Can we build an invisibility cloak?

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Speaker 1: Hey, Kelly, is there a superpower you see inviction that you'd like to have like in real life? You know, I think I'd like to be like the Flash, but instead of running anywhere, because I'm not a big fan of running, I want to get through my to do list super fast. Well, the world is on pause. I love it the Flash, but for paperwriting and parenting and household errands. Yeah exactly. But you know, I don't think a comic book about that would sell very well. To be honest, I also don't think that technology is likely to actually, you know, come to fruition. Really, you should have more faith in scientists, Daniel, Well, I think the problem is scientists already have too much to do, so they don't have time to create the Flash technology. Yeah, but once we have it, it'll be a snap to reinvent it. Hi. I'm Daniel. I'm a particle physicist and I have an infinitely long to do list. Hi. I'm Kelly. I'm a parasitologist and I haven't infinitely long to do list. Plus one and Welcome to the podcast. Daniel and Jorge Explain the Universe, a production of I Heart Radio in which we explore the infinity plus one things we'd all like to understand about the universe. We take a tour of everything that's amazing and crazy, from the inside of neutron stars to the tiniest particles out there, and we try to make sure you understand all of it. And as you might have, guest Jorge is not joining us to down the podcast, but we have a wonderful guest host, Kelly. Would you introduce yourself? Sure? So, I'm Dr Kelly Weener Smith. I am running a small ecology research site in central Virginia and I'm adjunct with Race University and only study parasites. But as my side job, I co author books with my husband, Zach Weener Smith. We've recently wrote a book called soon Ish Ten Emerging Technologies that will improve and or ruin everything. It isn't you're stuck inside because it's a pandemic. You should go read it. You should totally read it. It's a wonderful book. It's super fun to read. The comics are hilarious, and the science is super interesting and it's very relevant. Actually. For today's episode of the podcast. On this podcast, we'd like to talk about the physics of the universe and what's the inside black holes and how tiny particles work. But we also like to break down the physics of everyday objects and future technology. Previously on the podcast, we've talked about how solar panels work, can fusion reactors be real? Wood space elevators actually work? And the thing I think is super fun about that is understanding sort of like what's on the forefront of science? How are people actually solving these problems? You know, there's a wonderful interplay between science fiction where like crazy ideas about how the world might work are written into fiction and then actual scientists making it happen. Does that happen a lot in parasitology, Kelly, Oh jeez, we hope not not yet. You know, if the what is the name of the parasite in the expanse that takes over people's behavior? The proto molecule, the proto molecule. Yeah, so hopefully James S. A. Corey was totally wrong about that. I bet somebody out there in their lab is thinking, I wonder if I could actually build the protomolecule, and they're starting to write grant proposals for that. I hope not. I hope that's not fundable by the NSF. Well, today we are not talking about the protomolecule or anything that we think will lead to a more rapid demise of all of humanity. Instead, we're going to tackle a really fun concept that comes up a lot in fiction. We're asking the question today on the podcast, could we build a cloak of invisibility? So? Are you a fan of Lord of the Rings and Ry Potter Kelly? Yes, I can't say I watched them regularly, but yeah, I'm a fan. Yeah. So this kind of stuff pops up all the time in fiction, this idea of being invisible, of like sneaking around and having nobody see you, being able to listen to people's conversations or Rob Banks. Why do you think it's so fascinating? I mean, I think we all love the idea of of knowing what's going on when we're not in the room, and like, as a naturalist, I would love to be able to just sit in nature and watch stuff happening, because you know, when you're there, you're you know, impacting what happens when you know the deer are going to act differently if you're there, and just being able to observe the world as though you're not there. I think that's appealing in a lot of ways. Well, that's actually like a real science application for invisibility. I never even thought about that. Ecologist would love it, college just would love it. I would be terrified, though, to like sneak into a room and hear people talking about me. I'm pretty sure I don't want to hear what they have to say. I don't think i'd want to hear it either. No, that's why it appeals to me from a nature perspective. But I'll leave the humans alone, I think. So. This kind of stuff appears in Harry Potter and in Lord of the Rings and in much much older writing. And it's interesting to me how it first starts in sort of like magic and fantasy based worlds, and then in like the last hundred and fifty years, it appeared in like science fiction stories as authors have tried to figure out, like, what is the mechanism for this? How could this actually happen? And it appeared first in an eighteen fifty story by Fitz James O'Brien where there was a monster in a house that was alive and you know, not a ghost but invisible, so you couldn't see it. Was it cloaked? It was it just straight up invisible? It was just straight up invisible. Yeah, the whole monster itself was invisible. And then maybe the most famous early appearance was of course, in The Invisible Man by H. G. Wells, And this was right around the turn of last century, and it was just after X rays were discovered, and so people were sort of like getting their minds around the concept that there was different kinds of light and different kinds of visibility, right, that our bodies are opaque to normal light, but they could be transparent to other kinds of light. That was like a mind blowing idea. That is pretty awesome. Yeah, and so I think this god science fiction writers thinking about what is visibility and what is invisibility? Then a transition to science fictions. And now today maybe we'll talk about how it could actually be science. And so today on the podcast we will be answering that question and digging into all the various ways that you could have invisibility. The technology is underdeveloped, it's not actually there yet, but you might be impressed by how far people have gotten. So, as usual, I looked for volunteers for people who were willing to speculate baselessly on how something worked or answer a difficult physics question. So thank you to everyone who was brave and lent your