THE ANTHROPIC LANDSCAPE OF STRING THEORY

Leonard Susskind

I. A Fish Story

Once upon a time, on a planet completely covered by water, there lived a 
race of big-brained fish. These fish could only survive  at a certain depth 
and none had ever seen either the surface above or the bottom below. But 
their big brains made them very smart and also very curious. In time their 
questions about the nature of water and other things became very 
sophisticated. The most brilliant among them were called fyshicists. The 
fyshicists were wonderfully clever and in a few generations they understood 
a great deal about natural phenomena including fluid dynamics,  chemistry, 
atomic physics and even the nuclei of atoms.

Eventually some of the fyshicists began to question why the laws of nature 
are what they are. Their sophisticated technology allowed them to study 
water in all its forms, especially ice, steam and, of course,  the liquid 
state. But with all their efforts there was one thing that stumped them. 
With all the possible values from zero to infinity, how could they account 
for the fact that  background temperature, T, was fine tuned to be in the 
very narrow range that allowed H_2 O to exist in its liquid form? They 
tried many things including symmetries of various kinds, dynamical 
relaxation mechanisms, and many other ideas. But nothing could explain it.

Now closely allied with the fyshicists, another group, the cod-mologists 
were  also studying their watery world. The cod-mologists were less 
interested in the ordinary depths, where the big-brained fish  lived, than 
they were in discovering if there was an upper boundary to their 
water-world. The cod-mologists  were well aware that much of the 
water--world was not habitable, the pressure being wrong for their big 
brains. Journeying by fin to the upper reaches was by no means possible. 
Their big brains would explode if exposed to the very low water pressure in 
these regions. So instead, they speculated.

It happened that there was one school of thought among the cod-mologists 
that held a very radical (some said ridiculous) idea about the fine tuning 
of T. And they had a name for the idea -- the Ickthropic Principle. The 
I.P. maintained that the reason that the temperature was in the liquid 
water range was because only in this case could fish exist to observe it!

Garbage! said the fyshisits , that is not science. It's religion. It's just 
giving up. And besides, if we agree with you everyone will laugh at us and 
take away our funding.

Now not all of the cod-mologists  meant the same thing by the Ickthropic 
Principle. In fact it was hard to find any two who agreed. One thought that 
it meant that the Head Angel Fish had made the world just for the purpose 
of accomodating  big-brained fish. Another thought that the quantum wave 
function of the waterverse was a superposition of all values of T and only 
by observing it did some ancestral fish ``collapse the wave function."

A small number of cod-mologists  led by Andrei-The-Very-Big-Brained, and 
Alexander-Who-Swims-Deep held  a very extraordinary idea. They believed 
that a stupendously big space existed beyond the upper water-boundary. In 
this very big space there might be many other bodies similar in some ways 
to the their water-world but different in other ways.  Some worlds would be 
unimaginably hot. So hot that the hydrogen nuclei might even fuse to form 
helium and then perhaps grow even hotter. Other worlds would be so cold 
that frozen methane would exist. Only a tiny fraction of the bodies would 
be at temperatures conducive to the formation of fish. Then it would be no 
mystery why T was fined tuned. As every angler knows, most places are 
fishless but here and there conditions are just right. And that's where the 
fish are.

But the fyshisits  sighed, Oh Lord, there they go again with their fishy 
ideas. Just ignore them.


II. The Cosmological Constant

Steven Weinberg, in his very wise book, ``Dreams of a Final Theory"  says: 
``Thus, if such a cosmological constant is confirmed by observation, it 
will be reasonable to infer that our own existence plays an important role 
in explaining why the universe is the way it is." Weinberg,  whom Ernst 
Mayr called an ``uncompromising reductionist", a tireless enemy of 
creationism and a champion of the view that the laws of nature can all be 
traced to a small number of basic interactions between elementary 
particles, was saying  the unthinkable, possibly the most shocking 
admission that a modern scientist could  make: Man's place in the scheme of 
things may indeed be at the center. The laws of nature, at least in part, 
may be tailored to our own existence.

In fact Weinberg goes on to say: ``For what it is worth, I hope that this 
is not the case. As a theoretical physicist, I would like to see us able to 
make precise predictions, not vague statements that certain constants have 
to be in a range that is more or less favorable to life. I hope that string 
theory really will provide a basis for a final theory and that this theory 
will turn out to have enough predictive power to be able to prescribe 
values for all the constants of nature, including the cosmological 
constant. We shall see."

