Hi again physics fans.
In our last episode, I gave you a feeling
for how forces work in the microcosm.
Matter particles exchange force carrying particles,
like two people in boats tossing a sack back
and forth.
That’s true in general, but each of the
several known forces has its own unique features
and to understand the world of the ultra-tiny,
we need to know about all of them.
And that’s a perfect setup for what I’m
going to talk about in today’s episode of
Subatomic Stories.
Physicists know of four fundamental forces.
They are electromagnetism, gravity, and the
strong and weak nuclear forces.
As it happens, we don’t understand how gravity
works at the subatomic level, so I’m going
to ignore it for the moment.
So that leaves us with three.
Actually, in recent decades, we’ve learned
that the weak force and electromagnetism are
really the same thing and we should combine
them into the electroweak force.
And then there’s the Higgs field, which
we’ll talk about in another episode.
By some definitions, the Higgs field is another
force.
So, we see that even sometimes something as
simple as counting the number of forces is
a tricky business.
It depends on the energy at which you are
doing experiments.
For most researchers, the Higgs field is hidden
and the connections between the weak force
and electromagnetism aren’t obvious.
So, it makes sense to act as if the subatomic
world is governed by the three forces – the
strong and weak nuclear force and electromagnetism.
I talked in the previous episode about how
forces work at the subatomic level – how
force carrying particles jump back and forth
between matter particles, and how if you invoke
quantum mechanics and add up all possible
force carrying particles, following all possible
paths, you get simple attractive and repulsive
forces.
It’s all quite fascinating.
But the specific forces all have their little
idiosyncrasies.
Let’s take each one of them in turn.
Electromagnetism is the most familiar of the
fundamental subatomic forces.
The force is felt between two objects which
have electric charge.
The force can be attractive or repulsive.
The force carrying particle is called the
photon.
The photon has no electric charge and no mass.
It travels at the speed of light, which makes
sense, since a photon is a particle of light.
It also has infinite range, which is why you
can see galaxies billions of lightyears away.
I made a bunch of long videos on quantum electromagnetism,
and I put links to them in the description.
The strong nuclear force is quite a bit different.
It is exchanged between two objects that carry
the strong charge; what physicists call color.
I talked about color in episode two of subatomic
stories and I also made a couple of long videos
about it, again… they’re listed in the
description.
The particle that transmits the strong force
is called the gluon, because it glues the
proton and the neutron together.
It also acts like glue, meaning that the force
is strong when objects are in contact, but
the force is zero when they aren’t.
The gluons have no mass, but the gluons also
carry the strong charge, which means that
gluons can also interact with other gluons.
It’s this interaction between gluons that
makes the range of the strong force so short.
The weak force is maybe the most intriguing
force.
For one thing, it doesn’t have one force
carrying particle, it has three: a positively
charged one, a negatively charged one, and
a neutral one.
The charged ones are called the W plus and
W minus bosons and the neutral one is called
the Z boson.
All three of them are super heavy, which means
that the force has super short range – about
one one thousandth the size of a proton.
The lightest of the W and Z bosons has the
mass of over eighty protons.
Furthermore, the weak force is the only one
that can change a particle’s identity, for
instance a top quark emits a W boson and turns
into a bottom quark.
And, I’ve made long videos about the weak
force too.
The three fundamental subatomic forces have
different strengths and those strengths depend
on distance.
For instance, on the size scale of a meter,
the strong force and weak force have zero
strength.
However, if we pick a reasonable subatomic
size – say the size of a proton – we find
that the strong force is the strongest.
Electromagnetism is much weaker – about
one percent as strong as the strong force.
The weak force is weaker still, about 0.001%
as strong as the strong force.
There is a ton more to learn about the subatomic
forces.
I could talk about them for hours.
But, if I did, I wouldn’t be able to talk
about a bunch of other topics, which I’m
sure you’re wanting to hear about.
In the video description, I give lots of links
to other videos you can find on the Fermilab
YouTube channel, as well as a handful of some
of my favorite books.
You should look them over.
There’s some really great stuff there.
So, so far in this series, we’ve talked
about the quarks and leptons and the subatomic
forces, and we’re just getting started.
In my next episode, I want to talk to you
about a really mind-blowing subject – an
antagonistic substance that, when combined
with matter, will explode – releasing a
crazy amount of energy.
I’m talking, of course, about antimatter.
See you soon.
