Can we cancel light waves?

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1:34Hello. Welcome to CrowdScience. I'm Alex Laffbridge. Can you hear that? That noise? Apparently, if you're of a certain age, you might find that nostalgic. That hiss that you get when you're playing cassette tapes. You know what? I quite like it. It's this weird echo of the past. but sometimes you do really need clarity. Like now. Does this sound better? A bit easier on the ear? It took a very dedicated man to get recorded sound sounding so clean. He was annoyed by the noise from the refrigerator in the kitchen and took all the moving parts out of the refrigerator down into the basement and just had cold air coming into the refrigerator so that he wouldn't have these intermittent coming out of the fridge.

2:25from fridges to film sets that man turned his tinkering to recorded sound and changed how we hear the world when they were recording on records at the time there's a large hiss and so that background level means you can't ever be quiet so classical music could never be quiet you actually had loud classical music but also rock there were no rock ballads because you could not have anything quiet. He invented an analogue chip, a piece of electronics that would work fast enough to take away that hiss. What it allowed was this generation of rock ballads where you can sing quietly and quiet classical music and movies where we can hear a pin drop.

3:10And Ray Dalby's invention of noise cancellation is still driving innovation in sound today. Hey, have you ever tried noise cancelling headphones? I mean, I love my noise cancelling headphones. They're the ones where you can put them on and then the sound of traffic and the real world goes away. And you can just focus on the things that you love to hear, like music, podcasts, crowd science. Anyway, I'm not the only one who loves these things. I was using my noise cancelling headphones and it was working perfectly. and I got thinking how they work and I found that they use this amazing intelligent technology.

3:50This is listener Ahmed in Libya. As he marveled at how the background sound fell away, he had an idea and got in touch. I started wondering whether this is possible with light as well. Could we cancel light out in the same or similar way to how we cancel out noise? If you want to answer this, we have to start at the basics. How similar are sound and light? To answer that, we need to go back to 1818. The French Academy was running a contest to figure out new theories of light. This is Mary Lou Jepsen. She spent her entire career making all sorts of very cool tech that rely on manipulating light.

4:36Newton had a particle theory of light. So Sir Isaac Newton believed that light was made up of particles. It wasn't really a particle theory of light, but that's how people translated it. Oh, OK. Well, anyway, the point is that Newton didn't believe that light was made up of waves, but a scientist in France did. This guy Fresnel entered this submission saying light was a wave. What Fresnel tried to show was that in the shadows of disks that were opaque, meaning that they didn't let light through, there would be a giveaway sign inside the shadow that proved light travelled in the form of waves and not particles.

5:14If you illuminated an OPEC disk with rays that are all parallel, you'd see a dark spot behind the disk. And this guy Poisson said, well, that'll never work. We all know that doesn't happen, so light's not a wave. So this guy Argo, he did the experiment, showed you could see the dark spot behind the disk. And that proved light was a wave. Well, no, it didn't. Complicatedly, it proved that light travels in waves. It didn't prove that light actually is a wave, or if it's a particle. So which is it? What an annoyingly difficult question. This is Professor Matthew Middleton from Southampton University in the UK.

5:57It has taken science 400 years to arrive at our current picture, and I'm afraid the answer is probably going to be fairly unsatisfying. It's kind of both. It has the properties of particles and it has the property of waves. It kind of just depends upon the situation we're talking about. For our situation, in this program, we're going to use the term light waves. But if you want more on that, there is the crowd science clip, what is light? Is it a wave or a particle? Right now, though, because Ahmed has asked whether light waves can be cancelled out like sound waves in noise cancelling headphones, phones, let's get to grips with how noise cancelling actually works.

6:38We're walking past some of the old instruments from the lab, including equipment that discovered the electron, discovered the neutron here, discovered pulsars. Can we try and still discover it? Yeah. And I've discovered my woeful lack of fitness at Cambridge University in the UK, where I'm being taken deep into the Ray Dolby Center. But we're not meeting Ray. He passed away in 2013. We're talking to Jeremy Barmberg, Professor of Nanophotonics. I'll let him explain what that actually means. So, small things and light. And we're in the basement of this building. It's on these vibrational legs which sit on very strong concrete base.

7:22So everything here is very stable. Nothing through your feet. So, this is the Ray Dolby Center. Dolby is sound. Why is there an optics lab here? So, in fact, the physics of controlling sound is not so unlike the physics of controlling light. Many of the ideas from controlling sound can be carried over. So with sound, actually, we can only focus it to the size of the wavelength, from centimetres to metres. So we can only look at relatively big things with sound. But with light, it's a million times smaller. So we can look at really tiny things, and that's what we're trying to do here. So, listener Ahmed from Libya's question.

