Parallel Worlds (26 page)

DECOHERENCE

A way to
partially resolve some of these thorny philosophical questions, one gaining
popularity among physicists, is called decoher- ence. It was first formulated
by German physicist Dieter Zeh in 1970. He noticed that in the real world you
cannot separate the cat from the environment. The cat is in constant contact
with the molecules of air, the box, and even cosmic rays that pass through the
experiment. These interactions, no matter how small, radically affect the wave
function: if the wave function is disturbed to the slightest degree, then the
wave function suddenly splits into two distinct wave functions of the dead cat
or the live cat, which no longer interact. Zeh showed that a collision with a
single air molecule was enough to collapse it, forcing the permanent separation
of the dead cat and live cat wave functions, which can no longer communicate
with each other. In other words, even before you open the box, the cat has been
in contact with air molecules and hence is already dead or alive.

Zeh made the key
observation that had been overlooked: for the cat to be both dead and alive,
the wave function of the dead cat and the wave function of the live cat must be
vibrating in almost exact synchronization, a state called coherence. But
experimentally, this is almost impossible. Creating coherent objects vibrating
in unison in the laboratory is extraordinarily difficult. (In practice, it is
difficult to get more than a handful of atoms to vibrate coherently because
of interference from the outside world.) In the real world, objects interact
with the environment, and the slightest interaction with the outside world can
disturb the two wave functions, and then they start to
"decohere"—that is, fall out of synchronization and separate. Once
the two wave functions are no longer vibrating in phase with each other, Zeh
showed, the two wave functions no longer interact with each other.

MANY WORLDS

At first,
decoherence sounds very satisfying, since the wave function now collapses not
via consciousness but by random interactions with the outside world. But this
still doesn't solve the fundamental question that bothered Einstein: how does
nature "choose" which state to collapse into? When an air molecule
hits the cat, who or what determines the final state of the cat? On this
question, decoherence theory simply states that the two wave functions separate
and no longer interact, but it does not answer the original question: is the
cat dead or alive? In other words, decoherence makes consciousness unnecessary
in quantum mechanics, but it does not resolve the key question that disturbed
Einstein: how does nature "choose" the final state of the cat? On
this question, decoherence theory is silent.

There is,
however, a natural extension of decoherence that resolves this question that
is gaining wide acceptance today among physicists. This second approach was
pioneered by another of Wheeler's students, Hugh Everett III, who discussed the
possibility that perhaps the cat can be both dead and alive at the same time
but in two different universes. When Everett's Ph.D. thesis was finished in
1957, it was barely noticed. Over the years, however, interest in the
"many worlds" interpretation began to grow. Today, it has unleashed
a tidal wave of renewed interest in the paradoxes of the quantum theory.

In this
radically new interpretation, the cat is both dead and alive because the
universe has split into two. In one universe, the cat is dead; in another
universe, the cat is alive. In fact, at each quantum juncture, the universe
splits in half, in a never-ending sequence of splitting universes. All
universes are possible in this scenario, each as real as the other. People
living in each universe might vigorously protest that
their
universe is the real one, and that all the others are
imaginary or fake. These parallel universes are not ghost worlds with an
ephemeral existence; within each universe, we have the appearance of solid
objects and concrete events as real and as objective as any.

The advantage of
this interpretation is that we can drop condition number three, the collapse
of the wave function. Wave functions never collapse, they just continue to
evolve, forever splitting into other wave functions, in a never-ending tree,
with each branch representing an entire universe. The great advantage of the
many worlds theory is that it is simpler than the Copenhagen interpretation:
it requires no collapse of the wave function. The price we pay is that now we
have universes that continually split into millions of branches. (Some find it
difficult to understand how to keep track of all these proliferating universes.
However, the Schrodinger wave equation does this automatically. By simply
tracing the evolution of the wave equation, one immediately finds all the
numerous branches of the wave.)

If this
interpretation is correct, then at this very instant your body coexists with
the wave functions of dinosaurs engaged in mortal combat. Coexisting in the
room you are in is the wave function of a world where the Germans won World War
II, where aliens from outer space roam, where you were never born. The worlds
of
The Man in the High Castle
and
The Twilight Zone
are among the universes existing
in your living room. The catch is that we can no longer interact with them,
since they have decohered from us.

As Alan Guth has
said, "There is a universe where Elvis is still alive." Physicist
Frank Wilczek has written, "We are haunted by the awareness that
infinitely many slightly variant copies of ourselves are living out their
parallel lives and that every moment more duplicates spring into existence and
take up our many alternative futures." He notes that the history of Greek
civilization, and hence the Western world, might have been different had Helen
of Troy not been such a captivating beauty, if instead she had an ugly wart on
her nose. "Well, warts can arise from mutations in single cells, often
triggered by exposure to the ultraviolet rays of the sun." He goes on,
"Conclusion: there are many, many worlds in which Helen of Troy
did
have a wart at the tip of her nose."

I am reminded of
the passage from Olaf Stapledon's classic work of science fiction,
Star Maker:
"Whenever a creature was faced with several possible
courses of action, it took them all, thereby creating many . . . distinct
histories of the cosmos. Since in every evolutionary sequence of the cosmos
there were many creatures and each was constantly faced with many possible
courses, and the combinations of all their courses were innumerable, an
infinity of distinct universes exfoliated from every moment of every temporal
sequence."

