Many universes, many theories

Physics World, May 3, 2011http://physicsworld.com/cws/article/indepth/45854
 
Brian Greene on whether our universe is just one among many
– and whether theories of the multiverse will ever qualify as science
 
As a popular-science author, Brian Greene is nothing if not ambitious.
 
In his first book, The Elegant Universe, he set out to explain string theory
– one of the most complex and mind-bending areas of physics –
in a way that non-scientists could understand.
 
A few years later,
he tackled the nature of space and time
in The Fabric of the Cosmos.
 
But Greene's latest book
The Hidden Reality goes further still,
exploring one of the biggest questions of all:
is our universe the only one that exists,
or is it merely one among many?
 
In this special audio interview (below),http://physicsworld.com/cws/article/indepth/45854
Physics World reviews editor Margaret Harris
talks to Greene about some of the multiverse ideas
described in the book, and whether parallel universes
will ever become part of accepted scientific theory.
 
The book "does not argue for [the multiverse]
in a monolithic way", says Greene,
a physicist at Columbia University in New York.
 
Instead, he explains, it suggests
that the possibility of parallel universes
is something "worth thinking about,
worth taking seriously" even though
there are important questions
about how to turn such speculations into science.
 
One of the most compelling reasons
for taking multiple universes seriously, Greene says,
is the fact that many different areas of physics
seem to hint at their existence.
 
"People in physics who are trying to work out
the deepest laws that might describe
the small, the big and everything inbetween
are repeatedly coming upon the idea
that there may be other universes," he says.
 
"It's not that some
unidirectional fringe approach to physics
has suggested this possibility.
 
If you study cosmology, or relativity,
or quantum mechanics, or string theory,
and you follow these theories far enough,
they each bump into some variation
on the theme of parallel universes."
 
Of course, this common multiverse theme
could be a cosmic red herring.
 
Without solid predictions
or experiments to test such theories,
we cannot be sure.
 
Greene acknowledges that falsifiability
– the ability to prove a theory wrong –
is "the key question"
for proponents of the multiverse.
 
Indeed, he argues that some variants
of multiverse theory "stand outside science"
because they can never be falsified, even in principle.
 
However, he also notes that mathematics
has long provided a guide for exploring ideas
that are beyond our current ability to test,
and many of those ideas have later
been confirmed through experiment or observation.
 
In the near future, Greene hopes
that data from collisions
at CERN's Large Hadron Collider
will turn up what he calls
"circumstantial evidence"
that favours his preferred option
of a string-based multiverse.
 
He is also looking forward to getting back to physics.
 
"I find that these books are a nice change of pace," he says.
"But I think it will be a long time before I embark on another one – if ever."