How massive is a neutrino?

Daniel and Jorge talk about the mysterious mass of the neutrino and how we can measure it.

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, why do particle physicists obsessed so much about mass?

Speaker 2 (00:12):
Well, mass is one of the basic properties of a particle.
It's like part of its identity.

Speaker 1 (00:17):
Whoa is that? Healthy?

Speaker 3 (00:19):
Though?

Speaker 1 (00:20):
You think your mass should define who you are?

Speaker 2 (00:23):
I don't think we have to worry too much about
like particle mental health.

Speaker 1 (00:26):
Yeah, but shouldn't they be defined by their magnetism or
how colorful they are?

Speaker 2 (00:31):
Well, we're all made of particles, so I guess we
can just decide for ourselves how to identify with them.

Speaker 1 (00:35):
You are your particles, right, my particles are me? No,
I'm pretty sure it's the other way around.

Speaker 2 (00:41):
It depends if you believe in strong or weak emergence.

Speaker 1 (00:43):
That is a massive detail, right there. I am Poor Hamm,
a cartoonists and the author of Oliver's Great Think Universe.

Speaker 2 (01:05):
Hi, I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I really wish there was more
we could know about each particle.

Speaker 1 (01:13):
What do you want to know?

Speaker 2 (01:14):
I want to get to know them. You know, particles
are kind of like black holes. There's a few things
you can measure about it, the spin, the mass, the charge,
et cetera. But otherwise they're all totally identical. It's not
like this particle is Bob and that one is Sam
and this one is Juanita. You know, all electrons are
the same.

Speaker 1 (01:32):
What if they don't want to be known? What if
they're private particles?

Speaker 2 (01:36):
I see they're all spartacles.

Speaker 4 (01:38):
Huh.

Speaker 1 (01:38):
Yeah, they have secrets. They don't want the Walsh out
there on the internet.

Speaker 2 (01:43):
Well, like I've said before, I don't think the universe
deserves any privacy. You know, we are curious creatures and
we're part of the universe, so knowing ourselves is sort
of like knowing the universe.

Speaker 1 (01:53):
Are you saying. Physicists then are sort of like professional boxers.

Speaker 2 (01:58):
I like to think of as more as the detectives
maybe private snoops. Yeah, we are snoops for sure, and
we're out to solve the biggest mystery in the universe,
which is like, how does this whole thing all work?

Speaker 1 (02:11):
Did you change your jaw title then to a particle
snooper particle investigator? I'm a PI a PPI. I guess
I don't like having PP in my title. Yeah, PP's
not good on many things. But yeah, anyways, welcome to
our podcast Daniel and Jorge Explain the Universe, a production
of iHeartRadio in which we.

Speaker 2 (02:29):
Try to lift the level of discourse as best we can,
elevating your mind to the deepest, biggest, most ethereal questions
in the universe. How does it all work, what's it
all made out of? What are the rules of the game,
and how is the game played in such a way
to give us this crazy, amazing, visceral conscious experience of

(02:50):
such a real world, which in the end is made
up of tiny, little, almost massless particles.

Speaker 1 (02:56):
Yeah, because it is a pretty awesome experience to exist
in the universe and to look out there and appreciate
all the wonders and amazing things that are happening out
there in the universe that we can see and also
that we can't see.

Speaker 2 (03:08):
And as we drill down into the nature of reality,
taking things apart into molecules and atoms and nuclei and
protons and neutrons, we like to give names to these things.
We say, oh, this kind of thing is an electron,
and that kind of thing is a neutrino, and this
kind of thing as a quark. It's just part of
who we are to want to attach labels to bits
and pieces of the universe.

Speaker 1 (03:30):
Yeah, it's all part of humans quests to understand what's
going on out there, to get a handle on how
things work and how to predict what's going to happen
in the future.

Speaker 2 (03:39):
And as we look at these tiny little particles, we
want to describe them in ways that make sense to us.
You know, how much spin does it have? What can
it do? And maybe at the most fundamental level, part
of the identity of a particle is how much mass
does it have?

Speaker 1 (03:55):
Yeah, some particles have a little bit of mass, some
particles have a lot of and some particles have no mass. Right,
some particles adheres to a very impressive diet.

Speaker 2 (04:06):
Photons have no mass, while top quarks, the heaviest known
fundamental particle, have the mass of like one hundred and
seventy five protons. So there really is an extraordinary range,
which is something that we don't understand at all. But
mass is also part of how we tell which particle
is which. I think about an electron and a muon.

(04:27):
What are the differences there between the two? They're almost
identical particles, except that muons have more mass than electrons do.
And when we produce particles in our experiments. That's how
we tell what's what. We measure the masses of these
particles and we say, oh, this one's got to be
an electron because look at its mass. So it's not
just that we take the particles, we assign mass labels

(04:49):
to them. We use the mass to tell us who
is who.

Speaker 1 (04:52):
Yeah, and there are lots of particles out there. Some
of them are not shy at all about how much
mass they have. Some of them are a little bit
shy and don't necessarily want to reveal how much mass
he has.

Speaker 2 (05:03):
Some of the weirdest particles out there are neutrinos, these
ghostly little particles that are everywhere but very hard to spot.
And in the case of neutrinos, their identity is something
of a more complex story. They have sort of two
different kinds of clothing they can wear, who they talk to,
and how they move through the universe. And because their

(05:24):
mass is so weird and so hard to nail down,
it's not something we actually know very well.

Speaker 1 (05:29):
It's all a big mystery. And so today on the
podcast we'll be asking the question how massive is a neutrino?

Speaker 2 (05:40):
Or maybe we should have said, how massive isn't a neutrino?

Speaker 1 (05:44):
Wait? What why shouldn't we have not said.

Speaker 2 (05:46):
That because neutrinos have some masks, but they definitely aren't very.

Speaker 1 (05:52):
Massive, or how very little massive nutrina is? Is that
what I mean?

Speaker 2 (05:58):
How dainty is a neutrino?

Speaker 5 (06:00):
Yeah?

Speaker 1 (06:00):
I thought you meant, like, how significant in nutrino is?
Like how massive it is it in a universal scale
of awesomeness?

Speaker 2 (06:08):
Yeah. It actually turns out neutrinos are quite important and
play a big role in the physics of the universe
despite being almost invisible. So from a consequential point of view, right,
neutrinos are massive. Dude.

Speaker 1 (06:20):
Well, I think what you're saying is that the mass
of the neutrino is not known. We don't know how
much mass it has.

Speaker 2 (06:25):
We do not know how much mass the neutrino has.
We've only known that it has mass for a couple
of decades, which was a big shocker and sent quakes
through the theoretical community when we figured that out, and
it's still something that is very hard to pin down
and not something we know.

Speaker 1 (06:42):
It was a massive shock weighed heavily on the minds
of physicists for a long time.

Speaker 2 (06:47):
They didn't take it lightly, that's for sure.

