Christopher: In 1920, The New York Times published a scathing editorial mocking the physicist Robert Goddard. They claimed he lacked the basic knowledge taught in high schools, because everyone knew a rocket couldn't work in the vacuum of space—it had nothing to push against. The Times was so certain, they declared space travel impossible. Lucas: And they weren't just being mean; they were being logical, based on the physics they knew. But their logic was flawed. Forty-nine years later, as Apollo 11 launched for the moon, the paper quietly printed a one-sentence retraction, admitting their error. Christopher: That story perfectly captures the world Michio Kaku explores in his book, Physics of the Impossible. It’s a world where the line between absurdity and breakthrough is thinner than we think. Kaku argues that studying the impossible is not a waste of time; it's the very engine of scientific progress. Lucas: It’s a fascinating journey through technologies we think of as pure fantasy—force fields, teleportation, time travel—and grounding them in the actual laws of physics. It’s less about if and more about how and when. Christopher: Exactly. And today we'll dive deep into this from three perspectives. First, we'll explore why the very definition of 'impossible' is constantly changing. Lucas: Then, we'll deconstruct the real-world science behind creating an invisibility cloak. It’s not magic; it’s engineering. Christopher: And finally, we'll tackle the ultimate challenge: the physics and the profound paradoxes of time travel.

Lucas: So, Christopher, where does Kaku begin this journey? How does he even start to categorize these wild ideas? He can't just lump a death star in with a teleporter, can he? Christopher: He doesn’t. He starts by creating a framework, a way to sort the impossible. He proposes three classes of impossibility. Class I impossibilities are technologies that are impossible today but don't violate any known laws of physics. Think force fields or maybe even teleportation of a single atom. We can see a path to them, even if it's centuries long. Lucas: So that’s the stuff that’s currently in the engineering-problem-from-hell category. What’s next? Christopher: Class II impossibilities are technologies at the very edge of our understanding of physics. Things like time machines or travel through wormholes. To make them work, we might need to harness the energy of a star or find exotic forms of matter. They sit on the bleeding edge of theory. Lucas: And Class III? Christopher: Class III impossibilities are technologies that violate the known laws of physics. Things like perpetual motion machines. Kaku says that for these to become possible, our entire understanding of science would have to be overturned. But the core idea, and this is what I love, is that our definition of impossible is fluid. It's a moving target. Lucas: And he gives some incredible historical examples of this. It’s not just about future tech; it’s about how we’ve consistently been wrong in the past. Christopher: Exactly. Take the extinction of the dinosaurs. For decades, paleontologists had no good explanation for their sudden disappearance. Then, a wild theory emerged, mostly from outside the field, that a giant meteor had struck the Earth. The scientific establishment laughed. It sounded like something out of a pulp science fiction novel. Lucas: It was seen as a cheap, overly dramatic explanation. The prevailing theories were more gradual—climate change, disease. A space rock felt like a lazy plot device. Christopher: Right! But then, the evidence started piling up. Scientists discovered a thin layer of iridium—an element rare on Earth but common in asteroids—in geological formations all over the world, dating to precisely 65 million years ago. And then they found the crater, a hundred miles wide, off the coast of Mexico. What was once dismissed as absurd science fiction became the leading scientific consensus. Lucas: It’s a powerful reminder of our own intellectual arrogance. The great physicist Lord Kelvin, at the end of the 19th century, famously declared that "heavier-than-air flying machines are impossible" and that "X-rays will prove to be a hoax." He was a brilliant man, but he was trapped by the physics of his time. Christopher: So does this mean that anything we can imagine is eventually possible? If we just wait long enough? Lucas: Not quite. Kaku provides a crucial anchor: the known laws of physics. He's not saying anything goes. He's saying we should rigorously test the boundaries of what those laws allow. He references a quote from the physicist T.H. White that became a mantra in the field: "Anything that is not forbidden, is mandatory!" Christopher: I love that. It means if there isn't a fundamental law of nature saying 'you cannot do this,' then it's not only possible, but it's probably inevitable somewhere, sometime in the universe. It shifts the burden of proof. Don't tell me why it's possible; tell me why it's forbidden. Lucas: And if you can't find a law that forbids it, then the 'impossible' just becomes an engineering problem waiting for a solution. Which is the perfect entry point for some of the sci-fi gadgets he deconstructs.

