Unification
The idea
of unification of forces is far from
new. In the first half of the XXth century there were many
attempts at
constructing unified theories of gravity and electromagnetism.
For a
review see
Goenner.
The
failure
of
this program has led many physicists to regard such attempts
with
scorn. With hindsight, we now understand why this program
could not
have
succeeded: it ignored the existence of the weak and strong
interactions. Electromagnetism
gets first unified with the weak interactions. We are just
about to
make sure
that our understanding of this unification is correct. Then
these
electroweak
interactions may unify with the strong ones at some much
higher energy
scale. There
are hints in this direction, but there are huge uncertainties
about how
this
happens, if at all. Finally, the unified nongravitational
interactions
may
unify with gravity at the Planck scale. Clearly, any attempt
to put
together
only gravity and electromagnetism looks misplaced. Still, some
of the
ideas and geometrical
tools that were developed in the past could be of some use,
perhaps
after being
generalized from the abelian to the nonabelian setting.
Three routes
to unification: original
papers by Weyl, Kaluza, Einstein and Maier, each proposing a
different
geometrical way of unifying gravity and electromagnetism.
GraviGUT: a modern approach to the
unification of gravity with the other interactions, extending
the ideas
and methods that are used for the other interactions.
Some alternatives to the
standard
model Higgs (dated 10/01/2011) The origin of electroweak
symmetry breaking is the
only remaining part of the standard model that has not yet
received a
direct experimental confirmation. Waiting for the LHC results,
here are
a few alternatives to the standard picture.
(Added september 2011) There is a very
nice talk
by Christoph Grojean that bears almost the same title
and discusses in detail some of the options, and their
observational signatures. It is based al least in part on this paper.
Last
update 10/01/2011