RECENT THEORIES ON MATTER
Unlike Chapter 4 in the textbook, we now summarize the
most recent notions of matter, a picture which has really
only started to become clear in the last 40 years.
1.
The term anti-matter may be a source of some
confusion. Please note that anti-matter does not
mean negative mass! Rather, the “anti-particle”
corresponding to any particle would have the
same mass, but opposite charge (if any) and
opposite spin (if any). For example, since an
electron is assumed to carry a tiny negative
charge, an anti-electron (also called a positron)
would have an equal amount of positive charge.
Because of this, electrons and positrons (or any
other particle/anti-particle pairs) can
theoretically appear spontaneously (apparently
out of nowhere) and then re-combine,
annihilating one another and disappearing again.
2.
These particle/anti-particle pairs don’t really
“appear from nothing” and “disappear into
nothing”, of course. According to Einstein’s
famous equation E = mc2, the total mass m of a
particle/anti-particle pair can be “created” from
a burst of pure energy E (note that c = 3.00  108
m/s is the velocity of light), usually as a result of
colliding gamma rays (more about these later).
2
Likewise the same thing can occur in reverse,
with a particle/anti-particle pair spontaneously
combining and disappearing, releasing that same
amount of energy (again, usually as gamma
radiation).
3.
In theory, every particle has a corresponding
anti-particle, but these certainly wouldn’t be
expected exist in equal numbers at any one time.
Because electrons are a critical component of
every atom, and are not routinely being
annihilated by positrons, they obviously
outnumber positrons by a huge factor.
4.
Today, the tiniest fragments of matter (the
building blocks of everything in the universe) are
classified as fermions (after Enrico Fermi).
Fermions in turn are thought to be divided into
two subclasses: leptons and quarks.
5.
Leptons are by far the tinier particles of the two
subclasses. They have little (and possibly even
no) mass and they are “solitary”, i.e. they don’t
normally combine with one another or with other
types of particles (except for their corresponding
anti-lepton). There are believed to be six leptons
(and six anti-leptons) in all.
3
6.
Three of the six leptons (the electron, the muon,
and the tau) have a tiny mass and carry a small
negative charge. The electron is stable and never
decays while the muon and the tau are extremely
unstable and short-lived “exotic” particles which
won’t be mentioned further.
7.
The other three lepton types are collectively
referred to as neutrinos (with three subcategories, corresponding to the three types of
charged leptons identified above). Neutrinos are
electrically neutral (i.e. they have no charge), and
while their mass is still a somewhat open
question, most physicists today consider them to
be massless.
8.
Quarks have a great deal more mass and, unlike
leptons, they normally don’t exist alone. Rather,
they combine in threes to form hadrons (particles
which include the plentiful and familiar protons
and neutrons which make up atomic nuclei, as
well as some other “exotic”, short-lived particles
which won’t be discussed further).
9.
Quarks (and their corresponding anti-quarks) are
thought to occur in six distinct varieties (given
the interesting names up, down, charm, strange,
top and bottom). So far, all except the top variety
has been experimentally confirmed.
4
10. Each variety of quark has a different mass
(though an up quark’s mass is believed to be only
slightly less than that of a down quark). Three
varieties (up, charm and top) have a charge of
+2/3 and the other three have a charge of –1/3.
All six have spin 1/2 (though this is only
significant when combinations with anti-quarks
are involved). Also critically important is the fact
that each of the six quark varieties come in 3
colors (called red, green and blue).
11. The color of a quark isn’t a real “colour” at all,
but merely the name for another characteristic
that limits the types of combinations which can
occur. For example, protons and neutrons are
deemed to be colorless (i.e. “white”). So, when
three quarks combine to form a proton or a
neutron, one quark must be red, one must be
green and one must be blue. (This is because,
when it comes to real “colour”, we know that the
addition of the primary colours red, green and
blue produces white!)
12. We will only concern ourselves with those
combinations which produce the protons and
neutrons found in every atomic nucleus.
Specifically, one down quark (say red) and two up
quarks (say green and blue) produce a proton,
while two down quarks (say red and green) and
one up quark (say blue) produce a neutron. For
5
the proton, the net charge (+1) is correct, since

2 

2  1
      1 ,
3  3
while the neutron is electrically
 1 
2
neutral because   3    
2
  0 . In both cases,
3
the combined colors produces the required
colorless characteristic. Finally, the slight mass
discrepancy between up and down quarks
accounts for the slightly higher mass of the
neutron.
13. In the most current view of matter, the forces of
interaction between particles (gravitational force,
electromagnetic force, weak and strong nuclear
force) are thought to be transmitted by
“messenger particles” which somehow “carry”
the force between the particles. Such particles
can be thought of as massless bursts of pure
energy (though in some cases they could also
carry a charge). Overall, the entire class of forcecarrying particles have been called bosons (after
Satyendra Bose) This theory of bosons as force
carriers isn’t yet as well developed or as fully
confirmed as the theory of fermions (leptons and
quarks), but it is gaining in acceptance.
14. The strong nuclear force is assumed to be
responsible for combining quarks into protons or
neutrons, and for binding together the cluster of
6
protons and neutrons within an atomic nucleus.
The messenger particle bosons thought to do this
have been called gluons. These come in a number
of different varieties with characteristics called
color and anti-color. According to this theory, for
example a red quark approaching a green quark
could spontaneously emit a red/anti-green gluon.
As a result, the red quark would change to green,
leaving the (now green) quark and (red/antigreen) gluon combination appearing “red”
overall. The (red/anti-green) gluon can now be
absorbed by the other (green) quark, to produce
a red quark. And in the process, the original red
(now green) quark and the original green (now
red) quarks are “glued together” by virtue of the
gluon interaction.
15. The weak nuclear force is used to explain why
some atomic nuclei can spontaneously decay in
spite of the strong nuclear force holding the
nucleus together. The force-carrying particles in
this case come in several varieties (which can also
be positively-charged, negatively- charged or
neutral) and are called intermediate vector bosons.
(These won’t be discussed any further, though.)
16. The electromagnetic forces between charged
fermions (most notably electrons) are likewise
thought to be transmitted by another variety of
boson called photons. (This is the same name
7
given to “light particles” earlier, which makes
sense since, light is a form of electromagnetic
radiation, and if light can be seen to be a particle,
the particle would be massless.) For example, a
photon emitted by one electron and absorbed by
a second nearby electron would cause the
electrons to move away from each other (as
observed in electrostatic repulsion). For more on
this example, see 18, below.
17. It has likewise been suggested, though this has
still not as yet been detected or confirmed
experimentally, that there is another boson
responsible for transmitting the gravitational pull
which exists between any particles with mass.
The name given to this hypothetical forcecarrying boson is the graviton.
Finally, there is a widely-accepted principle called
“conservation of energy” which suggests that the total
energy within any closed system must remain constant.
While this should be expected hold in general across the
universe over time, the “uncertainty principle” of
quantum physics also allows for isolated and extremely
short-lived exceptions. This has given rise to the notion
of virtual particles. These can be either virtual bosons or
virtual particle-antiparticle pairs. The photon emitted by
one electron and absorbed almost immediately by a
second nearby electron is an example of a virtual photon.
Similarly, at some point in deep space with no apparent
8
nearby matter, a virtual electron-positron pair could
suddenly appear from nowhere, and then annihilate and
disappear again an estimated 10-21 seconds later. These
examples do not break the conservation of energy law
because it would be impossible to measure energy with
sufficient certainty over such a short time-period.