This video is sponsored by Brilliant.
Please sign up for free and support this channel at the link in the description.
There are thought to be just 4 fundamental forces, or interactions in nature.
Gravity which keeps you on the surface earth, the weak force which is responsible for some
kinds of radiation, electromagnetism which is responsible for all chemistry, and the
strong force which keeps protons and neutrons tightly bound together in the nucleus of atoms.
You are probably most familiar with gravity which at the scale of the earth, the solar
system and beyond dominates the universe.
But at the quantum scale, the other three forces dominate.
For example, electromagnetism is a whopping 10^37 times stronger than gravity.
It’s responsible for keeping the negatively charged electrons near the positively charged
protons in atoms.
So now you have to ask, since electromagnetism is so strong, then multiple protons in the
nucleus of any atom like Helium and beyond, should repel each other very strongly.
According to Coulomb’s law, the two protons in a Helium atom experience a repulsive force
equal to about 90 Newtons or 20 lbs.
For something as small as a Helium atom, this is a huge force.
They should repel each other and fly apart instantly.
The reason that these protons don’t fly apart is because they are kept glued together
by a force that is even stronger than electromagnetism – about a hundred times stronger, called
the Strong nuclear force.
It is the strongest force in the universe at the scale of the nucleus.
This force is not simply the opposite of electromagnetism, but works through an entirely different mechanism,
which unlike chemistry is typically not taught at the pre-college level.
So most people are not familiar with it.
I want to give you a glimpse of the fascinating world of quantum chromodynamics, and show
you exactly what goes on inside and in between protons and neutrons in the nucleus of atoms
that keeps them so strongly glued together against the enormous repulsive force of electromagnetism.
That’s coming up right now…
Even though we don’t directly experience the strong force, you can get a glimpse of
its power by watching any footage of a nuclear bomb explosion,
because the enormous energy that you see comes from the binding energy that keeps protons
and neutrons within the nucleus of atoms.
This binding energy is due to the strong force.
And in fact this binding energy is responsible for about 99% of the mass of ordinary objects
that you see all around you.
Contrary to what some people believe, only about 1% of the mass of objects come from
the Higgs Field. Almost all of it comes from the Strong Force.
So how does this force work?
We have to start with what protons and neutrons are made of.
They are made of more fundamental matter particles called quarks and force particles called gluons.
A proton and neutron each contain 3 quarks.
A proton is made of two Up quarks and one down quark.
While a neutron is made of two down quarks and one up quark.
Gluons within these nucleons are virtual particles so the number fluctuates.
We can’t say exactly how many are inside, but there are 8 total different types of Gluons
which keep quarks together.
Both the quarks and gluons carry what is called a color charge.
It is analogous to an electric charge but works via a completely different mechanism.
And although it is called a color charge, it has nothing to do with optical colors that
you can see with your eyes.
It is just a metaphor to describe a different kind of charge that these fundamental particles,
associated with the strong force, have.
It is just a property that some particles have.
And quantum chromodynamics provides a very good description of how the color charge works.
An important thing to remember is that the color charge carried by gluons is not the
same as electrical charge.
And here’s how color charge works.
There are three kinds of color charges, Red, green and blue.
The way to think of this is that red, green, and blue combine to make a neutral white,
analogous to the way it works for optical colors.
So just like electrical charges are conserved or balanced – positive with negative to
make neutral, color charges also have to be balanced by either a combination of the three
colors to make a neutral white, or by color-anticolor charges.
That’s right, there are anti-colors too,
like red and anti red, green and anti green, and blue-antiblue, which also combine to become
neutral.
So any combination of color charges has to be either red, green and blue together, or
a color and anticolor together such that we get a neutral color charge.
A combination of 3 quarks, in which 2 of them carry the same color cannot exist.
Both quarks and gluons carry these color charges.
And that’s why gluons interact with quarks, and bind them together.
The only fundamental particles that are affected by the strong force are quarks.
