11
Forces in the Standard Model and
Symmetries
So far we discussed the matter content (the fermions) of the standard model.
They describe the constituents of matter and are the fundamental units of all
matter. They come in three varieties of quarks and leptons (usually called three
generations), as we just saw. Now we turn to the forces acting on them. Recall
our earlier discussion that forces are caused also by particles, which are bosonic
in nature. Figure 11.1, in addition to containing the matter particles, also
contains the particles that carry the forces. They are denoted by the symbols γ
(for photon), the carrier of electric force, W
± and Z, the carriers of the weak
force, and g (the gluon), the carrier of the strong force. The gravitational force
is carried by a boson called a graviton (denoted by G). The photon is one
of the earliest force carrier to have been identified by physicists. As soon as
the electromagnetic waves were discovered in the last part of the nineteenth
century, many experiments were performed with light to understand it better.
The photoelectric effect, discovered by Hertz in 1887, was one of them. He
found some strange properties which Einstein explained by postulating that
the light comes in discrete units called light quanta (also called the photon).
That provided a simple way to understand the new features of photoelectric
effect found by Hertz. Light, which previously was assumed to be a wave, also
came to be called a particle. This dual nature of light was in the spirit of the
quantum ideas which became prevalent. Einstein was awarded the Nobel Prize
for this suggestion in 1921, and he received the award in 1922.
A beautiful aspect of the standard model is that the forces in this theory are
not put in by hand to fit what we observe, but are determined by a fundamental
physical principle. In some sense, that makes it more believable for the theory
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_11
77
Forces in the Standard Model and
Symmetries
So far we discussed the matter content (the fermions) of the standard model.
They describe the constituents of matter and are the fundamental units of all
matter. They come in three varieties of quarks and leptons (usually called three
generations), as we just saw. Now we turn to the forces acting on them. Recall
our earlier discussion that forces are caused also by particles, which are bosonic
in nature. Figure 11.1, in addition to containing the matter particles, also
contains the particles that carry the forces. They are denoted by the symbols γ
(for photon), the carrier of electric force, W
± and Z, the carriers of the weak
force, and g (the gluon), the carrier of the strong force. The gravitational force
is carried by a boson called a graviton (denoted by G). The photon is one
of the earliest force carrier to have been identified by physicists. As soon as
the electromagnetic waves were discovered in the last part of the nineteenth
century, many experiments were performed with light to understand it better.
The photoelectric effect, discovered by Hertz in 1887, was one of them. He
found some strange properties which Einstein explained by postulating that
the light comes in discrete units called light quanta (also called the photon).
That provided a simple way to understand the new features of photoelectric
effect found by Hertz. Light, which previously was assumed to be a wave, also
came to be called a particle. This dual nature of light was in the spirit of the
quantum ideas which became prevalent. Einstein was awarded the Nobel Prize
for this suggestion in 1921, and he received the award in 1922.
A beautiful aspect of the standard model is that the forces in this theory are
not put in by hand to fit what we observe, but are determined by a fundamental
physical principle. In some sense, that makes it more believable for the theory
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_11
77