How long does light last?

0:00This BBC podcast is supported by ads outside the UK. Pop quiz. What's in your kid's lunchbox? At Whole Foods Market, they've already done the studying. Over 300 food ingredients are banned from their shelves. No hydronated fats in the peanut butter and no high fructose corn syrup in the cookies. And for sandwiches, there are no synthetic nitrates or nitrites in any of their deli meat. So you can pack lunchboxes with peace of mind. Get back to school ready at Whole Foods Market.

0:59That's where your story starts.

1:12My name is Pascal Ibon. I am an astronomer working with one of the biggest telescopes in the world. Hello and welcome to CrowdScience from the BBC World Service, the show that searches far and wide across the universe for the answers to your science questions. We are going to open soon. This means 45 minutes before sunset. It takes 30 to 45 minutes to open a telescope. Let's go inside one and I will show you how we open. I am at the Paranal Observatory in Chile, far out in the Atacama Desert. over 2 ,600 metres above sea level. Here, the sky is crystal clear. I'm Anand Jagatia, and our show this week is all about the kind of work that Pascal does at night in the middle of the desert in Chile.

2:09It's one of the best places on Earth to carry out ground astronomy that does mean working throughout the night. So I am now inside the telescope, Dome, and you can see the primary mirror and on the top there is the secondary mirror. The primary mirror is an 8 meter diameter and the secondary mirror is a 1 meter diameter. To look closer at our own galaxy and the universe beyond it, I used a very large telescope, that's its official name. From here I can see some of the most distant objects in our universe.

2:56It's this ancient primordial light of the cosmos which inspired one of our listeners to get in touch. My name's Rob. I'm from North Devon in the UK. My question for CrowdScience is, how long does light last? This is a cool question. What's the thinking behind it? Why have you been wondering about the permanence or impermanence of light? Well, back in April, there was this article about a planet that was spotted 124 light years away. And I thought, well, if we're looking 124 years back into the past, and you hear about people looking at distant stars like 100 million light years away or whatever it is, how far can we actually look into the past?

3:40How long does this light actually last? Is there an expiration date for it? I've often wondered, as well as how long does it last, like, can we tell how old the light is just from looking at it? Or do we have to already know how far away the thing is that we're looking at? And then we can work out how old the light must be because we know how fast light travels and we know how far the distance is. Yeah, I was thinking about this earlier, like stars are meant to be slightly different colours if they're moving away from us or coming towards us. So could that tell you how old the light is? Does red last longer than blue, for example?

4:19I suppose the other aspect to your question is not just how old is the light that we're looking at, how far back can we go, but going forward, how long it can persist in the future. Yeah, if I went outside and I looked up at the stars, is my face being beamed into the stars forever?

4:38I really love that image. light bouncing off of Rob's face coming from stars on the other side of the galaxy that could have been travelling for, well, an astronomical amount of time before setting off into space again. But for how long exactly? Millions? Billions of years? Would it keep going forever? And how do we know how old the light actually is? Before we get into all that, Rob said that when we look at distant stars or planets, we're actually looking back in time. So astronomers like Pascal in Chile, who we heard from earlier, are in a way time travellers. If you go further and further in the universe, the light has more and more distance to travel towards us.

5:26And so that's why the time that the light from a very, very distant object arrived to us, the object is no more the same than it was when we received that light. So because the universe is just so big, it's taken so long for this light to reach us. We're obviously not looking at the thing as it is now. We're looking at the thing that it was when the light left it. Exactly. That's exactly that. Yes. Because when we are travelling through the lights in the universe, we are travelling through time. Which is crazy. I mean, they may not even be there anymore, right? I mean, what you see as like one point, which could be like a galaxy, is now like a gigantic galaxy.

6:10It has interaction with its environment and it has changed. It's very difficult to know. We'll be returning to Pascal as she works through her night shift at the Paranal Observatory throughout the show. First, though, the Very Large Telescope, hilarious name, that Pascal uses can detect visible light. But that's not the only kind of light there is. The light that we're familiar with, the one that we use every day, that our eyes are sensitive to, is optical light. This is Matthew Middleton, who's also an astronomer from the University of Southampton in the UK. But the spectrum of light goes way further than that, into what we call the electromagnetic spectrum.

6:50If you think of light as a wave, I always think of it as a wiggly line waving through the air, then at one end of the spectrum, you've got really big spaced out wiggles. And at the other end of the spectrum, you've got really small bunched up wiggles. The spacing between wiggles is called wavelength. Wavelengths of light can vary enormously along the electromagnetic spectrum, all the way through from the size of mountains to something that's smaller than the size of an atom. Astronomers like Matt can learn all kinds of different things about our universe by observing light with different wavelengths.

