Cosmic Queries – The Science of Invisibility with Greg Gbur

Can you make something invisible? Neil deGrasse Tyson and comedian Negin Farsad discover the science behind invisibility with professor of physics and optical science, Greg Gbur. What would real-life invisibility look like?

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Neil deGrasse Tyson?

  Welcome to StarTalk. Your place in the universe where science and pop culture collide. StarTalk begins right now. This is StarTalk. Neil deGrasse Tyson here, your personal astrophysicist. Today's edition is a cosmic queries on the subject of invisibility. We all want to be invisible at some point in our lives. And I got with me Nagin Fersah. Nagin, welcome back.

• 0:32Negin Farsad?

  Hello, Neil. I'm so excited to be here and to be talking about this crazy topic.

• 0:36Neil deGrasse Tyson?

  It's a crazy topic. And those who are meeting you for the first time on this show, all right, you're a host of Fake the Nation. You've got a podcast of your own there. And you're also a regular on Wait, Wait, Don't Tell Me. I think I've been on that twice. And I didn't do very well. They asked me questions. I got them all wrong. They never invited me back. And most-

• 1:00Negin Farsad?

  You know, they usually help guests just get the questions right. So they must have had something against you in particular, not to help you. They usually rig those tests.

• 1:11Neil deGrasse Tyson?

  Yeah, but it's a delightful moment of conversational comedy in Wait, Wait, Don't Tell Me.

• 1:16Negin Farsad?

  Absolutely.

• 1:17Neil deGrasse Tyson?

  On NPR, of course. Is it still on NPR?

• 1:18Negin Farsad?

  Absolutely, yes.

• 1:19Neil deGrasse Tyson?

  Excellent. And you're also a host of, recently, the Succession recap. You know, I've seen episodes of Succession. I don't want that recap. Okay, it's too weird. It's too, it's too, I don't know what's going on or why, or should it be happening? I don't understand it. I don't want to understand it. And you're going to recap it? No.

• 1:38Negin Farsad?

  I mean, Neil, you have no idea. These recaps are like, just out of control. We are, we get so deep into the psychology of billionaires. It is fun, ridiculous, and we have no idea what we're talking about because none of us are billionaires.

• 1:52Neil deGrasse Tyson?

  No, it's too weird.

• 1:52Negin Farsad?

  Tune in for the fun.

• 1:53Neil deGrasse Tyson?

  And the family tensions and everything. I don't want it, yeah, okay. All right. And I've read a little about this, but we have people out there who are experts in invisibility. And we're not talking about the CIA or anybody like that. We're talking about, we're talking about actual physical optics of invisibility. And so we combed the landscape and we found Professor Greg Gabor. Greg, welcome to StarTalk.

• 2:24Speaker 02

  Hey, thank you. Great to be here.

• 2:26Neil deGrasse Tyson?

  You're a professor of physics and optical sciences at University of North Carolina at Charlotte. So your research focuses on classical optics, the wave nature of light. So classical as opposed to quantum optics, I guess. Is that how you distinguish that?

• 2:41Speaker 02

  Yeah, that's exactly it. Light is a wave, not as a particle. And

• 2:47Neil deGrasse Tyson?

  So you're not in denial of it being a particle. You're just saying you care about it when it manifests as a wave and you're good with that?

• 2:54Speaker 02

  That, yeah, that's about correct. I find the, I find the wave properties just really elegant and beautiful and fascinating. And even though we've known lights away for like over 200 years, there's still so many interesting things to discover.

• 3:07Neil deGrasse Tyson?

  And you're author of a book that just has an audacious title, invisibility. Let me get the title right here. The history and science of how not to be seen. Oh my gosh, released in 2023. So you're the right guy for this, I'm guessing.

• 3:23Speaker 02

  I hope so. Yeah.

• 3:26Neil deGrasse Tyson?

  All right. So, so Nagin, we, we've collected questions from our Patreon members. Now that the, the barrier of entry to become a Patreon member has been reduced. So you can actually have access to our questions and be not only special recordings of our questions and answer sessions, but also you get to get one of your questions submitted. And that's for like $5 a month, which I'm sure costs less than all orders of coffee you could possibly put at Starbucks. I'm pretty sure. Is that right?

• 3:57Negin Farsad?

  I mean, absolutely. Well, yeah. And that includes tip, I think. It's a bargain.

• 4:01Neil deGrasse Tyson?

  Right, right. Totally, totally, totally. Okay. So what, so what do you have for us?

• 4:06Negin Farsad?

  Okay. So Dylan from Flagstaff asks, this question sounds weird, but can you be invisible in other spectrums of the electromagnetic field? It's one thing to be invisible from visible light, but how about ultraviolet or infrared?

• 4:22Neil deGrasse Tyson?

  I love that. And let me just preface that, Greg, by saying,

• 4:28Neil deGrasse Tyson?

  You know, technically, a window is invisible, right? Because you can't see the window if you see what's on the other side of the window. The window's just kind of not there. So this whole thing of invisibility, could you just put it on some kind of foundation? So that we can know how to think about the questions that are getting asked?

• 4:46Speaker 02

  Sure. I mean, I do draw usually a distinction between transparency and invisibility. So a window is very hard to see, and we all know that in a lot of cases, you just can't see it at all.

• 4:59Neil deGrasse Tyson?

  But how clumsy you are, yes.

• 5:02Negin Farsad?

  Yeah, ten-year-old Nagin definitely didn't see the sliding glass door that she walked into.

• 5:08Neil deGrasse Tyson?

  There's nothing more embarrassing than that.

• 5:11Negin Farsad?

  So embarrassing, so embarrassing.

• 5:12Speaker 02

  Well, now I feel bad because when I give talks, I often use a gif of people walking into like glass doors to highlight the point. But so, so yeah, I mean, so we tend to think of windows as transparent because they always do for, in almost every circumstances, reflect a little bit of light. And when we tend to think of invisibility, we think of something that isn't going to reflect light, and it isn't going to disturb the light that passes through it or around it, so that the object becomes, in principle, undetectable for whatever spectrum of light you're considering.

