Parallel Worlds (25 page)

After these and
other fierce debates with quantum physicists, Einstein finally gave in, but
took a different approach. He conceded that the quantum theory was correct, but
only within a certain domain, only as an approximation to the real truth. In
the same way that relativity generalized (but did not destroy) Newton's theory,
he wanted to absorb the quantum theory into a more general, more powerful
theory, the unified field theory.

(This debate,
between Einstein and Schrodinger on one side, and Bohr and Heisenberg on the
other, cannot be easily dismissed, since these "thought experiments"
can now be performed in the laboratory. Although scientists cannot make a cat
appear both dead and alive, they can now manipulate individual atoms with
nanotechnol- ogy. Recently, these mind-bending experiments were done with a
Buckyball containing sixty carbon atoms, so the "wall" envisioned by
Bohr separating large objects from quantum objects is rapidly crumbling.
Experimental physicists are now even contemplating what would be required to
show that a virus, consisting of thousands of atoms, can be in two places at
the same time.)

THE BOMB

Unfortunately,
discussions over these delicious paradoxes were interrupted with the rise of
Hitler in 1933 and the rush to build an atomic bomb. It was known for years,
via Einstein's famous equation
E
=
mc
²
,
that there was
a vast storehouse of energy locked in the atom. But most physicists pooh-poohed
the idea of ever being able to harness this energy. Even Ernest Rutherford, the
man who discovered the nucleus of the atom, said, "The energy produced by
the breaking down of the atom is a very poor kind of thing. Anyone who expects
a source of power from the transformation of these atoms is talking
moonshine."

In 1939, Bohr
made a fateful trip to the United States, landing in New York to meet his
student John Wheeler. He was bearing ominous news: Otto Hahn and Lise Meitner
had shown that the uranium nucleus could be split in half, releasing energy, in
a process called fission. Bohr and Wheeler began to work out the quantum
dynamics of nuclear fission. Since everything in the quantum theory is a matter
of probability and chance, they estimated the probability that a neutron will
break apart the uranium nucleus, releasing two or more neutrons, which then
fission even more uranium nuclei, which then release ever more neutrons, and so
on, setting off a chain reaction capable of devastating a modern city. (In
quantum mechanics, you can never know if any particular neutron will fission a
uranium atom, but you can compute with incredible accuracy the probability that
billions of uranium atoms will fission in a bomb. That is the power of quantum
mechanics.)

Their quantum
computations indicated that an atomic bomb might be possible. Two months later,
Bohr, Eugene Wigner, Leo Szilard, and Wheeler met at Einstein's old office at
Princeton to discuss the prospects for an atomic bomb. Bohr believed it would
take the resources of an entire nation to build the bomb. (A few years later,
Szilard would persuade Einstein to write the fateful letter to President
Franklin Roosevelt, urging him to build the atomic bomb.)

That same year,
the Nazis, aware that the catastrophic release of energy from the uranium atom
could give them an unbeatable weapon, ordered Bohr's student, Heisenberg, to
create the atomic bomb for Hitler. Overnight, the discussions over the quantum
probability of fission became deadly serious, with the fate of human history
at stake. Discussions of the probability of finding live cats would soon be
replaced by discussions of the probability of fissioning uranium.

In 1941, with
the Nazis overrunning most of Europe, Heisenberg made a secret journey to meet
his old mentor, Bohr, in Copenhagen. The precise nature of the meeting is still
shrouded in mystery, and award-winning plays have been written about it, with
historians still debating its content. Was Heisenberg offering to sabotage the
Nazi atomic bomb? Or was Heisenberg trying to recruit Bohr for the Nazi bomb?
Six decades later, in 2002, much of the mystery over Heisenberg's intentions
was finally lifted, when the Bohr family released a letter written by Bohr to
Heisenberg in the 1950s but never mailed. In that letter, Bohr recalled that
Heisenberg had said at that meeting that a Nazi victory was inevitable. Since
there was no stopping the Nazi juggernaut, it was only logical that Bohr work
for the Nazis.

Bohr was
appalled, shaken to the core. Trembling, he refused to allow his work on the
quantum theory to fall into Nazi hands. Because Denmark was under Nazi control,
Bohr planned a secret escape by plane, and he was almost suffocated due to
lack of oxygen on the plane trip to freedom.

Meanwhile, at
Columbia University, Enrico Fermi had shown that a nuclear chain reaction was
feasible. After he reached this conclusion, he peered out over New York City
and realized that a single bomb could destroy everything he saw of the famed
skyline. Wheeler, realizing how high the stakes had become, voluntarily left
Princeton and joined Fermi in the basement of Stagg Field at the University of
Chicago, where together they built the first nuclear reactor, officially
inaugurating the nuclear age.

Over the next
decade, Wheeler witnessed some of the most momentous developments in atomic
warfare. During the war, he helped supervise the construction of the mammoth
Hanford Reservation in Washington State, which created the raw plutonium
necessary to build the bombs that would devastate Nagasaki. A few years later,
he worked on the hydrogen bomb, witnessing the first hydrogen bomb blast in
1952 and the devastation caused when a piece of the Sun was unleashed on a
small island in the Pacific. But after being at the forefront of world history
for over a decade, he finally returned to his first love, the mysteries of the
quantum theory.