Speaker 1 (06:49):
So yeah, this is an interesting question. How much maths
does and theatrino have. Apparently it's kind of tricky to
find out. So as usually, we were wondering how many
people out there had thought about this question or have
an idea about the mass of a nutrino.

Speaker 2 (07:01):
So thanks very much to everybody who answers these questions
for this fun segment. If you'd like to hear your
voice speculating for everybody else's entertainment and education, please write
to us two questions at Danielandjorge dot com.

Speaker 1 (07:14):
So think about it for a second. How massive do
you think a neutrino is? Here's what people have to say.

Speaker 3 (07:20):
Not sure if the vibe was that there's more than
one type of neutrino. So maybe there's like some with
more mass. But I thought that neutrinos were like massless
or like had negligible mass and so like they travel
at the speed of light.

Speaker 6 (07:32):
I think there's different types of neutrinos that are different sizes.
You talked about one of NASA finding another universe by
seeing neutrinos pass through Earth. So there's some massive ones
but not so massive. How big maybe like fifty protons
big or something, if that even makes sense, And maybe
neutrinos are also dark matters what you also said in

(07:55):
one of your earlier podcasts.

Speaker 2 (07:56):
I would think that a neutrino is real light because't
interact with other particles, but it may interact with the
Higgs field. So I actually have no idea.

Speaker 7 (08:07):
Well neutrino so eno means very small in Italian or smaller,
so I would assume that the mass of a neutrino
is much much much smaller than that of a neutron,
and I'm tempted to say that.

Speaker 5 (08:25):
Neutrinos are massless.

Speaker 4 (08:27):
Maybe mass is just I think the amount of energy
that's required to move something, so gravitational mass is just
a unique form of inertial mass, wherein it's the gravity
which is pulling you and that changes according to where
you are, whereas inertial mass is just independent of that.

(08:49):
I guess I.

Speaker 5 (08:50):
Don't know how massive a neutrino is. I'm pretty sure
that I've heard that they have mass, and I think
it's extremely like neutrinos are very low mass, and it
would be great if they had the lowest amount of
mass allowed by quantum mechanics. That would be pretty.

Speaker 1 (09:09):
Neat, right. I think a lot of people seem to
know it had very little mass.

Speaker 2 (09:14):
I really like the linguistic analysis, reverse engineering the name
particle to infer what its mass has to be.

Speaker 1 (09:20):
What do you mean it has neutral mass?

Speaker 2 (09:23):
Well, you know, neutrino means little neutral particle. That was
the name given to it before we even really knew
what it was, because that's all we knew about it,
that it could be very massive and that it was
electrically neutral. So in that sense, you might even be
tempted to say that it's a well named particle.

Speaker 1 (09:39):
They were going to say it has the mass of
a newt. But also you kind of have to know
Italian to know that the io ending, you know, know,
means small, don't you not everyone speaks Italian.

Speaker 2 (09:50):
That's true. I guess if it had been named by
somebody who speaks Spanish, would be like nutrito.

Speaker 1 (09:54):
Yeah exactly. Or in English, I guess, how would you
call it new trini neutral?

Speaker 2 (10:00):
Do we have affectionate endings in English?

Speaker 1 (10:02):
Tiny neutron? There you go, like tiny tim?

Speaker 2 (10:05):
Or maybe we'd give it an ironic nickname you know,
like big.

Speaker 1 (10:08):
Neutron, Yeah, neutronizer or something, or how about just neutron.
I mean that sounds pretty massive now in comparison to neutrino.

Speaker 2 (10:18):
Neutron had already been discovered. Is the name of another particle?

Speaker 1 (10:21):
Oh well, there you go, that one's misnaming them. All right, Well,
let's dig into this mystery. What is the massive a neutrino?
But I guess far as Daniel talk to us about
what a neutrino actually is.

Speaker 2 (10:33):
Neutrino is a really fun particle because it's so weird
and yet so fundamental and so important and at the
same time not a part of the matter that's around us.
You know, if you take a part the stuff that
you're made out of, and that I'm made out of,
and that everything you've ever eaten is made out of,
you discover that it's made of atoms, and those atoms
are made of protons and neutrons and electrons. But the

(10:56):
protons and neutrons can be made out of quarks, up
quarks and down quarks. Specific that means that everything that
we know is made of two kinds of quarks, up
quarks and down corks, as well as electrons. So really
just three particles explain all of the matter that we know,
the stuff that the Earth is made out of, that
the Sun is made out of, that the visible matter
in the galaxy is made out of. Of course, put

(11:17):
dark matter aside because we don't know what that is
made out of. So those three particles sort of underlie
everything that exists. But there's another particle that's in the
same category as like one of the basic templates of
possible matter, and that's the neutrino. Because you notice that
the upcork and the down cork sort of have each other.
There's like a pair of quarks. You might wonder like, well,

(11:38):
who's the electrons partner, And the electron does have a partner,
it's the new trino. So it sort of like completes
the quartette of the fundamental bits of matter, even though
the neutrino doesn't appear in the atom and isn't used
to make up your lunch or your dinner or anything
you've ever eaten.

Speaker 1 (11:55):
Hmmm, I guess maybe the first question I would have
is why not why they aren't neutrino's part of the
matter that we're made at it, or why don't we have,
you know, neutrino bits inside of us.

Speaker 2 (12:05):
Yeah, it's a great question. You know, the universe has
these bits and pieces, and they have rules for how
they can come together, and then you get complex structures
emerging from that. You know, you have quarks bind together
to make protons and neutrons, which then bind with the
electron to make atoms, to make all sorts of other
complex stuff. I scream and stars and black holes and
all that stuff. And really it's the interaction. They're the

(12:27):
binding that's crucial. While quarks and electrons all have electric charges,
and quarks have strong charges, so they can use the
more powerful forces to build complex matter. Neutrinos are different
from the other three kinds of basic fundamental bits of
stuff in that they only feel the weak force, so
they have no electric charge, they're neutral, and they also

(12:49):
have no color, so they don't feel the strong nuclear force,
which means they're only left to interact via gravity, which
is basically negligible for a particle and the weak force.
So in order to bi old something out of neutrinos,
you'd have to have them bound together by the weak force,
but the weak force is just too weak to do that.

Speaker 1 (13:07):
Interesting, What do you mean too weak? Like you can't
stick to nutriinas together with the weak force.

Speaker 2 (13:13):
The weak force can be used to interact, but it's
really very, very shockingly weak. That's why, for example, if
you shoot a photon at the wall, it'll splat against
the wall and interact with all the electrons inside of it.
But if you shoot a neutrino against the same wall,
it will fly right through. It's not like it's finding
holes in the wall. It's not like the wall is
a screen or a mesh that it's slipping through. It

(13:35):
ignores all those particles because it doesn't interact with them.
So it's really all about the strength of the interactions.
And if you wanted to like bind two neutrinos together
into a more complex object, that have to be in
a bound state in order to be trapped together by
an interaction that's so weak, they would have to be
almost motionless. It wouldn't take very much energy to break
it apart, So you'd have to have very cold bits

(13:57):
fall together to make a bound state and then be
very easy to break it apart. So it's basically not
possible to build more complex structure using the weak force.

