This year's Nobel Prize is for building the largest telescope in the world in order to see the most elusive particle in the universe.

It's for neutrino astronomy, which is already poking at the secrets of black holes and at the edges of quantum theory.

The universe is firing bullets at us.

Mysterious processes accelerate particles to near light speed and send them careening through the cosmos, cosmic rays.

We see them slam into our atmosphere.

We watch the cascade of particles created.

The most energetic of them carry millions of times more energy than the particles in the Large Hadron Collider.

There are vast natural particle accelerators out there that presumably we could understand if only we could find them.

The problem is that these high-energy cosmic rays are electrically charged atomic nuclei, and they bend and bounce as they travel through the tangled magnetic fields of the galaxy and the cosmos beyond.

The bullets ricochet.

So by the time they reach us, there's no tracing them back to the smoking guns of their origins.

But there's good news and also bad news.

There is an uncharged particle produced in those same energetic events that produce the cosmic rays, a particle that's immune to magnetic fields that can travel at near light speed straight towards us.

It's the neutrino.

Unfortunately, the neutrino is immune to almost all interaction.

The same quality that makes a neutrino's path retraceable also makes it undetectable And figuring out how to detect the undetectable is what got Francis Halzen the Nobel Prize in Physics this year, and it was for decisive contributions to the IceCube Neutrino Observatory and the discovery of high energy neutrinos of astrophysical origin.

Now, hundreds of scientists and engineers worked on IceCube and its predecessors, and many of them were driving forces or provided critical expertise.

But Francis Halzen is unusual as the most persistent driver of this whole program, and that's why he's the first solo awardee of the Physics Nobel in 34 years.

Today, we're gonna talk about why Halzen and his IceCube experiment are Nobel-level contributions to our understanding of the universe.

But first, some neutrino 101.

Neutrinos interact only by the weak force and the even weaker gravity.

The former is weak because of its short range, which is due to the masses of the mediating W and Z bosons.

That means a neutrino has to get very close to another particle, typically an atomic nucleus, in order to have any chance of interacting.

Neutrinos don't just stream through matter unimpeded, they stream through atoms, the electron cloud, and even the nucleus.

But very rarely, they can interact with an electron or a quark when they have a head-on collision, to speak very loosely.

Although these aren't solid little objects bumping into each other, more accurately, they get close enough to exchange weak force bosons, and then things get interesting.

Let's say the interaction produces a muon, a common enough thing to happen, and one of the most useful, as we'll see.

That muon is born with a ton of kinetic energy, and it zips off at a good fraction of the speed of light.

If the interaction happens in some medium like air or water, then the muon finds itself traveling faster than the local reduced speed of light in that medium.

The result is something like a photonic equivalent of a sonic boom, Cherenkov radiation.

This cone of blue light trails the rapidly decelerating muon, and it's this Cherenkov radiation that is detectable.

It points to the fast-moving muon and by extension to the neutrino that created it.

And because the Cherenkov cone gives away the muon's direction of motion, it also tells us the neutrino's, which means we can learn where the neutrino came from in the first place.

The challenge is to get the neutrino to interact in the first place, and the key to that is just a numbers game.

Given a sufficiently large block or tank of whatever and a sufficiently large number of neutrinos, and some will eventually interact.

The first efforts used giant water tanks with extremely sensitive photon detectors.

By the '80s, such detectors like Japan's Kamiokande were using this technique to detect solar neutrinos in real time.

In 1987, Kamiokande caught neutrinos from the supernova 1987A.

Its much larger successor, Super-Kamiokande, and Canada's Sudbury Neutrino Observatory would go on to revolutionize neutrino physics.

But these relatively small tanks worked because the sun or, say, a nearby supernovae blast us with huge numbers of neutrinos.

But high-energy cosmic rays are rare, and so presumably are the high-energy neutrinos produced in the same types of events.

In order to detect enough high-energy neutrinos to really study the universe, we need a tank of water on the scale of a cubic kilometer that would weigh a billion tons.

Not really in the scope of human engineering, at least not yet.

And that's where Francis Halzen comes in.

In the late '80s, Halzen was one of the central people pushing the idea of using an Antarctic glacier as a neutrino detector.

The idea was that nature has already given us these kilometer scale H2O tanks frozen at the South Pole.

Lace them with photon detectors, and they should work as giant neutrino detectors.

