I am very pleased that Lenny Susskind has taken the time to respond to my paper on the Anthropic Principle (AP) ["Scientific alternatives to the anthropic principle"] and to discuss cosmological natural selection (CNS). Susskind is for me the most inspiring figure of his generation of elementary particle physicists. Indeed, the initial ideas that became loop quantum gravity came from applying to quantum gravity some of what I had learned from his work on gauge theories. And when in the late 1990's I began to work again on string theory, it was because of papers of his describing how special relativity was compatible with string theory.

I was thus extremely pleased when Susskind began arguing for a view of string theory I came to some time ago — that there is not one theory, but a "landscape" of many theories. But I was equally disturbed when he and other string theorists embraced versions of the Anthropic Principle that I had, after a lot of thought, concluded could not be the basis for a successful scientific theory. To see if we could do better, I formulated conditions that would allow a theory based on a landscape to be a real scientific theory. As an example I had invented the CNS idea. This was all described in my book, The Life of the Cosmos.

Susskind's papers on these issues led me to revisit them, to see if anything that had happened since might change my mind. So I undertook a carefully argued paper on the AP and alternatives to it [a]. The dialogue with Lenny began when I sent a note to him, asking whether he might have any response to the arguments in that paper. At first there were some misunderstandings, because Susskind responded only to a summary, and not the full paper. Nevertheless, some important points were raised, although nothing that requires modification of my original paper. This letter is my response to a paper Susskind put out in the course of our dialogue, making certain criticisms of cosmological natural selection (CNS) [b], and is mostly devoted to answering them.

We agree on several important things, among them that fundamental physics likely gives us a landscape of possible theories, while cosmology may give a multiverse containing a vast number of regions like our own universe. We disagree here mainly on one thing: the mechanism of reproduction we believe has been most important in populating the multiverse.

My main point is that string theory will have much more explanatory power if the dominant mode of reproduction is through black holes, as is the case in the original version of CNS. This is the key point I would hope to convince Susskind and his colleagues about, because I am sure that the case they want to make is very much weakened if they rely on the Anthropic Principle (AP) and eternal inflation.

Susskind believes instead that eternal inflation is the mode of reproduction. But suppose that everything Susskind wants to be true about both eternal inflation and the string theory landscape turns out to be true. What is the best thing that could reasonably be expected to happen?

Weinberg, Vilenkin, Linde and others proposed that in this case we might be able to explain the value of the vacuum energy, both during and after inflation. This is because it is the vacuum energy that determines how many universes are made in eternal inflation, and how large each one is.

However, a careful examination exposes two problems. The first is that the methods so far proposed to make predictions in this scenario are either logically flawed or ambiguous, so that the assumptions can be manipulated to get different predictions. This is explained in detail in section 5.1 of my paper. A second piece of bad news is that, even if this can somehow be made to work, you can't expect to explain much more than the vacuum energy. The reason, as I explain in some detail in section 5.1.4, is that a statistical selection mechanism can only act to tune those parameters that strongly influence how many universes get created. As the selection mechanism in eternal inflation involves inflation, which happens at the grand unified scale, the low energy parameters such as the masses of the light quarks and leptons are not going to have much of an effect on how many universes get created.

In order to tune the low energy parameters, there must be a selection mechanism that is differentially sensitive to the parameters of low energy physics. So we can ask, what possible mechanisms are there for production of universes within a multiverse, such that the number of universes made is sensitive to the values of light quark and lepton masses? I asked myself this question when I realized there would be a landscape of string theories.

The only answer I could come up with is reproduction through black holes. It works because a lot of low energy physics and chemistry goes into the astrophysics that determines how many black holes get made.

Susskind complains that this is complicated, but it has to be complicated. The reason is that we are trying to understand a very curious fact, which is that, as noted by the people who invented the anthropic principle, the low energy parameters seem tuned to produce carbon chemistry and long lived stars. This is explained if CNS is true, because the formation of stars massive enough to become black holes depend on there being both carbon and a large hierarchy of stellar lifetimes.

Thus, if you like eternal inflation because it has a chance of explaining the tuning the vacuum energy, you should like cosmological natural selection much more — because it has potentially much more explanatory power. It offers the only chance so far proposed to actually explain from string theory the parameters that govern low energy physics. Also, as I argued in detail in my paper, the selection mechanism in CNS is falsifiable, whereas those proposed for eternal inflation so far are too ambiguous to lead to clean predictions.

Moreover, because the selection mechanism is dominated by known low energy physics and chemistry, we really do know much more about it than about eternal inflation. We know the dynamics, we know the parameters, and we can use relatively well tested astrophysical models to ask what the effect on the number of universes is of small changes in the parameters. None of this is true for inflation, where unfortunately there are a large variety of models which all are in agreement with observation, but which give different predictions concerning eternal inflation.

