6. Related ideas
6.1. Multiple slEs
We
pointed out earlier that the boundary operators
2,1
correspond to the continuum limits of lattice curves which hit the
boundary at a given point. For a single curve, these are described by
SLE, and we have shown in that case how the resulting differential
equations also appear in CFT. Using the N-particle
generalisation of the CFT results of the previous section, we may now
‘reverse engineer’ the problem and conjecture the generalisation
of SLE to N
curves.
The
expectation value of some observable
given
that N
curves, starting at the origin, hit the boundary at (θ1, … , θN)
is
|
|
(67) |
where
.
This satisfies the BPZ equation
|
|
(68) |
where
is
the variation in
under
αj.
If we now write
and
use the fact that GjF1 = xΦF1,
we find a relatively simple differential equation for
,
since the non-derivative terms in Gj
cancel. There is also a complication since the second derivative
gives a cross term proportional to
.
However, this may be evaluated from the explicit form F1 = |ΨN|2/κ.
The result is
|
|
(69) |
where
the right-hand side comes from the variation in
.
The left-hand side may be recognised as the generator (the adjoint of the Fokker–Planck operator) for the stochastic process:
|
|
(70) |
|
|
(71) |
where
ρk = 2.
[For general values of the parameters ρk
this process is known as (radial)
,
although this is more usually considered in the chordal version. It
has been argued [37]
that this applies to the level lines of a free gaussian field with
piecewise constant Dirichlet boundary conditions: the parameters ρk
are related to the size of the discontinuities at the points eiθk.
has
also been used to give examples of restriction measures on curves
which are not reflection symmetric [38].]
We see that eiθj undergoes Brownian motion but is also repelled by the other particles at eiθk(k≠j): these particles are themselves repelled deterministically from eiθj. The infinitesimal transformation αj corresponds to the radial Loewner equation
|
|
(72) |
The conjectured interpretation of this is as follows: we have N non-intersecting curves connecting the boundary points eiθk,0 to the origin. The evolution of the jth curve in the presence of the others is given by the radial Loewner equation with, however, the driving term not being simple Brownian motion but instead the more complicated process Figs. (70) and (71).
However, from the CFT point of view we may equally well consider the linear combination ∑jGj. The Loewner equation is now
|
|
(73) |
where
|
|
(74) |
This is known in the theory of random matrices as Dyson’s Brownian motion. It describes the statistics of the eigenvalues of unitary matrices. The conjectured interpretation is now in terms of N random curves which are all growing in each other’s mutual presence at the same mean rate (measured in Loewner time). From the point of view of SLE, it is by no means obvious that the measure on N curves generated by process Figs. (70), (71) and (72) is the same as that given by Figs. (73) and (74). However, CFT suggests that, for curves which are the continuum limit of suitable lattice models, this is indeed the case.