Monday, April 11, 2005 ... Deutsch/Español/Related posts from blogosphere

Savas' colloquium

Savas Dimopoulos of Stanford University is not only an entertaining and pleasant (modern, not ancient) Greek physicist, but arguably one the most important persons behind the theories of phenomenology beyond the Standard Model. Nima summarized his achievements. The most striking - and perhaps a bit exaggerated - description was that "the joke is, whatever happens at the LHC after April 2007, Savas will be in good shape. The only question is which set of his collaborators will get to join in the fun - [Nima] hopes its [him]!" There are several choices

  • a pure Standard Model with a single Higgs - it's a nightmare scenario for particle physics because Savas did not discover this one
  • the Minimal Supersymmetric Standard Model, co-authored by Savas (well, yes, there has been a pre-history)
  • the old large dimensions by Savas, Nima, Gia
  • the warped large dimensions by Randall and Sundrum
  • the (extended) technicolor by Savas and Lenny
  • the Little Higgs model etc.
  • the landscape, as proved by split supersymmetry of Savas and Nima
  • a question mark - a new possibility which may or may not have been written down by Savas

Recently, the Landscape has unfortunately became Savas' most favorite scenario. I've discussed split supersymmetry in a text about Gia's paper here, and the related works about the friendly landscape here and here. The previous article about the anthropic reasoning described Vilenkin's seminar. Savas compared the conjectured huge multiplicity of the vacua to the statements of Giordano Bruno:

  • There are many stars and many planets like the Earth, and our civilization is not a center of the Universe.

These statements were somewhat controversial, Savas said and supported this statement by a picture of burning Bruno. However, the good news is that Bruno's ideas, applied to the Universes themselves - the idea of the landscape - is now supported by physicists all of whom are tenured professors. Therefore, the young people should be careful if they don't want to get fired like Bruno. Incidentally, that would really be an excellent joke if it were a joke. ;-) Savas explained the anthropic principle and the "entropic principle". Unfortunately, he did not use the term "entropic principle" along the lines of the "entropic principle" of Ooguri, Vafa, and Verlinde, but rather as Michael Douglas' demographic approach to the statistics of vacua.

Savas holds a more reasonable opinion about these matters - and he said explicitly that the "equal weight" probability distribution among the vacua is not the full answer. One can't count just the "entropy" but the cosmological counterpart of "free energy".

In the second part of his talk, Savas focused on split supersymmetry and its virtues. One of the "virtues" is that it shows that naturalness is wrong. Another virtue is that it can preserve the gauge coupling unification of the Standard Model - by construction. We take the smallest set of light superpartners that can do the job (therefore, supersplit supersymmetry is not enough & it's also not enough to obtain a dark matter candidate). Not surprisingly, if we make particular guesses about the family of superpartner fields that remain light, we can predict many relations between the observed parameters that could become strong indications that the scenario of split SUSY is correct. Savas also likes the long-lived gluino that could decay during vacations at CERN.

Savas argued that such a hypothetical confirmation of the split SUSY spectrum will almost be a proof of the landscape and the anthropic thinking. Chris Stubbs did not understand why - neither did I - so he asked how the reasoning leading to the proof of the landscape worked. Nima helped Savas to answer this question a bit. He argued completely rationally that once we see that some numbers - such as the Higgs mass - are indeed lighter than the naturalness based on effective field theories would lead us to believe, and there is no other new physics related to the hierarchy problem seen at the TeV scale, there will be two possibilities to explain it:

  • God
  • Nima and Savas

God - or another divine intervention - is not a rational explanation, as most of the audience happily agreed, and therefore we will end up, completely logically, with the Landscape, which is the only rational alternative to God. That's a really cute argument. (In discussions later, Savas and Nima agreed that the Landscape does not imply split supersymmetry, and split supersymmetry does not imply the Landscape.)

