The Cosmic Landscape

Susskind’s new book, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design is now out. It’s basically a lengthy version for the general public of the argument that he has been, with some success, trying to sell to the physics community for the last few years. In short, the argument is that the compatibility of string theory with an essentially infinite variety of different physics is not a bad thing (because it can’t predict anything), but a good thing (because it allows an anthropic argument for the small size of the cosmological constant).

Susskind devotes quite a lot of space to attacking the argument that the string theory picture of unification is “elegant”, instead promoting the idea that the properties of the universe come from some more or less random very complicated “Rube Goldberg” construction of a vacuum, one whose nature is just constrained by the anthropic principle. He asks:

But is String Theory beautiful? Does String Theory live up to the standards of elegance and uniqueness that physicists demand? Are its equations few and simple? And, most important, are the Laws of Physics implied by String Theory unique?

He answers these questions by first making fun of the supposed mathematical elegance of the theory:

Elegance requires that the number of defining equations be small. Five is better than ten, and one is better than five. On this score, one might facetiously say that String Theory is the ultimate epitome of elegance. With all the years that String Theory has been studied, no one has ever found even a single defining equation! The number at present count is zero. We know neither what the fundamental equations of the theory are nor even if it has any.

He goes on to argue that the laws of physics implied by string theory have turned out to be highly non-unique:

During the 1990s the number of possibilities grew exponentially. String theorists watched with horror as a stupendous Landscape opened up with so many valleys that almost anything can be found somewhere in it.

The theory also exhibited a nasty tendency to produce Rube Goldberg machines. In searching the Landscape for the Standard Model, the constructions became unpleasantly complicated. More and more “moving parts” had to be introduced to account for all the requirements, and by now it seems that no realistic model would pass muster with the American Society of Engineers — not for elegance in any case.

From this he draws the following bizarre conclusion:

Judged by the ordinary criteria of uniqueness and elegance, String Theory has gone from being Beauty to being the Beast. And yet the more I think about this unfortunate history, the more reason I think there is to believe that String Theory is the answer.

He remarks with surprise that no one has drawn the obvious conclusion that these arguments just imply that string theory is wrong:

What I have never heard is criticism based on the unfortunate inelegance or the lack of uniqueness of String Theory. Either of these tendencies might be thrown back at the string theorists as evidence that their own hopes for the theory are misguided. Perhaps part of the reason that the enemies haven’t pounced is that string theorists have kept their Achilles heel under wraps until fairly recently. I suspect that now that it is becoming public, partly through my own writings and lectures, the kibitzers on the sidelines will be grinning and loudly announcing, “Ha, ha, we knew it all along. String Theory is dead.”

adding a footnote to this paragraph in proof:

This remark was written in the spring of 1994, [presumably he means 2004] but by the time I completed writing ‘The Cosmic Landscape’ a year later, the vultures had descended in force.

He seems to have forgotten about at least one particular vulture, who back in 2003, tried to make this point at the question session after one of his colloquium talks.

Susskind’s argument that string theory’s compatibility with just about anything is actually an advantage is based on the fact that this makes a place for Weinberg’s 1987 anthropic principle argument for the size of the cosmological constant (which from what I’ve seen gets it wrong by at least one to two orders of magnitude if you only vary the CC, more if you vary other parameters). He addresses criticism of the anthropic principle as unscientific by denouncing the field of philosophy of science, and the criterion of falsifiability in particular:

Frankly, I would have preferred to avoid the kind of philosophical discourse that the Anthropic Principle excites. But the pontification, by the “Popperazi,” about what is and what is not science has become so furious in news reports and Internet blogs that I feel I have to address it.

He then goes on to quote from something he wrote for a debate with Smolin at the Edge web-site. For the bizarre story of how this debate came about, including the rejection by the arXiv of a submitted “paper” by Susskind about this, see here. He begins with:

Throughout my long experience as a scientist I have heard unfalsifiability hurled at so many important ideas that I am inclined to think that no idea can have great merit unless it has drawn this criticism. I’ll give some examples:

The examples he gives are:

1. Behaviorist psychologists like B. F. Skinner who argued that statements about emotions were unscientific. Here I think Susskind is confusing positivism (the philosophy that science should just deal with directly observable quantities), with falsifiability, which is different. A theory may be based on quantities that are not directly observable, and still make falsifiable predictions about experimentally observable quantities. In any case, physics is supposed to be a much “harder” science than that part of psychology dealing with human emotions, and it is pretty strange for a physicist to be arguing that what he is doing really is science using this as an example.

