From: nextc.Princeton.EDU!mdd (Mark D. Doyle)
Subject: Re: What do we know or believe about generations?
Followup-To: mdd@puhep1.princeton.edu
Originator: news@nimaster
Reply-To: mdd@nextc.princeton.edu
Organization: Princeton University
Date: Fri, 11 Sep 1992 03:24:01 GMT

In article <1992Sep11.020409.28542@nuscc.nus.sg> matmcinn@nuscc.nus.sg  
(Mcinnes B T (Dr)) writes:
> One of the really pressing problems in physics is the generation
> problem: why do we have three?
> What solutions have been proposed for this one?
> I know one: in string theory, the CAlabi-Yau people proposed that we
> have more than one generation essentially because of the way that the
> topology of the internal manifold affects the solutions of the Dirac
> equation. The number of generations turns out to be related to the Euler
> characteristic of the internal space. This extremely natural solution of
> the problem seems to be dying out along with string theory itself.

I wouldn't say that string theory is dying out. (Of course, I may be a 
bit biased since my thesis defense is tomorrow morning in string theory.) 
It has just become clear that there is a lot of things in string theory  
that need to be understood better. One big problem is to understand the  
nonperturbative definition of the theory so that one can address questions  
such as supersymmetry breaking, number of generations, the low energy  
spectrum (what we currently call high energy physics), etc. This has  
always been known to be a tough problem. Some progress has been made in  
the last few years with the introduction of matrix models and topological  
theories. The current trend is to set aside the harder problems of working  
with something like the heterotic string in ten dimensions and work with  
low dimensional string theories (D < or = 1) that are exactly solvable.  
The hope is to glean insight that can be applied to the more physically  
relevant case. The last few years have seen an explosion of interest in  
this approach and it has been quite fruitful. The most remarkable thing is  
the breadth of different areas of mathematics that come up in these  
pursuits. There is still a large contigent of people who are actively  
seeking ways to get down from string theory to more accessible (low)  
energies. What is clear is that it seems that it is no longer true that  
there are only a handful of consistent string theories. And there has  
always been a plethora of vacuum solutions (i.e. all those Calabi-Yau  
manifolds). The hope is now that a nonperturbative formulation of the  
theory will narrow down the choices. String theory is still the current  
favorite (certainly in these parts) for unifying gravity with the other  
forces. Anyway, I need to finish some transparancies, but I just want to  
reiterate that string theory is not dead; the emphasis has merely shifted  
in our approach to it and phenomenological questions have fallen into the  
background somewhat pending new insights and data. Hopefully a nice boost  
will come from the discovery of supersymmetry at the SSC. (Let's not start  
that thread again :^).) The other problem is more practical. Jobs in  
string theory are becoming very limited and the amount of activity will  
probably decrease as people switch over to other areas of physics. This is  
not to say that strings are not worth pursuing.

Mark Doyle, 11 hours to go...
mdd@puhep1.princeton.edu