The mind reels
when we realize that, according to this interpretation of quantum mechanics,
all possible worlds coexist with us. Although wormholes might be necessary to
reach such alternate worlds, these quantum realities exist in the very same
room that we live in. They coexist with us wherever we go. The key question is:
if this is true, why don't we see these alternate universes filling up our living
room? This is where decoherence comes in: our wave function has decohered with
these other worlds (that is, the waves are no longer in phase with each other).
We are no longer in contact with them. This means that even the slightest
contamination with the environment will prevent the various wave functions
from interacting with each other. (In chapter 11, I mention a possible
exception to this rule, in which intelligent beings may be able to travel
between quantum realities.)

Does this seem
too strange to be possible? Nobel laureate Steven Weinberg likens this multiple
universe theory to radio. All around you, there are hundreds of different radio
waves being broadcast from distant stations. At any given instant, your office
or car or living room is full of these radio waves. However, if you turn on a
radio, you can listen to only one frequency at a time; these other frequencies
have decohered and are no longer in phase with each other. Each station has a
different energy, a different frequency. As a result, your radio can only be
turned to one broadcast at a time.

Likewise, in our
universe we are "tuned" into the frequency that corresponds to
physical reality. But there are an infinite number of parallel realities
coexisting with us in the same room, although we cannot "tune into"
them. Although these worlds are very much alike, each has a different energy.
And because each world consists of trillions upon trillions of atoms, this
means that the energy difference can be quite large. Since the frequency of
these waves is proportional to their energy (by Planck's law), this means that
the waves of each world vibrate at different frequencies and cannot interact
anymore. For all intents and purposes, the waves of these various worlds do
not interact or influence each other.

Surprisingly,
scientists, by adopting this strange point of view, can rederive all the
results of the Copenhagen approach without ever having to collapse the wave
function. In other words, experiments done with the Copenhagen interpretation,
or the many worlds interpretation, will yield precisely the same experimental
results. Bohr's collapse of the wave function is mathematically equivalent to
contamination with the environment. In other words, Schrodinger's cat can be
dead and alive at the same time if we can somehow isolate the cat from possible
contamination from every atom or cosmic ray. Of course, this is practically
impossible. Once the cat is in contact with a cosmic ray, the dead cat and live
cat wave functions decohere, and it appears as if the wave function has collapsed.

IT FROM BIT

With all this
renewed interest in the measurement problem in the quantum theory, Wheeler has
become science's grand old man of quantum physics, appearing at numerous
conferences in his honor. He has even been hailed as a guru of sorts by New Age
advocates who are fascinated by the question of consciousness in physics.
(However, he is not always pleased with such associations. Once, he was distressed
to find himself on the same program with three parapsy- chologists. He quickly
put out a statement that included the sentence "Where there's smoke,
there's smoke.")

After seventy
years of contemplating the paradoxes of the quantum theory, Wheeler is the
first one to admit that he does not have all the answers. He continues to
always question his assumptions. When asked about the measurement problem in
quantum mechanics, he says, "I am just driven crazy by that question. I
confess that sometimes I do take 100 percent seriously the idea that the world
is a figment of the imagination and, other times, that the world does exist out
there independent of us. However, I subscribe wholeheartedly to those words of
Leibniz, 'This world may be a phantasm and existence may be merely a dream, but
this dream or phantasm to me is real enough if using reason well we are never
deceived by it.' "

Today, the many
worlds/decoherence theory is gaining popularity among physicists. But Wheeler
is bothered that it requires "too much excess baggage." He is toying
with yet another explanation of the Schrodinger cat problem. He calls his
theory "It from bit." It's an unorthodox theory, which starts with
the assumption that information is at the root of all existence. When we look
at the moon, a galaxy, or an atom, their essence, he claims, is in the
information stored within them. But this information sprang into existence when
the universe observed itself. He draws a circular diagram, representing the
history of the universe. At the beginning of the universe, it sprang into
being because it was observed. This means that "it" (matter in the
universe) sprang into existence when information ("bit") of the
universe was observed. He calls this the "participatory universe"—the
idea that the universe adapts to us in the same way that we adapt to the
universe, that our very presence makes the universe possible. (Since there is
no universal consensus on the measurement problem in quantum mechanics, most
physicists take a wait-and-see attitude toward It from Bit.)

QUANTUM COMPUTING AND TELEPORTATION

Such
philosophical discussions may seem hopelessly impractical, devoid of any
practical application in our world. Instead of debating how many angels can
dance on the head of a pin, quantum physicists seem to be debating how many
places an electron can be at the same time.

However, these
are not the idle musings of ivory-tower academics. One day they may have the
most practical application of all: to drive the economies of the world. One
day, the wealth of entire nations may depend on the subtleties of
Schrodinger's cat. At that time, perhaps our computers will be computing in
parallel universes. Almost all of our computer infrastructure today is based
on silicon transistors. Moore's law, which states that computer power doubles
every eighteen months, is possible because of our ability to etch smaller and
smaller transistors onto silicon chips via beams of ultraviolet radiation.
Although Moore's law has revolutionized the technological landscape, it cannot
continue forever. The most advanced Pentium chip has a layer twenty atoms
across. Within fifteen to twenty years, scientists may be calculating on layers
perhaps five atoms across. At these incredibly small distances, we have to abandon
Newtonian mechanics and adopt the quantum mechanics, where the Heisenberg
uncertainty principle takes over. As a consequence, we no longer know
precisely where the electron is. This means that short circuits will take place
as electrons drift outside insulators and semiconductors instead of staying
within them.