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 canceled 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 10500 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 pushes 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 Paparazzi.
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.
From issue 2530 of New Scientist
magazine, 17 December 2005, page 48