7:58He says, I was recently thinking about noise cancelling headphones. Is it possible to do something similar with light waves? And who better to ask than you? It's a great question, Ahmed. So the first thing we need to understand is how does noise cancelling headphones work? So we have a lot of ambient sound around us and often that's irritating and distracting. So the principle of noise cancelling headphones is that we have a microphone which records that and we use it in some clever way to predict how that background noise is going to carry on in the future and then we create a little loudspeaker which actually produces exactly the reverse sound.

8:38And you can think about it like waves, so we have waves which are a crest of a wave and what we do is we add a trough of a wave so that we get no wave at all so we don't hear any sound. so we have to use the ability to listen to sound to make the opposite to actually cancel it out so the tricky thing about the noise cancellation is trying to figure out what is the waveform that you need to feed in that takes it out and that requires some very clever electronics so in front of me right now you've got something that i know that that's a tuning fork you can do it i know you want to do it thank you i'm very happy with that no crowd science listeners will appreciate that So the idea of this is it's a very pure sound.

9:23So if I listen to any moment, I sort of know what the sound's going to be like the next second. So to take it out is really easy. So, simple sounds, it's quite easy to remove them. But when it comes to more complex sounds, it's a little bit more difficult. And so it is basically about choosing the amount of the inverse sound to sort of put in. Well, what it's doing there is it's probably using different ranges of sound. So sometimes on a plane, for instance, you get a very low frequency rumble or on a train, and then you want to concentrate on taking that out. But actually, if you're listening to somebody who has a deep voice, you don't want to take that out.

10:02So I think probably what it's doing is it's modulating what ranges of sound, what colours of sound, if you like, that you take out in different places. You're hearing this background hum. But let's do what noise cancellation does and take that away. Like that. Yeah, pretty good, huh? But listener Ahmed wants to know if we can do the same thing with light. So the first answer to that is yes. We're done. Easy. Quickest crowd science ever. Go home. Except when we're detecting sound waves, we can actually directly measure in time those crests and troughs, just like an ocean wave, to create the inverse of it.

10:45But with light, the problem is we don't have any way of measuring the crest and troughs easily. What we normally measure is the energy in the light beam at each colour. So measuring white light that's made up of the full visible spectrum of colours is just not currently possible. It's to do with the speed of light, and we'll talk about it more in a moment. But if we break up the colour spectrum, then we start to be able to understand the size and shape of waves a lot better. So if it's a very pure amount of light, we can do it. We can take a particular colour of light, which is falling on a particular spot, and we can actually add another light beam there that makes it go away.

11:26Oh, this is big news, Alisna Ahmed. It is possible to cancel out light. When you take one beam of light and you add another beam of light, it doesn't make two lights. But we just need one specific slice of light. How do you slice light? It's not bread. How are we going to do this? It's just come behind the curtain I'm gonna look at this spot on the screen So what I'm seeing here if you imagine a laser pointer but like a really big laser pointer held up That's almost completely solid. So that's light That's traveled down two different paths so we can think about one of them as being this pure beam of light that I'm going to try and get rid of.

12:06It's not sunlight, it's actually laser light which means it's got one colour as near as we can make it and it's as close as possible to that sound that came from the bell that we heard. Beautiful peaks and crests. So we can predict what it's going to do. Do you want to hear that again? What I've done is I'm adding in another light beam to that and what you'll see is that I'm just going to slightly change it to start with. So what you see now is that instead of being a big spot of light, it's got some lines through it and some of the lines are bright and some of the lines are dark. And that's exactly this principle of interference.

12:45Where the lines are dark, we're adding the two light beams together so that one is a peak and one is a trough and so they add together to give nothing. Picture this. It's a big red dot from a laser. and it looks a bit like a sun with hazy horizontal stripes slicing through. There's some stripes that are brighter than the rest of the sun, and some where there's no light, so it's just a dark stripe. The reason we get some bright stripes and some dark stripes all depends on how the two waveforms line up. Imagine it a bit like a zip on your clothes. A wave has tall bits and shallow bits, crests and troughs just like the teeth and gaps running along a zip when a zip works properly a tooth from one side drops cleanly into a gap on the other side with light waves if a crest lands in a trough the two waves cancel each other out that's destructive interference and is what's making jeremy's laser beam disappear to make those dark stripes so this is ahmed's idea can't i add light together to actually get it to go away and indeed that's what's happening but there's a problem it's just very close to it they're actually adding together to give us a peek.

