Hi guys.
How are you holding up?
Here at Chez Lincoln, things are just fine,
although it’ll be great to get back to the
laboratory.
In previous episodes, we’ve talked about
the quarks and leptons, which are the smallest
known building blocks of matter.
However, building blocks alone can’t describe
the rules of the universe.
We also need to know the forces, which is
what I’d like to tell you about in this
episode of Subatomic Stories.
We all know what the Force is, right?
It’s like that time that that Jedi did that
thing, right?
Obi-Wan even said that it surrounds us and
penetrates us; it binds the galaxy together.
Ok, that may be a bit Star Wars heavy…but
come on…some jokes are just begging to be
told.
I guess it’s strictly true that gravity
does actually hold the galaxy together, so
maybe Obi-Wan was kind of right.
But physicists have to be a bit more practical.
In physics, forces make other things move
or at least change them in some way or another.
There are four known fundamental forces.
Fundamental, by the way, means that they don’t
seem to arise from some deeper cause.
The four fundamental forces are gravity, electromagnetism,
and the strong and weak nuclear forces.
I’m going to hold off talking about the
specific forces until the next video.
In >>THIS<< video, I want to talk about how
forces work at the subatomic level.
Basically, each of the forces is caused by
a matter particle emitting a force particle
that is then absorbed by another matter particle.
Taking the simple case of electromagnetism,
for which the force carrying particle is a
photon, two electrons experience the electromagnetic
force when an electron shoots a photon at
another electron, which absorbs it.
This really isn’t all that hard to believe.
That’s one way forces work in the familiar
world.
Imagine two people standing in boats and one
of the boats has a heavy sack in it.
If the person in the boat with a sack in it
throws the sack, the boat will move in the
opposite direction.
Then, if a person in the other boat catches
the sack, that boat will move too.
This is kind of how the electric force will
repel two objects with the same charge.
People often ask how this analogy explains
attractive forces.
The version I use is one involving the same
people in the same boats, but throwing a boomerang
back and forth.
One person throws the boomerang away from
the other boat, but it circles around and
the other person catches it.
The result is that the boats move together.
Of course, this is just an analogy and it
isn’t perfect.
My advice is to accept it if it helps you
and if it doesn’t, I have a somewhat more
accurate explanation.
Richard Feynman was one of the bright physics
minds of the 20th century.
He was a leader in explaining how electromagnetism
works at the quantum level.
The theory that he helped develop is called
quantum electrodynamics, or QED.
In fact, he wrote a book for the public on
the subject called simply “QED.”
If you want to learn more, I recommend it.
It’s very good.
His explanation for how forces work is very
much tied into quantum mechanics.
Basically what he said is that when a photon
travels from one charge to another, it can
take literally any path, from the direct one,
to one that is slightly indirect, to one that
is truly bizarre…all paths must be considered.
Then you use some complicated math and add
them all up.
If you have two charges of the same sign,
the effect of adding up all of the paths results
in a concentration of energy between them.
Objects like to move to regions of lower energy,
so the charges move away from one another.
It’s like adding up the path of all the
particles makes a hill between them, and the
charges roll down the hill.
If you do the same exercise with two charges
of opposite sign, what happens is that the
energy between the two charges goes down.
Instead of a metaphorical hill between them,
it’s a valley and the result is that the
two charges again roll downhill, but this
time they move together.
That analogy is a little more accurate than
the boat one, but to understand it in detail
means you need to learn some very complicated
math.
It takes about two years for a physics graduate
student to learn, and that’s after they
already have at least one physics degree.
And, in the end, you’ll end up returning
to this analogy, just with a better understanding
of the math.
So that’s how forces work at the subatomic
level.
Very tricky.
In our next video, I’ll tell you about the
known fundamental forces, and how they’re
similar and how they’re different.
It’ll be great fun.
I’ll see you soon.
We’ve come to the part of the episode where
I answer viewer’s questions.