Weinberg was expressing the hope of almost all physicists, that the laws of 
nature, are somehow  impersonal and unique; that only one set of elementary 
particles, masses, constants of nature and rules of interaction will 
eventually survive the rigorous tests of mathematical and conceptual 
consistency that  theoretical physicists, of the future, will demand of 
their  final theory. He is expressing the longings for abstract Pythagorean 
simplicity,  consistency and  beauty that is the physicists creed; a kind 
of Myth of Uniqueness. But why then does he appeal to ``our own existence" 
playing ``an important role in explaining why the universe is the way it 
is"? Why the appeal to the  Anthropic Principle?  In part, the answer is 
that Weinberg is not just a physicist; he is also a cosmologist.

By contrast with physicists who, as a rule, focus on the simplest examples 
of nature's handiwork, cosmologists, with the help of astronomers look out 
and see a big messy world. From all that they have learned during the last 
century, there seems to be  one overriding  pattern. The world is very big 
and contains many stars and planets. In its early history it was lumpy 
enough to have formed galaxies but not so lumpy that all matter collapsed 
into black holes. It is expanding but not so fast that everything flew 
apart before galaxies, stars and planets could form. The laws of nuclear 
physics are just right for carbon nuclei to have formed in the interior of 
hot stars,  later to be recycled into new stars and solar systems. And the 
laws of atomic physics allow for the formation of the most unlikely helical 
tinkertoy molecules.  In other words the pattern that seems to trump all 
other considerations is that the natural laws are conveniently fine--tuned 
just to insure our own existence.

Physicists hate this idea. Especially string theorists.

What is this  remarkable  cosmological constant and why is it forcing 
physicists to rethink their entire paradigm? The cosmological constant has 
several facets. First of all it represents an energy content of the vacuum, 
i.e. empty space. That may sound like an oxymoron, empty space having 
energy, but in modern physics the vacuum is a complicated object in which 
violent quantum fluctuations are continually taking place. Electrons, 
positrons, photons and all the other elementary particles are constantly 
being created and annihilated so quickly that to our slow ponderous 
detectors they are usually invisible. But they are there and they give rise 
to an energy density in what we ordinarily think of as empty space. The 
terms vacuum energy density and cosmological constant mean the same thing 
and are used interchangeably.

Why does such vacuum energy make any difference? If it is always there in 
the background why don't we reset the zero of energy and just ignore it? 
The answer lies in the theory of gravitation.

According to Newton, the measure of an objects gravitational potency is its 
mass. Physicists describe this by saying that mass is the source of the 
gravitational field in much the same way that electric charge is the source 
of the electric field. Thus, the gravitational field surrounding the sun 
is  about a million times the field of the earth because the sun is a 
million times more massive. But according to Einstein, mass and energy are 
really the same thing. Every object has an energy content given by the most 
famous of all formuls, E=mc^2. Therefore we can equally well consider the 
gravitational field to be a response to the presence of 
energy.  This  means that the hypothetical energy of empty space would 
produce a gravitational field, i.e. it would cause particles to accelerate 
even in the absence of other objects.  It's effect would resemble a 
universal repulsion that increases with distance.

The best way to visualize  gravitational field caused by a hypothetical 
cosmological constant is in terms of the expansion of space, the expansion 
that cosmologists call the Hubble expansion of the universe. The universe 
is certainly expanding. But without a cosmological constant, the expansion 
would slow down with time. It might even stop expanding and start 
collapsing. But a (positive ) cosmological constant  would have the effect 
of accelerating the expansion causing the distant galaxies to forever 
recede away from one another at a faster and faster rate. The cosmological 
constant is a kind of pressure that expands space in a way that resembles 
the inflation of a balloon, the stars and galaxies being thought of as 
marks on the surface of the balloon. Accelerated expansion due to vacuum 
energy is often referred to as inflation of space.

The most striking thing about the cosmological constant is that it is 
either zero or extremely small. If it were at all sizable it would have a 
serious effect on the motion of astronomical objects. For example it would 
work against ordinary Newtonian attraction between the sun and  earth. If 
it were sufficiently big it would overcome the attraction and cause the 
earth to accelerate away from the sun. Because the effect of the 
cosmological constant increase with distance, it can be important 
cosmologically even if the cosmological constant is very small. For this 
reason the most sensitive determinations of the vacuum energy involve 
cosmological distance scales. Until recently the best that could be said is 
that the cosmological constant is smaller than about 10^{-8} Joules per 
cubic meter.

