Many Worlds In One

Alex Vilenkin has a new popular book out about cosmology, entitled Many Worlds In One. It’s mainly about the extremely speculative end of cosmology, and much of it is devoted to explaining the author’s ideas on eternal inflation, creating the universe by tunneling out of nothing, and the anthropic landscape, together with stories about how he came to these ideas. It contains various amusing anecdotes, especially about Alan Guth. Sean Carroll is credited with the following story:

One of the leading superstring theorists, Joseph Polchinski, once said that he would quit physics if a nonzero cosmological constant were discovered. Polchinski realized that the only explanation for a small cosmological constant would be the anthropic one, and he just could not stand the thought.

He also describes the reaction to his anthropic arguments back during the years when these were not all the rage like they are now:

After one of my seminars, a prominent Princeton cosmologist rose from his seat and said, “Anyone who wants to work on the anthropic principle – should.” The tone of his remark left little doubt that he believed all such people would be wasting their time.

Vilenkin’s book covers much the same ground as Susskind’s, although from the point of view of a cosmologist, not a particle physicist. A huge amount is made of the supposed anthropic “prediction” of the value of the cosmological constant (any news of the rumor from Sean Carroll of new work by prominent Princeton cosmologist Paul Steinhardt showing this is bunk?). Unlike Susskind, Vilenkin at least doesn’t seem to be on a campaign to attack the “Popperazi” and convert everyone to anthropics, but he demonstrates a similar lack of concern for the fact that the ideas he is discussing don’t lead to much if anything in the way of a testable experimental prediction.

Here’s his scientifc program for 21st century physics, which he hopes will be spent working on the anthropic landscape:

First, we will need to map the landscape. What kinds of vacua are there, and how many of each kind? We cannot realistically hope to obtain a detailed characterization of all 10500 vacua, so some kind of statistical description will be necessary. We will also need to estimate the probabilities for bubbles of one vacuum to form amidst another vacuum. The we will have all the ingredients to develop a model of an eternally inflating universe with bubbles inside bubbles inside bubbles… Once we have this model, the principle of mediocrity can be used to determine the probablility for us to live in one vacuum or other.

Unfortunately for this research program, it has yet to even begin to get off the ground, and there are very good arguments that it can never succeed. There are an infinite number of possible vacua, and trying to make this finite so one can do statistics requires putting in cutoffs, with results then strongly depending on the cutoff. The large numbers of these vacua make any attempts to identify ones that agree with the real world computationally completely intractable. Even if one could do this, all evidence is that one would end up with broad statistical distributions for many of the parameters of the standard model, providing no useful prediction of what new experiments will see, or any insight into why these parameters have the values that they do.

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37 Responses to Many Worlds In One

  1. MathPhys says:

    Look on the bright side, Peter. They have solved all our problems. In the immortal words of Alyosha Zamolodchikov when asked what he thinks of strings as the final theory of everything

    “That’s good. Now we can all relax and go and have a drink”.

  2. Hi,

    as a plot-inclined person, I would be soooo happy to see a graphical description of just one of the 19 parameters of the standard model as a function of any one of the possible needed cut-offs that allowed a mediocrity principle to work out… That would be a start.

    …Naah. It would only mean starting to get plot-bombed by 10^500 possible graphical descriptions…

    Cheers,
    T.

  3. Who says:

    A huge amount is made of the supposed anthropic “prediction” of the value of the cosmological constant (any news of the rumor from Sean Carroll of new work by prominent Princeton cosmologist Paul Steinhardt showing this is bunk?).

    maybe this is merely background to still newer work?

    http://arxiv.org/abs/astro-ph/0605173
    Why the cosmological constant is small and positive
    Paul J. Steinhardt, Neil Turok
    15 pages, 1 figure
    “… we show that a cyclic model of the universe can naturally incorporate a dynamical mechanism that automatically relaxes the value of the cosmological constant, … nearly every volume of space spends an overwhelming majority of the time at the stage when the cosmological constant is small and positive, as observed today.”
    ==================