Question time is my favorite time, but it
also makes me sad.
There are so many good questions and I can’t
get to all of them.
I have to figure out a solution forthat.
But, in the meantime, let’s at least answer
a few.
Leo Kastenberg asks how the strong force can
cause decay and he thought that the strong
force held atoms together.
Hi Leo.
Actually, it’s electromagnetism that holds
atoms together.
The strong force holds together both atomic
nuclei and individual protons and neutrons.
The way the strong force can cause subatomic
particles to decay is when quarks emit gluons
that then split into quark/antiquark pairs
and the particles rearrange into configurations
with lower mass, which means lower energy.
For instance, a neutral rho meson is a heavy-ish
particle that consists of an up quark and
anti-up antiquark.
You might think that the two would annihilate
into a gluon or photon, but that’s not what
happens.
What happens is that either the quark or antiquark
emit a gluon that splits into a down quark/antiquark
pair.
The down quark pairs with the up antiquark
and becomes a pi minus meson and the down
antiquark pairs with the up quark and makes
a pi plus meson.
The two mesons are lighter than the rho meson,
and they fly off.
Eventually the pi mesons decay via the weak
force.
Here’s another interesting fact.
The neutral pi meson also contains an up quark
andantiquark, but it can’t decay in the
same way the rho meson does.
That’s because the two charged pi mesons
weigh more than the neutral one, and that
would violate conservation of energy.
So, what happens here is an electromagnetic
decay, where the quark and antimatter quark
annihilate into two photons.
It has to be two photons in order to conserve
momentum.
One final interesting point is that the rho
meson decays in about five times ten to the
minus twenty four seconds.
That’s because gluon travels at the speed
of light and the two quarks are only about
a femtometer apart.
Divide that distance by the speed of light
and you get about the right number.
In contrast, the neutral pion takes about
ten to the minus sixteenth seconds to decay.
That’s just because quarks emit photons
less often than they do gluons.
Anthony Hargis points out why some Jedi’s
failed is that they were using the weak force.
I can’t decide whether Anthony gets kudos
for a high-grade dad joke, or if he’s a
Sith Lord and I shouldn’t cross him.
I think probably the second.
After all, I can’t help noticing that I’ve
never seen Anthony and Emperor Palpatine at
the same time.
Draw your own conclusions.
Future H asks what is the best introduction
for non-students to the mathematic of particle
physics.
That’s a hard one to answer.
You need to know algebra and calculus to even
get started.
But, when you have those, I advise Introduction
to High Energy Physics by Donald Perkins.
It’s a textbook, so it takes some work to
read, but it’s an excellent first introduction
to formal particle physics.
All of his textbooks are very good.
Daniel Nogueira Leitao asks two related questions.
Why is the universe electrically neutral and
why are there the same number of protons and
electrons in the universe?
Answering the first question is easier.
Probably the universe was created neutral
so when charges came into existence, the universe
should remain neutral due to charge conservation.
Why protons and electrons have equal charge?
Well, that’s much harder.
Nobody knows.
If we figure that out, we’ll figure out
something deep and fundamental about the rules
of the universe.
It could have been true that electrons had
half the charge of protons.
In that case charge neutrality would have
meant twice as many electrons as protons.
So, in summary, there is only one truthful
answer.
I dunno.
Surya Raju asks how we define the boundary
of a proton.
It’s actually similar to how we define the
boundary of the Earth’s atmosphere.
As we go up in altitude, we pick an arbitrary
air density and say that the atmosphere stops
there.
The same thing is true with protons.
The quarks inside protons can wander around,
but at a certain radius it becomes less and
less likely.
The point at which the probability is half
the maximum is considered the edge.
That’s at about 0.8 femtometers.
Bob Belas asks why electrons are so stable
and if it is related to conservation of charge.
Hi Bob.
So, yeah, basically.
Conservation of charge and conservation of
energy is what matters.
For any particle to decay, it has to decay
into an object of lower mass.
And, as you suggest, it has to conserve charge.
Since there is no electrically charged particle
with a lower mass than an electron, it can’t
decay.
So, you were right.
Okay- so those were great questions.
I wish we had time for more.
But keep them coming and maybe I’ll pick
yours next time.
If you enjoyed the video and the series, be
sure to like, subscribe, and share with all
your friends.
Why should we all have the fun?
Maybe your friends also like physics, and
how can they not?
Because, even at home, physics is everything.
