Christopher: Alright Lucas, I'm going to say a sci-fi technology, you tell me the first thing that comes to mind. Ready? Force field. Lucas: Easy. The little bubble shield in Super Smash Bros. that you can never time right. What else you got? Christopher: Okay, Teleportation. Lucas: The sound effect from Star Trek. And the existential dread of whether the 'you' that arrives is still 'you'. Christopher: Last one. Invisibility. Lucas: Harry Potter’s cloak, obviously. And the immediate, corrupting temptation to use it for very minor, petty crimes. Like stealing the last cookie. Christopher: That existential dread and petty crime are exactly what we're diving into today. We're exploring Physics of the Impossible by Michio Kaku. Lucas: Ah, the book that makes you feel like you could get a PhD in theoretical physics just by reading the preface. Christopher: Exactly. And Kaku is the perfect guide for this. He's a world-renowned theoretical physicist, one of the co-founders of string field theory, but he's most famous for making these brain-melting topics feel genuinely exciting and accessible. He wrote this book to show that the line between science fiction and real science is much thinner than we think. Lucas: It’s interesting because the book got really high ratings from readers, who loved how he could be both scientifically accurate and genuinely witty. But some critics have accused him of being a bit too optimistic, blurring the line between what's theoretically possible and what's just a fun idea. Christopher: And that's the tightrope he walks. To guide us, he does something brilliant right at the start: he classifies the 'impossible' into different levels of difficulty. It's like a video game, but for the laws of physics. Lucas: Okay, I’m intrigued. A difficulty setting for reality. What are the levels? Christopher: He calls them Class I, Class II, and Class III impossibilities. Class I are technologies that are impossible today but don't violate any known laws of physics. Think a hundred years out. Class II are things at the very edge of our understanding, maybe possible in millennia, like time travel. And Class III... well, those are the things that break the known laws of physics. Perpetual motion machines, seeing the future. Lucas: So Class III is basically, "Sorry, the universe has patched that exploit." Christopher: Pretty much. And it's that first category, Class I, where things get really wild, because the future he describes feels surprisingly close.

Christopher: So, let's start with something everyone 'knows' is impossible: a Starship Enterprise-style force field. A thin, invisible wall that can stop a laser blast. Lucas: Right, that feels like pure sci-fi magic. There’s no button you can press to just create a wall out of nothing. Christopher: But the concept isn't just fiction; it has roots in real, fundamental physics. Kaku tells this amazing story about Michael Faraday in the 1800s. Faraday was born to a working-class family, worked as a bookbinder's apprentice, and had very little formal education. Lucas: Okay, so not the typical resume for a physics pioneer. Christopher: Not at all. But he was obsessed with electricity and magnetism. And because he didn't have the formal math training of his peers, he didn't think in equations. He thought in pictures. He was the first to visualize what he called "lines of force" filling empty space—invisible fields that could push and pull things from a distance. Lucas: Wait a minute. So the entire concept of a 'force field' came from a guy who was just really good at visualizing things? Not some complex equation? Christopher: Precisely. He literally drew the fields. Kaku mentions that even today in physics, saying someone "thinks like a line of force" is a huge compliment. Faraday's intuition laid the groundwork for everything we know about electromagnetism. He basically invented the idea that empty space isn't empty. Lucas: That’s incredible. But how do you get from invisible magnetic lines to a solid wall that can stop a projectile? Christopher: Well, you can't, not directly with just magnetism. But his idea inspired other approaches. Kaku brings up a modern piece of technology called a plasma window. It was invented in 1995 by a physicist named Ady Herschcovitch at Brookhaven National Laboratory. Lucas: A plasma window? That sounds like something straight out of a video game. Christopher: It basically is. Herschcovitch was trying to solve a problem with welding. You can weld metals with an electron beam, but it has to be done in a vacuum, which is expensive and clunky. He wanted a way to separate a vacuum from normal air. So he invented a 'window' made of plasma. Lucas: How does that even work? Christopher: He heated gas to 12,000 degrees Fahrenheit—hotter than the surface of the sun—until it became a plasma. Then, he used a magnetic field to trap that superheated plasma in a thin, flat sheet. This sheet of plasma is dense enough to keep air from rushing into the vacuum chamber. It's a literal wall made of contained energy. Lucas: Okay, so that's not stopping a photon torpedo from the Klingons, but it's a real-life barrier made of pure energy! That's a huge step. Christopher: It is! And Kaku's idea is that a future force field wouldn't be one single technology, but a layered defense. You'd have a plasma window to vaporize incoming projectiles, then maybe a curtain of high-powered lasers to zap any remaining fragments, all layered together. It's an engineering problem, not