• 5:49Neil deGrasse Tyson?

  All right, so visible light, this questioner clearly knows about other bands of light in the electromagnetic spectrum. And I'm happy to, I'm always jumped to point out that windows are transparent to visible light, but are almost opaque to infrared light, right? So if our eyes were tuned to infrared, wouldn't the window just be opaque? It'd be like a wall? Is that a fair statement?

• 6:16Speaker 02

  I think so, yeah.

• 6:18Neil deGrasse Tyson?

  Yeah, okay, so now, in whatever you do to make something invisible, like, give us an example, and then tell me how you might do that in a different band of light. So what's your favorite band of light to render things invisible?

• 6:31Speaker 02

  You know, I'm always partial to visible, but I mean.

• 6:34Negin Farsad?

  Okay. Call him old fashioned Neil.

• 6:37Speaker 02

  Yeah.

• 6:37Negin Farsad?

  He's just partial to visible light.

• 6:40Speaker 02

  I'm a traditionalist. I do classical optics. None of those fancy waves for me.

• 6:46Speaker 02

  So actually, one thing I can say is that the first experimental test that somebody did to try and demonstrate at least the principles of making an invisibility cloak, they did it for microwave wavelengths, which are longer wavelengths, you know, closer to the radio wave spectrum than the visible spectrum. And they did that because the wavelengths are longer. It turns out it's easier to design the structures that'll do this light guiding and hiding of a hidden region.

• 7:17Neil deGrasse Tyson?

  Wait a minute, Greg. Greg, microwaves, that's what police radar guns use. So can I make my car invisible to the police radar gun?

• 7:27Speaker 02

  You know?

• 7:29Neil deGrasse Tyson?

  I don't want to, you know.

• 7:30Negin Farsad?

  But Neil, what are you trying to get away with out there? What is going on right now?

• 7:36Neil deGrasse Tyson?

  It sounds like this guy knows how to, he's never paid a ticket in his life. Okay, no, go on.

• 7:43Speaker 02

  Dang, I mean, you just gave me a new application. I had to pause for all this time.

• 7:47Negin Farsad?

  Fine.

• 7:49Neil deGrasse Tyson?

  Right.

• 7:49Speaker 02

  And one that's surprisingly plausible, actually.

• 7:54Speaker 02

  Because the little structures you need to design to fabricate sort of an invisibility cloak at microwave ranges are more on the order of millimeter sized structures or centimeter sized structures, which are a lot easier to do than to try and make structures for visible light, which is on a billionth of a meter scale of the wavelength kind of thing.

• 8:14Neil deGrasse Tyson?

  Oh, you're saying your, your, your tools and, and, and the bricks you're using to make the, the, the object, you know, the, the construction elements have to be on the size or smaller than the size of the wavelength itself?

• 8:30Speaker 02

  That's exactly right. And that's a great way to put it is sort of little building blocks. Because, and if I could already jump in to use the technical attention catching term metamaterials, cloaking devices and invisibility were part of this, what they called a metamaterial revolution. The idea that if you mess around with the structure of materials on the scale of a wavelength, you can suddenly get all sorts of optical behaviors that you don't get in a material in its natural state.

• 9:02Neil deGrasse Tyson?

  So, okay, so, all right, so what's an example? So now you, you, you meddle with my material and then I become invisible. What hap- what did the light do that was behind me that now comes straight to you to make you think I'm not standing here?

• 9:17Speaker 02

  The kind of the traditional invisibility cloak.

• 9:20Neil deGrasse Tyson?

  Traditional.

• 9:21Negin Farsad?

  Yeah, yeah, yeah.

• 9:22Neil deGrasse Tyson?

  You had one of these, the traditional ones.

• 9:24Negin Farsad?

  So classic, yeah. The OG, yeah.

• 9:27Neil deGrasse Tyson?

  Old fashioned, okay, go on.

• 9:30Speaker 02

  These are the ones that people first introduced in like 2006 theoretically. And those, the idea is you design your materials such that they guide light around a central hidden region. And kind of like the analogy that they often use is like water flowing around a rock in a stream. The idea is as the light goes in, it takes a detour around the hidden region. And then it goes on its way exactly like it came in, it goes out the way it came in. So that in principle, you don't know it took that detour.

• 10:02Negin Farsad?

  And you keep saying materials, are you like essentially talking about an invisibility cloak? Am I allowed to say cloak on this podcast?

• 10:13Speaker 02

  I think you can.

• 10:17Neil deGrasse Tyson?

  Right, was that a term before Harry Potter, like invisibility cloak?

• 10:20Negin Farsad?

  Yeah.

• 10:21Neil deGrasse Tyson?

  I don't know.

• 10:21Speaker 02

  Oh, this is one of my favorite little mind bending things, or at least for my mind.

• 10:28Speaker 02

  That when I was researching the book, I was kind of going through scientific papers, trying to find the earliest examples I could of scientists talking about invisibility. And I suddenly happened upon a paper from Science News in, I think, 1942 or so. And it was titled Cloaks of Invisibility.

• 10:48Neil deGrasse Tyson?

  Well, there you go.

• 10:50Speaker 02

  And my heart literally skipped a beat, I was just like. I thought I was gonna like rewrite two thirds of the book and kind of re-figure everything out, but it turned out they were talking about just regular camouflage.

• 11:03Neil deGrasse Tyson?

  Oh.

• 11:04Speaker 02

  You know, hiding, hiding things in camouflage, having somebody hiding in a spider hole under the ground.

• 11:09Neil deGrasse Tyson?

  Okay, and that's, and that's a wartime paper as well.

• 11:13Speaker 02

  Yeah.