2. The theory of quarks. Again, Susskind completely confuses positivistic objections (that if quarks are not directly observable you shouldn’t talk about them), with falsifiability. While quark theory was problematic until 1973 since there was no workable dynamics, it was taken seriously because it made some very impressive, highly falsifiable predictions. The best known example is the quark theory prediction of the mass, spin and charge of the Omega-Minus particle. If string theory had made some predictions like this, few people would be criticizing it.

3. The theory of inflation. Susskind claims that after Guth first came up with this in 1980, it was attacked as unfalsifiable. I don’t recall ever having heard such a criticism, although it was always clear and remains true to this day that experimentally distinguishing between the predictions of different mechanisms for inflation is difficult. From what I remember, there was actually a lot more optimism in the early 80s about this than now, since people were pretty enthusiastic about GUT models, and there seemed to be a good chance that one of the scalar fields in a simple GUT model would do the trick. Susskind writes: “It took 20 years to do the experiments that confirmed inflation.” As far as I know people were calculating the effects of inflation on the CMB and starting to design experiments to see them within a few years after Guth’s work. I don’t see the relevance of the fact that it took a while to get a sufficiently sensitive experiment working.

4. The theory of evolution. Susskind joins other string theorists like Lubos Motl and an anonymous Cambridge referee I dealt with in believing that the status of string theory is much like that of the theory of evolution. He seems to believe that fossil evidence is irrelevant to testing Darwin, writing:

And it took more 100 years or more for to decisively test Darwin (some would even say that it has yet to be tested).

I’ll leave it to a professional biologist like P. Z. Myers to argue this point with him, but it seems to me both nutty and irresponsible, given the ongoing battles over the teaching of evolution (which Susskind is getting himself involved with in the very subtitle of this book).

After attacking falsifiability as a criterion for a scientific theory, Susskind does admit that a theory has to make some predictions, even if they’re not the sort that could falsify the theory. He acknowledges difficulty in coming up with any predictions:

Is there any way to explain in which of these anthropically acceptable vacuums we live? Obviously, the Anthropic Principle cannot help us predict which one we live in — any of these vacuums is acceptable.

This conclusion is frustrating. It leaves the theory open to the criticism that it has no predictive power, something that scientists are very sensitive about.

He discusses the idea of using statistical arguments, acknowledging that there are severe problems with this due to the “measure problem” of not being able to compare sizes of infinite sets, as well as the problem of not knowing what a priori probability to assign to any given vacuum state. Finally he does try and come up with some suggestions of how the theory might be tested, they are:

1. Evidence in the CMB that our universe was formed by bubble nucleation:

If we are very, very lucky, the largest lumps in the CMB might date to a time just before the usual Inflation got started — in other words, just as the universe was settling onto the inflationary ledge…

If we are that lucky, then the Inflation did not go on long enough to wipe out evidence for the curvature of space… If our pocket universe was born in a bubble nucleation event, the universe must be negatively curved.

At the level of accuracy that the curvature of space has been measured, there is no indication of such curvature. This idea may fail because standard Inflation probably has been going on for a long time when the largest visible lumps were formed.

So, this is both very unlikely to be something we can observe, and even if we did it would only tell us that the universe was born in a nucleation event, still giving us just about zero information about the supposed landscape and none whatsoever about string theory.

2. Cosmic superstrings. Here Susskind is referring to claims by Polchinski and others that amidst the infinity of possible physics due to string theory, one can cook up special cases where certain kinds of superstrings of astronomical dimensions exist and have properties precisely such that we wouldn’t have seen them yet, but could see their effects in gravitational wave experiments like Advanced LIGO. As far as I’ve ever been able to tell, these are contrived constructions, with no reason at all that the vacuum state of the real world should be such as to support them. These are highly non-falsifiable “predictions” of string theory. No string theorist is going to give up on string theory just because Advanced LIGO doesn’t see these effects.

