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FLICKING through New Scientist, you stop at this page, think 鈥渢hat鈥檚
interesting鈥� and read these words. Another you thinks 鈥渨hat nonsense鈥�, and moves
on. Yet another lets out a cry, keels over and dies.

Is this an insane vision? Not according to David Deutsch of the University of
Oxford. Deutsch believes that our Universe is part of the multiverse, a domain
of parallel universes that comprises ultimate reality.

Until now, the multiverse was a hazy, ill-defined concept鈥攍ittle more
than a philosophical trick. But in a paper yet to be published, Deutsch has
worked out the structure of the multiverse. With it, he claims, he has answered
the last criticism of the sceptics. 鈥淔or 70 years physicists have been hiding
from it, but they can hide no longer.鈥� If he鈥檚 right, the multiverse is no
trick. It is real. So real that we can mould the fate of the universes and
exploit them.

Why believe in something so extraordinary? Because it can explain one of the
greatest mysteries of modern science: why the world of atoms behaves so very
differently from the everyday world of trees and tables.

The theory that describes atoms and their constituents is quantum mechanics.
It is hugely successful. It has led to computers, lasers and nuclear reactors,
and it tells us why the Sun shines and why the ground beneath our feet is solid.
But quantum theory also tells us something very disturbing about atoms and their
like: they can be in many places at once. This isn鈥檛 just a crazy
theory鈥攊t has observable consequences (see 鈥淚nterfering with the
尘耻濒迟颈惫别谤蝉别鈥�).

But how is it that atoms can be in many places at once whereas big things
made out of atoms鈥攖ables, trees and pencils鈥攁pparently cannot?
Reconciling the difference between the microscopic and the macroscopic is the
central problem in quantum theory.

The many worlds interpretation is one way to do it. This idea was proposed by
Princeton graduate student Hugh Everett III in 1957. According to many worlds,
quantum theory doesn鈥檛 just apply to atoms, says Deutsch. 鈥淭he world of tables
is exactly the same as the world of atoms.鈥�

But surely this means tables can be in many places at once. Right. But nobody
has ever seen such a schizophrenic table. So what gives?

The idea is that if you observe a table that is in two places at once, there
are also two versions of you鈥攐ne that sees the table in one place and one
that sees it in another place.

The consequences are remarkable. A universe must exist for every physical
possibility. There are Earths where the Nazis prevailed in the Second World War,
where Marilyn Monroe married Einstein, and where the dinosaurs survived and
evolved into intelligent beings who read New Scientist.

However, many worlds is not the only interpretation of quantum theory.
Physicists can choose between half a dozen interpretations, all of which predict
identical outcomes for all conceivable experiments.

Deutsch dismisses them all. 鈥淪ome are gibberish, like the Copenhagen
interpretation,鈥� he says鈥攁nd the rest are just variations on the many
worlds theme.

For example, according to the Copenhagen interpretation, the act of observing
is crucial. Observation forces an atom to make up its mind, and plump for being
in only one place out of all the possible places it could be. But the Copenhagen
interpretation is itself open to interpretation. What constitutes an
observation? For some people, this only requires a large-scale object such as a
particle detector. For others it means an interaction with some kind of
conscious being.

Worse still, says Deutsch, is that in this type of interpretation you have to
abandon the idea of reality. Before observation, the atom doesn鈥檛 have a real
position. To Deutsch, the whole thing is mysticism鈥攖hrowing up our hands
and saying there are some things we are not allowed to ask.

Some interpretations do try to give the microscopic world reality, but they
are all disguised versions of the many worlds idea, says Deutsch. 鈥淭heir
proponents have fallen over backwards to talk about the many worlds in a way
that makes it appear as if they are not.鈥�

In this category, Deutsch includes David Bohm鈥檚 鈥減ilot-wave鈥� interpretation.
Bohm鈥檚 idea is that a quantum wave guides particles along their trajectories.
Then the strange shape of the pilot wave can be used to explain all the odd
quantum behaviours, such as interference patterns. In effect, says Deutsch,
Bohm鈥檚 single universe occupies one groove in an immensely complicated
multi-dimensional wave function.

