Some readers may recognize the issue I’m talking about. Anyone who has read the recent New York Times article by Dennis Overbye knows that the ultimate fate of information falling into a black hole was the subject of an long debate involving Stephen Hawking, myself, the famous Dutch physicist Gerard ’t Hooft and many other well known physicists. Hawking believed that information does disappear behind the horizon, perhaps into a baby universe. This would be consistent with Smolin’s idea that offspring universes, inside the black hole, remember at least some of the details of the mother universe. My own view and ’t Hooft’s was that nothing can be lost from the outside world—not a single bit. Curiously the cosmological debate about cosmological natural selection revolves around the same issues that came to the attention of the press a week or two ago. The occasion for the press coverage was Hawking’s recantation. He has reversed his position.
Over the last decade, since Smolin put forward his clever idea, the black-hole controversy has largely been resolved. The consensus is that black holes do not lose any information. I’ll cite some of the most influential papers that you can look up yourself: hep-th 9309145, hep-th 9306069, hep-th 9409089, hep-th 9610043, hep-th 9805114, hep-th 9711200. Incidentally, the combined total number of citations for these six papers is close to 6,000. Another paper, coauthored very recently by the author of one of these classics, directly attacks Smolin’s assumption. In fact it was one of the 11 papers I found citing Smolin’s paper. If you want to look it up, here is the archive reference: hep-th 0310281. I warned you that I would say “And besides, so-and-so agrees with me.” I apologize, but at least you can go check for yourself.
The implication of these papers is that no information about the parent can survive the infinitely violent singularity at the center of a black hole. If such a thing as a baby universe makes any sense at all, the baby will have no special resemblance to the mother. Given that, the idea of an evolutionary history that led by natural selection to our universe makes no sense.
I’m sure there are physicists who are unconvinced by the arguments of the abovementioned papers, despite the number of citations. They have all the right in the world to be skeptical, but the average reader of this page should know that these people are swimming against the tide.
Finally let me quote a remark of Smolin’s that I find revealing. He says, “It was worry about the possibility that string theory would lead to the present situation, which Susskind has so ably described in his recent papers, that led me to invent the Cosmological Natural Selection (CNS) idea and to write my first book. My motive, then as now, is to prevent a split in the community of theoretical physicists in which different groups of smart people believe different things, with no recourse to come to consensus by rational argument from the evidence.”
First of all, preventing a “split in the community of theoretical physicists” is an absurdly ridiculous reason for putting forward a scientific hypothesis.
But what I find especially mystifying is Smolin’s tendency to set himself up as an arbiter of good and bad science. Among the people who feel that the anthropic principle deserves to be taken seriously are some famous physicists and cosmologists with extraordinary histories of scientific accomplishment. They include Steven Weinberg [2], Joseph Polchinski [3], Andrei Linde [4], and Sir Martin Rees [5]. These people are not fools, nor do they need to be told what constitutes good science.
References:
[1] Of course, you might say that the distance to the sun determines the temperature. But that just replaces the question with another, “Why is our planet at the precise distance that it is?”
[2] Professor of physics, University of Texas, and 1979 Nobel Prize winner.
[3] Professor of physics, Kavli Institute for Theoretical Physics.
[4] Professor of physics, Stanford University; winner of many awards and prizes, including the Dirac Medal and Franklin Medal.
[5] Astronomer Royal of Great Britain.
Lee Smolin’s “Final Letter”
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 1990s 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 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 b
e 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 [of] 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. 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 nontrivial, 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 e
vaporation, 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 decade. 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’m 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 black 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 as 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.
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