If you’re interested in learning about particle
physics, one of the key topics you need to
understand are the forces that come into play.
For instance, we know of four fundamental
forces, gravity, electromagnetism, the strong
nuclear force and the weak nuclear force.
Of these four, we have at least a little familiarity
with gravity and electromagnetism. So naturally,
I want to tell you something about the strong
nuclear force.
The first question you might want to know
the answer to is “How do we know that there’s
a strong nuclear force at all?” Well this
is something we’ve known for essentially
a century.
In 1911, Ernest Rutherford discovered the
basic structure of the atom, which is that
it consists of a small and dense core of positive
electric charge called the nucleus, surrounded
by a cloud of negatively charged electrons.
And in 1917, he discovered the proton, which
meant that the nucleus consisted not of a
big blob of electric charge, but a whole bunch
of smaller blobs held together. Remember that
each proton can be imagined as a tiny ball
with a positive electrical charge.
They say that opposites attract and that might
well work for couples, but it’s definitely
true for electrically charged particles. On
the other hand, like charges repel each other.
So let’s imagine a simple nucleus consisting
of just two protons. Since each proton has
a positive electrical charge, they push each
other apart.
And they don’t push each other apart a little
bit. The repulsion force is about 20 pounds.
Twenty pounds is a respectable force in the
world of you and me. In the particle world,
it’s huge! It’s so big, that if you let
the two protons go, they’d push until they
were moving away from one another at a speed
of over 8,000 miles per second.
And I just picked the simple case of two protons.
In the case of uranium, there are 92 protons
all packed together in a ridiculously tiny
volume and a proton on the surface is repelled
with a force of just over 70 pounds.
So if these protons are all getting pushed
apart from one another, what keeps them together?
There’s only one possibility. There has
to be a force that is even stronger that counteracts
the electromagnetic repulsion. And the name
for that force is? You got it- the strong
force.
What do we know about the strong force? Well,
we know that it’s strong. And it can’t
just be a little stronger than the electromagnetic
force. That’s because if the two forces
were pretty similar, it would be easy to knock
protons out of the nucleus of atoms. And we
know that’s not true because nuclei last
for millions and even billions of years. So
the strong force has to be way stronger.
From tests, we know that the strong force
is about 100 times stronger than the electromagnetic
force. But the story is a bit trickier than
that. After all, if the force is so high,
what keeps the strong force from continuing
to gather protons until the whole universe
consists of one big nucleus?
If we look at the periodic table, we see that
the highest stable element has no more than
100 protons in it. And yes, I know that nuclei
also contain neutrons. That doesn’t change
much in what we’re talking about here.
So it seems that with more than about 100
protons, the strong force can’t hold it
all together. What’s that all about?
It turns out that there is a property of the
strong force that is quite different than
the electromagnetic force.
Electromagnetism has an infinite range. In
principle, two electrically charged particles
will feel a force between them if they are
separated by the entire universe. Okay, the
force would be small- but in principle, it’s
true.
In contrast, the strong force is more like
a contact force. When two protons touch each
other, they feel the strong force. Pull them
apart a little bit and the strong force goes
to zero.
In a way, the strong force is a little like
Velcro. When two pieces of Velcro aren’t
touching, the two pieces feel no force towards
one another.
So that’s the strong force. It is stronger
than the electromagnetic force, but it has
a much shorter range. And those two properties
explain why low mass nuclei are stable and
heavy ones aren’t. In the heavy ones, all
the protons in the nucleus push against all
the other protons, while the strong force
only feels the force of its neighbors.
And this is all pretty cool. What I’ve told
you here is actually nuclear physics, not
particle physics. I’ll make another video
that brings the strong force into the particle
world.
The strong force is a fascinating subject
and it is the strongest known force in the
universe. Well… the second strongest force.
