure the values of fields, but does it mean that they objectively have no values?

You can measure fields (within some limits). See e.g. E. Goulielmakis et al., Science 305, 1267 (2004). For single photons, though, you deal with probability amplitudes for detection events, not with fields.

And lots of physics work this way - assume some objective state we cannot directly measure (like starting inflation), and find some far predictions we can measure (like hydrogen energy levels).

Yes, these are models. If your model does not make any testable predictions, it is rather an interpretation like de-Broglie Bohm vs. MWI vs. ensemble interpretation vs. many other things. Your initial proposal that single photons have a duration on the order of a single cycle is however ruled out by experiment. First, in that case single photons would always be spectrally very broad (broader than the whole visible spectrum) which does not agree with antibunching measurements. Second, you should not see interference in a Mach-Zehnder interferometer for delays larger than a single cycle. As experiments with single photons have shown interference for large delays, that hypothesis is ruled out.

If there are two observers in light year distance and one of them kills a cat depending on nuclear decay, this information needs a year to propagate to the second observer - so accordingly to his QM, meantime the cat is in quantum superposition ... but does it mean that it objectively isn't dead or alive?

You have some misconceptions about qm and decoherence. This is not how Schroedinger's cat works. The second observer will get a mixed state, not a superposition. The "dead" and "alive" possibilities should not interfere.

Returning to the single photon in vacuum, if it didn't travel in a line, changing direction means momentum transfer - with what?

You still cling to the idea that something localized is necessarily physically traveling somewhere. This may be the case, but there is no experimental evidence for that. The energy might just as well be delocalized over the whole field.

About the second answer, so how long the deexcitation process lasts?
Why not exactly this decohence time - delay making photon "misses itself" in interference?

I do not get the meaning of your second question. The sentence seems to be off. The deexcitation process has no fixed duration. It depends on the line width of your transition. This can be very narrow for single atom spontaneous emission or very broad for short processes like the attosecond pulses used in the paper you cite. Think about the emitter being in a superposition of the excited and the ground state which gives rise to some dipole moment.

Or what happens while 21as delay in photoemission? ( http://www.sciencemag.org/content/328/5986/1658.abstract ).

If people knew that in detail, there would be no need to write papers about that, but how is that related to the discussion at hand? Ferencz Krausz and coworkers are pretty much pushing optics to the shortest possible pulses and do great work, but they are far away from the single photon level. To answer your question: What people guess is that there are some interesting electron correlations going on during that delay, but I am not sure whether they did some follow-up experiments to check that. Attosecond optics is a pretty complicated field. The spectral width of the pulses is so broad that it would span pretty much the whole visible regime from 400 to 800 nm if its center was in the visible. They work in the extreme UV, where the same width is less intimidating, but still challenging.
 
  • #22
jarekd
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What? I have not said anything like that. I said that there are experiments showing that either locality or realism are violated - unless you assume some loopholes like for example free will not existing. You cannot make up something and claim I said it.
I apology, I just really don't like the need for free will of some orthodox quantum mechanists.
I also don't see the need for nonlocality or nonobjectivity - I am referring to concrete dBB intuitions, exactly like in Couder's experiments - you can literally see where quantum phenomenas come from, including interference.

You are referring to violation of Bell inequalities - the "unability theorem".
I didn't wanted this thread to go this direction - instead of focusing on what photon is.
Ok, Bell inequalities say that correlations of entangled photons are against our "evolving 3D" intuition ...
1) From one side, all physics we know is Lagrangian mechanics, which can be seen that the present moment is action optimizing equilibrium between past and future - we live in "full 4D" spacetime, what implies intuitions/correlations different from natural for us. And if we do thermodynamics in 4D - e.g. assume Boltzmann distribution among possible paths/trajectories: use euclidean path integrals/maximal entropy random walk, we get the squares connecting amplitudes and probabilities - leading to violation of Bell inequalities. Amplitudes correspond to probabilities on the end of half-paths toward past or future: to get randomly a given value, you have to get it twice: from past and future. Thermodynamics in 4D does not agree with Bell inequalities .
2) These correlations in EPR is just angular momentum conservation - it is Noether theorem: the whole filed guards this conservation ... while in Bell inequalities you look only at well defined spins ("classical arrows") - completely neglecting the field around, guarding this conservation.