14:00Stick with me on zips. If you slide one side of the zip just a tiny bit so the teeth bang into each other that would be a crest hitting a crest and that's what makes the light waves add together that's constructive interference and that's why we're seeing some lines of light that look brighter. So is there a way of cancelling out light, but not also ending up with peaks where we've made the light brighter? We'll be looking at that next. This is summer at its peak. Whole Foods Market Summer Fruit Fest is your invitation to eat the season. Fresh, organic and bursting with flavour. Start your day with peaches and organic blueberries and yogurt.

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15:35You're listening to CrowdScience from the BBC World Service, the show that sheds light on your science questions. I'm Alex Lathbridge and today we're exploring a question from listener Ahmed in Libya. Inspired by his noise cancelling headphones, he wants to know if you can get rid of light in a similar way. So far, we've managed to do it to laser beams. But, bit of a problem, we also ended up making the light next to it much brighter, which I feel defeats the purpose. So how do we solve that? Well, we need some more tech. Behold. The anti-laser. The anti-laser. It sounds like something you might deploy for like a space battle, but maybe this could help listener Ahmed in his quest for darkness here on Earth.

16:23The first thing to know about the anti-laser is that it bounces light using mirrors, a trick that we also see in nature to improve vision at night. Much of the light that comes to our eyes is actually not used for processing the information that we want to have in our brains, but actually much of the light is being lost. This is Stefan Rotter, professor of theoretical physics at Vienna Technical University in Austria. Think about the retina, which is at the back of your eye. This is also a very weak absorber in the sense that when light passes through the retina, it's not fully absorbed if the light just goes through the retina once.

17:08Now, many nocturnal animals have a way of recycling that light. They have behind the retina a mirror and this mirror makes the light go through the retina twice to make it more strongly absorbed which gives those animals a better eyesight at night and this is the reason actually why nocturnal animals where they have to deal with low light conditions have those shining eyes. Imagine you're in a field at night and you point your torch at some sheep, you'll see their eyes shining. But here's the thing, the fact that their eyes are shining shows that there's still light escaping, because that's what we're seeing reflecting back.

17:49If it absorbs perfectly, those eyes would be completely black. So how could we absorb more of the light? Could more light be absorbed if rather than bouncing the light twice, it reflected four times, eight times, a thousand times. Anti-lasers like the one Stefan and his team put together do just that because they send light bouncing in a perpetual loop. Think of it as two mirrors. Two mirrors, a bit like table tennis bats bouncing the light back and forth. And if you shine laser light onto this device what's going to happen is that this laser light will first get partially reflected by the first mirror, it will come back.

18:34But the light that hasn't been reflected takes a different course. It will enter into the region between the two mirrors and there it will bounce back and forth.

18:48That light in the middle is then directed so it cancels the other light thanks to destructive interference. And the device is constructed in such a way that the light that has spent some more time in between the two mirrors will perfectly erase or cancel the light that has already been back reflected from the first mirror without entering the region between the two mirrors. Then the anti-laser forces the light to be held hostage. The laser light will prevent itself from escaping so it traps itself in between the two mirrors it cannot escape and since it cannot escape it means that if you put even a weak absorber there.

19:31The light has no other way than being absorbed in this absorber and this leads then to the perfect absorption. So the light is absorbed by that absorbing material and as it does that the energy shifts from being electromagnetic energy as it was when it left the laser beam and it becomes heat energy and you can't see heat. The light disappears. is.

19:59Alright look, looky looky look, we're getting somewhere. We've cancelled light from a laser and we aren't left with the peaks of light that we saw with Jeremy. But we can do it with a laser beam because the light is coherent. It shines in a much clearer line than the light that surrounds us day to day. So could we go one bigger and cancel out natural light or a light bulb that Ahmed would find in his bedroom? You cannot just expect that like Dumbledore in Harry Potter could just say turn off the light in this room and then it goes dark because even if it worked for the whole spectrum which typically it doesn't would not be very cost effective because the light modulation techniques would be so expensive i feel as though the answer is no just buy some curtains buying a curtain is certainly cheaper than buying a light modulator and trying to work with it it would be even cheaper to just close your eyes

21:06okay so it's hard to argue the cost effectiveness of using light cancellation when there are cost-effective options such as curtains and eyelids. But let's not allow money to get in the way of a good crowd science question. Now, if I had pockets so deep that money was no object, could it be done? Like noise-canceling headphones get rid of sound. Here's Jeremy Baumburg again. So our ears, you can think of our ears as being a single pixel or two pixels. We have one pixel on either side of our head. If we go to light, because our eyes are millions of pixels, the problem is the light is different on every pixel.