3. High energy physics. Susskind talks about the LHC and the question of whether the fine-tuning problem of the Higgs mass will be resolved by supersymmetry or is anthropic. He acknowledges that, based on Landscape arguments:

My original guess was that supersymmetry was not favored, and I said so in print. But I have changed my mind — twice — and probably not for the last time.

This isn’t much of a argument for predictivity on this issue, but I guess his point is that sufficient study of the Landscape might somehow resolve this question, although all the evidence so far is that this is not possible.

The bottom line here is that Susskind is unable to come up with any remotely plausible way of ever getting any scientific predictions out of the string theory landscape framework, and yet he thinks it is a good idea to write a popular book designed to sell it to the public. He makes clear that he is doing this because he sees himself at war with that part of the theoretical physics community which still believes in the idea of continuing to try and do what theoretical physicists have always done: find a more mathematically beautiful, more compelling, more predictive theory than the one we have now. In one chapter he surveys the state of the ongoing political battle for the hearts and minds of his fellow theorists. He crows (with some justification) that Weinberg agrees with him, saying physicists have to give up the paradigm of how to do physics they pursued during the last century, that Witten is facing defeat and getting depressed, that Joe Polchinski says there’s no alternative, that the entire Stanford theoretical physics group are his allies, that ‘t Hooft won’t rule out anthropic explanations, that Maldacena believes in the Landscape, that Michael Douglas is on his side, that cosmologists Linde, Vilenkin, Rees and Tegmark are in his camp, and that Alan Guth is at least a fence sitter. One of the few active opponents that he sees left on the scene is David Gross, whose reasons for opposition he describes as “more ideological than scientific.” He sees Gross as dead meat: “the field of physics is littered with the corpses of stubborn old men who didn’t know when to give up.”

In coming weeks, it will be interesting to see how the physics community deals with the challenge presented by Susskind’s publicity campaign for changing how theoretical physics is done. So far the initial signs are depressing. Michael Duff’s review in Physics World just more or less respectfully repeats Susskind’s argument, not challenging it in any way. In a review of the Duff review, Clifford Johnson answers the question of whether this sort of thing is still science with “I have not yet made up my own mind whether it sits well with me or not…” He makes a distinction between postdiction and prediction that I don’t quite agree with (if Susskind’s framework accurately postdicted even a few of the known Standard Model parameters, I’d be a believer). He takes the usual stance favored by most sensible string theorists who want to keep working on the theory that they don’t understand the theory well enough yet to know whether they are stuck with the Landscape or not. Finally he thinks there’s a chance that maybe the structure of the Landscape is such that once one anthropically fixed the CC and some other constants, the remaining set of vacua would actually predict something. I don’t see the slightest evidence for this, but it’s the argument many are now using to justify exploring the Landscape and surrounding swampland instead of giving up on string theory and trying to find a better idea.

Update: I should have mentioned a recent well thought out review of Susskind’s book at Tech Central Station by Kenneth Silber. It’s quite sensible and worth reading if you’re following this story. Another review of the book has just appeared, this one by George Ellis at Nature. Ellis is much more critical of Susskind than Duff was, realizing that the crucial issue is that Susskind has no evidence for his claims, and writing in his final paragraph:

Physicists indulging in this kind of speculation sometimes denigrate philosophers of science, but they themselves do not yet have rigorous criteria to offer for proof of physical existence. This is what is needed to make this area solid science, rather than speculation. Until then, the multiverse situation seems to fit St Paul’s description: “Faith is the substance of things hoped for, the evidence of things not seen.” In this case, it is faith that enormous extrapolations from tested physics are correct; hope that correct hints as to the way things really are have been identified from all the possibilities, and that the present marginal evidence to the contrary will go away.

One peculiar thing about Ellis’s review is that he accuses Susskind of ignoring the fact that there is no experimental evidence for negative curvature of the sort one might get if the universe was formed by bubble nucleation. In Susskind’s defense, he does address this point, saying it is very unlikely we can see this negative curvature since inflation is likely to have gone on long enough to make it unmeasurably small.

