Date: Fri, 13 Sep 1996 12:07:58 -0300 (ADT)
Subject: Physical braids

Date: Thu, 12 Sep 1996 23:54:28 -0700
From: Vaughan Pratt <pratt@cs.stanford.edu>

Forwarded from Phillip Schewe's Physics News Update, #285:

BRAIDS PLAITED BY MAGNETIC HOLES. The study of braids
and knots is important for both mathematics (where it is a subfield
of topology) and for quantum physics, where it is used to describe
the interactions of particles in an abstract multidimensional phase
space.  Now physicists at the Institute of Energy Technology in
Norway (Geir Helgesen, geirh@ife.no) have demonstrated a
practical way to investigate complicated braids using tiny beads
confined between two plates and subjected to complex magnetic
fields.  The motion of these beads constitutes a three-dimensional
braid if you consider time as a third spatial dimension; a sequence
of photos of the beads at regular intervals is assembled into a plait-like
trajectory not unlike the smoke trails used in wind-tunnel
experiments, except that in this case the observed braid topology
can reveal information about the magnetic fields pushing the beads
around.  The researchers expect that the behavior of the beads
(actually non-magnetic spheres immersed in a ferrofluid) can be
used as a simple experimental tool for modeling complex
interactions in quantum field theory or chaos theory.  (P. Pieranski
et al., Physical Review Letters, 19 August 1996.)