Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Susskind/Steinhardt/Smolin discussion at Edge

  1. Dec 20, 2003 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed


    the December 15 2003 issue of Edge has an interview with Leonard Susskind (many anecdotes about beginning of string theory) and a discussion including remarks by Lee Smolin and Paul Steinhardt

    the above link is to the discussion including Susskind's replies to the comment of others

    there is also a link there to the original interview or talk given by Susskind. It is all pretty interesting. saw the link in a Thomas Larsson post at spr.
  2. jcsd
  3. Dec 21, 2003 #2


    User Avatar
    Gold Member

    this is a very interesting overview, but it remains true that
    tests for these theories are new and sometimes ambiguos, i think someone said "a theory is only good when it can be explained to
    a layman", or words to that effect, to my way of thinking
    string theories have been derived to exclude all but the most
    intelligent, and to ellude testing.
    but if ST prooves unfalsifiable then will anything take its place?
    if not LQG wil be the only viable theory and that is not without

    Note that even if the first possibility is true we cannot escape the implications of what Lenny is saying. The reason is that even if some day a unique solution to string theory is found that describes our world, we will never get rid of the large number of string theory solutions that do not describe our world. So whatever happens, if string theory is true we have to explain why the solution that describes our world is picked out of a large collection of solutions that describe very different worlds.
  4. Dec 21, 2003 #3
    So isn't this saying that our particular laws of physics are somewhat arbitrary?

    Does he have any proof of other universes or portions of this one that do not obey the same laws, etc?

    I think his views only express confusion and frustration. He should be looking for other principles that would restrict the solutions to the one that describes our universe. Perhaps that would be another symmetry principle, or perhaps it requires consistency of higher dimensional versions. Or perhaps the tension along a string should be a function and not a constant, etc.
  5. Dec 22, 2003 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Leonard Susskind is one of the founding fathers of string and M theories, as I expect you know. Relating to what you suggest about a sense of crisis, here is a quote from Susskind halfway down that page:

    "...the outsider to the subject should understand that string theorists watched with horror, not pleasure, the discovery of the gigantic landscape of solutions. And yet no string theorist that I know is prepared to say that these solutions don't exist. Like Steinhardt they quake in their boots and pray for deliverance. It is not impossible but all agree that it is unlikely..."

    Here you see that Susskind explicitly says that he wants outsiders to the subject (people outside string theory) to understand what he perceives as a crisis in the subject----an unpleasant horror felt by researchers in the string theory community at some prospect. Perhaps he over-dramatizes.

    But what he says is certainly consistent with the confusion and frustration I have seen expressed by other string theorists recently---dire dark quotes from Edward Witten (he's "afraid Susskind might be right"), also from David Gross---warnings in a paper by Tom Banks "Is there a String Theory Landscape?" Estimates by Mike Douglas of some 10100 distinct vacuums with no clear way to chose the right one.

    Here's a link to the Tom Banks paper
    He is one of the main people responsible for M theory, I believe.
    Another of the senior string figures.

    Here's a link to a Mike Douglas paper describing the dilemma Susskind was talking about
    "Statistics of string/M theory vacua"

    So, although it sounds to me as if Susskind is exagerating the current troubles in string theory, what he says is born out to some extent by recent articles and by worries expressed by other important string people.

    I dont concern myself much with such matters, as I'm more interested by Loop Gravity. But it seems to me that Susskind is talking about something real going on in string theory, that one should at least know about. Glad you took enough interest to read the page!
    Last edited: Dec 22, 2003
  6. Dec 22, 2003 #5
    Is it possible that the number of solutions to String/M theory may be reduced by principles that govern how the universe got started? Are there an great number of possible initial conditions to String/M theory? Or is there only one way the universe could have gotten started which results in the solutions at present?
  7. Dec 22, 2003 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    that strikes me as a really good question---tho I cannot really judge and certainly cannot answer! Since string/M is not a special interest of mine I will just fetch a quote from Tom Banks

    I believe it is a good idea to get an idea of the standing and reputation of people. He is one of a small group (with Polchinski, David Gross, Susskind) who give the plenary invited talks at main conferences, I gather, and maybe the person most responsible for M theory, and I guess he leads the group at Rutgers? or is it Brown?
    So his PhD students get the best postdoc gigs etc. AFAIK

    He also strikes me as kind of open and honest. BTW is a picture:
    One can't tell anything from a picture I guess but it gives
    something to hang random impressions on and imagine the person with.

    But everybody has to form their own impression of what sources are reliable----so if you have not already go to the arxiv search page and have it list all the papers of T Banks. See who he has colaborated with and what he has written about.

    I will get a quote from section 4. of Banks' "Is there a String Theory Landscape?" that seems to have some relevance to your question. You can check out the rest on line.

    Also good to look by at that paper Mike Douglas, another leading scholar. It will take a moment to get the Banks quote.
  8. Dec 22, 2003 #7


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Yes this is the Anthropic principle argument. We pick out the solution(s) that lead to us. A few string physicists support it, but most, I think are just as horrified of that as of the googles of vacua.

