Last revised on Thursday, 10-Jan-2008 06:44:14 EST
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In 1947 Cecil F. Powell photographed cloud chamber tracks
of charged particles from space descending upon the Andes
mountains. In those photographs Powell identified the
pi meson --- the particle postulated twelve years earlier by
the physicist Hideki Yukawa, as the intermediary for the
nuclear force. Two years later Powell would discover new
particles with his cloud chamber, and the pi meson would
play a crucial role in the postulation of a new force, the
weak interaction. Both of these discoveries were to
lead physicists of the mid-twentieth century to a contradiction.
Its resolution involved the overthrow of cherished
conservation law, and, as relativity challenged common
sense earlier, once again physicists would be shocked by
the fallibility of their intuitions.
Symmetries have long played a crucial role in physics. The
conservation laws of the past had more fundamental roots
within the symmetry of the universe. Such laws as conservation
of linear and angular momentum arise from an even more
fundamental requirement: physical laws are invariant under
translation and rotation[1]. The law of conservation
of parity arose from the symmetry between the left and right
hands.
In 1848 Louis Pasteur discovered, almost by sheer luck, the
property of optical isomerism. Two forms of the same
chemical compound, isomers, were found to rotate
polarized light in two different directions --- one to the left,
the other to the right. Isomers are essentially identical
chemical compounds. They have the same number and type of atoms
and the same structure, almost. The difference in the two isomers
of a compound is that one is the mirror image of the other.
This is the same symmetry that exists between the left and
right hands. Pasteur observed as well that living organisms
were able to synthesize and use only one isomer and never the
other. But nature itself appeared to have no preference over
which form it produced --- in reactions the isomers were produced
in equal quantities. That is, nature appears to exhibit complete
symmetry between the left and right. Until 1957 physicists
believed this symmetry to hold for all physical processes. A
mirror image of any reaction should be identical in every way
to the actual reaction. This idea was intuitive to the physicist ---
what could it mean if nature preferred left over right or
vice-versa?
To describe more precisely the symmetry between left and right,
physicists used the concept of parity. Parity originated
with the development of quantum mechanics. In 1924 O. Laporte
classified the wavefunctions of an atom as either even or odd,
depending upon the symmetry of the wavefunction. Laporte discovered
that when an atom transitions from one state to another and a photon
is emitted, the wavefunction changes from even to odd or vice-versa,
but never remains the same [1]. Even functions were
defined to have parity of +1 and odd functions a parity of
-1. In addition, the emitted photon was defined to have parity of
-1. Laporte's rule could then be stated as a conservation law, the
conservation of parity. For mathematical reasons, the parity of any
system is the product of the parities of the individual components.
Parity is conserved in atomic transitions. If the initial wavefunction
was odd (-1 parity), Laporte's rule asserts the final wavefunction
must be even (+1 parity). Since the initial system has -1 parity
and the final system has as its parity the product of parities
of the final wavefunction and the parity of the emitted photon,
(+1)(-1) = -1, parity is conserved in the transition. The
parities of the inital and final wavefunctions can be interchanged;
conservation of parity will still hold.
The importance of parity conservation, its fundamental nature, was
discovered in 1927 by the physicist Eugene Wigner, Wigner proved
that Laporte's rule was a consequence of right-left symmetry (or
mirror image symmetry) of the electromagnetic force[1].
Conservation of parity rested upon Maxwell's equations describing
electromagnetism, but more important, the intuitive idea that nature
should be left-right symmetric had been established on the quantum
level. Thus, when in 1949, the weak force was postulated to
explain disintegration of elementary particles, physicists could not
conceive that parity conservation would not hold for reactions
involving the weak force. It was a minor oversight however that
there was no direct evidence for the extension of this law to
the fourth force of nature. Seven years later physicists would
come full circle to question their acceptance of parity conservation.
The same war that would later finance the particle accelerators
in the United States would also bring together two young Chinese
students. In 1943 Tsung Dao Lee was a student in the Kweichow
province of China. It was the time of the Sino-Japanese War, and
the Japanese invasion of the mainland forced Lee to move to
Kunming. There he attended the National Southwest University where
he met Chen Ning Yang [2]. Lee and Yang had only a
nodding acquaintance then. In 1946 both students received
fellowships to study in the United States. Yang had pursued Enrico
Fermi from Columbia to the University of Chicago --- he was to have
a close association with Fermi. Lee, on the other hand, had little
choice. Only one school in the U.S. then allowed an undergraduate
to work towards the PhD without the intermediate degrees, the
University of Chicago. The two graduate students fast became friends.
For a while Yang had tried experimental physics, but it was not to be.
Other graduate students had teased him, "Where there was a bang, there
was Yang" [2]. Yang eventually did his doctoral thesis
under the supervision of Edward Teller. Lee on the other hand knew
he was a theorist from the start. He did his doctoral thesis under
Fermi. Yang recalls Fermi's advice on his career: As a young man,
work on practical problems; do not worry about things of fundamental
importance [3]. For all of his admiration of Fermi, Yang
chose to ignore this bit of advice. Both Lee and Yang graduated and
for awhile worked as staff members at the Institute for Advanced
Study in Princeton. Lee had become a reputable theoretical physicist,
invoking praise from J. Robert Oppenheimer as "one of the most
brilliant theoretical physicists then known" [4]. Thus the
individual physicists T. D. Lee and C. N. Yang had established
their reputations by 1956, when their work together would help
clear a mystery known as the theta-tau puzzle and topple of
the most fundamental conservation laws.
Within the cosmic rays in which C. F. Powell had discovered the
pi meson (pion) were other new particles. In 1949 Powell identified
a cosmic ray particle which disintegrated into three pions. He dubbed
this new particle the tau meson. Another particle called the
theta meson was also discovered. It disintegrated into
two pions. Both particles disintegrated via the weak force. Now,
a problem arose when the masses and the lifetimes of the tau and
theta particles were considered. The two particles turned out to
be indistinguishable other than their mode of decay. Their masses and
lifetimes were identical, within the experimental uncertainties.
Were they in fact the same particle? The problem itself was not that
the tau and theta, if indeed they were the same particle, decayed
in two different modes, one by two pions, the other by three pions.
The problem dealt with the more fundamental parity conservation law.
In 1953 the physicist R. H. Dalitz argued that since the pion has
parity of -1, two pions would combine to produce a net parity of
(-1)(-1) = +1, and three pions would combine to have total parity of
(-1)(-1)(-1) = -1. Hence, if conservation of parity holds, the
theta should have parity of +1, and the tau of -1. Hence,
they could not be the same particle [5]. Thus was born the
theta-tau puzzle. It's resolution would involve an almost unacceptable
proposition to the physicists of the time.
The events which led to the publication of Lee and Yang's historic
paper, Question of Parity Conservation in Weak Interactions,
began at the International Conference on High Energy Physics at the
University of Rochester in April 1956. Lee and Yang attended the
conference with a proposal for ending the theta-tau puzzle. Their
idea was that certain kinds of elementary particles occur in two
forms with different parities. The idea was called parity doubling
[5]. Also attending the conference was the theoretical physicist
Richard Feynman, who is renowned for his development of the field
of physics called quantum electrodynamics. Feynman's roommate
at the conference was the experimentalist Martin Block. Block suggested
to Feynman on the first night of the conference that parity just may
not be conserved in certain interactions. The next day, following
Yang's presentation of the parity doubling idea, Feynman brought up
the question of non-conservation of parity. Feynman himself later said,
"I thought the idea (of parity violation) unlikely, but possible,
and a very exciting possibility." Indeed Feynman later made a fifty
dollar bet with a friend that parity would not be violated [6].
Yang's reply was that he and Lee had considered the idea but had arrived
at no conclusions. During the discussion, Wigner, who had formulated the
law of conservation of parity in the first place, also suggested that
perhaps it did not hold in weak interactions [5].
Lee and Yang pursued the question further after the conference. "Early
in May, when they were sitting in the White Rose Cafe near the corner
of Broadway and 125th Street, in the vicinity of Columbia University,
it suddenly struck them that it might be profitable to make a careful
study of all known experiments involving weak interactions" [6].
After several weeks of reviewing past experiments, they had come to
two conclusions:
When Lee and Yang's paper appeared in the October 1, 1956 issue of
The Physical Review, physicists were not immediately prompted into
action. The proposition of parity nonconservation was not
unequivocally denied; rather, the possibility appeared so unlikely
that experimental proof did not warrant immediate attention. The
physicist Freeman Dyson wrote of his reaction to the paper:
"A copy of it was sent to me and I read it. I read it twice. I said,
`This is very interesting,' or words to that effect. But I had not
the imagination to say, `By golly, if this is true it opens up a whole
new branch of physics.' And I think other physicists, with very few
exceptions, at that time were as unimaginative as I." [6].
Hence, the initial reaction among most physicists to verifying parity
conservation was not enthusiastic.
In their paper, Lee and Yang stated, "To decide unequivocally whether
parity is conserved in weak interactions, one must perform an experiment
to determine whether weak interactions differentiate the right from the
left." [7]. And they proposed several experiments. One of the
simplest experiments (conceptually) invovled measurements on the beta
decay of cobalt-60. The idea involved orienting cobalt nuclei with a
strong magnetic field so that their spins are aligned in the same
direction. Beta rays (electrons) are emitted at the poles of the nuclei.
A mirror image of the system would also show beta rays being emitted from
the poles of the mirror cobalt nuclei, the only difference being that
the north and south poles of the mirror nuclei would be reversed since
they spin in opposite direction of their real counterparts. Hence parity
conservation demands that the emitted beta rays be equally distributed
between the two poles. If more beta particles emerged from one pole
than the other, it would be possible to distinguish the mirror image
nuclei from their counterparts. Thus an anisotropy in the emitted
beta rays would be tantamount to parity violation.
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, maintaing
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 [9].
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 [5]. 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" [9]. 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" [6].
Credulity of parity nonconservation had taken hold among
physicists.