voice, And if you would like to volunteer for future episodes, please write to us two questions at Daniel and Jorge dot com. Here's what people had to say. People have made really micro scale invisibility cloaks. I think invisibility cliques can be built, and I think at least one already does exist. I think it works by making light bend around it. I'm not sure if we can build a cloak per se, but I know we can make invisibility happen by using fiber optic cables. I think that's got to be a definite yes. And I think it really depends on what you mean by invisibility. But I think if if you consider a chameleon to be invisible, then yeah, I've seen cases where using cameras and displays and creatively arranged we can come close. Um. I think it's an engineering problem more than a physics problem at this point. Camouflage is something that has been figured out in the animal kingdom. I mean, if you look at an octopus then and a cuttle fish the way that they can completely blend into their background. So it seems like it's possible. Probably at some level. I really don't think we can. The problem with the invisibility clock is that if the wearer of the cloak is invisible to anyone on the outside, then the wearer also cannot see anything outside of the cloak. Probably not, i'd say, mainly because I'm guessing anything that can bend light around something with that force will probably have quite a horrendous effect on anyone or anything that it is put around. No, I don't think it's possible to build an invisibility cloak. I would say not likely, just because you can build it maybe invisible from one direction, but from multiple directions. UM, I can't imagine it. I have heard of some invisibility technology where it basically bends the light around something so that it looks like you're seeing through it. UM. I thought that it depended on the perspective of the viewer. So what do you think of those answers, Kelly? I thought they were really interesting answers. As a biologist, I thought it was interesting that chameleons and cuttle fish came up pretty often. And to be honest, when you told me the topic of this, my first thought was, woh, well, chameleons and octopuses can do it. I guess I sort of matched up with some of these responses. What did you think? Yeah? I was impressed. There's a huge variety of possible ideas in here, and I love in these responses hearing people sort of think on their feet. They don't know the answer, they're thinking about it, they're wondering, and that's like people doing physics right there, like taking their understanding of the universe and trying to use it to solve problems. Though. I love hearing people figure it out and actually making some progress on the fly. I'm impressed we have smart listeners totally alright. So let's dig into it. And first, maybe the thing we should talk about is what do we actually want to accomplish? If we're like talking to DARPA about building an invisibility cloak, what do we really mean buying visibility? What are the specs that we want to achieve? So you talked about invisibility in biology, and I'm super interested in that but don't know a whole lot about it. Could you tell us a little bit about how do octopy or chameleon or cuddlefish achieve some sort of biological camouflage. Sure, so I can't say that I'm an expert on this topic, but my understanding is that they have these chromatophors, which are cells that can expand and contract, and the cells are always filled with pigments, and whether they're expanding or contracted determines how much of the pigment you get to see. And so by making more of this pigment visible, for example, an octopus can become more brown by allowing more of the brown pigment up to the surface. And really complicated animals like chameleons might have different layers of these pigmented cells, and by sort of turning on some layers and turning off other layers, they can come up with really complicated and complex color patterns. But usually what happens when they're doing these things is they're staying like dead still. They're not moving at all. And so I'm guessing that invisibility cloaks, people who are wearing them expect to be able to to move while they're wearing them. Is that true? What do you think? I definitely think you want to be able to sneak around. Yeah, I don't think you want to just have to like squat somewhere and stay fixed. But also does that really count as invisibility? In my mind, what's happening is like an octopus is like sitting on a coral, and then it makes itself look like a coral, which is super amazing and impressive and not something I could ever do. But it doesn't look like it's not there. It just looks like it's more coral, right, yeah, fair enough, which I think would be a cool trick for people who want to sneak around, But I agree that it's probably not the same thing as being invisible. Yeah, Like, you know, you can make yourself look like a bush or whatever, but if you want to walk into a room and have nobody see that you're there, you can't just like, oh, I'm gonna look like more chairs or something. You need to blend in some way. So I think that's awesome and impressive, but not technically invisibility, right yeah, fair enough? All right, And then maybe we should disentangle invisibility from another similar concept, which is stealth technology. A lot of airplanes out there, you know, we have like stealth airplanes that can't be seen by radar. That's a really cool technology, but it's not also the same thing as invisibility. It's more like being totally black or like not reflecting anything. The way like a stealth airplane works is that when you shoot radar at it, and radar is just like another form of light, it doesn't bounce anything back. Like a normal airplane, you can think of like as a sphere, and if you bounce radar at it, it's going to bounce some back at you. Has a reflection, but a stealth airplane is like all these weird angles and so it has the profile so that if you shoot radar at it, the radar always bounces off in another direction. It's all these like sharp edges. There's no smooth curves that would always reflects something back at you. And so that's also not invisibility, right, It doesn't reflect any light. But it's sort of like equivalent to you know, painting yourself black. That doesn't make people invisible, right, I mean, it's it's pretty amazing that we figured out how to do that, but yeah, I agree. Also not invisibility, Yeah, not invisibility, like if you ordered an invisibility cloak that just gave you like a black sheet, you'd be pretty disappointed. Yeah, yeah, I want my money back for sure. All Right, So we want something that's actually invisible, which means it looks like you're not there, right. I want to be able to see basically what's behind you. I want to get an image in my eyes that would be the same image as if you