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32 Responses to The Cosmic Landscape

  1. Chris W. says:

    One hardly knows what to say. I find myself wondering whether this isn’t some kind of setup, with Susskind accusing dozens of people—some years from now—of allowing themselves to get sucked in by what was, in the end, just a ruse.

    Also, I sense an ironic twist in many of Weinberg’s comments on this subject that suggest he isn’t buying. He has merely made a modest investment which he would be happy to lose. (He hasn’t actually done much work on the Landscape himself, right?)

    Great post, Peter.

  2. D R Lunsford says:

    (aside) In contrast to this garbage, Cooperstock and Tieu have a followup paper on galactic rotation curves:

    http://arxiv.org/abs/astro-ph/0512048

    -drl

  3. D R Lunsford says:

    Speaking of Guth, one might well point out that the modern idiocracy rose to power along with the general acceptance of the intellectually bankrupt idea of “cosmic inflation”.

    We physicists need to take our subject back from these bright boys.

    -drl

  4. Arun says:

    A great essay, but profoundly depressing.

  5. MP says:

    Can anyone who knows these things comment on the braneworld scenarios? The claim is that braneworld predictions are testable in the not so distant future.

  6. hack says:

    “What I have never heard is criticism based on the unfortunate inelegance or the lack of uniqueness of String Theory”

    If he has never heard this criticism, it’s because his fingers were plugging his ears while he said “na na na na na…”

  7. Kasper Olsen says:

    @ MP: how braneworld theories can be tested: read Lisa Randall’s new book, Warped passages, or look at xxx.lanl.gov…

    regards, Kasper

  8. Adrian Heathcote says:

    Peter

    A great article, and for my money you are spot on in your critisisms of Susskind’s claims about falsifiability. Susskind seems to not have a clue about these things. And it’s interesting that he didn’t mention the one theory that has repeatedly faced claims of unfalsifiability, namely Freud’s theory of dreams and repression. And he doesn’t mention it because it would upset this silly idea that a theory could be perfectly good yet unfalsifiable.

    (Why do referees do such a poor job? they block the good and interesting, and let through confused stuff like this. The world’s going to hell in a handbasket.)

  9. Jose says:

    It seems to me that the book of Susskind is the best allegation against string theory!.

  10. Chris Oakley says:

    Why do referees do such a poor job? They block the good and interesting, and let through confused stuff like this.

    This may have something to do with the fact that LS has a prestigious job at a prestigious university.

    The same comment I made about Weinberg also applies here: if one is in a position of influence then one is duty bound to try to encourage people to do one’s subject rather than to try to scare them off. Telling people that String theory is nonsense but still the only idea in particle physics worthy of study is not going to make anyone want to sign up.
    Of course, there are alternatives to String theory, but the String-theory-dominated research establishment does its utmost to make sure that these particular suckers never get an even break. In the long run, though, if leaders like Susskind and Weinberg convince taxpayers that theoretical particle physics is not worth funding, then who are the real suckers?

  11. Juan R. says:

    Peter Woit said,

    Susskind’s new book, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design is now out. It’s basically a lengthy version for the general public of the argument that he has been, with some success, trying to sell to the physics community for the last few years. In short, the argument is that the compatibility of string theory with an essentially infinite variety of different physics is not a bad thing (because it can’t predict anything), but a good thing (because it allows an anthropic argument for the small size of the cosmological constant).

    That is, we can choose between two options:

    - Theory A. The theory predicts many things with great precision. By “predict” i mean that one knows the result of an experiment before it was done. For instance, one known approximation to theory A is the pair standard model more general relativity, even if both are incompatible, since both provide excellent result on the very big and the very small! Of course the true theory A may be internally compatible. String theorists often misunderstand the word “predict” and use it instead of “post-dict”. By “post-dict” i mean that one may know first the experiment and then modify/adapt the theory to the obtained value. Since we may know first the experimental outcome, of course, the practical utility of this kind of theories is zero. The theory cannot scientifically explain the cosmological constant.