To appreciate just how small such a cosmological constant  would be,  we 
need a benchmark; a theoretical expectation for how big it could be. Modern 
elementary particle physics gives us such a benchmark. As I mentioned, 
quantum field theory visualizes the vacuum as a complicated state of 
``virtual" particles and anti--particles constantly being created and 
annihilated. Each kind of particle contributes to the vacuum energy with 
some particles  footnote{Particles come it two types; fermions and bosons. 
Electrons, neutrinos and quarks are fermions while photons gluons and some 
others are bosons. Bosons contribute positively to the vacuum energy while 
fermions contribute with a negative sign. } giving positive contributions 
to the vacuum energy and others giving negative contributions.

How big is the contribution from the virtual photons? The answer depends on 
the precise assumptions but a typical calculation would give an answer 
10^{112} Joules per cubic meter;  one hundred and twenty orders of 
magnitude (10^{120}) too large! As many of my colleagues have remarked, 
this is by far  the all time worst discrepancy between theory and 
experiment.

However, there is the possibility that different contributions have 
different sign and cancel when they are all added up.  But the cancellation 
must be staggeringly precise. It   is as if we had several ordinary 
decimals, none of which are particularly small but when we add them up the 
result cancels to 120 decimal places! This seems like an absurd accident 
and we have no idea why it should happen.

Concerning this incredible failure, theorists have played a little 
psychological game with themselves. It goes something like this:

If the cosmological constant  is  that fine--tuned, it probably means that 
there is some as yet unknown principle that requires the vacuum energy to 
be exactly zero. Since string theory is our favorite theory, it must be 
some marvelous mathematical property of string theory that insures the 
cancellation.

But there are two flies in this ointment. First, no one has ever found 
anything in string theory (or any other theory for that matter) that 
suggests  that the cosmological constant vanishes, at least for  a 
realistic version of the theory. And second, astronomical observations in 
the last few years have turned up a very big surprise. The cosmological 
constant is not zero! It seems that the expansion of the universe is 
accelerating albeit with a very small cosmological constant . As absurd as 
it seems, the vacuum energy exactly cancels for the first 119 decimal 
places but then in the 120 place BINGO, a bit of vacuum energy. How 
can  such a situation possible be explained? And what does the partially 
understood String Theory really have to say?

III. The Anthropic Principle

In 1961  the great cosmologist, Robert Dicke, raised an interesting 
question. Given the infinite range of the real numbers, how do we explain 
the numerical fact that the universe is about ten billion year old? Why not 
some very different number? Dicke gave what at first sounds like 
a  ridiculous answer which goes as follows:

In order for anyone to be around to ask this question  intelligent life 
must exist. This implies that the universe has to be old enough for planets 
to have formed and intelligent life to evolve. According to standard 
astronomical and evolutionary theories, that would take about 10 billion 
years. On the other hand if the universe is very much older, the stars will 
all have burned out and could not support life. Therefore there is a finite 
window of time for us to be here to ask the question.


This kind of reasoning came to be known as the Anthropic Principle; this or 
that is true because if it weren't true there would be nobody to ask the 
question. Since Dicke's observations cosmologists have speculated about 
other applications of the Anthropic principle. The general idea is to find 
some numerical parameter in the laws of atomic physics, nuclear physics,... 
which, if slightly different, would not have permitted the existence of 
life as we know it. There are surprisingly many such coincidences. Then, if 
there is no other ``natural" explanation for its value, it becomes a 
candidate for an anthropic explanation. This idea is of course anathema to 
physicists, who see the existence of themselves as an accidental property 
of a universe determined by mathematical principles, to be discovered by 
disinterested analysis.

It was Steven Weinberg, wearing his cosmological hat, but deeply aware of 
the consequences for physics,  who raised the question: Is the improbable 
fine tuning of the cosmological constant  a consequence of the Anthropic 
principle? For the answer to be yes, he would have to show that a value of 
the cosmological constant much bigger than the tiny empirical bound would 
have prevented the formation of life! And indeed Weinberg found  this to be 
so. If the cosmological constant  were a ten or a hundred times bigger than 
the bound, the resulting accelerated expansion would have overcome the 
natural tendency of gravity to clump matter into gravitationally bound 
structures. All matter would have flown apart before it ever had a chance 
to condense into galaxies, stars and habitable planets.

This argument could be interpreted as a prediction: If  the cosmological 
constant  is so small for anthropic reasons alone, then there would be no 
reason for it to be very much smaller than required by that principle. This 
suggests that there ought to be a non--vanishing cosmological constant  at 
a level that could soon be measured. If the cosmological constant  was very 
much smaller then required by the anthropic principle, then the explanation 
must lie elsewhere.


