Hi again physics enthusiasts!
How are you guys?
I’m doing great and I’m super enthusiastic about today’s topic, although I should warn
you, this one is a mind-bender.
In our previous episodes, I spoke about pieces of the Standard Model, focusing on the various
particles and forces, but I barely mentioned a peculiar behavior of one of the subatomic
particles – the neutrinos.
Neutrinos are unique among the particles of the Standard Model in that they can actually
change their identity.
This is a peculiar behavior is called neutrino oscillation and it’s what I want to talk
about in this episode of Subatomic Stories.
In episode four, I introduced the neutrinos and talked a little bit about how neutrino
oscillations were discovered.
If you haven’t seen that one, you probably should watch it first.
Basically, there were two observations that needed explaining.
First, the Sun emits a bath of exclusively electron neutrinos and fewer electron neutrinos
hit the Earth than expected.
Second, protons from outer space slam into the Earth’s atmosphere and make muon and
electron neutrinos in a ratio two to one.
Experiments observed a ratio of one to one.
Definitive measurements made between 1998 and 2001 proved that these observations occurred
because neutrinos were changing their identity as they flew along.
For example, a beam of electron neutrinos would morph into a mixture of electron and
muon neutrinos and then back again into electron neutrinos.
The pattern then starts over again and again.
And, of course, tau neutrinos also get into the game.
If you start with a beam of pure electron neutrinos and then try to predict what fraction
of the beam will be electron neutrinos at different distances, the result is a very
complicated pattern.
You can see what is predicted in this plot here.
There is a big and slow oscillation, superimposed on a smaller and much faster oscillation.
So how does that work?
Well, there are two things going on.
The first is that if neutrinos can change their identity, it means that neutrinos have
mass.
They are not the massless particles that are embodied in the simplest form of the Standard
Model.
That’s an interesting thing to be sure, but it’s not so shocking.
Many subatomic particles have mass.
No, the tricky thing has to do with how the masses of neutrinos and identity of neutrinos
interplay.
It’s very counter-intuitive.To begin with, let’s think about the charged leptons – the
electron, the muon, and the tau lepton.
Each of them has a unique identity – what scientists call flavor, although it has nothing
to do with how they taste – and each of them has a specific mass.
The electron has a mass of 0.511 MeV, the muon has a mass of 106 MeV, and the tau has
a mass of 1,777 MeV.
A specific particle and a specific mass.
Pretty standard stuff.
The situation is different for neutrinos.
There are still three different flavor of neutrinos – the electron neutrino, the muon
neutrino, and the tau neutrino.
By the way, we write neutrinos with the Greek letter nu and a subscript to indicate their
flavors, so that’s nu sub e, nu sub mu, and nu sub tau.
They also have three masses.
We don’t know the exact mass of the neutrinos, but they have three masses, m1, m2, and m3.
We say that the three different mass neutrinos are nu sub one, nu sub two, and nu sub three.
We know that neutrinos one and two are similar in mass and neutrino three is quite different.
You’d think that the electron neutrino might be the same as neutrino one, or two, or three,
but that’s not how it works.
Each of the electron, muon, or tau type neutrinos is a mix of the three different numbered neutrinos.
Yes, that means that each of the uniquely flavored neutrinos doesn’t have a unique
mass.
The converse is also true.
If each flavor doesn’t have a unique mass, then each mass doesn’t have a unique flavor.
For instance, nu one is mostly electron neutrinos, but neutrino two is roughly an equal amount
of electron, muon, and tau neutrinos.
Nu 3 is about a fifty-fifty mix of muon and tau neutrinos.
So how can this be?
Basically, it’s a quantum mechanics thing and it’s not so different from Schrodinger’s
cat.
Remember that Schrodinger’s cat is in a box with a radioactive atom, a detector, a
vial of poison gas, and a hammer that drops and smashes the vial when the atom decays.
If you don’t open the box, you can’t know if the atom decayed or not, so the atom is
both decayed and undecayed and therefore the cat is both alive and dead until you open
the box.
There is a lot that’s obsolete about the Schrodinger’s cat example, but that’s
how it’s usually taught, and how most people are familiar with it.
With neutrinos, it’s the same.
For example, an electron, muon, or tau, neutrino could simultaneously have a mass of m one,
m two, and m three and and it’s only when you measure the neutrino’s mass that it
is actually determined.
Before the measurement, the electron neutrino simultaneously has all of the masses.
It’s just Schrodinger’s cat being alive and dead at the same time until you open the
box.
I said that we don’t know what the masses of the three neutrinos are, and that’s true.
Our measurements only know the differences between the three types.
For instance, the difference between the numbers zero and one are the same as between ten and
eleven.
We know the differences, but not the actual numbers.
Another weirdness is that we know that neutrinos one and two are similar and three is different,
but we don’t know if one and two are small and three is big, or if one and two are big
and three is small.
Working out which of these is true is something we’re still trying to figure out.
There are several experiments working on this, but I’m kind of partial to the ones done
by my colleagues at Fermilab who work on either the NOVA or DUNE experiments.
There are a couple of important takeaway messages from this episode.
First, neutrinos can change their identity over time.
Second, neutrinos of a specific type don’t have a specific mass and neutrinos of a specific
mass don’t have a specific type.
It’s all gloriously complicated and understanding this is why Fermilab is putting such a huge
effort into neutrino studies over the next decade or two.