Christopher: It is. And this is where the book gets really fun. Kaku takes these iconic sci-fi tropes that seem like pure magic, like an invisibility cloak, and he asks that exact question: "Is it forbidden by the laws of physics?" Lucas: And with invisibility, the answer is a surprising "no." It's not forbidden. It's just ridiculously, mind-bogglingly difficult. To understand how, you first have to think about what it means to see something. Christopher: Right. We see objects because light from a source, like the sun or a lightbulb, bounces off them and into our eyes. An apple is red because it absorbs all the other colors of light and reflects the red wavelengths. Invisibility, then, is the simple, or not-so-simple, act of preventing that light from bouncing off you and reaching an observer's eye. Lucas: The most obvious way is to just let light pass right through you, like glass. But Kaku points out that even glass is visible. You see the reflections, the distortions. True invisibility requires something much more clever. It requires you to bend light around an object, as if it were never there. Christopher: And for a long time, that was considered impossible. But Kaku introduces us to the game-changer: metamaterials. This is one of the most fascinating concepts in the book. Lucas: It really is. The analogy Kaku uses is perfect. Imagine a large rock in the middle of a flowing stream. The water flows around the rock and then joins together on the other side. Downstream, you might not even know the rock was there. Metamaterials are substances engineered on a microscopic level to do the exact same thing to light waves. Christopher: They are not materials found in nature. They're designed, atom by atom, with structures smaller than the wavelength of light itself. These tiny structures act as a guide, forcing the light to flow around a hidden object and then recombine on the other side, perfectly intact. To someone watching, the light would appear to have traveled in a straight line, and the object in the middle would be completely invisible. Lucas: And this isn't just a theory. Kaku tells the incredible story of the experiment at Duke University in 2006. Scientists created a flat, disc-like metamaterial made of tiny copper circuits. They placed a small copper cylinder inside it and then fired a beam of microwaves at it. Christopher: And what happened? Lucas: The microwaves flowed perfectly around the cylinder. It was, for all intents and purposes, invisible to the microwave beam. They had created the first working, albeit very limited, invisibility cloak. Christopher: So it's real! We have a working prototype for Harry Potter's cloak! Lucas: Well, let's temper that excitement just a bit. As Kaku explains, there's a huge difference between bending long-wavelength microwaves and bending the much, much shorter wavelengths of visible light. To do that, the components of the metamaterial would have to be engineered on the scale of nanometers—the atomic level. This is the challenge of nanotechnology, a field the great physicist Richard Feynman kickstarted with his famous 1959 lecture, "There's Plenty of Room at the Bottom." Christopher: So the physics allows it, but the engineering is a monumental challenge. We need to build with atoms as our building blocks. Lucas: Precisely. But the moment we start talking about achieving invisibility, Kaku pivots to a much older, more profound question. The physics is one thing, but the ethics are another. He brings up Plato's ancient thought experiment from The Republic: the Ring of Gyges. Christopher: I remember this. A shepherd finds a ring that makes him invisible. Lucas: And what does he do with this power? He sneaks into the palace, seduces the queen, murders the king, and seizes the throne. Plato's question, which Kaku brings into the 21st century, is this: is our morality just a performance for others? If you could do anything with no fear of being caught, would you still be a good person? Christopher: That's a chilling thought. The scientists are so focused on the 'can we?' that they might not be asking the 'should we?' The power of invisibility could be the ultimate tool for crime, for espionage, for terror. Lucas: It's a recurring theme in the book. The pursuit of these 'impossibilities' forces us to confront not just the limits of our technology, but the limits of our own wisdom.

