February 16, 2021 • 50 mins
Daniel and guest host Kelly Weinersmith break down the science and science-fiction of invisibility.
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Speaker 1 (00:08):
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
(00:30):
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,
(00:51):
but once we have it, it'll be a snap to
reinvent it. Hi. I'm Daniel. I'm a particle physicist and
(01:12):
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
(01:32):
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.
(01:53):
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
(02:14):
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
(02:37):
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
(02:58):
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.
(03:20):
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,
(03:42):
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,
(04:02):
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,
(04:23):
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
(04:45):
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
(05:05):
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
(05:26):
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
(05:48):
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
(06:09):
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,
(06:30):
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
(06:53):
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.
(07:17):
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
(07:40):
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
(08:03):
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
(08:24):
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
(08:47):
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
(09:09):
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,
(09:30):
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,
(09:53):
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
(10:16):
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
(10:37):
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,
(10:59):
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,
(11:20):
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
(11:42):
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
(12:03):
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,
(12:25):
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,
(12:47):
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
(13:07):
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,
(13:29):
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,
(13:53):
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
(14:14):
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
(14:35):
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
(14:57):
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,
(15:18):
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,
(15:40):
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
(16:00):
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
(16:23):
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
(16:43):
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
(17:04):
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
(17:25):
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,
(17:45):
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
(18:06):
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
(18:29):
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
(18:52):
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
(19:13):
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
(19:33):
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?
(19:56):
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
(20:17):
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
(20:38):
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
(20:58):
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
(21:21):
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
(21:44):
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
(22:05):
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.
(22:27):
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
(22:48):
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
(23:08):
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
(23:30):
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
(23:51):
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
(24:11):
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
(24:34):
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
(24:56):
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
(25:18):
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
(25:41):
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
(26:01):
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.
(26:21):
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,
(26:44):
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.
(27:04):
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
(27:25):
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
(27:45):
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
(28:07):
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
(28:27):
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
(28:48):
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
(29:08):
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,
(29:37):
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
(29:58):
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
(30:19):
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
(30:39):
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
(31:02):
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
(31:23):
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
(31:44):
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
(32:05):
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
(32:26):
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,
(32:48):
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.
(33:10):
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
(33:33):
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
(33:54):
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.
(34:16):
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
(34:38):
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
(35:00):
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
(35:22):
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
(35:44):
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
(36:08):
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.
(36:30):
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
(36:53):
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,
(37:15):
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,
(37:36):
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
(37:57):
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,
(38:18):
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
(38:40):
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
(39:02):
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
(39:25):
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
(39:46):
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
(40:06):
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
(40:26):
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.
(40:47):
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,
(41:08):
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
(41:28):
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
(41:48):
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
(42:10):
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,
(42:31):
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
(42:52):
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
(43:13):
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
(43:33):
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
(43:57):
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
(44:18):
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
(44:40):
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
(45:03):
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
(45:23):
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
(45:47):
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.
(46:08):
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
(46:28):
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
(46:51):
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.
(47:11):
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
(47:34):
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
(47:56):
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
(48:17):
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
(48:41):
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
(49:02):
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
(49:24):
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.
(49:47):
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
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
(00:30):
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,
(00:51):
but once we have it, it'll be a snap to
reinvent it. Hi. I'm Daniel. I'm a particle physicist and
(01:12):
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
(01:32):
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.
(01:53):
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
(02:14):
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
(02:37):
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
(02:58):
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.
(03:20):
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,
(03:42):
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,
(04:02):
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,
(04:23):
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
(04:45):
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
(05:05):
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
(05:26):
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
(05:48):
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
(06:09):
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,
(06:30):
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
(06:53):
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.
(07:17):
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
(07:40):
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
(08:03):
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
(08:24):
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
(08:47):
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
(09:09):
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,
(09:30):
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,
(09:53):
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
(10:16):
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
(10:37):
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,
(10:59):
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,
(11:20):
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
(11:42):
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
(12:03):
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,
(12:25):
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,
(12:47):
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
(13:07):
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,
(13:29):
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,
(13:53):
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
(14:14):
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
(14:35):
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
(14:57):
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,
(15:18):
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,
(15:40):
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
(16:00):
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
(16:23):
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
(16:43):
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
(17:04):
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
(17:25):
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,
(17:45):
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
(18:06):
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
(18:29):
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
(18:52):
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
(19:13):
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
(19:33):
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?
(19:56):
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
(20:17):
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
(20:38):
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
(20:58):
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
(21:21):
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
(21:44):
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
(22:05):
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.
(22:27):
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
(22:48):
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
(23:08):
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
(23:30):
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
(23:51):
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
(24:11):
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
(24:34):
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
(24:56):
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
(25:18):
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
(25:41):
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
(26:01):
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.
(26:21):
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,
(26:44):
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.
(27:04):
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
(27:25):
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
(27:45):
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
(28:07):
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
(28:27):
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
(28:48):
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
(29:08):
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,
(29:37):
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
(29:58):
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
(30:19):
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
(30:39):
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
(31:02):
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
(31:23):
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
(31:44):
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
(32:05):
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
(32:26):
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,
(32:48):
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.
(33:10):
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
(33:33):
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
(33:54):
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.
(34:16):
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
(34:38):
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
(35:00):
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
(35:22):
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
(35:44):
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
(36:08):
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.
(36:30):
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
(36:53):
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,
(37:15):
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,
(37:36):
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
(37:57):
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,
(38:18):
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
(38:40):
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
(39:02):
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
(39:25):
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
(39:46):
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
(40:06):
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
(40:26):
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.
(40:47):
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,
(41:08):
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
(41:28):
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
(41:48):
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
(42:10):
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,
(42:31):
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
(42:52):
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
(43:13):
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
(43:33):
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
(43:57):
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
(44:18):
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
(44:40):
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
(45:03):
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
(45:23):
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
(45:47):
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.
(46:08):
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
(46:28):
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
(46:51):
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.
(47:11):
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
(47:34):
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
(47:56):
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
(48:17):
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
(48:41):
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
(49:02):
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
(49:24):
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.
(49:47):
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
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