Measurement and Thermal Effects Similar?

  • #1
bushmonk
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TL;DR Summary
Light not only goes through the double slit, but is also either reflected or absorbed by the barrier. Contrasting a mirror and a flat black barrier suggests to me that even without a measurement a photon localizes when it is absorbed.
A weak (one photon at a time) beam of laser light strikes a double slit. In case 1 the barrier forming the double slit is a mirror. In case 2 the barrier is flat black. An array of detectors surround the experiment and can detect the photon whether it goes through the slit or is reflected. In case 1, the photon contributes to an interference pattern, whether it is reflected or transmitted. In case 2 the photon either contributes to an interference pattern beyond the barrier or it is absorbed by the barrier and contributes to heating it up. In case 1 each photon is explicitly measured to arrive at a specific point. In case 2 it is only explicitly measured if it is detected beyond the barrier. Is it correct to understand that a photon absorbed by the barrier is absorbed at a specific location on the barrier even though no explicit measurement of that location takes place?
 
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  • #2
bushmonk said:
In case 1, the photon contributes to an interference pattern, whether it is reflected or transmitted.
Why do you think the reflected photons in this case would form an interference pattern? What two alternatives are interfering?
 
  • #3
bushmonk said:
Is it correct to understand that a photon absorbed by the barrier is absorbed at a specific location on the barrier even though no explicit measurement of that location takes place?
What future predictions would you make differently if the answer to this question were "yes" vs. "no"?
 
  • #4
PeterDonis said:
Why do you think the reflected photons in this case would form an interference pattern? What two alternatives are interfering?
You get an interference pattern for both the reflected and transmitted photons. The interference is due to the superposition of the "Huygens's wavelets" coming from the sources at different locations. For the transmitted waves it's an interference pattern due to the two slits and the single-slit interference pattern and for the reflected waves the "inverse pattern", leading to a similar interference pattern.
 
  • #5
It's the path. If you can know the path(when the detector is 'on'), the photon lands classically(no interference).
If it's impossible to determine(know) the path(detector 'off'), the photon travels quantum mechanically(you get interference stripes).
Btw, the answer to your last question is that the photon is always absorbed at a specific location. The interference pattern does not arise from waves propagating through both slits but from wave-like behavior(this unique behavior is peculiar only to quantum objects)
 
  • #6
Of course, both cases are described by quantum (field) theory. If you deal with single photons you cannot describe it classically anymore. A single-photon Fock state is a generic quantum state.
 
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  • #7
vanhees71 said:
The interference is due to the superposition of the "Huygens's wavelets" coming from the sources at different locations.
I'm not following you. We are talking about mirror reflection of light. In the general case mirror reflection of light does not cause an interference pattern.
 
  • #8
If you have a large enought mirror there's of course no interference pattern.

However, I thought we discuss slits or gratings, which can be used with light going through as well as reflected light, because in the OP it was asked about the double-slit experiment.

https://en.wikipedia.org/wiki/Diffraction_grating
 
  • #9
vanhees71 said:
I thought we discuss slits or gratings
We're discussing a barrier with two slits in it, as is used for a double slit experiment, but with the barrier being a mirror instead of just absorbing photons that don't go through the slits. That's not at all the same as a diffraction grating or a barrier with slits everywhere in it.
 
  • #10
The principle is the very same. You'll get a (double + single-slit) interference pattern with both the transmitted and the reflected photons.
 
  • #11
bushmonk said:
TL;DR Summary: Light not only goes through the double slit, but is also either reflected or absorbed by the barrier. Contrasting a mirror and a flat black barrier suggests to me that even without a measurement a photon localizes when it is absorbed.

Is it correct to understand that a photon absorbed by the barrier is absorbed at a specific location on the barrier even though no explicit measurement of that location takes place?
Yes, that's correct.
 
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  • #12
Demystifier said:
Yes, that's correct.
1. Is this "correctness" generally acknowledged or is it something peculiar to the Bohmian interpretation.
2. I checked out the Bohmian link. The idea that position is the fundamental measurement seemed reasonable--I haven't come across any other. However I couldn't follow the math and so didn't know how it would help with the question.
3. A followup question: Is there a (relatively) simple theory of these two contrasting interactions that describes how in one case the light scatters coherently from a huge number of atoms, preserving phases, and in the other, triggers an irreversible transition at one and only one place (perhaps at one and only one atom)? What is the critical difference in the atoms in the mirror and the atoms in the black barrier that results in such dramatically different behaviour of the light.
 
  • #13
bushmonk said:
1. Is this "correctness" generally acknowledged or is it something peculiar to the Bohmian interpretation.
It's general.
bushmonk said:
2. I checked out the Bohmian link.
It's my signature, not a part of my reply to your question.
 