I see both violating Bell inequalities and conservation of angular momentum in EPR as expected, but I don't want to spoil this thread with such discussion.
So please let us stay in concrete models, like dBB/Couder's intuitions - please don't refer to general "unability theorems", but show concrete problems there.
You can measure fields (within some limits). See e.g. E. Goulielmakis et al., Science 305, 1267 (2004). For single photons, though, you deal with probability amplitudes for detection events, not with fields.
You can gain some information about the field, but as you disturb it by the way, this information will never be complete - what would be required to perform deterministic simulation.
Yes, these are models. If your model does not make any testable predictions, it is rather an interpretation like de-Broglie Bohm vs. MWI vs. ensemble interpretation vs. many other things.
dBB interpretation is just Schrodinger equation after Madelung transformation: substituting psi=R*exp(iS). Then for density (R^2) you get just continuity equation, for action (S) you get Hamilton-Jacobi with h-order correction from quantum phase: the wave nature. Intuition is exactly like in Couder's experiments and it is just equivalent to Schrodinger.
In contrast, MWI says that spacetime splits in every measurement - what disagrees with everything, like Lagrangian mechanics.
Your initial proposal that single photons have a duration on the order of a single cycle is however ruled out by experiment. First, in that case single photons would always be spectrally very broad (broader than the whole visible spectrum) which does not agree with antibunching measurements.
I don't understand the spectrum counterargument: imagine you place something flat in water and rotate it 180 degrees - you create twist-like wave, which can then twist something else.
Why optical photon isn't something like that - created from angular momentum of electron or twisting it 180 deg: from -1/2 to +1/2 spin?
Second, you should not see interference in a Mach-Zehnder interferometer for delays larger than a single cycle. As experiments with single photons have shown interference for large delays, that hypothesis is ruled out.
I think I have read somewhere something opposite - could you maybe refer to some source?
You have some misconceptions about qm and decoherence. This is not how Schroedinger's cat works. The second observer will get a mixed state, not a superposition. The "dead" and "alive" possibilities should not interfere.
It was supposed to be standard Schrodinger's cat experiment, but with perfect: spatial separation, but I agree that it is a bit different than superposition.
However, still wouldn't the external observer's description be (|dead>+|alive>)/sqrt(2) ... while objectively it would be dead or alive?
You still cling to the idea that something localized is necessarily physically traveling somewhere. This may be the case, but there is no experimental evidence for that. The energy might just as well be delocalized over the whole field.
Still, don't you need momentum exchange to change its direction?
I do not get the meaning of your second question. The sentence seems to be off. The deexcitation process has no fixed duration. It depends on the line width of your transition. This can be very narrow for single atom spontaneous emission or very broad for short processes like the attosecond pulses used in the paper you cite. Think about the emitter being in a superposition of the excited and the ground state which gives rise to some dipole moment.
The orthodox qm sees deexcitation as an instant process - and so the purpose of the next question was to show that it cannot be like that - there is at least some 21as process inside.
Also from quantum point of view, wavefunction collapse can be seen as interaction with environment - as a continuous/unitary process - there is some concrete evolution behind deexcitation and optical photon is "Noether's shadow" (due to conservation laws) of this process: wave carrying energy, momentum and angular momentum difference of deexcitating atom.
So time length of this deexcitation process, multiplied by c should be spatial length of single optical photon why it is not so simple?
 
  • #23
Cthugha
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I apology, I just really don't like the need for free will of some orthodox quantum mechanists.
I also don't see the need for nonlocality or nonobjectivity - I am referring to concrete dBB intuitions, exactly like in Couder's experiments - you can literally see where quantum phenomenas come from, including interference.

dBB is non-local, too. You can follow any interpretation you like. I do not consider dBB to be too intuitive in my field. I am not for it or against it. It is just important to notice that it is does not explain anything more than other interpretations.

You are referring to violation of Bell inequalities - the "unability theorem".
I didn't wanted this thread to go this direction - instead of focusing on what photon is.
Ok, Bell inequalities say that correlations of entangled photons are against our "evolving 3D" intuition ...

Then let us skip that topic. Your comments on it let me think it is better that way.

You can gain some information about the field, but as you disturb it by the way, this information will never be complete - what would be required to perform deterministic simulation.

You can get pretty far using homodyning and tomography, but of course this is indirect. You cannot perform a single shot measurement of both quadratures of the field. That is obviously true.

dBB interpretation is just Schrodinger equation after Madelung transformation: substituting psi=R*exp(iS). Then for density (R^2) you get just continuity equation, for action (S) you get Hamilton-Jacobi with h-order correction from quantum phase: the wave nature. Intuition is exactly like in Couder's experiments and it is just equivalent to Schrodinger.
In contrast, MWI says that spacetime splits in every measurement - what disagrees with everything, like Lagrangian mechanics.

Intuition is a personal thing. Personally, I prefer neither dBB, nor MWI.

I don't understand the spectrum counterargument: imagine you place something flat in water and rotate it 180 degrees - you create twist-like wave, which can then twist something else.
Why optical photon isn't something like that - created from angular momentum of electron or twisting it 180 deg: from -1/2 to +1/2 spin?

If the photon was that short temporally, it must be very broad spectrally. This is simple time-energy uncertainty. People can demonstrate antibunching - the hallmark for proving that one created a single photon state - for light emission which is way narrower in the spectral regime. Therefore, the temporal duration cannot be that short.

I think I have read somewhere something opposite - could you maybe refer to some source?

For example the paper I cited in my post #11 shows single photon coherence times in the nanosecond range. It is figure 3. Pretty much any paper on coherence in resonance fluorescence will show similar results.

It was supposed to be standard Schrodinger's cat experiment, but with perfect: spatial separation, but I agree that it is a bit different than superposition.
However, still wouldn't the external observer's description be (|dead>+|alive>)/sqrt(2) ... while objectively it would be dead or alive?

No. He should get a mixed state. A density matrix with dead and alive states on the diagonals occurring each with 50% probability. The difference lies in the fact that the cat state is a real superposition which can show interference, while the mixed state will not show interference and the 50/50 probability just describes a lack of knowledge.

Still, don't you need momentum exchange to change its direction?

Depends on the model you consider. If you consider it as delocalized, it does not have a well defined position anyway and there is no need for momentum exchange.
Note that I am not advocating one model over the other. All are plausible

The orthodox qm sees deexcitation as an instant process - and so the purpose of the next question was to show that it cannot be like that - there is at least some 21as process inside.