were not there. And this includes being able to move or is it just not look like you're there and you're just standing still? Like I guess I'm wondering if moving is part of the definition here. Now, I think moving is definitely part of the definition. I mean, let's be aspirational, right, we're setting the bar high. We want like, actual, real, useful invisibility, and so I think we want to be able to move. And also I think it's important that it works for different wavelengths of light. Right, it can't just be like, oh, you're invisible in red light or in green light, or in radio waves or in microwaves. Has to work broadband across the whole spectrum, otherwise you'd be pretty easy to detect. So more than just what humans and their machines can view, is what we're going for. Yeah, because other wrinkle, right, and that maybe people don't really appreciate or think about, is that humans are more than just visible. We actually glow like we give off light. Because every object in the universe that has a temperature is giving off infrared light. You're radiating heat, which is why, for example, night vision goggles, what they do is they see in the infrared, and like every object in the universe that has a temperature, you glow as some color. So the sun is super duper hot, which is why it glows. In the visible light. Where we take a piece of metal and heat it up, it glows red or blue or white or whatever. Your body is also glowing. So I think invisibility should mean not just that you look like the thing behind you, but that you don't give off any light also, so it has to somehow block your heat. I had not thought of that extra complication beforehand. That's really interesting. Yeah, it's complicated. And in addition, I think if we're gonna set the bar really high, it should be that you can't tell any difference in the light that comes to you, even like a time delay. Right, you can imagine, and we're gonna dig into this and talk about various solutions to invisibility. You don't want that the light that's coming from behind you is slowed down as it passes through you by your invisibility cloak, because then again, somebody could detect that right and then if you started moving around really fast, the image would be sort of flawed, sort of like the Predators camouflage and fix seen those movies totally, But I mean you can imagine if you're an early adopter, you can like get away with some of these things because people aren't expecting invisibility cloaks. But yes, as you go further on, I think you've got to fix all these problems first gen invisibility cloaks of lower standards. You know, I was thinking about this last night. I was talking to my thirteen year old about this, and he was thinking, invisibility would that even really be a good idea, you know, because could you even use it? He made this really interesting point I hadn't thought of, which is, if you're invisible, can you see the rest of the world. Because if light is like passing through you and not interacting with you, doesn't that mean that you're not seeing the rest of the world. So if you're invisible, are you also then blind? Oh? Well, that's an extra complicate ation. I mean, could there be like a little screen inside of your cloak that's showing you what's going on outside? Yeah, you need something like that, right, otherwise you'd be totally blind. And then I thought, hold on a second, even if you aren't blind, even if the world can't see you, but you can still see outside, you're probably not seeing yourself, right. You can't see your arms or your hands or anything. And I started thinking about, like, what is it like to walk up a flight of stairs if you can't see your own body? I mean, you're the biologists tell me, like, doesn't our brain rely on seeing where your feet are to be able to go upstairs? Like I can't walk up a flight of stairs with my eyes closed? Very well? Totally? Yeah, No, that's that's a great point. You. I guess you definitely need to do a lot of practice. So you know, like I think astronauts when they go into space, they sort of learned to live without some of the cues that they're used to on Earth. So I guess with enough practice, you could learn to stop relying on being able to see what your body is doing to accomplish that task. But I definitely think you'd be clumsy initially. You'd need some training in this cloak. You definitely would. So the first end click probably not super sophisticated, and the first and gen user probably pretty clumsy. I mean you'd be like banging around into stuff. And also like how would you walk through a crowd of people? Right, Nobody would see you, so they wouldn't know to avoid you, so they'd be like bumping into you all the time. The more I think about it, the more like complicated it would be to be invisible. Yeah, there might just be very specific situations where you can use it, and otherwise you're likely to get found out. Yeah, Like you can't stand in the rain if you're invisible. Right, we don't have invisibility cloth. That's gonna be like rain permeable. Also, anyway, it sounds really complicated. You need to carry a huge battery pack or a really long extension cord also invisible, which no one can trip over, so you need to make sure your path doesn't cross anyone else's all right, So I want to learn more about the technologies that have been developed so far. But first let's take a break, and we're back and we're talking about whether or not you could actually make an invisibility cloak Daniel, What do we know about how you can make invisibility cloaks? Has anyone done research on this? People have been doing this for a while, actually, and this is a really fascinating field where people had ideas like fifty years ago and then it's sort of lay dormant for a long time, nobody was doing anything. I think people sort of shrugged it off as basically impossible, like the flash technology. And then about fifteen years ago there was like a little renaissance where people had sort of like starting ideas for how you could actually tackle this and tried it and made some progress and now it's like a flourishing field again. So it's an exciting time actually to talk about invisibility. Was there anything fifteen years ago that kicked it off, like some technology that made people think it might be feasible or was it just sort of random, probably some scientists staying up late reading some science fiction novel and then just having an idea. You know, it's cool the way these things happen, that there's like progress and then it's abandoned for thirty years, and then somebody has an idea and then it moves forward anyway. That's sort of like three different ways you might achieve invisibility. There's like projecting your image on something, there's like recreating your image like adaptive camouflage, and then when I think is the most promising of like bending