• 11:13Neil deGrasse Tyson?

  Right, nineteen forty-two, I see. So, so you're not invisible, you just, no one notices you.

• 11:18Speaker 02

  Yeah.

• 11:19Neil deGrasse Tyson?

  Right.

• 11:20Speaker 02

  That, that's where you get kind of, for me, I always, I kind of say that you have to be careful. The word invisibility is incredibly suggestive and incredibly vague.

• 11:31Neil deGrasse Tyson?

  So, Nagin, have you ever seen those people who are painted into a pattern on the wall?

• 11:35Negin Farsad?

  Yeah, no, totally. That sounds like what that book was about, which is more just like an arts and crafts project.

• 11:42Neil deGrasse Tyson?

  Only if you move do you see them, but if they position themselves just right, all the patterns of paint, they just disappear into the wall.

• 11:48Negin Farsad?

  Yeah, yeah.

• 11:50Neil deGrasse Tyson?

  I've seen models do that and others, you know, who don't mind people painting on their bodies, you know, this sort of thing. All right, so, so Greg, what you're saying is you can do this in principle for any band of light, it's just easier for longer wavelengths of light. That's what you're saying here?

• 12:07Speaker 02

  That is exactly it.

• 12:08Neil deGrasse Tyson?

  Okay.

• 12:10Speaker 02

  The longer the wavelength, the easier it is to make the fundamental building blocks you need to make to kind of construct the light-guiding material.

• 12:19Neil deGrasse Tyson?

  Okay, so you just say this like, of course, you can do this with the material. So what precisely are you rearranging at the, you know, at the level of the size of, of your bricks to enable the light to go around it the way water goes around a rock?

• 12:37Speaker 02

  Okay, well, this is where it's, I still have a hard time coming up with a good example that visualizes it, but I'll give a historic example, which is the, the founder of metamaterial theory is John Pendry, and in the late nineteen nineties, he was contacted by a company that was making great-

• 12:54Neil deGrasse Tyson?

  I gotta interrupt. The way you say that, I'm thinking he's in the late eighteen, no, the late nineteen, nineteen. Dude, I was alive then, right?

• 13:02Negin Farsad?

  Yeah, but that was still the nineteen hundreds, you know what I mean?

• 13:05Speaker 02

  Yeah.

• 13:05Negin Farsad?

  It was a long time ago now.

• 13:07Neil deGrasse Tyson?

  Dude, I was alive then. Give it a little more respect in that place.

• 13:11Speaker 02

  We still had landlines.

• 13:15Neil deGrasse Tyson?

  All right, I'm sorry to jump in. All right, continue, please.

• 13:18Speaker 02

  So, um, so what happened is this company contracted John Pendry. They said, we made this great new paint and it's made out of carbon and it's really good at absorbing radar waves. But we do not know why it does this.

• 13:35Neil deGrasse Tyson?

  Radar is microwaves, right, basically?

• 13:37Speaker 02

  Yeah, or longer wavelengths.

• 13:41Speaker 02

  And, and that was sort of the funny thing is they made this great material. The only problem is they had no idea why it worked. So they brought in Pendry, who was this condensed matter theorist who looked at it. And Pendry realized that, you know, naturally, we tend to think of carbon. You've got diamond as one form. You've got graphite as another. And he found that the carbon that they had fabricated was on this very small, sub-microscopic level, a bunch of tangled little threads of carbon that were all woven together and tangled up. And I tend to describe it that the radar waves kind of hit this forest of carbon and get lost in it. And then they eventually get absorbed. And that was sort of the revelation

• 14:25Speaker 02

  that, hey, just because took the, took the carbon and instead of just having it in this sort of natural graphite form, they scrambled it up into this tangled mess, it suddenly had very different optical properties.

• 14:37Neil deGrasse Tyson?

  So this would be good for stealth airplane coatings.

• 14:40Speaker 02

  Yeah.

• 14:41Neil deGrasse Tyson?

  They don't reflect back the radar at all. This absorbs the signal. So you don't even know it's there.

• 14:46Speaker 02

  Exactly.

• 14:46Neil deGrasse Tyson?

  Okay, that's a form of invisibility, once again.

• 14:49Speaker 02

  Yeah.

• 14:49Neil deGrasse Tyson?

  Right?

• 14:50Speaker 02

  And I suspect that, I suspect that the company that Pendry was working with, that the material that they discovered is probably very similar to what's used in stealth aircraft. But the people that do the stealth aircraft aren't talking. So, so we don't know.

• 15:07Neil deGrasse Tyson?

  Well, let's get another question. See if we can slip one in before the break.

• 15:10Negin Farsad?

  From James Myers, um, they write, "Let's assume we have a full-spectrum cloaking device. If that device consumes energy, wouldn't it show up in the infrared?"

• 15:21Neil deGrasse Tyson?

  Oh, I love it. Oh my, wow, who, what kind of audience do we have?

• 15:27Negin Farsad?

  I mean, they're smart. It didn't even occur to me that a thing would require ener, like, some sort of energy consumption.

• 15:34Neil deGrasse Tyson?

  If you're gonna absorb energy, you're gonna get hot, period. That's pure thermodynamics. So, yeah, Greg, what about that?

• 15:42Speaker 02

  Starts to sweat.

• 15:46Speaker 02

  Yeah, uh, two things I can say about that. One, um, well, yes, uh, let's start with yes. That's one of the interesting things about sort of what has happened with cloaking research as, as the years have progressed now is that people start with these really idealized models of invisibility cloaks. And then they started to realize that there were all sorts of limitations to these idealized models and all these practical considerations. And yeah, infrared radiation would be a very big consideration. If your object is ending up hotter than the surrounding environment, you're probably not going to be able to completely hide it that way.

