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

Varying Fine Structure Constant?

  1. Oct 29, 2004 #1
    This question may have been asked a thousand times on here so please diredt me to the relevant post if that is the case.
    During a particle physics lecture I was told that although the fine structure constant is widely regarded as being 1/137 at normal lab energies, it seems to tend towards 1/128 at high energies.

    The lecturer refused to tell us why this occurred and even told us to ask other lecturers to watch them get uncomfortable.

    Anyway the best I got was look in Nature.
    Found out that there is evidence that it used to be lower in the past, but wondered if anybody could elaborate on which of the fundamental constants used to determine the fine structure constant varied at high energies.
  2. jcsd
  3. Oct 29, 2004 #2


    User Avatar
    Gold Member

    What is the question? Why does it vary with energy, or why does it seem to tend towards 1/128 (which is not exactly true, by the way)?
  4. Oct 29, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2018 Award

    There is only one "apparent" observation that the fine structure constant (alpha) might have been different in the past, and that was based on astrophysical observation by J.K. Webb et al. However, since then, there have been a number of measurements of alpha, that have contradicted that. For an overview, please read Oct. 2004 issue of Physics Today, Page 40. In fact, just this week, a more stringent upper limit on the possible variation of alpha was published in PRL [E. Peik et al, PRL v.93, p.170801 (2004)] that throws doubt into what Webb et al. measured.

    So if we are to go by on the experimental data alone to-date, there are more overwhelming evidence that alpha hasn't been changing.

  5. Oct 29, 2004 #4
    Sorry, but what does et al mean? Please tell me it's not part of his name.

  6. Oct 29, 2004 #5
    You have for example BSBM theory, a theory of varying alpha. In this theory, c and hbar are constants, while e varies. More info here

  7. Oct 30, 2004 #6


    User Avatar
    Science Advisor

    No, the fine structure constant does change, even in vanilla field theory (this drives me absolutely nuts, crackpots always like to assign a value to something that was never intended to have a 'constant' value) .

    There can be several well understood reasons for this.

    1) The renormalization group flow

    In QED coupling constants will run based on regularization schemes. Physically you can picture this as having a bunch of positron-electron pairs popping into existence around the vacuum, and creating a dielectric effect around the electron (shielding it from photon interactions).. The effective coupling constant will thus increase as energy goes up. The point.. Coupling constants scale with energy.

    2) Thermodynamic contamination
    For closely related reasons, sometimes (I should say always) vacuum values are contaminated by experiments outside absolute zero. Theoretically this is usually absorbed into the interaction constants (to first order). Thus, again, 'constants' will scale with temperature as well.
  8. Oct 30, 2004 #7

    Hans de Vries

    User Avatar
    Science Advisor
    Gold Member

    You should blame it all on Feynman then. He was perfectly aware of the vacuum polarization and charge renormalization issues you're mentioning here. After all, that was part of the work for which he earned the Nobel price. Still he urged physicist to investigate. In fact he mentioned both the shielded and unshielded coupling value when he did so, including the reason why they are different....

    The fact that a value seems to settle down with a precision of at least 10,11 digits throughout the universe and throughout time is not necessarily meaningless.

    Regards, Hans

    (edit) PS. The long distance value of the fine structure constant stays the same up to the GUT unification level. Where it changes is at the short distances corresponding to the energy.

    (Just imagine what it would mean, if the the proton's field was slighty stronger than the electron's field, for objects like the earth or the sun).
    Last edited: Oct 30, 2004
  9. Oct 30, 2004 #8
    Structure Lecture...

    While on QED topic, what is the equation that defines the unshielded classical Electroweak fine structure constant?

    I am looking for an equation that describes the classical case:

    [tex]\propto_s = \frac{f^2}{\hbar c}[/tex] - strong
    [tex]\propto_e = \frac{K_e q^2}{\hbar c}[/tex] - electromagnetism
    [tex]\propto_{ew} = \frac{g^2}{\hbar c}[/tex] - electroweak ???
    [tex]\propto_w = \propto_s \sqrt{ \frac{T_\Delta}{T_\Sigma}}[/tex] - weak
    [tex]\propto_g = \frac{G m_p^2}{\hbar c}[/tex] - gravity

    What is the actual value for the unshielded classical Electroweak fine structure constant?

    Is the actual value for the unshielded classical Electroweak fine structure constant equal to the Fermi coupling constant [tex]G_F[/tex]?

    [tex]\propto_{ew} = G_F[/tex] ???


    Last edited: Oct 30, 2004
  10. Oct 31, 2004 #9


    User Avatar
    Science Advisor
    Gold Member

    This is one of my personal favorites on this subject
    Many others also exist pointing to the conclusion the fine structure constant has not measurably changed for billions of years, if ever.
  11. Nov 6, 2004 #10
    et alanes (Latin)
    and others (<- not 100% sure about the translation)
    Last edited: Nov 6, 2004
  12. Nov 6, 2004 #11


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Should be et al = et alia. Alia is the plural of alium, meaning another thing. You can also have et alii with the plural of alius, another person. Strictly speaking you should pay attention to what kind of list you're summarizing: things or people? But nobody in these degenerate latter days gives a hoot about that.
  13. Nov 8, 2004 #12
  14. Nov 8, 2004 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2018 Award

    Consider that the paper that I cited earlier [E. Peik et al, PRL v.93, p.170801 (2004)] is more recent, is published in PRL, has a higher certainty than measurement from a quasar absorption, is doing exactly what they suggest should be done ("A laboratory measurement of how these transition frequencies vary over time is vital in determining whether alpha is varying today"), and contradicts their findings, which one would you put more credibility on?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook