[Paleopsych] NS: Is string theory in trouble?

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Is string theory in trouble?
http://www.newscientist.com/article.ns?id=mg18825305.800&print=true

      * 17 December 2005
      * Amanda Gefter

    Ever since Albert Einstein wondered whether the world might have been 
different,
    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 
candidates,
    is in trouble. A growing number of physicists claim it is ill-defined and 
based
    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 
physics.

    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 
cosmological
    constant, which describes the expansion rate of the universe, appears 
improbably
    fine-tuned for life. A little bigger or smaller and life could not exist. 
With an
    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 
means
    that the basic features of our universe are just a random luck of the draw. 
He
    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 
right.
    This theory that the universe expanded spectacularly in the first fraction 
of a
    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 
universe.
    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 
line
    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 
energy
    of empty space which contributes to the expansion rate of the universe - 
seems
    absurdly improbable, and nothing in fundamental physics is able to explain 
why. I
    remember when Steven Weinberg first suggested that the cosmological constant
    might be anthropically determined - that it has to be this way otherwise we 
would
    not be here to observe it. I was very impressed with the argument, but 
troubled
    by it. Like everybody else, I thought the cosmological constant was probably 
zero
    - meaning that all the quantum fluctuations that make up the vacuum energy 
cancel
    out, and gravity alone affects the expansion of the universe. It would be 
much
    easier to explain if they cancelled out to zero, rather than to nearly zero. 
The
    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 
different
    vacuums, which describe very different physical environments, tipped the 
scales
    for me. At first, string theorists thought there were about a million 
solutions.
    Thinking about Weinberg's argument and about the non-zero cosmological 
constant,
    I used to go around asking my mathematician friends: are you sure it's only 
a
    million? They all assured me it was the best bet.

    But a million is not enough for anthropic explanations - the chances of one 
of
    the universes being suitable for life are still too small. When Joe 
Polchinski
    and Raphael Bousso wrote their paper in 2000 that revealed there are more 
like
    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 
possibility,
    so I wrote a paper saying so. The initial reaction was very hostile, but 
over the
    past couple of years people are taking it more seriously. They are worried 
that
    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 
itself.
    Is it such a radical change?

    In a way it is very radical but in another way it isn't. The great ambition 
of
    physicists like myself was to explain why the laws of nature are just what 
they
    are. Why is the proton just about 1800 times heavier than the electron? Why 
do
    neutrinos exist? The great hope was that some deep mathematical principle 
would
    determine all the constants of nature, like Newton's constant. But it seems
    increasingly likely that the constants of nature are more like the 
temperature of
    the Earth - properties of our local environment that vary from place to 
place.
    Like the temperature, many of the constants have to be just so if 
intelligent
    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 
much
    bigger, richer and full of variety than we ever expected. And it doesn't 
seem so
    incredibly philosophically radical to think that some things may be
    environmental.

    In order to accept the idea that we live in a hospitable patch of a 
multiverse,
    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, 
or by
    Kepler's idea of nested Platonic solids. We simply have to reassess which 
things
    will be universal consequences of the theory and which will be consequences 
of
    cosmic history and local conditions.

    So even if you accept the multiverse and the idea that certain local 
physical
    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 
thing, the
    whole universe or multiverse. It's a scientific question: is the universe on 
the
    largest scales big and diverse or is it homogeneous? We can hope to get an 
answer
    from string theory and we can hope to get some information from cosmology.

    There is a philosophical objection called Popperism that people raise 
against the
    landscape idea. Popperism [after the philosopher Karl Popper] is the 
assertion
    that a scientific hypothesis has to be falsifiable, otherwise it's just
    metaphysics. Other worlds, alternative universes, things we can't see 
because
    they are beyond horizons, are in principle unfalsifiable and therefore
    metaphysical - that's the objection. But the belief that the universe beyond 
our
    causal horizon is homogeneous is just as speculative and just as susceptible 
to
    the Popperazzi.

    Could there be some kind of selection principle that will emerge and pick 
out one
    unique string theory and one unique universe?

    Anything is possible. My friend David Gross hopes that no selection 
principle
    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 
this
    view. Even most of the hard-core adherents to the uniqueness view admit that 
it
    looks bad.

    Is it premature to invoke anthropic arguments - which assume that the 
conditions
    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 
kind
    of life is the only kind possible. Why should we presume that all life is 
like us
    - carbon-based, needs water, and so forth? How do we know that life cannot 
exist
    in radically different environments? If life could exist without galaxies, 
the
    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 
exist
    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 
exist
    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 
surface of
    a sphere. It's a long shot but not as unlikely as I previously thought. 
Inflation
    tells us that our observable universe likely began in a different vacuum 
state,
    that decayed into our current vacuum state. It's hard to believe that's the 
whole
    story. It seems more probable that our universe began in some other vacuum 
state
    with a much higher cosmological constant, and that the history of the 
multiverse
    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 
background.
    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 
reason, the
    landscape turns out to be inconsistent - maybe for mathematical reasons, or
    because it disagrees with observation - I am pretty sure that physicists 
will go
    on searching for natural explanations of the world. But I have to say that 
if
    that happens, as things stand now we will be in a very awkward position. 
Without
    any explanation of nature's fine-tunings we will be hard pressed to answer 
the ID
    critics. One might argue that the hope that a mathematically unique solution 
will
    emerge is as faith-based as ID.

Leonard Susskind

    Leonard Susskind is the Felix Bloch Professor of Theoretical Physics at 
Stanford
    University in California. His book Cosmic Landscape: String theory and the
    illusion of intelligent design is published this week by Little, Brown 
($24.95,
    £14.33, ISBN 0316155799)