Speaker 1 (14:07):
I think you're saying that you can, but maybe matter
would have to be super duper cold to put together
things with the weak force.

Speaker 2 (14:13):
Yeah, matter would have to be super duper cold, and
they would have to not be other stronger forces disrupting it.

Speaker 1 (14:18):
Right, I don't know how does the weak force work.
Does it repel or attract or both? Does it have
positive negative charges to it?

Speaker 2 (14:26):
So the weak force is quite complicated. We talked once
about whether the weak force can attract or repel. It
actually can do both. There are two different charges for it.
They're called isospin and weak hypercharge, and so it's a
complex combination of all these different numbers that tells you
what the weak force is going to do. But in short,
it can attract and it can repel. So it's very
similar to electromagnetism. Actually, electromagnetism and the weak force together

(14:49):
are part of a larger idea called electro week And
the reason that one of them is more powerful than
the other has to do with the Higgs boson, which
breaks the symmetry between the two forces, leaving one of
them very powerful and one of them very very weak.

Speaker 1 (15:04):
So, like if I took two neutrinos, and I cooled
them down out there in space and I stuck them together.
Would they stick together due to the weak force?

Speaker 2 (15:12):
You could put two neutrinos into a bound state if
they were very, very cold, so they didn't have enough
kinetic energy to escape these bonds and there was nothing
else bothering them. Yes, you could. And you could even
add more.

Speaker 1 (15:23):
Yeah, you could add more. Maybe can you build a
whole planet out of neutrinos?

Speaker 2 (15:28):
You could build larger, more complex structures, but it would
be very fragile, and it certainly wouldn't look like a planet,
and the whole thing could probably pass through the Earth
without even noticing, because neutrinos, again don't interact with normal matter.
So even if you build more complex structures out of neutrinos,
it exists sort of in parallel to us, the same
way that like dark matter does. Dark matter is here,

(15:48):
dark matters everywhere. Dark matter might make complex structures that
we can't see, but they passed right through us, and
we passed right through them because we don't have any
interactions with them, the same way a neutrino can pass
through like a light year thick wall of lead without
even interacting. And so a whole planet of neutrinos would
do the same thing. Like, right now, there's one hundred

(16:09):
billion neutrinos passing through every square centimeter of the surface
of the Earth every second, and yet we don't feel them.
So somebody could throw a planet of neutrinos at us
and we wouldn't even notice.

Speaker 1 (16:21):
Would that neutrino planet break apart when it goes through
us or would it stay together?

Speaker 2 (16:25):
A tiny fraction of those neutrinos would interact with us,
so those a little bonds would break up, but most
of it would totally ignore us. Neutrinos have a very
very tiny probability of interacting with electrons or with quarks.

Speaker 1 (16:38):
So and then when you say weak, do you mean
like low probability or just that the force is weak?

Speaker 2 (16:44):
We mean low probability, not small momentum exchange, but low probability.
Like you shoot a neutrino at another particle, it's very
unlikely to interact. If it does interact, it can impart
significant momentum, it's just a low probability of it happening.

Speaker 1 (16:58):
Oh, that's interesting. So it's really called the weak force
because of its weak probability, not because like you wouldn't
feel it.

Speaker 2 (17:05):
Yeah, exactly. The very strength of the forces are more
about the probability of that interaction, which, if you integrate
over all possibilities, does end up playing a role in
like its impact on the world, basically how massive is
its impact.

Speaker 1 (17:17):
So then maybe like a better name for the weak
force would have been improbable for us, the unlikely.

Speaker 2 (17:23):
Force, the unlikely force that makes it sound like it's
going to go on a hero's journey and in the
end it become the most powerful force in the universe.

Speaker 1 (17:31):
That's right, the underdog for exactly What else do we
know about neutrinos.

Speaker 2 (17:36):
We know that there are three kinds of neutrinos, the
way that there's like three different kinds of electron. There's
the more massive version that's the muon and the even
more massive version that's the tau, So there's three different
flavorers of electron. There's also three different flavors of neutrino.
So there's a neutrino associated with the electron, the electro
and neutrino, and one associated with the muon and one

(17:58):
associated with the taw.

Speaker 1 (18:00):
What do you mean associated? What does that mean? They
signed a contract?

Speaker 2 (18:03):
Well, these guys interact via the weak force, and so
for example, if you want to make an electron, you
can make it from a w boson. A W boson
can decate to an electron, but also decays to a neutrino.
And when you create an electron, you also create an
electron neutrino. You create a muon, then you also create
a muon neutrino. So when we say associated with, we
mean like grouped together with by the weak force. It

(18:24):
groups these guys together. Remember that we count the number
of eleptons in the universe and that's conserved. So for example,
you can't just like make more electrons. If you make
more electrons, you also have to make more anti electrons
to balance out the number of electrons in the universe.
But electron neutrinos fall into that category. So you can
make an electron and then you make an anti electron neutrino,

(18:47):
and the universe's books are all balanced.

Speaker 1 (18:49):
Like an electron and a neutrino are sort of like twins,
like you can have you can make one without the other.

Speaker 2 (18:54):
You can make an electron either with an anti electron
neutrino or with an anti electron, So like a boson
will de kate to an electron and an anti electron
neutrino together, or a z boson will dekate to an
electron and an anti electron. You can't just make an
electron by itself.

Speaker 1 (19:11):
So there's it sounds like there are more electrons and
there are anti electrons and electron latrinos.

Speaker 2 (19:18):
There's definitely more matter than antimatter. So yeah, they're more
electrons than anti electrons. But when it comes to the neutrinos,
like we have these pairings. So there's three different flavors
of neutrino, the muon, the electron, and the town neutrino.
Each one is connected to one of these leptons because
the weak force likes to make those together.

Speaker 1 (19:37):
That's just something we've observed, right, Like we noticed that
the weak force, when it does things in the universe,
it creates these things in pairs. Like is there anything
else we know about them that associates them, Like do
they have the same quantum varible about it?

Speaker 2 (19:51):
I like the way you say, that's just what we observed,
Like that's basically science, right. We observe the universe and
then we describe it, and then we try to boil
that description down to a couple a set of rules
as possible, and think about what that means. So, yeah,
that's just what we've observed. We've never seen this be violated.
So there's an asterisk there. We'll talk about neutrino oscillation
in a minute, but yeah, really that's the only difference

(20:13):
we know about it from these different kinds of neutrinos
that the weak force associates them with different leptons, with electron,
a muon, or a taw. The other question, of course,
is about their masses, like what are the masses of
these particles. We know that for normal matter, all the
quarks and the electron, the masses tend to increase as
you go to their copies, like the muon is heavier
than the taw. The upcork has heavier versions the charm

(20:36):
and the top. The down cork has heavier versions the
strange and the bottom. When it comes to the neutrinos,
we don't know so much about what their masses are
and how that's organized.