Imagine that someone asked 100 years ago where did the large ratio between the unstable nuclei's lifetimes came from. It was completely unnatural and no one understood its explanation. At that time, a physicist could say that either the explanation was God, or the anthropic explanation proposed by that physicist 100 years ago that could give a large number of different Universes - in some of them, the lifetimes were large enough to allow for life.

Shockingly enough, today we understand the exponential hierarchy of the alpha decay rates of the nuclei using a simple effective model of quantum mechanics where the alpha particle tunnels through a barrier to get out of the nucleus, and the lifetime depends exponentially on the thickness and height of the barrier, which leads to very diverse scales for the decay rates. Large and small numbers can be explained and many of them have already been explained.

The bizarre statement that "the anthropic principle will be the only rational alternative to religion" may be formulated more accurately. It is an alternative to religion for those who want to answer any question that has not yet been understood by science. It's exactly this universal "applicability" of the anthropic reasoning and its hidden recommendation that we should not actually be looking for answers that depend on the question - the "anthropic reasoning" may be the universal answer for everything - which makes the anthropic reasoning effectively equivalent to religion - and the difference is just about the choice of the words.

However, Savas encouraged others to look for a truly scientific explanation of the hierarchy problem and perhaps also the cosmological constant problem. Bert Halperin noted that he had a solution to the cosmological constant problem, but based on obvious reasons, he could not have said what it was in front of Savas. ;-) Incidentally, I just solved the problem, too, but I can't tell you the whole story either.

Savas also admitted, completely fairly, that the landscape in string theory remains a controversial idea that can completely go away within a couple of years. Someone asked whether string theory was science and testable - because a certain gentleman named Laughlin gave a talk at the B.U. arguing otherwise - and Savas said that it was and many aspects of it may become testable very soon.

Michael Dine

Michael Dine of University of California at Santa Cruz visited us today. We had the opportunity for a brief discussion about the supersymmetry breaking scale, the validity of effective field theory, and the existence of landscape. Many of Michael's comments were very interesting. He also mentioned some interesting communication that followed their paper with Banks and Gorbatov.

Before he became a sort of landscape advocate, Michael has also been trying to prove that KKLT was incorrect. One of the conclusions that seems to have survived is that the complex structure moduli in the KKLT-like vacua were never guaranteed to be much/sufficiently lighter than the Kaluza-Klein modes. I've never noticed this thing. This should not happen if the effective field theory description is to be trusted, I/we think. If the background is to be viewed as a Calabi-Yau in the first approximation, then its moduli should be lighter than whatever lives on the Calabi-Yau space - especially the Kaluza-Klein modes. It's simply because it should be possible to integrate out local dynamics on the Calabi-Yau space - above the Kaluza-Klein scale - leaving just the overall parameters of the Calabi-Yau which includes the shape. Note that in flux-free vacua, the moduli are completely massless which is O.K. for the applicability of the effective theory.

On the other hand, such a thing - KK modes lighter than the complex structure moduli - generically occurs if the fluxes are large, which is what one needs to argue that there are many metastable vacua. We agreed that there is never a parameteric suppression that would justify the application of effective field theory (incidentally, such an ability to control things parameterically would imply that the number of vacua must be infinite), and Michael even argued that virtually all of the models that are abused to generate hypothetical large ensembles of vacua always need field strength that makes the description strongly coupled. I argued that this "fuzzy region" in the middle of string/M-theory should have a small volume, in a certain counting, i.e. a small number of vacua and possibilities. The large number of vacua that are supposed to be in the middle reflects a large number of different unjustifiable effective field theories that people write down, not true physics of string theory.

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reader Anonymous said...

hi lubos,
i always thought that
there was a hierarchy betwwen the
kk masses and the moduli masses
in the kklt scenarios for the
follwing reason :-

the moduli mass square go as G^{2} [ flux squared] thus

m_{moduli} ~{ 1 \over R^{3} }

while the kk-modes behave as

m_{kk} ~ { 1 \over R }

there is a factor of N ( units
of flux) in m_{moduli}, but none
in m_{kk}, but still at large
radius ( the setting of kklt )
one can have N take value within
a range. Since the number of vaccua is of the order of K^{N} ( where k is the number of 3 cycles
in the CY ) we still have a lot of

did you guys find a flaw
with this line of reasoning.

reader Luboš Motl said...

