Tuesday, December 14, 2004


Tom Banks, my (former) PhD advisor, has a new paper
Tom argues that the landscape is not a well-established feature of string theory. In a conceptual paper which does not bother you with too many equations (the Wheeler-DeWitt equation being an exception), he argues that
  • one must distinguish the 1PI and the Wilsonian effective actions more carefully: the 1PI action describes the whole field content, and you get the same action regardless of the point around which you expand
  • on the other hand, the Wilsonian effective action contains the low-energy degrees of freedom that depend on the point in the configuration space - and this is the type of the effective action that one obtains in string theory
  • Tom re-uses his statements that one cannot think about the bubble of another vacuum (such as a de Sitter bubble) as an excitation of the original vacuum, and interprets the Guth-Farhi results in this fashion - one cannot verify the existence of a dS bubble inside the bottle because the external observer sees that it is surrounded by a black hole
If I understand well, Tom does not propose new arguments that the numerous KKLT-like vacua don't exist. Instead, he says that they cannot be effectively used as vacua of the same theory, i.e. they cannot be combined into a big multiverse, and therefore there is no cosmological mechanism that would connect them and create a realistic ensemble of the Universes.

I would agree that this "disconnected feature" of the vacua prevents us from making a scientific claim about the probability measure of different ones. He argues that the "democratic" measure on the space of vacua is unjustified, and a more precise measure cannot be defined - and I agree with these statements. On the other hand, if the large number of stringy vacua exist (just imagine that!), we may perhaps be living in either of them, regardless of the existence of cosmological solutions (such as the eternal inflation) that interpolate between them. Of course that I tend to agree that even in this case, the full rules of string theory are more likely to pick some "special" vacua rather than the numerous ones, but we don't have any proof either way. In order to kill the current versions of the landscape idea completely, one would have to find a general enough and serious problem with the construction of the KKLT-like vacua.

Nima Arkani-Hamed, who has been converted to a landscape supporter much like many other people have been converted to Christianity ;-) (but otherwise he studies it much more scientifically than other landscapers!), argues very correctly that the anthropic idea is either colossally correct or colossally wrong. It is a bifurcation point that can direct physics in the next 10 years in vastly different directions, and one cannot decide which answer is correct by cheap attacks.


  1. Tom Banks, my (former) PhD advisorthanks for correction and update....my apologies

  2. String theory would loose much of its appeal and power if landscaping ideas were correct. If string theory at some in future can describe nature but only under the assumption that we happen to live in one of gazillions of vacua for anthropic reasons... How disgusting! I think it's a sign of frustration and lack of real progress that we study anthropic reasoning seriously. OK, we can't kill it -- and people like Nima love everything that can't be killed because they went lucky once -- but can we say anything in favor of it? It only just sidesteps much of the problem outside of the realm of good hard science. It's good to see that people like Tom Banks have better ideas and show that even if landscaping is correct, in its current stage it hasn't even been born yet.


    PS: What happend to your sci.physics.strings group, Lubos? Since the new (anthropic?) design was introduced, people have essentially stopped posting, it seems.

  3. Most of this landscape stuff in string theory over the last two years or so, appears to be a bit of an embarassment. Besides searching for a unique vacuum state and/or finding a nonperturbative mechanism for breaking the supersymmetry, what other approaches are there which can possibly kill off this landscape business?

  4. Hi Michael!

    Yes, I agree with you, including the guess that the dramatic decrease of the sci.physics.strings traffic is related to the previous Google interface being canceled - the new one requires new learning and registration etc. However I don't have time to revive the newsgroup right now.

    All the best

  5. intense!

    jason mulgrew
    internet quasi-celebrity

  6. How does the anthropic principle relate to Tegmark and the theory of mulitple universes?

  7. Lubos,

    What do you think will happen to string theory if this anthropic landscape stuff becomes the dominant area of research over the next few years, at the expense of everything else in the field?

  8. Hi!

    Good question. I would avoid predictions of disasters. People are working on this thing simply because right now they don't see any other obvious direction to make a significant, completely new type of progress.

    It's also far from being true that landscape is the only thing studied in string theory these days! ;-)

    As Nima Arkani-Hamed said, if you think in the anthropic framework (which he started to do), it really changes your mode of thinking. In my opinion, all these things are temporary. Once there is some clear progress on the experimental or theoretical non-anthropic front, these people who have turned anthropic will be able to unlearn it, I guess.

    Also, there is some probability that the anthropic thinking is correct in some sense. I don't think that it is logically impossible. It's just that this possibility is not very interesting. The anthropic mode of learning will simply reduce the excitement of everyone - it has already happened in a sense. But the real problem is that we don't have clear non-anthropic ways to accelerate the progress. I would not blame the anthropic people for this slowdown.

    Both sides agree that the question of anthropic thinking is a very dividing one: it's a guess about the very far future shape of our understanding of physical laws.

    I think that there has been too much time of this purely theoretical enterprise, and I hope that we will soon get some actual new data from experiments that will direct our thinking a bit. My guess is, of course, that the basic picture of the beyond-the-Standard-Model physics we obtained is correct: SUSY plays some role, interactions may be unified, matter can live on branes, they create warped geometry, maybe, and so forth. I don't trust any of the details too much. It's a very fair enterprise to study very detailed models quantitatively, but one must realize that at least 99% of this detailed work will turn out to be irrelevant in any case.


  9. Lubos,

    I guess I'm somewhat "conservative" when it comes to this issue. I certainly hope this anthropic landscape stuff is just a passing phase which will disappear in short period of time (ie. in a few years). Whether string or non-string related, physicists should be able to do better than this!