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Date: Thu, 21 Oct 2004 15:27:13 +0000 (UTC)

The tautologous nature of the current anthropic principle is due to the
fact that it is as incomplete as Dirac's Large Numbers Hypothesis was
flawed, is all:
www.anthropic-principle.ORG

Re: THE LANDSCAPE: A Talk with Leonard Susskind
Responses by Paul Steinhardt, Lee Smolin, Kevin Kelly, Alexander
Vilenkin, Lenny Susskind, Steve Giddings, Lee Smolin, Gino Segre, Lenny
Susskind, Gerard 't Hooft, Lenny Susskind, Maria Spiropulu

Paul Steinhardt
Well, the quote is right. I love Lenny, but I hate this recent landscape
idea and I am hopeful it will go away.
PAUL STEINHARDT is the Albert Einstein Professor in Science and on the
faculty of both the Departments of Physics and Astrophysical Sciences at
Princeton University.
Lee Smolin
I want to preface my remarks by saying that since my student days Lenny
Susskind has been for me a hero and a role model. The following remarks
are offered with great respect and admiration.
To start with, Susskind must be commended for courageously calling
people's attention to an apparently fundamental feature of string
theory: that it appears to allow for a huge number of different versions
(or, as some would prefer, solutions) each of which describes a universe
with different laws of physics. Basic features of a universe, such as
its dimensionality, the nature and strengths of the different forces and
the masses of the elementary particles vary from string theory to string
theory.
As Lenny says, this means that the old dream of a unified theory that
makes unique and falsifiable predictions appears no longer possible.
Much that physicists hoped to explain as necessary features of any
possible universe are just contingent, or environmental features of one
universe out of many possible ones.
Without in any way diminishing the importance of Susskind's recent
views, it should be said that several people have been making the same
argument, using very similar language, for many years. My book, The Life
of the Cosmos (1997), describes the same scenario of a landscape of
string theories, and explores the question of whether this situation is
inevitable and, if so, what this means for the future of science. One of
the main points it makes, however, is that the anthropic principle is a
wrong turn. There are alternatives which can resolve the worries of
those who don't like the anthropic principle, while taking into account
the surprising scenario described by Susskind.
Of course, the intelligent reader will want to know how strong the
actual evidence is that justifies the strong statements Susskind makes.
It may help first to explain why Susskind and other string theorists
have only recently begun to worry about these problems. Since the late
1980's it has been known that string theory has a great many solutions,
which describe universes with different properties. However, until
recently, all the known string solutions described universes that
disagreed with observations in one or more essential ways. For one
thing, most of them did not describe worlds with three macroscopically
large dimensions of space. But of those that did, they all had two
properties that disagreed with observation: unobserved symmetries
(called supersymmetries) and unobserved long range forces (in the
technical jargon, massless scalar fields.) To this was added in recent
years a third problem: the universe appears to have a positive vacuum
energy, but all consistent string theories then known had zero or
negative vacuum energy.
Thus, until very recently string theorists could hope that even if
string theory has many solutions, there would be only one solution
consistent with what observations tell us about the world.
A year ago there were new results that changed the situation quite a
bit. Very clever calculations by Shamit Kachru and collaborators gave
indirect evidence for the existence of string theories which agree with
the following observed aspects of our universe: 1) four large dimension,
2) positive vacuum energy, 3) no unbroken supersymmetry, 4) no massless
scalar fields. This was the first evidence for the existence of any
version or solution of string theory consistent with all these observed
features of our world.
But there was a twist. This new solution was not unique-quite the
opposite. Instead, Michael Douglas, Susskind and others argue that if
any string theories exist with these characteristics, so do at least
10(100) others. It is the vastness of this number that leads to the
apparently revolutionary implications Susskind speaks of.
For the sake of accuracy, it is important to stress that the evidence
for these string theories is indirect and not necessarily compelling.
Not a single one of these 10(100) string theories has actually been
constructed or otherwise shown to exist. Nor can any calculations be
done in any of these theories-even to the lowest order of approximation.
The results at hand are very far from an actual demonstration of the
existence of these theories-even at the loose level of rigor that
characterizes much work by theoretical physics.
In fact, no string theories-even the original five supersymmetric
theories in ten dimensions-have been conclusively demonstrated to exist.
There still remain unproven conjectures such as the finiteness and
consistency of any superstring theory, past the first three terms of a
certain approximation scheme. But, if a few issues remain unresolved in
the best cases, far less is known about the conjectured string theories
Susskind is talking about.
So the present results allow three possibilities:
String theory is true, but the string theories Kachru et al find weak
evidence for do not in fact exist. Some other way will ultimately be
found to construct at least one string theory that agrees with all
features of our observed universe. String theory is true and the string
theories Kachru et al find evidence for are genuine solutions to it.
String theory is false, because no consistent version of the theory
exists or no version agrees with all experimental results. One of the
alternative approaches to quantum gravity instead will turn out to be
the road ahead for physics. Note that even if the first possibility is
true we cannot escape the implications of what Lenny is saying. The
reason is that even if some day a unique solution to string theory is
found that describes our world, we will never get rid of the large