Speaker 1 (20:46):
All right, it sounds like a good cue for us
to dig deeper into the mass of the neutrino and
talk about how we know it has mass and how
we measure that mass. So let's get into that, but
first let's take a quick break. All right, we're talking

(21:09):
about the mass of a neutrino specifically, what is its mass?
Is it a lot, is it a little? And why
is it the way it is? So we talked about
what a neutrino is. They are ghostly particles that fly
around the universe without really interacting with the rest of
the matter in the universe. Daniel a quick question. Do
they interact with dark matter?

Speaker 2 (21:29):
Ooh, yeah, great question. We don't know. For a long
time we wondered if neutrinos were the dark matter, like
they kind of fit the bill because we can't really
see them and there's maybe a lot of them out there.
Turns out neutrinos can't be the dark matter because we
know the dark matter moves slowly, it's cold. We know
that from like how it's influenced the structure of the universe.

(21:50):
If dark matter moved faster, things would be less lumpy,
and neutrinos move really really fast, So neutrinos are too
hot to be the dark matter. Do neutrinos interact with
dark man? We don't think so, because we don't think
that dark matter feels the weak force or the improbable force,
as you'd like to call it, because if it did,
we would have seen it bump into some of our
big underground detectors. So because dark matter probably doesn't feel

(22:12):
the weak force, it probably doesn't interact with neutrinos.

Speaker 1 (22:15):
Yeah, I feel the same way. I think I'm too
hot to be dark matter.

Speaker 2 (22:19):
I'm always telling people that.

Speaker 1 (22:22):
And ironically, I'm also pretty cool.

Speaker 2 (22:24):
You're a paradox of physics.

Speaker 1 (22:26):
Yes, I'm an enigma wrapped in a cartoonist. But talking
about the mass of a nutrino, I guess the first
question is like, first of all, how do you know
it has mass? Like, there are particles out there without mass, Right,
how do we know that neutrino has mass?

Speaker 2 (22:38):
Yeah, you're right. There are particles out there that have
no mass, like the photon and the gluon. So it's
not impossible for the neutrino to have no mass, and
for a long time we assumed that it didn't. There's
even an argument about what we mean by the standard
model of particle physics, sort of our description of our
best understanding. Some people say that the standard model of

(22:59):
particle physics requires neutrinos to have no mass, though there
are extensions of it that allow them to have mass.
Some people say that's beyond the standard models. Some people
say that's the new standard model. As you might expect,
this big argument about how we name it. But for
a long time we assumed neutrinos had no mass. But
now we do know that they have mass, and we
know that in two different ways. We know that they

(23:21):
have mass even without knowing how much mass they have.

Speaker 1 (23:24):
Interesting do you know because they I don't know, pass
around heavy objects, or because you've weight them.

Speaker 2 (23:30):
So we know in a few different ways. Actually, one
of the first clues was looking at a supernova. There
was a supernova in nineteen eighty seven that was very,
very bright, and we saw a big flash of neutrinos
coming from that supernova. And the neutrinos actually arrived a
little bit before the photons because neutrinos come from the
center of the supernova and they aren't blocked by the

(23:50):
rest of the matter in the supernova, whereas the photons
come from the surface and it takes a while for
the energy to like propagate out and produce those photons.
But they looked at when the new trinos arrived and
realize that they don't all arrive at the same time.
We think they all leave the supernova basically the same moment,
but they don't all arrive at the same time. The

(24:11):
higher energy neutrinos arrive earlier than the lower energy once.
The higher the energy, the faster they go. That makes sense,
but it's actually a property you can only have if
you have mass. Massless particles like photons, all travel at
the same speed regardless of their energy. All photons travel
at the same speed because they're massless. Neutrinos have a

(24:34):
spread in their velocity, which means they have a mass.

Speaker 1 (24:38):
But I guess it tells you that they're not as
fast as photons, which means they have mass. Right, because
anything that doesn't have mass would move at the speed
of light exactly.

Speaker 2 (24:47):
Things that don't have mass always have to move at
the speed of light. There's no option there, right, Massless
objects always move at the speed of light.

Speaker 1 (24:55):
Okay, so neutrinos don't move at this speed of light,
which means they have some mass. But then is that
the main way that we know they have mass?

Speaker 2 (25:01):
So there's another really fascinating clue which comes from the
Big Bang. We think that a lot of neutrinos were
made in the Big Bang, Like all this energy was
hot and dense, and the quantum fields were frothing, and
as they cooled down, they sort of dribbled out into
all the different fields that are out there. So the
Big Bang made a lot of quarks and made a
lot of electrons, and made a lot of neutrinos as well.

(25:21):
And as those particles all mixed together, the amount of
photons and neutrinos and quarks determined like what kind of
stuff got made. Later as things cooled, like how much
hydrogen did you get and how much helium did you get?
And out of those things sort of sloshed together and
froth together in the Big Bang. So by studying the
relics of the Big Bang, the leftover bits of it,
we could actually get some clues as to like how

(25:43):
many neutrinos there were, and we can even figure out
something about the mass of those neutrinos.

Speaker 1 (25:49):
But wait, I thought, neutrinos don't interact with regular mass,
So how can like regular mass relics tell you about
how many the trinos there were in the Big Bank.

Speaker 2 (25:57):
Yeah, you're right. The neutrinos almost never interact with matter,
but if matter is dense enough, they will, Like the
probability is not zero, it's greater than zero. And actually,
back in the earlier times, when the universe was hotter,
when things were denser, the weak force was not as
weak as it is today. We think back in the
very early universe, the weak force and the electromagnetic force,

(26:19):
before the Higgs boson broke the symmetry, the two were
actually equally as powerful. So neutrinos used to interact with
normal matter more than they do today.

Speaker 1 (26:28):
I think what you're saying is that our models of
the Big Bang tell us that there were a lot
of neutrinos at the Big Band, and that they have passed.

Speaker 2 (26:34):
The models of the Big Bang tell us something about
how many neutrinos there were, like the number of neutrinos,
because neutrinos back then were moving really really fast, they
were very very hot, and so they helped like spread
energy out. They sort of acted like photons because everything
was so hot. And when we study the early universe
we can see these acoustic oscillations, like there were these

(26:56):
density waves. In the early universe. Things were hot and dense,
and the created pressure waves in the matter photons and
neutrinos helped us sort of smooth that out a little bit.
So by looking at those oscillations they're called baryon acoustic oscillations,
which make these ringing patterns in the early universe, we
can measure how many neutrinos and how many photons there were,
So that tells us something about the number of neutrinos.

(27:19):
Then we can do a second thing to figure out
how massive the neutrinos had to be. Like we know
how many neutrinos there were, and then we can figure out, well,
how much mass could the neutrinos have without causing the
universe to collapse? Right, We know that the universe has
been expanding since it was very very young, and that
tells us something about like how much matter and radiation
and energy there is in the universe, because if there

(27:41):
was too much, then gravity would pull everything back together
very quickly into a big bang. So we know something
about how many neutrinos there were, we could put an
upper limit on how massive they could be without collapsing
the universe.

