Rummaging for a Final Theory -- up to Lie group E8, Zeeya Merali, Scientific
American 2010 Sept: Rich Murray 2010.08.31
http://www.scientificamerican.com/article.cfm?id=rummaging-for-a-final-theory>From the September 2010 Scientific American Magazine, 4 comments
Rummaging for a Final Theory
Unifying gravity and particle physics may come down to an old approach from
the 1960s
By Zeeya Merali
MATHEMATICAL WEB: Visual representation of the Lie group E8. Such complex
symmetrical mathematical structures could help researchers weave together
the physics of particles and forces.
September 2010 Issue
News Scan
Turning the clock back by half a century could be the key to solving one of
science's biggest puzzles: how to bring together gravity and particle
physics. At least that is the hope of researchers advocating a
back-to-basics approach in the search for a unified theory of physics.
In July mathematicians and physicists met at the Banff International
Research Station in Alberta, Canada, to discuss a return to the golden age
of particle physics. They were harking back to the 1960s, when physicist
Murray Gell-Mann realized that elementary particles could be grouped
according to their masses, charges and other properties, falling into
patterns that matched complex symmetrical mathematical structures known as
Lie ("lee") groups. The power of this correspondence was cemented when
Gell-Mann mapped known particles to the Lie group SU(3), exposing a vacant
position indicating that a new particle, the soon to be discovered
"Omega-minus," must exist.
During the next few decades, the strategy helped scientists to develop the
Standard Model of particle physics, which uses a combination of three Lie
groups to weave together all known elementary particles and three
fundamental forces: electromagnetism; the strong force, which holds atomic
nuclei together; and the weak force, which governs radioactivity. It seemed
like it would only be a matter of time before physicists found an
overarching Lie group that could house everything, including gravity. But
such attempts came unstuck because they predicted phenomena not yet seen in
nature, such as the decay of protons, says physicist Roberto Percacci of the
International School for Advanced Studies in Trieste, Italy.
The approach fell out of favor in the 1980s, as other candidate unification
ideas, such as string theory, became more popular. But inspired by history,
Percacci developed a model with Fabrizio Nesti of the University of Ferrara
in Italy and presented it at the meeting. In the model, gravity is contained
within a large Lie group, called SO(11,3), alongside electrons, quarks,
neutrinos and their cousins, collectively known as fermions. Although the
model cannot yet explain the behavior of photons or other force-carrying
particles, Percacci believes it is an important first step.
One fan of Percacci's work is A. Garrett Lisi, an independent researcher
with a Ph.D. in physics from the University of California, San Diego. Lisi
hit the headlines in 2007 with his own attempt to embed a "theory of
everything" in the most complex and elegant Lie group, called E8. Percacci's
work, Lisi says, "provides a nice unification of gravity and the Standard
Model."
Lisi's ideas revived mathematicians' interest in this historical approach to
physics, which led to the Banff meeting, says Gregg J. Zuckerman, an expert
on E8 at Yale University. Lisi's attempt, he adds, "represents a more
general ideal about returning to Lie groups as a way to unify gravity with
the Standard Model." [ More... ]
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