鈥淭he question that pilot-wave theorists must address is: what are the
unoccupied grooves?鈥� says Deutsch. 鈥淚t is no good saying they are merely
theoretical and do not exist physically, for they continually jostle each other
and the occupied groove, affecting its trajectory. What鈥檚 really being talked
about here is parallel universes. Pilot-wave theories are parallel-universe
theories in a state of chronic denial.鈥�

Back and forth

Another disguised many worlds theory, says Deutsch, is John Cramer鈥檚
鈥渢ransactional鈥� interpretation in which information passes backwards and
forwards through time. When you measure the position of an atom, it sends a
message back to its earlier self to change its trajectory accordingly.

But as the system gets more complicated, the number of messages explodes.
Soon, says Deutsch, it becomes vastly greater than the number of particles in
the Universe. The full quantum evolution of a system as big as the Universe
consists of an exponentially large number of classical processes, each of which
contains the information to describe a whole universe. So Cramer鈥檚 idea forces
the multiverse on you, says Deutsch.

So do other interpretations, according to Deutsch. 鈥淨uantum theory leaves no
doubt that other universes exist in exactly the same sense that the single
Universe that we see exists,鈥� he says. 鈥淭his is not a matter of interpretation.
It is a logical consequence of quantum theory.鈥�

Yet many physicists still refuse to accept the multiverse. 鈥淧eople say the
many worlds is simply too crazy, too wasteful, too mind-blowing,鈥� says Deutsch.
鈥淏ut this is an emotional not a scientific reaction. We have to take what nature
gives us.鈥�

A much more legitimate objection is that many worlds is vague and has no firm
mathematical basis. Proponents talk of a multiverse that is like a stack of
parallel universes. The critics point out that it cannot be that
simple鈥攓uantum phenomena occur precisely because the universes interact.
鈥淲hat is needed is a precise mathematical model of the multiverse,鈥� says
Deutsch. And now he鈥檚 made one.

The key to Deutsch鈥檚 model sounds peculiar. He treats the multiverse as if it
were a quantum computer. Quantum computers exploit the strangeness of quantum
systems鈥攖heir ability to be in many states at once鈥攖o do certain
kinds of calculation at ludicrously high speed. For example, they could quickly
search huge databases that would take an ordinary computer the lifetime of the
Universe. Although the hardware is still at a very basic stage, the theory of
how quantum computers process information is well advanced.

In 1985, Deutsch proved that such a machine can simulate any conceivable
quantum system, and that includes the Universe itself. So to work out the basic
structure of the multiverse, all you need to do is analyse a general quantum
calculation. 鈥淭he set of all programs that can be run on a quantum computer
includes programs that would simulate the multiverse,鈥� says Deutsch. 鈥淪o we
don鈥檛 have to include any details of stars and galaxies in the real Universe, we
can just analyse quantum computers and look at how information flows inside
迟丑别尘.鈥�

If information could flow freely from one part of the multiverse to another,
we鈥檇 live in a chaotic world where all possibilities would overlap. We really
would see two tables at once, and worse, everything imaginable would be
happening everywhere at the same time.

Deutsch found that, almost all the time, information flows only within small
pieces of the quantum calculation, and not in between those pieces. These
pieces, he says, are separate universes. They feel separate and autonomous
because all the information we receive through our senses has come from within
one universe. As Oxford philosopher Michael Lockwood put it, 鈥淲e cannot look
sideways, through the multiverse, any more than we can look into the
蹿耻迟耻谤别.鈥�

Sometimes universes in Deutsch鈥檚 model peel apart only locally and
fleetingly, and then slap back together again. This is the cause of quantum
interference, which is at the root of everything from the two-slit experiment to
the basic structure of atoms.

Other physicists are still digesting what Deutsch has to say. Anton Zeilinger
of the University of Vienna remains unconvinced. 鈥淭he multiverse interpretation
is not the only possible one, and it is not even the simplest,鈥� he says.
Zeilinger instead uses information theory to come to very different conclusions.
He thinks that quantum theory comes from limits on the information we get out of
measurements
(New Scientist, 17 February, p 26).
As in the Copenhagen
interpretation, there is no reality to what goes on before the measurement.