Speaker 1 (27:54):
But I think the two are sort of tied together, right.
The number of neutrinos and how massive they are, right,
I mean you have to assume they have mass in
order for them to matter at the Big Bang, right.

Speaker 2 (28:03):
Well, they don't have to have mass in order to matter.
It's funny that we use matter because remember, general relativity
is sensitive to energy density, whether it's in the form
of radiation or in the form of matter. It really
is just sensitive to energy density. So the Big Bang
analysis tells us the number of neutrinos totally independently of
their mass. And the second step is to say, well,

(28:24):
if neutrinos do exist, how much mass could you give
them without causing the universe to collapse? So that tells
us something about how massive they could be. Like an
upper limit, Yes, exactly, it's an upper limit. That number
is actually really really low. The number is less than
the tenth of an electron volt.

Speaker 1 (28:41):
Which I guess to give us some context, how much
mass does an electron have?

Speaker 2 (28:45):
So an electron has like five hundred thousand electron volts.
It's half of an MeV half of a mega electron bolt,
and so five hundred thousand electron vaults. That's not very much, right,
Electrons are very very low mass. Particles compared to like
a proton. A proton has like one giga electron bolts
one billion electron bolts. So we know from the Big

(29:08):
Bang that all neutrinos added together have to have less
than a tenth of an electron bolt, less than one
ten billionth of the mass of a proton.

Speaker 1 (29:17):
You mean all the different kinds of neutrinos, not all
of the individual neutrinos in the universe.

Speaker 2 (29:22):
Right, Yeah, that's exactly right. There are three neutrinos. When
you add up all their mass together, it has to
be less than a tenth of an ev where an
electron is five hundred thousand ev and a proton is
about a billion ev m.

Speaker 1 (29:35):
Interesting, so then pretty light, very very light, Like how
much is a quark.

Speaker 2 (29:39):
It depends a lot on which cork you're talking about.
The lowest mass quarks have like a few MeV a
few million electronvolts. The most massive ones, like the top quark,
is like one hundred and seventy five billion ev. So
these neutrinos have mass much much closer to zero than
anything we've ever seen before. They're like shockingly low mass.

Speaker 1 (30:00):
Okay, so we have a sort of an upper limit.
You said for how much the three kinds of neutrinos
can add up together, But then how do we resolve
how much each one of them weighs?

Speaker 2 (30:09):
So then we have another really fascinating clue which tells
us about the mass difference between the neutrinos. So so
far we know something about the sum of their masses.
We know it's less than point one ev. We also
know there are three neutrinos, when we're wondering, like, well,
they all have the same mass, is it like with
the other particles, where there's one low mass and then
another one and then another one. So we can do
another kind of experiment to measure the differences between the

(30:32):
masses of the neutrinos. And this comes from how they
actually change their identities. Neutrinos are weird compared to the
other particles. In even another way. They're different from the electron,
the muon, and the towel, and that they can change flavor.
Like if you create an electron neutrino and shoot it
through space and then wait like a light year two

(30:52):
light years and try to measure it, you might discover
it's no longer an electron neutrino. It's now a muon
neutrino or a neutrino. This is called neutrino oscillation.

Speaker 1 (31:03):
M yeah, I think usually if you shoot anything the space,
no change flavor. But I guess how do we know this? Like,
how would we know if it changed flavors? And again,
flavor is kind of the charge of the weak force?

Speaker 4 (31:14):
Right?

Speaker 2 (31:14):
Flavor is actually which of these generations of particles? It
is like is it electron, is it muon? Is it tau? Right,
that's what we mean by flavor.

Speaker 1 (31:23):
Oh, is there a charge the weak force or is
it just the weak charge?

Speaker 2 (31:26):
The weak force does have a charge member, it's two
different charges. There's the isospin and the weak hypercharge, so
both of those count as weak charges. But the neutrinos
all have the same week charges where they have different
is this flavor? This different identity, But that identity actually
turns out to be different. When you create the neutrino
and when the neutrino flies through space, they have like

(31:46):
two different sets of identities. There's the identity we talked
about when a neutrino is made, like the weak force,
when it makes an electron, it makes an electron neutrino,
or if it makes a muan, it makes a muon neutrino.
But when neutrinos fly through space, they have three different
ideas entities, and those are their masses. So there's three
different kinds of neutrinos for the weak force, and there's
three different kinds of neutrinos for the masses. But those

(32:09):
are not the same. They're like a mixture of each other.
So if you imagine this like M one, M two,
M three are the three neutrino masses. When you create
an electron neutrino, it's not like it's M one. It's
some weird mixture of all the masses of the three neutrinos.

Speaker 1 (32:25):
You mean some kind of weird quantum mixture. Is that
what you mean?

Speaker 2 (32:28):
Yeah, it's a superposition. So you create an electron neutrino,
it's a quantum superposition of the three different neutrino masses.
You create a mule on neutrino, it's a different superposition
of those masses. It's like having two different set of
axes that are not aligned. It's like a rotation between
your set of axes.

Speaker 1 (32:46):
I guess maybe the question I have is so there's
three types of neutrinos, electronion, and town neutrinos, and the
only difference between them is the mass.

Speaker 2 (32:54):
The only difference between the electron, muon, and town neutrino
is how they interact with the weak force. Three different
kinds of neutrinos. There's two different ways to break them down.
One is how do they interact with the weak force?
The other is what are their masses? So you get
two different ways to categorize the three neutrinos.

Speaker 1 (33:11):
What do you mean how it interacts with the weak force?
Like like it's probability of interaction or its strength of interaction?
What do you mean by that?

Speaker 2 (33:19):
Like what it's made in association with? Like, if you
make an electron, what kind of neutrino do you make? Well,
you make an electron neutrino. If you make a tow
what kind of neutrino do you make? You make a
town neutrino.

Speaker 1 (33:30):
But if you already made it, does it matter or
does it matter in like what it can do later?

Speaker 2 (33:34):
It matters in the accounting of the number of electrons
or muons or towels in the universe.

Speaker 1 (33:38):
Yeah, But like if you just catch one in space.
How do you know what it is because you weren't
there when it was made.

Speaker 2 (33:44):
Yeah, good question. Well, electron neutrino is more likely to
make electrons and a muon neutrino will make a muon,
and a town neutrino will interact and make a tow.
One of our neutrino experiments can see electrons, it can
also see muons, and it can also see tows And
so you can tell which kind of neutrino it was
by how how it interacts. Does it create an electron,
does it create a muon, Does it created.

Speaker 1 (34:03):
A taw What it can do in the future kind.

