- Hello, and welcome to Wednesday Nite @ the Lab.
I am very excited to be here.
Again, my name is Ellie Feitlinger.
I work for Wisconsin IceCube Particle Astrophysics Center.
And outreach and events coordinator.
I'm super excited to be here
to learn a little bit about IceCube,
the biggest and strangest detector in the world.
And I'm really excited 'cause
we get to hear from Justin Vandenbroucke,
and actually, a little bit about Justin.
He is an assistant professor at UW-Madison
Wisconsin IceCube Particle Astrophysics Center.
He's actually based primarily in the physics department
with a joint appointment at astronomy department.
A little bit about him though is really cool.
He received his PhD actually at UC-Berkeley
and eventually moved here to Madison in 2013.
In addition to his work at IceCube Neutrino Observatory,
Justin leads construction of a camera for detecting
the highest energy photons in the universe
and is also the leader of the Distributed
Electronic Cosmic Ray Observatory, DECO,
Citizen Science Project, which enables users
to detect cosmic rays with their phones.
So, let's join and welcome Justin Vandenbroucke
as he talks about Ice Cube and all of our discoveries.
(clapping)
- Okay, good evening, thanks, Ellie.
Thanks to all of you for coming.
I'm also really excited to be here.
I've been waiting most of my career
to be able to announce some news like this.
It's really exciting time for our field.
So I'm going to try a little bit of an experiment tonight
in science and in physics and astronomy.
We don't often tell stories in chronological order,
but I'm going to attempt to do that here.
You can tell me how well that experiment works
or doesn't work, so I'm going to borrow some techniques from
other storytelling disciplines like theater.
So we need to start our storytelling by introducing
who the main characters of the story are
and then setting the stage and the
when and where it happened in addition to who.
So starting with who, we have four
main characters of the story, starting with
this giant galaxy across the universe.
That's part of the title, this is called a blazar.
It's a giant black hole, I'll tell you more about that later.
Another main character, you've heard about from
Ellie briefly, is the Ice Cube Neutrino Observatory
at the South Pole, another main character is this
NASA satellite in orbit around the Earth.
That's called the NASA Fermi Gamma-Ray Space Telescope.
And the final and the fourth witting character in the story
is called the MAGIC Gamma Ray Telescope.
These are both gamma ray telescopes
Fermi detects high-energy gamma rays.
Magic detects very high energy gamma rays,
even higher energy from the Earth.
So now you know who is involved in the main story.
The next question is when and where it occured.
So, the answer to that is like
when and where many of the best stories occur,
and that is a long time ago in a galaxy far, far away.
So that's, the stage is set now, we can dive into the story.
Okay, but to characterize,
this is still astronomy and physics
so we still need to be quantitative.
So what do we mean by long, long ago and far, far away?
So let's give a brief review of
distances and times in astronomy.
So, I think you're all on the same page.
To start off with, a year is a unit of time.
We're all familiar with that.
But then, how do astronomers measure distance?
Once you have the time, being one year,
the way we measure distance is actually also
based on that unit of time, so we talk about light years.
That might be confusing because a year is a unit of time.
A light year is a unit of distance.
So, it's a very convenient distance.
It's just equal to the distance
that a light travels in one year.
Light year is a unit of distance equal to the distance
that light travels in a time of one year.
We're going to talk about neutrinos today also.
Neutrinos because they have very, very small mass.
They also travel at the speed of light.
They travel the same distance in the same time as light.
So that's our basic building block
of both time and distance in astronomy.
We can also make that much longer and much shorter
by just multiplying by large and small numbers.
So a year is kind of a medium distance of time.
A light year is a pretty long distance of space
in astronomy because we have such large distances.
We use this large building block to measure distances.
We need even larger distances than light years,
so we have a billion light years.
That's obviously a billion times a single light year.
So a billion years is a very long time.
A billion light years is a very long distance.
So we kind of have this obvious scaling relations.
We can also go in the other direction, and that's convenient
because we can just use different units of time.
So a minute is a short time.
It should be singular, one minute, as a short time.
And we can also just again scale by the speed of light.
So a light minute, now depending on your perspective,
that's either a long distance or a short distance.
By astronomy standards, that's a short distance.
By most of our standards, that's a pretty long distance.
So a light minute, is an eighth of the distance
all the way to the sun, so it's still a very long distance.
Light travels one light minute distance
in one minute of time.
Okay, so now we have kind of a
time scales and distance scales to go forward.
Okay, so starting early on, I'll skip over to Big Bang,
but we're still going to start pretty far back.
Soon after the Big Bang, galaxies started to form.
So a lot of you have seen this picture.
This is called the Hubble ultra-deep field image.
This is a large number of galaxies
in a very tiny patch of sky, very, very far away.
So this is now five to 10 billion light years away.
And the universe had already had enough time
in the first few billion years of its existence
to form these massive galaxies.
Not only that, but today, we know that
most of these galaxies have large black holes
at the center of them, so an amazing fact.
When I was getting excited about astronomy first,
20 years ago in high school, I was reading
Stephen Hawking's books, and he was talking about how
black holes were amazing theoretical constructs,
but who knew if they were a reality
or if they would ever be detected.
Now we know that basically, every galaxy that we've looked at
has a giant black hole at the center, that includes our own Milky Way.
And most of these are very massive.
Millions or even billions of times
the mass of the sun crammed into a single black hole
at the center of every galaxy.
So we call that a galactic nucleus.
Kind of analogous to the nucleus of a cell.
