# Re: What is comparable and incomparable between casually disconnected universes?

`On Thu, Jan 10, 2019 at 2:30 PM John Clark <johnkcl...@gmail.com> wrote:`
```
> On Wed, Jan 9, 2019 at 9:49 PM Bruce Kellett <bhkellet...@gmail.com>
> wrote:
>
> > *There is no theoretical value".*
>>
>
> The measure value is 137.035999139, the value obtained from 12,672 Feynman
> Diagrams is 137.035999173. If you don't like the name "theoretical value"
> for that second number then call is something else. How about "Bob"?
>
> *> All the values that we have are measured -- often in different ways, or
>> from the results of different experiments to measure the same things, such
>> as g-2, so there can be a range of measured results.*
>
>
> Nobody uses 12,672 Feynman Diagrams to find a measured result.
>

The authors of the PRL paper did!

>
>
>>  > 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. But, because of the success of special relativity, they are
nowadays defined constants, as is the speed of light. So there is no such
comparison as between the calculated speed and the measured speed of light
-- the speed of light is a defined exact constant which is used to define
the relationship between the units of time and distance. One could,
therefore say that the speed of light is a theoretical value, not a
measured value.

and the fact that the calculated speed agrees with the measured speed tells
> us that Maxwell had a good theory. The reason scientists went to the
> considerable trouble of calculating the Fine Structure Constant from 12,672
> Feynman Diagrams when they already knew from measurement what the correct
> answer is was to test the theory and see if it still worked at that
> incredible degree of accuracy. And It did work. Why else would physicists
> have such enormous confidence in Feynman's diagrams? How else can you tell
> the difference between a good physical theory and a bad one?
>

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, together with a calculation via the theory of QED
that involved 12672 tenth-order Feynman diagrams
<https://en.wikipedia.org/wiki/Feynman_diagrams>:[6]
<https://en.wikipedia.org/wiki/Fine-structure_constant#cite_note-6>
*α*−1 = 137.035999173(35)."
So that value is obtained experimentally, i.e., a measured result. It is no
more a theoretical value than is the value of the mass of the electron (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.

Bruce

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