Of course it is possible that both mechanisms play a role. It might be useful to study this, so far no one has. It is premature to conclude, as Susskind does, that the production of universes by eternal inflation will dominate. Our universe has "only" 1018 black holes, but the total number of universes in CNS is vastly bigger than this, as there must have been a very large number of previous generations for the mechanism to work.

Susskind made a few direct criticisms of CNS, which are easy to answer, as they have been considered earlier.

He raises the question of how many new universes are created per astrophysical black hole. In the initial formulation of CNS I presumed one, but some approximate calculations have suggested that the number could be variable. I discussed this in detail on page 320 of Life of the Cosmos. The reader can see the details there, what I concluded is that if theory predicts that the number of new universes created increases with the mass, by at least the first power of the mass, the theory can easily be disproved. This hasn't happened, but it could, and it is one of the ways CNS could be falsified. This is of course good not bad, for the more vulnerable a theory is to falsification, the better science it is, and the more likely we are to take it seriously if it nonetheless survives.

One of the assumptions of CNS is that the average change in the low energy parameters when a new universe is created is small. Susskind says he doubts this is true in string theory. If Susskind is right then CNS and string theory could not both be true. But I don't share his intuitions about this. I would have to invoke technicalities to explain why, but all that need be said here is that so far there are no calculations detailed enough to decide the issue. But there could be soon, as I mentioned before, using methods developed recently in loop quantum gravity. These methods may help us study what happens to singularities in string theory and may also provide a better framework to understand eternal inflation.

The rest of this note concerns Susskind's comments about black holes. He says, "...we have learned some things about black holes over the last decade that even Stephen Hawking agrees with [13]. Black holes do not lose information." From this he draws the conclusion that "the quantum state of the offspring is completely unique and can have no memory of the initial state. That would preclude the kind of slow mutation rate envisioned by Smolin."

This is the central point, as Susskind is asserting that black holes cannot play the role postulated in CNS, without contradicting the principles of quantum theory and results from string theory. I am sure he is wrong about this. I would like to carefully explain why. This question turns out to rest on key issues in the quantum theory of gravity, which many string theorists, coming from a particle physics background, have insufficiently appreciated.

The discussion about black holes "losing information" concerns processes in which a black hole forms and then evaporates. Hawking had conjectured in 1974 that information about the initial state of the universe is lost when this happens. Susskind and others have long argued that this cannot be true, otherwise the basic laws of quantum physics would break down.

As Hawking initially formulated the problem, the black hole would evaporate completely, leaving a universe identical to the initial one, but with less information. This could indeed be a problem, but this is not the situation now under discussion. The present discussion is about cases in which a black hole singularity has bounced, leading to the creation of a new region of spacetime to the future of where the black hole singularity would have been. In the future there are two big regions of space, the initial one and the new one. If this occurs then some of the information that went into the black hole could end up in the new region of space. It would be "lost" from the point of view of an observer in the original universe, but not "destroyed", for it resides in the new universe or in correlations between measurements in the two universes.

The first point to make is that if this happens it does not contradict the laws of quantum mechanics. Nothing we know about quantum theory forbids a situation in which individual observers do not have access to complete information about the quantum state. Much of quantum information theory and quantum cryptography is about such situations. Generalizations of quantum theory that apply to such situations have been developed and basic properties such as conservation of energy and probability are maintained. Using methods related to those developed in quantum information theory, Markopoulou and collaborators have shown how to formulate quantum cosmology so that it is sensible even if the causal structure is non-trivial so that no observer can have access to all the information necessary to reconstruct the quantum state [c]. Information is never lost — but it is not always accessible to every observer.

So there is nothing to worry about: nothing important from quantum physics [d] is lost if baby universes are created in black holes and some information about the initial state of the universe ends up there.

A second point is that there is good reason to believe that in quantum gravity information accessible to local observers decoheres in any case, because of the lack of an ideal clock. In particle physics time is treated in an ideal manner and the clock is assumed to be outside of the quantum system studied. But when we apply quantum physics to the universe as a whole we cannot assume this: the clock must be part of the system studied. As pointed out independently by Milburn [e] and by Gambini, Porto and Pullin [f], this has consequences for the issue of loss of information. The reason is that quantum mechanical uncertainties come into the reading of the clock — so we cannot know exactly how much physical time is associated with the motion of the clock's hands. So if we ask what the quantum state is when the clock reads a certain time, there will be additional statistical uncertainties which grow with time. (In spite of this, energy and probability are both conserved.) But, as shown by Gambini, Porto and Pullin, even using the best possible clock, these uncertainties will dominate over any loss of information trapped in a black hole. This means that even if information is lost in black hole evaporation, no one could do an experiment with a real physical clock that could show it.