It's a massive thing at the center of the cell,
and even though it's smaller, it has very important mechanisms
to interact with the entire galaxy,
even though it's relatively smaller
mass and volume at the center.
Okay, so another fascinating thing about black holes
is that even though they're so massive and so dense
that light cannot escape from them, there's this amazing paradox there.
Despite that, they're actually the brightest objects
in the universe, how's that possible?
It's kind of like if you pull the drain on the bathtub
and the water spirals down into it,
there's matters spiraling down into these black holes,
and it's spiraling so strongly that it's glowing up
and it's heating up, and it's being accelerated.
And so it's generating huge amounts of matter and radiation
but actually escaping
and not falling down into the black hole,
but they're actually being accelerated.
and that's fundamentally because of
the gravitational force pulling all that matter
in a spiral down into the gravitational well.
And so a fraction of it, after being accelerated
doesn't fall all the way in, but it shoots back out again.
And often, it shoots out in these jets.
So you can see that this is the artist picture,
that jet coming up out of the black hole
to the upper right, is basically a particle accelerator
that's being accelerated by the black hole.
So kind of like the particle accelerators that
humans built here on Earth, but it's even more energetic
reaching even higher energies of particle acceleration.
So black holes are actually some of the
brightest objects in the universe.
Okay, so we've got some of the
building blocks of our terminology now.
You know what a galaxy is, it's like the Milky Way.
But other galaxies are much farther away.
You know, what the nucleus of a galaxy is.
That's that black hole, the center of
every galaxy as far as we know.
And then there's another third term that's important here,
and that's an active nucleus of a galaxy.
Some of these black holes are just sitting there
at the center of the galaxy.
Some of them have a lot of matter falling down into them.
And spewing out these accelerated beams
of matter and radiation, those are called active
as opposed to quiet or quiescent black holes.
So you put that altogether and you get some jargon, active galactic nucleus,
but it's just got three simple pieces.
So here's a side view of that, now a cartoon on the left.
You can see the black hole at the center.
You can see the two jets spewing out
perpendicular to the matter that's
spiraling down into the black hole.
If you look from along the side,
along the view of one of those jets from the right,
you have a top view of the same situation.
And at that point, this jet is pointed right at you.
And so that subcategory of these active galactic nuclei
is what we call a blazar.
It's basically like the barrel of a gun
just pointing straight at you,
having a particle accelerator being pointed straight at you.
And so a subcategory of these galaxies
with these giant black holes at the center
have a particle accelerator pointed straight at Earth,
and that's what we call blazars.
Okay, so now we've got kind of the basic background
of what happened here, and now we know the time scales
and the distance scales of all, so now we can start our story.
So the story started long, long ago
in a galaxy far, far away, and now we can say
exactly how long ago and how far away.
And the answer is it happened four billion years ago,
and the information about it just reached us.
It happened four billion years ago,
so the place that it occurred, the distance it occurred
was four billion light years distant from the Earth.
So that's when the story begins.
So a particular one of these active galactic nuclei,
that was also a blazar, emitted a neutrino.
So we'll come to what a neutrino is,
but just keep in mind that the distance and the time scale to begin with.
Okay, I'd like to go through all of the last
four billion years now, but I know you guys have other things to do,
so I'll try to compress it into an hour.
So we're going to pay attention to the next minute,
cause we're going to fast forward a lot very quickly.
So what happened over the last four billion years?
Okay, four billion years ago, starting point of the neutrino was emitted.
It was born at that massive black hole.
A little bit later, 3.8 billion years ago,
life began on Earth, the Earth had been
evolving as part of the solar system until that
cooling down from its original formation.
Fast forward more, this is an important part of our story later on.
Only a billion years ago, there were two black holes--
These are much smaller than the ones at the center of the galaxies--
but two black holes basically collided and merged with one another.
Okay, continuing with the story here on Earth,
half a billion years ago, fish evolved.
0.4 billion years ago, land plants
evolved for the first time.
0.2 billion years ago, dinosaurs and mammals
both evolved, roughly around the same time.
Dinosaurs and mammals here on Earth.
Another important astrophysical event,
0.1 billion years ago, two neutron stars merged.
So these are like black holes in the sense that
they're very massive and very dense,
not quite as dense as black holes.
So light can escape from them,
but they're basically like the giant nucleus of an atom,
similar density but on the scale of an entire star.
Okay, now things are a little more interesting for humans.
0.002 billion years ago, Homo sapiens evolved,
and that included astronomers.
You have to have a lot of digits here.
So this is, if you count the zeroes,
this is 0.2 million years ago, or only
200 thousand years ago, tiny fraction of the time
that this neutrino had been traveling across the universe.
I say astronomers were included in the Homo sapiens
because a lot of those Homo sapiens
looked up at the sky and so they were doing astronomy,
especially the ones who got more systematic
than just looking at it, but they tried to make
systematic observations and write down when things
appeared and when they changed and so on.
Okay, another important milestone for the astronomy side
is that a star in the large Magellanic cloud,
a nearby galaxy, exploded at the end of its life.
This also happened at the same time that the first
humans and the first astronomers appeared.
Okay, more recently, about 2000 years ago,
a particle physicist evolved, and notice
that they evolved more recently than the astronomers.
(laughing)
You can take from that what you want to.
Okay, so I'm counting as particle physicist,
the Greeks basically, there's one, especially Democrates,
who had a theory of atoms, and he believed that
matter was made fundamentally of particles.
