The Goldilocks Enigma

Paul Davies is an author of many popular science books, often dealing with topics in cosmology and particle physics. He has been based in Australia for the last sixteen years, but is now moving to the US, taking up a new position at Arizona State University, where he will establish a new center he describes as a “cosmic think tank”.

He also has a new book coming out, entitled The Goldilocks Enigma: Why is the Universe Just Right For Life?, and a major concern of this one is the multiverse and anthropic reasoning. I was asked to write a review of the book for the British Magazine New Humanist, and the review has appeared in their September/October issue. One reason I agreed to do the review (besides the fee in the upper two figures) was that I thought I might write about the book here anyway. Here’s the text of the review. It’s somewhat different than my other postings here, since it’s written for a much wider audience and constrained by space limitations to be rather short. As a result, it unfortunately doesn’t go as deeply as I would have liked into discussing some of the issues raised in the book.

Review for New Humanist

Paul Davies’ new book The Goldilocks Enigma wrestles with some of the deepest philosophical issues around, but concentrates on one in particular: “why is the world the way it is?” He approaches this question through a discussion of a hot topic in theoretical physics that most scientists refer to as the “Anthropic Principle”, but which Davies chooses to label the “Goldilocks Enigma”. This refers to the fact that the physical laws that govern the universe are “just right” for the development of life. Relatively small changes in certain parameters would make it uninhabitable by the likes of us and we wouldn’t be here.

What should one make of this? Religion has a quick explanation, that God set things up so that we can exist. “Intelligent Design” is the currently popular name for explanations of physics or biology that invoke a higher intelligence that chose to make the world the way it is. This explanation suffers from the lack of any way to ever test it.

Davies spends much of the first half of the book providing an introduction to the modern scientific view of physical laws and cosmology, working up to the latest and trendiest of these. For more than twenty years now, theoretical physics has been dominated by a very speculative idea known as “string theory”. Very roughly, this involves replacing elementary particles with objects more like loops, and it crucially requires six extra dimensions beyond the three space and one time dimension we’re familiar with.

One must do something like wrap up the six dimensions to make them unobservably small, but then the properties of particles and thus our physical laws depend on how this is done. Initially there was much optimism that there would be only a small number of consistent choices for how to handle the six dimensions, and one of these choices would agree with what we observe. Recent results in string theory appear to show that this isn’t the case; instead an unimaginably large number of possibilities exist. Indications are that if one can get our observed universe this way, one can also get just about any variation of it, and legitimate scientific predictions are not possible.

Instead of abandoning string theory as a hopeless cause since it can’t predict anything, some string theorists have chosen to promote the idea that our universe is just part of a “multiverse” of all the nearly infinite possibilities allowed by string theory. One of the few thingsone can then predict is that we must be in a part of the multiverse that is “just right” to allow our existence. Debate rages amongst physicists over whether or not this idea is really testable and thus scientific.

Davies provides a careful description of this currently popular multiverse scenario and its explanation for why things are the way they are, including some mind-boggling implications involving infinite numbers of copies of ourselves, and the possiblity that the universe is a simulation. He contrasts it with the common belief among many physicists that there is a simple unique mathematical structure underlying the physical laws that describe the universe. The problem he sees with this belief is that there’s no reason to expect that such a mathematical structure should pick out exactly the parameters that are “just right” for life. But then again, does it really make sense to have any expectations about this? It’s not clear that a sufficient answer to the question “Why is the universe just right for life?” isn’t simply: because otherwise we wouldn’t be asking the question.

The last chapter of the book moves away from conventional points of view among physicists to some much more speculative answers to the “why is the world the way it is?” question that Davies finds appealing. These involve some version of the idea that life itself is in some way or other built into the laws of the universe, that they inherently lead to the evolution of life. He looks to information theory and quantum mechanics for hints of how this might come about. Like the multiverse, this kind of speculation tends to suffer from a lack of any known way to test it. The hallmark ofthe scientific method is the insistence that theories have the property that one can confront them with experiment in a way that allows one to decide whether they work or not. One’s answer to the “why is the world the way it is?” question should be a theory of this kind.