21:47So this is our first challenge. We don't have to do it once, we have to do it a million times. That brings us to the second problem. If you look at sunlight, or the light from these LED lights around us, they're a million, billion times faster than the sound wave crests and troughs. We have to have electronics which is far faster than we can make at the moment. and then we would have to use that electronics to make a light beam and control then the crest and troughs of that light beam far faster than we can create at the moment. So our chances at the moment of just cancelling out the sun are really hard.

22:21They're big technical challenges. They're not completely insuperable. The physics should work, but it's the details about how you do it that is the technically really difficult thing. And that's how we learn in science. We can scratch our heads and think, OK, what could we do? Hmm, so what could we do? Well, I've got an idea. What if we could get our own really, really, really good absorber of light? I know just the thing. In the United States, engineers at the Massachusetts Institute of Technology, or MIT, have developed a material known as the blackest ever black. It's designed to absorb 99.995 % of incoming light.

23:03So what if Ahmed and I did a bit of redecorating. Could we cancel out all the light in his bedroom with that? We can certainly make a room which absorbs all the light but the problem is we live in the real world and then we start to get dust. That terrible thing yeah. So if I make a film with a black material I'm still going to get a reflection from the top surface. So it has to be designed in some way the light bounces around it has no chance of ever getting out. Then we start to get dust building up on it and that wrecks that absorption. So we could have a black room but we couldn't put you in it and we might have to suck out the air and you could never clean it because you can't wipe the surface because all of that structure which is sending the light in different directions is very vulnerable.

23:49So that's the other challenge about it. Okay Ahmed, maybe I'm not the interior designer that I thought I was. Maybe it's time to draw the curtains on this whole adventure and call it a day. But what's that? You want one last little story? Well, okay. You did ask about some of the ways light cancelling might be used by scientists. Do you remember earlier when we heard from Mary Lou Jepsen? I've really built my career on manipulating both the intensity and waves of light. Well, now she's got a plan to use her tech for healthcare. I thought it was pretty neglected what we could use using light and waves inside the body to diagnose thousands of diseases.

24:32To understand how you use light to diagnose disease, now if you have a phone on you, you might want to turn the torch on and shine it at your hand. It would help if you're in a dark room. When you put your smartphone camera on your thumb, for example, it looks red because that goes straight through. And it's why the sun feels so warm. It's infrared coming into you. And so it goes deep into your body. So Mary Lou doesn't use white light. She focuses on a particular segment of the electromagnetic spectrum just beyond what we can see. It's called near infrared light. But before she shines that into people's bodies, she sends in sound.

25:13You can use ultrasound to change the wavelength of light. Again, a wave phenomenon akin to cancelling. The ultrasound is focus to a specific spot and then the near infrared light is passed through it. It tunes the light. You change its colour slightly and that allows you to manipulate the wavelengths of just the changed colour. This means you can see the difference between the near infrared light and the surrounding scattered light. In a way, it's cancelling out the noise. To get the image back, they use a tiny camera that was invented to allow us to unlock our mobile phones with face recognition.

25:56Sometimes you get lucky. Apple ships a product that you kind of want to make yourself. They put a camera chip in a smartphone in about 2018 that sees in the infrared. That's the part that heats your body, but it has pixels the size of the wavelength of light. That tiny, high-speed camera creates a 3D image, a hologram, using the tuned light. When we pulse our laser, then we can see the cancellation or the non-cancellation on that camera chip, and it just looks like waves on the ocean. Mary Lou had been planning to use this to look for things like tumours, but without having to travel to a hospital.

26:36But then, she was asked to do a presentation about it, and the response she got ended up pushing her ideas even further. I gave this TED talk and I got all these calls from stroke doctors and they just said day in, day out, you don't know what it's like. They wanted Mary Lou to find a solution for rapidly measuring blood flow, which meant moving images, not just still pictures. The reason was because it would help diagnose a particular type of stroke. It's called large vessel occlusion stroke. it blocks the blood going downstream. And the quicker that you can see a blood vessel is being blocked, the quicker you can diagnose it.