This BBC podcast is supported by ads outside the UK. It's time to see what you can accomplish with Shopify by your side. Azer does it. So, we can now listen to your podcast. 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 blah blah blah blah coming out of the fridge. From 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 and ray dolby's invention of noise cancellation is still driving innovation in sound today 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. This 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. Newton 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, okay. 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. If 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. It 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. let's get to grips with how noise cancelling actually works. We'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 Centre. 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. So everything here is very stable. Nothing through your feet. So, this is the Ray Dolby Centre. 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 a million times smaller so we can look at really tiny things and that what we trying to do here So listener Ahmed from Libya question He 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. And 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. So 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 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. So 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. But with light, the problem is we don't have any way of measuring the crests 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. Oh this is big news listener 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? Let's just come behind the curtain. I'm going to 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. It'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. It'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. Where 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 stick 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 starting a business can be overwhelming you're juggling multiple roles designer marketer logistics manager all while bringing your vision to life shopify helps millions of business sell online build fast with templates and ai descriptions and photos, inventory and shipping. Sign up for your one euro per month trial and start selling today at Shopify.nl. That's Shopify.nl. It's time to see what you can accomplish with Shopify by your side. I understand that you want to listen to your podcast, so I'll keep it short. Because if you think it's important to make a lot of choices, maybe ASR can help. Now I hear you think, how then? Well for example it a waste of money that you love for Schade Will you know more about the insurance where a harm can be possible Go to asr slash duurzamekeuzes This is ASR for you and a more expensive community. ASR does it. So, we can listen to your podcast now. 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 a 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. The 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. Now 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 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 if 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. But the light that hasn't been reflected takes a different course. It will enter into the region between the two mirrors and that will bounce back and forth. That 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, the 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 all right 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. Okay, 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 cancelling 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. So 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 we 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 crests 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. They're big technical challenges. They're not completely insuperable. Well, the physics should work, but it's the details about how you do it that is the technically really difficult thing And that how we learn in science We can scratch our heads and think okay what could we do Hmm So what could we do Well I 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. So 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 chestnut. That terrible thing. So if I make a film of a black material, I'm still going to get a reflection from the top surface. So it has to be designed in some way that 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. So that's the other challenge about it. OK, 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. To 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. You can use ultrasound to change the wavelength of light, again a wave phenomenon akin to cancelling. The ultrasound sound is focused 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. Sometimes 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. But 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. The quicker that you can see a blood vessel is being blocked, the quicker you can diagnose it. For someone to have the best chance of not being permanently affected, they need to be diagnosed quickly. We said, OK, great, let's make something where we can do the diagnosis in an ambulance or just like they have for defibrillation. Let's see if we can see blood flow. In this case, the clearer the hologram that she could create, the worse the news was for the patient. Looking at the contrast of the troughs and the peaks and the size of the waves, if you get a really great hologram with really high contrast, that's really bad. That means there's no blood moving there. But just getting from stroke onset you to the right procedure, if they can just do that within two hours, pretty good results. Mary Lou was able to develop this new quick handheld technology for measuring blood flow and she's got big plans for moving beyond diagnosis into treatment, all using the principles of manipulating waves that we've been exploring thanks to this week's amazing question. Listener Ahmed, you asked if it's possible to cancel out light and we found out that in everyday life when it comes to sunlight, it's a lot easier to draw the curtains. But we did shed some light on some pretty fun physics along the way. Now, how about we draw this episode of CrowdScience to a close with the credits? You've been listening to CrowdScience from the BBC World Service. This week's enlightening question was from me, Ahmed Swidhan, in Libya. The presenter was Alex Slatbridge, and the producer was Tom Bonnet. If you got a question, email crowdscience at bbc.co.uk. Thanks for listening. Shukran jazeelam. Starting a business can be overwhelming. You're juggling multiple roles, designer, marketer, logistics manager, all while bringing your vision to life. Shopify helps millions of business sell online. Build fast with templates and AI descriptions and photos, inventory and shipping. Sign up for your one euro per month trial and start selling today at Shopify.nl. That's Shopify.nl. It's time to see what you can accomplish with Shopify by your side. This is murder. To the dark streets of Belfast. Put your humor on me. Good humor, slow misdades, lovely unfulfilled characters and precisely the right dose of sarcasm. Bingo. BBCNL, the place for the best British misdades series. Just on your Netherlands TV.