Weinberg's use of the Anthropic principle superficially  sounds similar to 
Dicke's but it is really far more speculative. Dicke was talking about the 
current age of the universe. The age of the universe is not a constant of 
nature. Assuming the universe lives forever, the age  will eventually 
attain every  numerical value. The only question is how long does it take 
for life to form and how long can it last? These are questions that a 
combination of biology, chemistry, physics and astronomy can answer.

By contrast the cosmological constant   is supposed to be a constant of 
nature having a unique  value. The question is; what agency chose that 
value once and for all and for what reason? But suppose for a minute that 
the situation is similar to  the one faced by  the big--brained fish. 
Suppose that as Andrei Linde, Alexander Vilenkin and many other 
cosmologists believe,  the universe is vastly larger than the region that 
has been astronomically explored. footnote{Astronomical observations are 
limited to regions which are close enough to us that light has had time to 
reach us.} Might it  be that the cosmological constant is not really a 
constant but varies throughout the unimaginably larger space. And might 
it  also be that the number of  possible values that it takes on is so 
large that practically every value occurs somewhere. If this were the case 
then the use of the Anthropic principle would be just as trivially correct 
as it was for Dicke. Life would  form only in those environmentally 
friendly regions where the cosmological constant permitted itfootnote{ 
Perhaps I should refer to this idea not simply as the Anthropic Principle 
but rather the Environmental--Anthropic Principle in order to distinguish 
it from other meanings of the term Anthropic Principle. But to spare you 
the constant repetition of this awkward construction I will simply say that 
from now on this is what I will mean by the A.P.}.

As I said, physicists, especially string theorists, are appalled by this 
idea. Believers in the Myth of Uniqueness, they would all have predicted 
that the cosmological constant is exactly zero although they didn't really 
know why. The Myth of Uniqueness is  very pervasive, especially among 
string theorists.

It was in this intellectual environment that the  most 
recent  observational discoveries  were made. I won't go into the details 
but suffice it to say that several different kinds of data now support the 
existence of a cosmological constant  at just about the level to be 
expected by Weinberg's argument. This is not small stuff. It has the 
potential to overturn hundreds of years of thinking about the laws and 
constants of nature. Things, maybe all things, that we thought were ``hard 
wired" into the equations may only be properties of our local environment, 
just like the temperature of of the sea.

My own view is that there is no alternative to some form of anthropic 
explanation of the almost, but not quite, vanishing  cosmological constant 
. footnote{To give my own horn a little  toot, I will mention that I first 
wrote the Story of the big--brained fish about ten years ago as a 
contribution to Martinus Veltman's 60th birthday festschrift.} On the other 
hand I also count myself among the ranks of the string theorists, having 
been associated with the theory since 1969 when Y.Nambu and I (and in a 
slightly different form, H.B. Nielsen) first proposed it. For obvious 
reasons, the discovery of a cosmological constant  in the range suggested 
by Weinberg, makes me more than a little uncomfortable. But about a year 
and a half ago I became rather skeptical that string theorists had the 
right idea about their own theory.  Perhaps I should say, the right 
expectations for their theory. Was Weinberg justified in his hope that   `` 
string theory really will provide a basis for a final theory and that this 
theory will turn out to have enough predictive power to be able to 
prescribe values for all the constants of nature, including the 
cosmological constant" or was his ``We shall see"  expressing a more 
suitable skepticism? The answer, I am now convinced,  is that the pursuit 
of the Unique was a quixotic quest.  In fact I would go as far as to say 
that the whole point of string theory may be the huge diversity of 
environments that it can lead to. It, among all theories, is uniquely well 
suited to an environmental/anthropic theory of the constants of nature.

In my view there two things are required of a theory in order that we can 
justly say that some or all of its constants are anthropically determined. 
The first is that the mathematical equations of the theory should have an 
enormous number of solutions each describing a different environment with 
different values of the constants. Physicists refer to these solutions as 
vacua. To a theoretical physicist a vacuum means more than just empty 
space. It is also a description of all the things that can be added to 
empty space; in other words all the particles and fields that can exist in 
otherwise empty space as well as the  cosmological constant  (and other 
constants). An anthropic theory must have a huge  number of vacuum 
solutions; so many that we can be reasonably sure of finding at least one 
with the desired fine tuned properties.