And the particle that mediates the strong force is the gluon.
Gluons bind quarks together by constantly exchanging color charge between them.
Gluons mediate the strong force, analogous to the way photons mediate electromagnetic
force between particles with electrical charge.
Very simply stated, it is this constant exchange of gluons carrying the color charge that causes
the quarks to stay bound to each other.
This exchange occurs continuously.
The animation you are seeing here is slowed down, to show what we believe is really happening.
But you should know that this is happening so fast and continuously that there is no
distinct gluons that could be detected.
The constant exchange of gluons creates a kind of continuous bridge between quarks called
a flux tube.
There is an important distinction between the way the strong force works with color
charges vs. the way electromagnetism works with electrical charges.
Electromagnetism, like gravity gets weaker as objects get farther apart.
The force between two quarks, however, actually gets stronger as they get further apart.
It works like a rubber band or a spring.
If you try to pull two quarks apart, the force between them gets stronger and stronger the
further apart they are.
This tends to pull a quarks back into the proton or neutron as they get further apart.
However, if the quarks get too far apart, then the rubber band breaks.
An interesting thing happens at this point, the quark is not released but the energy required
to break the rubber band creates a new quark and anti-quark pair.
This quark-antiquark pair is called a meson.
What I just described is called confinement in quantum chromodynamics because you cannot
have free quarks.
They are always confined together with at least one other quark.
Going back to the newly created meson from the breaking of the rubber band, this is key to understanding
how protons are kept glued together with other protons and neutrons in the nucleus.
By the way, what I am calling rubber band is really the quark gluon fluxtube which connects
quarks to each other.
It turns out that these mesons act as the force carrying particle for the residual strong
force that keeps them bound.
I will explain how this works.
But first let me explain why Mesons are formed in the first place.
What causes the quark-gluon flux tube to break and create a Meson?
What I didn’t tell you earlier is that although the color charges within the collection of
the three quarks within protons and neutrons combine to form a neutral color, such that
the net color charge cancels out, quantum fluctuations ensure that this cancelation
does not happen perfectly.
This is especially true when nucleons are very close together.
Some color charge remains.
This imperfect balance of color charge causes a residual strong force between nucleons.
This residual force is called the strong nuclear force, rather than the
strong force. It is mediated not by gluons, but by the exchange of mesons between two nucleons.
And by the way, nucleons in this context are either protons or neutrons.
Now let’s look at the details of how exactly this interaction happens using Feynman diagrams.
Quantum theories can be complicated, but American physicist Richard Feynman came up with an
ingenious way to represent these interactions in what is now called Feynman diagrams.
And even though these diagrams represent complicated mathematical equations, they can be visualized
intuitively and drawn for any process using some simple rules.
One of the simplest diagrams shows how a pair of quarks simply change color through the
exchange of gluons.
As I said earlier, this color charge exchange is the glue that binds the quarks together,
and keeps them confined to the inside of nucleons.
A blue and red quark, for example, can change colors via the exchange of a gluon.
So on this diagram, we have a red quark coming from the top left, it emits a gluon with a
combination red and anti-blue color charge. By the way, gluons always have a combination of color-anticolor
charges.
The outgoing antiblue of the gluon, immediately turns the quark into a blue quark.
The blue quark at the bottom absorbs this gluon.
Its blue color gets neutralized by the antiblue contained in the gluon.
And this quark turns red by absorbing the red that was also part of the gluon.
Note that color is conserved in this interaction.
Let’s now consider a more complicated Feynman diagram which represents the residual strong
force interaction that binds a proton and neutron together in the nucleus.
This is a critical interaction because without it, there would be no atoms heavier than hydrogen
because 2 protons could not bind together in the nucleus.
And life as we know it would not exist.
Essentially, this diagram shows how a proton and neutron interact with each other via the
exchange of mesons.
Mesons are a combination of a quark, anti-quark pair.
