[Paleopsych] NS: Is string theory in trouble?
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Fri Jan 6 18:03:21 UTC 2006
Is string theory in trouble?
* 17 December 2005
* Amanda Gefter
Ever since Albert Einstein wondered whether the world might have been
physicists have been searching for a theory of everything to explain why the
universe is the way it is. Now string theory, one of today's leading
is in trouble. A growing number of physicists claim it is ill-defined and
on crude assumptions. Something fundamental is missing, they say. The main
complaint is that rather than describing one universe, the theory describes
10^500, each with different constants of nature, even different laws of
But the inventor of string theory, physicist Leonard Susskind, sees this
landscape of universes as a solution rather than a problem. He says it could
answer the most perplexing question in physics: why the value of the
constant, which describes the expansion rate of the universe, appears
fine-tuned for life. A little bigger or smaller and life could not exist.
infinite number of universes, says Susskind, there is bound to be one with a
cosmological constant like ours.
The idea is controversial, because it changes how physics is done, and it
that the basic features of our universe are just a random luck of the draw.
explains to Amanda Gefter why he thinks it's a possibility we cannot ignore.
Why are physicists taking the idea of multiple universes seriously now?
First, there was the discovery in the past few years that inflation seems
This theory that the universe expanded spectacularly in the first fraction
second fits a lot of data. Inflation tells us that the universe is probably
extremely big and necessarily diverse. On sufficiently big scales, and if
inflation lasts long enough, this diversity will produce every possible
The same process that forged our universe in a big bang will happen over and
over. The mathematics are rickety, but that's what inflation implies: a huge
universe with patches that are very different from one another. The bottom
is that we no longer have any good reason to believe that our tiny patch of
universe is representative of the whole thing.
Second was the discovery that the value of the cosmological constant - the
of empty space which contributes to the expansion rate of the universe -
absurdly improbable, and nothing in fundamental physics is able to explain
remember when Steven Weinberg first suggested that the cosmological constant
might be anthropically determined - that it has to be this way otherwise we
not be here to observe it. I was very impressed with the argument, but
by it. Like everybody else, I thought the cosmological constant was probably
- meaning that all the quantum fluctuations that make up the vacuum energy
out, and gravity alone affects the expansion of the universe. It would be
easier to explain if they cancelled out to zero, rather than to nearly zero.
discovery that there is a non-zero cosmological constant changed everything.
Still, those two things were not enough to tip the balance for me.
What finally convinced you?
The discovery in string theory of this large landscape of solutions, of
vacuums, which describe very different physical environments, tipped the
for me. At first, string theorists thought there were about a million
Thinking about Weinberg's argument and about the non-zero cosmological
I used to go around asking my mathematician friends: are you sure it's only
million? They all assured me it was the best bet.
But a million is not enough for anthropic explanations - the chances of one
the universes being suitable for life are still too small. When Joe
and Raphael Bousso wrote their paper in 2000 that revealed there are more
10^500 vacuums in string theory, that to me was the tipping point. The three
things seemed to be coming together. I felt I couldn't ignore this
so I wrote a paper saying so. The initial reaction was very hostile, but
past couple of years people are taking it more seriously. They are worried
it might be true.
Steven Weinberg recently said that this is one of the great sea changes in
fundamental science since Einstein, that it changes the nature of science
Is it such a radical change?
In a way it is very radical but in another way it isn't. The great ambition
physicists like myself was to explain why the laws of nature are just what
are. Why is the proton just about 1800 times heavier than the electron? Why
neutrinos exist? The great hope was that some deep mathematical principle
determine all the constants of nature, like Newton's constant. But it seems
increasingly likely that the constants of nature are more like the
the Earth - properties of our local environment that vary from place to
Like the temperature, many of the constants have to be just so if
life is to exist. So we live where life is possible.
For some physicists this idea is an incredible disappointment. Personally, I
don't see it that way. I find it exciting to think that the universe may be
bigger, richer and full of variety than we ever expected. And it doesn't
incredibly philosophically radical to think that some things may be
In order to accept the idea that we live in a hospitable patch of a
must a physicist trade in that dream of a final theory?
Absolutely not. No more than when physicists discovered that the radii of
planetary orbits were not determined by some elegant mathematical equation,
Kepler's idea of nested Platonic solids. We simply have to reassess which
will be universal consequences of the theory and which will be consequences
cosmic history and local conditions.
So even if you accept the multiverse and the idea that certain local
laws are anthropically determined, you still need a unique mega-theory to
describe the whole multiverse? Surely it just pushs the question back?
Yes, absolutely. The bottom line is that we need to describe the whole
whole universe or multiverse. It's a scientific question: is the universe on
largest scales big and diverse or is it homogeneous? We can hope to get an
from string theory and we can hope to get some information from cosmology.
There is a philosophical objection called Popperism that people raise
landscape idea. Popperism [after the philosopher Karl Popper] is the
that a scientific hypothesis has to be falsifiable, otherwise it's just
metaphysics. Other worlds, alternative universes, things we can't see
they are beyond horizons, are in principle unfalsifiable and therefore
metaphysical - that's the objection. But the belief that the universe beyond
causal horizon is homogeneous is just as speculative and just as susceptible
Could there be some kind of selection principle that will emerge and pick
unique string theory and one unique universe?
Anything is possible. My friend David Gross hopes that no selection
will be necessary because only one universe will prove to make sense
mathematically, or something like that. But so far there is no evidence for
view. Even most of the hard-core adherents to the uniqueness view admit that
Is it premature to invoke anthropic arguments - which assume that the
for life are extremely improbable - when we don't know how to define life?
The logic of the anthropic principle requires the strong assumption that our
of life is the only kind possible. Why should we presume that all life is
- carbon-based, needs water, and so forth? How do we know that life cannot
in radically different environments? If life could exist without galaxies,
argument that the cosmological constant seems improbably fine-tuned for life
would lose all of its force. And we don't know that life of all kinds can't
in a wide variety of circumstances, maybe in all circumstances. It a valid
objection. But in my heart of hearts, I just don't believe that life could
in the interior of a star, for instance, or in a black hole.
Is it possible to test the landscape idea through observation?
One idea is to look for signs that space is negatively curved, meaning the
geometry of space-time is saddle-shaped as opposed to flat or like the
a sphere. It's a long shot but not as unlikely as I previously thought.
tells us that our observable universe likely began in a different vacuum
that decayed into our current vacuum state. It's hard to believe that's the
story. It seems more probable that our universe began in some other vacuum
with a much higher cosmological constant, and that the history of the
is a series of quantum tunnelling events from one vacuum to another. If our
universe came out of another, it must be negatively curved, and we might see
evidence of that today on the largest scales of the cosmic microwave
So the landscape, at least in principle, is testable.
If we do not accept the landscape idea are we stuck with intelligent design?
I doubt that physicists will see it that way. If, for some unforeseen
landscape turns out to be inconsistent - maybe for mathematical reasons, or
because it disagrees with observation - I am pretty sure that physicists
on searching for natural explanations of the world. But I have to say that
that happens, as things stand now we will be in a very awkward position.
any explanation of nature's fine-tunings we will be hard pressed to answer
critics. One might argue that the hope that a mathematically unique solution
emerge is as faith-based as ID.
Leonard Susskind is the Felix Bloch Professor of Theoretical Physics at
University in California. His book Cosmic Landscape: String theory and the
illusion of intelligent design is published this week by Little, Brown
£14.33, ISBN 0316155799)
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