number of string theory solutions that do not describe our world. So
whatever happens, if string theory is true we have to explain why the
solution that describes our world is picked out of a large collection of
solutions that describe very different worlds.
Thus, unless string theory is wrong, we cannot avoid what Lenny Susskind
is saying.
So does string theory imply the anthropic principle as Susskind seems to
suggest? Does it mean that we have to either give up string theory or
give up the dream of a fundamental theory that makes falsifiable
predictions for real doable experiments?
There is a simple and, so far as I know, irrefutable, argument that
leads to the conclusion that no theory that employs the anthropic
principle, as advocated by Susskind, could be falsified. This is because
it affirms the existence of an ensemble of "universes", at least one of
which has the properties already observed to be true of our own.
Furthermore, the total number of possible theories believed to exist is
so vast that it is reasonable to believe that the subset that agree with
all present observations will still be vast. Consequently, there will
likely be myriads of theories that agree with any possible result of
future experiments. Thus, there will be no way any conceivable
experimental result could contradict the theory.
I follow many philosophers and historians in believing that a necessary
part of what has made science a successful path to truth is that the
ethic of science requires that we study only falsifiable theories. We
only consider theories as possibly true if they are vulnerable to
falsification by real experiments, and we only believe them after they
have survived significant and stringent attempts to so falsify them.
This means that if science is to go on, we must find an alternative to
the anthropic principle.
Fortunately, it is not hard to find an alternative to the anthropic
principle in the scenario Susskind describes. All one needs to do is to
add to the theory two additional hypotheses, which may in fact be
themselves consequences of the fundamental theory.
The two hypotheses are: i) black hole and cosmological singularities
bounce, due to quantum gravity effects, and are replaced by the birth of
new universes, ii) each new universe that results is only slightly
different than its parent, in that the parameters of their physical laws
differ by small numbers.
As I described in my book, and related papers, these two hypotheses give
the "landscape" of theories the structure of a fitness landscape. These
are mathematical models from evolutionary biology. It is easy to see
that, once these are added to the theory, falsifiable predictions can be
obtained. For example, the observation of a single neutron star with a
mass greater than twice that of the sun would rule the theory out.
Of course, this means the theory may very well be proven false in the
near future. This means it is science. What we must avoid is the
situation Susskind describes, in which a theory is believed despite
there being not a single prediction for a genuine experiment whose
results could falsify it.
It can also be mentioned that recent work by Martin Bojowald and
collaborators provides strong evidence that hypothesis i) is a
prediction of at least one quantum theory of gravity (loop quantum
gravity). If Bojowald's techniques could be applied to string theory-and
I believe it likely they can be- one might very well be able to test
hypothesis ii).
To summarize, after the recent evidence summarized by Susskind, the key
question still appears to be the following: Is there any alternative to
either a) science proceeding without a falsifiable fundamental theory or
b) cosmology and physics relying on dynamical mechanisms like natural
selection to give falsifiable accounts of how our universe came to be
described by the laws we observe. If there are alternatives, I hope
someone will find one soon. If not, I certainly hope that b) is true,
because I believe strongly that rational argument about experimental
evidence is our only reliable path to truth.
Before closing, I want to inject a note of caution about Susskind's
claim that string theory has resolved the puzzles about black holes
posed by Hawking. Susskind makes the claim that, ""To this day, the only
real physics problem that has been solved by string theory is the
problem of black holes." I do not want to diminish the importance or the
beauty of the string theory results that pertain to black holes. As far
as they go, they are extremely impressive. But it should be noted that
many experts in quantum gravity are unconvinced that the problem posed
by Hawking has been solved by the actual results in string theory. The
reason is that the string theory results which give exact agreement with
the earlier work of Hawking are mainly restricted to a very special
class of black holes. These are black holes which have as much, or
nearly as much, charge as possible, given their mass. These do not
include real physical black holes, such as those the astronomers have
evidence for.
Furthermore, it is not yet possible in string theory to study directly
the spacetimes of even these very special black holes. The most precise
results are gotten by extrapolating very cleverly from certain systems
without gravity. These have similar statistical properties to these very
special black holes-but they are not actually black holes.
At the same time, there has been genuine progress understanding real
black holes in other approaches to quantum gravity, such as loop quantum
gravity. The fact that string theory has been unable to duplicate these
results is related to the fact that string theories so far can only
describe in any detail worlds with the unphysical characteristics
referred to above, such as exact supersymmetry. As a result, many
experts believe that the jury is still out on whether Hawking's
conjectures about black holes and information are true or not.
LEE SMOLIN. a theoretical physicist, is a founding member and research
physicist at the Perimeter Institute in Waterloo Canada. He is the
author of The Life of The Cosmos and Three Roads to Quantum Gravity.
Kevin Kelly
The best, most amazing Edge interview yet. It was educational beyond the
call of duty, full of insider gossip, and funny! I inhaled it in one
breath. Great going.
KEVIN KELLY is Editor-At-Large, Wired; Author of Out of Control: The New
Biology of Machines, Social Systems, and the Economic World; New Rules
for the New Economy;and Cool Tools.

