On 1/11/2019 6:01 AM, John Clark wrote:
On Thu, Jan 10, 2019 at 8:18 PM Brent Meeker <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:

    /> The fine structure constant is e^2/hbar*c.  Those three values
    are measured independent of any Feynman diagrams/

Absolutely correct. So if you use Feynman diagrams to predict what some physical system is going to do, such as a physical system of 2 electrons being hit by a photon of light with a wavelength small enough to contain enough energy to prevent the electrons repulsion, then you'd better get a number very close to the Fine Structure Constant. If you don't then Feynman Diagrams aren't any good.

They didn't use 12,672 Feynman Diagramsbecause they wanted to know what the Fine Structure Constantwas, they already knew what that number was to many decimal places from exparament, they used 12,672 Feynman Diagramsbecause they wanted to see if Feynman Diagrams worked. And it turned out they worked spectacularly well in that situation, and that gives scientists great confidence they can use Feynman Diagrams in other situations to calculate what other physical systems will do that involve the Electromagnetic Force.

There's always an interplay between theory and experiment.  It's completely analogous to Maxwell's discovery that light is EM waves. There were already experimental values of the permittivity and permeability of the vacuum and there were values for the speed of light.  Maxwell showed that his theory of EM predicted waves and using the permittivity and permeability values the speed of the waves matched that of light.  Now the speed of light is a defined constant and so are the permittivity and permeability of the vacuum.  So the connecting of the three values by a theory allows their values to be defined.  In the case of the anomalous magnetic moment of the electron, hbar and c are already defined constants. So quantum field theory (for which Feynman diagrams are just a calculational tool) linked them and e to g.


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