I cannot follow you. The 21 as delay happens between absorption of a photon and emission of an electron. Or do you talk about deexcitation of the em field? Do you mean the emission process?

Besides that, photon emission is usually not seen as instantaneous. For emission processes, the excited state and the ground state are in a superposition state. If you have a look at e.g. the simple harmonic oscillator, you will find that a superposition of two modes results in a spatial oscillation of the probability distribution function. As we are talking about a spatial oscillation of the probability density for a charged particle here, this also means that oscillation will give rise to a dipole moment and dipole radiation. This superposition can break pretty quickly or it can take a pretty long time, but it is this superposition and oscillation which is the emission process.

Also from quantum point of view, wavefunction collapse can be seen as interaction with environment - as a continuous/unitary process - there is some concrete evolution behind deexcitation and optical photon is "Noether's shadow" (due to conservation laws) of this process: wave carrying energy, momentum and angular momentum difference of deexcitating atom.
So time length of this deexcitation process, multiplied by c should be spatial length of single optical photon why it is not so simple?

Well, that gives you the coherence time. One should be careful to identify it with a length of a photon, but if you know what you are talking about, the idea may be ok. However, this deexcitation is very far from instantaneous as I noted above.
 
  • #24
kith
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You can tell that the detection event was localized. Thinking this tells you something about what happened in transit is jumping to conclusions. You took out one quantum of energy from the field locally. That does not mean that a bullet of energy traveled ballistically to this position or that the energy was localized in the field at the same position.
Just a quick question about localization: is the crucial difference between the electron and the photon that the photon can't be prepared in a localized state because it is always absorbed in measurements or is it something else?
 
  • #25
jarekd
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dBB is non-local, too. You can follow any interpretation you like. I do not consider dBB to be too intuitive in my field. I am not for it or against it. It is just important to notice that it is does not explain anything more than other interpretations.
See Couder's experiments (analogous to dBB) - without any nonlocality, they give clear intuition for:
- interference in double-slit experiment (particle goes a single way, but "pilot" waves it creates go through all paths - leading to interference): http://prl.aps.org/abstract/PRL/v97/i15/e154101 ,
- tunneling (the field depends on the whole history, making that getting through a barrier is practically random): http://prl.aps.org/abstract/PRL/v102/i24/e240401 ,
- orbit quantization (to find a resonance with the field, the clock has to perform an integer number of periods while enclosing an orbit): http://www.pnas.org/content/107/41/17515 ,
- "classical Zeeman effect" (Coriolis force instead of Lorentz): http://prl.aps.org/abstract/PRL/v108/i26/e264503.
Do you disagree that they give proper intuition about quantum phenomenas?
Please refer to a concrete quantum phenomena which indeed needs some nonlocality or nonobjectivity?
Then let us skip that topic. Your comments on it let me think it is better that way.
Thanks. Violation of Bell inequalities shows only that our natural past->future "evolving 3D" intuition is wrong ... what is also obvious from other fields of physics: using Lagrangian mechanics and is broken e.g. in quantum retrocausality: Wheeler's or delayed choice quantum eraser experiments.
This violation is not in disagreement with living in a field theory governed by Lagrangian mechanics (before or after quantization).
For example the paper I cited in my post #11 shows single photon coherence times in the nanosecond range. It is figure 3. Pretty much any paper on coherence in resonance fluorescence will show similar results.
Ok, http://prl.aps.org/pdf/PRL/v108/i9/e093602 [Broken] ... but it is about quantum dots: harmonic oscillators.
I completely agree that waves produced by harmonic oscillators should be very different from standard optical photons: results of deexcitation of excited atom ... what are "time lengths" for them?
No. He should get a mixed state. A density matrix with dead and alive states on the diagonals occurring each with 50% probability. The difference lies in the fact that the cat state is a real superposition which can show interference, while the mixed state will not show interference and the 50/50 probability just describes a lack of knowledge.
Sure, but still does the mixed state in his quantum mechanics means that the cat is not objectively dead or alive?
Intuition is a personal thing. Personally, I prefer neither dBB, nor MWI.
So when two paths in M-Z interferometer are indistinguishable, you get interference?
But are they really indistinguishable?
Reflecting photon by mirror means momentum transfer - doesn't final situation of photon going one or second path differ by final momentums of mirrors?
You could measure it by placing these mirrors floating in vacuum and finally measuring their positions after some time.
Depends on the model you consider. If you consider it as delocalized, it does not have a well defined position anyway and there is no need for momentum exchange.
Note that I am not advocating one model over the other. All are plausible
Quantum mechanics is h-order expansion of classical one by the wave nature ... in Feynman path integral you usually start with classical trajectory and make variation around it ...
So you say that this expansion allows energy to travel through curved trajectories without momentum exchange?
Like while reflecting by mirrors in M-Z interferometer?
If the photon was that short temporally, it must be very broad spectrally. This is simple time-energy uncertainty. People can demonstrate antibunching - the hallmark for proving that one created a single photon state - for light emission which is way narrower in the spectral regime. Therefore, the temporal duration cannot be that short.
But what's wrong is about having broad spectrum?
Take electron, twist it by 180deg, changing its spin by 1: e.g. from -1/2 to +1/2 ... Noether theorem says that the used angular momentum has to create twist-like wave.
Exactly like behind a marine propeller, but this time the medium (EM field) has no viscosity - such twist-like wave can travel without dissipating ... e.g. finally getting to another electron and twisting it 180 deg.
What's wrong with this picture?
I cannot follow you. The 21 as delay happens between absorption of a photon and emission of an electron. Or do you talk about deexcitation of the em field? Do you mean the emission process?
I meant only that everything in internal dynamics of a single atom takes time/evolution - and so we should be able to ask how much time deexcitation lasts - changing orbital angular momentum or twisting spin of electron.
Accordingly to Noether theorem, this angular momentum changing process should produce wave carrying this angular momentum (optical photon) - of spatial length being time length multiplied by the propagation speed (c).
Why it is not so simple?
... is photon something more than just a wave carrying energy, momentum and angular momentum - to compensate differences in deexciting atom?
 
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  • #26
Cthugha
Science Advisor
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See Couder's experiments (analogous to dBB) - without any nonlocality, they give clear intuition for:
- interference in double-slit experiment (particle goes a single way, but "pilot" waves it creates go through all paths - leading to interference): http://prl.aps.org/abstract/PRL/v97/i15/e154101 ,
- tunneling (the field depends on the whole history, making that getting through a barrier is practically random): http://prl.aps.org/abstract/PRL/v102/i24/e240401 ,
- orbit quantization (to find a resonance with the field, the clock has to perform an integer number of periods while enclosing an orbit): http://www.pnas.org/content/107/41/17515 ,
- "classical Zeeman effect" (Coriolis force instead of Lorentz): http://prl.aps.org/abstract/PRL/v108/i26/e264503.

They give analogies. The differences are huge. For example in the diffraction and interference paper, quantities which are depend on the field intensity in standard diffraction/interference experiments now depend on amplitude. The experiments are nice to look at, but they are just analogues, not more.

Do you disagree that they give proper intuition about quantum phenomenas?
Please refer to a concrete quantum phenomena which indeed needs some nonlocality or nonobjectivity?

Proper intuition? I would say no. The physics is not the same, though there are similarities. Concrete quantum phenomena? Antibunching and entanglement for starters.

Thanks. Violation of Bell inequalities shows only that our natural past->future "evolving 3D" intuition is wrong ... what is also obvious from other fields of physics: using Lagrangian mechanics and is broken e.g. in quantum retrocausality: Wheeler's or delayed choice quantum eraser experiments.
This violation is not in disagreement with living in a field theory governed by Lagrangian mechanics (before or after quantization).

Do you have any peer reviewed publications supporting your claim? Then please post it. Otherwise please stop making such claims. By the way, the delayed choice quantum eraser does not require any retrocausality. We have plenty of topics on this experiment here.

Ok, http://prl.aps.org/pdf/PRL/v108/i9/e093602 [Broken] ... but it is about quantum dots: harmonic oscillators.
I completely agree that waves produced by harmonic oscillators should be very different from standard optical photons: results of deexcitation of excited atom ... what are "time lengths" for them?

That is wrong. Quantum dots are the semiconductor equivalent of atoms. They are not harmonic oscillators (or just to the same degree that atoms are) and show pretty much the same mode spectrum as atoms do. The length for atoms depends on the transition used. See the references in the article above.

Sure, but still does the mixed state in his quantum mechanics means that the cat is not objectively dead or alive?

Loosely speaking, a 50/50 mixed state means that upon many repetitions, you get result A half of the time and result B half of the time, but within every repetition, the state is A and B all the time, not a superposition of both. It may be beneficial to read up on those basics before thinking about entanglement at all. There is not much chance to really understand it without knowing the difference between superpositions and mixed states.

So when two paths in M-Z interferometer are indistinguishable, you get interference?
But are they really indistinguishable?
Reflecting photon by mirror means momentum transfer - doesn't final situation of photon going one or second path differ by final momentums of mirrors?
You could measure it by placing these mirrors floating in vacuum and finally measuring their positions after some time.

Yes, they are indistinguishable unless you use a very light mirror. Distinguishability requires an irreversible interaction, which would mean that the mirror ends up in a state orthogonal to its initial state. If a photon gets redirected, the momentum transfer is well within momentum uncertainty of the mirror, so that it does not end up in a state orthogonal to the initial one. There are people in cavity optomechanics using very light cantilever mirrors where things may be different. For a typical MZ- or Michelson interferometer, however, the two paths are indistinguishable.

Quantum mechanics is h-order expansion of classical one by the wave nature ... in Feynman path integral you usually start with classical trajectory and make variation around it ...
So you say that this expansion allows energy to travel through curved trajectories without momentum exchange?
Like while reflecting by mirrors in M-Z interferometer?

I have nothing to add to what I wrote above.

But what's wrong is about having broad spectrum?

There would be nothing wrong with the photon having such a broad spectrum, but experiments show that it does not. So this idea is simply ruled out.

Take electron, twist it by 180deg, changing its spin by 1: e.g. from -1/2 to +1/2 ... Noether theorem says that the used angular momentum has to create twist-like wave.
Exactly like behind a marine propeller, but this time the medium (EM field) has no viscosity - such twist-like wave can travel without dissipating ... e.g. finally getting to another electron and twisting it 180 deg.
What's wrong with this picture?

I never said anything against this picture. It is a bit simplistic though.

I meant only that everything in internal dynamics of a single atom takes time/evolution - and so we should be able to ask how much time deexcitation lasts - changing orbital angular momentum or twisting spin of electron.

You claimed this "process" lasts just a single cycle of the em field. This is demonstrably completely wrong.