SUM OVER PATHS

One of Wheeler's
legion of students after the war was Richard Feynman, who stumbled on perhaps
the simplest yet most profound way of summarizing the intricacies of the
quantum theory. (One consequence of this idea would win Feynman the Nobel Prize
in 1965.) Let's say that you want to walk across the room. According to Newton,
you would simply take the shortest path, from point A to point B, called the
classical path. But according to Feynman, first you would have to consider all
possible paths connecting points A and B. This means considering paths that
take you to Mars, Jupiter, the nearest star, even paths that go backward in
time, back to the big bang. No matter how crazy and utterly bizarre the paths
are, you must consider them. Then Feynman assigned a number for each path,
giving a precise set of rules by which to calculate this number. Miraculously,
by adding up these numbers from all possible paths, you found the probability
of walking from point A to point B given by standard quantum mechanics. This
was truly remarkable.

Feynman found
that the sum of these numbers over paths that were bizarre and violated
Newton's laws of motion usually canceled out to give a small total. This was
the origin of quantum fluctua- tions—that is, they represented paths whose sum
was very small. But he also found that the commonsense Newtonian path was the
one that did not cancel out and hence had the largest total; it was the path
with the greatest probability. Thus, our commonsense notion of the physical
universe is simply the most probable state among an infinite number of states.
But we coexist with all possible states, some of which take us back to the
dinosaur era, to the nearest supernova, and to the edges of the universe.
(These bizarre paths create tiny deviations from the commonsense Newtonian
sense path but fortunately have a very low probability associated with them.)

In other words,
as odd as it may seem, every time you walk across the room, somehow your body
"sniffs out" all possible paths ahead of time, even those extending
to the distant quasars and the big bang, and then adds them up. Using powerful
mathematics called functional integrals, Feynman showed that the Newtonian path
is simply the most probable path, not the only path. In a mathematical tour de
force, Feynman was able to prove that this picture, as astounding as it may
seem, is exactly equivalent to ordinary quantum mechanics. (In fact, Feynman
was able to give a derivation of the Schrodinger wave equation using this
approach.)

The power of
Feynman's "sum over paths" is that today, when we formulate GUT
theories, inflation, even string theory, we use Feynman's "path
integral" point of view. This method is now taught in every graduate
school in the world and is by far the most powerful and convenient way of
formulating the quantum theory.

(I use the
Feynman path integral approach every day in my own research. Every equation I
write is written in terms of these sum over paths. When I first learned of
Feynman's point of view as a graduate student, it changed my entire mental
picture of the universe. Intellectually, I understood the abstract mathematics
of the quantum theory and general relativity, but it was the idea that I am in
some sense sniffing out paths that take me to Mars or the distant stars as I
walk across the room that altered my worldview. Suddenly, I had a strange new
mental picture of myself living in a quantum world. I began to realize that
quantum theory is much more alien than the mind-bending consequences of
relativity.)

When Feynman
developed this bizarre formulation, Wheeler, who was at Princeton University,
rushed over next door to the Institute for Advanced Study to visit Einstein to
convince him of the elegance and power of this new picture. Wheeler excitedly
explained to Einstein Feynman's new theory of path integrals. Wheeler did not
fully realize how utterly crazy this must have sounded to Einstein.

Afterward, Einstein shook his head and repeated that he still
did not believe that God played dice with the world. Einstein admitted to
Wheeler that he could be wrong, but he also insisted that he had earned the
right to be wrong.

WIGNER'S FRIEND

Most physicists
shrug their shoulders and throw up their hands when confronted with the
mind-bending paradoxes of quantum mechanics. To most practicing scientists,
quantum mechanics is a set of cookbook rules that yields the right
probabilities with uncanny accuracy. As the physicist-turned-priest John
Polkinghorne has said, "The average quantum mechanic is no more
philosophical than the average motor mechanic."

However, some of
the deepest thinkers in physics have struggled with these questions. For
example, there are several ways of resolving the Schrodinger cat problem. The
first, advocated by Nobel laureate Eugene Wigner and others, is that
consciousness determines existence.
Wigner has
written that it "was not possible to formulate the laws of quantum
mechanics in a fully consistent way, without reference to the consciousness [of
the observer] . . . the very study of the external world led to the conclusion
that the content of the consciousness is the ultimate reality." Or, as
the poet John Keats once wrote, "Nothing ever becomes real till it is
experienced."

But if I make an
observation, what is to determine which state I am in? This means that someone
else has to observe me to collapse my wave function. This is sometimes called
"Wigner's friend." But it also means that someone has to observe
Wigner's friend, and Wigner's friend's friend, and so on. Is there a cosmic
consciousness that determines the entire sequence of friends by observing the
entire universe?

One physicist
who tenaciously believes in the central role of consciousness is Andrei Linde,
one of the founders of the inflationary universe.

For me as a human being, I do not know any sense in which I
could claim that the universe is here in the absence of observers. We are together,
the universe and us. The moment you say that the universe exists without any
observers, I cannot make any sense out of that. I cannot imagine a consistent
theory of everything that ignores consciousness. A recording device cannot
play the role of an observer, because who will read what is written on this
recording device. In order for us to see that something happens, and say to one
another that something happens, you need to have a universe, you need to have a
recording device, and you need to have
us ...
In the absence
of observers, our universe is dead.

According to
Linde's philosophy, dinosaur fossils don't really exist until you look at
them. But when you do look at them, they spring into existence as if they had
existed millions of years ago. (Physicists who hold to this point of view are
careful to point out that this picture is experimentally consistent with a
world in which dinosaur fossils really are millions of years old.)

(Some people,
who dislike introducing consciousness into physics, claim that a camera can
make an observation of an electron, hence wave functions can collapse without
resorting to conscious beings. But then who is to say if the camera exists?
Another camera is necessary to "observe" the first camera and
collapse its wave function. Then a second camera is necessary to observe the
first camera, and a third camera to observe the second camera, ad infinitum. So
introducing cameras does not answer the question of how wave functions
collapse.)