If you don't count Democrates as a particle physicist
'cause he didn't have his own accelerator,
you can wait until about 40 years ago
when modern particle physicists evolved.
And so they had giant particle accelerators.
And that's when basically our modern understanding
of particle physics arrived, and that's called
the standard model of particle physics.
Okay, so what is that standard model of particle physics?
Here it is, I was pretty bad at chemistry.
The periodic table was way too big for me,
and I had a terrible memory, so I really liked physics,
with its very small version of the periodic table.
It's really simple, they're only a few building blocks.
And in fact, you can actually ignore most of this.
So what is matter made of?
It's made of atoms, the atoms are made of nuclei
consisting of protons and neutrons
and also, the atoms have electrons surrounding them.
So we have the quarks that make up the neutrons and protons.
Those are the up and the down quarks.
And then we have the electron.
So if you only care about normal matter,
about the pieces of normal matter,
you can basically only pay attention to these three.
Here, we care about more than just the building blocks of normal matter.
So it gets a little bit more complicated.
In addition to these three pieces the matter is made of,
there's also a neutrino, which is associated with those.
So it's also a kind of piece of matter,
but it's not what matter is made of.
It comes out from interactions between matter.
Nuclear interactions between matter.
And it's a very important part of how the sun shines.
So it is an important part of the behavior of matter,
just not what matter is really made of.
And then a really kind of strange
and a thing that's not totally understood today,
is that we basically have three copies
of that same strip of particles
which is repeated three different times.
So you have that up, down electron and electron neutrino
on the left column, and then you repeat that two more times
and that includes this muon and the tau.
And so the muon is basically like
a very heavy version of the electron,
very similar physics to the electron
but a much heavier version.
That muon is also an important part of the story here.
Okay, so I focused on the particles
that are in black for this talk.
We have on the left, the parts that make up normal matter.
In the middle is the muon and the muon neutrino.
These are basically heavier versions
of the electron and the electron neutrino.
And so they organize into these different columns.
And because neutrinos have very little mass,
they basically travel at the speed of light,
very, very slightly below the speed of light.
Okay, that brings us to another amazing fact
of particle physics, which is that
at the very highest energy, very highest energies of light,
light is actually absorbed by other light.
Something we never experienced in everyday life.
But imagine you're in a dark room, and you turn on a beam of a flashlight,
you turn on another one, and you make their beams cross
and one beam is blocked by the other beam.
Something you would never have any intuition with in everyday life.
But we've measured it in particle physics, and we know that that occurs.
We've also calculated it, and so it all
agrees with our understanding of modern particle physics.
So, the very highest energy type of light
is absorbed by crossing paths
with lower energy types of light.
And that's an important problem because
for the frontiers of physics and astronomy,
we want to understand the highest energies
and the longest distances, and we can't do that with light
because of this, so if you go to the highest energies
and longest distances, light simply
cannot travel across the universe.
It's blocked by other light, by low energy starlight.
So we need something else other than light.
And one thing we can use for that purpose is neutrinos.
They're kind of like super X-Rays.
X-Rays, you're familiar with from medical imaging
and other applications, they travel straight through
a lot of matter, so they let you image the interior of objects.
Similarly the neutrinos travel straight through the universe
including through matter and light.
So they're not blocked by other light or by other matter.
Okay, so where are neutrinos produced?
They're not part of,
they're not a building block really of normal matter.
Even though they're made of matter, they're produced by interactions,
by nuclear and particle physics interactions
around the Earth and around the universe.
So, this is a graph of, you can think of it as
how likely a neutrino is to collide with an atom
on the vertical axis as a function of how energetic the
neutrino is on the horizontal axis.
So they were produced abundantly in the Big Bang,
starting all the way on the left.
They're produced by radioactive reactions,
both man-made reactors, also naturally occurring
ambient radioactivity, terrestrial radioactivity.
They're produced in the sun as part of
the fundamental mechanism of how the sun shines.
They're produced by exploding stars, supernova.
And they're also produced when energetic particles
hit the Earth's atmosphere and trigger particle physics interactions
naturally high in the Earth's atmosphere.
We get a bright glow of neutrinos from that.
We can also produce them artificially not only from reactors
but also using particle accelerators on Earth.
And what we're interested in
in the field of neutrino astronomy
is a few of these, especially the very highest energies,
but starting off at medium energies,
the sun and the supernova, they glow in neutrinos.
But what I'm going to focus on for IceCube is the very highest energy range,
where there's galactic and extra galactic neutrinos
that we think are produced by
particle physics interactions throughout the universe.
Okay, to give you a benchmark scale for this
interaction probability on the vertical axis,
neutrinos at medium energies can travel huge amounts,
huge distances through dense matter.
So even if you have lead, one of the
best materials for absorbing X-Rays and other particles.
Even if you have a light-year of lead, a neutrino can travel all the way through it
at these intermediate energies.
If you go to even higher energies, they'll travel shorter distances,
but that's the scale at medium energies.
Okay, how do we detect the neutrinos?
We use Cherenkov radiation.
So, a good analogy for what's Cherenkov radiation,
if you have a boat or a duck traveling through the water,
and you don't see that boat or the duck
but you see the wake of it, you can basically triangulate
and say there must be a boat or a duck here
because there're some waves that are propagating
with this triangular pattern from a point that's right here.
The same thing happens with charged particles
that are traveling through matter.
So they make effectively a wake of electricity in their path
and so if you detect, and that wake of electricity produces
visible light, blue light and ultraviolet light.