Accordingly to Noether theorem, this angular momentum changing process should produce wave carrying this angular momentum (optical photon) - of spatial length being time length multiplied by the propagation speed (c).
Why it is not so simple?

I already responded to that. What exactly is the remaining question you have?

... is photon something more than just a wave carrying energy, momentum and angular momentum - to compensate differences in deexciting atom?

Yes. If it was "just a wave", there would be no need for quantization.

Just a quick question about localization: is the crucial difference between the electron and the photon that the photon can't be prepared in a localized state because it is always absorbed in measurements or is it something else?

It is a general problem of massless bosons. The problem arises due to moving at c and the absence of a rest frame. If you want the full math, you can show this by having a look at the representations of the Poincare group in the massless boson case, but I am sure that there are people on these forums way better at that than I am.
Needless to say, there are some alternative suggestions for constructing alternative position operators that still work for photons, but one has to keep in mind what they really mean. Although it sounds easy, position may become a tricky concept.
 
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  • #27
jarekd
111
2
They give analogies. The differences are huge. For example in the diffraction and interference paper, quantities which are depend on the field intensity in standard diffraction/interference experiments now depend on amplitude. The experiments are nice to look at, but they are just analogues, not more.
Sure, there are differences with microscopic physics, but they show how these quantum phenomenas are realized in dBB interpretation - without nonlocality or nonobjectivity (you claim we cannot avoid).

Your examples are "Antibunching and entanglement for starters".
I think we have already discussed the antibunching - for noninteracting atoms we would expect Poisson distribution of photons. Antibunching means that they are distributed in more regular way - there is synchronization between these atoms ... please explain why you claim that there is some nonlocality needed for this synchronization?

Which entanglement are you referring to? In EPR we have a couple of photons, which due to angular momentum conservation just have to have opposite spins - Noether theorem says the whole field guards it. So if one is spin up, the other had to be spin down.
Where is the nonlocality here?
You want to say that in Bell inequalities - but it says only that correlations are different than we expect.
So maybe it is our expectations what is wrong ...
Do you have any peer reviewed publications supporting your claim? Then please post it. Otherwise please stop making such claims. By the way, the delayed choice quantum eraser does not require any retrocausality. We have plenty of topics on this experiment here.
Reviewed publication that in Lagrangian mechanics physics chooses action optimizing configuration for given past and future situation?
That quantum field theories have to be CPT symmetric?
That while we cannot use it to send information back in time, in Walborn's delayed choice quantum erasure future choice influences past behavior?
These are very different causality relations from used in Bell equalities - here we have time/CPT symmetric situation, while in Bell inequalities we use asymmetric causality: assume fixed values in past and conclude correlations in future.

Meanwhile, field theories like QM of QFT allow to properly derive predictions of e.g. EPR - they violate Bell inequalities and this violation does not contradict field theories...
That is wrong. Quantum dots are the semiconductor equivalent of atoms. They are not harmonic oscillators (or just to the same degree that atoms are) and show pretty much the same mode spectrum as atoms do. The length for atoms depends on the transition used. See the references in the article above.
I apology, I have meant infinite potential well (3 dimensional) - still photons they produce have a bit different nature than produced by excited atom.
Loosely speaking, a 50/50 mixed state means that upon many repetitions, you get result A half of the time and result B half of the time, but within every repetition, the state is A and B all the time, not a superposition of both. It may be beneficial to read up on those basics before thinking about entanglement at all. There is not much chance to really understand it without knowing the difference between superpositions and mixed states.
Sure, the density matrix of external observer is e.g. diag(1/2,1/2) ... but objectively the cat is dead or alive.
It shows that one of reasons we use QM is representing our incomplete knowledge (like in thermodynamics).
Yes, they are indistinguishable unless you use a very light mirror. Distinguishability requires an irreversible interaction, which would mean that the mirror ends up in a state orthogonal to its initial state. If a photon gets redirected, the momentum transfer is well within momentum uncertainty of the mirror, so that it does not end up in a state orthogonal to the initial one. There are people in cavity optomechanics using very light cantilever mirrors where things may be different. For a typical MZ- or Michelson interferometer, however, the two paths are indistinguishable.
But when mirrors are floating in vacuum, if we would use 1000 times heavier mirror, wouldn't we just have to wait 1000 times longer to get the same displacement for momentum transfer of reflection of the same photon?
I never said anything against this picture. It is a bit simplistic though.
Yes, there is missing understanding why electrons have tendency to change projection of its spin by exactly 1 ... I think it is caused by that spins have tendency to align in parallel or antiparallel configuration ... but what about e.g. He+ ... ?
You claimed this "process" lasts just a single cycle of the em field. This is demonstrably completely wrong.
Multi-electron atoms have more complex internal dynamics, and so the exact shape of angular momentum carrying wave they produce can be more complex ...
Yes. If it was "just a wave", there would be no need for quantization.
Quantization is because energy of electron orbitals (and so their released differences) can obtain only a discrete set of values.
Why orbitals are quantized? Again Couder bring intuitive answer: the Bohr-Sommerfeld condition is finding resonance between the field and particle's internal periodic process (causing its wave nature).
It is a general problem of massless bosons. The problem arises due to moving at c and the absence of a rest frame...
You can also have solitary wave on water surface, like tsunami - they usually travel with propagation velocity of the medium ...
 