7:26At the bottom end of the electromagnetic spectrum, with the largest wavelength, are radio waves. They help us probe things like astrophysical jets. So when we have matter falling onto a black hole, some of it goes into the black hole and some of it gets fired from the accretion disk at nearly the speed of light. Then we have microwaves. The Event Horizon Telescope used microwave radiation to image black holes in M87 and Sagittarius A star in our own galaxy. Next is infrared. That's thermal. So when you have dust that's very hot, that tends to emit infrared light. We're using James Webb Space Telescope to get amazing views of the universe in the infrared, which can reveal things like very young stars or brown dwarfs, which are failed stars.

8:13We're using them to study exoplanets. Then we have optical light, which is what the very first telescopes observed. Things, you know, it's been our primary tool for centuries. And ultraviolet, which is actually not that useful for astronomers. Very hard to use anything in the ultraviolet because hydrogen, the thing that populates most of the universe, absorbs ultraviolet light, so that's a challenge. But you can look at things like aurora on Saturn using UV. That's kind of cool. After that, it's x-rays, which are Matt's favourite. And then you can go even higher energies and you're talking about things like particle acceleration.

8:46Listener Rob wanted to know how far we could look back into the past. What's the oldest light we can observe? And what part of the spectrum does it come from? OK, so the oldest light in the universe comes to us from the cosmic microwave background, which is over 300 ,000 years old into the universe's life. The earliest light we've ever detected are microwaves, nearly as old as the universe itself, from approximately 13.8 billion years ago. And way back then, our universe was a very different place. originally the universe was kind of a soup of photons and electrons and protons and as the universe expanded it cooled down now photons get scattered very easily by interactions with electrons so those photons couldn't get out but as soon as it got cool enough for the protons and electrons to combine to form a hydrogen atom then those photons were able to escape that radiation has been traveling to us ever since then so initially because the universe was like too compact, the electromagnetic radiation couldn't escape.

9:53But then once it expanded and cooled down, atoms formed and then could escape. Yeah, the universe became transparent for the first time. Wow. Okay. There's this sort of spike in the timeline of the universe where we can suddenly see loads of energy being released. And now that energy is in the microwave frequency. And it's everywhere. It is everywhere we look. And it's so important because it tells us about how the universe grew structure. So if you observe that with a microwave telescope or whatever, you're literally seeing something that has been travelling through the universe for 13 point whatever billion years until it got to our detector.

10:31Absolutely. It is the fingerprint of creation.

10:38This radiation really is everywhere. and you've seen it if you're old enough to remember the static on old analogue televisions. That white noise is in part from the cosmic microwave background radiation that Matt is describing. But those microwaves are spread out across the entire universe. What about the oldest individual object that we've been able to observe? For that, let's rejoin Pascal at the Very Large Telescope in Chile. The VLT is made up of four telescopes. They each have a different name, Ontu, Kriyen, Melipal and Yipul, which are their names in the Mapuche language. The telescope now is getting inclined to ensure that when we are opening nothing is falling on the mirror.

11:28This unit looks like big domes full of blue metal rigging that hold the telescopes in place. You have to consider here that we are moving and inclining tons of materials. The primary mirror weights approximately 24 tons and the secondary mirror around 50 kilos. This alarm is announcing the opening of the dome as you can see. It's now 6 pm which means my night shift is about to start. So I will head down to the operating room.

12:09I have to go down some stairs because the telescope is a very high facility. I am then going on the platform. It's where all of the telescopes are based and here you will start to hear some winds. The sunsets from here are particularly gorgeous, I have to say. Using the VLT, Pascal can use visible and infrared light to look beyond our solar system, past the Milky Way and deep into the universe. There is a lot of things that we can observe. We can observe new exoplanets and we call exoplanets the planets that are outside of our Milky Way, of our solar system too. There is new galaxies, the most distant galaxy in the universe.

13:00We can observe also the black hole that is at the centre of the Milky Way and understand better how the stars around the black hole are reacting. I mean, there is no limit in the science. So what's the oldest and furthest galaxy we've been able to observe? If we consider that the universe is 13.8 billion years old, the most distant object that has been found now is around 13 billion years old, yes, approximately. So it's not much older than the age of the universe, right? No, that's why it's extremely distant. So how do you work out how far away an object or a galaxy is? The oldest light, if we can call it like that, will be redshifted.

13:48So it will be more in the near-infrared or in the infrared.