an image around you. But let's talk about them in turn. So the first idea of this idea of image projection is pretty hokey, but I got a lot of attention In like two thousand and three, you win a lab in Tokyo put out a video showing how it would work, and the idea here it's basically, you have a camera behind you that's taking an image of what's going on behind you, and then you have a projector in front of you that's like shining that image onto your invisibility cloak. So then somebody standing basically next to the projector is technically seeing through you because they're seeing an image of what's behind you projected onto this cloak you're wearing. How smooth is the image, Like if there's any wrinkle in the cloak? Is there a wrinkle in the image, do you need to like stand there like a scarecrow. Yeah, you basically need to stand there like a scarecrow because you're right, like you're projecting this and so the image doesn't lie smoothly on the cloak, right, but it sort of works. If you stand there and hold your cloak pretty flat, then you basically look like a movie screen and it's sort of cool. It gives this sense of invisibility. You could definitely tell like, oh, where's the edge of the cloak, and you can see the guy's face because your face doesn't make a very good screen. But if you look at this video from this Tokyo experiment, it's invisibility ish, you know. It gives you that like the flavor of it. So you might be able to like stand still as somebody passes by and maybe they won't see you if they're not looking too hard. Yeah. Maybe. And also they have to like not pay attention to the fact that there's a projector they're showing this image onto you, and they have to be an exactly the right angle for this to work. So it's sort of like a cool demonstration that like maybe you could use to film a movie about invisibility to make that special effect, But it's never going to be like what we talked about, an actual invisibility cloak. You could us to walk around and do stuff. Yeah, right, if fails a lot of the criteria we were talking about. Yeah, but it's sort of like got people excited, and you know, there are some actual applications of it, Like it could be useful in some ways now when you want to sneak around in the faculty meeting and here where people really think about your work. But you know, you could imagine using in some scenarios, Like people have talked about using it for airplanes, Like if you can make the floor of the cockpit basically invisible, then a pilot would have like a better sense for where they are. It might be easier to fly if you like projected an image of the ground onto the floor of the cockpit, sort of a similar application. Do pilots have a lot of problems with that? I don't think pilots too much to the landing anymore. I think it's mostly AI these days, and AI doesn't care at all these images. But maybe it'd be cool, like for passengers, you know, if like the floor the airplane looked like it wasn't there, but they think i'd be cool with that would be like terrifying. I think it might be terrifying. It might depend on the person or the person you're sitting next to you. I think i'd be cool with it, but if the person next to me started to get nervous, that would make me nervous. Well. I remember Odd airplanes when they first had this feature that they had a bunch of cameras and you could do yourself like cycle through them and like look down or back or outside. I thought that was super cool, like connect to you with what's going on outside the airplane, because otherwise you feel like you're in this bubble. You get on it in l A, you get off it in New York. You don't really see experience the transition much. So I think it's cool to be connected to what's going on outside. But I think that would be pretty terrifying. When was that a thing that does sound awesome? Yeah, I think it's you know, in the last ten or fifteen years or so, when they you had like your own little screen that you could control yourself rather than like everybody watching the same movie, and so you could look at these cameras. It's like a camera in the nose and a camera in the tail. I thought that was pretty cool. I guess I've been flying the cheap airlines. I gotta I gotta upgrade. That does sound fun. You gotta get some of that government funding. There are other applications, like you could imagine painting the inside of your car with a view of what's going on outside, like, hey, maybe no more blind spots, right, Or you could look out of your car and actually see what's behind you. We have sort of a version of this already where people have like cameras on their license plates and you can just look at a screen in front of you and see what's behind you. But it would be sort of cool if the car didn't block your view at all. Yeah, that would be awesome. I bet that would save some lives. Yeah, And I read about some applications where in surgery, for example, you could paint an image of what's behind the surgeon's hand on his hand so he can hear she could see what they were doing better, like make their own hands invisible. So that would be I mean, if you got your hand inside a body, there's gotta be a lot of things getting in the way of the projection getting to the hands. So would it be like the gloves projecting, like would you have fancy gloves? Yeah, I guess if the body is opened, then you'd want, just like the gloves to have a projection on them. But if it's closed and you're doing it like laparoscopically, maybe you could like project an image of what's going on inside the body, like on the torso that would be kind of cool. So it would like look like you're opened up, even if you're not. That would be super cool. That does sound super cool, you know, that's fun. That's interesting, But it's definitely not real invisibility, right, and it's not a cloak, and it's not a cloak, right. And so moving on from that is a second category, which I think is closer to what we're trying to do, and this is like image recreation. So it's the same idea where you're like gathering the image on one side and creating it on the other side, but instead of using like a projector has to be like far away from you and then the image only works at one angle, you have basically like adaptive camouflage. Imagine like the cloak being made out of pixels. And it has cameras all over the place and pixels, and so it's taking pictures of what's behind you and creating that image on the pixels on the outside of the cloak. Well, and so you still have the problem with needing to make sure you're not bending or folding anywhere, right, Yeah, maybe, I mean it's potentially with like fancy computation and AI, you could imagine that the cloak would sort of know how it's arranged, and so it could know like where the cameras are pointed and how the cloak is folded to create the image in just the right way. A