  • #28
Cthugha
Science Advisor
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Sure, there are differences with microscopic physics, but they show how these quantum phenomenas are realized in dBB interpretation - without nonlocality or nonobjectivity (you claim we cannot avoid).

Ehm...no. Just no. Again: dBB is always and explicitly non-local. There is no dBB without it.

Your examples are "Antibunching and entanglement for starters".
I think we have already discussed the antibunching - for noninteracting atoms we would expect Poisson distribution of photons. Antibunching means that they are distributed in more regular way - there is synchronization between these atoms ... please explain why you claim that there is some nonlocality needed for this synchronization?

Synchronization between atoms? It does not seem like you know what antibunching is. You do not see it, when you have more than one atom. It is what you get for one single emitter. In the Hanbury Brown-Twiss measurement geometry you then use a beam splitter and two detectors. Detecting one photon in one arm immediately reduces the probability for detection of a photon in the other arm. One can show that the state is given by a superposition of the two possible pathways at the beam splitter (or its equivalent in your favorite interpretation). This points toward non-locality or non-realism. A mixed state would not.

Which entanglement are you referring to? In EPR we have a couple of photons, which due to angular momentum conservation just have to have opposite spins - Noether theorem says the whole field guards it. So if one is spin up, the other had to be spin down.
Where is the nonlocality here?
You want to say that in Bell inequalities - but it says only that correlations are different than we expect.
So maybe it is our expectations what is wrong ...

Sorry, but read up on the topic before making any claims. Pretty much all of them are plain wrong. There is obviously more to entanglement than just the conservation as measurements along several well defined polarization axes show.

Reviewed publication that in Lagrangian mechanics physics chooses action optimizing configuration for given past and future situation?
That quantum field theories have to be CPT symmetric?
That while we cannot use it to send information back in time, in Walborn's delayed choice quantum erasure future choice influences past behavior?

The last one. This is a very bad misinterpretation of what is happening in the experiment. It is also not Stephen Walborn's own opinion on the experiment.

I apology, I have meant infinite potential well (3 dimensional) - still photons they produce have a bit different nature than produced by excited atom.

No, both show 3d confinement, quantized states and (possibly) complex Coulomb correlations. Self-assembled QDs may show some differences due to the presence of the wetting layer continuum, but in a nutshell the physics is almost the same. The one difference is that we have electrons in atoms, while we have bound electron hole pairs (excitons) in QDs, but that does not have an influence on the nature of the photons. It just matters when trying to build devices.

Sure, the density matrix of external observer is e.g. diag(1/2,1/2) ... but objectively the cat is dead or alive.

This is the meaning of a mixed state. The state is really either one or the other, but we do not know. In a superposition state, it is not just our missing knowledge of the situation, but the system is really not in state A or B. (again, substitute the way your favorite interpretation handles the situation in case you favor an interpretation without superposition and collapse).

It shows that one of reasons we use QM is representing our incomplete knowledge (like in thermodynamics).

No. Absolutely not. Learn the difference between mixed states and superpositions. It is absolutely fundamental and trying to understand qm or even quantum optics without these basics is hopeless.

But when mirrors are floating in vacuum, if we would use 1000 times heavier mirror, wouldn't we just have to wait 1000 times longer to get the same displacement for momentum transfer of reflection of the same photon?

I do not get your point. A single photon can at most transfer a tiny amount of momentum. It does not do it 1000 times. You may be able to see an effect using a huge number of photons, but I assume you would melt the mirror first. Also, there should not be any cumulative effects as mirrors are mounted on the ground. So either you move all the earth around or you move the mirror from its equilibrium position. This change will dissipate faster than more kicks come in.

Yes, there is missing understanding why electrons have tendency to change projection of its spin by exactly 1

Group theory and qft tell us a lot about that. But there are people who really know that stuff and can write way better posts on that than I could.

Multi-electron atoms have more complex internal dynamics, and so the exact shape of angular momentum carrying wave they produce can be more complex ...

For single atoms it is usually not really complex unless you go to Rydberg atoms.

Quantization is because energy of electron orbitals (and so their released differences) can obtain only a discrete set of values.
Why orbitals are quantized? Again Couder bring intuitive answer: the Bohr-Sommerfeld condition is finding resonance between the field and particle's internal periodic process (causing its wave nature).

No. That is way too simple. You also only see quantized emission or absorption for absorbers with a continuous density of states and a mode spectrum which is not discrete. The intuition may be good enough for a limited number of cases, but it is very misleading to think it gives the full picture for general cases.
 
