Earlier in this module we reviewed the
fundamental forces that can act on particles.
As it turns out special
particles called exchange bosons
play a fundamental role in how at least three of the fundamental forces act.
Particles can feel each other's presence because they exchange these specific bosons with each other,
It's like they're sending
messages to each other as a way of
figuring out whether they should get
closer to each other or move further away.
Let's look at some of these
exchange bosons and think about how they
explain some of the fundamental
properties of each force.
We'll start with the electromagnetic force.
For the electromagnetic force 
the exchange boson is a photon,
sometimes known as a virtual photon,
since these exchanges happen so quickly
that we don't actually detect them.
The photon is a boson as the name suggests and is mass-less, charge-less, and has spin one.
They're only exchanged between charged particles.
We can draw this kind of
exchange in the form of a Feynman diagram, like this.
The diagram can be
translated into a specific mathematical
equation that describes the collision
process.
Don't worry we're not going to get into that here,
but the theory behind this is known as quantum electrodynamics or QED.
Next we'll talk about the strong force.
The strong force acts between quarks, and as you might recall only acts over very small distances,
about 10^ -15 meters (on the order of the size of the atomic nucleus.)
The exchanged bosons for the strong force are called gluons.
Like photons, they're massless and they're only exchanged between quarks and antiquarks.
This time though the  exchange process is a bit more complicated because
there's the third property known as color that determines whether certain quarks
can be drawn together via the strong force.
There are three possible colors that quarks can have: red, blue, or green.
The baryon that each set of quarks builds must be colourless,
which means you can have one of each type of quark: a red, a blue, and a green.
This is an analogy to light. If you
combine red, blue, and green light you get white.
This constrains how you can make
baryons.
Or you can have a quark anti quark pair of the same colour.
This constrains how you can make mesons.
Now quarks don't really have colour in
the visual sense. The colour here is a
visual tool we can use to think about
which quarks can bind together.
The theory that underlies this explanation
is known as quantum chromodynamics or QCD,
and the colour is basically another
quantum number.
It's a handy one too. It allows us to put
quarks which are fermions,
and therefore can't live in the same quantum states, 
in configurations that might be impossible otherwise.
These impossible configurations were in fact how colour was discovered in the first place,
but that's a story for another time, because we have one more force to go.
The weak force is next. If you recall, it's called the weak force because it only
acts over an extremely short range, about 10 to the minus 18 meters.
It happens to involve two exchanged bosons, the W and Z bosons.
These two, unlike the previous
exchange bosons we learned about, have mass.
Actually they're pretty heavy, about
a hundred times the mass of the proton.
This is one of the reasons the weak
force acts over such short ranges.
Imagine how hard it must be to check
huge bosons back and forth.
The W boson can be positively or negatively charged, and we denote both types: w+ and w- respectively.
The z boson is uncharged.
One good example of the weak force in 
action is in nuclear beta decay.
Let's take a look a bit more closely at how beta decay works on the quark level.
We'll choose neutron decay to focus on rather than look at the beta decay of a specific nucleus.
Recall that the neutron consists of one up and two down quarks, while the proton is two up and one down quark.
In the beta decay process, a down
quark emits a w- boson, causing it to
transform into an up quark. The w- boson
then decays into an electron and an
anti-electron neutrino, leading to the
familiar beta decay products
The W and Z exchange particles were actually predicted by Sheldon Glashow,
Steven Weinberg and Abdus Salam in their electroweak theory, which combined the
electromagnetic and weak forces into one
model. They won the Nobel Prize for this work in 1979
The W and Z bosons were discovered at CERN in 1983 by Carlo Rubbia and Simon van der Meer,
who won the Nobel Prize for this work in 1984.
I should note Carlo Rubia and Simon van
der Meer won the Nobel Prize for this work,
but there were actually quite a few
people involved in this experiment.
Now, we associated the weak forces range with the size of its associated exchange particles,
but the strong force is also short-range, and it's exchange particle the gluon has no mass
So what's going on here? 
Well the range of the strong force
has to do with that we never see isolated quarks in nature.
The force keeping them together
is indeed quite strong.
Now, back to the last force, gravity. 
It turns out to be the only force that isn't
part of the standard model. 
The reason for this is we don't have a quantum model
for gravity, and we have found no
evidence for a messenger particle for this force.
We do however already have a name
if such a particle has ever discovered, the graviton.
Now, evidence for
gravitational waves was just found by
the LIGO collaboration, including some
members of the ANU.
This means that Einsteins theory of general relativity was right about the existence of these
classical waves but we don't yet have an
idea of how to build up an
experimentally verifiable quantum theory
that is consistent with these
observations, as well as all the other
observations confirming Einstein's
theories of relativity. So there's still
more work to be done on this front.
Everything we've learned up until now
was first to put together into a
standard model of particle physics in
1974. To this day the standard model is
the most complete quantum model
explaining what matter consists of and how it interacts.
When we test for
physics beyond the standard model,
we're searching for clues that the
theory we've come up with thus far is
perhaps derived from a bigger, possibly
more elegant and more importantly experimentally verifiable story
In the next video we're going to learn 
about how the standard model of physics can be used
to understand what might have happened during the birth of our universe as we know it