OK, so that is a quick introduction into some of the most mind-boggling bits of neutrino
physics.
I’m sure that it will lead to some fascinating questions.
And, speaking of questions, let’s see what sort of questions were sent in from the last
episode.
Questions, so many questions.
I wish I didn’t have to pick, but pick I did, and here we go.
HL65536 asks if other particles can have different masses, like electrons with double or quadruple
their normal mass.
That’s a great question, with a lot of complication and nuance in the answer.
Let me tell you what.
Let me make a video about the Heisenberg Uncertainty Principle first and I’ll answer you in the
questions for that video.
Deal?
Al Cash likes my videos and asks what he should do when his neighbors hear his head exploding.
Duh…tell them to subscribe to the channel, of course.
Why you should have all the fun?
Don’t worry – you subscribed before them – you’ll still be my favorite.
Astro photography enthusiast asks if the quarks that make up us have been around since the
beginning.
Hi Astro.
Great question.
The answer is a bit complex.
Shortly after the Big Bang, the universe was full of energy in many forms.
As the universe expanded and cooled, quarks and antimatter quarks were created from the
energy.
For reasons that we still don’t understand, very early in the history of the universe
there was a super tiny imbalance between the number of quarks and antimatter quarks.
For every three billion antimatter quarks, there were three billion and one matter quarks.
The three billions annihilated and the one matter quark went on to form us.
And, of course, I’m talking ratios here, not literally just three billion particles.
The universe continued to expand and by about a millionth of a second after the Big Bang,
it had cooled enough for quarks to have clumped into protons and neutrons.
Those went on to become the hydrogen and helium of the early universe.
So that’s a big part of the answer.
Those quarks have been around since the beginning.
The dance of virtual particles inside protons complicates the situation, but that’s a
higher level of abstraction and a much longer answer.
Tarandeep Singh asks if the virtual particles experience the electromagnetic forces.
Hi Tarandeep.
Sure.
Virtual particles experience all of the known forces.
That’s how virtual particles emit even more virtual particles.
That’s one reason why the inner structure of protons is such a complex and ever-changing
mess of quarks, antiquarks, and gluons.
Everything gets involved.
Wotzinator basically asks why quantum mechanics and relativity don’t work together.
Hi Wotzinator.
Actually, your question isn’t quite right.
Quantum mechanics and Einstein’s theory of special relativity work together just fine
and have since Paul Dirac unified them back in 1928.
His efforts led to the prediction and eventual discovery of antimatter.
I talked about this in episode 7.
What >>IS<< true is that quantum mechanics and Einstein’s theory of general relativity
haven’t been unified.
The reason for that is a bit complicated.
When quantum mechanics and special relativity were combined to form quantum field theory,
the result seemed flawed because it predicted a couple of infinities.
Predictions of infinities by a theory is usually fatal to the theory.
However, physicists came up with a clever trick called renormalization, which allowed
the infinities to be combined together and replaced with parameters that could be measured
in data.
For quantum electrodynamics, the infinities could be hidden and replaced by two measured
parameters – the mass and the charge of an electron.
Admittedly, renormalization seems kind of dodgy, but it works.
Now when quantum mechanics and gravity are combined, what arises is not just a couple
of infinities, but rather an infinite number of infinities.
And an infinity of infinities breaks the renormalization idea.
Researchers would need to make an infinite number of measurements to replace the calculated
infinities with physical quantities.
And, of course, that’s not useful.
That’s the reason that general relativity and quantum mechanics can’t be combined.
Angelo Marcio asks if muons and tau leptons can replace electrons in atoms.
Hi Angelo.
The answer is yes, at least partly.
Researchers have replaced electrons with muons.
Atoms surrounded by muons are smaller than atoms surrounded by electrons.
If you want to learn more about it, google the term “Muon catalyzed fusion.”
Atoms surrounded by tau leptons haven’t actually been made, but it’s possible, at
least in principle.
Klaus Ole Kristiansen asks why physicists are so fond of TLAs?
IDK Nate Watson asks why we think that dark matter
is so simple given that ordinary matter is so complicated.
Hi Nate.
The answer is simple.
A model with a single type of antimatter is just easier to work with.
But theorists have thought about a more complicate kind of dark matter with many kinds of dark
particles.
If you want to learn more about it, I coauthored an article about the idea in the July 2015
issue of Scientific American with my colleague Bogdan Dobrescu.
And, if you’re interested in a deeper dive into the idea, check out Lisa Randall’s
book “Dark Matter and the Dinosaurs.”
The title premise is a bit silly, but the physics in the book is excellent.
Lisa is a leading theoretical physicist.
She also writes very well for the public.
In general, I recommend all of her science popularization books.
XtReMz 98 asks if my latte coffee would taste better with a little quantum foam.
Hi extremes.
Actually, I’m more of a cappuccino guy, so yes.
Definitely.
Foam is better.
Of course, it can get you in trouble.
I once ordered a cappuccino in Italy after 11 AM, which I soon learned was a serious
social faux pas.
Luckily, I wasn’t banned from the country, but apparently word got around and now the
Italian border guards giggle a little every time they check my passport.
Who knew?
OK, so that’s all the time we have for questions today.
If you are enjoying the series, please like, subscribe, and share with your neighbors…especially
you, Al Cash.
Physics videos are fun and should be shared with everyone, because, of course, even at
home, physics is everything.