Speaker 2 (34:06):
Of yeah, what it can do in the future, because
the universe keeps track of this, accounting how many electrons
are there, how many muons are there, how many towels
are there. But again, that's just one way to see
these things. Another way to see these things is how
much mass do they have? And for most particles it's
the same thing. The weak force creates an electron, the
electron has a mass. All electrons have the same mass.
It's just a number. And if you ask, like what

(34:28):
are the masses of the eleptons, you get three different numbers,
those align with the flavors of the e leptons, but
when it comes to the neutrinos, they don't. So when
you create an electron neutrino, it's a weird mixture of
these different masses, and as it flies through space, those
that mixture can change because mass tells us how things
move through space. So these electron neutrinos and mew neutrinos
and town neutrinos, because they're made of three different masses,

(34:50):
and those masses are different, those masses like fly through
space slightly differently, and they can turn from one into another.

Speaker 1 (34:57):
I think what you're saying is that, like, if you
make an electron neutrino like in the center of the
Sun and it's flying to us, and it has the
identity of an electron neutrino, it might have that identity,
but it might not necessarily have a particular mass, like
it might have one of three different masses exactly. Or
if you like find a neutrino during space with like
one of the masses, like the highest mass for neutrinos,

(35:19):
then that could still be either an electron neatrino or
town latrino or a unutrino.

Speaker 2 (35:25):
Yes, that's exactly right. In mathematical terms, if you have
a weak eigen state, if you have an electron neutrino,
that's something produced by the weak force. In a pure
electron state, it's a mixture of the mass states. If
you have a pure mass state, it's a mixture of
the flavor states.

Speaker 1 (35:41):
I think basically nutrino's kind of have an identity crisis
going on, both a mass crisis and an identity crisis,
Like it doesn't quite know what it is, or it
could be different things, but it could also weigh different things,
and it could also call itself different things. And it's
sort of like up in the air, like it can
change its fluid between these identities exactly.

Speaker 2 (36:00):
Neutrinos have two different kinds of identities, and they do
not align. For most particles, these things align very well
for neutrinos.

Speaker 1 (36:07):
They don't like an electron. For example, if it's born
an electron, it's going to have the mass of an electron.
It's not somethingly going to have the mass of a
Taue electron or a Newon electron.

Speaker 2 (36:16):
Right, Yeah, And this calls in the question what I
was saying at the very beginning of the podcast about
mass being part of the identity of a particle, because
neutrinos can't really be defined by their mass, Like, well,
it depends are you talking about who I interact with
or how I fly through space? Because the same neutrino
can give you two different answers to that question.

Speaker 1 (36:33):
Interesting, all right, Well, let's dig into how we actually
measure the mass of a neutrino and what those results
have found. But first, let's take another quick break. Right

(36:55):
we're talking about the mass of a neutrino. How massive
is this ghostly part of that flies through space, barely
interacting with everybody else in the universe, ignoring everyone. It's
kind of a snobby particle.

Speaker 2 (37:06):
It's just got its own stuff to do, you know.
It just can't stop and chat with everybody. It's got
its list of barrens.

Speaker 1 (37:12):
It's very aloof.

Speaker 2 (37:14):
It's just busy, man, It's just busy.

Speaker 1 (37:16):
Just more neutral, has less opinions. I guess it's not
as interesting, all right. And so we're talking about how
much mass it has, and we know from the Big
Bang models that we have that it nutrina has very
little mass, and the different kinds of nutrino thos can't
have a lot of masks combined. We talked about how
the nutrina kind of has an identity crisis, doesn't quite

(37:39):
knows for real what kind of neutrino it is and
how much it weighs. It's so sort of fluid and quantumy,
kind of complex and superposition. So then I guess the
big question is what can you do with that? How
do you measure these masses if the neutrino so wishy washing?

Speaker 2 (37:55):
Yeah, So the fact that neutrinos can change flavor was
a big mystery in particles physics for many decades. Like
we count the number of neutrinos we see from the
Sun electron neutrinos, and we don't see as many as
we thought we should, which is a big puzzle. For
a long time, we predicted a certain number of electron
neutrinos being created in the Sun, and we just didn't
see as many. We saw like a third as many

(38:15):
as we expected. Now we understand that's because they're oscillating.
They're changing from electron neutrino to something else, and so
we're not seeing them because they're not interacting with our electrons.
But we can also use that to measure the differences
in the masses of the neutrinos. It's because there's a
mass difference between the neutrinos because they fly differently through
space that they're changing their identity as they go. So

(38:38):
what we can extract from this are two numbers, the
mass differences. Like you imagine this M one, M two,
M three. We can measure the separation between those three.
We can't tell the overall mass, but we can tell
how different they are. But the gaps are between the
neutrino masses.

Speaker 1 (38:54):
I guess the question is can we why can we
measure the absolute value of these masses.

Speaker 2 (38:59):
Because this oscillatetiontion doesn't depend on the absolute value. It
only depends on the difference. Like if all the neutrinos
had the same mass, then there wouldn't be any oscillation.
If the mass differences were really really large, they would
oscillate more. So by measuring how much they oscillate, we
can measure this mass difference, but the oscillation doesn't depend
on the total mass. This's a separate experiment we'll talk
about in a minute, called the Caturing experiment, which is

(39:21):
going to try to measure the overall mass of the neutrino.
But this oscillation, something which is quite well established, gives
us a precise measurement only of the differences between the masses.

Speaker 1 (39:31):
I guess maybe I didn't quite understand why we can
only measure the differences.

Speaker 2 (39:34):
Because the oscillation comes from the differences, Like if there
weren't any differences, you would see no oscillation. And the
larger the difference, the more the oscillation. It's kind of
like measuring interference between two laser beams. If they're in sync,
you see no interference. If one of them is delayed,
then they're out of phase and they interfere with each
other and giving an effect you can measure. But all

(39:55):
you can measure from the interference is the difference between
the beams, because that's what causes the interference. And neutrino
is a mixture of different masses, and each of those
masses flies through space differently, and it's that difference that
causes them to change flavor to oscillate.

Speaker 1 (40:11):
But then how do we measure the oscillations, Like we
can only measure one neutrino at a time, we don't
know what it was before. How do we know what
it was after.

Speaker 2 (40:20):
Well, we don't measure oscillations for an individual neutrino, You're right.
What we do is measure them statistically. So we have
like a bunch of neutrinos made in the Sun, and
we know those are all electron neutrinos because the Sun
has electrons in it and not muons and towels, So
we can measure how many of those have disappeared by
the time they get to Earth. We can also make
a bunch of muon neutrinos and a particle beam on
Earth and then see how often they disappear. So we

(40:41):
can make a bunch of these measurements of neutrino oscillation,
not by looking at an individual neutrino and seeing it oscillate,
but by making a huge number of neutrinos and seeing
how many of them disappear from their original identity.

Speaker 1 (40:53):
Because you're saying, like the way you measure them, when
you catch a neutrino, you sort of know what it was,
or at least the detectors can only as you're one
kind of neutrino at a time.

Speaker 2 (41:02):
Exactly, and all you can do is measure is flavor.
That's the way we detect them is we interact with them.
The only way to interact with them is through the
weak force, and that means using electrons, viuans, and towels.
That's how we interact with them.

Speaker 1 (41:12):
And then how does that tell us their mass differences?
Like if I catch in the trino, can I just
infer its mass from like how much energy it has
and how fast it was going.