The prototype experiment was AMANDA, the Antarctic Muon And Neutrino Detector Array.

Boreholes were drilled to around a kilometer depth, really melted with hot water, and photomultiplier tubes were strung down.

The sensors saw Cherenkov light.

But instead of preserving the clean timing geometry of a track, the light arrived hopelessly scrambled.

It turns out that glacial ice is full of trapped air bubbles, and that scatters light, destroying useful directional geometry of these Cherenkov cones.

It was physicist Buford Price who stepped in to see if it was worth actually going deeper.

He and his colleagues figured out that below around 1.4 kilometers, the increased pressure would cause bubbles to disappear into air hydrate crystals whose refractive index is almost the same as that of the surrounding ice.

No refraction means no scattering, and so ice at that depth should be clear enough to be used in this experiment, and presumably in the most hipster of cocktail bars.

So 10 new holes were bored down to two kilometers, and transparency increased enormously.

But AMANDA was still only a few hundred meters across.

Francis Halzen led the movement pushing for a full cubic kilometer observatory.

The National Science Foundation agreed, and in 2004, the full IceCube Observatory began construction with Halzen as principal investigator.

It incorporated all of the learnings from AMANDA in terms of drilling, the detectors, and most of all, the size and needed depth.

The original phase consisted of over a cubic kilometer of photomultiplier chains starting at 1.5 kilometers beneath the South Pole glacier.

And that includes more than 5,000 optical sensors suspended on 86 vertical strings.

It was really Antarctica transformed into a telescope.

Let's go into a little more detail on how the full scale IceCube works.

Different neutrino interactions produce different event shapes.

Neutrinos come in three varieties, one for each of the charged lepton family.

So there are electron, muon, and tau neutrinos.

On interacting with matter, a neutrino will tend to produce the corresponding charged lepton type.

Each looks different in IceCube.

The muons produced by muon neutrinos often create long tracks, which are especially useful for determining direction of origin of the neutrino.

Electron neutrinos result in more compact bursts, which provide a better measure of the amount of energy deposited, and tau neutrinos can produce distinctive double flashes that reveal the flavor composition of the cosmic neutrino beam.

Now, IceCube doesn't directly detect neutrinos.

It detects Cherenkov photons produced by electrons, muons, and taus, and they may not have been produced by their neutrino counterparts.

So how do we know that these heavy leptons were created by a neutrino collision in the first place?

Well, the answer is especially neat for muons.

These particles are produced in large numbers as cosmic rays strike our atmosphere IceCube sees tons of these characterized by Cherenkov cones tracking downwards through the glacier.

It's almost impossible to pick out the ones that are sourced from neutrino strikes.

But IceCube also sees a much smaller number of muon tracks coming from below.

These are essentially all neutrino induced because the literally rock solid Earth blocks almost everything except neutrinos.

Except at the most extreme energies, a neutrino will slip through the Earth as though it wasn't even there.

A tiny fraction gets intercepted within the Earth's interior, and an even tinier fraction makes it through the Earth and then collides within the IceCube's glacier, producing a muon and providing proof positive that a muon neutrino came from below.

For electron and tau neutrinos, simply having the Cherenkov burst start within the detector is a big part of the solution because unlike muons, electrons and tau particles can't travel long distances through the atmosphere or ice after they first form.

Pretty much right after IceCube was completed, the scientific payoff came.

In 2011 and 2012, the detector recorded two extraordinarily energetic events, each depositing around a petaelectronvolt of energy.

They were named Bert and Ernie.

And more events followed.

Before long, dozens of high energy neutrino events that could not be explained by the atmosphere background alone were seen.

And this represents IceCube's first major scientific discovery, the continual bombardment of Earth by very high energy neutrinos coming from beyond the solar system, not from the sun, not even from supernovae.

So from what?

The first clue came on September 22nd, 2017, when IceCube detected a very high energy muon neutrino, an event called IceCube-170922A.

The Cherenkov path was reconstructed relatively well enough so that IceCube could send an alert to observatories around the world and above the world.

Telescopes swiveled to that location on the sky, and there they found the potential culprit, a blazar.

That's a supermassive black hole feeding on a whirlpool of gas, generically an active galactic nucleus.

The most well-known type of those is the quasar, but the blazar is a very special type.

It has a jet launched in the magnetic fields around the rotating black hole that happens to be pointing right at us.