Davies concludes with the admission that, in the end, he finds all the different answers he has examined to be wanting. He notes that we’re the evolutionary products of the pressures of a specific environment, and only recently beginning to be liberated from these. Our minds may still be far too crude and our knowledge of the universe too fragmentary to allow us to perceive the correct answers to these existential questions. In the meantime, Davies has provided an engaging and very readable account of the range of answers we have come up with so far.

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35 Responses to The Goldilocks Enigma

  1. alex says:

    When you write:


    Is that you stating your opinion, as opposed to you quoting Davies? I read it that way, but it’s anthropic reasoning…

  2. Peter Woit says:


    That’s my opinion, and I guess it’s anthropic reasoning, but the question in my mind is whether anthropic reasoning here is anything but tautology. Is it sensible to ask “why X?” when “not X” implies that you can’t even formulate the question.?

    My problem with anthropic reasoning isn’t that it is never a legitimate form of logical argument, the problem is that it may not be science, since you can’t use it to make falsifiable predictions.

  3. urs says:


    We can have such a simple unique mathematical structure underlying the laws of the universe, and still have many possible solutions obeying these laws.

    And this is how it has been for all our existing theories – always. Anything else would be rather shocking.

    Alain Connes proposes a unique mathemtical structure behind the laws of the observable universe (the spectral action principle #). And yet, our world is described by just one out of infinitely many possible solutions (here: spectral triples) of this principle.

  4. Cristina says:

    Hello, I wandered here from John Baez’s blog 🙂

    I agree with your comment above, because the anthropic principle has always struck me as a vicious circle.

    So the physical laws are good for the development of life as we know it — good. But it’s a mistaking of cause and effect, because the Universe could be entirely different, composed of other basic building blocks, obeying entirely different physical laws, and still contain life, albeit absolutely different from what we call “life”. (And those beings would probably have an anthropic principle of their own! :D)

    The Universe is not here to allow us to exist — it is we who are here because the Universe in which we live is the kind of Universe which enables life similar to us to exist.

    It all depends on how one defines life, really.
    It’s like wondering how come that it was exactly my parents who got to be together and, as a result, how come that it was exactly me who was born 🙂


  5. Vicky says:

    Nice review. It makes me want to read the book. (I hope that was consistent with your intention.)

    I was unclear, however, about your concern regarding the notion of building the life into the fundamental laws of the universe. I can see why this might be directly untestable, but isn’t it possible that such a dependence could eventually be logically derived from other, testable propositions? Surely valid science can permit untestable conjectures that are inevitable consequences of testable ones.

    I ask because it sounds, from your brief description, like a line of thought that may be worth pursuing, and it beats some of the alternatives. I will have to read the book to see if his musings are pure conjecture or if they may have some merit.

  6. alex says:

    Thanks Peter,

    If I may give my answer to the question in your reply…

    In general whether or not “why X?” is a sensible or fruitful question is unrelated to any implications that “not X” may have for ones existence. For instance, I can learn something from asking why I survived the accident that might have killed me a few years ago. But I probably won’t learn much if I am content with the answer that if I hadn’t I wouldn’t be asking the question. The question may or may not have a useful answer, but that’s a different issue.

    “Why is the universe just right for life?” and its big brother “Why is there anything at all?” may or may not have answers within human reach, but “Otherwise we wouldn’t be asking” isn’t an answer. It probably isn’t even much of an indication as to whether they are likely to turn out to be fruitful questions.

  7. Tim says:

    May I make a brief comment on your claim “Religion has a quick explanation, that God set things up so that we can exist… This explanation suffers from the lack of any way to ever test it.”

    This suggests an unreasonably limited epistemology. A claim of Christianity, for example, is that God has intervened in history, supremely as a person (Christ), and communicated with people on topics including why we exist. Whether God has indeed intervened in world history can be subjected to tests in a similar way to a claim such as “Emperor Hadrian intervened in British history.” For example, we can examine the quality of witness accounts and other documents, and we can look at the results of alleged interventions. Additionally, in the case of Christianity, one can ask whether the revelations amount to an unreasonably profound understanding of the human condition and whether prophesies have been fulfilled. If one concludes God has indeed intervened and spoken to us, one can seek an answer to whether He set things up so that we can exist, within what he has said.

