emulsions, and a series of brave researchers dragged their equipment
up to high elevations to search for possible new signals. Many
cosmic rays interact and disappear before reaching sea level, so this
group and others interested in exploring this wondrous new source
of particles coming from the heavens had no choice but to seek
higher elevations. Here cosmic rays would have traversed less
distance in the atmosphere and might be more easily detected.
The former Italian mountain guide turned physicist Giuseppe
Occhialini had been invited from Brazil to join a British team
working on the A-bomb during the war. As a foreign national, he
couldn’t work on the project, so instead he joined the cosmic-ray
physics group at Bristol. Occhialini’s mountain training proved
useful as he dragged photographic emulsions up to the Pic du Midi
at twenty-eight hundred meters in France. Today you can travel to
the observatory on top of this peak by cable car, and it is a
terrifyingly exciting ride. But in 1946 Occhialini had to climb to the
top, risking his health in the effort to discover signals of exotic new
physics.
And he and his team did discover exotic new physics. As Cecil
Powell, Occhialini’s collaborator at Bristol (and future Nobel
laureate, while Occhialini, who had done the climbing, did without),
put it, they saw “a whole new world. It was as if, suddenly, we had
broken into a walled orchard, where protected trees flourished and
all kinds of exotic fruits had ripened in great profusion.”
Less poetically, perhaps, what they discovered were two examples
in which an initial meson stopped in the emulsion and gave rise to a
second meson, just as had been suggested by the theorists. Many
more events were observed with emulsions taken to an elevation
almost twice as high as Pic du Midi. In October of 1947, in the
͢͜͝
journal Nature, Powell, Occhialini, and Powell’s student Cesare
Lattes published a paper in which they named the initial meson the
pion—which seemed to interact with the nuclear strength
appropriate to Yukawa’s meson—and the subsequent meson the
muon.
It seemed at long last that Yukawa’s meson had been discovered.
As for its “partner” the muon, which had been confused with
Yukawa’s meson, it was nothing of the sort. Not spinless, it instead
had the same spin as the electron and the proton. And its
interactions with matter were nowhere near strong enough to play a
role in nuclear binding. The muon turned out to be simply a heavy,
if unstable, copy of the electron, which is what motivated Rabi’s
question “Who ordered that?”
So, the particle that made Yukawa famous wasn’t the particle he
predicted after all. His idea became famous because the original
experimental result had been misinterpreted. Fortunately, the Nobel
committee waited until the 1947 discovery of the pion before
awarding Yukawa their prize in 1949.
But, given the track record of errors and mislabeling, it is natural
to wonder if the pion was in fact the particle Yukawa had predicted.
The answer is both yes and no. Exchange of charged pions between
protons and neutrons is indeed one accurate way of trying to
estimate the strong nuclear force holding nuclei together. But in
addition to charged pions—the mesons that Yukawa had predicted—
there are neutral pions as well. Who ordered those?
Moreover, the theory that Yukawa wrote down to describe the
strong force, like Fermi’s theory to describe neutron decay, was not
fully mathematically consistent, as Yukawa had conceded when he
proposed it. There was, at the time, no correct relativistic theory
involving the exchange of massive particles. Something was still
amiss, and a series of surprising experimental discoveries, combined
͢͝͝
with prescient theoretical ideas that were unfortunately applied to
the wrong theories, helped lead to more than a decade of confusion
before the fog lifted and light appeared at the end of the tunnel. Or
perhaps at the mouth of the cave.
͢͝͞
C h a p t e r 1 2
M A R C H O F T H E T I TA N S
The wolf also shall dwell with the lamb, and the leopard shall lie
down with the kid.
—ISAIAH 11:6
The relationship between theoretical insight and
experimental discovery is one of the most interesting aspects of the
progress of science. Physics is at its heart, like all of science, an
empirical discipline. Yet at times brief bursts of theoretical insight
change everything. Certainly Einstein’s insights into space and time
in the first two decades of the twentieth century are good examples,
and the remarkable theoretical progress associated with the
development of quantum mechanics by Schrödinger, Heisenberg,
Pauli, Dirac, and others in the 1920s is another.