And so in the case of energetic particles
traveling through matter on the right,
that's actually a picture of a nuclear reactor
which is emitting a lot of these particles,
and they're glowing in this blue light, which is the Cherenkov radiation,
analogous to the wake of the duck.
And so you can figure out there must be a,
energetic particle or a neutrino that produced that.
Okay, so this is now the foundation of
what we call multi-messenger astronomy.
So ever since those first Homo sapiens,
those first astronomers started looking up at the sky,
they were doing astronomy with different types of light.
They expanded to other types of light,
including energetic light, like X-Rays, low-energy light
like radio waves, but those are all different types of light
Now with particle physics, we can use
other types of particles to do astronomy,
rather than only light, and so we can use
cosmic rays, these are energetic nuclei
that are accelerated in these large,
naturally occurring accelerators,
like the black hole that I showed earlier.
We can also use neutrinos as messengers to do astronomy.
And finally, we can use gravitational waves.
That's another exciting, new part of this field
of multi-messenger astronomy.
There's a lot of different science we can do
with this new type of multi-messenger astronomy.
One of the most important topics that we can study
is the origin of cosmic rays and what their sources are.
So we know that they're these very energetic particles
we call cosmic rays, hitting the atmosphere
and hitting the Earth all the time.
We don't know exactly what's producing them
or how it's producing them, but we think that those sources--
Here's a cartoon on the upper left again of this galaxy
with the jet of matter and radiation coming out of it.
We think that that's producing not only
cosmic rays but also types of light and types of neutrinos.
And so by studying those, we can understand the sources
and the accelerators out in the universe.
And it has, in the case of neutrinos,
has this advantage of being a kind of super X-Ray
that can go straight through any intervening matter
or any intervening light, even though the
high energy light would be blocked
by what's in between us and the particle accelerator.
Okay, so that's the idea of multi-messenger astronomy.
So how does neutrino astronomy,
how does neutrino astronomy work?
Here's a cartoon of what's going on in the atmosphere when
one of these particles, these cosmic rays hit the Earth.
So you can see for example a proton
coming from outer space as a cosmic ray
colliding with the atmosphere,
and it undergoes a particle physics interaction
to make a bunch of particles.
Some of them are absorbed in the Earth, but some of them travel all the way down,
absorbed in the atmosphere.
Some of them travel all the way down
to the bottom of the atmosphere and reach the ground
and even keep going into the ground.
And those are neutrinos and muons.
So those are some of the particles we're most interested in.
Now we can detect both of those with a detector like IceCube.
We can detect both muons and neutrinos.
One challenge is that we have a much higher rate
of these muons compared to the neutrinos
that we're interested in, so we detect about
70 billion muons per year.
We have to detect these atmospheric neutrinos,
the neutrinos that are produced in the atmosphere
as a very tiny fraction of those, about one in a thousand.
And then out of those, we're looking for the
astrophysical neutrinos that started
above the Earth's atmosphere, out in these cosmic sources.
So that's only one in 10 to the eight
out of those atmospheric muons.
And so we have to find a needle inside a haystack,
and so it's a big experimental challenge.
And of course, that's on top of the fact
that these neutrinos are very unlikely to interact
with matter to detect them.
Okay, coming back to our timeline and our story.
So, 31 years ago, after kind of this original idea
of developing the field of neutrino astronomy,
people-built detectors that could use
Cherenkov light to detect the neutrinos.
And they built one in Japan called the Super-K detector.
And if you remember that star that exploded,
around the time that humans were first evolving
in the large Magellanic cloud,
the humans had evolved, the particle physicists had evolved.
They had built this Cherenkov detector.
And then that burst of neutrinos from that exploded star
arrived at that detector, so that happened 31 years ago.
So that was one milestone in neutrino astronomy,
that we could detect this first source
of extra terrestrial and extra galactic neutrinos.
You could argue that this is not neutrino astronomy
because those detectors couldn't figure out
where the neutrinos were coming from exactly.
They knew they were from this supernova
because they saw the supernova in bright light at the same time
but they were using timing rather than direction.
Okay, continuing forward with neutrino astronomy,
there was another milestone where another
astronomical object was detected 20 years ago and this is the image of that.
Any guesses of what this astronomical object is?
- [Audience Member] Sun?
- The sun, yeah.
I mentioned there were two medium energy sources
of neutrinos, the supernova and the sun.
So this is an image of the sun and neutrinos.
The sun is actually much smaller than you can see here,
not because the neutrinos are glowing in a halo much bigger than the sun,
but just because of the angular resolution of the detector.
So they can't actually image with the same resolution
as the emission region of the sun.
But at least you can say if they're coming from
in that direction in the sky, and that is where the sun is.
Okay, moving along in the timeline.
27 years ago, it was proposed to do this in ice
rather than in water, and the detector
that was led by the University of Wisconsin
along with a couple of other universities,
they installed some sensors into Greenland ice,
and they detected Cherenkov light
from muons from the atmosphere.
So they proved that you can do this technique
in ice, not only in water.
That was an important milestone led
right here at UW-Madison.
17 years ago, another mile, another important milestone.
They realized that the ice was even better at the South Pole
than in Antarctica, it was even clearer,
and there was more of it, and so
they installed an array of detectors.
You can see it's a bigger group of
collaborating institutions and funding agencies at this point,
but it was especially enabled by WARF here at UW-Madison.
So they built a detector called Amanda,
and that Amanda detector at the South Pole
detected neutrinos for the first time
using this Cherenkov detector in the ice.
So it's just a flash of blue light
due to a neutrino colliding with the ice.