a physics impossibility. Lucas: That makes so much more sense. It’s not magic, it’s just incredibly advanced, layered engineering. What about invisibility, then? Harry Potter's cloak. Kaku puts that in the same 'almost possible' category, which seems way more far-fetched. You can't just make light... not see something. Christopher: You can if you can bend it. This is where we get into one of the most exciting fields in modern physics: metamaterials. The idea is to create a material with properties that don't exist in nature. Specifically, a material with a negative index of refraction. Lucas: You lost me at 'index of refraction.' What is that? Christopher: Think of a straw in a glass of water. It looks bent, right? That's because light travels at a different speed in water than in air. The index of refraction is just a measure of how much a material bends light. All natural materials bend it in a predictable way. But what if you could create a material that bends light in the opposite direction? Lucas: It would be like the light is actively avoiding something. Christopher: Exactly. It would flow around an object like water flowing around a rock in a stream, and then continue on its path as if nothing was there. The object inside would be completely invisible. Lucas: Okay, that sounds like pure theory. Has anyone actually done this? Christopher: They have, on a small scale. Kaku highlights the groundbreaking experiment at Duke University in 2006. Researchers, funded by DARPA, created a metamaterial made of tiny copper circuits arranged in concentric circles. They managed to make a small copper cylinder completely invisible to microwaves. Lucas: Microwaves, okay. Not visible light. That feels like a big catch. Christopher: It's a huge catch. The problem is that to bend a wave, the components of your metamaterial have to be smaller than the wave itself. Microwaves are several centimeters long, so the copper circuits could be relatively large. The wavelength of visible light is measured in nanometers. Lucas: So you'd need to build an impossibly tiny structure. Christopher: You'd need nanotechnology. You'd have to engineer a material atom by atom to create these tiny circuits that can steer visible light. And this is where some of the criticism of Kaku comes in. Lucas: Right. This is where some critics say Kaku gets a bit loose with the term 'impossible.' It's one thing to hide a cylinder from a microwave in a lab, but a person from the full spectrum of visible light? That seems like a giant leap. Christopher: It is a giant leap. And Kaku acknowledges that. But his point is that the path is there. We have the blueprint. We know it's not forbidden by the laws of physics. It's just an incredibly, monumentally difficult engineering challenge. For him, that's what separates a Class I impossibility from a true impossibility. The path, however long, exists.

Christopher: And that leap from 'almost possible' to 'mind-bendingly difficult' is what defines Kaku's Class II impossibilities. This is where we get into things like faster-than-light travel. Lucas: The holy grail of science fiction. The warp drive, the hyperdrive. The thing that lets you get from one end of the galaxy to the other before the movie ends. Christopher: Exactly. But this is where we run into a hard wall: Albert Einstein. Kaku tells this very humanizing story about Einstein's early life. In 1902, he was a recent Ph.D. graduate, but he couldn't get a teaching job anywhere. His own professor wrote him negative recommendations. His family disapproved of his girlfriend. He was so broke and felt like such a failure he considered becoming a traveling salesman. Lucas: Wow. It’s hard to picture Albert Einstein having to cold-call people to sell them vacuum cleaners. Christopher: But it was from that place of desperation, working a low-level job at the Swiss Patent Office, that he revolutionized physics. And his theory of special relativity put a universal speed limit on everything: the speed of light. Lucas: The famous E=mc². Christopher: That's part of it. But the core idea is that as you approach the speed of light, strange things happen. Your mass increases, time slows down for you, and your length contracts. To actually reach the speed of light, your mass would become infinite, and it would take an infinite amount of energy. Lucas: So, it's a hard 'no' from Einstein. The universe has a speed limit, and you can't break it. How can Kaku even classify this as 'possible' in any sense? Christopher: Because Einstein gave us two theories of relativity. Special relativity says you can't travel through space faster than light. But his second theory, general relativity, which is our theory of gravity, suggests you might be able to cheat by warping spacetime itself. Lucas: Cheating Einstein with Einstein. I like it. How would that work? Christopher: Kaku explains the most famous idea, which was proposed by physicist Miguel Alcubierre in 1994, directly inspired by Star Trek. It's called the Alcubierre drive, or a warp drive. Lucas: So a real physicist tried to build the Starship Enterprise's engine? Christopher: In theory, yes. The idea is that you're not actually moving fast at all. You're inside a 'warp bubble' of spacetime. The drive would compress space in front of your ship and expand space behind it. The bubble of space you're in moves, carrying you with it, potentially faster than light. Lucas: That’s a brilliant workaround. It's like you're standing still on a moving walkway