But it is not enough that the theory should have many solutions. The 
big--brained fish might  realize that the equations of chemistry and 
physics permit a wide variety of different kinds of planets but still 
not  realize that these possibilities  all occur somewhere out  in space. 
To make a convincing case Andrei-The-Very-Big-Brained, and 
Alexander-Who-Swims-Deep would also need a theory of the formation of the 
universe which would explain why space is filled with such diversity. 
Accordingly, the cosmic evolution of the universe should lead in a natural 
way to an extraordinarily large universe filled with every possible 
environment.

IV. What Does String Theory Really Say?

One of the earliest discoveries about string theory was that the dimension 
of space--time is not arbitrary. Most mathematical theories make sense in 
space--times of any dimension. Neither Newton's nor Einstein's theory of 
gravity dictate three dimensions of space. Nor does Maxwell's theory of 
electromagnetism (By the way, the existence of life probably does require 3 
spatial dimensions). But string theory is far more demanding. Depending on 
one's viewpoint it either requires nine or ten dimensions of space and one 
of time. On the face of it this would immediately preclude the theory both 
on anthropic and observational grounds. But string theorists have a good 
trick up their sleeves. Nothing says the  extra six or seven dimensions 
have to big enough for our large selves to move around in the way we move 
in ordinary space. These dimensions may be far smaller than even the minute 
sizes of the particles that we ordinarily consider to be elementary. They 
may be rolled up into tiny spaces such as a microscopic six dimensional 
torus or more complicated geometric six dimensional shapes called Calabi 
Yau spaces. This process is called compactification. There are millions of 
distinct Calabi Yau spaces to choose from. But in this business a million 
possibilities is not a large number.

Choosing a Calabi Yau space is only part of the story. String theory has a 
large number of different kinds of ``moving parts". The shape and size of 
the compact space are described by a set of variables called moduli. A 
typical Calabi Yau space must be supplemented with a couple of hundred 
moduli to  completely specify it. Furthermore there are objects called 
Branes that can wrap around the compact dimensions. There are other objects 
called fluxes that can also be stuck in. All in all a generic 
compactification requires several hundred variables to fix it. These 
variables are not  constants. They may vary either with time or with 
location in ordinary space.

Because the compact dimensions are so small, one might think that the 
particulars of the compactification don't have any effect on large scale 
physics. But this is not correct. They completely determine the microscopic 
laws. Thus one choice might lead to a world with no electrons but six types 
of quarks and a cosmological constant so big that the earth itself would 
fly apart if indeed it ever formed. Another choice might have quarks and 
electrons but no photon. Varying the parameters of compactification changes 
the type of particles, the nature of forces and constants of nature. To 
plot out the possibilities we could draw a set of axes, one for each of the 
several hundred variables. This of course could not be done on a sheet of 
paper. It would take a space of several hundred dimensions to embed all 
those axes footnote{This space should in no way be confused with ordinary 
space.}.

One of the  many things that vary as we move around in this space of 
possibilities is vacuum energy. But not every value of the parameters is a 
legitimate solution of string theory. The real solutions or vacua are the 
local minima of this horrendously complicated energy function. And of great 
importance, the cosmological constant  is different for each of these 
vacua. In fact it is nothing but the value of the energy at the minimum.

Think of the energy function as defining a landscape on the space of the 
parameters. The energy is like the altitude. The landscape is of course in 
a space of several hundred dimensions but nonetheless it has 
mountain  peaks (maxima), valleys (minima) ridges and even flat plains. Due 
to the large number of dimensions, the landscape is wildly diverse. One can 
make a rough estimate of the number of minima that such a function will be 
likely to have and the answer is astonishingly big. A typical function of 
500 variables may have as many as 10^{500} distinct minima. Each of these 
is an environment with its own cosmological constant as well as other 
properties.  The evidence points, not to a unique vacuum but to an 
unimaginably large number. With that many vacua it would be very surprising 
if there was not one with a cosmological constant in the range indicated by 
Weinberg's anthropic argument.