    Granted, this is not straight-forward! But I simply want to make the point that there are other reasonable routes to knowledge than the scientific method and those routes may be pertinent to the question you are addressing.

    The anthropic principle is an emotive subject because it relates to the presence of a greater purpose. Perhaps one more example (due to John Lennox) might help in this context. Imagine Aunt Joan bakes a cake. Chemical analysis can determine the ingredients. You can write papers on the material properties of sponge and icing sugar. But whatever tests you perform on the cake, you will not discover why Aunt Joan baked it until she tells you it was to celebrate her grandson’s birthday.

  8. Who says:

    this says that the sales rank of Goldilocks Enigma is currently 745 in the UK (among all books)

    this is a pretty high standing. people in the UK are pre-ordering the book which is supposed to be available 28 September.

    the amazon price is about 13 pounds have paired it with Dawkins “the God Delusion” to make the usual two-for-less package deal.

  9. Paul Davies CB (Order of the Bath) will be a huge addition to ASU and the USA too. He believes that the fundamental parameters are the result of a deeper principle. He has also written in Nature about the changing speed of light!

  10. John Baez says:

    I wonder if Davies raises the question: to what extent is the universe really “just right” for life?

    If we saw life teeming throughout the universe, on every planet, asteroid and comet, then I’d say the universe was “just right” for life. In fact we’ve only seen it on one planet.

    Maybe in fact the universe is not “just right” for life. Maybe life is not so tender and delicate as we think, either. Maybe life arises whenever conditions permit a sufficiently complex set of reactions. Maybe the Earth is the only place in the universe where this happened, maybe not.

    It seems way, way premature to start wondering about why the universe is “just right” for life, before we know whether it is.

  11. TruthSeeker says:

    What a beautiful, well-written message, Tim. You have a way of explaining abstract concepts to the general public that even children can understand.

    Sometimes we scientists do have a tendency to overanalyze everything. But perhaps – just perhaps – there is a simple explanation of why we are here, and of other related questions?

    They say that it is incompatible to be a good scientist and believe in a Creator at the same time. But I don’t see why it needs to be the case. The scientific method has been proven to be an appropriate tool to study the natural world. But perhaps it is the wrong tool to use to test anything in the supernatural world, where supreme beings reside? It would be like using a ruler to measure temperature, for example. Just because something cannot be tested by science does not mean it does not exist.

  12. John says:

    The scientific method can’t hope to answer every question in fact one can’t show, via the scientific method, that any question pertaining to the to the universe can be answered via the scientific method.

    In simpler terms, one cannot use the scientific method to prove the validity of the scientific method, as a tool to answer questions about the universe. Hence, the answer to some questions about the universe may reside outside the scientific method.

  13. dave tweed says:

    @Tim: I think you’re missing the point whe you say “The anthropic principle is an emotive subject because it relates to the presence of a greater purpose.” The anthropic principle is emotive because it’s not remotely clear whether it contains such a big vaguely defined concept (that we know what properties life has and what lower-level physics leads to/is incapable of leading to them) that any current _application_ of it seems to many to be untenable.

    To give a completely impractical thought experiment: if we could somehow run simulations of a suitably huge sampling of all possible “rules for existance substrates” for suitably long “simulated times” AND we had artificial intelligences that were able to hunt through for any sign of “life” even if it didn’t have the form we’re used to
    AND they found no sign of anything like life, THEN I’d be more comfortable with the anthropic principle. (Note the point of the simulations isn’t to test possible physics but to see if we can generate something that we’d class as life that doesn’t have the characteristics of all the life we’ve seen so far.) If you’d firmly established this level of understanding that is presupposed by the anthropic principle, I wouldn’t have any problems accepting as legitimate questions about whether this leads to “greater purpose” or “supreme beings”.