Alexander Vilenkin
I would like to comment on Lee Smolin's view, that anthropic arguments
are unpredictive, unfalsifiable, and therefore unscientific. There has
been a lot of confusion about what the anthropic approach is and how it
should be used. Here I will argue that, when properly used, this
approach does yield testable predictions, and thus meets all the
standards of a scientific theory. Let me first clarify what I mean by
the anthropic approach. The definition Lenny Susskind gives in his
article is a bit too simplistic: "The kind of answer that this or that
is true because if it were not true there would be nobody to ask the
question is called the anthropic principle". In other words, if some
constant of Nature has certain values which do not permit the existence
of intelligent observers, then the "anthropic principle" says that such
values are not going to be observed. This "principle" is, of course,
guaranteed to be true. If this were all there is to anthropic arguments,
I would have to admit that Lee Smolin has a point. But there is more to
it than that.
Suppose our theory predicts that the constants of Nature vary from one
part of the Universe to another, and we want to extract testable
predictions from that theory. Then, instead of looking for extreme
values of the constants that make observers impossible, we can try to
predict what values will be measured by a typical observer. In other
words, we can make statistical predictions, assigning probabilities to
different values of the constants. If any principle needs to be invoked
here, it is what I call "the principle of mediocrity" – the assumption
that we are typical observers in the Universe, so the values of the
constants we observe should be close to the maximum of probability. If
instead we measure a value very far from the probability peak, this
should be regarded as evidence against the theory. For example, if the
observed value has probability of 1%, we can say that the theory is
ruled out at 99% confidence level.
To illustrate my point, it's best to look at a specific example. Let us
consider the parameter that Lenny mentioned in his article: the
cosmological constant that causes the Universe to expand with
acceleration. The larger this constant is, the earlier the accelerated
expansion begins. And once this happens, the process of galaxy
formation, which is crucial for the evolution of observers, comes to a
halt. If the cosmological constant varies from one part of the Universe
to another, then regions where it is larger will have fewer galaxies.
This point was recognized by Steven Weinberg, who showed that regions
where the cosmological constant is more than 100 times greater than the
present density of matter in the Universe would have no galaxies at all,
and therefore no observers. Clearly, such values will never be observed.
To improve on this analysis, we can use the theory of galaxy formation
to determine the probabilities for different values of the cosmological
constant. If we pick a galaxy at random, we can ask, what is the
probability that this galaxy is in a region where the cosmological
constant has such and such a value. The answer is that the cosmological
constant measured by most observers in the Universe should be a few
times greater than the present density of matter. Observations in our
local region show that it is greater by a factor of about 3, as
expected. Remarkably, the prediction was made in 1995, more than two
years before the cosmological constant was actually measured. If the
value turned out to be much greater or much smaller than it actually is,
the anthropic explanation would be ruled out at a high confidence level.
ALEXANDER VILENKIN is Director, Tufts Institute of Cosmology.