But Deutsch insists that his picture is more profound than Zeilinger鈥檚. 鈥淚
hope he鈥檒l come round, and realise that the many worlds theory explains where
the information in his measurements comes from.鈥�

Why are physicists reluctant to accept many worlds? Deutsch blames logical
positivism, the idea that science should concern itself only with objects that
can be observed. In the early 20th century, some logical positivists even denied
the existence of atoms鈥攗ntil the evidence became overwhelming. The
evidence for the multiverse, according to Deutsch, is equally overwhelming.
鈥淎dmittedly, it鈥檚 indirect,鈥� he says. 鈥淏ut then, we can detect pterodactyls and
quarks only indirectly too. The evidence that other universes exist is at least
as strong as the evidence for pterodactyls or quarks.鈥�

Perhaps the sceptics will be convinced by a practical demonstration of the
multiverse. And Deutsch thinks he knows how. By building a quantum computer, he
says, we can reach out and mould the multiverse.

鈥淥ne day, a quantum computer will be built which does more simultaneous
calculations than there are particles in the Universe,鈥� says Deutsch. 鈥淪ince the
Universe as we see it lacks the computational resources to do the calculations,
where are they being done?鈥� It can only be in other universes, he says. 鈥淨uantum
computers share information with huge numbers of versions of themselves
throughout the multiverse.鈥�

Imagine that you have a quantum PC and you set it a problem. What happens is
that a huge number of versions of your PC split off from this Universe into
their own separate, local universes, and work on parallel strands of the
problem. A split second later, the pocket universes recombine into one, and
those strands are pulled together to provide the answer that pops up on your
screen. 鈥淨uantum computers are the first machines humans have ever built to
exploit the multiverse directly,鈥� says Deutsch.

At the moment, even the biggest quantum computers can only work their magic
on about 6 bits of information, which in Deutsch鈥檚 view means they exploit
copies of themselves in 2⁶ universes鈥攖hat鈥檚 just 64 of them. Because the
computational feats of such computers are puny, people can choose to ignore the
multiverse. 鈥淏ut something will happen when the number of parallel calculations
becomes very large,鈥� says Deutsch. 鈥淚f the number is 64, people can shut their
eyes but if it鈥檚 10⁶⁴, they will no longer be able to pretend.鈥�

What would it mean for you and me to know there are inconceivably many yous
and mes living out all possible histories? Surely, there is no point in making
any choices for the better if all possible outcomes happen? We might as well
stay in bed or commit suicide.

Deutsch does not agree. In fact, he thinks it could make real choice
possible. In classical physics, he says, there is no such thing as 鈥渋f鈥�; the
future is determined absolutely by the past. So there can be no free will. In
the multiverse, however, there are alternatives; the quantum possibilities
really happen. Free will might have a sensible definition, Deutsch thinks,
because the alternatives don鈥檛 have to occur within equally large slices of the
multiverse. 鈥淏y making good choices, doing the right thing, we thicken the stack
of universes in which versions of us live reasonable lives,鈥� he says. 鈥淲hen you
succeed, all the copies of you who made the same decision succeed too. What you
do for the better increases the portion of the multiverse where good things
丑补辫辫别苍.鈥�

Let鈥檚 hope that deciding to read this article was the right choice.

You can see the shadow of other universes using little more than a light
source and two metal plates. This is the famous double-slit experiment, the
touchstone of quantum weirdness.

Particles from the atomic realm such as photons, electrons or atoms are fired
at the first plate, which has two vertical slits in it. The particles that go
through hit the second plate on the far side.

Imagine the places that are hit show up black and that the places that are
not hit show up white. After the experiment has been running for a while, and
many particles have passed through the slits, the plate will be covered in
vertical stripes alternating black and white. That is an interference
pattern.

To make it, particles that passed through one slit have to interfere with
particles that passed through the other slit. The pattern simply does not form
if you shut one slit.

The strange thing is that the interference pattern forms even if particles
come one at a time, with long periods in between. So what is affecting these
single particles?

According to the many worlds interpretation, each particle interferes with
another particle going through the other slit. What other particle? 鈥淎nother
particle in a neighbouring universe,鈥� says David Deutsch. He believes this is a
case where two universes split apart briefly, within the experiment, then come
back together again. 鈥淚n my opinion, the argument for the many worlds was won
with the double-slit experiment. It reveals interference between neighbouring
universes, the root of all quantum phenomena.鈥