Hi, thanks! It is not so difficult to decide to believe that the scalings you write are correct, but it's hard to view "N" as a parameteric separation. "N" is never really large. It's like 30 which is more or less of order one and can be beaten by a 4.pi^2 if it appears there.

A related worry - it is not clear whether you have written down all the powers of N that appear anywhere. The maximal allowed value of "N" depends also on "k" and other things, and I am afraid that you may have neglected similar factors in both formulae for m_{moduli} and m_{kk}. It's just too much about pure numbers, and I was told that if you try to look at particular examples with finite fluxes, the (clear) separation is not there.

Moreover, one needs to increase "N" to a large value to make the picture based on a Calabi-Yau or a G2 manifold well-separated, but then it's the same regime in which the flux is so large that you should not be treating the background as a small perturbation of the flux-free background either. The energy density is too large, and so forth.

reader Anonymous said...

Hi Lubos,
Of course, Savas is a great guy, but please be careful not to credit him for other people's (perhaps bad) ideas.
SUSY extension of SM was pioneered by Fayet et al long before Howard and Savas wrote down their SUSY GUT. While low-energy SUSY may be discovered at LHC, GUTs won't. Similarly, large extra dimensions were considered by many people in various contexts in the early years of string theory, most notably by Savas' compatriot Antoniadis. LHC can see SUSY partners, maybe even a tower of KK's but let's not fool ourselves that some detailed "everything goes" models like ADD or RS are testable.

reader Anonymous said...


m_{moduli} ~ N/R^3,


m_{KK} ~ 1/R

there is also the point that R is never really that large. The explicit examples all involve Vol ~ O(100) giving R ~ 2, and KKLT solutions can never give parametrically large volumes. (as V ~ ln(W_0)), where W_0 is the flux-tuned bit.

reader Quantoken said...

Lubos said:
"Shockingly enough, today we understand the exponential hierarchy of the alpha decay rates of the nuclei using a simple effective model of quantum mechanics where the alpha particle tunnels through a barrier to get out of the nucleus, and the lifetime depends exponentially on the thickness and height of the barrier, which leads to very diverse scales for the decay rates. Large and small numbers can be explained and many of them have already been explained."

No, I think that question is far from settled today, for Three reasons.

One, we would expect in general heavier nuclears Would have a higher and thicker barrier. But we see absolutely no correlation between the atomic size and decay lifetime. Both heavy and light elements have isotops of very different decay lifetime. The two seem totally unrelated.

Two, lots of nuclears are completely stable and do not decay at all. You would have to have an infinitely high barrier or infinitely thick barrier to lead to zero decay possibility, which is not possible. Obviously you can not use barriers to explain why these elements are completely stable.

Three. Barriers work both ways. If it is hard for the alpha particle to get out because of the barrier, consider the reversed process, it should also be exponentially hard for an alpha particle to penetrate the barrier and get in. But we see no difficulty at all shooting an alpha particle into such nuclears.


reader Anonymous said...

hi lubos,
what do you have to say the following:-

II B on 10-d space is a \underline{perfectly good} perurbative string theory,
but has nothing to do with the
real world. Would you say that
when non-perturbative effects are
properly taken into account we
would compactification would happen
dynamically and there is just
one string theory ?

reader Luboš Motl said...

Hi! That was a possibility that people could have considered 15 years ago, but today it seems clear that the qualitative behavior of the 10D SUSY vacua is under control and nothing drastic - such as forced compactification - can occur for any coupling. The known strong coupled limits and the stability resulting from the huge SUSY pretty much guarantee that the flat vacua are exact and stable. The same holds for all vacua with 8 or more supercharges, I think.

The N=1 and N=0 vacua in d=4 are different and various things may in principle happen non-perturbatively and spontaneously.

All the best, Lubos

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