The physics of invisibility cloaks

Every morning, you see your reflection in the mirror. Every summer, your friend looks squiggly in the bottom of the pool. But how can we use this to make invisibility cloaks? Boo. What?! Or explain the shiny wings of a butterfly? The answer is 'photonics', which is all the crazy ways to mess with light! Crazy way number one: bouncing Take a look at yourself in a mirror - you probably do this often enough, but do it again anyway. What do you see? It's yourself, of course, but have you ever wondered WHY this happens? You may have heard that light travels like a wave, sort of like these water waves. The water ripples have high parts and low parts that travel out and if they hit a wall, they bounce right back. That's the same sort of thing going on with the mirror - light coming from your cell phone is hitting your face and bouncing off of it. This bouncing is called 'reflection'. A lot of that reflected light bounces off the mirror and goes into your eyes. Crazy way number two: bending Now let's look at this pencil underwater - why does the part of the pencil that's IN the water look bent compared to the part ABOVE it? Of course, for you to see the pencil at all, light needs to reflect off of it. But it also needs to travel through different materials before it hits your eye. Light generally takes the fastest path to get between two points, and that might always be a straight line. Light travels slower in water than in air, so the fastest path is actually bent. It's just like when you travel from your house - the shortest path isn't always the fastest. Like here you could travel the shorter route along slower, smaller roads, or you could go a little farther and take the fast highway. Light does that too - this bending is called 'refraction'. That's why the pencil looked bent in the cup, and your friend looked squiggly in the pool. Crazy way number three: mixing Let's look at water waves, which actually behave pretty similarly to light waves. I made these waves by dropping to droplets some distance apart. Each droplet forms a set of waves going out and these waves meet and mix with each other. This mixing is called 'interference'. At some meeting points, the mixed wave is bigger than each of the original two waves. At other meeting points, the two waves cancel each other out. We can see the same thing happening with light waves in this experiment, where I shine light through two tiny slits with a small separation. Just like with water, there are some parts of light meeting where the mixed light is brighter, and other parts where the mixed light is darker. But what does this have to do with invisibility cloaks and butterflies? Let's bring these crazy ways to mess with light together. How could this cloak make someone invisible? Let's say this camera is your eye, and this green fabric is the cloak we want to make invisible. You're probably thinking, "Let's make the cloak itself invisible so that light would pass through it." But if we did that, light would still be able to bounce off what the cloak covers, pass through the cloak, and get to your eyes. And that's no good. What the cloak really needs to do is take light from the opposite side of the camera, channel it through the fabric and out the front, unchanged. No light would bounce off the cloak OR what it covers inside, so you would just see what's on the opposite side of the cloak, kind of like camouflage. And every other side of the cloak would just channel light to its opposite side, so none of the light would even get to what's inside the cloak, and none of the light hitting the cloak would be reflected into the camera lens. So there'd be nothing to see! All of this depends on CHANNELING light, which is not easy. But, nature can help. I'm here with some Morpho butterflies at the Boston Museum of Science. Their wings are a super shiny bright blue color. The front of their wings are made of alternating layers of air and chitin, and chitin is what lobster shells are made of. These alternating layers take advantage of how light bounces, bends, and mixes, to channel the reds, yellows, greens, oranges - basically every color of light EXCEPT blue - all the way to the back of the wing. The blues don't get channeled through, so this is why you see blue on the surface, but no other color. If you could take the color-channeling structures of a butterfly wing and weave them together into a fabric, you would have a cloak that would be invisible to every color of light except blue. If we lived in a world with no blue light, no blue could ever be reflected, so our cloak would work great! But if someone shined a blue light on you and your cloak, you would look kind of silly. To work in our world, your cloak would have to channel ALL colors, including blue. So how DO you invent a true photonic invisibility cloak? Well, any photonic cloak should be possible, right? Because you CAN mess with how light bounces, bends, and mixes. All right, well, I'M going to invent a photonic cloak first. I already have. You know, that's just a green cloak... Oh.