Less heralded is another period, from 1954 to 1974, which, while
not as revolutionary, will, when sufficient time has passed, be
regarded as one of the most fruitful and productive theoretical
physics eras in the twentieth century. These two decades took us, not
without turmoil, from chaos to order, from confusion to confidence,
and from ugliness to beauty. It’s a wild ride, with a few detours that
might seem to come from left field, but bear with me. If you find it a
tad uncomfortable, then recall what I said in the introduction about
science and comfort. By putting yourself in the frame of mind of
those involved in the quest, whose frustration eventually led to
insights, the significance of the insights can be truly appreciated.
͢͟͝
This tumultuous period followed one in which experimental
bombshells had produced widespread confusion, making nature
“curiouser and curiouser,” as Lewis Carroll might have put it. The
discoveries of the positron and quickly thereafter the neutron were
just the beginning. Neutron decay, nuclear reactions, muons, pions,
and a host of new elementary particles that followed made it appear
as if fundamental physics was hopelessly complicated. The simple
picture of a universe in which electromagnetism and gravity alone
governed the interactions of matter made from protons and
electrons disappeared into the dustbin of history. Some physicists at
the time, like some on the political right today, yearned for the
(often misremembered) simplicity of the good old days.
This newfound complexity drove some, by the 1960s, to imagine
that nothing was fundamental. In a Zen-like picture, they imagined
that all elementary particles were made from all other elementary
particles, and that even the notion of fundamental forces might be
an illusion.
Nevertheless, percolating in the background were theoretical
ideas that would draw back the dark curtains of ignorance and
confusion, revealing an underlying structure to nature that is as
remarkable as it is strangely simple, and one in which light would
once again play a key role.
It all began with two theoretical developments, one profound and
unheralded and another relatively straightforward but brilliant and
immediately feted. Remarkably, the same man was involved in both.
Born in 1922 to a mathematician father, Chen-Ning Yang was
educated in China, moving in 1938 from Beijing to Kunming to
avoid the Japanese invasion of China. He graduated four years later
from National Southwestern Associated University and remained
there for another two years. There he met another student who had
been forced to relocate to Kunming, Tsung-Dao Lee. While they
͢͝͠
only had a marginal acquaintance with the United States, in 1946
both of them received scholarships set up by the US government,
with funds received from China to allow talented Chinese students
to pursue graduate study in America. Yang had a master’s degree and
therefore had greater freedom to pursue a PhD, and went with Fermi
from Columbia to the University of Chicago. Lee had less choice, as
he did not have a master’s degree, but the only US university where
he could work directly toward a PhD was also the University of
Chicago. Yang did his PhD under the supervision of Edward Teller
and worked directly with Fermi as his assistant for only a year after
graduation, while Lee did his PhD with Fermi directly.
During the 1940s, the University of Chicago was one of the
greatest centers of theoretical and experimental physics in the
country, and its graduate students benefited from their exposure to a
remarkable set of scientists—not only Fermi and Teller, but others
including the brilliant but unassuming astrophysicist Subrahmanyan
Chandrasekhar. When he was nineteen, Chandra, as he was often
called by colleagues, had proved that stars greater than 1.4 times the
mass of the Sun must collapse catastrophically at the end of their
nuclear-burning lifetime, either through what is now known to be a
supernova explosion, or directly in what is now known as a black
hole. While his theory was ridiculed at the time, he was awarded the
Nobel Prize for that work fifty-three years later.
Chandra was not just a brilliant scientist but, like Fermi, a
dedicated teacher. Even though he was pursuing research at the
Yerkes Observatory in Wisconsin, he drove one hundred miles
round-trip each week to teach a class to just two registered students,
Lee and Yang. Ultimately, the entire class, professor included,
became Nobel laureates, which is probably unique in the history of
science.
͢͝͡
Yang moved to the venerable Institute for Advanced Study in
Princeton in 1949, where he nurtured his budding collaboration
with Lee on a variety of topics. In 1952 Yang was made a permanent
member of the institute, while Lee moved in 1953 to nearby
Columbia in New York City, where he remained for the rest of his
career.
Each of these men made major contributions to physics in a
variety of areas, but the collaboration that made them famous began
with a strange experimental result, again coming from cosmic-ray
observations.
In the same year that Yang moved from Chicago to the IAS, Cecil
Powell, the discoverer of the pion, discovered a new particle in
cosmic rays, which he called the tau meson. This particle was
observed to decay into three pions. Another new particle was
discovered shortly thereafter, called the theta meson, which decayed
into two pions. Surprisingly, this new particle turned out to have
precisely the same mass and lifetime as that tau meson.