    - String ‘theory’. Or its M unknown generalization! The ‘theory’ predicts nothing of this world and is incompatible with physics we observe in laboratory. Since string ‘theory’ is not useful, people of the real world continue using well-tested scientific theories. For example, astronomers continue using general relativity on the prediction/explanation of phenomena. Chemists continue using quantum mechanics of particles on the computation of reaction constants, biologists continue using thermodynamics to understand metabolism, etc. Using antrophic principle, string theorists can ‘explain’ the cosmological constant. The antrophic principle basically says that cosmological constant has value we measure because if has other value life would be imposible and we would not measure it! Of course, this is not a scientific explanation. At the best, it is phylosophical one, but of no real utility on science.

    Therefore, the option opened by string theory reduces to either scientifically understand/predict/explain a ‘billion’ of things or scientifically understand/predict/explain nothing and just phylosophically explain ONE single thing.

    Of course, the phylosophical antrophic principle can be used with theory A!

    Since my position is well-known i add no more comments!

    Juan R.

    Center for CANONICAL |SCIENCE)

  12. Pingback: It’s Equal but It’s Different » Blog Archive » Physics news…

  13. DMS says:

    Nice, but depressing review.

    It is quite pathetic to see some of the arguments used by Susskind, like comparing string theory to QCD, in terms of falsifiability. Whom does he think he is fooling with this blatant lie?

    I am afraid the post-War (generally very favourable) perception of the public of physics(and physicists in general), not just string theorists, will be severely damaged. This will not be without consequences.

    It is quite amusing to see the kid glove treatment of Susskind by other string theorists and contrast it with the reaction to Krauss; he obviously touched a nerve. The irony is that Susskind’s book(right from the title onwards) is more damaging to the field than Krauss’. I suppose, intellectual dishonesty is in all fields, but the extent of it in string theory may be unrivalled.

    I was quite surprised to note the many eminent physicists who are behind this anthropic nonsense, or at least not outright rejecting it. I was under the impression that it was merely a fringe in the community. No longer, it seems…

    DMS

  14. a says:

    I think you are discussing two different issues.

    1) The anthropic issue. Here you seem too negative: experiments still have something important to say. If LHC will find that the Higgs mass hierarchy problem is solved in some “natural” way, anthropic interpretations will be mostly abandoned. If LHC will find nothing, anthropic intrpretations will be a good alternative to harakiri.

    2) The string issue. Here it is easy to guess the impact of LHC results: whatever LHC will find (nothing? supersymmetry? extra dimensions?) will be used to argue in favor of string theory, and you are doing a good work to make the situation more healthy.

  15. Dumb Biologist says:

    Dear. Dr. Woit,

    I’ve been an admiring lurker for quite some time*, but this review prompted me to contribute. Superbly written, and I am always deeply appreciative of scientists who can not only think clearly, but also communicate their thoughts with equal parts clarity and cogency to a lay audience (in physics, at least). I suppose the depressing starkness with which Susskind’s bereft position is revealed is also due, in no small part, to his efforts to reach a broad audience as well.

    I’ve no doubt Dr. Susskind is my intellectual superior, as are most, if not all of the String “theorists”, but all the powers of all the greatest physicists joined together will never conjure experimental verification out of thin air, and for the assay and its fruits, logic is simply no substitute in the realm of skeptical inquiry. Now that we’ve seen a preeminent physicist’s attempt to herald a new paradigm shift, by eschewing testability altogether in the name of “progress”, I fear for the integrity of the natural sciences in general, and physics in particular. I’d laugh it off in happier times, but particularly the ID “theorists’” faith-based pollution and perversion of the public scientific discourse has revealed how truly vulnerable the edifice of skeptical inquiry, built as it has been, brick-by-torturous-brick since the Renaissance, can be battered by the rams of fundamentalist faith and dogmatic philosphizing on nothing. It’s painful to see science assaulted from without, but positively excruciating to see it eat at itself from within.

    Despite the sometimes overly-pejorative nature of the conversation here (which it seems you’ve recently taken more assertive steps to limit, which I appreciate), I’m glad people like you are willing to stick their necks out and protest this corruption of the scientific method. I hope the message gets through someday.

    *I chose my handle to preempt any insults hurled my way by Dr. Motl, should he deign to swat at a biognat such as myself if something I might ever say touches a nerve.