Leonard Susskind
First I want to thank Paul Steinhardt for his concise summary of the
views of the other side in this debate.
As to Smolin's less concise summary I am afraid of getting into an
endless debate so I will say what I usually say to the students in my
premed class: Hear me carefully because I will not explain again.
Smolin is correct. He did recognize the kind of diversity in the laws of
physics that string theory suggests. He is also correct that string
theorists can not prove that any of the solutions to string theory are
really solutions, even the supersymmetric ones. Nor can anyone prove the
sun will rise tomorrow. The level of confidence that string theorists
have for their theory is based on a web of interconnected pieces of
evidence that is so compelling that genuine mathematicians have no doubt
about it's validity.
More relevant is Smolin's claim about the new non-supersymmetric
solutions of my colleagues at Stanford and the Tata institute, KKLT.
They have not undergone sufficient scrutiny. But the outsider to the
subject should understand that string theorists watched with horror, not
pleasure, the discovery of the gigantic landscape of solutions. And yet
no string theorist that I know is prepared to say they these solutions
don't exist. Like Steinhardt they quake in their boots and pray for
deliverance. It is not impossible but all agree that it is unlikely.
As for Smolin's speculations about the evolution of the universe, let me
say that almost all cosmologists would agree that the universe is
reproducing. But they would not agree that the dominant mechanism is
universes inside black holes (talk about unobservable!).
The most efficient mechanism according to cosmologists and one that is
gaining strong observational support, is eternal inflation. Inflation is
the exponential reproduction of the universe due to a cosmological
constant. Perhaps black holes add to the process but I doubt it. In any
case it is absolutely clear that we do not live in the fittest kind of
universe which would be the universe with the largest cosmological
constant. We live in a universe, which is fit to live in and a large
cosmological constant would render our universe fatal to nuclei, atoms
and life.
Alexander Vilenkin is a hero of the revolution and I always listen very
carefully to what he says. He says that my statement "The kind of answer
that this or that is true because if it were not true there would be
nobody to ask the question is called the anthropic principle" is
simplistic. Yes it is and it was intended that way. It's a definition
that entirely misses the subtleties that Vilenkin explains. However it
does express a broad-brush definition that covers the many things that
are called the anthropic principle.
My own view is that we don't yet know enough to use the A.P. in a
predictive way. Vilenkin disagrees. But what I am sure we, and also
Paul, would agree is that we will be in a much better position to argue
the merits of the AP when the landscape is more thoroughly explored.
This is probably a job for the string theorists.

Steve Giddings
Some thoughts on the landscape and the anthropic principle:
I'm not a big fan of the anthropic principle. But physics is not
designed for you or me to like—it is what it is, and that may mean
certain features of our physical world are explained by anthropic
reasoning.
If true, this is simply one more step down the Copernican path.
Copernicus taught us that the Earth is not the center of the universe.
If the idea of the "string landscape" and its population through effects
like eternal inflation hold true, then the entire visible universe is
not particularly special or unique, but rather is just a small and
unremarkable part of an even larger universe. The constants of nature in
our region aren't specially tuned to any particular a priori values.
Rather we must take a more Darwinian view: life evolves where it can,
and in our particular region of the larger universe, or "megaverse," it
evolved because the conditions—the strength of electromagnetism, the
magnitude of the cosmological constant, and so on—allow life to
evolve. Our kind of life couldn't have evolved in a region where these
constants took a significantly different value.
I find this viewpoint no more disturbing than the simple observation
that life didn't evolve in the center of the sun. There are regions of
the visible universe that are hospitable to life and those that aren't,
and the same could hold for the megaverse.
One of the thing that disturbs many physicists with this picture is it's
apparent lack of predictability. There are many different possible
values for the many physical parameters, and figuring out what region of
this space is the "L=1" surface, where life has unit probability of
emerging, is an enormously complicated and perhaps not wholly tractable
problem. No longer can we follow the dream of discovering the unique
equations that predict everything we see, and writing them on a single
page. Predicting the constants of nature becomes a messy environmental
problem. It has the complications of biology.
But I feel the views of some, that such a picture is unscientific, or a
cop-out, are extreme. In particular, understanding the laws that give
rise to the megaverse is a very scientific question, and one that I
think is well worth studying further. For example, in a paper with
Kachru and Polchinski, we outlined a lot of the basic structure
underlying one piece of the megaverse that people are talking a lot
about today. But we have a ways to go in fully understanding even this
piece of the megaverse—indeed its internal consistency has been
questioned by Banks and Dine, and it's conceivable the picture could
collapse entirely. And assuming that this piece is eventually well
understood, it may well be the tip of the iceberg, with many other
interesting pieces of the megaverse yet to be explored.
This may force us to rethink the kinds of questions that we hope to
answer—such as trying to predict the precise value of the cosmological
constant. But it does open up the possibility of investigating other
kinds of questions, and could well be testable, once we figure out how
to test string theory experimentally. If we're very lucky that could
even happen with the Large Hadron Collider, cosmological observations,
or perhaps other ways we haven't thought of.
Another fascinating part of the picture is a generic feature of the
"landscape." Indeed, this feature would appear to be present even if
string theory proves not to be the correct theory of quantum gravity.
This feature regards the ultimate fate of the Universe. Indeed, as long
as there are extra dimensions of space, and the presently observed
positive value of the cosmological constant, it appears that the extra
dimensions of space will ultimately become unstable, and can begin to
grow. Having a positive cosmological constant is like being in a high
mountain valley, and sooner or later, through quantum effects or
otherwise, the universe should find its way down to the plains. Thus
whether or not we find the extra dimensions of space, ultimately they
will find us.
STEVE GIDDINGS is a theoretical physicist at University of California,
Santa Barbara.

