### Video Transcript

In this lesson, we’re talking about
the fundamental forces. There are four such forces, and
we’re going to learn what causes each one as well as how the forces compare to one
another. These four include gravity and
electromagnetism. And as we get started, perhaps the
most important as well as surprising thing we can say about them is that fundamental
forces are caused by particle exchange. This means, for example, that if we
have two objects, and say both of these objects have some amount of mass, then we
know there will be a gravitational force on each object due to the other. We’ve often thought of this force
as being due to one of these particles existing in a gravitational field created by
the other.

If we get down to a very small
scale though, subatomic distances, we find that what actually allows, say, the force
of gravity to act between these two objects is the exchange of a certain kind of
particle. So if we’re thinking of this object
being attracted gravitationally by this one, we can say that that will only happen
once this mediating particle is sent from the one to the other. We say that it’s that exchange that
allows a gravitational force to exist between our two objects. Whatever the properties of this
particle then, we hold that it is responsible for the gravitational force between
massive objects.

Now, gravity, we can recall, is
just one of the four fundamental forces. The other three, in no particular
order, are the electromagnetic force, the weak nuclear force, and the strong nuclear
force. Just like gravity, each of these
fundamental forces are also caused by particle exchange. And interestingly, each force has
its own specific particle that mediates that force’s interaction. This may raise the question though,
just what sorts of particles are these that cause these fundamental forces?

Consider the kinds of subatomic
particles we’re already familiar with. One such group of particles is the
six kinds of quarks. These, along with their
antiparticles, make up all hadrons. Another separate class of
particles, of which there are also six, is the leptons. These include electrons, muons,
tauons, and their corresponding neutrinos.

As it turns out, these various
force carrier or force transmitter particles, we could call them, are neither quarks
nor leptons. Rather, they belong to a third
class called bosons. There are many different types of
bosons, most of which have nothing to do with carrying forces from one particle to
another. We’re going to focus though on the
four that do. These include the photon,
symbolized with the Greek letter lowercase 𝛾; a particle called a gluon,
represented by a lowercase g; a pair of particles called W bosons, represented,
respectively, by a W with a positive and a negative superscript; and lastly a Z
boson, represented by a Z with a zero superscript, representing that it is neutrally
electrically charged.

Now, since there are four
fundamental forces and we have four bosons here, we might think that one goes to
each force. But interestingly, here’s how it
works out instead. Both the W and the Z boson are
known to be particles responsible for the weak nuclear force. This means that it’s by the
exchange of W and Z bosons that the weak nuclear force can act between
particles.

The next boson up on our list is
the gluon. And this is known to be the force
carrier for the strong nuclear force. Recall that it’s the strong force
that helps atomic nuclei, protons and neutrons, stay tightly packed together.

And then another sort of boson is
the photon. Because we associate this particle
with light, we might expect it to be the force carrier for electromagnetism, and
that’s correct. It’s by the exchange of photons,
which we now know are bosons, that charged particles are able to influence one
another through this force.

Conspicuously absent from this list
is a boson that mediates the force of gravity. Although a certain particle has
been hypothesized as the gravitational force carrier, none has yet been
experimentally discovered. This points to the fact that our
physical model of how the universe works is not yet complete.

Now that we know the names of the
bosons that serve as force carriers for three of the four fundamental forces, let’s
compare these four forces one to another in terms of their relative strength as well
as the distances over which they act. If we were to arrange these forces
from strongest to weakest, then the strongest force of all is the strong nuclear
force. Next in strength comes the
electromagnetic force, then the weak nuclear force, and then way down at the bottom
of our list is gravity. Gravity is actually much weaker
than any of the other three.

A couple other interesting
attributes to consider are the range over which a force acts, that is, the distance
over which it acts, as well as whether that force attracts, repels, or both. Starting out with our strongest
fundamental force, we know that the strong nuclear force is responsible for keeping
atomic nuclei together. Even though this force is very
strong, it only acts over a very short range, about equal to the diameter of an
average-sized nucleus. Over these very small distances
between particles, the strong nuclear force is able to overcome electrostatic
repulsion between, say, adjacent protons in a nucleus.

When it comes to whether this force
attracts or repels or both, the strong nuclear force is only attractive. If we then consider the
electromagnetic force, we know that the mathematical equation for this force is
expressed by Coulomb’s law. This law tells us that the
electromagnetic force between two charged particles is proportional to one over the
distance between their centers squared. In other words, no matter how big
𝑟 gets, up to the point that it’s infinitely large, there will still be some
nonzero electromagnetic force between charged particles. We say then that the range of this
force is unlimited; it’s infinite. And then, as far as attracting or
repelling, we know the electromagnetic force can lead to both. That’s because two like charges
repel one another, while two unlike charges attract.

Moving on to the weak nuclear
force, it’s this fundamental force that’s responsible for nuclear decay
processes. And like the strong force, it only
acts over very short ranges. In fact, 10 to the negative 18th
meters is smaller than the diameter of a single proton. This force, it turns out, is the
shortest-range force of them all. And when it comes to whether it
attracts or repels or both, because the weak nuclear force is responsible for
nuclear decay, there isn’t a clear intuitive picture of which direction or
directions in which it might act. For this force then, we won’t
specify whether it’s attractive, repulsive, or both.