I believe this answers the worries about quantum theory, but I haven't yet addressed Susskind's assertion that "we have learned some things about black holes over the last decadeŠ.Black holes do not lose information."

I've found that to think clearly and objectively about issues in string theory it is necessary to first carefully distinguish conjectures from the actual results. Thus, over the last few years I've taken the time to carefully read the literature and keep track of what has actually been shown about the key conjectures of string theory. The results are described in two papers [g].

In this case, I am afraid it is simply not true that the actual results in string theory — as opposed to so far unproven conjectures — support Susskind's assertions [h].

There are two classes of results relevant for quantum black holes in string theory. One concerns the entropy of very special black holes, which have close to the maximal possible charge or angular momenta for black holes. For this limited class of back holes the results are impressive, but it has not, almost ten years later, been possible to extend them to typical black holes. The black holes that were successfully described by string theory have a property that typical astrophysical black holes do not have — they have positive specific heat. This means that when you put in energy the temperature goes up. But most gravitationally bound systems, and most black holes have the opposite property — you put in energy and they get colder. It appears that the methods used so far in string theory only apply to systems with positive specific heat, therefore no conclusions can be drawn for typical astrophysical black holes.

A second set of results concerns a conjecture by Maldacena. According to it, string theory in a spacetime with negative cosmological constant is conjectured to be equivalent to a certain ordinary quantum system, with no gravity. (That ordinary system is a certain version of what is called a gauge theory, which is a kind of generalization of electromagnetism).

Even if Maldacena's conjecture is true, that is no reason to assume there could not be baby universes where information was kept apart from an observer in the initial universe for a very long, but not infinite, time. This can be accomplished so long as all the different regions eventually come into causal contact so that, if one waits an infinite time, it becomes possible to receive the information that has gone into the baby universes.

But in any case, Maldacena's conjecture has so far not been proven. There is quite a lot of evidence showing there is some relation between the two theories, but all of the results so far are consistent with a far weaker relationship holding between the two theories than the full equivalence Maldacena conjectured. This weaker relationship was originally formulated in a paper by Witten, shortly after the one of Maldacena. Except for a few special cases, which can be explained by special symmetry arguments, all the evidence is consistent with Witten's weaker conjecture. We should here recall a basic principle of logic that when a collection of evidence is explained by two hypotheses, one stronger and one weaker, only the weaker one can be taken to be supported by the evidence.

But Witten's conjecture requires only that there be a partial and approximate correspondence between the two theories. It does not forbid either baby universes or the loss of information by black holes. For example, Witten shows how some black holes can be studied using results in the other theory, but again it turns out that these are atypical black holes with positive specific heat.

This discussion is related to a conjecture called the Holographic Principle (HP), an idea proposed by 't Hooft (and a bit earlier Crane) that Susskind brought into string theory. Susskind proposes a strong form of the HP, which holds that a complete description of a system resides in the degrees of freedom on its boundary. He takes Maldacena's conjecture as a demonstration of it. I believe here also the evidence better supports a weaker form (proposed with Markopoulou) according to which there is a relation between area and information, but no necessity that the boundary has a complete description of its interior [i].

I would urge a similar caution with respect to Susskind's claim, "As repeatedly emphasized by 't HooftŠ black holes are the natural extension of the elementary particle spectrum. This is especially clear in string theory where black holes are simply highly excited string states. Does that mean that we should count every particle as a black hole?"

As I mentioned, the only results in string theory that describe black holes in any detail describe only very atypical black holes. In those cases, they are related — at least by an indirect argument — to states described by string theory, but they are not in fact excitations of strings. They involve instead objects called D-branes. So Susskind must mean by "a highly excited string state" any state of string theory. But in this case the argument has no force as stars, planets and people must also be "highly excited string states". In any case, until there are detailed descriptions of typical black holes in string theory, it is premature to judge whether Susskind and 't Hooft have conjectured correctly.

Susskind attempts to invoke Hawking's authority here, and it is true that Hawking has announced that he has changed his view on this subject. But he has not yet put out a paper, and the transcript of the talk he gave recently doesn't provide enough details to judge how seriously we should take his change of opinion.

Next Susskind refers to a paper by Horowitz and Maldacena, of which he says that "The implication [14] is that if there is any kind of universe creation in the interior of the black hole, the quantum state of the offspring is completely unique and can have no memory of the initial state. That would preclude the kind of slow mutation rate envisioned by Smolin."