And those neutrinos now were coming from the atmosphere.
So they're not just those background muons
that were easy to detect,
now we're detecting neutrinos from the atmosphere.
But they're still background in the sense
that they're not the neutrinos coming from outer space.
They're still from the atmosphere.
That was 17 years ago.
Okay, so that brings us to our second player in the story.
We have the super massive black hole, the blazar.
And this is now the IceCube Neutrino Observatory.
So this is not to scale, but there's a big detector
in the middle of Antarctica, this is now the successor
of the Amanda experiment, a bigger version.
It's sitting right at the South Pole,
in the middle of Antarctica, and it's using
a large volume of that ice as a detector
for this Cherenkov light, why ice?
Because you can straight through it.
It's one of the clearest naturally occurring
large volumes of material on Earth,
so that you can monitor huge volume of ice
for this very rarely interacting neutrinos.
Okay, so this is the ice, diagram of the IceCube detector.
It's basically an array of individual light sensors.
They're about 5000 individual light sensors.
And continuing with our timeline, this detector
was completed about 10 years ago.
A huge effort by a lot of people around the world,
a really successful construction project
that culminated in 2010.
So it's a simple, pretty simple concept
but very difficult engineering to achieve,
installing 5000 different light sensors,
spanning a cubic kilometer of ice,
monitoring a billion tons of ice for this neutrino interactions.
Here's how the detector was constructed.
It's again a simple concept, but an enormous
engineering challenge, and this was
a success thanks to the physical sciences lab here,
in Stoughton, just outside of Madison.
So the idea is that you heat up water
to very high temperature and very high pressure,
and you squirt it on the ice.
And then you make a hole, you lower it farther,
you keep squirting, and you just do that
for a couple of days, and you drill down
a mile and a half by doing that.
And then you'll just do that 86 different times
over seven years. (laughing)
Really, again, really simple concept,
a very huge engineering challenge.
So there're about 50 drillers on this team
working around the clock in harsh conditions
at the South Pole to pull this off.
Here's just a movie of what that process looks like.
A cartoon, so squirting that hot water,
First you start with kind of a hot butter knife approach
because there's a layer of snow on top.
If you squirt hot water that'll just seep out into the edges.
Then you pull that hot butter knife approach out again
and go with the hot water hose approach.
And now this is the fully-densified ice,
that's been compactified after it fell as snow at the top, and it's just solid ice.
Squirting that hot water and just continue squirting it
all the way down a mile and a half deep in the ice.
So those holes are a mile and a half deep
and about 67 centimeters across.
And it's hard to have a feeling for what those numbers are like
until you're standing over one of the holes
and looking down, and they're just barely wider than my shoulder.
So I had the chance to work there for three seasons during construction,
a lot of fun but a little bit scary to stare down one of those holes.
(laughing)
Here's what it looks like from the top.
So imagine a mile and a half all the way down
to the bottom of the ice there.
Here's what it looks like from the surface.
There's the new South Pole station.
This is the Amundsen-Scott National Science Foundation
South Pole Station, so that's all here.
It houses scientists and support staff.
There's a runway where airplanes can land and take off.
And then across that is where IceCube is.
So it's deep under the ice here, a mile and a half down,
but there's a computing building at the top
where all of the data come.
Okay, so this is a large collaboration.
This is big science, and it takes a huge team of
great people to pull off all of this engineering and science.
As of today, consists of 12 countries on four continents.
It's about 49 different universities and labs
around the world and we have about 300 scientists
who are collaborating together.
Why is IceCube so deep at the South Pole?
It's buried a mile down.
First of all, you need a large volume,
but even that large volume actually displaced
down in the ice, we don't instrument the top of the ice
and that's for a couple of reasons.
The ice at the lower depths is actually bubbly.
It's more like the ice that you would get in your freezer.
It's kind of milky and opaque because it's got little bubbles.
But if you go deeper, if you go at least a kilometer down,
it's crystal clear, and you can see for hundreds of feet through the ice.
And that's what you need to detect this Cherenkov light.
We're also using the upper layers of the ice
as a shield for this background muons.
Remember we're more interested in the neutrinos than the muons.
And so those muons are absorbed by the upper part of the ice.
Okay, there are two different types of signals
that we detect in IceCube, one is what we call a track
and one is what we call a cascade.
And you can see they look different, they have different shapes.
I won't get into the technical aspects of it,
but they have different advantages and disadvantages
in terms of the science that we can do with them.
But they're both signals that are produced by
neutrino interactions in the ice.
Okay, so continuing with our timeline,
we were constructing IceCube 10 years ago.
We completed it about seven years ago.
And very soon after we completed construction,
we made the first discoveries.
We discovered the first astrophysical neutrinos.
Remember these milestones, atmospheric muons,
atmospheric neutrinos, now finally, astrophysical neutrinos.
That was on the cover of these two
scientific journals that are important in our field.
You can see the pictures of these,
of these depositions of energy
from these neutrinos interacting in the detector.
Remember this is a huge volume, right?
It's a kilometer across and these neutrinos are so energetic
even though they're a subatomic, fundamental particle
much lower mass than an electron,
they still have so much energy that they light up
this entire kilometer across of light instrumentation.
If you want to know what a cubic kilometer
across neutrino interaction looks like for human scale,
you might recognize this landmark that its overlaid on,
on top of, this is the Memorial Terrace,
and this is Lake Mendota, and this is
a large fraction of UW Madison campus and Madison.