at the airport. You're not breaking the 'no running' rule, but you're covering ground incredibly fast. Christopher: That's the perfect analogy. You are locally at rest inside the bubble, so you don't violate special relativity. But to make this work, you need something... exotic. You need what physicists call 'negative energy' or 'negative matter.' Lucas: Negative energy? That sounds completely made up. Like something a supervillain would be after to power his doomsday device. Does that even exist? Christopher: It's not made up! It's been proven to exist, just in tiny, tiny amounts. Kaku points to something called the Casimir effect. If you take two uncharged metal plates and put them incredibly close together in a vacuum, they will actually be pushed together. Lucas: Why? What's pushing them? Christopher: The vacuum itself. Quantum physics tells us that empty space is actually a bubbling, frothing sea of 'virtual particles' popping in and out of existence. Between the two plates, only certain wavelengths of these virtual particles can fit. Outside the plates, all wavelengths can exist. This creates a pressure difference, and the space between the plates has slightly less energy than the vacuum outside. It has negative energy density. Lucas: My brain just did a somersault. So 'nothing' has energy, and you can have 'less than nothing' energy? Christopher: Welcome to quantum mechanics! The problem is, the amount of negative energy needed for a warp drive is astronomical. Some calculations suggest you'd need the energy equivalent of the entire mass of Jupiter. Lucas: Okay, so we're back to 'monumentally difficult.' And this is similar to the problem with time travel, right? Christopher: Exactly the same class of problem. To travel back in time, you need a time machine. And a time machine, in Einstein's theory, is a wormhole—a shortcut through spacetime. But to keep a wormhole open and stable enough for someone to travel through, you'd need... Lucas: Let me guess. A Jupiter-sized amount of negative energy. Christopher: You got it. And that's before we even get to the paradoxes. Lucas: Right, the classic Grandfather Paradox. You go back in time, you accidentally run over your own grandfather with your time-traveling DeLorean, you cease to exist, but then you couldn't have gone back in time to do it in the first place. My head hurts. Christopher: Physicists have a few ways around this. One idea is that the river of time is fixed. You can go back and watch the past, but you can't change it. Any attempt you make to kill your grandfather will mysteriously fail. The gun will jam, you'll slip on a banana peel. The universe self-corrects. Lucas: So, no free will in the past. That's one solution. What's another? Christopher: The other popular one is the 'many worlds' interpretation. When you go back in time and change something, you don't alter your own past. You create a new, parallel timeline. So in one universe, your grandfather lives and you are born. In the other, he doesn't, and you're never born in that reality. Lucas: So you can kill your grandfather, you just have to deal with the fact that you've created an orphan timeline where you don't exist. That's... a lot of cosmic paperwork. Christopher: It is. And Kaku's point is that to even begin to solve these problems—the energy requirements, the stability, the paradoxes—we'd need a 'theory of everything' that unites Einstein's gravity with quantum mechanics. We're talking about a civilization thousands, maybe millions of years more advanced than us. That's why it's a Class II impossibility.

Lucas: So after all this, from force fields to time machines, what's the big takeaway? Are we all getting lightsabers and warp drives, or is this just a fun thought experiment to sell books? Christopher: I think Kaku's ultimate point, and it's a beautiful one, is that 'impossible' is a temporary state of being. He quotes the physicist T.H. White, who said, "Anything that is not forbidden, is mandatory!" Lucas: That’s a powerful idea. What does it mean in this context? Christopher: It means the laws of physics, as we understand them, draw a hard line around what is truly forbidden—like a perpetual motion machine that creates energy from nothing. But everything else? Force fields, invisibility, teleportation, maybe even time travel? Those things aren't forbidden. They're just engineering problems waiting for a solution. Lucas: So the book isn't a promise that we'll have these things. It's more of a roadmap of what's allowed by the rules of the universe. Christopher: Exactly. It's an invitation to see the universe not as a set of limitations, but as a place of immense, structured possibility. Kaku is trying to get us to ask the right questions. Not "Is it possible?" but "What would it take?" He wants to reignite that sense of wonder that drove people like Faraday and Einstein. Lucas: It really makes you wonder what 'impossible' things we take for granted today will be normal in a hundred years. Or a thousand. The idea that we could be on the verge of manipulating matter and spacetime is both terrifying and exhilarating. Christopher: It is. And it leaves you with this profound sense of humility and optimism. We know so little, but the rulebook of the universe seems to allow for some truly incredible things. Lucas: What 'impossible' technology do you think we'll see first? Let us know your thoughts on our social channels. We'd love to hear what you think. Christopher: This is Aibrary, signing off.