Lee Smolin
Regarding Susskind's always vivid comments, I am glad we agree about the
basic point that string theory leads to a landscape of theories. The
issue I have been concerned with for some time is the same Susskind
closes with: how can we get predictions from a theory of this kind? Two
possible answers are the anthropic principle and cosmological natural
selection. The conclusion I have come to after a lot of thought is that
the latter is likely to lead to a larger number of falsifiable
predictions.
To avoid confusion it must be emphasized that the term "anthropic
principle" is used with several meanings. I agree that the definition
Alex Vilenkin gives is nothing but commonsense logic and that, "when
used properly", in conjunction with physical hypotheses, it can lead to
some falsifiable predictions. However, in these cases, what is
falsifiable is not the commonsense logic, but the physical hypotheses it
is combined with. This is the case in the example Alex, gives, regarding
the cosmological constant. Here the calculations depend on hypotheses
about quantum cosmology and the physics of galaxy formation. If his
predictions are proved wrong, he will want to amend those hypotheses,
and not the logic used in his reasoning.
My comments were addressed to a different version of the anthropic
principle in which someone posits a multiverse model, and then claim its
predictions are verified because the ensemble of universes contains at
least one universe that has the properties we observe ours to have.
Problems with falsifiability arise when the ensemble is so vast, that
there will be members that agree with any possible future experiments.
No falsifiable prediction are possible, because whatever is observed
will be true of some members of the ensemble.
But even if we agree to employ Alex's weaker definition, there are
further questions. Can we predict the value of any parameter we can
measure, or are we restricted to making predictions about just a few
parameters ?
For example, as pointed out by Anthony Aguirre, there are many possible
universes that contain life, but are very different from our own, such
as universes where the big bang was cold rather than hot. The anthropic
principle cannot explain why we do not live in one of these universes.
Hence there are basic features of our world it cannot explain or
predict.
There are also problems when the anthropic principle is used to save a
theory that otherwise makes incorrect predictions. This can happen when
very few members of the ensemble of universes predicted by the theory
resemble our world. In such cases, to make reliable predictions about a
parameter , x, both the a priori probability given by the theory to
members of the ensemble and the probability for life, must depend
strongly on x. The cosmological constant is one case in which this is
satisfied. But there will be many cases in which it is not satisfied. In
these cases the theory cannot make predictions. A good example of this
is eternal inflation. In eternal inflation the probability, or fitness
depends as Susskind says, strongly on the cosmological constant.
However, the probability depends only very weakly on most measurable
parameters such as the masses and charges of the stable elementary
particles. This is because their values have little effect at the
physical scales at which the reproduction of universes takes place
(which are much higher in energy than those so far probed
experimentally.) Thus, eternal inflation, by itself, cannot explain or
predict the values of these observable parameters. Even when the
anthropic principle, in Alex's sense, is added, it is still very
difficult to make predictions for future measurements, having to do with
unstable particles, whose existence and properties affect neither the
probability for observers or the probability for inflation.
Let us compare this with the cosmological natural selection scenario in
which the mode of reproduction is through black holes. The rate of
reproduction of universes through black holes does depend very
sensitively on many observable parameters. This is because the
properties of ordinary matter determine the rate of formation of massive
stars that become black holes. As a result, almost all members of the
ensemble generated will, if the theory is true, resemble our universe.
There is no need to call on the anthropic principle to extract a
sub-ensemble consisting of otherwise extremely improbable universes.
Hence, if black hole formation dominates reproduction of universe, we
have an opportunity to explain the values of all those parameters,
without relying on the anthropic principle. As a result, the theory
gives falsifiable predictions, testable by observations of things like
neutron stars. This gives this theory, if true, much more potential
explanatory power.
Regarding string theory, here also my intention is to be constructive. I
think it is useful in the development of a theory to keep clearly in
mind exactly what has been proved, and what remains open and still
requires proof. It is unfortunately the case that many key links in the
"web of interconnected pieces of evidence" that support string theory
remain unproven conjectures, even at a physicists level of rigor,
despite many years of study by many very smart people. It is true that
some, "genuine mathematicians have no doubt about it's validity". But
other genuine mathematicians who have studied the technical issues
involved do have serious doubts. Given that the theory so far makes no
contact with experiment, it is to be hoped that further work will
improve this situation.
Similarly, Susskind's claim that the fittest universe is the one with
largest cosmological constant depends on internal inflation being true.
But eternal inflation is much more than just the claim that the universe
inflated at early times. It is a large step from present observations to
the claim that eternal inflation has strong observational support. That
step requires a number of assumptions, which we can hope will be checked
as both theory and observation become more precise.