    As for straightforward debunking, Abraham Loeb has presented an observational test to show that the CC is not of anthropic origin:

    http://arxiv.org/abs/astro-ph/0604242
    An Observational Test for the Anthropic Origin of the Cosmological Constant
    Abraham Loeb (Harvard)
    JCAP 0605 (2006) 009
    “…Here we propose a simple empirical test for this anthropic argument within the boundaries of the observable Universe. We make use of the fact that dwarf galaxies formed in our Universe at redshifts as high as z~10 when the mean matter density was larger by a factor of ~10^3 than today. Existing technology enables to check whether planets form in nearby dwarf galaxies and globular clusters by searching for microlensing or transit events of background stars. The oldest of these nearby systems may have formed at z~10. If planets are as common per stellar mass in these descendents as they are in the Milky Way galaxy, then the anthropic argument would be weakened considerably since planets could have formed in our Universe even if the cosmological constant was three orders of magnitude larger than observed. For a flat probability distribution, this would imply that the probability for us to reside in a region where the cosmological constant obtains its observed value is lower than \~10^{-3}. A precise version of the anthropic argument could then be ruled-out at a confidence level of ~99.9%, which constitutes a satisfactory measure of a good experimental test.”

    Loeb disposes of any a priori argument that the CC is anthropically determined, since it is plausible that habitable zone planets will be detected in the descendents of dwarf galaxies (this is what he proposes searching for) which would rule out an anthropic CC, as he explains. One has every reason to expect to find evidence of planet formation in dwarf galaxies just as one finds it in our own galaxy.

    I don’t know if Steinhardt has any current work debunking anthropic CC but it does not seem that he NEEDS debunk it. Loeb has taken care of that. And Steinhardt has proposed a mechanism by which, in his cyclic scheme, the CC value could be explained.

  4. steve says:

    The original Weinberg mechanism never really worked. The anthropically-favored value of the CC depends on the prior distribution of magnitude of density fluctuations. We were not the only ones to notice this…

    Anthropic Distribution for Cosmological Constant and Primordial Density Perturbations

    Authors: Michael L. Graesser, Stephen D.H. Hsu, Alejandro Jenkins, Mark B. Wise

    Journal-ref: Phys.Lett. B600 (2004)

    Abstract: The anthropic principle has been proposed as an explanation for the observed value of the cosmological constant. Here we revisit this proposal by allowing for variation between universes in the amplitude of the scale-invariant primordial cosmological density perturbations. We derive a priori probability distributions for this amplitude from toy inflationary models in which the parameter of the inflaton potential is smoothly distributed over possible universes. We find that for such probability distributions, the likelihood that we live in a typical, anthropically-allowed universe is generally quite small.

  5. Chris W. says:

    MathPhys, your comment reminds me of a remark of Stalin: “Death solves all problems (…).”

  6. MathPhys says:

    Chris W,
    Your comment on my comment explains many things to me. Keep the immortal sayings of wise Russians coming, please.

  7. MathPhys says:

    and Georgians too.

  8. To the extent that anthropic arguments are intended to answer “why” questions about constants (such as the cosmological constant) that have a continuous range of possible values, they are surely flawed because they can never yield a single answer.

    You can have anthropic arguments for the number of dimensions of space – where the possible values are integers – but surely any anthropic argument applied to a continuous-valued constant will still yield an uncountable infinity of allowable values?

  9. Stalin says:

    actually the situation is worse: different regions of the landscape will give different broad distributions; heretic versions of the antrhopic religion will proliferate, unless we obey Slatin’s order 227: Ни шагу назад.

  10. annoying lurker says:

    Peter, since the attacks are against the anthropic principle, perhaps you should comment on what you think about Hoyle’s use of that principle to successfully predict the fusion of 3 alpha particles into carbon in stars. Hoyle realised that for the observed amounts of C-12, there must be a resonance at 7.65 MeV in C-12. Experiments confirmed it.

    Hoyle thought he didn’t win the Nobel Prize because he criticised the fact that the Nobel Prize for the discovery of pulsars was awarded to the PhD adviser (Hewitt) of the person who actually made the observation (Bell). However, perhaps it was because of the anthropic principle. Hoyle wasn’t predicting anything in the usual sense, just showing that one set of observations (measured carbon 12 abundances in stars, people, etc.) are consistent with a particular nuclear reaction rate.

    I think the cancer of corruption in physics is that in default of real understanding, obfuscation is preferred. String theory and its landscape are the ultimate obfuscation with which to sink any questions anyone asks about physics.

    When exactly was it that crackpotism won? Bohr at Solvay in 1927, Aspect “falsifying” causality in 1982, or M-theory in 1995?

    Do you ever just feel like giving up and embracing the extra dimensional multiverse? It is so much easier to be a crackpot, peter!

  11. sunderpeeche says:

    Hoyle showed that other mechanisms would not work. For our universe to exist (people etc) there had to be a resonance in C-12, and he calculated its energy (correctly). Observations confirmed his claim. That’s more than just saying the existence of people is consistent with a particular reaction rate.