    The discomfort with the anthropic principle for many people is the jump from “I can’t imagine any life which doesn’t have this property” to “it is not possible for there to be life which doesn’t have this property” in order to “get answers to question x” rather than accept “we can’t get the true answer to the question x yet”.

  14. Tim says:

    TruthSeeker – thank you for your comment.

    dave tweed – I think the anthropic principle is found to be emotive by different people for different reasons.

  15. George says:

    RE: The Goldilocks Enigma.

    I think the right question is really – Why is life just right for the universe?

  16. Gumbi says:

    Seems to me that the anthropic principle is not there so much to offer an explanation, as to suggest that no explanation is needed.

    We don’t wonder too much why earth is suitable for life, because we see that there are at least a few and probably very many other planets out there that are suitable to life to varying degrees.

    But if there were only one planet in the universe, and it were earth, then the question of why this one planet was suitable for life would demand an explanation.

    The anthropic principle, combined with multiverse concepts, suggests that some mysteries are not mysterious at all, and don’t need explanations, just like the case of earth being so suitable for life.

  17. TheGraduate says:

    I think in general the anthropic principle is somewhat redundant and political. We all know that we may fail to find an answer to any question we seek to find an answer to. In that sense, the anthropic principle is redundant. Because it is so redundant, the only reason to bring it up is to convince people not to try and in that sense it seems political.

    I consider the anthropic principle something like the ‘god did it’ principle. As long as this kind of idea is not part of making concrete progress, it seems a waste of time to focus on it.

  18. Chris W. says:


    Right. More specifically, it seems to me that the underlying premises of Christianity in treating these questions have been (1) apologetics – convincing unbelievers and doubters that God really exists, and (2) that rational investigation of nature without presupposing God’s existence is tantamount to sin, and is the sort of thing that got us kicked out of Paradise. Therefore we have no business doing it. The existence of the world and ourselves as part of it ceases to be a mystery. Instead, it becomes a moral drama.

    Returning to the subject of whether the existence of life tells us anything about the structure of physical law, the key point which always seems to be ignored is the role of physical law in accounting for any kind of stability in physical systems, and also the limits of that stability. Clearly, living things need some degree of stability or reproducibility in their environment and in their own constitution, combined with an allowance for change, the latter requiring some degree of instability. Some sort of balance needs to be struck, and the laws of physics are central to how it is struck and whether such a balance is possible. Of course there is something deeply reflexive about this, because the laws of physics are, by assumption, stable themselves. This begs the question, how do we account for the stability and universality of the laws?

    There has never been an absolute answer to this question. When putative laws have been discovered to be limited in scope, ie, to fail under some circumstances, we have sought to understand this failure and their prior success in terms of a deeper and more all-encompassing law. The central question about a failed law is always “why and how did it ever work?” The relevance of relative velocities and the speed of light only became fully apparent with the advent of special relativity. Without it, the problems of reconciling Newtonian mechanics and Maxwell’s electrodynamics and various observational anomalies could only be puzzled over.

    I think we’re at a point in the history of physics where the question of why nature has any law-like structure at all must be squarely confronted if we are to understand why it has the particular law-like structure that it does. In this connection, I should note that John Stachel has argued that Einstein’s well-known objections to the indeterminacy of quantum mechanics were not due to its indeterminacy as such, but rather to the fact that the degree and form of the indeterminacy was unexplained. If we’re going to admit some indeterminacy, then why not go all the way? Again, the subtle balance between indetermininism and determinism is the core issue, and calls for an explanation. Life seems to require it, almost by definition, but this fact throws little light on the fundamental basis of the balance.

  19. Arun says:

    The questions which currently interest us may not be answerable by application of the scientific method at this time. Progress in science as it is at a particular time depends on asking the right questions. If science is seen to be turning into philosophy, it is perhaps because we are not asking the right questions. Perhaps it is because the right questions at this time are relatively tame and boring compared to having theories of everything, and HEP theorists are no longer psychologically suited to tame and boring.

  20. TheGraduate says:

    Chris W. ,

    Great comments. I think that to tackle the anthropic principle scientifically is at the moment a very lightweight approach and one really needs to read some of the analyses put forward by the philosophers to get to ideas with any kind of heft.

