many respects, the history of physics is the history of our
growing comprehension of nature’s simplicity—the
astonishing fact that the entire universe is governed by
just a handful of rules, and that the boiling, bewildering
diversity of the world around us conceals a profound underlying
Indeed, the quest to understand that unity has now become
one of the defining challenges of physics. By the
late 20 th century, that quest had led researchers to the
realization that everything in the visible cosmos is built
from just two kinds of matter: leptons,
a family of lightweight particles that includes the familiar
electron; and quarks, a family of somewhat heavier
particles that are the building blocks of protons and neutrons.
Furthermore, the scientists had described the particles that
carry three of the four fundamental
forces, namely: electromagnetism, the strong force that holds
the components of nuclei together, and the weak force involved
in radioactivity and nuclear fission. (The carrier of the
fourth force, gravity, is presumed to be a particle called
the graviton.) Additionally, the researchers had defined the relationships
among those particles and forces in a mathematical framework
called the “standard
model,” which now stands as the most accurately
confirmed theory in the annals of science.
Yet the quest continues—because the standard model
is almost certainly incomplete. In particular, it says nothing
about how leptons relate to quarks, or how either one of
them relates to the fundamental forces.
Gaps like these have given physicists every reason to suspect
that there are even deeper unities – that each apparently
unique type of matter is actually a different manifestation
of some irreducibly basic entity, and that each force is
really just a different form of a single “super force.” If
that is the case, it could also help explain puzzling disparities
such as symmetry
violation, a puzzling difference in the behaviors of
closely related particles.
NSF supports a wide range of research related to such “grand
unified theories.” One is the hunt for effects
predicted by theory, such as the elusive Higgs field
that is thought to give the known particles their perplexing variety
of masses. Another is the search for evidence of a supersymmetry,
the idea that even particles as different as photons, electrons
and quarks are actually closely related siblings in a heretofore
undetected “superfamily” that comprises all
Part of that effort involves the experimental study of utterly
new phenomena and physics revealed in particle accelerators as
matter and antimatter collide at ever-greater energies. At
the same time, theorists are working to reconcile two of
the 20 th century’s epoch-making ideas: quantum
mechanics, which describes the behavior of matter and
energy at the smallest scales; and Einstein’s general
relativity, which explains gravity as curvature of space-time.
Both theories have been verified by experiments to exquisite
accuracy. But so far, neither is compatible with the other.
Bridging that gap will require bold new concepts, and may
reveal a universe with seven or more dimensions beyond the
four – three of space and one of time – that
are familiar in our daily lives.
Two research areas supported by NSF hold particular promise
for unification: string
theory and loop
quantum gravity. (See also Scientific American, Jan.
2004.) Each offers a theoretical structure for connecting
quantum mechanics and gravity. And other explanations may
arise as physicists draw closer to the fundamental basis
of matter and energy.
Physics of the Universe [Next]