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

Insights Struggles with the Continuum - Part 6 - Comments

  1. Feb 12, 2016 #1

    john baez

    User Avatar
    Science Advisor
    Gold Member

    Last edited: Sep 5, 2016
  2. jcsd
  3. Feb 12, 2016 #2


    Staff: Mentor

  4. Feb 12, 2016 #3


    Staff: Mentor

    Another wonderful paper - many thanks to John.

    I would like to mention a series of lectures I enjoyed very much and re-watch them every now and then with equal enjoyment:

    It examines perturbation theory, extracting finite answers from divergent series, and some of the things touched on in the paper. The most startling for me was the origin of quantisation which naturally comes out of this approach. Fascinating. Oh - he touches on its relation to renormalisation - not in depth though.

  5. Feb 12, 2016 #4


    User Avatar
    Science Advisor

    Hi John,

    I'm not familiar with the 2-loop calculation. Does the Landau pole appear at the same energy as for 1-loop, or does it move?
  6. Feb 13, 2016 #5


    Staff: Mentor

    Almost everything in this Continuum series of Insights articles is way over my head. Nevertheless, I manage to learn something, and to gain a glimpse of the outstanding problems in this field when I read them. I can only attribute that to excellent writing.

    Kudos and thank you John Baez.
  7. Feb 13, 2016 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    One cannot stress this enough. These Insights are written in a way to explain the issues discussed "as simple as possible but not simper". It's really great!
  8. Feb 13, 2016 #7
    If we are considering imaginary values of charge, then logically we must also consider imaginary values for the EM fields. A classical plane wave with imaginary amplitude would have a (real-valued) Poynting vector opposite the direction of propagation- the wave radiates negative energy. I am guessing that the QED equivalent would be a negative-energy photon. Now if all of the electrons & positrons have only imaginary charge, I don't see how real-valued fields could be generated. It would seem that in such a pair production, the only photons that could be produced would have negative energy, so energy conservation should rule out this scenario!
  9. Feb 15, 2016 #8
    @john baez , is my question above meaningful?
  10. Feb 18, 2016 #9
    (excuse in advance if this question is too newbie)
    I was reading Penrose's Road to Reality, chapters 7, where he discusses how some complex functions have Taylor expansions that are only correct within a small domain. The expansion converges within that domain but diverges outside of it. This is usually when the complex function has singularities, or is multi-sheeted, like y = log z. To get a description of the whole domain you need analytic continuation (patch together Taylor expansions around different points). The same is true for more general Reimannian surfaces (chapter 8).

    Anyway, my question is whether this property has anything to do with the QED problem. It sounds quite similar to me. Maybe the latter iterations are somehow actually sampling from outside the domain of convergence.
  11. Feb 18, 2016 #10
    In this power series, the "complex valued independent variable" is the fine-structure constant alpha. By the definition of a power series, or any function for that matter, all the terms must be evaluated for the same value of the variable. The important question is whether the actual value, about 1/137, is within the domain of convergence. Dyson argued that it must not be, because domains of convergence are symmetrical around the origin. If the series converges for 1/137, it must also converge for alpha=-1/137, That would imply that in a hypothetical universe with imaginary-charged electrons, we would have finite values for all the measurements that are finite in real life. But in such a universe the vacuum should instantly generate infinite numbers of electrons, positrons, & photons, so it seems that the series should diverge. The same argument implies that the domain of convergence should simply be zero.
  12. Feb 18, 2016 #11


    Staff: Mentor

    The answer is finite answers can be extracted from divergent series - see the videos I linked to.

    Just as a start consider the following series 1 + s + s^2 + s^3 ++++++.

    Complex analysis shows its divergent if s > 1. But consider the following:

    S = 1 + s + s^2 + s^3 ++++++ = 1 + (s + s^2 + s^3 ++++) = 1 + s(1 + s + s^2 +++++) = 1 + sS or S-sS = 1 or S = 1/1-s. This gives an actual value if s is not 1. For example if s =-1 you have 1 - 1 + 1 ....... =1/2.

    Its called generic summation:

    For more detail of this interesting issue see the videos.

    It may seem like a mathematical quirk, but as John says it leads to results consistent with experiment so there is obviously somethin going on.

  13. Feb 18, 2016 #12
    @bhobba, once you're on the thread, would you mind responding to my question re. Dyson's argument against the possibility of actual convergence (that it would imply convergence for negative values of alpha i.e. imaginary electron charge, & in such a universe infinite numbers of pairs could be produced)?
  14. Feb 18, 2016 #13


    Staff: Mentor

    Sorry - but I don't know enough about it to respond.