This might not seem that strange. Might they be the same
particle, simply observed to decay in two different ways? Remember
that in quantum mechanics, anything that is not forbidden can
happen, and as long as the new particle was heavy enough to decay
into either two or three pions—and the weak force allowed such
decays—both should occur.
But, if it were sensible, the weak force shouldn’t have allowed
both decays.
Think for a moment about your hands. Your left hand differs
from your right hand. No simple physical process, short of entering
through the looking glass, can convert one into the other. No series
of movements, up or down, turning around, or jumping up and
down, can turn one into the other.
͢͢͝
The forces that govern our experience, electromagnetism and
gravity, are blind to the distinction between left and right. No
process moderated by either force can turn something such as your
right hand into its mirror image. I cannot turn your right hand into
your left hand merely by shining light on it, for example.
Put another way, if I shine a light on your right hand and look at it
from a distance, the intensity of reflected light will be the same as it
would be if I did the same thing to your left hand. The light doesn’t
care about left or right when it is reflecting off something.
Our definition of left and right is imposed by human convention.
Tomorrow we could decide that left is right and vice versa, and
nothing would change except our labels. As I write this on an
airplane, flying economy class, the person to my right may be quite
different from the person to my left, but again that is just an accident
of my circumstances. I don’t expect that the laws governing the flight
of this plane are different for the right wing than for the left wing.
Think about this in the subatomic world. Recall that Enrico Fermi
found that, given the rules of quantum mechanics, the mathematical
behavior of groups or pairs of elementary particles depends on
whether they have spin ½, i.e., are fermions. The behavior of groups
of fermions is quite different from the behavior of particles such as
photons, which have a spin value of 1 (or any integer value of spin
angular momentum, i.e., 0, 1, 2, 3, etc.). The mathematical “wave
function” that describes a pair of fermions, for example, is
“antisymmetric,” while one describing a pair of photons is
“symmetric.” This means that if one interchanges one particle with
another, the wave function describing fermions changes sign. But for
particles such as photons, the wave function remains the same under
such an interchange.
Interchanging two particles is the same as reflecting them in the
mirror. The one on the left now becomes the one on the right. Thus
ͣ͢͝
an intimate connection exists between such exchanges and what
physicists call parity, which is the overall property of a system under
reflection (i.e., interchanging left and right).
If an elementary particle decays into two other particles, the wave
function describing the “parity” of the final state (i.e., whether the
wave function changes sign or not under left-right interchange of
the particles) allows us then to assign a quantity we can call parity to
the initial particle. In quantum mechanics if the force that governs
the decay is blind to left and right, then the decay will not change the
parity of the
quantum state of the system.
If the wave function of the system is antisymmetric under
interchange of the particles after the decay, then the system has
“negative” parity. In this case the wave function describing the initial
quantum state of the decaying particle must also have negative parity
(i.e., it would change sign if left and right were interchanged).
Now, pions, the particles discovered by Powell and hypothesized
by Yukawa, have negative parity, so that the wave function that
describes the quantum state of their mirror image would change sign
compared to the original wave function. The distinction between
positive and negative parity is kind of like considering first a nice
spherical ball, which looks identical when reflected in the mirror,
and hence has positive parity:
Versus, say, your hand, which changes character (from left to right)
when reflected in a mirror and could therefore be said to have
negative parity:
ͤ͢͝
These somewhat abstract considerations made the observed data
on the decays of the new particles that Powell discovered perplexing.
Because a pion has negative parity, two pions would have positive
parity, since (−1)2 = 1. A system of three pions, however, would, by
the same consideration, have negative parity, since (−1)3 = −1.
Therefore if parity doesn’t change when a particle decays, a single
original particle cannot decay into two different final states of
different parity.
If the force responsible for the decay behaved like all the other
known forces at the time, such as electromagnetism or gravity, it
would be blind to parity (it would not distinguish between right and
left), so it shouldn’t change the original parity of the system after the
decay, just as shining a light on your right hand will not cause it to
look like your left hand.
Since it seemed impossible for a single type of particle to decay
sometimes into two, and sometimes into three, pions, the solution
seemed simple. There must be two different new elementary
particles, with opposite parity properties. Powell dubbed these the
tau particle and theta particle—one of which could decay into two
pions, and one into three pions.
Observations suggested that the two particles had precisely the
same masses and lifetimes, which was a bit strange, but Lee and
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