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ The mathematical 
evidence for this humongous landscape of string theory is mounting. The 
techniques for exploring it are being developed by string theorists and all 
signs point toward diversity of exactly the kind required for an anthropic 
theory. Although this is not  what string theorists originally envisioned, 
the reader should make no mistake; The mathematical tools of string theory 
and all the discoveries of the last  few decades are not only fascinating 
in themselves but are  essential to exploring the landscape. Turning the 
Anthropic principle into hard science can only be done if and when the 
landscape is quantitatively understood. For now our best hope  lies in the 
mathematics of string theory. 
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

V. Populating the Landscape

But will the equations of cosmology and string theory lead to a universe 
that populates all, or at least enough, of the landscape to be sure that 
somewhere there is a region of space that has the desired fine features? 
Although these equations are incompletely formulated the answer is likely 
to be yes. The starting point for cosmology is a small universe with an 
exceedingly large energy density; a small ``seed" at some point of ``high 
altitude" footnote{By high altitude I mean a large energy density} in the 
landscape. The large energy density caused the seed to rapidly expand, thus 
diluting the energy. Alternatively, the universe, while growing, rolled 
down the landscape toward regions of lower altitude.

What we find at these lower altitudes is a landscape full of  valleys and 
hills where it is overwhelmingly likely to get stuck in some valley where 
the vacuum energy is at a local minimum. But the likelihood of arriving in 
a valley which is hospitable to life is neglibible. The overwhelming 
majority of the valleys have a vacuum energy far outside Weinberg's window 
of opportunity for life to form.


But that's not the end. First of all as we learned many years ago from the 
work of Alan Guth and others, a universe with a positive vacuum energy will 
begin to inflate at an exponential rate, doubling its size in a very small 
time. It will soon be many orders of magnitude bigger than when it began. 
By this time the original details of the seed have long been forgotten. 
That is one of the effects of accelerated expansion; it tends to dilute 
away all traces of the starting point.

But this is still not all. It is a feature of string theory that there will 
always be regions of the landscape with smaller vacuum energy. Every 
physicist knows that this indicates an instability. The nature of this 
instability was worked out years ago by Sidney Coleman and  ....De Lucia. 
What typically happens is that small bubbles slowly form in the inflating 
vacuum. These bubbles are regions in which the cosmological constant is 
smaller than in the ambient inflating background.  If the bubbles are too 
small they will shrink and disappear. But every so often a larger bubble 
will nucleate and then start to grow. Because the seed has grown so large 
there is enough  volume for a huge number of bubbles.  These bubbles are 
regions where the universe has made a transition to a neighboring region of 
the landscape. But although the vacuum energy is smaller inside the bubble 
it is not likely to be zero. Thus the space inside these bubbles  inflates. 
The bubbles themselves soon become prodigiously large and the process 
continues with new bubbles forming inside the old, effectively populating 
the entire landscape with every possible kind of vacuum. Although this 
phantasmagoric image seems like something out of the mind of a madman, it 
is hard to see how it could be wrong. Given the diverse  string theoretic 
landscape of possibilities, and the well known the rules for quantum 
nucleation of bubbles in an inflating space it is inevitable that an 
indefinitely large number of bubble-universes will form. Some small 
fraction will be within the anthropic window. And it is in one of these 
regions that we find ourselves.

To say that all of this  rigorously  follows from the precise mathematics 
of string theory is not justified at the present time. We don't know with 
certainty what the landscape is really like and how many valleys at each 
altitude are there. We also don't have the technology to rigorously study 
the evolution as the population of bubbles spreads out over the landscape. 
But the individual elements all seem increasingly plausible and I think we 
will know, in not too long a time, if string theory really does have the 
properties that I am suggesting.  If it does then I think it will not be 
long until string theorists readjust their sights and begin to see the 
non--uniqueness and diversity as the best and most convincing argument for 
their theory.

Direct observational confirmation of the vastness and diversity of the 
landscape is probably not possible. The space between bubbles is typically 
expanding so rapidly that no signals can reach  one of them from any other. 
Physicists would say they are out of causal contact. They are  outside each 
other's cosmic horizons. But our own region,  has passed through several 
valleys in the landscape before standard cosmology had its start. In 
principle those earlier epochs left an observable imprint on the relic 
microwave radiation from the big bang. The problem is that the enormous 
inflation of the universe dilutes most of these imprints to the point where 
they can not be observed. However if we are very very lucky, the last 
episode of inflation which began  our current big--bang cosmology might not 
have lasted long enough to obliterate all evidence of a transition from 
some neighboring valley. In the words of Weinberg, ``We shall see."

If this view of nature is correct then there is cold comfort for those who 
look to the anthropic principle for a deeper meaning to their own 
existence. As Darwin's,   principle of survival of the fittest, eliminated 
the need for the hand of god to guide evolution, so too does the 
environmental intepretation of the anthropic principle eliminate the 
necessity for a guardian angel to fine tune the laws of nature. But the one 
thing that we can be sure of is that the last word on these matter is yet 
to be said.