• 16:30Speaker 02

  But one thing I can mention, which is kind of mind-boggling in and of itself, is people have theoretically speculated that you can actually apply some of the mathematics of cloaking to the equations of thermal heat flow as well. And they've theoretically said you can make a thermal cloak, that, uh, heat diffusion works, heat diffusion works very different from the way waves propagate, but the equations are close enough that you can kind of adapt it. You can design an object that kind of keeps the heat away from the middle of the region. For a period of time, like a superthermos.

• 17:08Negin Farsad?

  Like, like air conditioning for an object? Is that?

• 17:12Speaker 02

  Kind of, kind of more like a do not enter sort of barrier that it's sort of just that it kind of, it kind of forces the heat, instead of the heat going into the middle of the region, it goes around the region first and it eventually kind of seeps in. Not exactly the same thing as hiding infrared radiation, but.

• 17:32Neil deGrasse Tyson?

  But if you redistribute it that way, you can, you can, in fact, you could probably create the shape of something that people would think is not dangerous to you.

• 17:40Speaker 02

  Yeah.

• 17:40Neil deGrasse Tyson?

  Yeah, if you, you can sculpt something out of the radiate, that's interesting. That's interesting. Well, when we come back, we're going to talk more about going invisible on Cosmic Queries with my co-host Nagin Farzad and our special guest Greg Gaber, who is a world's expert on going invisible when StarTalk continues.

• 18:10Speaker 03

  Hey, I'm Roy Hill Percival and I support StarTalk on Patreon, bringing the universe down to earth. This is StarTalk with Neil deGrasse Tyson.

• 18:28Neil deGrasse Tyson?

  We're back, StarTalk Cosmic Queries, the invisibility episode. Whoever thought I'd get to say that? We've got an invisibility expert, Professor Greg Gaber, professor of physics and optical science at the University of North Carolina Charlotte. And of course, I got Nagin Farzad. Nagin, you got all the questions for us.

• 18:51Negin Farsad?

  Absolutely. And should I just dig right in?

• 18:54Neil deGrasse Tyson?

  Yeah, yeah, just give me another one. Give me another one.

• 18:56Negin Farsad?

  From Tom B. Knight, we have a sort of foundational question. He asks, how could you make something invisible to all parts of the electromagnetic spectrum? So just nuts and bolts, how do you do it?

• 19:08Neil deGrasse Tyson?

  So, yeah, based on what you've already told us, Greg, it sounds like you're only restricted to one part versus another. Can you take out every part of the electromagnetic spectrum? Is that something you could do?

• 19:20Speaker 02

  That's actually been one of the big challenges in invisibility. Even trying to make something invisible to the whole of the visible spectrum turns out to be a bit of a challenge.

• 19:33Neil deGrasse Tyson?

  That would be red all the way through violet, right?

• 19:36Speaker 02

  Yes, yes.

• 19:37Neil deGrasse Tyson?

  The full visible wavelengths, yeah?

• 19:38Speaker 02

  Yeah, and there's kind of one thing we could say is there's a practical challenge, which is that in order to make it invisible to all parts of the spectrum, you'd have to make it a cloak that has properties for every wavelength separately, and the light's all going to propagate a little differently. There's also a fascinating fundamental issue, which is that, this goes back to my picture of a cloak guiding light around a central region and sending it on its way. Well, that means the light's taking a detour through the material, and that means that compared to a light that would have just gone straight through, the light that takes

• 20:21Speaker 02

  a detour is going a longer distance. However, so in order for it to be truly undetectable, it's going to have to go faster than light that went in a straight-line path all the way through. Now, the problem with that is if you're trying to make an invisibility device that works in air, the speed of light in air is pretty much the speed of light in vacuum, which means that your invisibility cloak would have to have the light be going faster than the vacuum speed of light in the cloak.

• 20:55Neil deGrasse Tyson?

  So what you're saying is, just to recap, you're saying for the light to go around the object and come out the other side, and you have no way to detect it, it has to come out the other side at the same time it would have come out the other side without having to go around the detour?

• 21:11Speaker 02

  Exactly.

• 21:12Neil deGrasse Tyson?

  Otherwise, there's a time delay from that blob of light relative to all the light around it, but why would you even notice that?

• 21:19Speaker 02

  Well, you would actually note, you would actually notice it in an image at least some, some way downstream of your light beam, because there's this delay in the light coming out, it would act kind of like a lens. What a lens does when it focuses is it delays part of the light selectively to the other, which gives you some sort of curvature. So if you have this time delay imbalance, that's effectively going to give you some sort of focusing effect.

• 21:45Negin Farsad?

  Greg, can I just tell you, as a person with regular eyeballs, maybe this puts you at ease, I wouldn't notice. You know what I mean? If that light was just like a touch slower, I'd be like, oh, that light's doing its own thing. I'm not here to judge, take your time, whatever you gotta do. I just, I'm here to look, that's it. You know?

• 22:04Neil deGrasse Tyson?

  Yeah, that's, that's, that's, there you go. There you go. You get, you get enough people with messed up eyesight. No one's gonna notice. That's what you're saying.

• 22:13Speaker 02

  That's actually been part of the trend in a lot of the invisibility research since these sort of early theoretical papers, as people said, "Well, you know, we've been talking about can we make something perfectly invisible? And in principle, it's possible. In practice, there's a lot of problems. But we don't really need something perfectly invisible. If we make something 90% invisible, that's probably going to be pretty good." Or 99% or something like that.

• 22:40Neil deGrasse Tyson?

  That's people walking into play class windows.

• 22:43Negin Farsad?

  Yeah, yeah, yeah, yeah, yeah.

• 22:45Neil deGrasse Tyson?

  Okay, but not to put words in your mouth, but if you were to make the, getting back to the question, if you were to make the entire electromagnetic spectrum invisible through an object, then you'd have to have materials serving every band of light in that electromagnetic spectrum, because they, the one, there's not, there's no one solution fits all. So that would be a problem. It would be a huge surface, a very thick skin around the object, because every layer of the skin is going to handle different wavelengths of light differently. Is that, is that a fair statement?

