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Let’s say you are driving a car going 100 km/hr. It should be easy to see that your car has energy,
the energy of movement. This is called kinetic energy. In classical physics,
this is a simple formula ½ mv^2 or one half times mass times velocity squared.
And since you are sitting in that car also moving at 100 km/hr, you have energy too. If you weigh
80 kg, you can use the equation to find that your energy is equal to about 3.1*10^4 Joules.
But did you know that the energy you have standing still on the ground,
not moving at all, is more than a hundred trillion times that, over 6*10^18 joules.
How is this possible? Yes, I know what you are already thinking. I’m going to
invoke E=mc^2. Yeah, yeah, mass is the same as energy. So the energy is in my
mass. You’ve heard this a thousand times. You may have also heard that most of that
mass it tied up in the form of energy within the nucleus of atoms. But have you ever thought about
what that energy actually is? How and why is it in the nucleus. Is it due to movement? It is due
to forces acting on something? What exactly is the nature of this energy and why is it there? How is
it that mass can be the same as this energy. If it’s all energy, then is mass even a real thing?
I am going to ask how and why questions, over and over again, like a child,
until we get a final answer. And this will take us on a scientific and philosophical journey into the mind of the universe.
Let’s go on this journey together, coming up right now…
An 80 kg person might weigh the
same as this rock. Nothing’s moving, so there’s no kinetic energy. And there’s no heat emanating from
the rock that we can see. But they both have about the same amount of energy. So what’s happening?
Where is this energy? Well, it’s in the mass. Where is this mass located? And where
is the energy in this mass exactly? Let’s answer the first question first,
then we’ll answer the second question before we go even deeper.
What we call mass is made up of all the atoms in the rock. Almost all the mass of an atom,
typically more than 99.99%, is located in its tiny center, in its nucleus, which makes up a
tiny fraction of its overall volume. The rest is in the electrons that surround the atom,
making up the rest of the volume. There are two sources of mass. The
known mass of fundamental particles, that is elementary particles which
is what everything in the universe is made up of, comes from their interaction with the Higgs Field.
You can think of this field as like an energy grid that is everywhere in the universe. The different
elementary particles take different amounts of energy from this grid. The more energy they
take the more their mass. So for example, a down quark which has a mass of 4.7 MeV or Mega Electron Volts interacts more
intensely and takes more energy from the Higgs Field, than an electron which has a mass of about
0.5 MeV. Similarly, all elementary particles that have mass interact with the Field to some degree.
Why do some particles interact more with the Higgs field more than others causing them to
have more mass? We don’t know the answer to that. It just seems to be a fundamental
property of different particle types. But not all particles interact with
the Higgs field. Photons and gluons which have no mass, do not interact with it. Neutrinos are a bit
of mystery. They don’t seem to interact with the Higgs field, but experiments seem to indicate that
they must have a very slight mass. We don’t know why yet. That’s another story for another day.
The second source of mass lies in the nucleus of atoms. It is not contained in the mass of
the elementary particles that make up the components of protons and neutrons,
but in the energy keeping them together, and keeping them from falling apart.
What do I mean by this? Let’s take the simplest nucleus, that of the hydrogen atom. This atom
consists of just one proton, which is the nucleus, surrounded by one electron. The
mass of this nucleus is about 938 MeV. I can also state this as 1 atomic mass unit, or in grams,
it would be about 1.67*10^-24 g. But to keep things consistent and less confusing,
I will state everything in terms of MeV. A proton is made up of 2 up quarks and one
down quark. An up quark as a mass of about 2.2 MeV, and a down quark has a mass of about 4.7
MeV. So if we add up all the masses of each of three component parts, we get a total
of only 9.1 MeV. This is only around 1% of the measured mass of the proton,
938 MeV. So where is the other 99% of the mass? We just demonstrated that the Higgs Field
is responsible for only about 1%. The remaining is still in the nucleus somewhere.
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Now, back to the mystery of the hidden 99% of mass.
Now we are getting to the meat of our video.
The 99% is due to the force that keeps the quarks glued together inside the proton.
What the heck am I talking about? There is fundamental force of nature called
the strong force that keeps these quarks bound together to form the proton nucleus.
This force is mediated by gluons. We know that gluons are massless,
so how are they responsible for so much mass? Well, gluons bind quarks together. And both
have a kind of charge called the color charge. This charge is how the strong force operates.
This charge is how the strong force operates. It’s somewhat analogous to an electrical charge
where a negatively charged electron for example surrounding a positively charged proton makes
the atom neutral. With quarks however, we deal with red, blue and green color charges
that combine to form a neutral color. I want to point out that these are not
literally optical colors, but we use the term “color charge” because this charge
metaphorically speaking, works similarly to the way red, blue and green optical colors
can combine to form a neutral or white color. The way the color charge works is that each
quark’s color is constantly changing. They exchange colors between each other.
You’ll notice that while the colors are always changing inside a proton or neutron, they change
in such a way that the three combinations are consistently red, green, and blue to form neutral.
This exchange of colors is mediated by gluons, that is the gluons carry a combination of color
and anti-color charges such that they can change the colors of quarks. And it is this interaction,
that is the constant exchange of colors between quarks, that keeps them bound together.