Gino Segre
It may well be that we are part of a megaverse, as Lenny says. This may
be the next step in a 500 year progression of our thinking. In 1543
Copernicus proposed that the Earth was not the center of the Universe.
Some 70 years later, Galileo showed with his telescope that those milky
looking objects in the sky were made up of many stars. From this the
notion of many galaxies eventually evolved, but humans still clung to
the idea that Earth was at the center of their own galaxy. That notion
was finally disproved by Shapley in the 1920s .
We now believe we live on an ordinary planet, one of many, circling an
ordinary star, one of many, in an ordinary galaxy, one of many. Perhaps
we need to take the next step, admittedly a revolutionary one, of saying
we live in an ordinary universe, a very small part of an enormous
megaverse. However , as controversial as each one of those earlier
proposals was, they were all confirmed unambiguously by scientific
observations. Science has both a revolutionary and a conservative side,
revolutionary in the proposing of dramatic new possibilities and
conservative in the requirement of demanding experimental evidence
before they are accepted.
As with past notions, the idea of a megaverse will require experimental
confirmation before it is accepted. Superstring theory and the existence
of extra dimensions will likewise have to clear the same hurdle.
Megaverse may be the right path and it may not— the existence of a
cosmological constant has caught us all by surprise and some genius may
yet calculate its value in a way we cannot even imagine right now,
showing us a new road to follow.
Whatever happens, we are all grateful that some very exciting
experiments in both particle physics and cosmology will be taking place
in the coming years. Hopefully they will help us sort it out.
GINO SEGRE, a professor of Physics and Astronomy at the University of
Pennsylvania, is the author of A Matter of Degrees.

Lenny Susskind
A year or two ago most theoretical high energy physicists would have
dismissed any talk of the anthropic principle as anti-science. However,
as I said in the interview; "because of unprecedented new developments
in physics, astronomy and cosmology these same physicists are being
forced to reevaluate their prejudices about anthropic reasoning." The
attitude among the more thoughtful physicists has softened to "hmmm,
maybe we better think about this." The messages of Steve Giddings and
Gino Segre reflect this less biased mindset. Segre correctly emphasizes
the importance of experimental tests of theoretical ideas. In this
connection I want to point out that Weinberg predicted that if the AP is
correct, the cosmological constant would turn out to be non-zero.
Moreover he predicted the correct order of magnitude. This was more than
a decade ago. Finally I want to re-emphasize that it's not just the
cosmological constant that is pushing us in the "anthropic landscape"
direction. The success of inflation strongly suggests that we live in a
very big universe. The other clear fact is that string theory gives rise
to a stupendously rich landscape with perhaps 10(500) vacua with no
reason to prefer one over the other. Sure it's possible that some genius
will come along and explain the cosmological constant by some
mathematical magic but things sure don't seem to be going in that
direction.

