Hiding in the Mirror (15 page)

An expanding universe is in fact precisely one
of the two possibilities allowed by Einstein’s general relativity.
Indeed, a frustration that Einstein first encountered after
developing his theory and attempting to apply it to the nature of
the universe as a whole was that it did not allow for a static
universe unless that universe was devoid of matter. He tried to get
around this problem by introducing an extra ad hoc element into his
equations—called the “cosmological term”—which he thought could
allow for a static solution with matter. The effect of the
cosmological term was to add a small repulsive force throughout
space that Einstein thought could counteract gravity on large
scales, holding distant objects apart. Unfortunately, however, he
blundered. His static solution with a cosmological constant was not
stable. Had Einstein had more courage of his convictions in 1916,
he might have predicted either an expanding universe or a
collapsing one, because these are the only two options allowed by
general relativity. Once Hubble had discovered our cosmic
expansion, Einstein was overjoyed and even went to visit him at
Mount Wilson in 1931 so he could look through the famous telescope
himself. George Gamow, physicist and author, later said Einstein
confided to him that he thought his introduction of a cosmological
term into his equations was his “biggest blunder.” As we shall
later see, being willing to discard this term immediately after it
seemed unnecessary may have been yet a bigger blunder. In any case,
Hubble’s discovery of cosmic expansion changed everything about the
way we think of “universal” history. If the universe was now
expanding, it was once smaller. Assuming the expansion has been
continuous, following its history backward meant that ultimately
all objects in our visible universe would have been located at a
single point at a finite time in the past. This implied, first of
all, that our universe had a beginning. Indeed, when Hubble
initially used his measured expansion rate to determine the age of
the universe, he found an upper limit of two billion years. This
was embarrassing, because the earth was, and is, known to be older
than that, except by school boards in Ohio, Georgia, and Kansas
perhaps. Fortunately, Hubble’s original measurement was actually
off by almost a factor of ten, establishing a now noble tradition
in cosmology. With current and thankfully more precise measurements
of its expansion history, we now know that the age of the universe
is about fourteen billion years.

But a finite age for the universe was not the
only startling implication of the observed Hubble expansion. As we
continue to move back in time, the size of the region occupied by
the presently observable universe decreases. Originally,
macroscopic bodies such as stars and galaxies would have been
crowded together in a volume smaller than the size of an atom. In
this case, the physics that would have governed the earliest
moments of what has now become known as the big bang would involve
processes acting on the smallest scales. On these scales the
strange laws of quantum mechanics reign supreme, at least as far as
we know. But, as we peer back to the very beginning itself, when
all the matter in the observable universe existed together at
virtually a single point, the very nature of space itself, and
possibly even time as well, may have been dramatically different.
Perhaps the entire universe as we know it emerged from behind the
looking glass, from another dimension of sight and sound. Suddenly,
faced with a possible singularity at the beginning of time, truth
was stranger than fiction.

While the past remains a compelling subject,
the future is usually of more practical interest. And a currently
expanding universe could have one of three possible futures: Either
the expansion continues unabated, or it slows down but never quite
stops, or it stops and the universe recollapses. Determining which
of these fates awaits the cosmos, by determining the magnitude of
each of the terms in Einstein’s equations for an expanding
universe, became one of the principal items of business for
cosmology for the rest of the twentieth century. In the 1990s we
thought we finally had the answer down pat. But the universe, as it
has a way of doing, surprised us. As we shall see, it turns out
that empty space—not matter, and not radiation—holds the key to our
future. Thus, just as trying to understand our cosmic beginnings
has forced us to ponder the ultimate nature of space and time, our
very future may depend upon whether there is much more to empty
space than meets the eye.

These revolutions in our picture of the
universe at fundamental scales, from the existence of antimatter
and virtual particles, to the apparent population explosion of
particles and forces, and ultimately to the dynamic nature of space
itself, completely transformed the landscape of physics and
affected the very questions about nature that physicists might ask.
Happily, many of the confusions raised by these unexpected
discoveries have been resolved, as we shall see. But not all of
them have been, and in the process other puzzles have arisen that
have made the preliminary thrusts of physics at the beginning of
the twenty-first century bear an odd resemblance to the
philosophical speculations that so inspired Poincaré, Wells,
Picasso, and others at the beginning of the previous century.

After a storm comes a
calm.