Now you might ask, why does color exchange keep them bound. Well, that’s a good question. This
has to do with an idea called confinement. Essentially what this means is that a color
charged particle cannot exist on its own. Quarks and Gluons are color charged particles.
All experiments have confirmed that free quarks or free gluons do not exist,
they are always found bound to other quarks and gluons. Why? Because color charges exist
in a way such that the combination of colors is always neutral. This need for the combination of
charges to exist as neutral, attracts red, blue and green color charged particles to each other.
Color charged particles can also exist as color/anti-color pairs. So for example,
a blue charged quark can exist with an anti-blue charged quark to also form a
neutral color. This is similar to the way that in electromagnetism,
a positively charge particle is always attracted to a negatively charged particle.
If you are curious like a child, you might ask, why do colors have to combine to form neutral?
Well, this just seems to be the way color charges work, just like positive and negative electrical
charges work. You could further ask, well, what is a darn color charge anyway? Well,
it’s a quantum number that helps us explain the behavior of some quantum particles. We can
explain it using mathematics, specifically gauge theory, but fundamentally, what it really is,
as in, what is it about the particle that causes this strongly attractive behavior, we don’t know.
Just like we don’t really know what a negative or positive electrical charge
fundamentally is. It is just a property of some particles that helps us explain their behavior.
If you understand what I’ve said so far, then congratulations you understand the essentials
of quantum chromodynamics. It’s the science of the way the strong force works to keep the quarks
inside the nuclei of atoms tightly bound together. And it’s this force that is responsible for 99%
of the mass of an atom. Now the question is, how is this force creating mass? Well,
to understand this, you have to realize that mass is really just bound energy. In other words,
the rest mass of an object is its intrinsic energy-content. It’s a type of potential energy.
Any form of energy contributes to the mass of a body, potential or kinetic. The nature of
this energy doesn’t matter. So for example, a compressed spring is a little bit heavier
than the same spring without the potential energy stored in its compression. A hot pan
is heavier than the same pan when it’s cold. A fully charged battery on your
phone is slightly heavier than a dead battery. You won’t notice this mass difference because
at our scales, these energies contribute very little to the mass, but they are there. This
might not even be measurable because, you have to remember that mass = E/c^2 – so any increase
in energy is divided by a very large number. You can imagine that a
few joules of extra energy are not going to make a huge contribution to the mass.
The strong force that binds quarks together is a form of potential energy. It is called the
“Strong” force because it is more powerful than all the other fundamental forces,
electromagnetism, the weak force, and gravity. So when you put three quarks together to create
a proton or neutron, you end up binding up an enormous energy density in a small region in space.
Any kind of energy bends spacetime, in other words, it creates gravity. And so bounded energy,
which we have defined as the mass of an object, creates gravity. It is the
energy inside the object, that is its intrinsic energy content, that really defines its mass.
Interestingly, E=mc^2 never appeared in Einstein’s famous 1905 paper. What appeared was m=E/c^2,
that is mass is a measure of the energy content of a body. That’s the important take-away.
Ultimately 99% of the mass of objects comes from the energy of the gluons
confining quarks inside the nuclei of atoms. They are massless particles. But when they
are locked up in the confines of protons or neutrons, their energy translates to mass.
In general, any energy confined in one spot becomes a kind of rest energy of the object,
which is what mass is. This is why a hot object is heavier than the same object when it’s cold.
One philosophical way to think of this is that the entire mass of the universe is nature’s way
to confine much of the vast energy of the universe, in such a way as to make useful
particles that can create things, and ultimately create conscious living beings like us. The
energy needed to keep color charged particles from existing freely by themselves is ultimately what
most the mass of the universe really is. And this strong force has resulted in life as we know it.
One thing I didn’t cover, which is related to the strong force is the force that keeps Protons and
neutrons glued together inside the nucleus of larger atoms. This is usually referred
to as strong NUCLEAR force, and is distinct from the Strong force which keeps quarks glued together.
This is also a form of potential energy, and also makes up part of the mass of an
atom. So let me cover that briefly here. As I described so far, the strong force
mediated by the exchange of gluons holds charged quarks together inside a nucleon,
that is a proton or a neutron. But if a quark is pulled out of a nucleon far enough,
the potential energy created by that stretch is enough to create a new quark-antiquark pair.
When this happens, the stretched quark drops back into the nucleon, but the quark/anti-quark
pair forms a short-lived new particle called a meson. This meson has a slightly longer range
than a gluon — typically about the diameter of a nucleus — so it can be exchanged between
nucleons as a force-transmitter. This force, transmitted by mesons in the nucleus, is the
“strong nuclear force.” And it holds protons together along with neutrons,
against their electromagnetic repulsion. This force or energy keeping the protons from flying
apart is also part of the mass of atoms. It is considered a residual effect of the strong force.
In fact, it is this mass, that is the mass due to the strong nuclear force
holding nucleons together, that is released as energy in a nuclear bomb. The mass due the
strong force holding quarks together is not the energy released in the bomb.
So what you are seeing, when you see pictures of a nuclear bomb is due to the strong nuclear force.
If you found this video interesting, be sure to check out my detailed videos on
quantum chromodynamics. I’ll see you in the next video my friend.