    In defense of some christian philosophers, I think the motivations for their investigations are as you describe. However, I would not say that all their arguments fall into the categories you outlined.

    One thing that Christians sometimes argue is that their way of looking at things is a complete theory of everything in that it attempts to describe all phenomena both physical and mental. In the sense that science has nothing to say on the question of ‘what is a good life?’ which is of incalculable importance to most human beings, Christians view the scientific enterprise as a quest toward a partial theory of everything where ‘everything’ for them includes intellectual, sensory, moral, emotional and physical phenomena in both the objective and subjective perspectives.

    In a way, the scientific conclusion of the ‘consistency’ of physical systems is quite conditional in that one may take certain drugs; one may have dreams; one may have what is defined as a ‘mental illness’; one may simply experience a state of mind that one tends to retroactively define as ‘confused’; and during these times the rule of ‘consistency’ is thrown out the window.

    To even approach the scientific method, one must lay down much conceptual frame work. For instance, one has to hypothesize that human beings have a certain frame of mind in which it is possible to do science and that the ‘reality’ of other frames of mind are invalid.

  21. John says:

    The question itself raises some problems; we are assuming that “natural processes”, as we know them, held before or at the “creation” of the universe. In addition, physics can only describe the behavior of preexisting processes and material, but not “creation” where creation means something out of nothing. This goes deeper than the appearance of particles out of the quantum void, for you need laws to govern that void before anything can come from it, hence something exists.

    Modern science always presupposes the existence of an underlying law governing the behavior of material objects. The problem is what happens when we ask about the origin of those very laws? We are in a quandary, we presuppose an underlying law, but wait the origin of the universe is the origin of all physical laws, so what kind of underlying law can govern the “creation” of the universe? We can’t say its a physical law for ontologically speaking it must come before the physical laws, hence its a “meta-physical” law. Even if we did find an explanation via physical laws, we can always ask why those laws came about as they did and so on to infinity.

    This is what I mean when I say the scientific method cannot validate itself; it always needs a preexisting underlying structure.

  22. Neznaika says:

    Thank you for the interesting review. My question is about your earlier comment that one cannot use anthropic principle to make falsifiable predictions.
    How about “Principle of Mediocrity”? I found a very interesting recent article on the subject by A. Vilenkin on ( In that article and apparently in his new book, Many Worlds in One –
    Vilenkin argues that one can make testable STATISTICAL predictions using the Principle of Mediocrity. He also claims that his (and Weinberg’s) prediction for the cosmological constant has already been confirmed.
    What do you think about those arguments? Is that Science in your view?

  23. Neznaika says:

    Correction to my post:
    Vilenkin’s article has been published on, not on Sorry about that.

  24. Who says: has The Goldilocks Enigma in stock and is shipping.

    they guarantee delivery by 1 PM Tuesday 26 September, if ordered now, so I guess they mean business.

    Goldilocks has UK salesrank #716 at the moment, which is very high for a physics book. For comparison, Not Even Wrong had UK amazon salesrank #1318 last time I looked.
    And Hawking Brief History of Time (paperback) had rank around #4700.
    so Davies book having UK salesrank 716 is really quite good

  25. Chris W. says:


    The strength of Weinberg’s argument has been questioned a number of times. See hep-th/0407174 for a recent example. It has also been questioned whether the argument is truly “anthropic”, ie, whether the existence of life plays anything more than an incidental role in the argument.


    This statement strikes me as quite unfounded. As my comment should have indicated, science can be seen as a natural outgrowth of grappling consciously with a problem that faces all living things. We must find and learn to rely upon some stability in the world for the sake of simple survival, if nothing else. The growth of science was galvinized by the gradual and surprising discovery that a certain kind of stability—stability of certain deep patterns of change—could be found in the world, and could be precisely and testably described, well beyond what seems practically necessary.

