How To Identify Black Holes By Catching Neutrinos

Thank you to Private Internet Access for supporting PBS. Neutrinos are one of the most bizarre of known particles. Black holes are probably the most bizarre of astrophysical objects. So it makes sense that we should use one to study the other, right? Well, today we're going to do just that.

There's been a lot of hype about our shiny new observatories and the new windows to the universe that they open. There's the James Webb Space Telescope and its infrared supervision. And of course LIGO, with its ability to see gravitational waves. And then there's neutrino astronomy. It gets far less attention in the media. In fact, it's almost as elusive as the particle it depends

on. And yet mapping the neutrino sky will surely unlock secrets that no other method can access. Today we're going to look at some of the first neutrino astrophysics in this new astronomical error, as we discuss a new result from the IceCube collaboration. They report seeing neutrinos produced in the colossal magnetic field surrounding a black hole

with a mass of 10 million suns. No big deal. But first up, let's review what neutrinos are. These elementary particles are fermions, so particles of matter rather than the force-carrying bosons, like the photons of regular astronomy. And the neutrino-fermion type is lepton, So they are cousins of the electron, the muon, and the tau particle.

There's one neutrino type corresponding to each of these three, with the most striking difference being that neutrinos are electrically neutral and have much lower masses than their charged counterparts. The extremely low mass of neutrinos means that they tend to travel at very close to the speed of

light, just because it doesn't take much energy to get them close to the cosmic speed limit. But perhaps the most characteristic property of the neutrino is that they only interact via the weak nuclear force and gravity. That makes them very very difficult to detect.

For reference, if you want to stop a low energy neutrino with a wall of lead, the wall would need to be a light year thick to have even a 50-50 chance that the neutrino gets close enough to a lead nucleus to interact. So you can see how doing neutrino astronomy might be challenging.

Given that each neutrino has a minuscule interaction probability, most of them pass straight through your detector. But minuscule isn't zero. With enough neutrinos, you'll get interactions.

And space does indeed generate a lot of neutrinos. By far the brightest neutrino source on the sky is the sun. It creates 10 to the power of 38, or 100 trillion trillion trillion, neutrinos every second in the fusion reactions in its core.

spread out in every direction, so a mere 100 trillion pass through your body every second. In fact, the vast majority pass straight through the entire planet. To have a chance of catching a neutrino you need a big detector, and the biggest neutrino detector is Ice Cube.

There are two famous musicians who are also astrophysicists. Those are Brian May and Brian Cox. So no, Ice Cube is not named after the rapper. It's named after the fact that It's literally an ice cube. It's a full cubic kilometer of glacial ice at the South Pole.

Let's see how ice can be used to see neutrinos. When neutrinos pass through the ice of ice cube, one in a million interact with the water molecules via the weak force. For high energy neutrinos, interactions are with an atomic nucleus, and that interaction

can transmute the neutrino into its high mass lepton counterpart, an electron, muon, If the neutrino becomes a tau then it decays almost instantly. But an electron or a muon will continue through the ice, emitting light as it interacts with other charged particles.

This is seen as a cone of blue light that trails the particle. It trails behind because the lepton is traveling faster than light. See, the speed of light is reduced in any medium that's not a vacuum. In ice it's around 25% slower.

But neutrinos are not slowed down, so the electron or muon that it creates also start out with a speed faster than the reduced speed of light in the ice. The result is the electromagnetic analogue of a sonic boom. The expanding EM waves created by the charged particle expand slower than the particle itself

is moving forward. So those wave fronts overlap each other and you get constructive interference in this cone shape that follows the particle. This is Cherenkov radiation, and it's ultimately how we detect the neutrinos.

The actual detectors are sensitive photomultipliers, basically extremely sensitive light detectors, suspended in deep boreholes. A grid of over 5,000 photomultipliers span a cubic kilometer of the glacier, starting at a depth of 1.5 kilometers.

In this way, IceCube sees the Cherenkov radiation from neutrinos generating both electrons and muons. But it's the muons that are really useful. Electrons interact very strongly with the water molecules and so begin to bounce around immediately,

leading to a bubble of Cherenkov near their creation point. But muons can travel for kilometers without changing course, so there Cherenkov cones trace out a straight line in the direction of the original neutrino path. In this way it's possible to figure out where on the sky the neutrino came from.

And with enough neutrinos you can even build up a rather blurry image. We need to sort out one more thing to understand how IceCube actually takes neutrino pics of the sky. It turns out that our environment is very noisy with neutrinos.

The sun produces an enormous number, but those are easy to distinguish because they come from the direction of the sun. More challenging are confounding particles from our own atmosphere. When cosmic rays hit molecules in our atmosphere, many different particles can be produced,

but the most annoying are muons and neutrinos. Those muons can cause Trenkov-Kerns, but they're easy to sort out because almost all of them come from above. If we only pay attention to signals from below, we eliminate atmospheric muons.