Re: CPT theorem

From: John Baez (
Date: 08/18/04

Date: 18 Aug 2004 12:28:43 -0400

In article <>,
Perfectly Innocent <> wrote:

>We agree that every particle with spin determines a plane in space.
>Well, every plane uniquely determines two opposing directions. If a
>rare type of particle manifests P symmetry violations by having the
>direction of emission of a daughter particle favoring the two opposing
>directions symmetrically, then that's not terribly inconceivable, is

Heh, no, it's not inconceivable. Not anymore. In fact this sort
of thing was first seen in 1956, and people won Nobel prizes for it -
although the actual discover of the effect, Madame Chien-Shiung Wu,
did not:

> It wouldn't be a violation of anything.

Yes, it would be a violation of P symmetry, the symmetry between
left and right - as you mention yourself! Once people thought
the world had this symmetry. It came as a huge shock to discover
it didn't. Now we know it doesn't. Now it's not so surprising anymore.

Lee and Yang proposed the first experiment to detect parity
violation, and Wu did it. It was one of the most dramatic episodes
in the history of physics. To quote the above site:

Even before Lee and Yang's paper had been submitted to The Physical
Review, Lee had discussed the experiment with Wu. At the time, Wu and
her husband had planned a trip to Europe and the Far East. But she
chose instead to remain and perform the experiment rather than lose the
opportunity to other physicists who might recognize its importance.
However, the experiment could not be performed with only her expertise.
Reaching the low temperatures necessary to be able to orient the cobalt
nuclei spins required equipment few laboratories possessed. Nevertheless,
one such laboratory existed in the United States --- the Cryogenics
Physics Laboratory at the National Bureau of Standards in Washington.
Early in June of 1956, Wu sought the help of Ernest Ambler at NBS.
Ambler accepted enthusiastically. Indeed his doctoral thesis dealt
with the orientation of cobalt-60 nuclei. In addition, Ralph Hudson,
with expertise in cryogenics, and Raymond Hayward and Dale Hoppes,
with experience in radiation detection, joined the team. By early
October they began to assemble and test their equipment. The same
month saw the publication of Lee and Yang's paper.

The experimental problems were enormous. Temperatures as low as one
hundredth of a Kelvin were necessary to attain a high degree of spin
orientations for the cobalt nuclei. While such temperatures could be
reached through a process called adiabatic demagnetization, maintaining
the super coldness posed quite a problem for the group. Another problem
was leaks in the apparatus --- the experiment required the detectors and
cobalt sample to be placed in a vacuum. Nevertheless, after reconstructing
their equipment, several trials, and the use of cotton thread, the
experiment finally succeeded. The day was December 27, 1956.

News of the success reached Lee and Yang. At Columbia, in those
days, many of the physicists would gather on Fridays for "Chinese
lunch" under the supervision of T. D. Lee. When Lee, during such
an occasion, announced that positive results to parity violation
were being given by Wu's group, the physicist Leon Lederman was
among those present. Lederman, who worked with Columbia's cyclotron,
realized that he could perform an independent test of parity with
the cyclotron. His experiment, which involved the decay of pi and
mu mesons, had also been proposed by Lee and Yang in their paper.
Soon, Lederman, along with his graduate students, Marcel Weinrich,
and Richard Garwin began their experiments. At the same time, the
group under Wu was running into problems. Wanting to verify their
results from December 27, they repeated the experiment. Their
original finding of a large asymmetry in the beta ray distribution
was not consistently reproducible. However, after a week of solving
problems with the apparatus, consistent results were obtained. And
the results pointed to parity violation. Much consideration was
given to the question of the origin of the beta ray asymmetry ---
was it really an indication of the failure of parity or some result
intrinsic to the experiment? "The group worked around the clock,
assembling the apparatus many times, and took their breaks for a
few hours sleep when the superfluid helium spoiled their vacuum by
finding its way around the stopper at the bottom of the cryostat.
Hoppes then slept beside the apparatus, telephoning to the others as
soon as its temperature was low enough to begin their experiments again.
Finally, on Januray 9th, at 2 o'clock in the morning, Hudson brought
out a bottle of Chateau Lafite-Rothschild, 1949, and they drank to
the overthrow of the law of parity". As the closing door to the question
of parity violation in weak interactions, results from Lederman's
group at the cyclotron came quickly. They too had obtained distinct
evidence for parity violation. Both groups submitted their papers
together to The Physical Review on January 15, 1957. On that day,
Columbia called for a press conference.

As newspaper headlines told of a physics principle demolished,
startled reactions emerged from the physicists. Feynman had lost his
bet (and fifty dollars). From Zurich, Wolfgang Pauli wrote to Victor
Weisskopf at MIT, "Now after the first shock is over, I begin to
collect myself. Yes, it was very dramatic." At Columbia's press
conference, Isador Rabi said, "A rather complete theoretical
structure has been shattered at the base and we are not sure how
the pieces will be put together". Credulity of parity nonconservation
had taken hold among physicists.