    ? crackpotism Bohr Solvay 1927 ?

  12. annoying lurker says:

    “Hoyle showed that other mechanisms would not work.”

    Similarly, I can “predict” the width and smoothness of a road from knowing the widths and suspension systems of cars!

    Other “mechanisms” (like flying cars through the sky) can be shown simply not to work. Hence, for cars to exist, the roads must possess very constrained, predictable features. Where’s my Nobel?

    BTW, Bohr wasn’t actually awarded a Nobel for disproving causality (despite his long philosophical arguments with Einstein in 1927).

  13. Peter Woit says:

    All,

    Unless you really have something new to say, please resist the temptation to start rehashing various aspects of the anthropic principle every time the landscape gets mentioned here. At this point, all this is doing is adding to the noise level.

  14. Peter,

    You said “There are an infinite number of possible vacua, and trying to make this finite so one can do statistics requires putting in cutoffs…”

    Could you briefly explain this comment and maybe mention the rationale for the cutoff used?

  15. secret milkshake says:

    Stalin: “In the Soviet Army it takes more courage to retreat than advance.”

    You tell me: Is ST getting like SA? Anybody shot in the back?

  16. z3 says:

    NY Times, “Physics Awaits New Options as Standard Model Idles” describes the lack of progress in theoretical HEP in the past 30 years and portraits the sense of frustration in this field. String theory received a passing mention that’s sort of dismissive.

  17. woit says:

    CIP,

    You should look at Douglas’s papers for the details. One part of the story is that he wants to put a bound on the volume of the compactification manifold, arguing that if it is too big, we would see its effects already (i.e., we’d be aware that we live in more than 4 space time dimensions by various experimental results).

    This kind of bound (together with others), allows him to argue that possibly the number of vacua given by various flux compactification constructions is finite (because the size of the manifold grows as you add fluxes). One problem with putting in a cutoff though, is that, even if you make the number of vacua finite, the distribution of them may be peaked near the cutoff (lots more ways to put in fluxes at the maximal number of them).

    If you look at his sequence of papers on counting vacua, you’ll see that he started off hoping that statistical distributions of vacua would allow a landscape prediction of whether the supersymmetry breaking scale was low or high, depending on which was statistically favored. He seems to have given up on this, partially because of this problem. People who talk about statistical calcuations of these vacua possibly providing predictions are just ignoring the fact that this idea has been tried, and now is dead for a good reason.

  18. Who says:

    z3,
    could you provide a date for the article in the NYT?
    I am not a regular Times reader. Any help with finding it? Author name?

    NY Times, “Physics Awaits New Options as Standard Model Idles” describes the lack of progress in theoretical HEP in the past 30 years and portraits the sense of frustration in this field. String theory received a passing mention that’s sort of dismissive.

  19. woit says:

    The NYT article is an essay by Dennis Overbye (who was in Beijing for Strings 2006), and it is at

    http://www.nytimes.com/2006/07/04/science/04phys.html

    Unlike many articles in the press full of hype about extra dimensions, etc., Overbye gets the mood in physics right, and emphasizes the lack of any experimental data that could help us figure out how to get beyond the standard model.

  20. Visitor says:

    Secretmilkshake, the answer to your question “Is ST getting like SA? Anybody shot in the back?” is “Motl will certainly be willing to act as an Obstacle Detachment.”

  21. andy says:

    Dennis Overbye is an editor at the NYT. What other qualifications does he have to comment on the current state of particle theory?

  22. Not A Nobel Laureate says:

    “Dennis Overbye is an editor at the NYT. What other qualifications does he have to comment on the current state of particle theory?”

    Given that he understands that physics is an experimental science driven by the discovery of new data, Overbye already more qualified to comment on the current state of particle theory than many string theorists.

  23. Tony Smith says:

    secret milkshake said:
    “… Stalin: “In the Soviet Army it takes more courage to retreat than advance.”
    You tell me: Is ST getting like SA? Anybody shot in the back? …”.

    What about Glashow, who in a just world would (along with Coleman) be senior guru of Harvard Physics, but,
    due to his views of superstring theory,
    had to go to Boston U. to set up a physics program connected to reality ?

    What about Smolin, who, as he said in his 2001 book “Three Roads to Quantum Gravity”, “… was one of the first people to work on loop quantum gravity … before then I [Smolin] worked on string theory …”, and
    has recently, for advocating LQG, been a target of superstring community attack dogs ?