And this blazar happened to be in a very active state at that time.

The Fermi satellite saw that it was popping off in gamma rays.

Now, matter blasted out in one of these jets is one way to make high energy cosmic rays the type that can't be traced back to the source.

But it's also a way to make high energy neutrinos, which can be traced, and in this case, this one event pointed to this blazar This clue led IceCubers to do a search of archived neutrino detections, and they found that several past neutrinos came from the same general direction.

Other blazars have also been associated with these sorts of neutrino excesses, but they're probably not the only source of IceCube neutrinos.

For example, in 2022, the collaboration reported strong evidence for neutrino emission from NGC 1068, also known as M77.

This galaxy contains a feeding supermassive black hole also.

But unlike blazars, its central engine is heavily obscured, in this case making it a Seyfert galaxy.

That matters because the dense radiation and matter surrounding an active black hole can absorb high-energy gamma rays and other light before they escape.

Some of the best cosmic particle accelerators may therefore hide themselves from conventional high-energy telescopes.

But neutrinos don't have that problem.

Neutrinos can escape directly from regions where photons are absorbed.

So NGC 1068 showed that neutrino astronomy was not merely another way to look at the same objects we already knew.

It could reveal hidden parts of the high-energy universe that light itself might not be able to show us.

More recent IceCube work has strengthened this possibility with growing evidence that X-ray bright Seyfert galaxies as a population may contribute significantly to the cosmic neutrino flux.

That is an especially interesting direction for the field because it suggests that some of the universe's most important particle accelerators might not be the spectacularly bright jets that first drew our attention, but rather be deeply buried environments close to actively feeding black holes.

So far, we've just talked about IceCube finding the neutrinos created in the vicinity of giant black holes, but in 2023, the collaboration reported the first detection of diffuse high-energy neutrino emission associated with the plane of our own galaxy.

Now, those blazar neutrinos come from other very distant galaxies, which are distributed randomly across the sky.

So a correlation with the Milky Way's disk suggests a different source, probably from galactic cosmic rays colliding with interstellar gas.

These collisions create pions, which then decay into gamma rays and neutrinos.

So we've come from finding that a cosmic neutrino flux exists to finding individual sources to probing those sources in ways that we couldn't previously, and now to mapping the Milky Way galaxy in neutrinos, and that's what the birth of a new type of astronomy looks like.

But IceCube isn't just a telescope and an astronomy experiment.

It's also a particle physics experiment.

One particularly beautiful example comes from the Glashow resonance.

In 1960, Sheldon Glashow showed that an electron antineutrino with just the right energy, around 6.3 petaelectronvolts, could collide with an electron and produce a W boson.

Nice idea, Sheldon.

If only we could build a neutrino beam that gets us literall.. close to that energy to test this.

But wait, the universe can.

In 2016, IceCube saw an enormous six peta electron volt-ish cascade that was consistent with a Glashow resonance interaction, and it's now broadly agreed that we've actually seen this now in IceCube.

Another cool particle physics result is to measure the interaction cross-section of extreme energy neutrinos.

Probability of interaction actually increases with energy.

Solar neutrinos pass straight through the Earth with basically 100% efficiency.

But above a certain energy, neutrinos start getting blocked by the Earth.

From around 100 tera electron volts, long passages through the middle of the planet start getting blocked, and by a few peta electron volts, only glancing passages make it to IceCube.

And that's enabled physicists to figure out the interaction cross-section of high-energy neutrinos, and that's something that could never be done with a human-made neutrino beam.

Neutrinos are already the most mysterious particle in the standard model, and maybe they're our best bet for pushing us beyond the current understanding of particle physics.

And IceCube is our best bet for exploring the behavior of the most extreme neutrinos, which is often where current theories break.

I've really just given you some of the IceCube highlights, and there's plenty of other science besides, both in astro and particle physics.

All by turning a cubic kilometer of Antarctic ice into a telescope.

That's how Francis Halzen and the IceCube collaboration allowed us to catch and track messages from the edges of black holes and the sites of the most extreme particle collisions.

They helped usher in a new type of astronomy and a regime of particle physics beyond any human engineering.

Worth a Nobel?

Well, being able to see the universe in neutr.. So here's to IceCube and to Francis Halzen for leveraging the clarity of the ice to the clarity of our view of spacetime.