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 10100 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 10100 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 A) 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.