How can matter ever go faster than light? What happens when it does? Who discovered this, and what is it good for? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPT (AUTO-GENERATED)

I wanted to employ one of my overly torturous metaphors for this episode, and it was going to involve a crowd of fans and paparazzi waiting for a celebrity to make an appearance on a red carpet. But I didn't want to date myself because I'm only vaguely aware of people who are famous in the present moment. And I also wanted to future proof myself so I couldn't just name someone from the present moment that will be forgotten in the unknown future when you'll actually listen to this. So here's what I decided. The metaphor stays because, of course, it does. But our celebrity is Brad Braddington. I want you to imagine this scene. It's the red carpet. It's the night of the Oscars or the Emmys or the participation trophy ceremony for your kid's soccer game. That's not the essential part of the metaphor. What matters is who's here. Brad Braddington's adoring fans, curious onlookers, and, of course, the paparazzi ready to take their shot. In our analogy, Brad Brannington is a particle, an electron, a proton, even a neutrino if it felt like it.

The crowd of onlookers and fans, that's the material. It's a substance like air or water or diamond or the inside of your eyeball, which is mostly water and hopefully not much diamond. Oh, and those paparazzi, they're the most important part. I'm talking today, with the help of Brad Braddington, about something called Cherenkov Radiation, which I prefer to call Light Boom, but as usual, nobody listens to me much. It's called Cherenkov Radiation because it's named after Soviet physicist Pavel Cherenkov. He didn't really understand what he was seeing, but he took great notes, and by the time we figured out what was going on, his name was already attached to it, so not bad Pavel. And he did his work in 1934. He was working in a lab in Moscow, and he's doing some really super serious, incredibly important science work. He is shining gamma rays into a bottle of water. That's it. That was his experiment. In the 1930s, a lot of particle physics basically boils down to shining and or shooting as object X into target Y.

So it's not as lame as it sounds. But he finds that when he shoots the gamma rays into the water, it glows. It's blue. It's faint. It's barely there, but it is there. Now, here's the thing. This wasn't the first time anyone had seen this. Marie Curie's lab had noticed the same glow years earlier. Other physicists had seen it, too. And every single one of them had looked at it and shrugged and written it off as fluorescence, like some fluorescence. impurity in the water was absorbing the radiation and re-emitting it as blue light. Some silly secondary effect, not interesting, move along. Pavel looks at it and thinks, in the great hallmark of most scientific discoveries, huh, that's weird. And I do have to mention that it's said that good scientists don't discover new things, they look at old things in a new way, and that's exactly what Pavel did. He's not sure why he's suspicious, but But he's suspicious or incredibly bored. So he does what any good, curious and or bored experimentalist does when something doesn't sit right.

He starts poking at it. He has this blue light in the water when he shines gamma rays on. What is what's going on? So he tries purifying the water and the glow stays. He tries different liquids. The glow changes. Yeah. He varies the energy of the radiation. The glow responds. Oh, neat. He changes the geometry of the experiment. And the glow has a direction. Wait. Wait, wait, wait, wait. Back up a bit. What was that last bit? Fluorescence. Glows in all directions equally. It doesn't care which way you're looking at it. But this glow that Pavel was looking at was asymmetric. It was stronger in some directions than others. It was doing something that fluorescence doesn't do. He doesn't know what this is, but it is definitely not fluorescence. Maybe it's Patreon. Patreon.com slash PM Sutter is how you can contribute to this show. And maybe it was first discovered in a Moscow lab in 1934. No one can say for certain. Actually, we can. Totally safe for certain because Patreon is a software platform for and you get the idea.

Patreon.com slash PM Sutter. Thank you so much for your contributions. Trenkov spends the next three years characterizing this thing with obsessive precision. He's not a theorist. He can't tell you why it's happening, but he can tell you everything about what it's doing. He measures its intensity, its direction, its dependence on the speed of the incoming particles, its behavior in different materials. He builds up a complete portrait of this phenomenon that nobody understands. He publishes his results, and the physics community is mildly interested. I mean, this is the 1930s. There's kind of a lot going on. Quantum mechanics is still being sorted out. Nuclear physics is exploding, literally. The faint blue glow in a bottle of water is not exactly front page news. A few years later, a couple other physicists pick up the mystery, and they realize what's going on. What's going on is that it's Brad Braddington showing up at the red carpet. Now, we need to talk about the red carpet and who's in it.

A bunch of atoms, molecules, and stuff. And all this stuff changes how light moves. And if this change didn't happen, then the whole magic around Brad Braddington slash Cherenkov radiation wouldn't happen. Now we all know what light is. It is waves of electricity and magnetism. At least in the classical picture, yes, there is a quantum description, but we don't necessarily need that quantum description to understand Cherenkov radiation. It is, you've got some electricity, you've got some magnetism, The electricity changes, the magnetism changes, they reinforce each other, and off they go. They travel, they propagate, just like waves on the top of the ocean propagate. And good old James Clerk Maxwell, when he realized that changes in electric fields can induce changes in magnetic fields and vice versa, and then they reinforce each other and they start traveling, he figured out what that speed was, and voila, it's the speed of light. And that's just a number. It falls out of our understanding of electricity and magnetism.

The speed of light is the speed of light. This became the cornerstone of our entire search for relativity, that no matter where you looked or how you looked, the speed of light was always the same. But that's the speed of light in vacuum. But what we're talking about is light moving through a substance, moving through a material, something, if I'm remembering correctly, we all learned in kindergarten. The speed of light is changes when it moves through a material. In air, it's just a little bit slower. In water, it's only 75% of its maximum speed. In diamond, it's only half. Half! We've even developed materials that can slow down light to walking pace. Now, this slowdown is caused by all sorts of interesting and fun interactions between light and atoms and molecules. I did a whole episode on this before, and the basic thing is you have three different pictures of describing how this all works. One picture is you imagine the waves of electricity and magnetism interacting with the molecules, and then those molecules start wiggling around.

