Science, Culture and Integral Yoga :: 2) The Anthropic Principle: Lee Smolin's #2 to Leonard Susskind #1

I am grateful to Lenny for taking the time to respond to my paper. I will be as brief as I can in replying, especially as the key points are already presented in detail in my paper hep-th/0407213 ["Scientific alternatives to the anthropic principle"] or in my book, Life of the Cosmos or previous papers on the subject.

For clarity I had in section 5.1.6 identified two arguments in Weinberg's papers. The first is the one I criticized in the summary. Susskind reponds, reasonably, by agreeing, and then raising the second argument. This argument is also criticized in detail in my paper, and it was perhaps a mistake not to include this in the summary I sent to Susskind.

This second argument is based on a version of the AP called the "Principle of Mediocrity" by Garriga and Vilenkin, who have done the most to develop it. Their version states that, "...our civilization is typical in the ensemble of all civilizations in the universe."

This argument is discussed in full in sections 5.1.5 and 5.1.6. There I argue that the mediocrity principle cannot yield falsifiable predictions because it depends on the definition of the ensemble within which our civilization is taken to be typical as well as on assumptions about the probability distribution. I establish this by general argument as well as by reference to specific examples including Weinberg's use of it.

Can this be right if, as Susskind claims, Weinberg's prediction was found to hold? In fact, Weinberg's prediction did not work all that well. In the form that he made it, it led to an expectation of a cosmological constant larger than the observed value. Depending on the ensemble chosen and the assumptions made about the probability distribution, the probability that Lambda be as small as observed ranges between about 10 % and a few parts in ten thousand. In fact, the less probable values are the more reasonable, as they come from an ensemble where Q, the scale of the density fluctuations, is allowed to vary. While I am not an expert here, it appears from a reading of the literature [references in the paper] that to make the probability for the present value as large as 10% one has to assume that Q is frozen and fixed by fundamental theory. It is hard to imagine a theory where the parameters vary but Q does not, as it depends on parameters in the inflation potential.

But, because there is so much flexibility-and an absence of strict, up or down, falsifiable predictions, anyone who wants to continue to use the AP in this context is free to modify the assumptions about the prior probability distribution to raise the probability for the observed value of the vacuum energy from 10^{-4} to order unity. No one can prove they are wrong to do so-and this is precisely the problem.

I do believe it is important to insist on falsifiability, because it alone prevents we theorists from keeping theories alive indefinitely, by freely adjusting them to match data.

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.

The CNS idea was invented, not for itself, but to give an existence proof that shows that the Anthropic Principle can be replaced by a falsifiable theory, that explains everything the AP claims to. The reason I chose the term "landscape" of string theories, is to anticipate the transition to "fitness landscapes", a term that comes from mathematical models that explain why the mechanism of natural selection is falsifiable.

As the theory of CNS is falsifiable it is vulnerable to criticisms of the kind Susskind makes. Let me briefly address them.

The last first. Susskind claims that life is exceptional in the ensemble of universes. This is not true in CNS. The whole point of cosmological natural selection is that it follows the logical schema described in section 5.1.4 and 5.2. There, and in more detail in the book and papers, I show that falsifiable predictions can be gotten from a multiverse theory if the distribution of universes is very different from random. CNS results in a distribution peaked around small regions of the parameter space-so that a typical universe in this distribution is very untypical in any randomly chosen ensemble. I show in detail why falsifiability follows from this. I also show why reproduction through black holes leads to a multiverse in which the conditions for life are common-essentially because some of the conditions life requires-such as plentiful carbon- also boost the formation of stars massive enough to become black holes.

Next, there is a big difference between the explanatory power of the reproduction mechanisms of eternal inflation and black holes bouncing. This stems from the fact that any selection mechanism can only operate to tune parameters that strongly affect the rate of reproduction. Given that standard inflation acts on grand unified scales, the differential reproduction rate due to eternal inflation is only sensitive to the parameters that govern GUT scale physics plus the vacuum energy. Thus, eternal inflation cannot explain the values of any of the low energy parameters such as the masses of the light quarks and leptons. This means it cannot explain why there are long-lived stars or many stable nuclear bound states leading to a complex chemistry.

Reproduction through black holes explains all the puzzles and coincidences of low energy physics because carbon chemistry, long lived stars etc, are essential for the mechanisms that lead to the formation of massive stars-those that become black holes. There is a long list of observed facts this turns out to explain, and a few genuine predictions. These are summarized in the paper and discussed at length in the book.

With regard to cosmological natural selection and the cosmological constant; if both mechanisms of reproduction exist, there is a competition between them that determines Lambda. Eternal inflation favours a larger cosmological contant, as Lenny says. But-by Weinberg's first argument, if Lambda is too big there are no galaxies, hence many fewer black holes. To my knowledge, nobody has attempted to do a detailed analysis including both mechanisms. Vilenkin has pointed out that too small Lambda can hurt black hole production in a single universe, by making mergers of spirals more common. This is briefly discussed in section 6.2, where I propose that the observed value may maximize the production of black holes-but this has not been analyzed in any detail.

Certainly, if the only mechanism of reproduction is eternal inflation, cosmological natural selection is wrong. But we know that there are black holes and we have reasonable theoretical evidence that black hole singularities bounce. I expect that in the next year we may have reliable quantum gravity calculations that will settle the issue, building on the methods Bojowald and collaborators have used to study cosmological bounces. So the consequences of reproduction through black holes seem reasonable to explore. Not only that, we are on firm ground when we do so because star and black hole formation are observed and controlled by known physics and chemistry.

We know much less about eternal inflation. We cannot observe whether it takes place or not, and there is little near term chance to check the theories that lead to it independently, as there are alternative early universe theories-inflationary and not-that agree with all the cosmological data and do not yield reproduction through eternal inflation.

I do not know if CNS is the only way to get a falsifiable multiverse theory, it is just the only way I've been able to think of. As I have been saying for more than ten years, if CNS can be proved wrong, and if people are stimulated to invent more falsifiable theories to explain the observed parameters, this would be all to the good. So far CNS has not been falsified, but I read astro-ph every day looking for the discovery of a very massive neutron star that will disprove it.

I am very glad that Susskind has been able to give these issues much more visibility. But it would be very unfortunate if string theorists finally accept there is an issue with predictability, only to fall for the easy temptation of adopting a strategy towards it that cannot yield falsifiable theories. The problem with non-falsifiable theories is nothing other than that they cannot be proven wrong. If a large body of our colleagues feels comfortable believing a theory that cannot be proved wrong, then the progress of science could get stuck, leading to a situation in which false, but unfalsifiable theories dominate the attention of our field.

Thanks,

Lee

Keywords: Susskind, Smolin, Anthropic

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