Speaker 2 (41:22):
So there are experiments that are going to try to
do exactly that, which we can talk about in a minute.
The oscillation experiments are just counting how many neutrinos have disappeared.
Neutrinos have such low mass that's very, very difficult to
measure them individually on a per neutrino basis. But there
is an experiment in Germany which is trying to do
exactly that.

Speaker 1 (41:40):
Okay, so then you're saying that we have measured kind
of the differences between the masses. So what are those numbers?

Speaker 2 (41:45):
Those numbers are really small. There's two numbers there. One
of them is ten mili electron volts. A milli electric
bolt is one thousands of an electron volts. The other
one is fifty milli electron volts. So some of them
has to be less than one hundred and twenty milli
electron volts. We know that the gaps between them are
ten and fifty.

Speaker 1 (42:03):
This feels like a fourth grade logic problem, like Sally, Paul,
and John money in their pockets and it adds up
to a dollar twenty. But the difference between Sally and
Paul is fifty cents, and between Sally and John is
Paul sense how much does Sally have exactly?

Speaker 2 (42:22):
And so we know that there's two possible solutions. We
know that two of the neutrinos are close to each other.
This is a small gap ten MeTV. We also know
that the third one is further away, it's fifty MeTV away.
We don't know if the two ones that are closer
are heavier or lighter, So like are the two ones
that are near each other at the top of the
spectrum or the bottom of the spectrum. We don't know.
There's two possible answers there. We also don't know quite

(42:44):
how it adds up, like the number we have from
the early universe is an upper limit, and they could
all still be very very low values. So there's a
lot of open questions there. We'd love to know the
sum of the masses of all the neutrinos.

Speaker 1 (42:57):
Well, you sort of just need to know one of
the masses right, and then that would click the other
ones in place.

Speaker 2 (43:02):
Well, there's still two possible solutions if you just know
one of them. You don't know if you have like
the inverted hierarchy where the two close ones are the top,
or if you have the other hierarchy where the two
close ones are at the bottom.

Speaker 1 (43:11):
Oh, I see, but you're saying, we know this very precisely.
Like our models of the neutrino. When you shoot a
bunch of them out and you see how many transform
into different kinds, dat somehow tells you the difference in
their masses because it, I guess it affects the probability
of these transformations.

Speaker 2 (43:26):
Yeah. And we've been doing these neutrino oscillation experiments for decades,
and we've done them in all sorts of ways, with
all sorts of different combinations. Make this kind of neutrino
disappear that kind of trino, make this kind of measure
the appearance of the other one. We've triangulated that whole matrix,
and we know exactly how these numbers work out. What
we don't know is the overall mass, only the differences.
So the differences are very precisely known. The overall mass

(43:48):
is limited by this Big Bang cosmology stuff to less
than one hundred and twenty mili electron bolts. But now,
this is really cool experiment in Germany called the Katrine experiment,
which is going to try to measure the mass of
the electron neutrino as precisely as possible.

Speaker 1 (44:03):
All right, let's talk about this experiment, and now what
is it? How does it work?

Speaker 2 (44:07):
So this experiment is called the Carlsrua tritium neutrino experiment,
which is a tortured way to make catrin as an
acronym to.

Speaker 1 (44:15):
Say the least.

Speaker 2 (44:17):
But it starts from tritium and tretium decays two helium,
which is like two protons a neutron, and then it
also produces an electron and a neutrino.

Speaker 1 (44:26):
And tretium is just an element, right.

Speaker 2 (44:29):
Yeah, Tritium is two neutrons and a proton, so it's
like an isotope of hydrogen. Basically, what happens is one
of those neutrons turns into a proton and then emits
an electron. And a neutrino, And this is a nice
way to measure the neutrino mass because the electron neutrino
don't have a lot of energy. The comp moving really
really slow, and so basically you can see the effect

(44:49):
of the mass of these particles on how fast they're moving.
It's like, not a whole lot of energy made in
this reaction, so not a lot of despair. So if
the electron and the neutrino have a lot of mass,
they'll come out moving slower. They have less mass will
come out moving faster, And so we can't see the
neutrino directly, but we can measure the electron energy very

(45:09):
very precisely. So that's why this experiment does. It measures
those electrons really really precisely, and if it sees electrons
moving with more energy, it means that the neutrino mass
hasn't taken up some of that energy budget. And if
it doesn't see electrons moving with sort of near the
maximum possible energy that this decay can make, it means
that the neutrino has used up some of the energy

(45:30):
budget that otherwise could have made the electron go faster,
and that means the neutrino has some mass, so it's
sort of like a way to measure the neutrino mass
by seeing how much energy it slurps out from this reaction.

Speaker 1 (45:43):
Okay, so let me see if I got this straight.
You start with an isotope of hydrogen called tritium, which
is two neutrons in the nucleus surrounded by an electron,
and then you just let it hang out and eventually
it's going to decay into a hydrogen atom like right,
Like one of those neutrons is just going to disappear,
transformed to something else. And you're saying that this reaction
shoots out an electron and an antineutrino, and the electron

(46:05):
we can measure its mass and speed because it's an electron,
and so whatever's left because we I guess you assume
a certain amount of energy at the beginning.

Speaker 2 (46:14):
We know very well how tritium decays and how it
turns into helium and how much energy is available. Yeah,
and we know that energy has to go to the
electron and the neutrino, and.

Speaker 1 (46:23):
So the difference between what you started with and how
much you measure the electron is the energy that goes
into the neutrino exactly, But then how does that tell
you the mass? It could just be like something light
moving fast or something heavy moving slow.

Speaker 2 (46:35):
Yeah, So there's a spectrum of possibilities, and what we're
looking for is the maximum scenario, Like are there any
cases where the electron takes all of the energy available?
There's like a certain energy budget for producing this subtract
out the electron mass, and then we wonder like, are
there scenarios where the electron takes all of the energy.
If we see cases where the electron takes all of

(46:56):
the energy, that means the neutrino hasn't taken any So
it's sort of like a budget. You have a budget
for the whole thing. The electron mask gets taken out,
then we wonder like does the neutrino take a cut?
If the neutrino takes a cut, that leaves a smaller
budget for the electron, and you'll never see an electron
having energy higher than that limit. The neutrino doesn't take
a cut, it leaves more energy for the electron and

(47:17):
you'll see faster moving electrons. So you look at the
tail of the distribution, like what's the fastest electron you
ever see, and that'll tell you how much the neutrino
is taken from the budget.

Speaker 1 (47:27):
M I think I get it. So, like you start
with let's say one hundred units of energy, and you
measure how much energy electron that comes out has, and
you look for, like, what's the maximum energy that the
electron can take away from this? And let's say it's
like ninety nine out of a whole bunch of times,
did you do this? Ninety nine is the maximum, which
means like the minimum amount of energy the neutrino can

(47:47):
take is one, which, since it's the maximum for the electron,
it must mean that it's like it created a neutrino
that wasn't moving at all.