    Very interesting situation. We live in fascinating times.
  9. Dec 22, 2003 #8


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Banks section 4 "Scientific Explanation"

    Banks page 17, section 4 called "Scientific Explanation in the Discretuum"

    The discretuum is the presumed 10100 or so discrete separate solutions from which it is difficult to choose by means of empirical observation. Here is an exerpt:

    ---------quote from Banks paper--------------

    4 Scientific Explanation in the Discretuum

    There are two possible modes of scientific explanation in the discretuum. The first is to simply to ask whether there are states in the discretuum with properties identical to those of the Standard Model and observational cosmology, and to ask whether the physics of the ensemble of such states is sufficiently similar that one can make further predictions[9], for as yet unmeasured quantities*.

    At the crudest level, the problem is one of counting. In order to explain the cosmological constant alone, one needs of order 10^63 − 10^120 states (depending on one’s assumptions about supersymmetry and supersymmetry breaking). Additional requirements, such as a suitable hierarchy, the correct values of the quark and lepton masses and mixings, the correct amount of inflation, and so on, are likely to increase this by an enormous factor, quite possibly just as large or larger.

    Refererence [7] provided some estimates of the number of flux vacua, and [9] examined some prototype counting problems, which suggest that there could conceivably be enough flux vacua.

    The problem of actually determining the correct ground state is hopeless, from this viewpoint. It is not clear that there is any sort of small expansion parameter in the flux vacua. If there is, it is unlikely to be smaller than a part in 100. This means that, if one works say to tenth order in this parameter (far more precise than any precision QED calculation) one would calculate the cosmological constant, at best, to an accuracy more than 40 orders of magnitude less fine than the observed value of the dark energy. In other words, at this stage of the computation, there must be at least of order 10^40 states with properties otherwise identical to those of the Standard Model; determining which of these described nature would require an impossibly difficult calculation.

    *Footnote: Another problem here is the inevitable imprecision of quantities associated with a particular state in the discretuum, due to its finite width. For example, all energy levels should be broadened. Is this consistent with an enormously precise tuning of the value of the c.c. associated with this state?
    --------------end of Banks quote--------------------

    Banks reference [9] is to a couple of papers by Mike Douglas, one of which I believe I already gave a link for

    [9] M. R. Douglas, JHEP 0305, 046 (2003) [arXiv:hep-th/0303194]

    S. Ashok and M. R. Douglas, arXiv:hep-th/0307049.
    Last edited: Dec 22, 2003
  10. Dec 22, 2003 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

  11. Dec 22, 2003 #10


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    unfortunately I have to rush out and get more raw materials for my wife to use in baking Christmas cookies
    so I cannot participate here for a while
    but this is not my beat anyway
    thought the edge interview with Susskind was interesting,
    several smart feisty guys in one room
    may not agree with them but enjoy the sparks
  12. Dec 22, 2003 #11
    Actually, I was asking if it were possible that the unique solution of our present observable universe may be determined or selected by initial conditions of the universe. So I guess I'm asking if String/M theory has been able to explain what happened at the very first instant of time? I've not heard of it portrayed as such. But it occurs to me that the present is a result of the past, and so the initial condition is what determines the present state.

    For example, can the initial string or p-brane emerge instantaneously, or must it evolve continuously from a point?
  13. Dec 22, 2003 #12


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    I don't think there is any direct calculable conection from string physics to the "first moment of the universe". Of course initial conditions don't have to be taken then, they can apply to any spacelike surface you find convenience.

    BTW, Lubos Motl has a very nice clear exposition of the multiple vacua issue in a post on today's s.p.r..
    It's all about _compaction_...
  14. Dec 22, 2003 #13
    So string/M theory does not describe how the universe came to be? Are you saying string/M theory is only valid from some time after the beginning?

    So how far back can we go before we run into the problem of multiple vacua? Does the problem appear with some sort of symmetry breaking for which string theory has predictive power to a time earlier than that? As I recall, a symmetry can break spontaneously in some arbitrary direction. Is it this arbitrariness of symmetry breaking that gives rise to the many possible solutions? Or asked in reverse, does the many possible solutions point to the existence of some symmetry heretofore not considered?
  15. Dec 26, 2003 #14
    The Anthropic principle argument raises problems generally because it tends to sound like invoking a religious kind of solution. One reference to this is found at "The Teleological Argument and the Anthropic Principle." In The Logic of Rational Theism: Exploratory Essays, pp. 127-153. Edited by Wm. L. Craig and M. McLeod. Problems in Contemporary Philosophy 24. Lewiston, N.Y.: Edwin Mellen, 1990.

    Here being raised by a Theologian. But in general it also depends upon the actual version of the argument used.
  16. Dec 27, 2003 #15
    Anthropic Principle

    There are several mysteries about the Universe:
    Horizon problem
    Flatness problem
    Matter/Anti-matter asymmetry
    Cosmological constant
    to name a few.

    This makes it appear as though we live in a special universe. Some quantities seem to be highly improbable values, for example, the flatness of the Universe is disturbing. Several questions are:
    Are these special values simply the way our Universe is?
    Is there a deeper level to the physical nature of the Universe which we do not understand?

    These questions are interesting in the sense that if we can understand the origins of the above problems, this would imply that we understand how the Universe was constructed. But they may not be problems at all are imply something unique and special about this universe at all. The may, once we understand how this universe was formed, be simply the only possible outcome for such a formation in which case there really is no Anthropic Principle at play at all. Basically, our Universe would be the most probable universe from the plethora of all possible universes, almost akin to Hawking's attempts at determining the wave function of the Universe in which what he was after was a function where the Universe appears at the medium between all possible wave functions.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?