QED & Partial Reflection

  • Thread starter Malakai
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Hope this is the right forum. Rather daunting task to pick the right forum when you don't understand what differentiates each topic...

Anyhow, I've been reading Feynman "QED The Strange Theory of Light and Matter" (lectures). Actually, I've read it about 3 times, cover to cover. I get some of it, but I have two questions I hope maybe someone here could shed some light on. That wasn't a pun.

With regards to partial reflection, Feynman states "The situation today is, we haven't go a good model to explain partial reflection by two surfaces; we just calculate the probablity that a particular photomultiplier will be hit by a photon relfected from a sheet of glass."
ok, so Newton figured maybe light knew what kind of surface it was hitting, and whether it was the only surface, and therfore gave the partial reflection you would expect (or canceled out reflection completely). Well that theory is circumspect. I don't think science believes that today. But Feynman talked about shooting a laser through 50 meters of glass and getting the 0 to 16% relfection at the exepected intervals, based on the thickness of glass......

so can I use this for Faster Than Light communication? (i hope not).

Or, in these excessive tests with extremly thick glass, does it takes longer and long for the initial reflection to occur? That seems weird.

If there wasn't a delay (tied to the thickness of glass) before reflection, then given a 10 lightyear think piece of glass, I could communicate faster than light by shaving off one end of the glass, which would instantaniously change the partial reflection given at the face of the glass. And that just doesn't seem right.

well, that was my first question.

My second question has simply to do with the notion of "arrows" in QED. The so-called stopwatch hands, which spins faster or slower based on the color of light. While I like the abstraction, and it certainly helps in understanding whats going on, I now would like to know what it actually corrosponds to in the physical world. I'm guessing it can be tied to the frequency of light (because in his lectures he mentions that the 'blue' light stopwatch is faster than the 'red' light stopwatch).

Thanks all,

-Frank O'Connor
"A fire-breathing dragon lives in my garage."
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I don't know the answer to the question 1. But we need to think particle-wave duality seriously. We don't have a clear view about this based on language we use in a usual life. The second question is clear. The arrow indicates the phase for path integral. Feynman refers to a formulation of QED based on a path integral.
The reflection happening only on the front surface and the back surface is an approximation. There is reflection in the bulk of the glass (RF says it in the first chapter if I remember well, he also says that it is not the same photon entering as leaving the glass, it has been emitted and absorbed, re-emitted and re-absorbed...). So with a very thick glass you will have reflection before anything reaches the other side of the glass. Patterns will start forming and will change shape until the time t=thickness x velocity of light in the glass and steady states is reached

for question 2, I think the aim of the book is to make you admit that the theory is not intuitive but should be accepted because of its high efficiency to predict experimental results (there is no better theory so far !). Your question is "why is the theory like that ?" The clock is a way to avoid the writing of the probability function in mathematical symbols. We should not try to interpret it as something happening in the photon
About the first question, I haven't got a clue.

As regards the second one, I think the arrows have something to do with Huygens-Fresnel principle, but I’m not sure (and if it was the case, I would like to know what’s the link between them) … All I know is that every time a photon covers the distance of one wavelength, this arrow turns one turn.

(correct me if I am mistaken)
yes it's linked to the WL but the point is you shouldn't look at it like that. WL is the wave point of view (obviously...) and RF says photons are particles. A wave is everywhere, a quantum particle is somewhere with a certain probability. Huygens-Fresnel did not know about quantum behavior.
Huygens-Fresnel did'nt know about quantum behaviour indeed, but I read in the third lecture (that you can watch here http://vega.org.uk/video/subseries/8" [Broken] (link given by Fredrik), that if you consider a source of light that sends photons at t=T1, and others at t=T2, then at t=T2, the second wave of photons will begin with a stopwatch's hand that has already turned of an angle. I think I saw the same phenomenon in electromagnetism with the vector E. So I was wondering whether this vector could have a link with the hands??
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Feynman's stopwatch hands are merely an analogous device to describe the results in a wavelike fashion, as amplitudes. Imagine plotting the position of the hands of a stopwatch, they would create a continuous wave, with peaks at the 12'o clock, and troughs at 6o'clock.

If the 'stopwatch hands' already are turned through an angle, the probability amplitude is offset, just as a typical wave would not be "in-phase" with another.

Anyway, regarding the first question, FTL transmission of information is impossible, even with the probabilities given by QED. In the example described using a light-year-thick piece of glass, remember that the partial reflection for any thickness of glass follows a sequence that repeats as the thickness increases.
From the 0% of the thinnest glass to 16% maximum which is the same for all thicknesses of glass, so there would be no real difference, and therefore impossible to transmit any information.
I think the hand watch has to do with the coherence of the unique source of photons. It can be describe in a non-quantum theory (waves in phase)
Hi, I'm reading QED for the second time now, and sure enough I am having the same problems as Malakai had.

From what I gather in the previous posts, the stop watch basically correlates the wave-like nature of the photon with it's particle-like behaviour.

Now, I have 2 question in this reference-
1. What does the rotation of the stopwatch hand represent?
2. Why should Richard Feynman want to correlate the wave-like and particle-like natures of a photon--he declares right in the first few pages that the entire theory of QED is based on the assumption that light is definitely particle in nature.

I also have a doubt in regard to partial reflection and thin-film interference--in the example that Richard Feynman states about the oil in mud,and a beautiful spectrum being obtained when light is shone through it,this seems to be thin film interference---so is there no difference between thin film interference and partial reflection?
I was doing some googling,and I found a partial answer to my questions in one of the websites....in this entire seven page website,the real answer lies in the last two pages....however,I have still not understood it completely,and I have difficulty in understanding especially the two different kinds of spin stated(page 7)and also, the fact that the author of the web-page says that the length of the probability amplitude of the photon being reflected off any of the surfaces (0.4) is determined by the refractive index of the glass.

The website is: http://www.wbabin.net/mathis/mathis53.pdf

If anyone could make time to read through the website and try and help me with the two areas that I failed to understand (written in bold in this post),I would be really grateful.

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