Gerard 't Hooft
During the '80s, a number of physicists became more and more excited
about what was called "super string theory". The rather bizarre
mathematical equations that emerge if one attempts to subject
"relativistic strings" to the laws of Quantum Mechanics, had previously
appeared to be inconsistent, but are now recognized as possibly
describing fundamental elementary particles together with gravitational
forces quite similar to those of Einstein's general theory of
relativity. Even so, inconsistencies continued unless one postulated
very special kinds of projection schemes and symmetries, such as
supersymmetry.
Supersymmetry of the type needed has not yet been detected among the
real particles of Nature, and also other predictions of the theory could
not yet be checked against experiment. These are by themselves no
reasons to dismiss the theory; supersymmetry is also predicted by other
arguments, and the domain of physics where the theory should apply
directly, the so-called Planck domain, is so far separated from what can
be observed under controlled circumstances, that one should really
admire these deep and stimulating ideas than try to ridicule them, as
some other physicists are sometimes seen doing.
However, when I hear Lenny say that "this theory is going to win, and
physicists who are trying to deny what is going on are going to lose",
then to my opinion he is going too far. I have several reasons for
advising my friends to practice caution, modesty and restraints when
they air their suspicion that this theory "is" the everlasting and
complete theory of the Universe. If this theory indeed allows for 10^500
distinct solutions out of which we somehow have to choose—some say it
is 10^1000 solutions, nobody really seems to know—then this must be
seen as an enormous setback. Less than a decade ago we still hoped that
some stability argument could be used to single out the single, "
correct" solution; apparently this hope has been abandoned. Now, they
are invoking the "anthropic principle", which really means: try all of
these solutions until you find a Universe that looks like the world we
live in. This is not the way physics has worked for us in the past, and
it is not too late to hope that we will be able to find better arguments
in the future.
On top of this, there are even more serious objections against
"superstring theory". It has already been recognized now that
superstring theory itself only describes a tiny corner of our world, the
corner where these strings happen to interact only weakly, because as
soon as they interact more strongly, nobody can follow the equations
anymore, let alone solve them. In the past, whenever I complained about
this, my voice was hardly heard, but now all string theorists say: "O,
yes, but then the theory can be reformulated in terms of another theory
that is related to the previous version by what is called 'duality'."
And, for convenience, it is then forgotten that this new theory, called
'M-theory', again only exists in a few tiny little corners of the world.
How do we plan to formulate and understand the complete picture? Can one
obtain a complete picture along such lines at all? String theorists are
so confident of their expectations that such questions are usually
ignored.
This is because the duality schemes that have been discovered are
extremely suggestive. Indeed the mathematical equations repeatedly turn
out to show a magnificent degree of perfection. But what does all of
this really mean? String theorists say: " this can only mean that our
theories are true, and this is the scheme used by God to create our
Universe."
It is hard to argue with that, since such arguments have some religious
overtones. My own "religion" tells me that theories of this sort can
never be more than approximations. Perhaps the approximations contain
some truth, but the ultimate laws of Nature must contain a fundamental
and simple, concise relation between 'cause and effect', between past
and future, between close-by and far-away. Such principles could not be
built in whatever formulation of 'M-theory' people could give. This is
because the duality arguments that are being used do not refer to the
local equations, but to their symmetry properties instead. This should
be recognised as a weakness of the theory. Take the proud boasts
concerning black holes; the resulting picture leaves no shred of
locality or causality in the laws controlling these mysterious objects.
But this is what I am waiting for. Such a simple demand is unfortunately
far too much to ask from what is now called superstring theory or
M-theory, and as long as I don't see any progress in this respect I
treat the claims with caution and restraint.
GERARD 'T HOOFT, Nobel Laureate, is Professor of Theoretical Physics at
University of Untrecht.