  • #29
jarekd
111
2
Ehm...no. Just no. Again: dBB is always and explicitly non-local. There is no dBB without it.
But this "nonlocality" is fully determined by underlying field, which evolves in local and deterministic way (Hamilton-Jacobi equation with modified potential).
Sure if we would like to model reality by just classical mechanics (like seeing spin in classical way in Bell inequalities), there will always be something missing - some "nonlocality".
But I am talking only about field theories - does they still need some nonlocality?
Synchronization between atoms? It does not seem like you know what antibunching is. You do not see it, when you have more than one atom. It is what you get for one single emitter. In the Hanbury Brown-Twiss measurement geometry you then use a beam splitter and two detectors. Detecting one photon in one arm immediately reduces the probability for detection of a photon in the other arm. One can show that the state is given by a superposition of the two possible pathways at the beam splitter (or its equivalent in your favorite interpretation). This points toward non-locality or non-realism. A mixed state would not.
Ok, now you are talking about Hanbur-Brown-Twist effect, what is something different ... in the wikipedia article we can read:
"Hanbury Brown and Twiss resolved the dispute in a neat series of papers (see References below) which demonstrated first that wave transmission in quantum optics had exactly the same mathematical form as Maxwell's equations albeit with an additional noise term due to quantisation at the detector, and secondly that according to Maxwell's equations, intensity interferometry should work."
Indeed correlations can be more complex than naive thinking, but where do you need nonlocality?
Sorry, but read up on the topic before making any claims. Pretty much all of them are plain wrong. There is obviously more to entanglement than just the conservation as measurements along several well defined polarization axes show.
We know that field theory like QM (wavefunction is a field) properly predicts the outcomes - field theories can generally violate Bell inequities - not true?
So does this violation contradict field theories?
The last one. This is a very bad misinterpretation of what is happening in the experiment. It is also not Stephen Walborn's own opinion on the experiment.
Choosing polarizer angle in the future, modifies statistics in the past (indeed those in coincidence with future events) ...
Apropos ... could you maybe explain that while laser can be used to send information forward in time, why CPT analogue of laser doesn't allow to do it in reverse direction? ( https://www.physicsforums.com/showthread.php?t=715019 )
No, both show 3d confinement, quantized states and (possibly) complex Coulomb correlations. Self-assembled QDs may show some differences due to the presence of the wetting layer continuum, but in a nutshell the physics is almost the same. The one difference is that we have electrons in atoms, while we have bound electron hole pairs (excitons) in QDs, but that does not have an influence on the nature of the photons. It just matters when trying to build devices.
Generally the space of localized, dynamical EM field configuration is huge, including different knotting configurations ( http://physicsworld.com/cws/article/news/2013/oct/16/physicists-tie-light-into-knots ) ... let us maybe try to focus here on understanding configuration behind photons from deexcitation of simple atoms ...
This is the meaning of a mixed state. The state is really either one or the other, but we do not know. In a superposition state, it is not just our missing knowledge of the situation, but the system is really not in state A or B. (again, substitute the way your favorite interpretation handles the situation in case you favor an interpretation without superposition and collapse).
We are finally getting to what I have initially written here (I do understand the difference between superposition: single wavefunction and mixed state: probability distribution among multiple wavefunctions) ...
So you are saying that if a light year from me a cat was killed or not depending on nuclear decay, because for this year in my quantum mechanics the density matrix would be diag(1/2,1/2) ... objectively the cat would be neither dear nor alive?
I do not get your point. A single photon can at most transfer a tiny amount of momentum. It does not do it 1000 times. You may be able to see an effect using a huge number of photons, but I assume you would melt the mirror first. Also, there should not be any cumulative effects as mirrors are mounted on the ground. So either you move all the earth around or you move the mirror from its equilibrium position. This change will dissipate faster than more kicks come in.
Yes, such reflecting photon transfers the same momentum, so 1000 times heavier mirror will gain 1000 smaller velocity - we will have to wait 1000 times longer to get the same displacement ... as I have written.
I don't see why you say that it would require very light mirror - whatever its weight, the final situation with electron traveled one or the other trajectory differs e.g. by momentum distribution - these two scenarios have physically always a bit different outcome.
For single atoms it is usually not really complex unless you go to Rydberg atoms.
Looking only at this table, I think it is already quite complex ...
No. That is way too simple. You also only see quantized emission or absorption for absorbers with a continuous density of states and a mode spectrum which is not discrete. The intuition may be good enough for a limited number of cases, but it is very misleading to think it gives the full picture for general cases.
Sure in some cases spectrum can be continuous, like for thermal radiation ... but can single excited atom have continuous spectrum?
Anyway, of course this intuition would grow in complexity e.g. for larger atoms ... or situation when we produce much more complex dynamical EM field configurations ...
 
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  • #30
kith
Science Advisor
1,414
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Needless to say, there are some alternative suggestions for constructing alternative position operators that still work for photons, but one has to keep in mind what they really mean. Although it sounds easy, position may become a tricky concept.
I know a bit about the mathematical arguments, but I find it difficult to get a physical intuition here. In the double slit, the situation for photons and electrons is quite similar. In both cases, interactions at certain regions on the screen create the interference pattern. This doesn't seem to be enough to make "position" an observable. So I was thinking what I can do with electrons but not with photons.

Maybe I should open an own thread on this.
 
  • #31
jarekd
111
2
Kith, indeed the localization of electrons seems even much stronger as elementary charge is indivisible - e.g. not only the final situation in M-Z interferometer differs by momentums of mirrors while choosing different paths by a single photon ... the corpuscular part of electron: mainly charge, changes electric field all around - there are tiny EM differences all around between situation when electron chooses one path or another.

ps. In the Wikipedia article about angular momentum of light, there is a nice picture agreeing with that changing spin of electron is just twisting it by 180 deg:
800px-Sam-oam-interaction.png


To summarize this discussion, I think it is safe to say that optical photon is just EM twist-like wave: carrying energy, momentum and angular momentum - to compensate differences in deexciting atom (due to Noether theorem).
... however its shape can be more complex for multielectron atoms or different ways to produce localized pulses of EM waves, like quantum dots ... or even knotted configurations.
 