—Fourteenth-century proverb

T
he 1950s are
remembered by many to be a period of relative peace and stability,
at least compared to the World War and subsequent recovery that had
occupied the previous decade, and the tumultuous era that was yet
to come. Memories, of course, can be deceiving, and I suspect that
the families of the many thousands of Korean and U.S. soldiers
killed in the Korean War, and of those who lost their lives or
became trapped in Communist Hungary in 1956, may think otherwise.
Whatever one’s assessment of the political situation of the time,
in physics it was a period of growing but exciting confusion as the
implications of the remarkable discoveries of the 1930s became
manifest. Part of this excitement was generated by the availability
of gargantuan tools that were part of the emergence of “big
science” in late 1940s, following the mammoth Manhattan Project
that led to the development of the atomic bomb and an immediate,
and gruesome, end to World War II. During this period the
unprecedented power of atomic weapons raised scientists up on a
pedestal. While general scientific education in the United States
did not become a priority until the crisislike reaction following
the Soviet Union’s launching of the
Sputnik
satellite in 1957, the public began to appreciate the possibly
dramatic impact of what would otherwise be considered rather
esoteric physical phenomena. The newfound knowledge of the inner
workings of atomic nuclei had manifested itself in the devastation
wrought by nuclear weapons. But almost as if to balance the scales,
physicists also invented the transistor and, with it, solid-state
electronics, exploiting the strange laws of quantum mechanics to
positively revolutionize our daily lives in almost every way. Today
it is hard to imagine going for even an hour without depending at
some time on transistors and the technology that has been developed
around them.

Even biology was benefiting from knowledge on
atomic scales. X-ray crystallography was enabling scientists to
piece together the atomic structure of many materials, and in April
1953, Watson and Crick discovered the remarkable double-helix
structure of DNA and, with it, the very basis of life itself. Or,
as they put it in the concluding sentence of the paper announcing
their results, in one of the most celebrated understatements in the
history of science: “It has not escaped our notice that the specific
pairing we have postulated immediately suggests a possible copying
mechanism for the genetic material.”

The potential for the future seemed endless,
limited only by our imagination. And imagination was in no short
supply. But at the same time, on fundamental scales at least,
nature seemed to be outpacing our ability to keep up.

The onslaught had begun slowly, as early as
1937, when once again cosmic rays produced a surprise. Recall that
Carl Anderson had discovered the existence of the positron in
cosmic rays in 1932 by using a cloud chamber. Shortly thereafter,
in England, Patrick Blackett and his young Italian colleague
Giuseppe Occhialini set out, in Blackett’s charming terms, “to
devise a method of making cosmic rays take their own photographs.”
They hooked up electronic sensors above and below a cloud chamber,
which produced signals when cosmic rays passed through them. These
signals were transmitted to the device that controlled the
expansion of the vapor in the cloud chamber, causing the tracks to
be visible. In this way, instead of expanding the cloud chamber at
random, as had been done previously, and catching a cosmic ray in,
on average, one out of fifty such expansions, they caught a cosmic
ray in each expansion. Using this technique, physicists could study
cosmic ray properties more comprehensively, and within a few years
it was observed that cosmic rays appeared to be more penetrating
than one would expect based on theoretical estimates for the energy
loss by electrons propagating through matter. It was natural for
some—particularly experimenters, perhaps—to question whether the
new quantum theory predictions of energy loss rates were, in fact,
correct. Ultimately, however, the problem was demonstrated to lie
elsewhere when, in 1937, two different teams of researchers (one of
which included Anderson) demonstrated unambiguously that the cosmic
rays being observed were not electrons, but new elementary
particles, almost two hundred times heavier than the electron, and
about ten times lighter than the proton and neutron. The world of
elementary particles was becoming even more crowded.

Theorists, not to be outdone, pointed out that
in fact one of their clan had earlier “predicted” such a particle.
The soon to be famous (and infamous) U.S. physicist J. Robert
Oppenheimer and his colleague Robert Serber explained that in a
little-known Japanese journal, in 1935, the physicist Hideki Yukawa
had proposed, by analogy to the force of electromagnetism—which
operates by the exchange of electromagnetic radiation (which
quantum mechanics implied could also be represented by particles,
i.e., photons) between charged objects—that the strong force that
must bind neutrons and protons together inside of nuclei might also
operate by particle exchange. Because the nuclear force is very
short range, operating over only nuclear distances, Yukawa used the
Heisenberg uncertainty principle to argue that the particles
responsible for transmitting this force would have to be heavy,
about two hundred times the mass of the electron.

Everything seemed to be falling into place . .
. except that nature would not let physicists off so easily.
Experiments performed over the next decade demonstrated the
somewhat strange behavior of this new particle, at the time called
the “mesatron.” (The term
Yukon,
after
Yukawa, was briefly considered but quickly abandoned as too
frivolous.) Yukawa’s strong nuclear force carriers should interact
strongly with nuclei, and it was therefore predicted that the
negatively charged mesatron should be captured by the positively
charged nuclei in matter well before it could itself decay into
lighter particles such as electrons and neutrinos. By 1947 it was
clear that this particle interacted millions of times less strongly
than these predictions suggested it should.

Instead, it turned out that this new particle,
now renamed a “muon,” behaved exactly like an electron, except it
was two hundred times heavier. This completely unexpected
development caused the famous experimental physicist and Nobel
laureate I. I. Rabi to make his now often repeated remark, “Who
ordered that?” We are still wondering that today!

While 1947 brought the demise of the mesatron,
it also heralded the discovery of the long sought particles
proposed by Yukawa. Using a new technique involving photographic
emulsions to record particle tracks—a technique that was claimed to
be “so simple even a theoretician might be able to do it”—the
British physicist Cecil Powell and Blackett’s erstwhile
collaborator Occhialini were able to go to high altitudes to search
for new cosmic ray signatures.