• 23:24Speaker 02

  That's, yeah, I think that pretty much hits it exactly. So you usually don't see people looking at sort of super broadband invisibility cloaks. They're really happy right now if they can study a cloak that'll, you know, block or make invisible some parts of the spectrum and try to learn from that.

• 23:48Neil deGrasse Tyson?

  So I tell people, you know, the, is it the B-2 bomber, which looks like the Bat Plane, that has, I've read, the radar cross-section of a bumblebee, right? So you're looking for this bomber and nothing's showing up, right? And however, you could just go outside and look up. Because it's not, it's not stealth in visible light. It's only stealth in radar light, right? And so I, so it will matter. It can matter ultimately, right? What, you have to know how they're trying to detect you to then be prepared for that.

• 24:27Speaker 02

  Exactly.

• 24:29Neil deGrasse Tyson?

  Okay. Just want to verify, verify. All right, Nagin, what else you got for us?

• 24:33Negin Farsad?

  Um, well, I have from, from Chris Plots. He writes, "If invisibility is bending light around an object, wouldn't that mean that someone who was made invisible would be unable to see in the spectrum that bends around them? I suppose another type of invisibility where the image behind a person is projected in front of them would get around this, but that itself, but that itself would only work against people probably in a pretty narrow band directly in front of the invisible person. They'd be obvious from the sides. Seems like invisibility has issues."

• 25:04Neil deGrasse Tyson?

  Man, we got good people out there.

• 25:06Speaker 02

  They're thinking about it.

• 25:07Neil deGrasse Tyson?

  What happens to the person being made invisible? Can they see anything?

• 25:11Speaker 02

  Well, if the invis, that, if I, the way I say it is, yeah, if the invisibility cloak works as intended, then yes, nobody can see you, but you can't see anything.

• 25:22Negin Farsad?

  So then it's like the least fun superpower.

• 25:28Negin Farsad?

  I don't even know. It's, I guess you can eavesdrop pretty great, but you can't see anything? That sucks.

• 25:34Speaker 02

  Yeah, there, there are kind of a couple of workarounds of this.

• 25:37Neil deGrasse Tyson?

  So the questioner is correct.

• 25:40Speaker 02

  Yeah.

• 25:40Neil deGrasse Tyson?

  The person would not be able to see outside of their own eyeballs in the wavelength that you're using to make them invisible.

• 25:46Speaker 02

  Yeah.

• 25:47Neil deGrasse Tyson?

  Yeah, that's crazy. That's crazy. Okay.

• 25:49Speaker 02

  And, I mean, there are a couple of workarounds, and science fiction authors have done a good job of this, too. One workaround is you simply have some sort of detector or sensor. And now you start to imagine a vehicle instead of, you know, something you're wearing on yourself.

• 26:03Neil deGrasse Tyson?

  A periscope of some kind.

• 26:04Speaker 02

  Yeah, that detects in, you know, infrared or ultraviolet, that in a spectrum that's not being shielded.

• 26:12Neil deGrasse Tyson?

  Okay.

• 26:13Speaker 02

  Now, another thing that people have proposed, and I should stress that a lot of this is all, if it's been done, it's been done on a tiny, tiny scale. So we're not talking about somebody looking over your shoulder right now without you knowing. But people have also proposed that you can, you can actually build the cloak, the cloaked structure as part of your invisibility device. If you know, if you know how your, the thing you're trying to hide is scattering light, you can design your cloak to cancel out that scattered light. It becomes a wave interference effect.

• 26:54Speaker 02

  So you can kind of in principle, design a cloak where you get some of the lighted as a detector, but the cloak itself has been designed to account for the fact that part of the light has gone inside.

• 27:07Neil deGrasse Tyson?

  And therefore what?

• 27:10Speaker 02

  Well, ideally, the cloak itself is now, is still invisible because it's blocking any of the light scattering off of you. But some of the light from outside is coming in to where you can see it.

• 27:24Neil deGrasse Tyson?

  Oh, okay. All right. So this would be a solution to that problem?

• 27:28Speaker 02

  Yeah.

• 27:29Neil deGrasse Tyson?

  Okay.

• 27:30Negin Farsad?

  How far are we from this being available at H and M just for purchase?

• 27:35Neil deGrasse Tyson?

  In the back corner.

• 27:37Negin Farsad?

  Yeah.

• 27:38Neil deGrasse Tyson?

  Ponchos invisibility cloaks.

• 27:40Negin Farsad?

  And does the cloak have pockets? I'm just thinking about practicality here.

• 27:46Speaker 02

  Good question. One thing, though I, I always go out on a limb and I always get these things wrong. It's really hard to predict. Right now, it seems like. It's really, really hard to try and make something perfectly invisible with this cloaking sort in this cloaking sort of sense. So I'm not sure whether anyone will ever completely get it to work in practice. It might just be too hard and not worth the trouble to try and assemble these building blocks to make this thing. Though somebody will still probably try. And there'll probably still be some limitations like we've talked about. I use that to reassure people because when I tell when I tell them I do little work

• 28:29Speaker 02

  on invisibility, you know, they can start to worry that I'm dooming us all. But

• 28:36Neil deGrasse Tyson?

  But in fact, if you had perfected it, how would we know?

• 28:39Speaker 02

  Yeah.

• 28:40Neil deGrasse Tyson?

  We would never know.

• 28:43Speaker 02

  My skating coach keeps telling me I need to give her an invisibility cloak and I keep telling her, I already gave it to you. Don't you know where it is?

• 28:52Neil deGrasse Tyson?

  She can't find it. Okay. All right. So you're the author of the newly released book, Invisibility: The History and Science of How Not to Be Seen. So, so is that out yet at the time of this recording?

• 29:08Speaker 02

  The book just was released on April 11th of this year.

