Good morning John, I've given up on hat.

So the way that you're watching this video right now is through two things. One is waves in sound, uh, so that's an important part of the video. The other is waves in the electromagnetic spectrum. Particularly in the part of the spectrum that is visible light which turns out to be a very narrow piece of the spectrum. Now sound and light are very important here on Earth. There's a bunch of other senses as well, but when we start to look at the universe, the only tool we have had to do astronomy with for the entire history of humanity has been the electromagnetic spectrum. It's just been photons. When Galileo looked up and first saw the moons of Jupiter, he was looking at visible light photons. And when Jocelyn Bell Burnell first saw the rapid radio blinking of what turned out to be pulsar, she was also looking at photons (or at least the telescope she was using were looking at photons because we can't see those photons). In fact, a lot of the story of astronomy, and thus the story of how we understand like what's going on around us, has been the story of expanding from the visible light spectrum in into other wavelengths of light.

We've just celebrated a year since the first photos from JWST, and its superpower is being able to see in further sections of the infrared which is hard to do because anything that has any heat is emitting infrared radiation. So in order to detect certain infrared photons, you have to be very cold because otherwise the mirror itself that's collecting the light is gonna be emitting a bunch of those photons into the detector and thus washing out whatever it is you might be able to detect. So there's infrared and ultraviolet and x-rays and gamma rays and microwaves and all these things, all this electromagnetic radiation, gives us different clues into how the universe works. We can see different stuff with different wavelengths, different events make different kinds of light, different kinds of light are able to travel through different objects more easily or worse. And thus we have learned all of the things that we know about our universe just by looking at this one kind of thing.

But in the last decade, and especially in the last couple of months, we have got got two new ways to observe our universe. I don't really know if I can express how big of a deal that is. It's like in the last 10 years having Galileo look at the moons of Jupiter for the first time twice.

Now the first one of these got pretty good press, and I talked a ton about it. I was part of the press that it got and it was very exciting. And it is gravitational waves. Ripples in spacetime that we are able to detect. Have been able to detect for a while, but now we've got this new extraordinarily powerful way of detecting gravitational waves by looking at the timing of pulsars that are distant in our galaxy. Basically, turning the entire galaxy into a giant gravitational wave detector. Because when there are waves in spacetime, things actually move a little bit closer or further away, like a meter per light year but detectable because of how regular pulsars are at flashing. If they move a little bit toward us, we actually detect that, which is wild! It has allowed us, for the first time, to detect the gravitational wave background which is extraordinarily cool, but I had talked a lot about it already.

But there is another thing that's happened over the last 10 years that had a huge announcement, like on the same weekend as the gravitational wave announcement, which is kind of a shame. Because it got a little bit overshadowed and I want to talk about it because of course I do. Now certain high-energy things that happen in the universe, or even low energy things that happen in the universe, emit particles. They emit photons, is one of the kinds of particles they emit, but also particles with mass can be ejected, like you get electrons and you get, like, protons and pieces of nuclei, and that's amazing but those things get tossed all around and get absorbed like crazy by the gas and dust in the galaxy so they're not very useful.

But another kind of particle is also emitted. It's called a neutrino. It's like an electron, but it's neutral. Like 3% of the energy emitted by the Sun is emitted as neutrinos. It's a lot. Like there's a hundred trillion of them going through you every second right now and me and everyone. But the thing is, because they are neutral and because atoms are mostly empty space for the most part, neutrinos just fly right through stuff. And when I say for the most part, I mean basically all of them. Detecting neutrinos has been a big deal in the last, you know, decades of science. Like several Nobel Prizes have been won about this. Because when they do hit a nucleus of an atom, things can happen, detectable things. Like, some light can be emitted. In very weird situations, one element can be transformed into another element. Like just one atom, but it can happen.

So we've had some ways of detecting neutrinos, but if we want to detect a lot of neutrinos what you would need is a giant block of an extremely transparent substance. I'm talking about like kilometers on a side, that had inside of it a bunch of detectors that could detect the light created when a neutrino collided with a nucleus. But the substance we're talking about would have to be, like, more transparent than anything we've ever created in a lab. And so you would be right to believe that there is no way to create such a neutrino detector.

BUT! It turns out that deep down, as Antarctic snow gets compressed and compressed and compressed, it turns into a kind of ice that is more transparent than anything we have. And also that ice is in an extremely dark place because it's under a bunch of other ice and snow. So we have on Earth, I mean by chance because Antarctica doesn't need to be there, like, it does-like it could just not be there but it is, so by chance, we have on Earth the perfect neutrino detector. All we had to do was drill a bunch of holes in it with like hot water drills and then lower instruments in to be able to detect the slightest flashes of light from neutrinos hitting atomic nuclei. And we did that. It's called, and I love this, the IceCube Neutrino Observatory. And no, that doesn't stand for anything, they just called it ice cube because it's an ice cube. It's just a really big one. It's actually not a cube, you don't care, but it's more like a hexagonal prism but not exactly. It's this shape.

But we are not done with the problems yet. So the IceCube Observatory has detected over a hundred thousand neutrino collisions, which seems like a lot. It's not that many when you considered that it's a kilometer of ice and there's a hundred trillion of them going through just your body every second, so it's a very small percentage of the total neutrinos flying through that cube. The other problem though is the vast majority of those neutrinos originated from within our solar system, they came from the Sun and that's not that useful. What we want is the neutrinos that are coming from our galaxy and maybe even from other galaxies. That would allow us to see things in our galaxy that we could never see because, of course, photons get absorbed by stuff. Like if you go in a place that doesn't have a lot of light pollution, you can look up at the Milky Way and you can see the clouds of dust that are preventing the light from the Milky Way to reach us. That's what those dark areas of the Milky Way are, but neutrinos are just gonna fly right through that. If we could map our galaxy using neutrinos, we could see stuff that we've never been able to before. 

But to do that, we have to separate out, like, the 99.9% of neutrinos that are coming from the Sun from the ones that are coming from our galaxy. But to do that, we have to actually detect some of those neutrinos coming from our galaxy, and there's far fewer of them. But with machine learning, we are now able to do it and we were thus able to use those neutrino collisions to create the first ever map of our galaxy that did not depend on photons. It's very low-resolution right now, but as more time passes, more collisions are detected. Hopefully, we make this thing even better at detecting neutrino collisions. That map will get more high-resolution and this observatory, this most bizarre of telescopes! This giant hexagonal prism of ancient Antarctic ice under thousands of meters of other ice (oh my-are you serious). This way of observing our galaxy is entirely new. And as more time passes and we get better at the tech, these collisions gon-it's gonna tell us so much stuff about how our galaxy works. It could, along with the gravitational wave back background information, it could help us rewrite the laws of physics.

And this is just the beginning. I mean the wild thing about neutrinos is that they aren't like photons, they change because they're particles. So they can go different speeds, they can have different characteristics, and all those things tell you stuff about where they came from and what they've experienced along the way. And their experience thus gets transferred into our experience, our experience and understanding of our universe and how it works. It has been, but continues to be, just a remarkable time in science.

John, I'll see you on Tuesday.