Professor Dave here, let's take a look at
the standard model.
Thousands of years ago, we thought that
the universe was made of earth, air, water
and fire. Once science came along, we
realized that these are not fundamental
elements at all, and that everything on
earth is made of atoms of different
elements, like carbon, oxygen, hydrogen, and
dozens more. Soon we realized that these
atoms were also not the smallest things.
They themselves are made of protons,
neutrons, and electrons. The development
of quantum theory brought along with it
the discovery of completely new
particles. New theories had to be
developed to describe these particles
and these theories ended up predicting
even more particles. Modern-day
experiments in particle accelerators
use the concept of E = mc^2
to collide tiny particles together with
incredible energy to see what other
kinds of particles manifest out of the
chaos. Fundamental particles are spotted
and confirmed and the cycle continues.
So many particles were hypothesized and
confirmed in the 20th century that their
ever-growing catalog was dubbed the
particle zoo. But with so many particles
it becomes very important to be able to
define and categorize them, and the model
that describes all of these particles is
called the standard model of particle
physics. Let's quickly go through the
taxonomy of the particle zoo so that we
can make sense of all these particles
and how they relate to one another.
All of the particles in the universe can be
divided into two categories: fermions and
bosons. Fermions are particles that make
up matter, and bosons are particles that
mediate force. Within the family of
fermions there are two types of
particles: quarks and leptons. Quarks make
up the subatomic particles we are
familiar with, the protons and neutrons
found in every atom in the universe,
and leptons are other massive particles
that are not made of quarks, like
electrons. On the other hand, in looking
at quantum field theories we became
familiar with particles that mediate
force, which we call bosons. These include
the photons that mediate the
electromagnetic force, the W and Z bosons
that mediate the weak nuclear force, the
gluons that mediate the strong nuclear
force, and the still hypothetical graviton,
which ought to mediate gravity. So once
again we've got fermions, the particles
that don't mediate forces, and bosons, the
ones that do. Fermions, as we said, can be
split up into quarks and leptons. Leptons
are stable by themselves but quarks are
not, so quarks combine to make other
particles called hadrons. Hadrons
themselves can be split up into mesons
and baryons, the latter being the
nucleons we are familiar with. Protons
and neutrons are examples of baryons.
But there are many more given all the
possible combinations of three quarks.
So far that leaves us here. We can see the
up, down, charm, strange, top, and bottom
quarks. We see the leptons including the
electron, muon, tau, and their respective
neutrinos. We see the gauge bosons that
mediate the three forces we have quantum
field theories for, and the Higgs boson,
the particle that bestows massive
particles with their mass. The rest
masses of these particles are often
listed in mega electron volts, a unit of
energy due to mass-energy equivalence.
But wait, we're not through yet. Remember
when we talked about virtual particles
that can exist because of the Heisenberg
uncertainty principle? These particles
appear without cause simply due to their
mere probability of existing, but in
doing so they always obey symmetry, in
that they arrive in pairs. One of these
particles is considered regular matter
and the other is called antimatter, so when
an electron forms it is not without its
antimatter counterpart, called the
positron. These have the same mass but
they are opposite in charge, and just as
they are born out of energy, like
experiments in particle accelerators,
when they collide they annihilate back
into pure energy, returning the energy
that was borrowed by their existence.
Protons and neutrons have antiprotons
and antineutrons that are made of anti
quarks. All the leptons have anti
particles too, like antineutrinos. When we
include antimatter, the particle zoo
gets much bigger, because we can then
include mesons,
which are short-lived particles
comprised of one quark and one anti
quark. There are plenty more hypothetical
particles, but we can pause here and
review what we've learned. Once again, matter
is made of fermions. Fermions are
comprised of quarks, which come in six
varieties, and leptons, of which there are
also six. Quarks combine to make hadrons,
which are either mesons or baryons,
depending on the quark combination.
Protons and neutrons are the baryons
that make up atomic nuclei, which come
together with electrons from the lepton
family to form atoms, which then form
molecules and all of the matter we see
every day. On the other hand, we also have
the force carrier particles called the
bosons. These are the gluons that mediate
the strong nuclear force, the W and Z
bosons that account for the weak nuclear
force, the photons that carry the
electromagnetic force, and the as of now
hypothetical gravitons that we believe
mediate gravity. Quantum theory has been
successful in showing that these
fundamental forces are actually
different manifestations of the same
force. At very high temperatures, the
electromagnetic force and the weak
nuclear force become the same force, the
electroweak force. At higher temperatures
still, these combine with the strong
nuclear force to become a single unified
force. This force is described somewhat
successfully by several models called
grand unified theories, though there
remains much work to be done in this
area. That leaves just one more force to
unify: gravity. So let's move forward and
talk about that next.
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