  26. TheGraduate says:

    Chris W.,

    I am not quite sure what you mean. Could you elaborate? I understand that by stability you mean being able to replicate the results of experiments. But I think there is definitely a difference between the sort of investigative method that leads to science and the sort of reasoning people employ when they conclude things like: the sun will rise tomorrow, God is always on the side of the righteous and that poor people always steal. Nevertheless, on a subjective level, I think there is a sense for the people that believe these things that these are stable, replicable rules.

  27. John Bussoletti says:

    I’ve always wondered why those who espouse Intelligent Design seem to get so involved with questions of natural selection and the ideas of Darwin, when there really are some very basic aspects of nucleosynthesis that have even given me pause on occasion. Not that I’m espousing anthropic or Intelligent Design principles, but consider the following:

    There is evidence that the chemical elements that make up our universe are created through nucleosynthesis processes in stars. “Hydrogen burning” creates helium. “Helium burning” critically depends on a “triple alpha” process, which is predicated on the existence of a 7.65 Mev excited state of Carbon to allow the production of Carbon. And from Carbon, production of all other elements follows

    So there is in the nucleus of Carbon an excited state with zero net angular momentum at an energy level of 7.65 MeV, which is juist above the dissolution state of the nuclues into Berillium 8 (itself unstable to decay into two Helium nuclei) and an alpha particle. This excited state has an electromagnetic decay branch which emits a photon and decays to the 2+ excited state of the Carbon 12 nucleus, which is a bound state and itself decays by photo emission to the ground state.

    Were the energy of this state of the nuclear system somewhat higher, the probability for a triple alpha interaction within solar interiors would drop precipitously, greatly reducing (eliminating?) the production of Carbon in our universe. Were it lower in energy, there might be no path for creation of Carbon. And without carbon, most other nuclei don’t get produced.

    The fundamental electromagnetic, strong and weak nuclear forces, and the values of the various undetermined constants in our various theories of particles and the universe as we know it, are just so, to allow the existence of this excited state, allowing synthesis of Carbon. Without the interplay of the various strong, weak and electromagnetic forces, there would be no carbon, let alone carbon-based life anywhere in this universe.

    So, the Intelligent Design contingent, rather than worry about Darwinism, really ought to call greater attention to the very existence of Carbon.

    Now, as much as the scientific method would like to argue against such an anthropic point of view, there is another fundamental problem, which was first explained to me by Jeremiah Ostriker in an Astrophysics course I took many years ago. In the early days of the course he explained the Copernican point of view by saying there are two fundamental assumptions that we make in Astrophysics: “First, we do not live an any special place in the universe. Secondly, we do not live at a special time.”

    With these two assumptions, we can make observations of that universe, build theories based on those observations, and have some confidence that those theories reflect some elements of truth.

    The trouble with this is that these two assumptions are basically unprovable. That is, we’re not able to subject them to the experimental test that the scientific method would dictate must be done. So as much as we might like to subject the universe to test, we’re limited in time, space and even energy to explore only the very lowest excitations of the system. Our theories are nothing more than models that reflect those observations.

    Robert Pirsig in “Zen and the Art Of Motorcycle Maintenance” quoted Bertrand Russell’s description of science as “If bread is a stone and stones are nourishing, then I can eat bread and be nourished. So I do the experiment, I eat some bread and find that, indeed I am nourished, thus proving my theory that bread is a stone.”

    This is an extreme caricature of the scientific method, but unfortunately, it’s also pretty accurate.

    No matter what “fundamental” theory that one might propose, even should it explain “everything”, the reality is that all we know how to do is construct models of the low level excitations of whatever it is that is in our universe at the particular time that we occupy it and in the particular place where we observe.

    Landau had it right. All we can ever observe are effective interactions and so we’re free to model them in whatever way is consistent with our observations.

    But we can never prove correctness. We can only achieve utility.

    That is, if our model is “good”, it will allow us to build or control something. Basically, allow us to be engineers. But “Truth” and “Proof” elude us always.

    So, the anthropic point of view directly opposes the Copernican assumptions. Both are unprovable points of view and both are largely irrelevant with respect to “utility”. That’s why Peter and others like him (I include myself among them) are so adamant that one must make predictions, testable ones, with whatever theory one creates. Without some ability of a model to make predictions, it has no “utility” and is, in the end empty. As much as science would like to establish proof, the reality is that the scientific method is largely a consistency argument, and in the end, is not provable.

