Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What can be called a fundamental theory?

  1. Jul 21, 2009 #1
    The usual perception is quantum field theory is not fundamental since it has divergence. A fundamental theory must always be finite, like string theory.

    But in the physical sense, a fundamental theory means it is valid at any arbitrary energy scale. We know that non-renormalizable theories are not fundamental because they must break down at a finite cut-off; QED is not fundamental because it blows up at the Landau pole. However, QCD is asymptotically free and I see nowhere it must break down. Suppose we have only QCD in our world. There is no EW theory or gravity. do we still worry about the divergence in QCD and set off looking for a fundamental mother theory for QCD? In other words, is divergence really physical so that we must find something to cure it?
     
  2. jcsd
  3. Jul 21, 2009 #2
    I would typically consider this example to be an academic problem showing unreasonable conditions for a "fundamental theory". The landau pole is even above the Planck scale. We have quite a different perspective on renormalization since Landau came up with this pole.
     
  4. Jul 21, 2009 #3
    As long as it requires renormalization, it's not fundamental.
     
  5. Jul 21, 2009 #4
    The book was written in 2003. Landau died in 1968.
     
  6. Jul 21, 2009 #5
    Thank you for mentioning A Zee. It was exactly what he said in his book(QFT in a Nutshell) made me bring up this question. I still don't understand why this "divergence" criterion is reasonable.

    Let's put it in this way. I remember Zee also said in his book, "theories in physics have the ability to announce their own eventual failure and hence their domains of validity". So even if we put aside all the divergence or renormalization criterions, we can ask one question: Is there any where that QCD cries out its failure by its own? There are several possible answers:

    1 QCD can be fundamental in principle so it cannot show its domain of validity by itself. Only conflicting with other theories (Eg. gravity) can bring it down.

    2 QCD cannot show its domain of validity because it's not a good physical theory. We can get a reasonable result (finite, respecting unitary, perturbative, etc. ) at an arbitrarily high energy, however it conflicts with experiments. (Even worse than a EFT in this sense.)

    3 QCD can show its boundary and we'll somehow see that the cut-off cannot be pushed to infinity. (However, I can't imagine how this would happen. )

    4 This question is nonsense. Gravity can never be avoided in any complete fundamental theory, so we can never separate QCD or whatever "good" field theory from gravity. And gravity says, all field theories are doomed.

    Which one might be the most reasonable answer?
     
    Last edited: Jul 21, 2009
  7. Jul 21, 2009 #6
    Since we're quoting things, here's one more quote:

    - Supreme Court Justice Potter Stewart, quote slightly modified

    But seriously, even if we forget about renormalization and divergences within QCD itself - QCD does not incorporate gravity, and QCD has to be unified with electromagnetism, which does blow up near Landau Pole. Therefore, QCD is not fundamental.
     
  8. Jul 22, 2009 #7
    Did you notice that we already know something occurs with EM at the so-called electroweak scale ? There was a nice plot from ZEUS
     
  9. Jul 22, 2009 #8
    To answer the OP: QCD alone, is mathematically elegant and as far as we know, free from UV problems. However, QCD is manifestly not sufficient. The other bits do have UV problems. It is currently not understood whether the non-QCD bits and the QCD are linked at some high energy, but it is widely thought to be so; thus any problems they have should be related.

    As a condensed matter theorist I'm never going to believe any claim that a theory is UV complete. The very essence of universality is that a wide variety of theories tend to have the same low energy behaviour, and it is pure hubris if we believe our current HEP theories to be exempt --- QCD included.
     
  10. Jul 22, 2009 #9
    In fact I am very glad to see that QFT must break down at some point. Maybe my question is too conceptual to get a concrete answer. Let's just leave it as it is.

    A different but related question:
    Can anyone remind me why gravity cannot be quantized as a field theory? Is there any simple reason? Maybe by answering this question can we understand why QFT is not fundamental.
     
    Last edited: Jul 22, 2009
  11. Jul 23, 2009 #10
    Since I have too little strength to answer seriously to this thread, I'd like to post a link to a paper by Weinberg which is more than 10 years old and tells it much better than I can.

    http://xxx.lanl.gov/pdf/hep-th/9702027v1

    Note that GR is not renormalizable from a power counting point of view only.
     
  12. Jul 24, 2009 #11
    Thank you, Humanino.
    I read this paper one year ago, however I did not fully understand it. Think I should read it again.
     
  13. Jul 24, 2009 #12
    I'm note exactly sure what you are trying to nail down, whether it is about QFT or renormalization. The above paper by Weinberg sums up rather well what seems to me is the modern point of view on QFT. For renormalization, there is a very good introductory paper which does not depend on QFT by itself :
    A hint of renormalization
     
  14. Jul 27, 2009 #13
    It is innapropriate of you to quote the Independent Research forum here.
    Please provide a published reference to your work.
     
  15. Jul 27, 2009 #14
    I hear a lot of that. Well, sorry but I'll be frank with you : if your work had any value, you would not have any difficulty to publish it in English for everybody to read and it should be your DUTY to publish it in English so that it does not fall in forgetfulness. Besides, I do not believe it is common practice on PF to accept foreign language publications, for obvious reasons.

    Please take into account that it is already too difficult to read what is published, so please do not expect people to pay attention to things not published, such as "I posted it on arXiv".
     
  16. Jul 27, 2009 #15
    They were translated into English but are not available on internet (too old publications).
    Do you think the Russian science is wrong or different?
    Your reference to "Renormalization hint" points out to arXiv. The article is written by a french researcher, like me. You are too hostile to me personally.
    What I do is answering questions that persist despite a huge amount of mainstream answers.
    I have a unique and helpful experience in solving problems. You even cannot imagine to what extent you are unjust to my efforts.

    P.S. I removed my answers from this thread.
     
  17. Jul 27, 2009 #16
    Yes I think it is different, and I have a lot of respect for the Russian school, as a matter of fact my best teachers were Russians.
    Then do not be unjust to your own efforts, and publish them in English ! Seriously, I am quite interested by the renormalization procedures, and although I am not researching professionally I try to read as much as I can.
     
  18. Jul 27, 2009 #17
    As soon as it's been published already in western journals (and present at some libraries) I prefer to submit English translations to arXiv and refer to the journal versions in the Literature chapters.
     
  19. Jul 28, 2009 #18
    "Reformulation instead of Renormalizations" is even simpler and more relevant than "A hint of renormalization". It's also been written with pedagogical purposes and contains an explanation of the anatomy of renormalizations.
     
  20. Jul 28, 2009 #19

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    This thread has gone seriously off-topic, so I assume that there's nothing more to be discussed with respect to the original topic.

    Zz.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook