M2L9d.txt

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So this paper is 1933.
Fifteen years before the Lamb Shift.
And it says that one possible explanation
for some discrepancy of the structure of the Balmer Lines
is the effect of the interaction between the radiation field
and the atom has been neglected.
That's QED.
You cannot just calculate the structural of the hydrogen atom
from the Coulomb Field.
You have to allow the radiation field, all the modes
of the vacuum to be included.
So this insight is not due to Lamb, it was there in already,
1933.
Same year, look at the title, "On the Breakdown
of the Coulomb Law for the Hydrogen Atom."
People speculated or discussed that the Coulomb Law will not
be valid at very small distances.
And this is ultimately what QED, radiative correction, the Lamb
Shift, vacuum polarization, and all that brings about.

Finally, people had an understanding of the hydrogen
atom, and they measured-- and I want you to sort of keep
that in mind-- they did optical--
they measured the Balmer lines of hydrogen and deuterium.

And then they couldn't fully resolve it
because of the finite lifetime of the P state.
But there was some hunch when you
tried to get the envelope from the underlying structure that
was a discrepancy.
It was just not significant enough to say for sure
there is an additional line shift, which
is not accounted for by theory.
There was a discussion that there
is a deviation of the Coulomb Law, but here is the insight.
Or as was indicated by previous authors,
the interaction required to change the Coulomb Law
at small distances is much too large
to be accounted for by the assumption of a finite size
of electron and proton.
So the Coulomb Field has to be modified
at short distances in a much stronger way than just
the finite size of the proton.
We talk about the finite size of the proton in a minute.
And then of course, 1947, new RF oscillators
have been developed in the pursuit of radar,
experimental tools are there now.
High power RF sources, tunable sources, and such.
And then Lamb and Retherford in this landmark paper
look at the fine structure of the hydrogen atom,
and this is the famous result. They measured transitions
as a function of magnetic field, and you
see the solid line which I think was the theory without the Lamb
Shift.
The dashed line is the hyper fine structure of hydrogen,
and the lines converge.
And the difference is 1,000 megacycles.
The first determination of the Lamb Shift.
So this was 1947.
It's interesting it's just one or two weeks later
there is a theoretical paper by Hans Bethe providing
an explanation for the Lamb Shift.
So already coming up with the first model
how to account for QED.
I didn't look it up in detail, but I thought the spirit
was similar to what I presented to you
in class, that the electromagnetic zero point
energy is shaking the electron and leading to corrections.
But it's amazing that within weeks, theorists
figured really out, yes, this is the explanation and this is how we have to extend the theory.