simpler version, which is like painting image on the cloak, assuming it's flat, but that might not always work. But if you knew how it was curved, you might be able to adjust how you're painting the image to still make it work. So that is totally epic, and it sounds like it would require a lot of computing ability, So like to do this, do you need to have like a big battery or carrying a computer inside the cloak with you. I'm gonna imagine you're carrying like one of those old tower computers from like the year two thousand inside and people are like, I hear the fan worrying on your computer, man, Like that would give you away. Yeah, you need sound canceling something. Yeah. But you know, in terms of like the general possibility, you could imagine maybe solar power and super micro computers of the future could potentially pull this off. And I think this is the basis of the Predators invisibility in this sort of like adaptive camouflage that projected the image behind him. It created the image on his surface so that you could see it. And I think one cool thing about that is that I think it should work from any angle. Right. It shouldn't matter necessarily what direction you're looking at the cloak from. It should still be able to work as long as you create the right image. You've got from all sides you need cameras looking out, and then from all sides you need pixels so that if somebody walks behind you, you're still invisible. So is that the idea. That's the idea. Yeah, And people are actually working on this, and there's a lab I've seen one of their papers where they have these thermo chromic liquid crystals. Liquid crystals are these things that are very cool. A lot of people have l c D displays on their computers or on their calculators or whatever. But these can be flexible, so they can be like woven into fabric and they can respond, of course, you know, to computation, and so you can create basically any image. And these are thermo chromic, which means that they change their color based on the temperature. Remember those mugs that if you poured hot water into them, an image appeared or disappeared or something. I have one that says best daughter in law. Wait, and what happens when you pour water into it changes to worse daughter in law or something. You know it's black, and then it says best daughter in law. But yeah, maybe there's a secret message in there. If you pour hot enough water in there, you see the secret message. I know how they really feel. Then yeah, no, I don't want to get you in trouble with your family. So I've seen an example of this. They have a small patch like a few inches by a few inches that can effectively create an image of what's behind the user. But you're already takes a lot of computation, and if you want it to be very effective, it has to work quickly, right, has to like really quickly update the image, otherwise it will be obvious if you move or somebody changes their perspective. So if this depends on temperature, if somebody had like a temperature sensing device, would they be able to detect your cloak? No, because the cool thing is that this thing can also cloak your temperature. These pixels can create an arbitrary color, but they can also heat up or cool down to cloak your temperature. For example, if you're in a warm environment, you want to give off a lot of heat. If you're in a cold environment, you want to not give off a lot of heat. So that's actually a really cool application of this sort of like adaptive camouflage that it can cloak you not just in terms of visible light, but also in terms of infrared light, effectively your temperature. What is the largest size of one of these that they've been able to make. Are we talking about cloaks? Are we talking about, you know, postage stamps where this has been tried out so far. We're talking about like basically a napkin. So it's an invisibility napkin, so you might be able to like hide a small road in behind it, you know, but you're definitely not squeezing behind this napkin. But you know it's also chunky, Like the pixels are not very small, they're like a centimeter and by a centimeter. But you know, these are engineering problems, and so as I often like to say, you know, the physics has been figured out, the idea is there, and we just hand it off to the engineers and they can make the pixels smaller and make it the battery more efficient, and make the cameras better and you know, it's a it's a tech problem now exactly. There'll be an app for it eventually, no doubt. So that's image projection, which I think is pretty hokey, and then like image recreation or adaptive camouflage, which I think has some promise, but also there's definitely some issues. They're like the one that you identified, all this computation that's happening inside the cloak that would get pretty warm. I'm joking around, but that's not a small tech problem to solve. This seems like a good place to stop, So let's take a quick break and we're back. Okay, So, Daniel, you said that there were three different sort of types of technologies for invisibility cloaks. Let's tackle the third, all right, and this is I think the one that might actually one day achieve like actual invisibility and real cloaking, so I'm pretty excited about it more. The idea here is basically image bending. Instead of projecting the image onto you or recreating the image, interrupting it, digesting and computing and recreating it, it's like, just take the light that was gonna hit you and instead of having it hit you, have it flow around you instead. So the common analogy that's used is think about a rock that's in a river. The water is flowing, it flows around the rock, and downstream, it's not easy to tell that the rock was there all the water is flowing basically in the same direction as it was before. So if you could somehow get light to bend around you and then reform the original image that was behind you, you could effectively be invisible. Out of all the technologies you've talked about so far, this one seems the most sci fi to me. Really hard for me to wrap my brain around this being a thing that we could actually do in a lab. Well, it's really cool and there's a lot of people working on this kind of stuff, and there's a few different technological approaches that people are taking, but it's the one that while most science fiction e actually I think is most realistic. So while I'm a physicist, I'm not an expert in optics. So I reached out to an expert, Greg word, and asked him to talk to us about this new promising technology. My name is Greg Gaboram, a professor of optical science and engineering at the University of North Carolina and Charlotte, and my specialty is theoretical classical optics. Basically, light is a wave. So I was really grateful to Greg for giving us some time and talking about the prospects of invisibility. So I asked