SuperExtra

David Gross was invited to our department for 3 days. During one public lecture, one colloquium and some informal chat, there are something he said which may be interesting to hear. One thing to remind you, Gross is a very conservative reductionist, so you may not like some of his words. Welcome to leave your comments. I just quote them (not word by word) in a random order:

• Clues that SUSY may be right (at TeV scale)
  1. It helps Grand Unification (MSSM)
  2. It helps to get rid of mass hierarchy of Higgs
  3. It predicts a candidate for dark matter

  However, none of the clues is indispensable, the LHC can never say that SUSY is dead, either. SUSY can give up all these jobs and stay safe with string at plank scale.

• String theory is both radical and conservative
  How do we get string theory? Just change all the word “particle” to “string” in your QFT book and keep all the other principles of physics unchanged.

• String theory first started by explaining mesons, and nowadays it’s still under study, that is QCD string. Of course, string is only a mathematical tool in QCD, the key is AdS/CFT (large N) , so the mesons are like free bosonic strings and may be solved.

• The biggest triumph of string theory is the black hole. Only string provides a tool to count the microscopic entropy of black holes. (NB: We asked Gross about Witten’s recent work on 3D gravity. He said he did not read the paper and not know much about it. )

• The problem of cosmological constant is not that , but that we do not know the mechanism to determine .
  Actually we have other hierarchies, for example, , which seems more severe than cc. Dirac tried to use large number theorem to explain it, but predicted running of and , which is ruled out. However, QCD came to the salvation. Asymptotic freedom indicates that the mass of proton is at the same scale of , and the hierarchy of and plank scale can be calculated from coupling running. So the hierarchy between and is natural. Similarly, the hierarchy between EWSB and GUT may be solved by SUSY and the hierarchy between neutrino mass and GUT may be solved by see-saw mechanism. As for fermion mass hierarchy, it is a problem, but not so severe as cc, in some solutions of string, quark mass hierarchy comes out naturally from topology of space.

• The nature of spacetime — are they emergent?
  Space is easy to emerge as a large distance classical approximation, so are the dynamics like gauge interaction. Examples: AdS/CFT, matrix theory, etc. Relativity tells us space and time are equivalent. But no one knows how to emerge time, because we can’t find a physics without time.

• Testing string ( I do not understand here)
  1. Transport coefficient of quark-gluon fluid, which is related to correspondence of N=4 SUSY on M4 and IIB string on AdS5.
  2. Quantum phase transitions in strong correlation system.

• When may quantum mechanics fail? (Gross believes never)
  t’Hooft: at very short distance
  ?: In large complex systems
  Penrose: In conscious system
  As a description of the universe? How can we find a universe wave function?

• Is physics an environmental science?
  Are all the parameter and laws are calculable or determined by historical or quantum accident?
  Can Einsteins’ good old dreams be realized?