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  • #14
bushmonk said:
3. A followup question: Is there a (relatively) simple theory of these two contrasting interactions that describes how in one case the light scatters coherently from a huge number of atoms, preserving phases, and in the other, triggers an irreversible transition at one and only one place (perhaps at one and only one atom)? What is the critical difference in the atoms in the mirror and the atoms in the black barrier that results in such dramatically different behaviour of the light.
Every material partially reflects and partially absorbs. The absorption is a quantum effect. First, it requires that the atoms/molecules/crystals have empty excited states the energy of which (relative to the nonempty state) is equal to the energy of photons. Second, even if such excited states exist, it still doesn't mean that a photon will be absorbed because the quantum probability of absorption is smaller than 1. Hence some fraction of photons will remain unabsorbed. The reflection, on the other hand, is basically a classical effect; light that was not absorbed gets reflected, simply because it has nowhere else to go (if the material is not transparent).

See also https://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission
 
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  • #15
What's really a quantum effect is spontaneous emission. Absorption and induced emission can also be described in the "semiclassical theory", i.e., where you quantize only the charged particles (electrons in atomic physics) but leave the em. field classical.
 
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  • #16
Demystifier said:
Every material partially reflects and partially absorbs. The absorption is a quantum effect. First, it requires that the atoms/molecules/crystals have empty excited states the energy of which (relative to the nonempty state) is equal to the energy of photons. Second, even if such excited states exist, it still doesn't mean that a photon will be absorbed because the quantum probability of absorption is smaller than 1. Hence some fraction of photons will remain unabsorbed. The reflection, on the other hand, is basically a classical effect; light that was not absorbed gets reflected, simply because it has nowhere else to go (if the material is not transparent).
The reason for my followup question is that I want to know what quantum mechanics says about why some interactions preserve coherence (and therefore the capacity for interference) and others produce localization. In introductory quantum mechanics, I was taught that the localization occurs when the location of the photon is measured. Measurement is a purposeful activity of a conscious being. However, if it is generally acknowledged that the photon is localized in thermal absorption, then purposeful measurement is not the cause of localization, but instead it is the nature of the interaction of the photon with the material. In both the case of a mirror and the flat black barrier, the wave interacts with a huge number of atoms. The combined effect of those interactions is, in the mirror, a coherent reflection from all the atoms at (or near) the surface, and, in the flat black material, the effect is the absorption of the photon by a specific atom at (or near) the surface.

The link you provided suggests to me that for absorption two things must happen. 1. There must be a match between the photon energy and a transition energy of the atom. So a black material must have a continuous range of transition energies available. 2. The energy of the excited atom must be shared with its neighbours promptly so that it becomes random thermal motions. Is that the full answer? Is there a simple quantum explanation for these two things--a continuous range of transition energies and rapid sharing of the energy.

The link also endeavoured to explain reflection and transmission by positing that 1. there are no matches between the photon energy and the transition energies of the atoms and 2. if the material is transparent or opaque, there is transmission or reflection. There must be some quantum mechanical explanation of why some arrangements of atoms are transparent and others are opaque. Is this explanation accessible to the introductory student.
 
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  • #17
bushmonk said:
The reason for my followup question is that I want to know what quantum mechanics says about why some interactions preserve coherence (and therefore the capacity for interference) and others produce localization.
To see why reflection from the mirror does not produce decoherence (i.e. why it preserves coherence) see e.g. my http://thphys.irb.hr/wiki/main/images/5/50/QFound3.pdf page 15.
 
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  • #18
bushmonk said:
There must be some quantum mechanical explanation of why some arrangements of atoms are transparent and others are opaque.
I guess in the end, you just have to determine the complex dielectric constant of your material for your given wavelength. If its imaginary part is very small, then the material will be transparent at your given wavelength. Solid state physics textbooks contain theory explaining and deriving such frequency dependent complex dielectric constants under some circumstances. More generally applicable theory is available too since the 1970s, but not discussed or mentioned in the solid state textbooks I have read or browsed so far (which reminds me, I definitively want to browse Kittel at some point).
My solid state physics textbooks are also better at understanding crystaline materials than they are at understanding amorphous glass like materials. Again, more recent research is available that overcomes those limitations.

bushmonk said:
Is this explanation accessible to the introductory student.
The basics would be just to understand the differences between insulators, semiconductors, and metals. That should be totally within reach to the introductory student.
But already the Lindhard formula for the longitudinal dielectric function ##\epsilon(\bf{q},\omega)## is unsuitable to the introductory student, if you ask me. Or maybe it is just me. For example, I could not tell you why wikipedia has thrown in "longitudinal" there, or whether ##\bf{q}## should be very small for photons, because they have very little momentum.
 
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