him what he thought was the most promising technology. The approach that really gained prominence starting with some papers in two thousand six is the idea of of invisibility cloak. That it's not like a Harry Potter cloak, where it's a flexible material that you can strape over yourself. It's a structure, probably a rigid structure made of some very specialized material that we don't completely know how to make yet, that would guide light around some hidden interior region and send it along its way as if it had encountered nothing at all. One analogy that was used by those original authors in two thousand six was like water going around a rock and a stream, which, amusingly is also a description that a science fiction author used in the nineteen thirties used exactly the same analogy. I think the best way to understand this is to think about the way you might see this otherwise in your life, Like if you're on a road on a hot day, you can see this reflection on the horizon. It's sort of like the desert mirage effect, where it looks like there's a puddle of water in the desert, and what's happening there is just that light is bending, Like you're not actually seeing water on the highway or water in the desert. Where you're seeing is a reflection of the sky because the air that's just above the hot sand creates this effect where light from the sky gets bent towards your eye. What you're seeing is an image. You're not actually seeing the reality, so it fools your eye into thinking there's a puddle of water on the ground. So the same basic idea of finding a way of bending the light around something. Is the physics behind this approach, and so why and we do this in a lab, it won't have the like wibbly look that you get over, you know, a highway on a hot day. It'll it'll be a lot more controlled than crisp. Is that the idea, that's the goal. And people are doing this in several different ways, which I think is pretty cool. There's like really three or four totally different approaches to bending the light around you. Number one, which is sort of science fictionally cool but probably not very practical, is using nanotubes. Nanotubes are these really weird constructions of carbon that were invented a couple of decades ago. These carbon nanotubes have lots of really interesting electrical and other kinds of effects, and they're very very thin, and they're very very strong. Actually, they're like one candidate for how you could build a space elevator. Right. They're super strong and thin and they can hold a lot of weight. But also they have weird optical properties and when you heat them just right, light bends around them. And just this way that we're talking about that blows my mind. Because they're just carbon, it seems like such a simple molecule, but yeah, it would be very cool if it could do this. Yeah, and that's the base principle. It's a simple molecule, but complex arrangements of simple molecules can have really interesting optical effects. I think the best example is like silver. Take a piece of silver and you polish it. What do you get? You get a mirror, right, silver makes like some of the best mirrors out there. But take that same silver, chop it up into nanoparticles and suspended in water. What does it look like? It's jet black. In fact, a lot of the negatives that you see the black in that comes basically from suspended silver nanoparticles and so like the same materials, just the same silver can have very different optical properties based on how you build it because the optical effects depend on how light interacts with it and the surfaces inside and exact arrangement of electromagnetic fields can really change how that happens. So that's the key idea. Yeah, but nanotubes aren't really very practical. They did a little demonstration where they have a really small bit of it and it kind of works, which is cool sort of in principle, but you've got to heat this thing up and only really works for like a very very thin sheet. So if you want to wear a very hot, thin sheet of nanotubes, then you know, maybe you could look a little invisible, but it's not something that can easily be scaled up about how hot are we talking. We're talking like a hundred degrees celsius. Yeah, that's probably know, that's not gonna work out. It reminds me of that Nathan for You episode where he puts on the chili suit and then he realizes he doesn't want to walk around in a suit of hot chili. And now there's another approach which has gained a lot of attention on the internet, probably because they put the name quantum in it for no good reason. And that's this thing called the Quantum stealth invisibility cloak. And if you just google invisibility technology, probably the top ten videos you'll see are people demonstrating this stuff. And at first glance, it looks pretty cool. It's like a sheet of stuff and if you stand behind it, people can't see you. And he's like a guy and he's holding a sheet of the stuff. It's like a real size sheet of it. And when you look at the sheet. You only see the thing behind him, the wall behind him. You don't see his body, so it's like you see the top of his body and then the middle of its invisible, and then you see his legs. So it appears pretty impressive at the beginning, So it's not actually really invisibility in my mind, because the way it works is that actually has like a grooved surface. It's sort of like you know holograms that have these grooves on them and they give you like a weird image as you look at them, or you know, like a wall of glass that has like a rough edge to it, so it doesn't project the image behind it very clearly. These have these lenticular lenses. He's like very narrow grooves. So it happens is that when the light passes through it, it gets spread out, but only in one direction. So, for example, the light that from his body he's standing behind this invisibility shield, the light from his body gets smeared out in one direction, and so you actually still can see him through the invisibility shield. It's just that his body has been smeared out, so you don't really notice him anymore. It's like diluted his effect by smearing it in one direction. So it sounds like for some of these ideas, it really sounds like if they're used very rarely, you might be able to get away with it people aren't expecting it. But if it becomes a popular tech, it's just not going to work because you've noticed the smeared guy. If you need to look out for him, that's right. And then once people know that it's out there, they'll be on the lookout for it, and it wouldn't work very well anymore. And this, for example, it's pretty easy to fail. It works at some levels, but if you're looking at it from the right angle or actually from the wrong angle, then it actually makes what's behind the cloak look bigger and larger. It amplifies it. Awkward, Yeah, awkward if you're