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Amateur question about QED and the modern understanding of light

  1. Jun 16, 2011 #1
    I'm reading Feynman's QED, which is a nontechnical overview of quantum electrodynamics. In this book Feynman makes three claims that I am curious about. I have a math background but regrettably not much in physics.

    1. Light is a particle, period. It's not true that light is "sometimes a particle and sometimes a wave."

    Is this the consensus interpretation these days? In high school or college physics do we no longer teach people that light is a wavicle? Or is light a wavicle in high school but a particle in grad school?

    2. Over short distances, photons can go slower or faster than the speed of light. The speed of light is constant only over sufficiently large distances. This one really surprised me. Is that true?

    3. It's not known whether renormalization is mathematically consistent. Feynman's book is from 1983 I think. Is this still true? I know that renormalization was mathematically questionable originally, but is it true that it's still never been properly formalized?

    Thanks for any insight.
  2. jcsd
  3. Jun 16, 2011 #2


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    Unfortunately, there are no simple answers to these questions.

    A photon is discreet, true, but depending on your definition of a wave it could be a wave. This is true for any particle. It has an associated wave function which may not be well localized in spacetime. That would give it properties like a wave. It follows the Heisenberg Uncertainty Principle.

    As to c (speed of light): I don't really know about the faster than light part, but the effects resulting from the many possible paths it can take from the source to the destination definitely affect its average speed.
  4. Jun 16, 2011 #3
    I don't know how extensive your math background is so let me know if this goes over your head. Anyways, it seems a rather irrefutable fact that "particles" in the sense of infinitely small points zooming around with set trajectories do not exist. As an electron moves through space it is not an infinitely small billiards ball shooting through, rather it is a WAVE PACKET propagating forward. In other words it is like a Fourier transform which has an infinite number of coefficients that represents a localized but not infinitely localized packet which is constrained to follow certain restrictions (i.e. follow a certain equation). Now ultimately this is the observed behaviour. Feynman (actually apparently the idea was originally Dirac's) realized he could replicate this math by considering a point-like particle whose movement forward in time was actually considered as a weighted sum over all possible paths. This is mathematically IDENTICAL to the wave-packet obeying Schrodinger's equation approach, however, it has some advantages both conceptually and in ease of making certain calculations.

    Therefore, since both description are mathematically IDENTICAL and predict EXACTLY the same result for any possible experiment, there is no way one could ever tell the difference between the two. So to answer your question, are particles points who explore all possible paths resulting in a sort of blurring out? Or are they blurred out wave-packets obeying a certain equations? Well, the answer is: *shrug* which would you like it to be? Sometimes calculations are easier in one framework and sometimes they're easier in the other. People have their favorites but objectively we're assured that no experiment could ever, even in theory, tell the difference. That being said, the path integral approach (Feynman's approach) much more easily lends itself to a lot of more advanced stuff (which again doesn't necessarily mean it's "more correct").
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