  • #32
Cthugha
Science Advisor
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But this "nonlocality" is fully determined by underlying field, which evolves in local and deterministic way (Hamilton-Jacobi equation with modified potential).
Sure if we would like to model reality by just classical mechanics (like seeing spin in classical way in Bell inequalities), there will always be something missing - some "nonlocality".
But I am talking only about field theories - does they still need some nonlocality?

Well, field theories where changes in the field at, say, Alpha Centauri have an immediate effect here on earth are non-local, even if they are deterministic. Field theories, where the changes in the field move at some finite speed are not.

Ok, now you are talking about Hanbur-Brown-Twist effect, what is something different ...

I am talking about the setup. HBT used the setup first to measure the (classical)effect named after them, but the same setup is routinely used to characterize antibunching.

Indeed correlations can be more complex than naive thinking, but where do you need nonlocality?

Single photon detections create anticorrelations at spatially separated detectors.

We know that field theory like QM (wavefunction is a field) properly predicts the outcomes - field theories can generally violate Bell inequities - not true?
So does this violation contradict field theories?

I am not sure what you are aiming at. Why should it?

Choosing polarizer angle in the future, modifies statistics in the past (indeed those in coincidence with future events) ...

You do not change the statistics in the past. The detections once made stay exactly the same. The decision you make afterwards is more like a filtering decision. You have several possible subsets of detections afterwards. This corresponds to "throwing away" a lot of the earlier detections as you just have a look at coincidence counts with a tiny part of them. The DCQE experiment is rather about filtering a dataset measured earlier, but not about actually changing something which happened in the past. Of course it is sometimes presented in a manner which sounds very differently, but these presentations are rarely done by people in physics. Stephen Walborn himself calls the essence of the experiment "bookkeeping". But that might be a bit off-topic here.

Apropos ... could you maybe explain that while laser can be used to send information forward in time, why CPT analogue of laser doesn't allow to do it in reverse direction? ( https://www.physicsforums.com/showthread.php?t=715019 )

I do not get your setup. Sorry.

We are finally getting to what I have initially written here (I do understand the difference between superposition: single wavefunction and mixed state: probability distribution among multiple wavefunctions) ...

Ok, thanks for clarifying.

So you are saying that if a light year from me a cat was killed or not depending on nuclear decay, because for this year in my quantum mechanics the density matrix would be diag(1/2,1/2) ... objectively the cat would be neither dear nor alive?

The cat would start out in a true superposition state which is loosely interpreted as "neither dead nor alive" and would develop towards a mixed state. For the mixed state, if you repeat the experiment very often (poor cat), you get both a dead and alive cat 50% of the time, but it is objectively dead or alive in every repetition. You just have a mixed state due to lack of knowledge. The timescale over which the cat develops from a true superposition towards the mixed state is its decoherence time. This is roughly the time scale on which some interaction with the outside world or even the within the constituents of the cat occurs, which breaks the superposition. It scales exponentially with system size (which is also the problem of creating many qubits for quantum computing). As a cat is pretty large, I would assume it has a decoherence time of next to nothing. Only stuff which almost does not interact with anything has been shown to get to (moderately) long coherence times. I think nuclear spins do pretty well or NV centers in diamond.

Yes, such reflecting photon transfers the same momentum, so 1000 times heavier mirror will gain 1000 smaller velocity - we will have to wait 1000 times longer to get the same displacement ... as I have written.

Ok, assuming that there is no restoring force and you somehow have mirrors freely floating in vacuum, you will get some displacement. The question is whether this displacement will be larger than the minimal displacement of the mirror you will get anyway because the initial momentum of your mirror is not precisely known, but only to within the bounds of uncertainty.

I don't see why you say that it would require very light mirror - whatever its weight, the final situation with electron traveled one or the other trajectory differs e.g. by momentum distribution - these two scenarios have physically always a bit different outcome.

But the final states usually still have significant overlap in phase space. If the differences are so large that this overlap is gone, the two pathways become distinguishable, though. In practice that is rarely the case. You need a lot of momentum transfer to go past uncertainty. And even so, what matters is the single electron case.

Looking only at this table, I think it is already quite complex ...

Well, but each single transition is still pretty well defined. I would consider transitions in chemistry or even solids as complex. But that is just a matter of opinion.

Sure in some cases spectrum can be continuous, like for thermal radiation ... but can single excited atom have continuous spectrum?

Single atom? Not really. But already simple things like a plasma or bremsstrahlung can.
 
  • #33
kaplan
44
1
Single photon detections create anticorrelations at spatially separated detectors.

No. Single photon detections are anticorrelated with detections at spatially separated detectors. That does not imply the detections caused the correlations. In the MWI, for example, there is no non-locality.
 
  • #34
Cthugha
Science Advisor
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The standard experimental way is still to consider photon detection events at some detector heralded by detection events at a second detector. The interpretation of what that means is of course up to you. I never implied that the experimental procedure necessarily means a causal connection. I already pointed out several times in this thread that the explanation and whether it involves non-locality, non-realism, both or something more exotic is up to your interpretation of choice.
 
  • #35
kaplan
44
1
The standard experimental way is still to consider photon detection events at some detector heralded by detection events at a second detector. The interpretation of what that means is of course up to you. I never implied that the experimental procedure necessarily means a causal connection. I already pointed out several times in this thread that the explanation and whether it involves non-locality, non-realism, both or something more exotic is up to your interpretation of choice.

OK - then we are in agreement. The word "create" made it sound as though you were asserting causality.
 

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