Hi there, physics fans.
Welcome to another visit to my den, where
I want to tell you about another fascinating
building block of matter.
I’m talking about the leptons.
The word lepton comes from the Greek word
leptos, which means small or light.
While all subatomic particles are far removed
from people’s day to day experience, at
least one of the leptons affects us every
day.
Let’s learn more about them in today’s
episode of Subatomic stories.
The leptons are generally much lighter than
the quarks.
There are two classes of leptons, the top
row, which experience two of the subatomic
forces, and the bottom row, which experiences
only one.
Since they’re different in this way, I’m
going to make two videos about them.
This video will talk about the top row, which
have electric charge and are governed by the
force of electromagnetism.
There are three charged leptons, all with
the same charge as the electron.
These leptons are called the electron, the
muon, and the tau lepton.
Basically, you can think of them as cousin
particles of increasing mass.
The muon is about 207 times heavier than the
electron, while the tau lepton is about 3,500
times heavier than the electron.
The muon and tau lepton are unstable.
The muon decays in about two microseconds,
and the tau lepton decays in 0.3 trillionths
of a second, so you need good equipment to
measure them.
The electron is the most familiar subatomic
particle.
It is responsible for atoms emitting light
and all of chemistry.
Our technological world depends on it.
To understand its behavior requires you know
something about quantum mechanics.
Two of my longer videos cover some of those
topics and I put links to them in the description.
The muon is ephemeral, but it is super important.
The muon acts like a spinning electric charge
and, when you put a spinning charge in a magnetic
field, it will precess like a top.
Scientists have calculated the rate at which
the muon will precess to twelve digits of
accuracy and they’ve measured about as well
as that.
The calculation and measurement disagree enough
to be exciting.
If a more precise measurement supports this
discrepancy, it will be a huge discovery.
And guess where that measurement will occur?
At my own Fermilab, with the g-2 experiment.
I made a long video about the g-2 experiment
and I put the link in the description.
If you’re interested in what could well
be the biggest discovery of the decade, take
a look.
It’s hard to know when the g-2 researchers
will make their announcement, but it is likely
to be in under a year – maybe sooner.
I certainly hope sooner.
So that’s a quick introduction to the charged
leptons.
There’s more to learn, of course.
There’s always more to learn.
But it’s time to move on.
In our next episode, I want to tell you about
the bottom row – the neutrinos.
They have some very mind-blowing properties.
See you soon.
So this is the part of the episode where I
answer your questions.
So, let’s get started.
Myutubechannel likes my book “History of
the Vikings” and asks “Wait!
Were the Vikings particle physicists too?”
Only the very smart and attractive ones.
Lief Erickson asks “are you sure about that?”
Yes.
Jerry Miller notes that the force is strong
in the quarks.
Of course, the force is strong in quarks.
Why do you think lightsabers are red, blue,
and green, huh?
On second thought, never mind.
This isn’t the answer you’re looking for.
Oheng75 says your jokes are so quarky and
funny.
Thank you, thank you.
I’m here all week.
Be sure to tip the wait staff.
Okay, on a more serious note, Mozib Boss asks
“Do quarks decay?”
That’s a very, very good question.
Quarks only decay via one force, the weak
force.
For instance, the top quark can emit a particle
called the W boson.
The W boson is a particle of the weak force.
When it emits a W boson, it can transform
itself into a bottom quark.
Similarly, a botton quark can transform into
a charm quark.
A charm can transform into a strange.
The only quark that doesn’t decay is the
up quark and we’ll learn a little more about
that when we have the episode on the weak
force.
Debabrata Kalita asks about the quantization
of charge.
How can quarks have a fractional charge?
To say a charge is quantized simply means
that it comes in discrete lumps.
Now, in the old days, we thought that the
electron was the smallest charge, so that
it had a charge of minus one and that made
perfectly good sense.
However, when quarks were discovered, we realized
that there were charges smaller than that;
for instance, the down quark has a charge
of minus one-third.
So you could have just as easily changed your
definition of charge and said the down quark
had a charge of minus one.
In that case, the electron would have a charge
of minus three.
So, it’s still not a problem.
The charge is quantized.
It’s just is quantized in smaller allotments
than you would think if you use the electron
as your base charge.
Good question.
RIKI 101 asks “Do quarks have mass?”
That’s a great question.
I didn’t talk about the mass of the quarks
in my quark video, but there’s actually
a huge range of masses of quarks.
In units where the mass of the proton is a
thousand, the mass of the up and down quarks
are more in the range of two to three to four,
whereas the top quark has a mass of a hundred
and seventy two thousand, which is a huge
amount of mass.
Why do the various quarks have different masses?
We actually don’t know the answer to that.
The mass of the quarks is given by the Higgs
boson and we’ll talk about that in an upcoming
episode.
Tutul asks what kind of matter can be built
with quarks?
They know that the proton and neutron is three
quarks, but what about two quarks and four
quarks?
What about other ones?
In my previous video, I talked about how quarks
can come grouped in threes.
And, when they come grouped in threes, that’s
a class of particles called the baryon.
The most familiar baryons are the proton and
neutron.
However, there are other types of particles
called mesons, which are a quark and antimatter
quark pair.
So, those are the two most common ways in
which quarks can be combined.
The three quarks in the baryon and the matter/antimatter
pair of quarks in the mesons.
But they’re not the only ways that quarks
can combine.
For instance, even in the original paper,
things like two quarks and two antimatter
quarks could combine.
Another thing is what we call a pentaquark,
which is actually four quarks and an antimatter
quark.
Those have been observed.
Other predictions are things called hexaquarks,
which are six quarks all together.
Those haven’t been seen.
So, there are many ways in which quarks have
been predicted to combine and make known particles,
but the big ones, the most common ones, are
the baryons and the mesons.
The other types – so many of them have been
seen and we’re looking for others.
Siddharth Mukkanaar asks “are the protons
and neutrons only made of quarks, or is there
anything else?”
Well, that’s actually very interesting.
In the simplest model, we say the proton and
neutron are made of three quarks.
However, that’s not the entire truth.
Actually, if you really look at a proton carefully,
you see that it does have those three quarks,
but there’s also a bath of particles called
gluons, which govern the strong force.
And, even more confusingly, the energy in
the gluons can make, for a very short amount
of time, matter and antimatter quarks which
are appearing and disappearing.
So if you really look at a proton in high,
careful technical detail, you see the three
known quarks.
You see gluons.
You see matter and antimatter quarks appearing
and disappearing.
It’s a total complicated mess and people
spend their careers studying the details of
how the energy and matter and charge and spin
is all combined inside things like the proton
and the neutron and other particles that we’ve
seen.
Dolphin Whale “can an electron and quark
annihilate and I don’t know if this is a
dumb question?”
That’s not a dumb question at all, but it
turns out that the answer is no.
The only things that can annihilate when you
have matter and antimatter particles is when
they are the same.
So a quark and an antimatter quark can annihilate.
An electron and an antimatter electron can
annihilate, but a quark cannot annihilate
an antimatter electron, nor can an electron
annihilate and antimatter quark.
Just can’t happen.
Okay, so that’s all the time we have for
questions today.
I hope you liked the episode.
If you do, please like it.
Subscribe to the channel and share us on social
media.
In our next episode, we will talk about neutrinos,
which is one of the most fascinating subatomic
particles of all – one of the most interesting
topics in all of physics and, I’m excited
to learn more about it and hope you are too;
because, even at home, physics is everything.