like sneaking up on your enemy, you know, for military applications, and all of a sudden like boom, you know, they can see you. That could be embarrassing. But maybe this could work in like the animal Kingdom, you know, where you like try to look bigger than your enemies. Maybe you could give like small chickens this, uh, this device, and then they could be you know, top of the pecking order. Yeah, exactly. So in my mind, this is basically just the same thing that like a fuzzy shower door does. You know, like somebody's taking a shower and the door is fuzzy, you can't really see them, right, It basically blurs them out. This is that, but it just sort of like maximized like maximum blurs. You don't really noticed that they're there. But again, anybody who's really on the lookout is going to tell. So this guy has been trying to sell his quantum stealth technology to the military for like a decade, and they have been buying it, I guess for a reason. Yeah, yeah, and so now he's offering it to the general public. So anyway, it's cool. It's definitely not something I could have invented. It's pretty simple stuff, you know, achieves something of what you want to do. But again, it's not real invisibility. It's not really bending the image around the person. Yeah, I certainly wouldn't want to go to war with a sort of smudgy mirror in front of me, smugy piece of glass. So the most promising approach came after a breakthrough in two thousand and six when people realize that if you made a really weird new kind of material, a substance called a meta material, then you could use that to bend the light around an object because had a really strange optical property of having a negative index of ref action. What does that mean? So index with refraction tells you how a material bends light. So, for example, you know that when light hits glass, it passes through, but it also gets bent. Right, that's how prisms work. Or if you look at somebody who's half in the pool and half out of the pool, the bottom half looks like shifted in one direction. That's because the light bends a little bit as it passes through the material. Well, that's an index of refraction. It's usually measures like one or above. But if you had a negative index of refraction, the light would do something really weird. Instead of entering at an angle and just changing its angle a little bit, it would enter and then go the other direction, so it would make like more than a ninety degree turn and bend really weirdly inside the material. Well this is cool because if you wrap an object in this negative index of refraction material, then it basically bends the light in such a way that it passes around the object, and then when it comes back out, it's going in exactly the direction was originally. So the idea was, oh, maybe if you had this negative index of refraction material, it could actually achieve this invisibility. Yeah. Well, the coolest part of the story is that the idea comes from like the sixties. Somebody wrote a paper in the sixties being like, hmm, this seems impossible, but if you could make a material that had this weird property nobody's ever seen before, then you could achieve invisibility. Ignored for four decades until in two thousand and six, people realized if you could create this meta material, and a meta material is a material that has a property that you don't usually find in nature, like negative index of refraction. They realized there might be a way to do it by constructing a really strange material. And so, for example, to have this material with really thin layers of silver and then really thin layers of magnesium fluoride. And there's another way to do it, making these like silver nano wires. And the way to understand it and to think about it is to remember that light is just an electromagnetic wave. So what we're talking about is bending light. We're talking about electromagnetic interactions. So you just create these atoms in a special arrange mints, so they have these weird electromagnetic properties that bend light in whatever way you want. And this is possible now and it wasn't fifty years ago, because now we're much better like building these nano materials, like engineering super duper small structures and then making a large sample of them. So has anyone tried making a cloak out of this or is this still pretty preliminary? It's still pretty preliminary. Like they got it to work, and where they built basically was like a cylinder of this stuff, and you can't see the cylinder, Like you shine a bunch of light at it, and the light comes out the other side going exactly the same direction that it entered. Yeah, it's like basically invisible, but it only works in certain wavelengths. So far it works in microwaves, or maybe it works in radio waves. It doesn't work in visible light. You can still see this thing in the visible light, but you know, if somebody shines a microwave laser at it or shoots radio waves at it, then it's invisible to that kind of radiation. So this would not require the like giant batteries and computers that we were talking about for the other one. But would it cover your own heat or is that another problem? Yeah? That is another problem. But you're right, this thing is like static. It doesn't need to be powered. It's just like a structure of material that bends light in a certain way. And if you're inside of it, you're like walking around with this cylinder of meta material. Then imprinciple, people can't see you in the microwave or the radio wave. But you're right, it doesn't mask your heat, right, doesn't like absorb all of your heat. I don't know how you could do that, right, because you're putting out a lot of heat and if there's no way to vent it, you're just going to basically roast inside this thing. Yeah, and is that the state of the art right now? Is that the fanciest solution we have? And that is basically what we've achieved. And by we, I mean you know, the whole physics community actually working hard on it, and me just sort of like reading articles about it. But hey, that's a contribution. And people are trying to think about how to make it work for visible light. Also, like, is it possible to build a material which could do this for all kinds of frequencies? And that's sort of the next generation. That's what people are working on now. And I read a paper from lad Ear that has a really cool title on it. It talks about developing a tachyonic invisibility cloak. What the heck is that? Yeah, attack eon is something that travels faster than the speed of light. So we talked in the podcast once about like what is a tacky on It's a hypothetical particle that might travel fast than the speed of light. We don't think it exists. It violates special relativity. But the phrase tachi on it, it doesn't just exist in star trek is referred to anytime something moves faster than the speed