  16. woit says:

    a,

    All I was doing here was reporting on what Susskind is saying, he’s the one bringing claiming that string theory both allows and requires anthropic arguments, and that, perversely, this is a good thing.

  17. Chris Oakley says:

    I chose my handle to preempt any insults hurled my way by Dr. Motl, should he deign to swat at a biognat such as myself if something I might ever say touches a nerve.

    No-one is going to take you seriously in this forum until you have been savaged by Lubos Motl. Look on it as a rite of passage.

  18. Brett says:

    While I disagree with Susskind’s ideas about anthropic reasoning, I have to say that I find Smolin’s arguments on the subject equally unconvincing. Smolin’s (and everyone else’s) main example of how the anthropic principle is used is Weinberg’s examination of the cosmological constant. Smolin’s seems to mistake the fundamentally inductive character of this argument. The deductive stage is unambiguous, that because there are galaxies, the cosmological constant must be small. But this tells us nothing about why it is small. Part of the purpose of science is to determine causes for what is observed, and what Weinberg wanted to argue was that the size of the cosmological constant could provide evidence that the cause of that smallness was purely anthropic. While I do not consider his evidence to be all that strong, the problem is a numerical, not philosophical one.

    I believe that Susskind is correct that bringing heavy philosophy into things is simply misleading, and Popper’s ideas, in particular, do not really match with how many real scientists have always done science. Popper rejected almost entirely the notion of positive evidence, which is really one of the cornerstones of the scientific method. Falsifiability is fundamentally a deductive criterion; we can deduce which theories are false, but not which are true. The earliest modern philosophers of science, Descartes and Bacon, ridiculed the deductive reasoning of the scholastics. Scholastic reasoning assumed known (and theological) causes and tried to work of predictions from that. There was no apparatus for determining any new causes. But finding new causes was exactly what a scientist like Newton had to do. This is a fundamentally inductive process, and it is based, ultimately, on the intuitive Bayesian fact that an observation of a cause’s effects makes that cause more likely. This kind of framework allows anthropic causes, and so there is no a priori reason to reject them as unscientific.

  19. Dumb Biologist says:

    I think if you simply substitute “testability” for “falsifiability”, many these pernicious philisopical issues simply vanish. Science, simply put, involves putting ideas to the test experimentally. Perhaps there are scientifically relevant concepts which cannot be falsified by any means available to mortals, but we become comfortable (or only ought to, anyway) with such assertions only after the most vigorous experimental assault. That’s how science keeps itself honest. Surely scientists utilize everything from induction to deduction to dumb luck in the process of discovery, but the process becomes rather suspect when the very meaning of “discovery” is subverted, and at the end of the day one no longer feels beholden to nature to demonstrate the predictive power of one’s ideas. Perhaps we invent hypotheses, but we verify the accuracy of those hypotheses and build theoretical frameworks through observation. THAT is the scientific method.

    If what I have read in the “popular science media” about String “theory”, it is right to say it’s not a theory at all, but a hypothetical model. There’s nothing wrong with hypothetical models, but mistaking or conflating such a thing, no matter how rigorous or ambitions, with the TESTED predictive power of a true theoretical framework is irresponsible. Look no further than the “controversy” over Intelligent Design to see why respecting the meaning of words like “theory” is vitally important.

    And yet: Because of its purported beauty and profundity, what started as hope does seem, at least in the minds of some, to have become a kind of faith among some physicists. String Theory simply must be true, because of all its wonderful symmetries, and so forth. Ergo, if it cannot be falsified (or worse, even tested), the need for anything remotely resembling experimental verification is shown to be superfluous, quaint, even obstructive. Such is the “progress” we’ve achieved with deductive reasoning, it would seem. Perhaps the sciences are coming full circle to mere philosphy once again. It’s especially rich when the very definition of science, speciously derided as some “Popperian dogma”, is being deliberately reworked to defend a so-called theory that apparently can’t predict anything (or rather, predicts everything?). The scientific method isn’t just any philosophy we can discard when no longer fashionable (like postmodernism). At least, I hope it isn’t, because we’ve tried promulgating “truth” after bouts of deep thought, and that program of “discovery” has rather failed to deliver.

