M2L9c.txt

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What I want to discuss next is give you
a little bit of an historic summary how spectroscopy
of hydrogen has developed.
In particular, also focusing on one important discovery,
the discovery of QED through the Lamb shift.
And of course, we all love hydrogen
because it's the simplest atom, but it
has so much interesting physics in it.
So I've summarized for you here some papers on hydrogen.
And I used it to illustrate several points.
I will show you that actually the discovery of the Lamb shift
had precursors.
10 years before the Lamb shift was discovered,
people had even some idea that something
may be wrong with the understanding
of the structure of hydrogen.
So you can say they came so close
-- people 10 years before --- in realizing the Lamb shift.
There were people who may have missed the Nobel Prize by just
a tiny little bit.
They had all the insight that there
may be quantum / QED corrections in hydrogen. They
just didn't have the technology to measure it
accurately enough.
The second example I want to show
is that we always talk about fundamental limitations.
But fundamental limitations can disappear in time
because they may not be as fundamental as they appear.
So for instance, there were limitations in with
the measurement of the Lamb shift because you had a short
lifetime of p states But with the advent of two-photon
transition, you can go from s to s and s to d
states, and therefore, map out Lamb shift with much,
much higher precision not limited by the finite lifetime
of p states.
Or finally, you would say Lamb shifts are small splittings.
And for many, many years, Lamb shifts
were measured by making radio frequency transitions
between 2s and 2p states.
Well, today, the most accurate measurement of the Lamb shift
is with an optical transition where
you need a much, much higher relative precision
to see the tiny Lamb shift.
But optical metrology with direct frequency measurements
and frequency combs has so much improved in precision that
now, an optical measurement, even if it comes to tiny
difference is more accurate than a direct RF
measurement.
So I think that the history of hydrogen
shows you that technology can completely
change the paradigm how measurements are made.
Fundamental limits disappear because new tools
or new insight is available.
And also, I find it interesting that discoveries often
have precursors, and people have a hunch, know about it,
and then finally it is discovered.
So let me just step you through some historic papers.