The best theory in physics today tells us
that there are 4 known fundamental forces or interactions
of nature that control every action in the
universe you can think of - everything from
the sun shining, to your brain activity, to
water evaporating, to planets rotating - every
process, every chemical reaction, every movement,
every thought.
In a previous video, we looked at the function
and origin of the 4 fundamental forces and
showed how they are believed to have come
from a common origin.
But we were also left with several questions.
How exactly does a force between particles
work? What is the underlying mechanism? What
causes an attraction or repulsion? And, why
does electromagnetism and gravity have infinite
range, but the strong and weak force have
such a small range? The answer to these questions
and more is coming up right now…
Before we get into the 4 forces, I’d like
to thank Magellan TV, today’s sponsor for
making this episode possible. I just watched
a two episode series called the secrets of
quantum physics, hosted by one of the world’s
great science educators, Jim Al-Khalili.
The second episode is a fascinating look at
how quantum entanglement and the uncertainty
principle have impacted our evolution as well
as the evolution of other animals.
Magellan TV is a new type of streaming documentary
service that was founded by filmmakers and
producers who bring you premium quality documentary
content. I think you’re going to love it
Featured subject include history, nature and
of course science and space. You can watch
it on any of your devices at any time with
no interruptions, and in 4K.
I’m excited that Magellan TV has a special
offer for Arvin Ash viewers. If you use the
link in the description, you will get a free
one-month trial. I highly recommend Magellan
TV, but be sure to use the special link.
So far, there are thought to be just 4 fundamental
forces, or more precisely "interactions" in
nature. Gravity which keeps you on the surface
earth, and keeps the earth orbiting the sun.
Electromagnetism which is the essence of light
waves, and is also the force responsible for
chemistry. The strong force which binds
the protons and neutrons in the nucleus of
atoms together, and the weak force which is responsible for some kinds of radiation.
So how do these forces work?
First, all forces, with the possible exception
of gravity are mediated by force carrying
particles called gauge bosons. Gravity is
also theorized to be mediated by a particle.
It’s carrier particle would be the graviton.
All four forces operate by way of the same
basic mechanism: the exchange of virtual particles,
usually the gauge bosons of the standard model.
These gauge bosons are: the  photons that carry
the electromagnetic interaction, gluons which are responsible for the strong force, and
the W and Z bosons for the weak force. The graviton would also be a boson responsible for
gravity.
Real versions of these gauge bosons have been
observed with the exception of the graviton.
Since there is no quantum theory of gravity
yet, let’s first look at the other three
forces that do have a quantum theory to support
them.
First, what is a virtual particle? These are
particles that exist due to the Heisenberg
uncertainty principle.
One version of the uncertainty principle states
that the change in energy times the time of
a particle’s existence must be equal to
Planck’s constant over 4pi.
But the probability laws of quantum mechanics
are such that virtual particles, not real
particles, can exist, if their energy times
time is LESS than h/4pi. This essentially
means that nature allows fleeting particles
to exist.
And since energy and mass are equivalent,
nature creates particles with mass by borrowing
energy from the vacuum, but for a really short
time so that it gets destroyed before it can
be detected. They are really just excitations
of the underlying quantum fields, but appear
only as forces, not as detectable particles.
Now, you might say, come on, how do you know it’s there if you can’t detect it. Well we can’t
detect individual virtual particles, but we
can detect the effect of many particles being
created and destroyed. One such effect is
called the Casimir effect where a physical
force between two closely held plates is created when the pressure of virtual particles
outside the plates is higher than the pressure
between the plates.
So now, if you buy that, the next question
is how does an exchange of these virtual particles
result in a force? There are two ways to look at this.
Let’s use electromagnetism as the example.
When two magnets attract or repel each other,
it is due to an exchange of virtual photons.
Here is the simplest analogy.
Imagine two people on a boat throwing basketballs
at each other. As the two throw the balls
away from each other, there is a transfer
of momentum that occurs. This momentum transfer
moves the boats apart. The more this happens,
the further apart they go.