We have a single proton up top and a single neutron at the bottom.
Time flows from left to right.
Keep in mind that color must always be conserved.
That is, the three quarks inside a proton or neutron must be a combination of red, blue,
and green which combine to make a neutral color charge.
Let me clarify again, that this is the same way that optical colors combine to make white, but
color charge like I said earlier is just a metaphor and has nothing to do with colors
that you can see with your eyes.
Inside the proton, the blue up quark interacts with the red down quark via a gluon.
This turns the up quark red, and the down quark blue.
As this down quark tries to leave the proton, the “rubber band-like” stretching of the
flux tube results in energy that creates a quark-antiquark meson.
These newly created mesons consisting of a blue down quark and anti-blue anti down quark
form a color and charge neutral pi meson.
Note that the quarks of the proton change colors, but remain within the Proton.
The pi meson now interacts with the neutron.
The anti-down quark of the meson annihilates the down quark of the neutron.
The down quark of the same meson joins the neutron by taking the place of the annihilated
down quark.
Note that exchanges of colors are taking place with the overall result that both the neutron
and proton above remain color-neutral, that is, their three quarks maintain a combination
of red, green, and blue colors.
These virtual meson interactions are what keep protons and neutrons bound together in
atoms.
It is a kind of residual force of the strong force. And it's called the Strong NUCLEAR force.
But note that this is distinct from the strong force which works within the nucleons keeping
the quarks tightly bound together.
And that is mediated by gluons.
But the strong nuclear force which keeps protons tied to other protons or neutrons is mediated
Not by gluons, but by mesons.
Remember that this force is caused by the not-quite-perfect neutrality of color charges
within nucleons.
That’s why it’s referred to as a residual force.
Although this force is much weaker than the strong force keeping quarks tied together
in nucleons and mesons, it is still much stronger than the electromagnetic repulsive force between
two positively charged protons.
So protons are still glued together even though they feel a strong electromagnetic
repulsion due to both of them having a positive electric charge.
But this strong nuclear force diminishes rapidly with distance.
It’s effective only at very short distances, on the order of one femtometer, which is about
the diameter of a proton.
When distances between nucleons are much larger, this residual strong force has almost no effect.
And this aspect of the strong nuclear force that limits the size of atoms because nuclei
beyond a certain size are not stable.
You can think of this like Velcro.
If you bring it close together, it grips tightly, but move them apart and it has no effect.
Now, I explained this in the simplest way I could think of.
Obviously, there are more complicated details involved in Quantum Chromodynamics.
But I hope this gives you some idea of how this very important phenomenon of nature works,
without which there would be no objects we could see, nor life as we know it.
And if you enjoy science like this but sometimes feel lost because perhaps you never learned
the fundamentals, then let me suggest a couple of courses that I think you’ll find very
useful and enjoyable.
They’re available on Brilliant.org, today’s sponsor.
One of them is a brand new course called Quantum Mechanics with Sabine.
It was developed as a collaboration with my friend and fellow Youtuber, Sabine Hossenfelder.
What I found really great about this course is that it walks you through the fundamentals
of quantum mechanics including superpositions and entanglement through a series of short
interactive quizzes.
This is so fun that you might not even realize you’re learning some pretty high level stuff.
But if that seems a bit advanced and you want to brush up on more fundamental concepts first,
than Brilliant's Scientific Thinking course would be a great place to start.
This course teaches you essential scientific principles, not through number crunching but
physical insights.
It’s a great way to learn physics without being overwhelmed with math.
Brilliant has a special offer for Arvin Ash viewers right now - Get started for free by
clicking the link in the description.
the first 200 people will even get 20% off their subscription.
This is a great opportunity to give Brilliant a try.
So be sure to click the link in the description below.
And You’ll be supporting this channel when you do that, so many thanks for that!
And if you enjoyed this video, then be please smash the like button and feel free to share
this video.
I will see you in the next video my friend.