Quantum Weirdness in Glass

  Perfect Transmission

One of the weird aspects of photons involves reflection from glass of varying thickness.  Send a laser pointer beam perpendicular to a pane of glass and about 4% of it will reflect back, on average, but, by carefully selecting glass of various thicknesses, the reflections vary from 0% to 16%.    Glass a foot thick can be slightly adjusted in thickness to not reflect at all!   All the light goes into the glass – perfect transmission. 

 QED easily shows how this works for light beams.   Rays from the back of the glass interfere with the rays coming in the front so as to cancel the reflection if the wavelength is a multiple of ½ wavelength.

However, the cancellation at ½ wavelength also works for individual photons for thick glass, and there seems to be no answer other than “quantum weirdness”.    QED cannot explain it for single photons.

Photon on Glass






How does an individual photon know how thick the glass is the instant it hits the front surface when the back surface is thousands of wavelengths away?   The reflected photon would be six feet away before a copy could make a round trip through a foot thick piece of glass.  (Two feet round trip at 1/3 speed of light in air).

The photon has a number of options to reflect.  It can reflect at the front surface of the glass,  the back surface, somewhere within the glass, be absorbed or go on through.  The reflected and absorbed cases mean it does not exit the other side, and the latter one means it does exit.  

If the glass is exactly the right thickness, it does not reflect and shoots right through.   The rub is this: a photon that reflects off the front surface of a foot thick piece of glass has to make that decision at the front surface before it goes in.   As soon as it hits that surface, it begins to move away in the other direction.  The probability for reflection depends on the thickness of the glass.  However, the photon cannot know the thickness in advace.  Or can it?  I have a theory that tells how this works.

This is only one of the glass problems.   Next I will tell you about the weird effects for photons that hit tilted glass


4 responses to “Quantum Weirdness in Glass

  1. I’ve only read a few of your pages so far, but I intend to follow your path through all of them. I wish I had encountered your work sooner, because I have for many years wondered about how the world looks from the point of view of a photon. From what little I have read so far, it appears this is the basis for the theory you will get to later on. I might very well have some comments on that when I get there.

    For this particular section of your work, I just want to point out that you have included contradictory information. The diagram suggests that a front-surface reflected photon would move 3 feet from the glass in the time it takes a back-surface reflected photon to traverse the glass in both directions. The text says the front-surface reflected photon will be 6 feet from the glass, rather than 3 feet, because the speed of light in glass is only 1/3 the speed in air. For typical glass the speed of light is close to 2/3 the speed in air; the diagram has the correct information, but the text is incorrect.

  2. You are correct. As I pointed out in response to a similar comment of yours about this 1/3 speed error of mine that it is a serious error on my part and I’ve reconstructed how it came about. I knew and wrote down that the speed of light was “1/3 less in glass” than in a vacuum but then used 1/3 to refer to the speed itself, losing the “less” part. I will find time to correct the mistake here and in other places I might have used it.

    I’ve noted an error on this very topic in another post in a diagram as well.

    I hope readers will read the comments and adjust their thoughts in the interim.

    Thank you for pointing this out to me.

  3. Oh, and both the diagram and the text are wrong.

    The diagram refers to 3 feet before the photon reaches the other side of the glass, implying the far side where it is shown in the diagram. The text refers to 6 feet before the photon could return to the near side. Same thing, both wrong.

    • You are right; they are both wrong. I failed to notice the disconnect between the phrases “other side of the glass” in the diagram, and “return to the near side” in the text. The diagram would be OK if it were talking about the round trip of the “copy photon” covering a distance of two feet at a speed of 2/3 c, while the primary photon moves 3 feet at speed c.

      Still, if we think of a photon as a thing with no length (at least in free space), as you later describe it, we are still perplexed by the fact that a “copy photon” would not be able to detect the far surface before the original photon “knows” that it is supposed to reflect at the first surface. I am thinking about this notion of the “flat” photon, and hope to offer a comment to the article that focuses on the photon itself.

      Your statement “However, the cancellation at ½ wavelength also works for individual photons for thick glass, and there seems to be no answer other than ‘quantum weirdness’. QED cannot explain it for single photons,” sent me off to look for some information on QED, which is a topic I probably knew more (but far from ALL) about in the distant past, but much of that knowledge has leaked away over many years of non-use. I managed to get a copy of Feynman’s very readable little book, based on a series of lectures for a general audience “QED: The Strange Theory of Light and Matter” in which he addresses this very problem. As I suspected, it turns out that QED has an explanation for this phenomenon. This does not separate it from the realm of ‘quantum weirdness’ of which you speak; weirdness is at the very heart of the theory of QED, and Feynman makes no bones about that.

      Very simply stated, my interpretation (and I hope I am doing Feynman justice in this description) is that Feynman is saying that the arrival of a photon at a point outside of the glass on the approach side (first surface reflection) is an event that can be observed with a probability that depends on the likelyhood of a photon being reflected by an electron within the glass, and the thickness of the glass plate. However, as in the double slit experiment, we cannot know the exact path the photon took to arrive at that point. The basic idea of QED is that one has to consider ALL of the possible paths it could have taken to arrive there, and these include all paths corresponding to slightly different emission times in the source with corresponding “reflections” from atoms at all possible points within the crystal. (In his treatment of the reflection from a glass plate he only talks about possible reflections from different depths, but when talking about reflections from a mirror he talks about possible reflections from all points on the mirror and shows how the only non-cancelling reflections are the ones that give rise to the familiar specular reflection. If one applies that same logic to the glass, every layer in the glass could be thought of as a partially reflecting mirror, and the contributions from all such layers could then be added.) The remarkable outcome of this more-in-depth analysis is that it yields the same result as thinking in terms of interference between a front surface reflected path and a back surface reflected path, as you have described. But for either description (many-layer reflections vs. two-layer reflections) to make sense, you have to allow for different emission times of “flat” photons (or for them to travel at different speeds, which is something that is not strictly forbidden in QED as long as the speed of what survives in the sum does not exceed c; this goes beyond my level of understanding of the theory at this point).

      I should probably also mention that “all paths” in QED theory includes all paths that involve multiple reflections within the glass. Fortunately, these multi-reflection paths are far less likely than single-reflection paths, so they can be ignored to a good approximation.

      In summary, I don’t think it is accurate to say that “QED cannot explain it for single photons.” What you can say is that QED provides no SIMPLE explanation that avoids bringing the strange concepts of superposition of many possible paths into consideration. I assume that your work is an attempt to find an alternative explanation as to how these weird things can happen based on relativity theory.

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