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.