    Tony Smith
    http://www.valdostamuseum.org/hamsmith/

  24. BE PREPARED says:

    HEP-PH/0607028 ANNOUNCED THAT OUR IMPURE UNIVERSE, SUSONIA, WILL SOON MOVE INTO SUSYRIA, WITH PURE 10-DIMENSIONAL SUPERSYMMETRIES AND EXTENDED LIFE EXPECTANCY. That work was partially supported by the DOE under grant number DE-FG02-96ER-40967.

  25. Well, Tom, the Harvard team kept Georgi, so it is not a completely lost department.

  26. woit says:

    BE PREPARED,

    I think you mean hep-ph/0607029, which is an impressive piece of science fiction.

    Andy,

    Overbye’s profession is to spend his life talking to the best scientists he can find, asking intelligent questions, and writing about what he has learned from them. The results are only going to be as good as the scientists he talks to, but they’re as good a way of coming up with a summary of what scientists in a field think about the current state of their subject as any.

  27. D R Lunsford says:

    Peter,

    That is so funny I spilled my coffee. It’s like Star Trek plot.

    -drl

  28. island says:

    Andy… long ago in a land far away from here it was understood that you should learn enough about physics that you can judge what you’re looking at for yourself, so that you don’t need to make lame appeals to higher authority.

  29. >It’s like Star Trek plot.

    It reminds me a lot of “Schild’s ladder” by Greg Egan. Except that Egan’s story is much more credible.

  30. MathPhys says:

    I wonder what Clavelli was thinking when he wrote these papers. But — it’s a free country.

  31. Bee says:

    From the present to the past

    Cambridge physicist Stephen Hawking and his CERN colleague Thomas Hertog have proposed a radical new approach to understanding the universe[…]

    The new theory aims to get round a fundamental problem of string theory — the most popular candidate for a “theory of everything” — which is that it allows the existence of a multitude of different types of universes as well as our own. Each possible universe in this “landscape” has its own fundamental constants and even different numbers of space-time dimensions. Moreover, string theory does not favour any particular universe over another, which is not a good state of affairs as we clearly live in a universe with a particular set of physical properties.

  32. Benni says:

    Peter woit wrote:
    People who talk about statistical calcuations of these vacua are just ignoring the fact that this idea has been tried, and now is dead.

    What do you think about these papers from Luest:
    I think this is pretty solid work!

    The Statistics of Supersymmetric D-brane Models
    http://arxiv.org/abs/hep-th/0411173

    http://arxiv.org/abs/hep-th/0510170
    One in a Billion: MSSM-like D-Brane Statistics

    Abstract
    Continuing our recent work hep-th/0411173, we study the statistics of four-dimensional, supersymmetric intersecting D-brane models in a toroidal orientifold background. We have performed a vast computer survey of solutions to the stringy consistency conditions and present their statistical implications with special emphasis on the frequency of Standard Model features. Among the topics we discuss are the implications of the K-theory constraints, statistical correlations among physical quantities and an investigation of the various statistical suppression factors arising once certain Standard Model features are required.

    We estimate the frequency of an MSSM like gauge group with three generations to be one in a billion.

  33. Peter Woit says:

    Benni,
    What is dead is the idea of extracting any physical predictions out of such calculations. There are no predictions in the papers you mention.

  34. Benni says:

    This papers are the reason why Douglas has given up counting vacua. He referes in his talk at Solvay directly to Luests work.
    The Idea of counting vacuas is not death, it was solved.
    However one could say the chance of 1 over a Billion to find the standard model is a prediction! At least the only prediction statistical arguments can make, since their only sense is to answer the question “How possible is it to find the standart model”

    This is answeared by Luest.

    But then we have the case that at the end, one has a certain possibility in string theory to describe our world. Since no one can find the solution explicitely (because of Douglas Complexity paper)
    I think it is indeed fair to say the whole subject is at its end.

    (as Luest has said to me in person: If the LhC doesn’t find supersymmetry, then we need some good advise)

  35. Benni says:

    to be more concrete, you wrote Peter:
    Once we have this model, the principle of mediocrity can be used to determine the probablility for us to live in one vacuum or other.

    Unfortunately for this research program, it has yet to even begin to get off the ground, and there are very good arguments that it can never succeed.

    The point why I send you these papers was, that I think this program has succseeded in the papers from Luest! The answer is one over a billion!

  36. JC says:

    The anthropic string folks surely have more than enough rope to hang themselves with. 😉

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