  20. Dumb Biologist says:

    Oh, and apologies for some of my typos and other poor writing. I failed to see the preview unfolding below the entry field (first time posting on a blog and all…simple use of the scroll bar would have revealed the Preview), and hence didn’t proof-read my posts very well. I’ll may just shut up now, but, anyway, thanks again Dr. Woit for your contributions to a worthy subject of debate.

  21. Chris W. says:

    From Leonard Susskind, in his email exchange with Lee Smolin published on Edge.org:

    Good scientific methodology is not an abstract set of rules dictated by philosophers. It is conditioned by, and determined by, the science itself and the scientists who create the science. What may have constituted scientific proof for a particle physicist of the 1960′s—namely the detection of an isolated particle—is inappropriate for a modern quark physicist who can never hope to remove and isolate a quark. Let’s not put the cart before the horse. Science is the horse that pulls the cart of philosophy.

    A quote from Popper, via this page:

    “We all take our philosophies whether or not we are aware of this fact, and our philosophies are not worth very much. But the impact of our philosophies upon our actions, our lives, is often devastating. This makes it necessary to try to improve our philosophies by criticism. This is the only apology for continued existence of philosophy which I am able to offer.” (from Objective Knowledge)

    (I don’t think this transcription is completely accurate, but it’s very close to the original. I remember the passage well from my own reading of Objective Knowledge.)

  22. As a somewhat newcomer to the subject of quantum gravity, and an astrophysicist by formation, I find this all exceedingly interesting. Congratulations to Dr. Woit for making this very relevant and timely debate possible.

    I would like to take the opportunity, if not considered a diversion, and quote the following paragraph from Barrow & Tipler – The Anthropic Cosmological Principle, page 275:

    At present there is no theoretical understanding of why just three dimensions have expanded to a large size if the others are indeed confined to minute extent. However, the Anthropic arguments we gave concerning the special properties of three-dimensional space and four-dimensional space-time show that there would be a Weak Anthropic explanation for this observation also; but, for all we know, it may also be a consequence of the unique topological properties that four-dimensional manifolds have recently been found to possess. The fact that only they admit more than one distinct differentiable structure may well turn out to have something to do with the fact that observable space-time has four dimensions.

    Is that view compatible or incompatible with the current anthropic landscape issue? In any case, how can this really be useful?

    Thank you,
    Christine

  23. Clark Goble says:

    Good post, although I’d quibble with your discussion of logical positivism. Logical positivism, at least none of the figures I’ve read, would say science can “just deal with directly observable quantities.” Rather logical positivism was more trying to take the typical science of the day and move it towards philosophical problems. That is, if it can’t be verified empirically, it is meaningless. That tendency goes back well into the 19th century. In America we have figures like C. S. Peirce espousing a theory fairly similar, for instance. All would admit theoretical entities, although they’d say our justification for theoretical entities comes from this verification principle.

    Once could adopt both a verification principle and a principle of falsification. Peirce does this, for instance, although many more argued that one could verify directly through empirical knowledge.

    Logical positivism does start diving kinds of science when we get into psychology, history and related disciplines. But with a few caveats, I think one could be a logical positivist and not have trouble with most physics.

    Where the logical positivists part ways is with people who tend to espouse a coherency theory of truth. The positivists were very much tied up into a correspondence theory of truth.

  24. Quantum_Ranger says:

    The clarity of your post peter, portrays a just confusion with respect to Susskind’s “state of mind”.

    It is by no coincedence that the ‘boom and bust’ mentality of the 80′s, was perfectly matched with stringtheory’s creation and evolution.

    The sober thinking these days, albeit with hindsight, needs the guiding voice and eye’s such as you attain in your varied writings.

    I do not think it is all doom’n'gloom though, but it is apperaing more and more that there is a lot of “dis-information” contained within the Anthropic Landscape, by fact of his positional stance, Susskind rule’s supreme, over the scientific kingdom that is apparently untouchable as well as unaccountable?..if one takes the vacuum background as literally de-facto?

