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  • QFT videos

    From Stefan Ram@21:1/5 to All on Sat Sep 23 14:08:36 2023
    I prefer to learn from books. But videos have two advantages: One
    can watch/listen to them while doing household chores, and one also
    learns the correct English pronunciation of the technical terms.
    So I listened to some videos of QFT lectures by Prof. Susskind.

    However, after about three videos so far, I am rather disappointed.
    I have the impression that Susskind deliberately wants to
    counteract all too flowery gobbledygook with a "don't talk, but
    calculate" approach. I have always found such an approach absurd
    in physics, but especially devastating in teaching. He does some
    math, "We put this in here, and then we get this," "I'm not going
    to say why, I'm just going to do it this way," and then he says,
    "And this is the simplest example of a quantum field." (these are
    all not literal quotations). He doesn't explain what a "quantum
    field" is supposed to be. This doesn't seem very educational to me.

    As an example of an approach that I like (at least according to the
    few pages I have read so far), I would like to mention "Quantum Field
    Theory" by Mark Srednicki (which is a written text, not a video).
    He first explains that it is about combining quantum mechanics and
    relativity. In order to do this, space and time must be treated
    "on an equal footing at the outset". In quantum theory, time is a
    label (parameter), location is an operator. So to treat them equally,
    one can either treat location as a label, or one can treat time as
    an operator, says Srednicki. Since the second is a bit complicated
    (Srednicki says it would lead to string theory), Srednicki follows
    the way to make the location a label. Each location x is associated
    with an operator phi(x). And this is a quantum field.

    So Srednicki first explains what requirements a quantum field
    should satisfy and why, and then he shows how these requirements
    can be satisfied, so that one can grasp the concepts. Susskind
    lacks such an explanation (though I have not seen all the
    videos in the series, so I may be missing something).

    [[Mod. note -- It would be useful to have references to the specific
    videos and books under discussion. -- jt]]

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  • From Richard Livingston@21:1/5 to Stefan Ram on Mon Sep 25 15:42:00 2023
    On Saturday, September 23, 2023 at 4:08:40=E2=80=AFPM UTC-5, Stefan Ram wrote:
    I prefer to learn from books. But videos have two advantages: One
    can watch/listen to them while doing household chores, and one also
    learns the correct English pronunciation of the technical terms.
    So I listened to some videos of QFT lectures by Prof. Susskind.

    However, after about three videos so far, I am rather disappointed.
    I have the impression that Susskind deliberately wants to
    counteract all too flowery gobbledygook with a "don't talk, but
    calculate" approach. I have always found such an approach absurd
    in physics, but especially devastating in teaching. ...

    Stefan,

    I share your frustration and opinions. I've been trying to "understand"
    QFT for almost 50 years now and still have some issues. Almost
    everyone treats it as a math problem and it gets very abstract. After transforming to momentum space and doing a Wick rotation I'm not
    sure what we are calculating anymore. I have some more fundamental
    questions that may or may not be valid (I'm undecided on these):

    -Transforming the path integrals to momentum space fundamentally
    changes the integration. I'd have no issue with this in Newtonian
    space-time, but the Fourier transform in Minkowski space-time does
    not cover the same space-time volume as the spatial integration. My
    real concern here is that the high momentum parts of the integration
    is covering the same physical paths over and over again, and I
    wonder if weighting the momentum integration proportionalto the
    volume of space it represents might reduce the infinities calculated
    and give a more reasonable result?

    -I'm not sure how much this would affect the calculation of
    macroscopic problems, as the wave function outside the light cone
    is clearly attenuated, but for very small distances and very high
    energies there is a bit of "fuzzyness" to the light cone, and I wonder
    if that is part of the source of the infinities calculated particularly at higher energies and short distances.

    -The Lagrangian used universally has F_{\mu\nu}F^{\mu\nu}, which
    includes an electromagnetic field energy, which integrated over all
    space gives an infinite result. This has been an embarrassment for
    over 100 years now. I wonder (and this is by far an unconventional
    opinion) if the concept of EM field energy is a mistake. It has been
    known since at least the 1920's that if you do EM via potentials that
    the energy of interaction has to be on the charges, not in the fields.
    Wheeler and Feynman advocated for this briefly in the 1940's but
    gave it up in light of the Lamb shift results. I wonder if maybe they
    were too quick to give up on it?

    Part of the problem is that QFT is necessarily a very advanced mathematical theory, and the people who can do the math easily are mathematicians.
    But I'm a Physicist and I want to understand what the math is modeling,
    and I'm often suspicious that mathematical transformations are changing
    what is being modeled.

    If you are inclined, I'd be interested in discussing issues and ideas, and
    I'll try not to bother you too much with my heretical thoughts. We might
    be able to help each other.

    Rich L.

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