IMAGINE an infinite number of realities. In one, you never opened this copy
of New Scientist. In another your parents never met so you weren’t
born. In yet another, the Sun did not congeal out of a cold cloud of gas so
there was no Earth.
It sounds strange, but multiple realities can explain some of the most
baffling aspects of our Universe like how an electron can be in two places at
once.
Although this so-called “Many Worlds” interpretation of quantum theory has
been mired in controversy for years, it is gaining increasing acceptance. In a
poll taken at a physics conference in Baltimore in August, less than half the
participants supported the standard Copenhagen interpretation. “People were
stunned to find that Many Worlds was the main challenger,” says physicist Max
Tegmark of the Institute for Advanced Study in Princeton. What’s more, Tegmark
has come up with a way to prove whether or not it is right, albeit a
double-edged one. “It involves standing in front of a quantum machine gun,” he
says. “Unfortunately your colleagues see you die in the process of discovering
the truth, so you can never convince anyone but yourself.”
Advertisement
One reason there are several interpretations of quantum theory is that there
are different ways of interpreting the underlying mathematical reality of
quantum theory in terms of everyday experience. That reality, called the “wave
function”, encapsulates everything knowable about a quantum system and evolves
in time according to the Schrödinger equation. The most remarkable
prediction of this equation is that quantum measurements may have no definite
outcome. A pencil balanced on its tip and then falling onto a table is predicted
to land in a “superposition” of all directions at the same time.
The fundamental problem physicists have had to wrestle with is why we
perceive a definite outcome. The different interpretations of quantum theory are
rival attempts at providing an answer.
Until now, the Copenhagen interpretation, formulated by the Danish physicist
Niels Bohr in the 1920s, has been the most popular. Crudely expressed, it
maintains that “the wave function obeys the Schrödinger equation, except
when it doesn’t”. At a certain point, the wave function may “collapse”, forcing
our pencil to select one, and only one, state from all the possibilities. But
the Copenhagen interpretation does not predict when the Schrödinger
equation goes off duty and the wave function collapses.
Parallel puzzles
The Many Worlds interpretation, invented in 1957 by the late Hugh Everett of
Princeton University, sidesteps the whole problem of wave function collapse in a
most extraordinary way—by denying that it collapses at all. According to
Many Worlds, the wave function always obeys the Schrödinger equation. The
pencil will occupy more than one place at a time, with each position
corresponding to a “parallel reality” within the single wave function. However,
the person watching the pencil also enters a superposition of many states, each
perceiving the pencil to be in a different, but definite place.
But there is a puzzle that long prevented physicists from taking the Many
Worlds idea seriously: if quantum theory really allows our pencil to be in two
places at once, why don’t we ever perceive parallel states? The answer, says
Tegmark, is a phenomenon known as “decoherence”.
The source of all quantum weirdness is the interaction between a system’s
different states. But such an interaction can occur only if there is “coherence”
between the states, that is if the system is isolated. Just one photon bouncing
off the pencil shatters that isolation, destroying the coherence between the
states and preventing the possibilities from interacting.
Decoherence explains why even though the wave function does not collapse in
the Many Worlds interpretation, it “seems to collapse”, so we never perceive
weird superposition states. According to Tegmark, decoherence is a key reason
why a lot of physicists are willing to come out of the closet and plump for Many
Worlds.
If they were prepared to try out his extraordinary test, they would know for
sure. The bold experimenter stands in front of a quantum machine gun. The gun is
controlled by a particle whose “quantum spin” can point either up or down, each
with a 50 per cent probability. If the spin is measured to be up, the gun fires
a bullet; if it is down, it doesn’t. The particle exists in a superposition of
the two states, and according to the Copenhagen interpretation, when the
measurement is made, the particle chooses either up (bullet) or down (no
bullet).
In this view, the subject of the experiment has a 50 per cent chance of
surviving after one measurement, 25 per cent after two, and so on. After 10
measurements, the chance of surviving has dropped to just 0.1 per cent.
However, if the Many Worlds idea is correct, the particle is not forced to
choose between up and down. All possible outcomes happen, each in a parallel
reality. After one measurement, the experimenter is dead in one parallel
reality, but alive in another. After a second measurement, and a third, the same
principle applies. In all of the parallel realities where the experimenter died,
that is the end of the story: a dead person perceives nothing with 100 per cent
certainty. But in one reality the experimenter survives, and so continues to
perceive the world with 100 per cent certainty. “If the Many Worlds idea is
correct, the experimenter will discover that she is immortal,” says Tegmark.
The catch is that she will never be able convince anyone else of this. In
99.9 per cent of the parallel realities, her assistant saw her die, and even in
the one where she is alive and well, he will have no reason to prefer either
interpretation. He knows that if the Copenhagen interpretation is right, she had
only a 0.1 per cent chance of surviving. But he also knows that if Many Worlds
is right, he himself has only a 0.1 per cent chance of finding himself in the
reality where she survived.
His boss, on the other hand, has every reason to choose. She has only a 0.1
per cent chance of surviving if Copenhagen is right, but if Many Worlds is right
she has a 100 per cent chance of finding herself in the one reality where she
survives.
So would it be worth the gamble? Although Tegmark is a strong believer in the
Many Worlds idea, he has reservations about the experiment. “I’d be OK but my
wife, Angelica, would become a widow,” he says. “Perhaps I’ll do the experiment
one day—when I’m old and crazy.”
- The Interpretation of Quantum Mechanics: Many Worlds or Many Words
by Max Tegmark - Fundamental Problems in Quantum Theory
eds M. H. Rubin and Y. H. Shih, Wiley, 1997 - http://www.sns.ias.edu/~max/everett.html