  25. Chris W. says:

    Christine,

    Regarding the significance of the special features of 4-dimensional manifolds, see the excerpt from a paper by Hendryk Pfeiffer quoted in this comment on an earlier post on this blog.

    There is much of interest in this post and many of the ensuing comments.

  26. Who says:

    when Susskind’s views on the String Landscape are discussed, together with Anthropic, and related, reasoning, I think you miss half the argument if you dont mention this article by Smolin (which upset Susskind quite a bit when Lee sent him a copy)

    http://arxiv.org/abs/hep-th/0407213
    Scientific alternatives to the anthropic principle
    Lee Smolin
    Contribution to “Universe or Multiverse”, ed. by Bernard Carr et. al., to be published by Cambridge University Press.

    “It is explained in detail why the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science. Cases which have been claimed as successful predictions from the AP are shown to be not that. Either they are uncontroversial applications of selection principles in one universe (as in Dicke’s argument), or the predictions made do not actually logically depend on any assumption about life or intelligence, but instead depend only on arguments from observed facts (as in the case of arguments by Hoyle and Weinberg). …

    We show however that it is still possible to make falsifiable predictions from theories of multiverses, if the ensemble predicted has certain properties specified here. An example of such a falsifiable multiverse theory is cosmological natural selection. It is reviewed here and it is argued that the theory remains unfalsified. But it is very vulnerable to falsification by current observations, which shows that it is a scientific theory.
    The consequences for recent discussions of the AP in the context of string theory are discussed.”

  27. Christine says:

    Chris W. wrote:

    see the excerpt from a paper by Hendryk Pfeiffer

    Thank you very much!

    Best wishes
    Christine

  28. JC says:

    Chris W.,

    In your link “http://www.math.columbia.edu/~woit/wordpress/?p=206#comment-3795″ in a previous post, check out the talk of one of the authors (Prakash Panangaden) of the paper you linked to gr-qc/0407094. The last page of the talk is hilarious.

  29. island says:

    Lenny doesn’t realize it, but he is advocating a natural design theory if he’s using the landscape to rationalize away the significance of the fact that the appearance of [intelligent] design is undeniable… because it isn’t just an “appearance” if multiverses don’t exist to lose the significance in.

    He isn’t just correct about that, he’s…

    NOT EVEN RIGHT

  30. Chris W. says:

    The same talk is available as a PDF file.

    —————————
    It seems that a number of computer scientists studying distributed systems have formulated theories of what might be called asynchronous causality, and some have noted the potential value of connecting these ideas with Lorentzian geometry. In Panangaden’s recent papers with his collaborator Keye Martin this point of view has been well developed.

    Also see Discrete Quantum Causal Dynamics (gr-qc/0109053).

    [By the way, asynchronous processor designs have been the subject of a growing R & D effort in the computer industry for the last several years. (Sun Microsystems' effort has been led by Ivan Sutherland.) Synchronous clocking of highly integrated digital circuits is threatening to become non-viable in the fairly near future, for performance and power consumption reasons as well as the sheer difficulty of implementation at very high clock rates.]

    (* professor of CS at McGill)

  31. island says:

    Finally, he thinks there’s a chance that maybe the structure of the Landscape is such that once one anthropically fixed the CC and some other constants, the remaining set of vacua would actually predict something. I don’t see the slightest evidence for this

    Universes with other exaples of the commonly speculated forms of life, like; Silicon based life, Nitrogen and Phosphorus based life. Chlorine or Sulfur based life, where differently tuned constants produces a different set of eco-balances, and a different ratio in the cosmic abundance of any of the mentioned elements to carbon, of 10 to 1 in their favor, rather than the other way round like it is in our universe… for examples of how far your imagination can run if…

    … if ours is an anthropic universe that undergoes an evolutionary-like leap/bang to a higher order of symmetry and efficiency, in terms of more uniform form of energy-disseminating capability…

    …but that’s not going to happen in the one that exists this go-round, and there certainly isn’t the slightest bit of evidence that a “pocket” of naturally selected near-identical bubble-universes in a “would-be” multiverse, would or should be preferred over an evolving biocentric universe, for which there is pre-existing observed justification for all the way down to your ancestors, the apes.

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