Leonard Susskind
Gerard advises caution and restraint. That's hard to argue with. I
consider myself to be a cautious, rather conservative physicist. I
really don't like new ideas. But I also find wisdom in a quote from
Sherlock Holmes; "When you have eliminated all that is impossible,
whatever remains must be the truth, no matter how improbable it is." A
couple of times I have reached the point where I felt forced to a very
unconventional idea, because I could see no way out of it. One case that
particularly comes to mind is the "Holographic Principle." This was a
crazy idea but I would guess that Gerard felt the same way as I did; all
conventional alternatives led to paradox or inconsistency. That is
exactly the way I feel about the cosmological constant.
I've watched for 40 years as people tried this scheme, and that scheme,
to explain the absence of vacuum energy, but they all failed. I've also
seen string theorists fail over and over in trying to find a "vacuum
selection" principle that would pick out a particular version of the
theory. Add to this the fact that astronomers find that the cosmological
constant is non-zero but just barely small enough for galaxies to form,
I personally feel that we have come to a point where "whatever remains
must be the truth, no matter how improbable it is." Here's what we know:
The cosmological constant is probably not zero but falls in the narrow
range of values that allows galaxies, stars and planets to form. The
evidence for this is empirical.
There is growing empirical evidence confirming the inflationary theory
of cosmology. It follows that the universe is much larger than what we
can observe.
Theories of inflation tend to produce domains of space with varying
vacuum properties such as the vacuum energy (cosmological constant).
This is from theoretical studies.
String theory has a very large number of vacuum solutions. Some are
supersymmetric but these do not support ordinary chemistry. In addition
there appear to be a huge number of non-supersymmetric vacua with non
zero cosmological constant. As Gerard says, the numbers could be as
large as 10 to the 500 power or bigger. The evidence for this is
mathematical but not rigorous.
Gerard may not find a pattern here but I do. It's a matter of taste and
judgement.
My comments about the "theory winning" and "theorists in denial" was
mainly aimed at those string theorists want to avoid the facts. Their
own theory is pointing in a very different direction than what they
hoped. I did not have in mind people like 't Hooft who remain skeptical
of string theory. However I do take exception to his claim "the
resulting picture leaves no shred of locality or causality in the laws
controlling these mysterious objects." Here I can only say that I
believe Gerard is wrong.
Finally, I would ask Gerard; do you have a better idea?_____ I want to
add one technical comment to the above response. In Gerard's message he
says "the ultimate laws of Nature must contain a fundamental and simple,
concise relation between 'cause and effect', between past and future,
between close-by and far-away. Such principles could not be built in
whatever formulation of 'M-theory' people could give." I completely
agree with the first sentence in quotes. I don't agree with the second.
The present formulation of (uncompactified) M-theory is called M(atrix)
theory. It is a conventional quantum mechanical theory with a
Hamiltonian and a Shroedinger equation. The relation between past and
future, cause and effect are exactly the same as in any other quantum
mechanical system. While I certainly agree that there is a lot missing,
I think it is too much to say, "the resulting picture leaves no shred of
locality or causality."

Maria Spiropulu
I don't know how else to understand the anthropic principle other than
the "simplistic" way. Does anybody have a scientifically precise
definition of this principle and how to apply it?
In the physics I have learned there were many examples of where the
mathematics was giving infinite degenerate solutions to a certain
problem (classical mechanics problems e.g.). There the problem was
always a mistake in the physics assumptions. Infinity is mathematical
not physical, as far as I know.
There lies the difference between math and physics. In math you have the
equation and you look for the solution—the solution can be a set of
solutions-infinite solutions. In physics you start from the answer—the
real world (scale by scale as I learned from Polchinski) and you seek
the equation. There are measurements (well there are many measurements,
many experiments, resulting in one arithmetic value for this or that),
and you look for the equation. If the equation gives you nonsense, then
it is not the measurement that it is wrong but the equation.
In other words one should not expect to derive the uniqueness of the
universe starting from an infinite set of solutions to a beautiful
equation. One should start from the universe, which is the one universe
that we measure, and try to find a theory that describes it.
I don't understand anthropic remarks like the sun-earth distance is just
right to allow the appropriate chemistry for humans to be. Of course it
does. But before the chemistry was there, the distance was the same. It
is more interesting to research the thermonuclear reactions in the sun,
discover something about the neutrinos, understand the radioactive
warming of the earth's core, study the earth's atmosphere, and in
general find why the temperature and chemistry is what it is—not for
us to be here but for the phenomena to be what they are. And I find it
rather absurd to believe that if we were not here the sun-earth distance
would be different and the universe would be upsidedown.
The whole anthropic thinking seems to me intellectually decadent. It
takes obviously true positive statements, then negates them to makes a
conditional negative argument, which is then regarded as profound or
scientific.
The argument "The environment has to be right for us to exist" is
obviously right. But scientifically I find it is a redundant statement.
Of course I cannot be in an environment that I cannot survive in, and
study that environment at large. But I can study the enviroment I live
in and this is what I do. The life-centric view of the works of the
cosmos seems to me too mystical to be able to deal with scientifically.
MARIA SPIROPULU, a physicist, is currently at CERN. She has been working
at the Tevatron with UCSB and was an Enrico Fermi Fellow at the
EFI/University of Chicago.
Back to THE LANDSCAPE: A Talk with Leonard Susskind
John Brockman, Editor and Publisher
Russell Weinberger, Associate Publisher
contact: editor@edge.org
Copyright © 2003 by Edge Foundation, Inc
All Rights Reserved.

"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well with it."

Stephen Hawking

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