So you can imagine the single subatomic particle
lighting up the entire sky over Madison.
It's got energy that-- Each one of these neutrinos
has energy that's about a thousand times
more energetic than the largest energies
we reach on Earth with human accelerators.
Okay, where are they coming from?
That's the most important first question.
We discovered that there is this glow of neutrinos from the universe.
The next thing we want to do is actually do astronomy with them,
answer the question what's producing them.
So we do that the way astronomers answer that question,
and you just make a map of where everything's coming from.
You image the sky in neutrinos rather than in light.
And this is what we get. This is a simple projection which is just taking the Earth
and projecting it out into the sky.
And so the North Pole would be up on the top,
the South Pole on the bottom.
So the galaxy goes across that view.
It's marked in that as that solid black line,
and you can see the galactic plane there.
The first conclusion you can draw really simply from this picture
is that they're not coming from the galaxy.
They're not all following that line.
So they must be coming from beyond the galaxy
because they're distributed evenly across the entire sky.
So that's a really important first conclusion about what's producing them.
But for the most part, we have not been able to figure out
what's producing them, that's remained a mystery.
Beyond this first step of saying that they're
evenly distributed across the sky,
therefore, they're beyond the galaxy.
We also did another technique.
So this first technique was using
what we call those cascades, that one spherical deposition of light
that I showed you a few pictures of.
We also use this other technique, which is when
a neutrino converts into a muon inside the detector.
And that produces this long, straight line of light,
and we can do that, we can image neutrinos
coming all the way through the Earth.
Remember, they can travel through large distances of matter.
The neutrinos travel through the Earth,
then they interact in the ice to produce a muon,
and then we can measure that muon in a long, straight track
and calculate the direction of the original neutrino.
That's a powerful technique because we block out
all the muons because the original muons from the atmosphere
cannot travel all the way through the Earth.
Only a neutrino could travel all the way through the Earth.
So in this cartoon, we're looking through the Earth
in order to shield ourselves from
the muons produced in the atmosphere
in order to only detect neutrinos
that can travel all the way through the Earth.
So we did that approach, and we also discovered
the signal again basically with the complementary method.
Okay, continuing with our timeline
and the story of multi-messenger astrophysics,
if you remember this in that first fast forward,
there was a billion years ago,
two black holes that collided and merged,
and when they did that, they launched ripples in
basically in the fabric of space and time.
And those are known as gravitational waves.
Those spent the last billion years propagating across the universe,
and they finally reached the Earth three years ago.
And at that point, the astronomers
and the particle physicists had evolved highly enough
that they could build a really good detector,
and they were ready and they detected that
ripple of gravitational waves.
And so that was a huge discovery three years ago,
and it got the Nobel prize last year.
That's an important part of the story of multi-messenger astronomy.
Okay now, coming up to two years,
we're getting closer to the present.
Two years ago, IceCube had basically established
this diffused glow of neutrinos from the cosmos,
done a lot of different analyses
to try to understand what's producing them,
and that's using combining the neutrinos
with different types of other signals
including cosmic rays and different types of light.
And basically, we've been slowly ruling out
individual types of sources, and so it's growing
as a bigger and bigger mystery
of what's actually producing these neutrinos.
And so we realized a couple of years ago,
we can do this better if we partner with our
collaborating astronomers at other telescopes,
not only just looking at data we've collected
but to do it in real time.
So instead of taking years of work,
to analyze the data to figure out
when and where the neutrinos came from,
can we do that all within a few seconds
rather than a few years.
It was a big effort by a lot of people in IceCube
to get this infrastructure up and running.
We're now, when a neutrino hits the ice,
we can calculate the direction that it came from
within a few seconds, and not only calculate it
but release it to the world in real time
and tell them when and where to look on the sky
in case there's something still interesting happening in that direction.
So we implemented that system two years ago,
and since then, there've been 14 interesting neutrinos.
Each one of them, within a minute of the time
the neutrino collides with the Earth,
IceCube automatically calculates where it came from
and publishes a website where astronomers can basically say,
"Let's point our telescope in that direction."
It's totally open, so any professional or backyard astronomer
can point their telescope in that direction.
Okay, we're still in the meantime trying to figure out
what's producing these neutrinos.
So a year ago, we published our
most stringent search for that.
And we basically do that by looking for clustering
in individual directions on the sky.
And so this color scale indicates clustering,
if there's any clustering of particular direct,
in particular directions on the sky of neutrinos.
And that would indicate an individual source of neutrinos.
And even though your eye can pick out
individual clusters here, none of them
are significant statistically.
It's kind of like how you can see shapes in clouds,
but they're just random shapes.
And so as of a year ago, we had still never identified
a single source of this very high energy neutrinos.
So we're getting closer up to the present now, a year ago.
Here's a cool movie, showing those 14 different
alerts that we've published to the web.
So you can see over the last couple of years
how they've occurred at different positions on the sky
at different times, and so you can see,
we're detecting them from all over the sky
and at a roughly random time along the horizontal axis.
Okay, that brings us to next milestone.
If you remember in that fast forward timeline
over the last four billion years,
there was also a really astronomical event happened
which is that after those two black holes merged,
there were two neutron stars that merged.
And that light traveled across the universe,
and it finally just reached us 12 months ago.
That was another really big milestone
in multi-messenger astronomy.
You've probably heard about that.
That was exciting because not only
gravitational waves were detected from
that merging of two neutron stars
but also flashes of light from across the entire
electromagnetic spectrum, so for the first time,
we've seen both gravitational waves
and light from the same astrophysical event.