  15. Feb 22, 2016 #14
    "If these power series converged for small positive alpha, they would have a nonzero radius of convergence, so they would also converge for small negative alpha. Thus, QED would make sense for small negative values of alpha [which is problematic since it describes unstable vacuum]."

    I don't understand the above logic. Why is it a problem that QED with negative alpha results in a physically nonsensical picture? Why would anyone care? This is not a model of our Universe.
  16. Feb 22, 2016 #15
    The problem is that a convergent power series would imply that QED should work, i.e. give finite results, in that nonsensical universe.
  17. Feb 22, 2016 #16
    And that is a problem... why?

    The vacuum in that fictitious Universe would decay, creating more and more electrons. The electrons would feel finite forces and have finite charges at any given time, there just would be more and more of them as time progresses, which is clearly not what we see in our Universe. But why should I care? I didn't expect QED with negative alpha to describe our Universe, thus the mismatch is *expected*.
  18. Feb 22, 2016 #17


    User Avatar
    Staff Emeritus
    Science Advisor

    I think the reasoning is that
    1. If the power series for QED were convergent, then the behavior of particles and fields would be an analytic function of [itex]\alpha[/itex].
    2. If the behavior was an analytic function of [itex]\alpha[/itex], then the behavior for [itex]\alpha[/itex] slightly less than zero would be only slightly different from the behavior for [itex]\alpha[/itex] slightly greater than zero.
    3. But the behavior for [itex]\alpha < 0[/itex] is nothing like the behavior for [itex]\alpha > 0[/itex].
    In other words, if the vacuum is stable for [itex]\alpha > 0[/itex] but unstable for [itex]\alpha < 0[/itex], then that implies that something weird happens at [itex]\alpha = 0[/itex].
  19. Feb 22, 2016 #18


    User Avatar
    Staff Emeritus
    Science Advisor

    But I guess I'm not completely convinced, myself. In general, if you have a physics problem with a parameter [itex]\lambda[/itex], some quantities might be analytic in [itex]\lambda[/itex], and others might not. So the fact that QED completely falls apart with [itex]\alpha < 0[/itex] means that not everything is analytic in [itex]\alpha[/itex], but does it mean that the quantities of interest fail to be analytic in [itex]\alpha[/itex]?
  20. Feb 22, 2016 #19
    What do you mean by "the quantities of interest"? I thought the question was whether a particular power series converges. According to this argument, it does not. What other quantities are relevant?

    Once you are here, would you perhaps respond to my question in post 7 above? In a universe with only imaginary-charged electrons, how could positive-energy photons, corresponding to real EM field values, be generated to balance the negative energy of the pairs?
  21. Feb 22, 2016 #20
    This is not true in many cases.
    Trivial examples: behavior of 1/x is quite different for x>0 and x<0. Behavior of exp(1/x) even more so.
  22. Feb 22, 2016 #21
    These functions are not analytic at zero, and therefore cannot be expressed as power series in x.
  23. Feb 22, 2016 #22


    User Avatar
    Staff Emeritus
    Science Advisor

    [itex]1/x[/itex] and [itex]exp(1/x)[/itex] are not analytic at [itex]x=0[/itex], so those are not counterexamples. The definition of a function [itex]f(x)[/itex] being analytic at [itex]x=0[/itex] is that there is a convergent Taylor series of the form [itex]f(x) = a + b x + c x^2 + ...[/itex]. [edit] (With only positive powers of [itex]x[/itex])
    Last edited: Feb 22, 2016
  24. Feb 22, 2016 #23
    My point was not about functions being analytic.

    My point is that qualitative differences are more likely to appear when you go from 0.0001 to -0.0001 than when you go from 0.5001 to 0.4999. Often, sign matters a lot.
  25. Feb 22, 2016 #24


    User Avatar
    Staff Emeritus
    Science Advisor

    Right, but the sign change making a huge difference is a sign (no pun intended) that things are not analytic in whatever parameter you're considering. Not analytic implies that whatever it is can't be expanded in a convergent power series.
  26. Feb 28, 2016 #25
    I do not know much about this stuff but according to the wiki article on Landau poles (https://en.wikipedia.org/wiki/Landau_pole), some russian guy recently showed that the beta function does not blow up in QED. Isn't this in contrast with the calculations discussed in the text?

    Also somewhat related, in the appendix of Robert Helling's http://homepages.physik.uni-muenchen.de/~helling/classical_fields.pdf he shows that in ordinary ## \phi^{4} ## theory with a weird sign we have solutions in 1d to the equations of motion given by ## \phi \sim tanh(x) ##. He then makes the comment

    Is there any way to make these comments a bit more concrete? A simple calculation for example.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted