Take a look at anything that’s around you. Everything that you can see is made of up
of the same stuff – the same fundamental particles of nature. This means that remarkably
a tree is made up of the same basic stuff as your
phone, your pet, your house, and even your body.
These fundamental particles are so tiny that you can’t see them with your naked eye.
These are quantum particles obeying quantum laws.
We call them fundamental because they are the smallest particles we know of. In other
words, there is nothing smaller that they are composed of, as far as we know.
The best theory we have to describe these fundamental particles, and how they interact
with each other is called the Standard model of particle physics. This is the best, most
accurate theory we have today. And this is what you’ll find if you Google the
standard model. This table can be thought of the same way as the periodic table of elements
in chemistry, listing all the known atoms we know of.
Although this is what you see, the standard model isn’t just the particles shown here,
but is an umbrella term that denotes everything we know about the microscopic world – the
world that operates by the rules of quantum mechanics. It encompasses the sets of rules
about how the fundamental particles we know of interact with each other, and how
these interactions are mediated by the different fundamental forces.
It’s more precisely a collection of quantum field theories describing the strong, weak,
electromagnetic force, and more. In this video I will show you a different
way to visualize the standard model, such that you can more clearly see not only the
uniqueness of each particle, but also how they interact with each other, and how the
fundamental forces and the Higgs boson connect everything to everything else.
In this way I hope to show you visually how particle physics works, as best we understand it
today. And that’s coming up right now!
Some of you might wonder what inspires me to make videos on a particular topic.
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First I want to give credit to theoretical physicist, Chris Quigg, who is a distinguished
scientist Emeritus at Fermilab. He is the one that conceptualized the visualization
I’m about to show you. You can visit his website at the link in the description.
Let’s start by throwing away the table of the standard model and begin with a clean
slate. We will start with nothing and go step by step, while exploring the standard model.
First, let’s build an atom. To do this we need quarks. The nucleus of atoms contains
neutrons and protons. These are made of either a combination of 2 up and one down quarks
in the case of the proton, or 2 down and one up quarks for the case of the neutron.
These quarks come in two values of electrical charge. The up quark has a charge of +2/3
and the down quark has a charge of -1/3. So the total charge of the proton is +1 if you
add up the charges from 2 up and 1 down quark. Similarly, the total charge of the Neutron
adds up to zero. We have two classes of matter particles in
the standard model, Leptons and quarks. Matter particles have this special property of spin.
They can spin clockwise or counterclockwise. And Depending on what way the particle spin is
in relation to its direction of motion, it is classified as either right or left-handed particle. Quarks
come in both types, left-handed and right-handed. The aspect of handedness is very important,
because it turns out to be crucial for how the particles interact. This is something
that is completely omitted from the typical representation of the standard model.
Left handed up and down quarks can transform into each other via an interaction of one
of the fundamental forces of nature, called the weak force. This is called a charged weak
force interaction, and is mediated by a W+ boson which has a charge of +1 or a W- boson,
which has a charge of -1. This line represents the W boson charged interaction.
Now for some unknown reason, nature turns out to not treat left- and right-handed particles
the same way, because this interaction does NOT happen with right handed quarks. It is
not known why. This means that right-handed quarks cannot
change into other quarks and remain the same particle always. There is no charged weak
interaction between right-handed particles at all!
The quarks also have another charge called color charge, or more precisely, each type
of quark exists in three different colors, red, blue, and green. The color charge allows
quarks to interact via another fundamental force of nature, the strong force.
The strong force is mediated by another boson called the gluon. This strong force interaction
allows 3 quarks of different colors to be bound together to make a proton or neutron
in the nucleus of atoms. We can represent this force by showing a triangle between the different colors.
Note that the triangle is the same for left and right handed particles because the strong force unlike the
weak force does not discriminate between left and right handed particles. It treats both types of particles
the same way. The colors of these quarks are constantly
changing by emitting and absorbing gluons. But all three quarks must be of the three
different colors, red, green and blue to make a neutral or colorless charge.
Any two of the three quarks can't be the same color at the same time.
So far, we have the following for quarks: there are up and down quarks. These quarks
can be further split in two groups depending on if they are right or left-handed. In each
group there are 3 colors of each quark. So as of now, we have 12 different quarks, 6
different up quarks, and 6 different down quarks.
Again, looking at the standard table of the standard model you might be fooled that there
is just one up and one down quark! But there are really 12 as shown here.
You should know that quarks by themselves can never be observed. The theory tells us
that only colorless particles can be observed. A colorless particle can only be formed in
two ways. You can either combine quarks of three different colors, red, green and blue
to make a neutral color combination, like in neutrons and protons which can be observed.
Or, you can combine quarks anti-quarks. Yes, there are antiquark that carry an anti-color.
So there are anti-red, anti-green, and anti-blue antiquarks. When a quark binds to an antiquark,
the particle that's formed is called a meson which can be observed.
These color rules come from the strong force.
This strong force is mediated by a particle called the gluon.
Like glue, it binds quarks tightly together. There are actually
8 different gluons with different combinations of colors. Gluons act as a mechanism to transfer
colors from one quark to another. This transfer of colors is what binds quarks together. I
made a very detailed video of how this happens using Feynman diagrams, if you are interested
in knowing the mechanics of this. The gluons are not colorless, but theoretically
you could combine them such that you can make a composite ball of gluons that is colorless
and could be detectable. But so far no one has ever detected a glue ball.
Now, let’s look at the second class of matter particles called leptons.
Again, we have two types of leptons in terms of charges. We have the electron with a charge
of -1, and the neutrino with a neutral charge of 0.
Like with the left handed up and down quarks, the charged weak force can transform left
handed electrons and neutrinos into each other too, via the exchange of the W boson.
While both left-handed and right-handed electrons exist, when it comes to neutrinos, for some
reason, only left handed neutrinos exist. Right handed neutrinos have never been detected.
Unlike quarks leptons do not have any color charge, and so they are not affected by the
strong force. This lack of color charge is the main distinction between leptons and quarks.
With that, let’s do a quick summary of what we have learned about matter particles thus
far. There are 2 classes of matter particles – quarks
and leptons. For the quarks, there are 6 up quarks, and 6 down quarks. They carry both
electrical charge and color charge. For leptons, there are 2 electrons – left
handed and right handed, and 1 neutrino - left handed only. Electrons only carry electrical
charge and do not have a color charge. Neutrinos do not carry any electrical or color charges.
So to sum it up, we have 15 particles so far. Recall that physics is about explaining what
we see, and not about creating something that we decide. We just observe. We don’t make
the rules. I say this, because for some strange and unknown reason the universe appears to
contain what we call 3 generations of particles. What we have discussed so far was only the
first generation of matter particles, but there are 2 more generations. Each generation
contains the same number of particles; thus, we get another 15 from the second generation
and 15 more from the third generation. In total. we have 45 particles. The only difference between
the generations is that the particles of generation 2 are heavier than generation 1 and the particles
of generation 3 are even heavier than generation 2. There is no other difference that we can
tell. To this day scientists have no clear understanding
of why there are 3 generations, except the theory just works this way, and the three generations
match our observations. It turns out that the left-handed quarks can
not only transform between particles within the same generation, but also across generations
to a lower mass particle. So for example, the charm quark which is the heavier 2nd generation
version of the up quark, can decay into a first generation down quark.
In terms of leptons, there is nothing in the theory that prevents them from transforming
between particles of different generations, but so far, this has never been observed. This
is however an area of current research, and there are signs that this kind of transformation
could exist. But as of right now, evidence shows that only quarks transform across generations.
Let’s move now to the non-matter particles – the bosons. We discussed in some detail
the strong force, mediated by gluons, which binds the quarks together into composite particles,
such as neutrons and protons.
We talked a little bit about electric charge – the fact that quarks have a charge of
either 2/3 or -1/3. The electron and its heavier generations all have a charge of -1 and all
neutrinos are neutral with a charge of 0. The force responsible for this charge is the
electromagnetic force, and it interacts with all matter particles except the three neutrinos.
So, we can draw a connection between all of these particles. Just like gluons mediate
the strong force, photons mediate the electromagnetic force. In other words, the exchange of photons
is what confers an electromagnetic force between two electrically charged objects.
We also talked a bit about the weak force. As I said earlier, only left-handed quarks
can transform into one another via the weak force. And only left-handed electrons and
neutrinos can transform into each other via the weak force.
And these interactions that change one particle into another are made possible by the charged
W+ and W- bosons. But these are not the only particles that
confer the weak force, a third boson involved in weak interactions is the Z boson. However,
it does not have any electrical charge. The exchange of a Z boson acts to only transfer
momentum, spin and energy between particles, while leaving any other properties of interacting
particles unaffected. Since there’s no transfer of electrical
charge, this is referred to as a "weak neutral current" interaction. Note that the word current
just means that a Z particle is exchanged. There is no flow of current like in electricity.
Also, note that unlike the charged weak force, the neutral weak and electromagnetic forces can
interact between both left and right-handed particles. This is because the neutral weak
force and the electromagnetic force appear to come from a single force that existed near
the beginning of the universe called the electroweak force.
I have a video on this if you want to know more details.
At this point we’ve discussed all particles, except one critical one. The grand poohbah
of fundamental particles, that was predicted more than 50 years ago, but only confirmed
in 2012 at the Large Hadron Collider in CERN, Switzerland.
That particle is of course the Higgs boson. This is the particle that gives mass to all
massive fundamental particles. And since all matter particles are massive,
the Higgs particles connects to all of them. But there is one exception - the neutrino.
The Higgs does not interact with them, according to the standard model. So how neutrinos acquire
mass remains a mystery. The Higgs particle doesn’t care about right
and left-handedness and treats both equally. The only bosons that do not interact with
Higgs are the gluons of the strong force and the photons of the electromagnetic force.
These two particles thus have zero mass. The amount of mass conferred to these fundamental particles
depends on how much they interact with the Higgs field, the greater the interaction, the
higher the mass. So for example, the top quark interacts more
with the Higgs field than the charm quark, thus gains a higher mass. Likewise, the charm
quark interacts more with the field than the up quark, and so it has a higher mass than the up quark.
For simplicity, you can think of this interaction with the Higgs field as slowing the particles
down. The more the interaction, the slower the particles become, and the more mass is
conferred. Massless particles like photons do not get slowed
down, and so travel at the maximum speed allowed by nature, the speed of light.
So the final count is the following: 45 matter particles, 8 gluons, 3 weak bosons, 1 photon
and 1 Higgs particles. In total 58 particles with different handedness, charges, colors,
and interactions. This is a full picture of the standard model of particle physics.
But you should know that this is not the end of the story. First, while we have 58 particles,
we also have an equivalent amount of anti-particles. Anti-particles are generally the same as regular
particles, except they have opposite properties, such as charge. So for example, the anti-electron
called a positron has a positive charge, whereas the electron has a negative charge. Handedness
also changes, so for example anti-particles of left-handed particles are right handed.
This means that while only left handed neutrinos exist, only right handed anti-neutrinos exist.
So the charged weak force only works with right handed antiparticles just like it does with left
handed regular particles . To account for antimatter
we would have to add 45 antimatter particles, increasing the total count to 103.
Second, we also know the standard model is incomplete.
This model only tells us what the visible universe is made of, but there is more to
the universe than just that. So for example, dark matter and dark energy
are not accounted for in this theory. And other mysteries like why Neutrinos have
mass, how gravity enters the quantum picture, and why there is a matter-antimatter asymmetry in the universe
are still unanswered. These are what keeps the standard model from being a theory of
everything. So although this is the best, most accurate
model of the universe we’ve got. there is more to be added to this story. And If you
are an aspiring physicists, there are a lot of things waiting for you to discover. If
you want a deeper more mathematical look at the standard model using Feynman diagrams,
check out my video on that above, and in the description. I will see you in the next video
my friend.