• 29:12Neil deGrasse Tyson?

  April 11th, 2023. Okay. Excellent. So Nagin, Nagin, tell me what else you got?

• 29:17Negin Farsad?

  Well, we have from Kayla Hunter, the question, in the Avengers movie, they had a helicarrier that turned completely invisible. In this scene, it looked like the ship was covered in a layer of individual panels that caused this visual effect once the stealth mode was activated. What's the science behind that? And is it possible with the technology we have today to execute something similar?

• 29:41Neil deGrasse Tyson?

  So this is like a switch. You can just turn it on and make, I think James Bond had a version such as this, where his Aston Martin could just on command could go invisible because of something on the skin. So have you have you researched the authentic physics of these documentaries called The Avengers series?

• 30:01Neil deGrasse Tyson?

  These science documentaries.

• 30:03Speaker 02

  A little bit, I can say, yeah. And here I can distinguish between what we often call passive invisibility versus active invisibility. Passive invisibility is where you design the structure to guide the light, but you don't mess with it. You just let the light go around the hidden object and go on its way. And so the object just sits there and the light just does what it does. Active invisibility is this idea, like the James Bond car, like the Avengers, actually like the 2020 Invisible Man movie, where the idea would be you'd have a bunch of cameras on one side, and on the other side, you'd have a bunch of projectors. And so if you have the object completely surrounded on one side, it'll always be taking

• 30:48Speaker 02

  in the scene, it'll, a computer will process it all and put it all out on the other side. Now, one trick with that is it's a little more difficult than a lot of people would think, because, you know, we take a photograph, we get the still image of a scene, but if you're trying to recreate the actual true light field that's hitting the side of an object, you don't just record how much light is it in a location, you need to know which way the light is coming from. And this is kind of like the person who talked about the projection image, the questioner, earlier, that you can project an image on somebody and it'll look correct from one

• 31:31Speaker 02

  direction, but if you step two feet to the side, it doesn't look good anymore.

• 31:36Neil deGrasse Tyson?

  Right, because you're not capturing the entire reality of what's going on on the other side, which is every light beam from every angle coming in. Interesting. So, and it wouldn't be a projection, I wouldn't think, in these modern times. The side coming towards you would just be clad in some high-resolution pixel LEDs or something, right? Isn't that what that would be?

• 31:57Speaker 02

  Yeah, and the projection that comes out the other side would also have to project light in all these different directions, you know? These pixel would be recording light coming from all these different directions. On the other side, it would have to send them all out on other different directions.

• 32:15Neil deGrasse Tyson?

  Well, let's close out this segment and come right back for segment three after this break. And we're talking about invisibility, not only now, but possibly in the future and even in science fiction. When StarTalk returns.

• 32:41Neil deGrasse Tyson?

  We're back. StarTalk, Cosmic Ferries, the invisibility episode with Professor Greg Gabor from the University of North Carolina, Charlotte, and of course Nagin Farzad. Nagin, I also saw you. I said this last time, but I just want to recapture my emotion when I saw I'm channel surfing and there you are on TV chilling with Hillary Clinton.

• 33:05Negin Farsad?

  Yep, yeah.

• 33:06Neil deGrasse Tyson?

  Just remind me what show that was?

• 33:08Negin Farsad?

  Hillary Clinton and Chelsea Clinton have a show on Apple TV called Gutsy Women, and it's about gutsy women. They wrote a book about gutsy women in history, and then the show is an attempt to kind of show gutsy women who are alive.

• 33:23Neil deGrasse Tyson?

  And you're a living gutsy woman?

• 33:25Negin Farsad?

  And I'm a living gutsy woman, so.

• 33:27Neil deGrasse Tyson?

  If you wrote a book called How to Make White People Laugh, that's gutsy.

• 33:33Negin Farsad?

  Yeah, so it was one of the honors of my life. You could check out that show on Apple TV.

• 33:38Neil deGrasse Tyson?

  And what captured their interest? Was it your book, How to Make White People Laugh?

• 33:42Negin Farsad?

  I mean, it was probably my book, and it was partly I've sued the Metropolitan Transit Authority in New York City for the right to put up funny posters about Muslims. I've done a lot of work around like activism, comedic activism, around immigrant rights and Islamophobia, and so that's something that I think caught their interest.

• 34:02Neil deGrasse Tyson?

  Wow. Wow, I didn't know any of that about you. Okay.

• 34:06Negin Farsad?

  I'm multi-layered. I'm not wearing no invisibility cloak. There's a lot going on here.

• 34:12Neil deGrasse Tyson?

  Exactly. You were in visibility cloak. That's a different kind of cloak.

• 34:17Negin Farsad?

  Exactly.

• 34:18Neil deGrasse Tyson?

  So Greg, I'm gonna have to call you out on this right now, okay? So far, you've been all talk. Have you built one of these things yet? Have you, or are you just Mr. Theory?

• 34:30Negin Farsad?

  Yes, spurning question.

• 34:32Speaker 02

  Making like the cloaks as people are trying to do them is actually very difficult, and people have made some simplified cloaking devices. The biggest one I've seen was big enough to hide a cat. Which made me very happy.

• 34:46Negin Farsad?

  That's pretty big.

• 34:46Speaker 02

  Yeah. But it was also an imperfect cloak. Now, one thing that I have done is, and I wish I brought it, and I wish I'd thought of doing that, is you can do a very nice light-guiding trick using prisms. And I guess it's a classic trick that a friend introduced me to. You put together eight prisms in just the right way, and this is sort of me using my fingers as the top view, and the light rays will reflect off the prisms and do a tour around the central region. So you can stick your hand in the central region, you can stick your fingers in there, you can, but while you're looking through the prisms, you can see everything on the other side.

• 35:25Neil deGrasse Tyson?

  Oh, I see. So the light is internally reflected within the prisms as it comes around the curve?