of light. And so that's pretty cool. And the idea behind this was they did a bunch of theoretical calculations to figure out could you make a material which number one passes all frequencies of light through it in the same way that the meta material does, right, having a negative index of refraction, but works for all kinds of frequencies, not just microwave or radio wave. And number two, and this is an important and tricky bit that also didn't have a time delay. One problem with bending the image around you is that then the path length of the image is longer. It takes more time for the light to travel around you than if it had flowed through you. Somebody with like really sensitive devices might be able to measure that time difference and deduce that you're there. So they wanted to develop a cloak that was take ionic so that the light traveled on that longer path was going faster effectively than the speed of light, in such a way that it emerged on the other side of the material exactly the same time it otherwise would have if you weren't there. And is that possible? Well, they claim that it is, and they do all these calculations, and your first thought must be like, hold on a second, what about special relativity? Right? Doesn't that violate causality? How could you possibly get from one side of material to the other and do it faster than the speed of light. Well, as usual in physics, there are loopholes, and if you read the rules really really carefully, you can discover that there might be ways around them. For example, what actually is the rule about traveling faster than the speed of light, it's that no piece of information can go through space faster than light travels through space. So, for example, we've talked in the podcast before about how that doesn't mean that you can't get from one place to another faster than light could if you could like bend space itself. Right, That's like how warp drives and wormholes work. So the spirit there is to look for loopholes, and the loophole here is to think about how the wave propagates through this stuff. And there's two actually different velocities to think about when you think about the wave. There's the phase velocity. That's like how an individual piece of the stuff is moving, like the thing that's doing the waving. How fast is that being actually moving? And that can't move fast in the speed of light because it's the thing and moves through space. But then there's the group velocity. The group velocity tells us about like how the speed of the whole wavefront is moving. Is not actually a requirement for the group velocity to move slower than the speed of light. So this is pretty tricky. So I asked our friend Greg Gaber to see if he could explain it to us. If you have a material that has optical gain that you can pump so has energy built into it, you can basically have a wave that effectively moves faster than the vacuum speed of light, kind of like an avalanche. That the center of gravity of an avalanche can move faster than the front of the avalanche because it's picking up extra rocks as it goes and leaving some behind. So the shape of the wave itself is moving, so you're sort of like losing the back edge of the wave and adding new stuff to the front edge of the wave, which effectively makes the wave move faster. So there's a lot of technical stuff in this paper. If you're really interested, I suggest you look it up. But the basic idea is that you use some loopholes and trickery to try to get that light to pass through the material in such a way that it appears with no time delay on the other side. So is this something that is working on a computer but not working in real life yet. Yeah, this is just a calculation and they've done in simulation, and they say, if you could build this thing and it has these properties, then you could actually achieve that that's exciting, and the people I've talked to you said, wow, this is a cool idea. And in the paper they don't actually build it, right, they just talk about how you might be able to and to do that you need this thing called a semiconductor optical amplifier, and that's what Bird was talking about. It needs some energy, like requires a little bit of optical pumping. So this is a technology that people are working on, have developed. They haven't quite made it do this thing yet, but it's sort of like, you know, at the theoretical stage, like, hey, this idea might be possible. Let's get some folks in the lab and try to actually build this thing. Awesome. I hope it gets funded. Probably dark, but would be all about it. So I think that covers most of the topics of like how you could do invisibility and what people are doing and the progress that's been made. And I'm pretty excited that people are making progress. Like we've gone from this is a ridiculous thing in science fiction to hey, here's how you might actually be able to do it, to like, look, we have some basic working prototypes that you know, don't satisfy all the requirements, but satisfy some of them. I'm torn between thinking it's totally awesome that we're making progress on this, being excited about seeing the world of sci fi come to real life, and also being a little bit uncomfortable about a world where this technology exists and people can go sneaking around. So I guess, I guess I can see the good and bed of this well. As Professor Gooberg what he thought the prospects were for invisibility in the future, if you want to make a device that would work very well to hide things from visible light, you would need to design something that has a controllable structure on the order of the wavelength of light, which means very very small sizes, and we don't quite know how to do that yet. So we might start seeing interesting applications not in visible light. But something that I've noticed is there's a trend of looking at how can we use this same cloaking technology for other types of waves. People have talked about making earthquake cloaks or cloaks that guide water waves around structures, And I love his idea there that we could use invisibility for more than just like sneaking around and snooping on people, but for like making objects invisibles to other kinds of waves, like earthquakes or water waves or sound waves. That's a really cool idea. So maybe one day the whole Bay Area will be cloaked in some sort of earthquake invisibility device. Alright, So that's the science of invisibility. Thanks for taking this trip with us to understand how people are pushing the forefront of physics to make science fiction real. Thanks tune in next time and don't forget. If you have something you'd like us to break down, send it to us to questions at Daniel and Jorge dot com. Thanks for listening, Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio or more podcast for my heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. M