This is analogous to the way that photons
transfer momentum from one charged particle
to another, resulting in a repulsion of like
charges. The only difference is that the photons
are virtual photons.
Now, you might say, ok, I can understand this,
but how does this exchange of particles result
in an attractive force. Well, It happens through
the exchange of negative momentum. This can
only be approximated in our boat example,
but here is a simplified way to visualize
it.
Imagine the same two boaters, but this time
instead of holding a basketball in their hands,
they are holding boomerangs. And instead
of facing each other, they are facing away
from each other. When one person throws the
Boomerang, it circles back in such a way that
the other person catches it facing the opposite
direction.
This also results in an exchange of momentum,
but this time such that the boats pull closer
together. This is roughly analogous to the
way an exchange of negative momentum can occur
between two dissimilarly charged particles.
Now, let me explain it in a less intuitive way,
but one that is probably more accurate. We
will use the same example of electromagnetism
and exchange of virtual photons.
When two similar charges are near each other,
quantum theory shows that the exchange of
virtual particles creates an energy gradient
between the two particles such that there
is a higher energy state between the charged
particles. Since particles want to exist
at their lowest energy state, they move apart.
The opposite happens when dissimilar charges
are near each other. A lower energy state
is created between the two particles, so the
charges move closer together to be at a lower
energy.
Now the question is why does electromagnetism
have an infinite range? This is directly due
to the fact that photons have zero mass. If
we take the same uncertainty equation we saw
earlier and substitute MC squared gamma instead
of E, from Einstein’s famous equation E
equals MC squared gamma, where gamma is the following,
and since distance is C times time, if you
do the math, you find that distance has the
following value. This means that the higher the (rest) mass
of the particle, the shorter the distance that
it can travel. Range is limited by its mass.
Since photons have no rest mass, they can
theoretically travel and infinite distance
before being absorbed by another charged particle.
But this presents a conundrum when it comes
to the strong force, because it’s mediating
virtual particle, the Gluon is also massless,
but the strong force has a very limited range,
to only the width of a proton. So what gives?
Why does it also not have infinite range?
The answer involves quantum chromodynamics
or QCD. Here’s a simplified version.
First understand that protons and neutrons
are made of three quarks each. Gluons are
responsible for binding the three quarks together.
So it is the reason protons and neutrons exist.
The main difference between photons and gluons
is that photons are electrically neutral so
they do not interact with themselves. They
only interact with the charged particles when
mediating the electromagnetic force.
Gluons on the other hand, carry a charge so
that they end up interacting with each other
in addition to interacting with the quarks. This limits their range because they form a kind
of link or rubber band with not only the quarks,
but also with themselves.
Now here is the shocker. These gluons are
electrically neutral. They do not have a positive
or negative charge like protons and electrons.
They have a completely different kind of charge
called a color charge. Wow! What the heck is a color charge? It has nothing do with colors that
you and I can see with our eyes. It is just
a metaphor to describe a new kind of property
of certain particles.
Physicists kind of understand how color charges
work, but to be honest no one really knows
what a color charge actually is. In fact,
no one knows what a positive or negative electrical
charge is either. We know how it works, but
what it actually is, no one knows.
It is just a property that some particles
have. But the important thing to remember
is that the color charge carried by gluons
is not the same as electrical charge.
There are three kinds of color charges, Red,
green and blue. The way to think of this is
that red, green, and blue combine make a neutral
white. So just like electrical charges are
conserved or balanced – positive with negative,
color charges also have to be balanced by
either a combination of the three colors to
make a neutral white, or by color-anticolor
pairs. Like red and anti red, or green and
anti green which combined, also become neutral.
These color charges are also a property of
quarks that make up protons and neutrons.
And that’s why gluons interact with them,
and bind them together. Only particles with
color charge are affected by the strong force.
There is an important distinction between
the way the strong force works with color
charges vs. the way electromagnetism works.