• 35:32Speaker 02

  That's exactly it. It's total internal reflection, so all of the light is perfectly reflected inside. And it's not, it's not really an invisibility cloak, the way that we would like to imagine.

• 35:43Neil deGrasse Tyson?

  It's a fun optical trick, yeah.

• 35:44Speaker 02

  Yeah. And it demonstrates that idea that, yeah, we can send light around a central region, and it'll still work very convincingly.

• 35:52Negin Farsad?

  By the way, Greg, I love that your friends are showing you, like, you know, optical tricks, because my friends are the type of people that are teaching me to take Alka-Seltzer when I have a hangover. So, I need to hang out with better your friends.

• 36:07Neil deGrasse Tyson?

  You need people who have eight prisms in their pocket, right? That's what you need.

• 36:11Negin Farsad?

  Exactly.

• 36:13Neil deGrasse Tyson?

  All right. And again, you got another one. Give it to me.

• 36:14Negin Farsad?

  Yes, from Christopher Bax. They write, "Can you explain a bit about what the interaction between light and matter is that makes some wavelengths of light reflect off matter and other wavelengths pass through? For example, visible light reflects off skin, but X-rays can look through it. Are there existing or theoretical materials that would not interact with visible light and would be visible to the naked eye?"

• 36:38Neil deGrasse Tyson?

  I love that. Yeah, so what's going on on the surface? This is, this is all classical wave optics, right, for you?

• 36:44Speaker 02

  Yeah. Yeah, and, yeah, and so, first of all, with X-rays, part of what happens with X-rays is there are such high-energy particles, if I dwell into the quantum room for a moment.

• 36:57Neil deGrasse Tyson?

  He said light was particles. I'm warning!

• 37:01Speaker 02

  My whole life is a lie.

• 37:04Neil deGrasse Tyson?

  Warning. No, no, in all fairness to him, as an astrophysicist, we want to detect objects in the universe that emanate in all, that that radiate in all these different bands of light. And it turns out, it's more sensible to reference high-energy light as particles and low-energy light as waves. Right? So no one thinks of a radio wave particle, radio particles. We don't think of it that way. It's hard to think of gamma way, or X-ray, X-ray.

• 37:35Speaker 02

  X-rays.

• 37:36Neil deGrasse Tyson?

  Elmer Fudd. So, so the fact that particle slipped out of your mouth is very consistent with anything I have to think about when we talk about bands of light and detecting them. But go on.

• 37:46Speaker 02

  Yeah. And, and I have a little historical side note on that, but I can come back to that in a moment. Otherwise, the, for visible light, part of the trick, people actually, including my own research, I did my PhD work on crude invisibility in 2001, before it was cool. And there was a...

• 38:09Neil deGrasse Tyson?

  What, you said crude invisibility?

• 38:12Speaker 02

  Yeah.

• 38:12Neil deGrasse Tyson?

  Crude, okay.

• 38:14Speaker 02

  So like...

• 38:15Neil deGrasse Tyson?

  As opposed to refined.

• 38:16Speaker 02

  Yeah, yeah. Like, the modern invisibility theories are really elegant and powerful, and you can do all this stuff. Back in the day, I was doing very primitive versions of that sort of stuff.

• 38:27Neil deGrasse Tyson?

  Back in the aughts.

• 38:29Speaker 02

  In the aughts, yep. I remember it well.

• 38:31Negin Farsad?

  We were young and innocent then.

• 38:34Neil deGrasse Tyson?

  Back in the aughts, yeah, aught three. Okay, go on.

• 38:38Speaker 02

  But part of it, part of, so back then, there were, it was widely thought that you couldn't make things invisible. And part of it was this reflection problem, that there didn't seem to be perfectly reflected, perfectly non-reflecting materials that would be non-reflecting for any direction that light's coming in. But people found out and demonstrated, or, or at least rediscovered that... If you make the right material that has, and I'll get a little technical, has a magnetic response in addition to an electrical response, and also is what we call an anisotropic material, then you can design materials that don't reflect any light at all.

• 39:22Speaker 02

  Even if their optical properties are different from air, light can just go in without reflecting. And that was a big piece of making invisibility work.

• 39:31Neil deGrasse Tyson?

  All right, and so, what was the question, Nagin?

• 39:37Negin Farsad?

  The question was about explaining about the interaction between light and matter.

• 39:43Neil deGrasse Tyson?

  Yeah, so if light gets absorbed and doesn't get completely from all angles, but X-rays go straight through you.

• 39:52Speaker 02

  Yep.

• 39:52Neil deGrasse Tyson?

  So why aren't they interacting with our skin the way visible light is?

• 39:55Speaker 02

  Yeah, and that goes back to the particle thing. That X-rays are so high energy, you know, they're kind of... What do you use as an analogy for something just going right through you without stopping? A freight train going through your body? It just charges right through. Yeah, there's nothing... An X-ray is so high energy, it just, for the most part, doesn't interact with anything in your body, and it just goes straight through because it's got so much energy and it's so out of... It oscillates at a frequency that is much higher than any of the frequencies that the atoms in your body are oscillating at.

• 40:34Neil deGrasse Tyson?

  So they don't even notice each other at that level?

• 40:36Speaker 02

  They don't, yeah.

• 40:36Neil deGrasse Tyson?

  Right.

• 40:37Speaker 02

  They pass in the hall, they don't talk. And so that's the advantage of X-rays, is that they're... They just are so high energy, they don't interact with matter very well because, yeah, the best way to describe it is this frequency difference. Atoms tend to have these natural frequencies they vibrate at, and it so happens that those natural frequencies they vibrate at are comparable to the frequencies of visible light. So we see a lot of stuff. X-rays have super-high frequencies, very different from the frequencies at which atoms are vibrating, so there's very little interaction.

• 41:13Neil deGrasse Tyson?