They create their own waves of electricity and magnetism, which then interfere with the initial wave, slows everything down. Another picture lets you imagine tiny little quantum particles bouncing around like a game of pachinko. And another picture involves things called phonons, which is my favorite because it's both the nerdiest and the most accurate. But the how doesn't matter for the story of Pavel Cherenkov and Brad Braddington. What matters is that it happens. The most important point of this is that light moves slower in materials. That's what we care about. That's what produces Cherenkov radiation. And it can produce Cherenkov radiation because inside of a material, You can outrace light. In the vacuum of space, you can't do it. Nothing is faster than the speed of light in vacuum. But inside of a material, it's a different story. Light gets all caught up on itself. But a particle can just barrel on through, punching its way past all the atoms and molecules while the light is busy getting tangled.

So it is possible to go faster than light. You just have to cheat. If light is the Usain Bolt of particles... You can never beat Usain Bolt in a sprint, but what if you filled up the stadium with molasses? You might stand a chance if you change the rules. I would still lose, but maybe you have a shot. Okay, enough setup. We have our material. That's the crowd at the red carpet. We have our star particle itself, Brad Braddington. And remember those paparazzi, the ones I said were the most important part? The flashes from their cameras? That is the Cherenkov radiation. That's the light boom. That's what Pavel saw glowing in his little bottle of water in Moscow. So here's how it goes down. Let's say Brad Braddington steps out of his limo and walks at a nice, slow pace. There's a huge crowd surrounding the red carpet, absolutely crowding. It's so packed that the people in the middle of the crowd and at the edges can't really see him. The paparazzi out there don't even know he's there. So when Brad Braddington steps out, it's only the people right next to the limo that know he's finally stepped out.

And what do they do? They scream, they holler, and they take out their phones and start snapping pictures. But it's only the people nearest to him that can do this because they're the only ones who can see him. As long as Brad Braddington walks slowly through the crowd, this person This triggering of screaming and taking pictures, the flashes from the paparazzi cameras, moves out in all directions from him. He's the focal point. He's the center of the action. Wherever he is, the people nearest to him are reacting and taking pictures. If you had a bird's eye view of the red carpet, you would see Brad Braddington making his way through the crowd with these rings of paparazzi flashes around him on all sides. Inside a material, Brad is a charged particle. He's moving, but he also carries with him an electric field, a celebrity aura, if you will. That electric field influences all the atoms and molecules around him, and then they react. They stretch, they squeeze, they twist, they do all the things that atoms and molecules do.

And then they snap back into place and release a flash of light. These flashes of light move out in all directions from wherever the particle happens to be as it's moving through. If the charged particle slash Brad Brannington is moving slowly, then the molecules in front of him learn about his presence right around the same time that the ones to the side do and the ones behind him do. They all take pictures at the same time. They all release light at the same time and then everything cancels out in the wash and and you don't get any special glow. But let's say Brad Braddington is in a hurry. Maybe he's late. Maybe he hates crowds. Maybe he really has to pee. Instead of walking slowly, taking his time, letting all the paparazzi get their shots from all directions, front, side, and back, he leaps out of the limo and absolutely barrels for the entrance. He's shoving people, elbowing. He doesn't care. All the physics here is the same. The same charged particle is moving through the material.

It's the same material made of the same atoms and molecules. It's the same flashes of light from molecules near the particle. The only thing that's changed is the speed. And that's because the paparazzi have a reaction time. They have to notice Brad's presence before they can react and snap a picture. That reaction time is governed by the speed of light. If we want to get literal, then the light from Brad Braddington, from his skin... has to literally reach the paparazzi's eye for them to know he's there, at which point they take out the camera and take a picture. But the reaction of the paparazzi, the reaction of the atoms and molecules, is governed by the speed of light. As long as Brad Braddington, or the charged particle, is moving slower than the speed of light, everything is in order. All the paparazzi all around him have more than enough time to take their pictures. And for the charged particle... All of the molecules, as the charged particle is moving, all the molecules have more than enough time to feel the influence of the particle's electric field, to reconfigure themselves, snap back and release a flash of light.

And it's happening in a circle uniformly around the particle as it's traveling. But it's possible if the charged particle and or Brad Braddington is determined enough to move faster than the speed of light in the materials. That means that the paparazzi in front of Brad Braddington don't know he's there until he's already gone. By the time they are aware of his presence, respond, pull out their cameras and take a picture, it's only ever from the sides and back. No one gets a front view picture of Brad Braddington. In our material, the flashes of light created by the passage of the charged particle are now only ever behind or to the sides of the particle, which means they don't get to cancel out, which means they pile up on each other in a cone. This is a sonic boom, but made of light, hence a light boom. This is Cherenkov radiation. And what does Cherenkov slash Brad Braddington get us? One of the most visceral sources of Cherenkov radiation is are reactor pools in in nuclear power plants where you have high energy fission reactions releasing tremendous amounts of high energy charge particles and then the water is being used to cool and moderate the reactions to regulate the temperature and so you have a material where the speed of light is slower than the speed of light and vacuum because it's not a vacuum it's water You have charged particles screaming through coming out of the reactor core into the water.

They're racing through that material. As they race through, they leave behind a wake of flashes of light of the molecules in the water. The water itself reacting to the presence of these charged particles, stretching out, twisting, and then snapping back into place and releasing a bit of light.

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