Lastly, we come to gravity, which
as we said is by far the weakest of these four forces. This may seem strange when we
consider that as we make astronomical observations, gravity seems to be the dominant
force. But we can recall that both the
strong and the weak nuclear forces are very range limited and also that very large
macroscopic objects, such as moons or planets or stars, are typically electrically
neutral. And so for very large masses over
very large distances, gravity is the most influential force.

Just like with electromagnetism,
the force of gravity is proportional to one over 𝑟 squared, where in this case 𝑟
is the distance between the centers of mass of two masses. So gravity, too, has an infinite
range and will only become zero when two masses are literally infinitely far
apart. And in terms of attracting or
repelling or both, we know that the gravitational force is always attractive.

Now that we know about how the four
fundamental forces compare with one another, as well as the specific particles that
carry these forces, which we learned are called bosons, let’s get some practice with
these ideas through an example.

Which of the following symbols
represents a gluon? Uppercase G, lowercase 𝛾,
uppercase 𝛤, lowercase g, lowercase y.

Okay, to answer this question, we
can be helped by remembering that a gluon is part of a class of particles called
bosons. Specifically, it’s one of the four
bosons that are known as force carriers. These are bosons that particles
exchange so that forces can be experienced between the particles. The force carrier bosons are the
photon, the gluon, the W boson, and the Z boson.

We want to know which of these five
symbols is typically used to represent the gluon. A photon, we know, is represented
by the symbol 𝛾. And so we can cross off option (B)
on our list. The W boson consists of a particle
and antiparticle pair represented with a capital W and a plus and a minus sign
superscript. And a Z boson is represented by a
capital Z with a zero superscript. None of these last three symbols
appear on our list. So we’re not able to cancel out any
more options.

To answer this question, we’ll
simply need to recall that a gluon is represented by a lowercase g. We do see that symbol on our list
as option (D). And so our answer is that a
lowercase g is the symbol that represents a gluon.

Let’s look now at a second example
exercise.

Which of the following symbols
represents a photon? Lowercase 𝜐, 𝜈, lowercase y,
uppercase Y, or uppercase 𝛶, lowercase 𝛾.

Alright, a photon, we can recall,
is a massless particle that serves at least two functions. First, it’s the name we give to the
smallest possible packet of light energy. So when electromagnetic radiation
or light is transmitted from one location to another, we say that that happens via
photons. But then a second function of
photons is to serve as what’s called a force carrier. If we have two electrically charged
objects, say this one and this one, then the way that there’s an electromagnetic
force from one object to the other is via or by the exchange of photons.

In this case, the photon is not
transmitting light but rather transmitting force, electromagnetic force. Anyway, the symbol we use to
represent this particle is the lowercase Greek letter 𝛾. Among our answer options, we see
that as choice (E), and so that will be our answer. It’s lowercase 𝛾 that represents a
photon.

Let’s look now at one last example
exercise.

List the four fundamental forces
from greatest relative strength to lowest relative strength.

Okay, to begin putting these forces
in order of strength, let’s start by recalling what they are. In no particular order, the four
fundamental forces, that is, the forces behind all other forces we observe, are
gravity, the electromagnetic force, the weak nuclear force, and the strong nuclear
force. We want to put these forces in
order from the strongest, we’ll say that’s number one, to the relatively weakest,
number four.

One of the nice things about the
names that have been given to these fundamental forces is that the strongest force
actually has that word in its name. The strong nuclear force, which is
responsible for holding the nuclei of atoms intact, acts over only very short
ranges, about the diameter of an average-sized nucleus, but over those distances is
more powerful than any other force.

The next strongest fundamental
force is the one that the strong nuclear force needs to overcome to keep the protons
in a nucleus together. These positively charged particles
naturally want to repel one another via the electromagnetic force.

Now, if we’re unsure of which force
goes here in the third slot, whether gravity or the weak nuclear force, it can help
us to remember that the force of gravity is actually by far the weakest of these
four forces. This can seem strange because, to
our eyes, gravity may be the most apparent of these forces. But nonetheless, from a strength
perspective, it is the weakest, by a lot. This means that our third spot will
be occupied by the weak nuclear force, the force responsible for nuclear decay
processes. And so from strongest to weakest,
the four fundamental forces are the strong nuclear force, the electromagnetic force,
the weak nuclear force, and gravity.

Let’s summarize now what we’ve
learned about the fundamental forces. In this lesson, we learned that the
fundamental forces are mediated or carried by particles called bosons. These bosons are the photon, which
mediates the electromagnetic force; the gluon, which mediates the strong nuclear
force; and the W and the Z bosons, which mediate or carry the weak nuclear
force.

We noted that the boson thought to
mediate the force of gravity is hypothetical and hasn’t yet been discovered. Along with this, we learned that of
the four fundamental forces, the strong nuclear force is the strongest. Then comes the electromagnetic
force, then the weak nuclear force, and finally gravity. We learned the ranges over which
these forces act and for three of the forces whether they attract, repel, or
both.