Christopher: Speaking of uncomfortable questions and wisdom, Kaku doesn't stop with Class I impossibilities like invisibility. He pushes into the truly mind-bending territory of Class II—and there's nothing more mind-bending than time travel. Lucas: And he starts with that famous, almost playful question posed by Stephen Hawking: "If time travel is possible, then where are the tourists from the future?" It sounds like a joke, but it's a genuinely profound physical problem. The absence of time travelers is, in itself, a piece of data. Christopher: Kaku breaks down time travel into two distinct problems. First, there's travel to the future. And surprisingly, he says this is not only possible, it's been proven. It's a consequence of Einstein's theory of relativity. The faster you move, the slower time passes for you relative to everyone else. Lucas: It's a tiny effect in our everyday lives, but it's real. He mentions the Russian cosmonaut Sergei Avdeyev, who holds the record for time travel. After spending 748 days in orbit aboard the Mir space station, traveling at 17,000 miles per hour, he returned to Earth having aged about one-fiftieth of a second less than the rest of us. He traveled 0.02 seconds into the future. Christopher: So, traveling to the future is just an engineering problem. Build a fast enough ship, and you can jump ahead years, decades, centuries. The real headache, and the heart of the impossibility, is traveling to the past. Lucas: This is where the paradoxes come in. The most famous, of course, is the Grandfather Paradox. Let's say you build a time machine, go back in time, and accidentally prevent your grandparents from meeting. If they never meet, you are never born. But if you are never born, you could never have gone back in time to stop them from meeting in the first place. It's a complete logical breakdown. Christopher: It makes my brain hurt just thinking about it. So how do physicists even begin to resolve this? Is time travel to the past simply forbidden? Lucas: This is where it gets wild. Kaku explains that physicists, working within Einstein's equations, have proposed a few potential solutions to the paradox. Each one is stranger than the last. Option one is what you could call the 'Fixed River of Time' model. Christopher: Meaning what? Lucas: Meaning the river of time is fixed and unchangeable. You can travel back into the past, but you have zero free will to change anything. You can't kill your grandfather because, well, you didn't. The fact that you exist is proof that you failed. Any action you try to take to alter the past will mysteriously fail. The gun will jam. You'll slip on a banana peel. The universe conspires to protect its own timeline. Christopher: So you're just a ghost. You're watching a movie that's already been filmed, and you're part of the cast, but you can't deviate from the script. Lucas: Exactly. It preserves causality, but it kills free will. But then there's option two, which is even stranger and is gaining traction in theoretical physics: the 'Forking River' model, based on the 'many worlds' interpretation of quantum mechanics. Christopher: Don't tell me this is about parallel universes. Lucas: It's absolutely about parallel universes. This theory suggests that the moment you go back and successfully change the past—the moment you prevent your grandparents from meeting—the universe splits in two. In the original timeline, you were never born. But you have now created a new, alternate timeline, a parallel universe, where you exist as a kind of temporal refugee, but your grandparents back in this universe's future are gone. Christopher: So you don't erase your own history; you just create a new one. You solve the paradox by creating an entirely new reality. Lucas: You solve the paradox by multiplying reality. Every quantum decision, every choice, creates a new branch of the universe. It's a staggering, almost incomprehensible idea, but it's a mathematically consistent way to resolve the paradoxes of time travel. Christopher: It feels like the universe's ultimate loophole. You can't break the rules of one reality, so you just create another one where the rules are different. Lucas: And that's the essence of what Kaku is exploring. The universe may have loopholes. Finding them is the work of physics. Deciding what to do with them... that's the work of wisdom.

Christopher: So, Lucas, when you step back from it all, we've taken quite a journey. We've gone from seeing the 'impossible' as just a moving target, to actually drawing up a scientific blueprint for an invisibility cloak, and finally to questioning the very fabric of time and reality. Lucas: It's a powerful arc. Kaku starts with a simple, inspiring premise: don't be afraid to question what seems impossible. He shows how science fiction has consistently inspired real science, from H.G. Wells's prediction of the atomic bomb inspiring Leo Szilard, to the warp drive of Star Trek inspiring the Alcubierre drive. Christopher: But it's not just a fun tour of sci-fi tech. He grounds every single idea in real physics, showing the immense challenges but also the tantalizing possibilities. The book gives you a genuine appreciation for the elegance of the physical laws that govern our universe. Lucas: I think the most profound takeaway for me, though, is the final question the book leaves you with. It's not 'Can we do these things?' but 'Should we?' As we get closer to turning science fiction into fact, whether it's invisibility or manipulating the building blocks of life, the biggest challenge isn't the physics—it's our own judgment. Christopher: It's the H.G. Wells story, "The Man Who Could Work Miracles." An ordinary man is given unlimited power, and he starts with good intentions but ends up destroying the world because he lacks the wisdom to wield it. Lucas: Exactly. And that's the ultimate question Kaku leaves us to ponder. As our science gives us powers that previous generations would have considered god-like, are we prepared to develop the god-like judgment to match? That's the real 'impossible' we need to solve.