by Roger Highfield
September/21/2007
from
TheTelegraphNewspaper Website
Parallel universes really do exist, according to a mathematical
discovery by Oxford scientists that sweeps away one of the key
objections to the mind boggling and controversial idea.
The work has wider implications since the idea of parallel universes
sidesteps one of the key problems with time travel. Every since it
was given serious lab cred in 1949 by the great logician Kurt
Godel, many eminent physicists have argued against time travel
because it undermines ideas of cause and effect to create paradoxes:
a time traveler could go back to kill his grandfather so that he is
never born in the first place.
But the existence of parallel worlds offers a way around these
troublesome paradoxes, according to David Deutsch of Oxford
University, a highly respected proponent of quantum theory, the
deeply mathematical, successful and baffling theory of the atomic
world.
He argues that time travel shifts between different branches of
reality, basing his claim on parallel universes, the socalled
"manyworlds" formulation of quantum theory.
The new work bolsters his claim that quantum theory does not forbid
time travel.
"It does sidestep it. You go into
another universe," he said yesterday, though he admits that
there is still a way to go to find schemes to manipulate space
and time in a way that makes time hops possible.
"Many sci fi authors suggested time travel paradoxes would be
solved by parallel universes but in my work, that conclusion is
deduced from quantum theory itself", Dr Deutsch said, referring
to his work on many worlds.
The mathematical idea of parallel worlds
was first glimpsed by the great quantum pioneer, Erwin
Schrodinger, but actually published in 1957 by Hugh Everett
III, when wrestling with the problem of what actually happens
when an observation is made of something of interest  such as an
electron or an atom  with the intention of measuring its position
or its speed.
In the traditional brand of quantum mechanics, a mathematical object
called a wave function, which contains all possible outcomes of a
measurement experiment, "collapses" to give a single real outcome.
Everett came up with a more audacious interpretation: the universe
is constantly and infinitely splitting, so that no collapse takes
place. Every possible outcome of an experimental measurement occurs,
each one in a parallel universe.
If one accepts Everett's interpretation, our universe is embedded in
an infinitely larger and more complex structure called the
multiverse, which as a good
approximation can be regarded as an evermultiplying mass of
parallel universes.
Every time there is an event at the quantum level  a radioactive
atom decaying, for example, or a particle of light impinging on your
retina  the universe is supposed to "split" into different
universes.
A motorist who has a near miss, for instance, might feel relieved at
his lucky escape. But in a parallel universe, another version of the
same driver will have been killed. Yet another universe will see the
motorist recover after treatment in hospital. The number of
alternative scenarios is endless.
In this way, the "many worlds" interpretation of quantum mechanics
allows a time traveler to alter the past without producing problems
such as the notorious grandfather paradox. But the "many worlds"
idea has been attacked, with one theoretician joking that it is
"cheap on assumptions but expensive on universes" and others that it
is "repugnant to common sense." Now new research confirms Prof
Deutsch's ideas and suggests that Dr Everett, who was a Phd student
at Princeton University when he came up with the theory, was on the
right track.
Commenting in New Scientist magazine, Prof Andy Albrecht,
a physicist at the University of California, Davis, said of the link
between probability and many worlds:
"This work will go down as one of
the most important developments in the history of science."
Quantum mechanics describes the
strange things that happen in the subatomic world  such as the way
photons and electrons behave both as particles and waves. By one
interpretation, nothing at the subatomic scale can really be said to
exist until it is observed.
Until then, particles occupy nebulous "superposition" states, in
which they can have simultaneous "up" and "down" spins, or appear to
be in different places at the same time.
According to quantum mechanics, unobserved particles are
described by "wave functions" representing a set of multiple
"probable" states. When an observer makes a measurement, the
particle then settles down into one of these multiple options.
But the many worlds idea offers an alternative view. Dr Deutsch
showed mathematically that the bushlike branching structure created
by the universe splitting into parallel versions of itself can
explain the probabilistic nature of quantum outcomes. This work was
attacked but it has now had rigorous confirmation by David
Wallace and Simon Saunders, also at Oxford.
Dr Saunders, who presented the work with Wallace at the Many
Worlds at 50 conference at the Perimeter Institute for
Theoretical Physics in Waterloo, Canada, told New Scientist:
"We've cleared up the obscurities
and come up with a pretty clear verdict that Everett works. It's
a dramatic turnaround and it means that people now have to
discuss Everett seriously."
Dr Deutsch added that the work addresses
a threecenturyold problem with the idea of probability itself,
described by one philosopher, Prof David Papineau, as a
scandal.
"We didn't really know what
probability means," said Dr Deutsch.
There's a convention that it's rational
to treat it for most purposes as if we knew it was going to happen
even though we actually know it need not.
But this does not capture the reality,
not least the 0.1 per cent chance something will not happen.
"So," said Dr Deutsch, "the problems
of probability, which were until recently considered the
principal objection to the otherwise extremely elegant theory of
Everett (which removes every element of mysticism and
doubletalk that have crept into quantum theory over the
decades) have now turned into its principal selling point."