Occhialini, who had been a mountain guide,
ascended to the Pic du Midi at 2,867 meters in the French Pyrenees
and exposed his film to cosmic rays high in the atmosphere. Later
that year, when he and Powell examined the developed emulsions in
London and Bristol, Powell remembered feeling as if they had
entered a whole new world. As he later wrote; “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.”

I have rarely read a more poignant description
of the joy of scientific discovery, of seeing something absolutely
new, something that no human has ever witnessed before. It is what
drives individuals to scale mountains, metaphorical or literal: the
hidden universe, previously unknown and unobserved, but actually
present, that we all seem hardwired to crave so deeply.

Powell and his collaborator’s discovery of the
particles that became known as pions was not the end of the road,
merely a new beginning. An even stranger discovery occurred in the
same year, although it took until 1950 before it was independently
confirmed. In 1947, working at Manchester, George Rochester and
Cecil Butler observed two unusual events involving forked tracks in
cloud chambers that appeared to be due to the decays of new
particles, about five times heavier than the newly discovered pions,
and half as heavy as protons. In 1950, again at Pic du Midi, using
a cloud chamber carted up to this high altitude just for this
purpose, Blackett’s group observed similar events. The situation
still remained somewhat confused until 1952, when a new refined type
of cloud chamber resolved that there were actually two different
types of these new sorts of particles. What made the decay events
associated with these objects so strange, literally, is that while
the particles involved were indeed strongly interacting, they lived
about ten million million times longer than one would estimate for
unstable, strongly interacting particles. Whatever property caused
them to live so long was dubbed by physicists, in an act of
linguistic creativity worthy of a primary school student,
“strangeness,” and the mysterious entities themselves became known
as “strange” particles. Powell’s cosmic ray data produced yet one
more shock for the physics community, much higher on the Richter
scale than even the discovery of strangeness itself. In 1949, in
one of the observations associated with the discovery of
strangeness, Powell noticed a strange particle, which he dubbed a
tau meson, that decayed into three pions. (We now call it a kaon.)
Shortly thereafter came the discovery of the theta particle, which
decayed into two pions. This in itself was not especially
surprising, but when careful measurements were later made, it was
found that the two particles had identical masses and identical
lifetimes. Why should two such different particles be otherwise so
identical?

One suggestion was that they were, in fact, the
same particle. However, that was impossible because the final states
of the two decays behaved very differently in one crucial
respect—indeed, a respect that is of great significance in the
context of this book. If Lewis Carroll’s Alice were to observe the
three-pion outgoing particle state in her looking glass, it turns
out that it would be distinguishable from the three particle state
as seen in her own room, just as a left hand in her world becomes a
right hand when viewed in the mirror. The three different particles
arrange themselves to have a certain “handedness,” just as pointing
three fingers in the x, y, and z directions with your right hand
produces a “right-handed coordinate system,” while pointing three
fingers from your left hand in these three different directions
produces a coordinate system that is left handed. Try it. There is
no way you can rotate one configuration into the other.

By contrast, it turns out that the two-pion
state would look identical in the mirror to the state as observed
in the real world. There is no “handedness” to this distribution.
Now, there is no way that a single particle could on the one hand
produce a final state that was distinguishable from its mirror
image, and on the other hand decay into a state that was identical
to its mirror image, at least as long as the fundamental laws of
physics governing the decays themselves don’t distinguish left from
right. The tau-theta puzzle, as it became known, persisted for over
five years until two young theoretical physicists, Tsung-Dao Lee and
Chen Ning Yang, working for the summer at the new Brookhaven
National Accelerator Laboratory in 1956, asked a remarkable
question: What evidence was there that the new force responsible
for the decay of these particles, the socalled weak force, which
was also responsible for the decay of the neutron, actually didn’t
distinguish left from right?

It is hard to overstate the striking boldness
of this question. After all, everything we experience about nature
suggests that the world in the mirror behaves identically to our
own world. Being able to distinguish left from right is simply an
accident of our location. If one was out in the open ocean on a
cloudy night, for example, so that one couldn’t see the stars to
navigate, there would be nothing on the horizon that would suggest
one direction was different than any other. Or, to take a more
modern example, if one was in empty space and performed any physics
experiment, it would be ridiculous to expect that somehow its
results should distinguish between right and left.

But there it was. In 1956 Lee and Yang realized
this assumption was so ingrained in people’s psyches that no one
had ever bothered to test it for the weak interaction. By contrast,
for both electromagnetism and for the strong interaction, this
property had been verified by a host of detailed measurements. Not
only did Lee and Yang recognize that no tests of left–right
symmetry (or, as it has become known,
parity
) had been performed for weak interactions,
they also proposed several experiments that could be performed to
verify it. Within a year of the publication of their paper, two
studies, both performed by physicists at nearby Columbia
University, had been carried out, and both revealed the same
startling conclusion: The weak interactions indeed distinguished
left from right!