  So it's fascinating to think about it that way. That means for any two things to interact at all, there's got to be some resonance between the frequencies. Otherwise, it's ships passing each other in the night.

• 41:25Speaker 02

  That's a lot of the way it works.

• 41:28Neil deGrasse Tyson?

  Wow, okay, pretty cool.

• 41:30Negin Farsad?

  I'm learning that like these, that what these, all these wavelengths are very standoffish, very hard to befriend, you know?

• 41:38Neil deGrasse Tyson?

  Yes, they are. You gotta, you just have to know the right frequency. You have to, no, no, no, I got it. You have to be on their wavelength. Same, it's the same concept, right?

• 41:48Negin Farsad?

  All right, well, Tom Lindelius actually writes from Sweden. He asked, would it be possible to light a room in such a way that everything in it would appear black and white? About 15 years ago, my physics teacher claimed that it was possible, but she never told me how to actually do it.

• 42:04Neil deGrasse Tyson?

  Ooh, I think so. I think so. Let me take, I'll take a stab at this, Greg. So if you know in advance what color everything is, okay? If you know, oh, black and white?

• 42:19Negin Farsad?

  Yeah.

• 42:19Neil deGrasse Tyson?

  Oh, okay. I'm thinking of something different because what's very cool is if you get a very deep red light, for example, like one of these emergency lights that are light a hallway, and you take a Coke can, a red and white Coke can, and bring it up to that lamp, you cannot see the white stripes because the red reflects the red light and the white reflects the red light. So the whole can just turns red. And you can do fun light experiments this way. And if you take that Coke can and put it in front of a deep blue light, the red only knows how to reflect the red. Blue, it's going to absorb it.

• 43:03Neil deGrasse Tyson?

  You're not going to see any, like, it turns black. The white turns blue and the can becomes black and blue. So the weird thing is, the color something is, is not what it absorbs. It's whatever comes back to your eyes. If nothing comes back to your eyes, it's black. Now, that's my starter here, Greg. Can you turn a room into only things that are black and white? By choice of wavelength of illumination? I don't know if that's possible. You can do that?

• 43:29Speaker 02

  I'm not sure. I've never really thought about it before. I think you might have been on the right track. That if you have a room that has only certain colors in it, you might be able to tailor the illumination to sort of selectively ignore or reflect or absorb certain colors.

• 43:49Neil deGrasse Tyson?

  You find a frequency that's not represented by any object. And then all of those colors turn black.

• 43:55Speaker 02

  Yeah, and so it would be.

• 43:57Neil deGrasse Tyson?

  However, that frequency would be reflected just as itself from anything that's white. And so everything that's white would be the color of the light. Everything that's not the color of the light would be black. But you wouldn't get white. Because if you got white at all, the white has all the colors and it would show you the colors of the stuff that's turning black. So I don't believe, I don't believe your science teacher.

• 44:20Negin Farsad?

  Your teacher was lying. But also, I feel like if you could do, basically, we're talking about Instagram filters here on like a room,

• 44:34Negin Farsad?

  like in, through to like in real life, then you'd be rich, Greg. Maybe this should be your next venture is how to do Instagram filters in real life.

• 44:42Neil deGrasse Tyson?

  That's, you know, that's, Greg, you know, you missed out.

• 44:47Speaker 02

  I've missed out on a lot.

• 44:48Negin Farsad?

  Did we just, did the three of us just form a business?

• 44:54Neil deGrasse Tyson?

  So Nagin, one last question. That might be all we have time for, if that.

• 44:57Negin Farsad?

  Here we go from Kenneth Von Smelzmore from Atlanta, Georgia. They write, if invisibility cloaks work by bending or manipulating light and if we can see into the past due to the time and distance it takes for light to reach us, then might it be possible to make time invisible?

• 45:16Neil deGrasse Tyson?

  Ooh.

• 45:17Negin Farsad?

  Yeah.

• 45:18Neil deGrasse Tyson?

  Ooh.

• 45:19Speaker 02

  I actually do have a good answer for that. And in a sense, which may not be exactly what the questioner is asking, people have made what they call time cloaks. And what they did is so you picture, you picture this picture that I made where I said, okay, there's this hidden region, there's a cloak around it and the light goes around it like that. And so imagine this is like the horizontal position, this is the vertical position. Replace one of those axes with time.

• 45:53Speaker 02

  And now, at least in a picture, you've made a time cloak. You could make, you can design a structure where it kind of hides events in a certain time regime from light.

• 46:09Neil deGrasse Tyson?

  And now you sound dangerous.

• 46:16Neil deGrasse Tyson?

  Now, I don't, you know, I worry about Nagin, I don't know about this guy.

• 46:18Negin Farsad?

  I know. The main message, I think, for all of us is that Greg is, is, is that we should all be very scared right now of Greg. He's a very dangerous man.

• 46:27Neil deGrasse Tyson?

  Especially of Greg, yes.

• 46:29Speaker 02

  People occasionally say that about me and then I say, but I am kind of lazy, so it all kind of balances out.

• 46:34Neil deGrasse Tyson?

  There you go. A lazy, diabolical nemesis to the superheroes. Yeah. Yeah. Well, Greg, it's been a delight to have you. And the next time you have a full up cloak that they can sell in H and M, Nagin will be online to buy it.

• 46:50Negin Farsad?

  Absolutely.

• 46:50Neil deGrasse Tyson?

  You want to come back and tell us about it. But it's a delight to get this kind of insight because we've seen it discussed and we've heard about it, but we didn't know people were actually doing it. So good to know that you exist in this world. And Nagin, great to, great to have you again.

• 47:05Negin Farsad?

  Oh, always fun to be here. And I cannot believe we talked about invisibility cloaks. This was a dream.

• 47:11Neil deGrasse Tyson?

  For a whole show. That's right. All right. This has been StarTalk Cosmic Queries, the invisibility episode. Neil deGrasse Tyson here. Keep looking up.