9 The Standard Model of Fundamental Particles
183
saw in Chap. 8 how Feynman lamented that the Fine Structure Constant
cannot be calculated by physicists, but must be measured by experiment and
inserted into the equations of QED. Philosophically this is quite unsatisfactory because even a comparatively small change in the value of this constant
would result in a very different universe that would not support life as we
know it.
However, the Standard Model has nineteen independent constants which
have to be inserted into it arbitrarily. Examples of these constants are
the masses of the elementary particles and of the Higgs boson, as well
as constants, analogous to the Fine Structure Constant, which govern
the strength of the various interactions. Such a degree of arbitrariness is
clearly not appropriate, and physicists are continually searching for other
approaches.
Two avenues that have been explored to extend the Standard Model are
a Grand Unified Theory (GUT) and Supersymmetry. We have discussed
these approaches briefly in Appendix 9.3. Both of these theories predict the
existence of new particles. However, unfortunately these new particles are
so massive that it is beyond the capabilities of existing particle colliders to
produce them.
String Theory is another contender that has received a lot of attention in
the research literature and in the popular media. It was first developed in the
nineteen-sixties as a possible explanation of the strong nuclear force. It fell
from favour as Quantum Chromodynamics became popular, but has found
another life as a candidate for a Theory of Everything (TOE), perhaps the
Holy Grail of physics (see Chap. 4). The attraction of String Theory is that
it treats gravity on the same footing as the other forces, and can therefore be
regarded as a theory of quantum gravity.
As the name might indicate, in String Theory “zero-dimensional” pointlike particles are replaced by one dimensional “strings”. These may be either
lengths of string, or loops of string. The properties of the particle (mass,
charge, etc.) are determined by the vibrational states of the string. One
of these vibrational states corresponds to the graviton, the long soughtafter carrier particle of the gravitational field. Originally String Theory only
included bosons, but has now been extended into Superstring Theory, to also
include fermions.
Five different versions of String Theory were developed over the years until
it was found that they were all variants of one overriding single theory, called
M-theory. M-theory is formulated in eleven dimensions, which represents an
improvement on the original bosonic string theory, which required twenty-six
dimensions.
183
saw in Chap. 8 how Feynman lamented that the Fine Structure Constant
cannot be calculated by physicists, but must be measured by experiment and
inserted into the equations of QED. Philosophically this is quite unsatisfactory because even a comparatively small change in the value of this constant
would result in a very different universe that would not support life as we
know it.
However, the Standard Model has nineteen independent constants which
have to be inserted into it arbitrarily. Examples of these constants are
the masses of the elementary particles and of the Higgs boson, as well
as constants, analogous to the Fine Structure Constant, which govern
the strength of the various interactions. Such a degree of arbitrariness is
clearly not appropriate, and physicists are continually searching for other
approaches.
Two avenues that have been explored to extend the Standard Model are
a Grand Unified Theory (GUT) and Supersymmetry. We have discussed
these approaches briefly in Appendix 9.3. Both of these theories predict the
existence of new particles. However, unfortunately these new particles are
so massive that it is beyond the capabilities of existing particle colliders to
produce them.
String Theory is another contender that has received a lot of attention in
the research literature and in the popular media. It was first developed in the
nineteen-sixties as a possible explanation of the strong nuclear force. It fell
from favour as Quantum Chromodynamics became popular, but has found
another life as a candidate for a Theory of Everything (TOE), perhaps the
Holy Grail of physics (see Chap. 4). The attraction of String Theory is that
it treats gravity on the same footing as the other forces, and can therefore be
regarded as a theory of quantum gravity.
As the name might indicate, in String Theory “zero-dimensional” pointlike particles are replaced by one dimensional “strings”. These may be either
lengths of string, or loops of string. The properties of the particle (mass,
charge, etc.) are determined by the vibrational states of the string. One
of these vibrational states corresponds to the graviton, the long soughtafter carrier particle of the gravitational field. Originally String Theory only
included bosons, but has now been extended into Superstring Theory, to also
include fermions.
Five different versions of String Theory were developed over the years until
it was found that they were all variants of one overriding single theory, called
M-theory. M-theory is formulated in eleven dimensions, which represents an
improvement on the original bosonic string theory, which required twenty-six
dimensions.