I read that paper and had some correspondence with its authors about it; unfortunately Susskind misstates its implications. In fact, that paper does not show that there is no loss of information, it merely assume it and proposes a mechanism — which the authors acknowledge is speculative and not derived from theory — that might explain how it is that information is not lost. They do not show that information going into baby universes is precluded, in fact Maldacena wrote to me that "If black hole singularities really bounce into a second large region, I also think our proposal is false [j]."

Finally, Susskind suggests that loop quantum gravity will be inconsistent unless it agrees with his conjectures about black holes. I should then mention that there are by now sufficient rigorous results (reviewed in [k]) to establish the consistency of the description of quantum geometry given by loop quantum gravity . Whether it applies to nature is an open question, as is what it has to say about black hole singularities, but progress in both directions is steady.

Let me close with something Susskind and I agree about — which I learned from him back in graduate school: an idea called string/gauge duality according to which gauge fields, like those in electromagnetism and QCD, have an equivalent description in terms of extended objects. For Susskind, those extended objects are strings. I believe that may be true at some level of approximation, but the problem is we only know how to make sense of string theory in a context in which the geometry of spacetime is kept classical — giving a background in which the strings move.

But general relativity teaches us that spacetime cannot be fixed, it is as dynamical as any other field. So a quantum theory of gravity must be background independent. We should then ask if there is a version of this duality in which there is no fixed, classical background, so that the geometry of spacetime can be treated completely quantum mechanically? Indeed there is, it is loop quantum gravity. Moreover, a recent uniqueness theorem [l] shows essentially that any consistent background independent version of this duality will be equivalent to loop quantum gravity. For this reason, I believe it is likely that, if string theory is not altogether wrong, sooner or later it will find a more fundamental formulation in the language of loop quantum gravity.

Indeed, what separates us on all these issues is the question of whether the quantum theory of gravity is to be background independent or not. Most string theorists have yet to fully take on board the lesson from Einstein's general theory of relativity; their intuitions about physics are still expressed in terms of things moving in fixed background spacetimes. For example, the view of time evolution that Susskind wants to preserve is tied to the existence of a fixed background. This leads him to propose a version of the holographic principle which can only be formulated in terms of a fixed background. The strong form of Maldacena's conjecture posits that quantum gravity is equivalent to physics on a fixed background. The approaches string theory takes to black holes only succeed partially, because they describe black holes in terms of objects in a fixed background. Eternal inflation is also a background dependent theory, indeed, some of its proponents have seen it as a return to an eternal, static universe.

On the other hand, those who have concentrated on quantum gravity have learned, from loop quantum gravity and other approaches, how to do quantum spacetime physics in a background independent way. After the many successful calculations which have been done, we have gained a new and different intuition about physics, and it leads to different expectations for each of the issues we have been discussing. There is still more to do, but it is clear there need be — and can be — no going back to a pre-Einsteinian view of space and time. Anyone who still wants to approach the problems of physics by discussing how things move in classical background spacetimes — whether those things are strings, branes or whatever — are addressing the past rather than the future of our science.

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[a] Lee smolin, Scientific Alternatives to the Anthropic Principle, hep-th/0407213.

[b] Leonard Susskind, Cosmic natural selection, hep-th/0407266

[c] E. Hawkins, F. Markopoulou, H. Sahlmann, Evolution in Quantum Causal Histories,
hep-th/0302111.

[d] In particular, global unitarity is automatically present whenever there is a global time coordinate, but need not be if that condition is not met. Quantum information accessible to local observables is propagated in terms of density matrices following rules that conserve energy and probability, because a weaker property, described in terms of completely positive maps, is maintained.

[e] G.J. Milburn, Phys. Rev A44, 5401 (1991).

[f] Rodolfo Gambini, Rafael Porto, Jorge Pullin, Realistic clocks, universal decoherence and the black hole information paradox hep-th/0406260, gr-qc/0402118 and references cited there.

[g] L. Smolin, How far are we from the quantum theory of gravity? , hep-th/0303185; M. Arnsdorf and L. Smolin, The Maldacena conjecture and Rehren duality, hep-th/0106073.

[h] This is one of several key cases in which conjectures, widely believed by string theorists, have not so far been proven by the actual results on the table. Another key unproven conjecture concerns the finiteness of the theory.

[i] F. Markopoulou and L. Smolin, Holography in a quantum spacetime, hepth/9910146; L. Smolin, The strong and the weak holographic principles, hep-th/0003056.

[j] Juan Maldacena, email to me, 1 November 2003, used with permission.

[k] L. Smolin, An invitation to Loop Quantum Gravity, hep-th/0408048.

[l] By Lewandowski, Okolow, Sahlmann and Thiemann, see p. 20 of the previous endnote.