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
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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.
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.