On Wed, Jan 9, 2019 at 10:53 PM Bruce Kellett <bhkellet...@gmail.com> wrote:

> > *There is no theoretical value".*

>From a book on spectrography by Norman Ramsey discussing the lamb Shift,
the very thing that led to the discovery of the Fine Structure Constant in
the first place:

"*The atomic hydrogen hyperfine separation can be accurately calculated
from theory so comparison to the experimental value provides a crucial test
for a fundamental theory. Historically it was the disagreement between the
theoretical and experimental value of this quantity that stimulated
Schwinger's development of relativistic quantum electrodynamics (QED)*."

Lamb Shift

>> Nobody uses 12,672 Feynman Diagrams to find a measured result.
> >The authors of the PRL paper did!

I don't know what the "PRL paper" is, I'm talking about the same paper I
referred to before:

Improved Value of the Fine Structure Constant

And that paper specifically discusses what they need "*To compare the
theoretical prediction with the measurement*". It also makes clear the
importance of testing the theoretical predictions made by QED against
actual measurements:

"*The anomalous magnetic moment a e ≡ ( g −2) /2 of the electron has played
the central role in testing the validity of quantum electrodynamics (QED)*"

They also make it clear then are not the ones who made the experimental

 "*On the experimental side the measurement of the Harvard group has
reached the astonishing precision*"

But they've calculated a even more accurate value than the experimenters:

 "*we have managed to evaluate it with a precision which leads to theory
more accurate than that of the measurement*"

So now it's up to the experimenters to do better and see if the theoretical
prediction is still correct.

>  > *in the final analysis, the fine structure constant is an arbitrary
>>> physical constant that must be measured*
>> So is the speed of light, but Maxwell's theory can calculate that speed
> > *Maxwell's theory gives the speed in terms of the permittivity and
> permeability of the vacuum, both of which were measured quantities in
> Maxwell's day. *

And they still are. if those 2 values were different the speed of light
would be different and the General Theory of Relativity would be different.
And if the Fine Structure Constant were different then Feynman's theory
would have to be different because the existing rules on how the diagrams
work would have produce a result that disagreed with experimental results.

Feynman didn't devise his rules specifically to produce a number close to
137 and there is no obvious reason to think that it would, and yet it does
and it agrees with the measured value to a astonishing degree; 40 years ago
he bragged that his theory was like predicting the distance between New
York and LA to the distance of a human hair, but measurement has gotten
about 60 times better since then so today it more like the distance between
New York and the moon. It's the most accurate prediction in all of science.
He found all this very mysterious and so do I.

> *> But, because of the success of special relativity, they are nowadays
> defined constants, as is the speed of light. *

Maxwell's theory requires modification because of Quantum Mechanics but as
far as special relativity is concerned absolutely nothing about Maxwell
needs to change.

> > *One could, therefore say that the speed of light is a theoretical
> value, not a measured value.*

Yes, people can say anything regardless of how silly. If the speed of light
or any other law of physics were different our theories would be different,
they'd have to be because theories must dance to experiment's drum not the
other way around.

*> You really ought to read the Wikipedia article more carefully, rather
> than just using it to obtain the CODATA best-fit value, and the value
> measured by the latest g-2 experiment. (Yes, the one calculating all 12,672
> Feynman diagrams to the tenth order.)*
> *"The most precise value of α obtained experimentally (as of 2012) is
> based on a measurement of g using a one-electron so-called "quantum
> cyclotron" apparatus,*

Right, that's how you get the measured value and it turns out to be

> > together with a calculation via the theory of QED that involved 12672
>  tenth-order Feynman diagrams:[
> *α*−1 = 137.035999173(35)."

Right, that's how you get the value Feynman's diagrams predict and it's
137.035999173 in excellent agreement with measurement and therefore we
conclude that Feynman's theory was a good theory.

I asked this before but got no answer, if checking a theoretical prediction
against a measured value is not the way to tell the difference between a
good physical theory and a bad one what on earth is?

> *So that value is obtained experimentally, i.e., a measured result.*

Obviously a value obtained experimentally is a measured result.

> *> It is no more a theoretical value than is the value of the mass of the
> electron*

Before Feynman the theoretical value of the electron's mass was infinite
because it had infinite self energy, and that is obviously ridiculous and
was a embarrassment to physics.  QED still can't calculate the electron's
mass but at least it can show reasons why it's not infinite.

> > *(or the mass of the sun, for that matter.)*
> *As Feynman says:*
> *Immediately you would like to know where this number for a coupling comes
> from: is it related to pi or perhaps to the base of natural logarithms?
> Nobody knows. It's one of the greatest damn mysteries of physics: a magic
> number that comes to us with no understanding by man. You might say the
> "hand of God" wrote that number, and "we don't know how He pushed his
> pencil." We know what kind of a dance to do experimentally to measure this
> number very accurately, but we don't know what kind of dance to do on the
> computer to make this number come out, without putting it in secretly!*
> *So even Feynman knew that there was no theoretical value for the FSC,
> alpha.*

No,  he knew very well there was a theory that could come up with a value
because his own Feynman Diagrams could do it. But what he didn't know and
what nobody knows is why his theory came up with that particular pure
number when he never specifically stuck that number into the rules on how
the diagrams should operate.

Before QED you could make first order approximations about what a electron
would do but when you tried to be more precise you'd get nonsense results
like infinite energy or negative probability or all probabilities not
adding up to one. This problem had been know for 20 years before Feynman
but until Willis Lamb discovered very fine lines in the spectrum of
hydrogen the crude approximations were good enough for all
experimental results.

Feynman and others (although Feynman's method was by far the easiest to
calculate) found a way to actually get finite numbers out and probabilities
that always added up to one. So his ideas made mathematical sense and they
produced finite numbers, but did they produce the correct finite numbers,
did it make any physical sense? The only way to know is to perform
experiments and see if predictions conformed with observation. They did and
did so to a spectacular degree.

Feynman was never entirely comfortable with his method even though it
produced the correct numbers, he referred to it as "*sweeping the
infinities under the rug*". He also said:

"I still get nervous with it. [...]  It has not yet become obvious to me
that there's no real problem. I cannot define the real problem, therefore I
suspect there's no real problem, but I'm not sure there's no real problem."

John K Clark

You received this message because you are subscribed to the Google Groups 
"Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.

Reply via email to