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found close together. These problems do not occur in string theory
because particles have an entirely different origin, related not to
points but to the oscillatory modes of the strings .
There are an enormous number of elementary particles that
have been discovered in addition to the photons and electrons
familiar from high-school physics . The discoveries have been made
with the use of large machines, called accelerators, by colliding
beams of particles bent by magnets . The bigger the accelerators , the
deeper inside nature and the shorter the distance that the machine
can probe. The particles can be classified according to the way they
interact , their mass, their spin, and their other so-called quantum
numbers . They display regularities when classified by certain sym-
metry schemes. However, the prediction of their existence, their
properties and their behavior from first principles is an open prob-
lem. One can compare this problem to the state of chemistry before
Mendeleev developed the periodic table of the elements . One could
classify the elements according to the regularity of their behavior;
however, nobody had an explanation from first principles, like the
atomic theory, for these regularities. The existence of such unex-
J
plained experimental regularities is one indication that a branch of
science is ripe for a new deductive approach with wide-ranging,
perhaps revolutionary consequences . Physicists have searched for
many years for the equivalent of the periodic table for electrons ,
neutrons and the many other particles observed in the laboratory.
We already know that neutrons and protons are made of objects with
the unusual name (derived from a Joycean line using the word for a
German cheese) of quarks. One of the hopes for a fundamental
theory of physics is that all of the other properties would be deter–
mined . Everyone would then be able to calculate the electron mass
from some clear and simple rules. However, rather than a new
general principle like string theory , it might be that new discoveries
in larger machines are needed before physicists will understand
these rules.
As we have stated before , string theory was first developed as a
fundamental theory of strongly interacting hadrons , like the proton
and neutron, with the observed spectrum of particles obtained as
oscillatory modes of a string. However, one mode found in string
theory was not observed as a hadron. This initial failure is what is
now the most interesting aspect of string theory. The unobserved
hadron is now considered to be the mediating particle for gravity .
Gravity is the weakest of the four forces, making its presence felt