Speaker 2 (47:56):
Maybe, And so they're looking for those scenarios like when
you make a motionless neutrino and the electron takes all
of its energy, that reaction reveals the mass of the
neutrino in the energy of the electron.

Speaker 1 (48:09):
It reveals I guess, the mass of an electron neutrino.

Speaker 2 (48:12):
Yes, it's revealed the mass of a neutrino created with
an electron. What does that really mean? Remember, the electron
neutrino doesn't have a definite mass, So actually what it's
measuring is a combination of all the masses of the neutrinos.
It's just like incoherent some of the distinct neutrino mass
values weighted by how much of each one is in

(48:33):
that electron neutrino. So remember electro neutrinos don't have a
definite mass, So you're measuring this like weird average mass
of a neutrino.

Speaker 1 (48:42):
If you're going to sort of for like the minimum
amount of mass that the neutrino has, then must be
giving you the minimum mass for one of them. Right.

Speaker 2 (48:48):
Yes, it's a bit of a subtle point of quantum mechanics.
The mass of that neutrino is not actually determined, right,
It's not like it has a certain number and we
don't know it. What we know is it's an electron neutrino,
which means we don't know what its mass is. And
so overall, on average, what you'll be sensitive to is
the average mass of those neutrinos. But you're right, what

(49:11):
we're doing is looking for the most energetic electron, which
means we'd be sensitive to the lower end of the
neutrino masses of that electron neutrino.

Speaker 1 (49:21):
Which would maybe give you like the lightest of the
three neutrino masses.

Speaker 2 (49:24):
Yeah, And what we're looking to do is combine this
with our measurements from neutrino oscillation, which tells very precisely
the separation between the neutrinos, and now we want to
anchor the overall scale and slide it up or slide
it down.

Speaker 1 (49:38):
But I guess even if you do, like you said,
there's two possibilities for the other two, right, so like
you might know the massive one one of the masses,
but you wouldn't necessarily know the mass of the other two.
But I guess you would narrow it down to two possibilities.

Speaker 2 (49:51):
Yeah, we'd narrow down to two possibilities. You're right, This
would still leave ambiguity for which higherarchy we have, Like
are the two close ones the top? Were the two
close ones at the bottom. So this experiment's been running
for a couple of years and they have some preliminary results.
Their measurement says that this mass they're measuring is less
than eight hundred milli electron bolts. Now that's not much
information because we already know from the Big Bang that

(50:13):
it's less than one hundred and twenty. This is just
sort of like their first result. They're going to keep
running the collecting more data, and they hope they'll be
able to measure this thing more precisely.

Speaker 1 (50:22):
Wait, so we know that they can be more than
one hundred and twenty, but the first measurements say it's
less than eight hundred.

Speaker 2 (50:28):
Yeah, so this is not as sensitive as the Big
Bang measurement so far.

Speaker 1 (50:32):
But it would be really weird if they found that
the mass in the try now is eight hundred million
electron bolts, because that's way too much.

Speaker 2 (50:38):
Yeah, exactly, this sets an upper bound of less than
eight hundred. We already know they're less than one twenty,
so it'd be pretty weird to measure it at like
six hundred or five hundred, you know. But these are
very very different measurements, right, the Big Bang versus like
experiments we're doing here on Earth. So it's not always
the case that they're going to agree. There's a lot
of theoretical assumptions that go into both of them. But
the good thing about this one is we keep running,

(50:59):
and so we can keep getting more and more precise measurements,
and so they're hoping by twenty twenty four twenty twenty
five they can get their sensitivity down to like two
hundred MEB and then they can push even further.

Speaker 1 (51:11):
Because I guess it's all statistical, right, and so just
the longer you run it, the more accurate you can
say what the minimum is.

Speaker 2 (51:18):
And this experiment is also super fun because it involves
this huge metal container. They shoot these electrons into this
mammoth vacuum chamber to measure their energy super duper precisely
this spectrometer. It required a really specialized shop to build
this thing. You should go online and google a picture
of this thing. It's like a big steel blimp basically,

(51:38):
and it was so big that it was really hard
to transport from the factory where they built it, like
three hundred kilometers to the experimental site. They actually had
to put it on a boat and float it down
river through the Mediterranean, out through the Atlantic over to
the Netherlands, and then up another river to the experiment.
So it's only like three hundred fifty twolometers away, but

(52:00):
I have to take like a nine thousand kilometer long
detour because it was too big to like put on
a flatbed truck and drive around.

Speaker 1 (52:08):
Wow, sounds like they should have thought about it before
they built it. I mean they have built it on site.

Speaker 2 (52:14):
Yeah exactly. But you know, you take specialized techniques just
to build this thing, and then specialized techniques just to
move this thing. There's some awesome videos of it making
its last seven kilometer journey across land from the docks
to the laboratory. They like squeezed it through these old villages,
you know, with like a centimeter to spare on each side.
It's pretty awesome, all right.

Speaker 1 (52:34):
Well, again, a neutrino is part of our standard model
of denvers and so, and it's also kind of like
one of the last frontiers in terms of what we
know about the standard model, right, Like, once we found
the Higgs boson and we know about all the matter particles,
the neutrino is sort of one of the last big
questions we have about it, right, and which means it
sort of helps complete our understanding of matter particles in

(52:56):
the universe.

Speaker 2 (52:57):
Yeah, you're absolutely right. It's the frontier particle physics, and
the US specifically has decided to double down on neutrinos.
We didn't build the next greatest best particle collider to
compete with CERN. Instead, the US has decided to build
big neutrino experiments to measure these masses, to measure the
neutrino interactions, to understand this weird sector of the universe
in more detail. We think there's probably a lot more

(53:18):
interesting hints.

Speaker 1 (53:19):
There, and so learning more about the nutrino what would
that tell us about the universe.

Speaker 2 (53:23):
Well, understanding the neutrino mass will help us understand the
Big Bang and like what was going on and the
neutrino contributions there. We also don't really know how the
neutrino gets mass, like does it get mass from the
Higgs boson the way other particles do, or does a
neutrino give itself mass? Like it might be that there
is no anti neutrino, that the neutrino is its own
anti particle. This is a fun story about a physicists

(53:45):
called Mayorana who thought about these Mayorana particles that might
be their own anti particles and give themselves mass in
this weird way. So it might even teach us about
what mass is for a particle.

Speaker 1 (53:55):
HM cool, And that's very important because it it would
tell us why we have right.

Speaker 2 (54:00):
Yeah, absolutely, it would tell us more about what the
meaning of mass is. They might also give us some
clues about the nature of dark matter. We know that
these three neutrinos are not the dark matter, but there
might be a fourth kind of neutrino, hysterile neutrino that
could be out there, and understanding the neutrino masses and
how they mix and interact with each other might clear
up some nagging questions about whether there are other flavors

(54:23):
of neutrinos out there.

Speaker 1 (54:25):
That would be massive. All right, well, we hope you
enjoyed that. Thanks for joining us, see you next time.

Speaker 2 (54:41):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.