So that brings us up to 12 months ago.
Now we're coming towards the culmination
of this four-billion-year story.
After that four-billion-year-old neutrino had traveled
across those four-billion light-years of distance,
it was really tired after that long travel. (laughing)
It was finally ready to lay down to rest,
and it collided with a molecule of ice
inside the South Pole next to the--
inside Antarctica in the middle of this--
in the middle of Antarctica, near the South Pole,
outside the IceCube detector.
It produced a very high-energy muon.
That muon traveled through the detector,
and it lit up the neutrino, it lit up the detector.
So here you can see all the different light sensors
in IceCube that were lit up by that detector.
And you can see just by eye that
if you draw a line through this energy pattern,
the neutrino must have come from over here.
The color here represents that
it came first in this position, the red,
and it came last in this position, the blue.
And so we can do a more sophisticated than just
drawing a line by eye, more sophisticated analysis.
And we can figure out it came from this particular direction
seven degrees below the horizon or six degrees below the horizon
within one degree of uncertainty.
We also measured its energy as a very high energy,
again hundreds of times more energetic than the
highest energy particles we've produced
at accelerators on Earth.
And so again this whole infrastructure was set up
so that we were ready after this four-billion-year journey
to detect this neutrino, so it collided with the ice
that made the detector light up.
The computers on top of the detector
calculated what direction it came from.
Those computers published a website to the public.
It did all that within 43 seconds.
And then some other humans read that website
and said, "That's really exciting. Let's point our telescopes over there."
Some other telescopes didn't even need a human.
They just said, "That's really interesting.
"We're going to point ourselves over there without a human,"
because there are actually some robotic telescopes today.
So you might say that's the next step
in the evolution of astronomers.
Okay, so here's what came out of that.
So 23 different observatories around the world
pointed in that direction.
IceCube's lucky because it can see the entire sky.
A lot of traditional telescopes can only point
at a particular direction in the sky.
That's why it's really important to
release this information within a minute.
So that they can point in that direction
while it's still interesting.
A lot of different telescopes around the Earth
and also in orbit around the Earth,
those ones on top of the Earth are in orbit,
a lot of them pointed in that direction, and most of them
detected something from that same direction.
There's something interesting going on
in the direction of that neutrino.
So, that brings us, we've gone through
the blazar, we've gone through the IceCube detector.
Those are the first two characters in the story.
That brings us to the third character
which is the Fermi Gamma-Ray Space Telescope
operated by NASA, so that's in orbit around the Earth.
And it's basically scanning the whole sky all the time.
Scientists looking at that data
pointed out that there was one of these blazars
in the same direction as that neutrino.
And so that's how we identified that it's likely
that it was a blazar, this massive black hole,
that's what actually produced the neutrino
coming from that direction,
thanks to this NASA satellite, Fermi.
That's detecting high-energy gamma rays.
There's another telescope on the ground,
the MAGIC Gamma-Ray Space Telescope,
which motivated by the IceCube Neutrino signal
in the MAGIC high-energy gamma-ray,
and the Fermi Gamma-Ray, high-energy gamma-ray signal,
it searched for even higher energy gamma rays,
very high energy gamma rays.
I know we use technical language, but in this field.
There are numbers associated with that,
but MAGIC detected even higher energy gamma ways
coming from the same direction.
So the Fermi telescope identified
that there was a known blazar in that direction.
We already knew there was a blazar in that direction.
But no one had ever detected a neutrino from it.
And so that's the first time that we've identified
an individual source at these very high-energy neutrinos.
Here's just a map of what that looks like,
a map of that region of the sky.
You can see the Fermi data is in the color scale here.
So purple and orange represents
the brightest gamma ray emission from that,
from different directions.
You can see these contours indicate where
we think the neutrino came from.
So with high confidence, it was within this red circle.
With medium confidence, it was within this gray circle.
Our best estimate of where it occurred was
at this green position, and this NASA satellite
knew that there was this blazar at this circled position
based on this bright distribution of
individual gamma ray events from that direction.
Okay, so you might ask, there are a lot of these black holes
on the sky, isn't it possible that that neutrino
just happened to come from the direction
of one of those black holes, it's just a chance coincidence.
And that's a really important question in astronomy because
unfortunately, to astronomy, the world is basically
two-dimensional, so everything is collapsed
onto a projection of the sky, right.
We don't know for the most part,
how far away something is that we detect.
It could be that it came from something
that's just roughly the same direction on the sky,
but it might be far behind or far in front of
the actual source that we're interested in,
the black hole that we're interested in.
So we basically spent the next several months--
A lot of astronomers in this field
spent the next several months after September of last year
calculating what's the probability
that this is just a random chance coincidence.
And we estimated that it's about one in a thousand.
So it's pretty unlikely that this is
a false association that we've determined.
But it's pretty unlikely, and as scientists,
we'd like to be more confident.
So we'd like to be, like to beat that number down
and say that it's like one in a million to be,
you know, you never prove it, you never have one in infinity
but the higher you can make that number,
the lower you can make the probability that you're wrong
about understanding what's going on here.
So that's what we want to do next, is basically increase our confidence
that we understand what's going on.
We also know that there are--
By doing a quantitative analysis,
we know that there are at least 100 different sources
producing the diffused glow of neutrinos that we've detected
So this is just the first one that we've detected.
So we want to understand this one better.
We want to detect the other hundred or more that are out there.
We want to understand what's producing all of the neutrinos.
