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.