    But it can be very, very useful (sic).

  28. Adrian Heathcote says:


    Just a comment about John Baez’s post.

    I agree with this wholeheartedly. This is the point that no one seems to make in discussions of the Anthropic Principal. The most obvious thing about the Universe that we observe is that we seem to be alone in it. This may not be the case, of course. But that is the current evidence. People who talk about the fittingness of the universe to life must confront the difficult truth that it doesn’t seem to be.

  29. Adrian Heathcote says:

    I think most people can see that the Anthropic Principle is a fallacy, the only real disagreement is what kind of fallacy it is. So let me add my 2 cents on this question. I think it is an example of what logicians call a “modal fallacy”—of which there are many examples in ordinary thinking. It is confusing a fact about the universe—that intelligent life exists—with a modal claim—that the universe *must* be such as to sustain life. But this is a case of inserting the necessity operator (“must”) into the wrong position in the sentence. Fallacies that are generated by misplacing necessity and possibility operators are called modal fallacies.

    The traditional statements of fatalism (“what will be must be”) are likewise thought to be modal fallacies.

  30. Adrian Heathcote says:

    There is a second strand to the AP, evident in the post of John Bussoletti. The claim is that if the universe were different in some particular respect X then life would not be able to exist. Therefore since it does exist the universe *must* have this property X. This is the modal fallacy that I mentioned. The only thing that really follows is that the universe does have this property X.

  31. Neznaika says:

    To Chris W. and Peter,
    Thank you very much for responding, Chris, since Peter is simply ignoring my question.
    Actually, I don’t care “whether the argument is TRULY “anthropic”, (it’s “anthropic” enough for me). Vilenkin’s claims that ‘The Principle of Mediocrity’ is testable, since it can make VERIFIABLE statistical predictions and some of those predictions, for the cosmological constant specifically, have already been confirmed. I very much want to know if those claims are correct. I read the article on and thought that Vilenkin makes a very convincing case but I am just a layman, I can’t judge…
    I am interested in Peter’s opinion very much – he is extremely smart and seems to be the expert, so I asked him this very specific and simple question in response to his comments to Alex. I didn’t receive any answer. Is he hiding, or he doesn’t have an answer?

  32. Peter Woit says:


    I didn’t answer your question because Chris already did, and I’ve written many, many times about these issues here. I’m just really tired of repeating the same points in response to the same overhyped claims. Once more:

    The anthropic principle by itself is useless. It gives “predictions” that are tautologously true, so can’t be falsified. The “principle of mediocrity”, or more generally, the use of a multiverse model that gives an a priori statistical distribution of values of observables, combined with the anthropic principle as a selection effect, can in certain cases give predictions. If your multiverse model predicts a statistical distribution strongly peaked at a point, and that point is in the anthropically allowed range, then you should observe something near that value or the model is (probably) wrong.

    The problem is that people are working with models (like the string theory landscape) that they have no control over and seem to have no useful structure, so people are just assuming the statistical distributions are flat. This is exactly the same assumption you make when you throw up your hands and say “I have no idea what is causing this”, so, a priori, the distrbution of expected values is flat. You can’t get something for nothing, and claim to be doing a serious non-trivial test of a model when the model’s “predictions” are identical to those of just admitting you have no idea what is going on, so anything is equally likely. (I should note that Wilczek et. al have a calculation involving axions where they have some control over the a priori distribution, and it isn’t flat, so maybe there is something more there).

    The Weinberg “prediction” has been seriously overhyped, in many ways. First of all, it involves a flat a priori distribution of the CC values, so suffers from the problem mentioned above. It “predicts” a generic value of the CC in the anthropically allowed range, but that is also the “prediction” that comes from the model “the CC is determined by something purely mysterious such that I’ll never know anything about its origin”.