I’m sorry for those I couldn’t get to.
You have a better chance of being selected
if your question is concise.
I tend to skip the longer or meandering ones.
So, let’s get started.
Deepak Soni asks how the nucleus can exist,
in spite of the electric repulsive force between
protons.
So that question has a long history.
A similar observation was the first indication
that there might be forces beyond electromagnetism
and gravity.
For instance, two protons touching each other
in the nucleus of an atom feel a repulsive
force of about ninety newtons or twenty pounds
or the weight of ten kilograms.
Take your pick.
The only way they will remain stationary is
if there is an even stronger force holding
them together.
Historically, this was called the nuclear
force, but it’s now called the strong force.
It’s a hundred times stronger than electromagnetism
and it’s why atomic nuclei are stable.
I’ll talk more about it in the next video.
Chris Clarke asks what determines the longevity
of a decaying particle.
In T.H.
White’s 1958 book “The Once and Future
King,” he described the principle of totalitarianism
as “Everything which is not forbidden is
compulsory.”
That’s pretty much how particle decay works.
Decay is inevitable.
However not all decays are possible.
Some decays are forbidden because they don’t
conserve energy or spin or charge.
Knowing which decays are forbidden and which
aren’t would take a long time to explain,
however I can tell you that decays caused
by the various subatomic forces take different
amounts of time to proceed.
Decays via the strong force take about ten
to the minus twenty three to ten to the minus
twenty seconds.
Decays via electromagnetism take ten to the
minus twenty to ten to the minus sixteen seconds.
Finally, decays via the weak force take more
than ten to the minus thirteen seconds.
So, if a particle can decay via the strong
force, it never has a chance to decay via
the weak force, because the strong force works
first.
Vazgen Ghazaryan asked a long question, which
I’ve edited for brevity.
He wanted to know if I think that the patterns
of quarks and leptons exist because they are
made of even smaller particles called preons?
Well Vazgen, let me tell you.
There are two answers to that.
The fact that there are identical copies of
the up and down quark and electron and neutrino
just bugs the heck out of me.
I’m sure it’s telling me something important.
Personally…and I’m just talking for me
here…I think that it is likely that the
quarks and leptons are made of smaller particles.
I even wrote an article in the November 2012
issue of Scientific American if you want to
learn more about the idea.
But what I think isn’t important.
It’s what the data says that matters, and
the simple fact is that there is exactly zero
experimental evidence that quarks and leptons
have constituents.
Zero.
And I have personally looked at the most relevant
data known to mankind.
And the data is completely consistent with
quarks and leptons having no particles within
them.
So, the bottom line is that while something
inside me says that quarks and leptons are
made of smaller objects, it’s not something
that I truly believe.
I need confirming data to believe.
But I’ll keep looking.
Maybe I’ll get lucky.
By the way, I made a long video on this idea
as well.
I put the URL in the description below.
Kevstar19 asks how we can identify which type
of neutrino interacts in our detector.
That actually turns out to be easy.
Since neutrinos have an affinity for their
associated lepton, what happens is that a
neutrino hits a proton or neutron and breaks
it apart.
If the neutrino interacts by emitting a W
boson, then it turns into its associated lepton.
So, if you see a proton or neutron shatter
and a muon appears, it was a muon neutrino.
If you see the same thing, but an electron
appears, it was an electron neutrino.
Edart Nelis notes that they are a chemist,
but that they are now interested in learning
more fundamental physics.
Well >>of course<< you are interested in physics,
because…well, never mind…it would be rude
to point out the obvious and I don’t want
to get ahead of myself.
And, besides, we’ve run out of time.
If you’re enjoying this video series, please
like, subscribe, and share on social media
– as widely as possible if you can.
These topics continue to fascinate me after
all these years, because I still want to know
all about how the universe works.
That’s why I study what I do, and because…and
this one is for you Edart…even at home,
physics is everything.