Electromagnetism and gravity get weaker as
objects get further apart. The force between
two quarks, however, actually gets stronger
as they get further apart.
It works like a rubber band. If you try to
pull two quarks apart the force between them
gets very strong. When they are close together,
the force is weak. This tends to pull the quarks
back into the proton or neutron. However,
if you knock a quark very far away from the
neutron or proton, the energy becomes so large
that the rubber band breaks.
And the energy released creates a new quark/ani-quark
pair. And one quark is pulled back into the
proton. The newly created quark-anti quark
pair is called a meson. These are another
type of particle. And these are the residual particles that actually mediate the
velcro-like force that keeps protons and Neutrons
tied together in the nucleus.
The gluons are involved only indirectly in
Keeping neutrons and protons together
because they are involved in the process of creating mesons. Now since
mesons do have mass, if you do the math, the
range of the force keeping neutrons and protons
glued together is very short, about the diameter of a proton.
Now if you're with me so far, you have just earned a diploma
In introductory QCD.
That’s the strong force. Now, how does the
weak force work? It is a stretch to call this
a force. It is more like a power that certain
particles have to turn themselves into different
particles. An example of this is when a neutron
turns into a proton, by emitting an electron
and antineutrino.
What is the mechanism? A neutron is composed
of two down quarks and one up quark. A proton
is composed of two up quarks and one down
quark. Any neutron can turn into a proton
by converting one of its down quarks to an
up quark.
It does this by emitting a negatively charged virtual W boson. This turns the down quark into an
up quark, changing the particle from a neutron
to a proton. Since the W minus boson carries
a negative charge, the Neutron which had no
charge, now becomes a proton with a positive
plus 1 charge to balance charges.
Charge is always conserved in the universe.
The W boson then almost immediately decays
into an electron and antineutrino. So what is detected in the decay of Neutrons to protons
is an electron and an antineutrino. The antineutrino
acts to balance out the momentum in this exchange.
The reason this interaction has a very short
range is because the W boson is quite massive.
Consequently, the W boson just doesn’t survive
for very long. If you do the math using our
uncertainty equation, the range turns out
to be only about 10^-18 meters, which is about
1/1000th the diameter of a proton.
This kind of decay is rather rare in quantum
mechanical terms. A lone Neutron takes almost
15 minutes to decay into a proton. This is
an eternity in the tiny world of quantum mechanics.
The interesting thing about the weak interaction
is that its strength a range of 1/1000th the
diameter of a proton is almost the same as
that of electromagnetism. And it was this
insight that led to the unification of electromagnetism
with the weak force – the electroweak theory.
I will have dedicated video on this theory
in a future video.
I will have dedicated video on this theory
in a future video.
How does all this apply to gravity? Well,
since it has infinite range, its theoretical
carrier particle the Graviton should also
have no mass.
The reason we don’t have a quantum gravity
theory is because at quantum scales, It is
completely overwhelmed by the other three
forces. It would be like figuring out the weight
of a dust particle on your skin while weighing
yourself on a scale. So for now gravity is
treated almost purely in geometrical terms
using general relativity.
And I have a dedicated video on this, if you want more details.
But like I said in the previous video, at
some fundamental level, all 4 forces should
all be one and the same. Figuring out this
unification is the holy grail of physics,
and would lead us to a theory of everything.
Will be ever discover it? I think we absolutely
will. It is just a matter of time.
Stephen Hawking said the theory of everything
would be the ultimate triumph of human reason,
for then we would truly know the mind of God.
I happen to think that the theory of everything
would not be the end of our pursuit, but in
many ways its beginning. How the theory results
in the complexity of the universe, life on
earth, the organization of our brain, and
a myriad of other mysteries would still need
to be figured out. But at least with a theory
of everything instead of poking a stick at
a dark world like a blind person, we would
see the universe in all its glorious colors.
And then at least we would know the ground rules of the universe.
I’d like to thank my generous supporters
on Patreon and Youtube. If you like my videos,
consider joining them. I will see you in the
next video my friend.