We think that these black holes are actually only
producing a small fraction of the total flux,
the total emission of neutrinos.
And so we want to understand what the other
mysterious classes of sources are there out there.
Okay, but there were actually two--
The story continues because there were actually
two different papers that were published
on the same day back in July.
Some of you might have heard the--
seen the newspaper articles and seen the headlines
that were sparked by these two papers.
The first one describes the story that I've told so far.
And that was kind of old news for a lot of astronomers in the field
because of these public webpages.
So all these telescopes had been writing public webpages
saying we identified something interesting
from the direction of that neutrino.
But there was another exciting part of the story
which is that within IceCube, we had also gone
back in our data, not only looking at that
2017 single neutrino that we detected
but saying if there's a neutrino coming
from this giant black hole in this direction,
what about the last decade of data that IceCube's been collecting?
Were there ever previous neutrinos from that same direction?
And that's a hard question to answer because of this
very high rate of background nuance
and background neutrinos from the atmosphere.
But once you have a precise direction to look in,
you can do a sophisticated statistical analysis,
and we did that and we found there was
an excess of neutrinos from that direction
even before this 2017 arrival of this neutrino.
So we detected a burst of neutrinos that happened.
It lasted for several months, and it came from the same direction.
Here's kind of a picture of what that burst looked like on the sky.
It had only about 13 neutrinos associated with it.
But that's enough to be an excess
above the background rate of other neutrinos
from that position on the sky.
So it lasted for about five or six months
during 2014 and 2015, long before
we detected this high-energy neutrino in 2017.
So that boosted our confidence that we really understand what's going on here,
that there's a black hole that's producing all these neutrinos.
But that's just the first step.
I should say that we don't really understand because
in this case, there were actually no gamma rays
detected by the gamma ray detectors
from the same direction at the same time.
So there's something very subtle and exciting
going on here that we don't understand yet.
Okay, so to recap, here's kind of the timeline
of neutrino astronomy focusing on the story in Antarctica.
This idea of doing astronomy using neutrinos,
using Cherenkov light, using the huge
naturally occurring ice at the South Pole,
that idea first started in 1988.
Then we had this predecessor detector, Amanda.
That reached an important milestone of detecting neutrinos
from the atmosphere for the first time,
proving the concept of this technique.
2011, we finished building this large
engineering and physics undertaking.
An important milestone five years ago then the first
of these neutrinos from beyond the atmosphere were detected.
And finally just this year, we've announced the first
identification of an individual source.
So I put a dot-dot-dot here because
this is kind of still the beginning of neutrino astronomy.
It's the first time we've identified a single source
of these very high energy neutrinos.
It's kind of like detecting a star for the first time in astronomy.
So we think it's the beginning of a new field of neutrino astronomy.
So how do we go farther from here?
We want to keep taking more data with IceCube of course,
understanding the other hundred sources that we think
are out there, understanding other types of sources
beyond these giant black holes.
But it's pretty slow progress with these very
small probability of neutrinos hitting an atom.
And so we want to build an even bigger detector that's called
the next generation of IceCube, IceCube Gen 2.
You can see in the middle here,
that dense array, that hexagonal array
of detectors in the middle viewed from the top
is IceCube, and we want to surround it with
additional light sensors surrounding it
in order to have a bigger volume to collect these neutrinos.
Okay, before I conclude, I just want to give one advertisement.
Ellie mentioned this quickly.
If you want to do particle physics,
if you want to do astrophysics,
you don't need to build IceCube yourself or join us.
You can actually use your own detector in your pocket.
That's called DECO, the Distributed Electronic
Cosmic-Ray Observatory, and here are some pictures
that we've collected with DECO.
The idea is that you run an app on your cellphone.
When a particle travels through the
camera sensor on your phone,
it works very similar to IceCube on a much, much smaller scale.
And it can detect individual particles.
And so here are some examples, these pictures on the right
are individual particles we have detected with DECO.
And on the right is a picture of all the different places
around kind of the region of Madison
and the Midwest and the east where
users are detecting these particles.
You can download it at that website if you're interested.
Okay, so there's a cliché in multi-messenger astronomy
which is that we're opening new windows on the universe.
But clichés of course are coming to become in place
because they're true, so I think
in this new era of multi-messenger astronomy,
we really are opening these new windows.
It's new ways of seeing the universe
using all of our senses rather than only using sight.
We're also starting to use hearing and taste
and all these different senses, feel maybe,
beyond just looking, and of course,
an analogy to our everyday experience,
if you can do that in astronomy, then you can learn much more about the universe
than just using one of our senses.
So here's a cool image of that,
kind of taking the analogy literally.
New windows on the universe.
These are stained glass windows on the universe.
On the left, it's a neutrino detector.
In the middle, a gravitational wave detector,
and on the right, a gamma-ray light detector.
So to conclude, neutrino astronomy is providing
a new view of the universe, it's kind of this
super X-ray vision of the universe.
It's giving us totally complementary information,
new information beyond what we can do with light.
And when we do this new type of astronomy,
we found already that the universe is full of these other signals.
It's not just full of light, it's also full of
gravitational waves and neutrinos and particles.
We've identified the first source. That's an important milestone.
But, we still have important questions about that source.
We want to beat down that probability,
that false probability to much smaller levels
so that we can be really confident that we understand
that this is a true detection, even though we think it's
unlikely to be an accidental detection.
And we want to detect more of these sources and more of the classes of sources.
And this new era of observing the universe
with all of our senses is just beginning, thanks.
(clapping)