    You can argue that this “prediction” is falsifiable: you may find that the CC has a non-generic value, e.g. very close to zero. If you observe this, then, you do have some information about the origin of the CC, it’s not something random, but some unknown physics is giving it the non-generic value.

    Despite the over-hyped claims you hear, this appears to be what is happening in the CC case. If you fix all cosmological parameters except the CC, you find that the observed value is somewhat smaller than expected, since it is one to two orders of magnitude below the top of the allowed anthropic range. If you allow other cosmological parameters to vary, the CC is much too small, many orders of magnitude below the top of the anthropic range. Vilenkin and Susskind’s claims that the observed value of the CC is decisive evidence for the multiverse and landscape are absurdly overhyped.

    If you take the string theory landscape seriously, there are lots of other similar “predictions” it makes which are just completely wrong. There appears to be no reason for the proton lifetime to be anything in particular on the landscape, but it is observed to be not generic, many orders of magnitude below the anthropic limits on proton decay. Same for CP violation, and lots of other things. If the people hyping these anthropic landscape “predictions” were honestly willing to give up their model when these “predictions” failed, that would be one thing. They’re not, they don’t take their own “predictions” that they are getting from these vacuous models seriously, so I don’t see why anyone else should either.

  33. Neznaika says:

    Thank you very much for taking time and answering this truly primitive question. I didn’t realize that you’ve done it many, many times before, and I apologize for that. Your explanation is very detailed and clear; with your help I now understand the issue much better.
    I agree that people make exaggerated, over-hyped claims about anthropic predictions but I find Vilenkin and Susskind’s positions to be very different: they are interested in different kind of models. Susskind is interested in superstring theory and landscape, Vilenkin doesn’t care about that. His only interest is cosmology: he uses ‘The Principle of Mediocrity’ for cosmological predictions.
    I went to Vilenkin’s colloquium recently (I am an alumni) about The Principle of Mediocrity. He made 3 points which I found interesting. Vilenkin is using a 2-step process to calculate the statistical distribution. He does assume AT FIRST that the statistical distribution of all possible CC values is flat. His argument that the range of all possible CC values in the multiverse is enormous and the allowed anthropic range is tiny by comparison. If you select a tiny range of ANY distribution curve, it appears flat as Earth surface looks flat to us. Next he goes on to CALCULATE the statistical distribution based on the number of galaxies, and that one is NOT FLAT. The PREDICTED (since it was done way BEFORE any experiments) and the observed CC values agree at 75% confidence level which looks pretty impressive to me.
    Peter, I am very interested in Davis’s book which is not available yet. Your review is great, so I have a question: I read your old Vilenkin’s book review, you didn’t like it at all. Davis’s book review is much better but the subjects look similar. What are the differences and/or similarities in your view? Do they have a difference of opinions? Which one is better written and easier to read? Thank you very much.

  34. Peter Woit says:


    Both books are written at a very low level, for a very unsophisticated audience. If one has a serious interest in these subjects, I’m not sure that either one is all that helpful. The Davies book is more about philosophical issues than the Vilenkin book. Its virtue is that it’s even-handed: Davies explains the whole range of views on these topics, as well as what their problems are. Vilenkin’s book is hyping one specific kind of theory, one that I find highly problematic since it is not really testable and thus not really science. On the whole, he also doesn’t really bother to explain what the problems are with what he is pushing.

    I can guess at what the “75% confidence level” claim is about, and I think it’s heavily overhyped. For one thing, it completely ignores the problem I mentioned that if you allow other cosmological parameters to vary, the CC comes out much too small (probably so small that the standard experimentalists way of characterizing the situation is that Vilenkin’s scenario is ruled out at greater than 99% confidence level).

  35. Neznaika says:

    It’s for ME! I AM that unsophisticated audience. I am trying to understand the science behind it but I don’t want to fall asleep in the process, I want an interesting, well written book.
    On a different subject: I read in your earlier post that Lubos Motl wrote a ‘one-star’ review of your book and “when his review is there, sales improve”. I’ve noticed that Motl gave Vilenkin’s book 5 stars (which is extremely rare for him). Do you think that Vilenkin’s book sales are in real trouble?

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