13:55Redshifting is when light waves get stretched out. Their wavelength becomes elongated and so shifts towards the red part of the spectrum. It happens because the universe is expanding. Here's Matt again to explain. Well, when you're listening to, say, a police car go past, you'll hear the pitch of the siren change. And it becomes lower as it's moving away from us. That's redshifting, essentially. You're taking acoustic waves and you're stretching the wavelength out. The same thing happens with photons that are travelling, or light that's travelling very large distances across the universe. The universe is expanding, we know that.

14:36And so those wavelengths are also getting stretched out. So I might have started off with photons in the optical, but now I have to look in the infrared if I want to see them. The further light travels from distant objects, the more it gets redshifted. In fact, for researchers like Pascal, it's actually easier to think in terms of redshift rather than light years when measuring distances. I never speak in billionaires. I always go through redshift because it's such a smaller number. I mean, when you think about it, I can remember 12 and it's easier. Like the most distant object, you'll achieve 12 and that's it.

15:14But that doesn't speak to anyone. As technology improves, the hope is we'll be able to peer even further into the universe. And in fact, an even bigger telescope is currently under construction in the Atacama Desert in Chile. And it's called, wait for it, the Extremely Large Telescope. we were thinking two decades ago, has been proven to be different. So it's very difficult to say, are we reaching this object because they are really the first? Or is it the only one that we can go right now with that technology? And maybe in 20 years we will say, oh no, we were wrong and we can go even further away.

15:56And so that means we are not even at the birth of the universe, you know? So who knows? So, Rob, you wanted to know how far back into the past we can look. And the answer is that we can see objects and detect background light that's nearly as old as the cosmos itself, from over 13 billion years ago. New telescopes are pushing this boundary back further and further, shining a light, quite literally, on what our universe was like after the Big Bang. But you also wanted to know how long light actually lasts. Does it just keep going on forever? That's what we'll be finding out next. Pop quiz. What's in your kid's lunchbox?

16:39At Whole Foods Market, they've already done the studying. Over 300 food ingredients are banned from their shelves. No hydronated fats in the peanut butter and no high fructose corn syrup in the cookies. And for sandwiches, there are no synthetic nitrates or nitrites in any of their deli meat. So you can pack lunchboxes with peace of mind. Get back to school ready at Whole Foods Market.

17:29Technos Homes. Download the app today.

17:41You're listening to CrowdScience from the BBC World Service, the show that goes the distance to answer your science questions. And this week, we're exploring the colossal intergalactic distances of our universe. We've heard that scientists have detected light that's been travelling almost since the dawn of time. Microwaves from when the universe first became transparent, when it was just 300 ,000 years into its 13.8 billion year history. So much for the past. But what about the future? Does light have an expiration date? Photons are complicated. We heard earlier that light is like a wave, but it's also like a particle, called a photon.

18:26Thinking of light in this way can help us get an answer to Rob's question. Astronomer Matt Middleton again. You don't need to go that deep to appreciate that energy is conserved in any closed system. That's the first law of thermodynamics. So energy doesn't go away, it just changes its form. So a photon is a form of energy and that energy will always exist in some form. So photons might change, but the energy remains. In what way can a photon, I guess, change into something else? You can get photons that produce matter, matter-antimatter particles. So you can actually get literally light turning into matter.

19:07That's, by the way, why we exist. But also photons could be absorbed. So you can excite atoms or you can excite electrons off atoms altogether. And then that energy is bound up in that electron and nucleus. but the energy has remained but the photon has gone away but it may well reappear at a later date just in some slightly different energy it changes, it moves but it never gets fully destroyed so the light itself might turn into a different form of energy or matter it may not be light anymore so here's another way of thinking about it if I created something that launched one photon out into the universe and it didn't interact with anything ever, it would forever be a photon it will never just turn off At least if it did, that would really, really mess up some physicists.

19:54So, yeah, in principle it goes on forever.

20:00So it turns out light doesn't have an expiration date. If it were to travel through empty space, it would keep on going forever. And even if it did interact with something, it wouldn't be destroyed, just converted into a different form of energy. And this got us wondering, if light can, in theory, last forever, could we somehow capture it? Could we hold onto it for eternity? Well, to do that, you'd have to find a way of slowing light down. And that's not easy because light speed is famously very, very fast. In vacuum, in free space as people call it, the speed of light is 3 times 10 to the 8 metres every second.

20:47This is Myles Padgett, Professor of Astronomy at the University of Glasgow. He works in quantum optics, so he's interested in manipulating the behaviour and speed of light. So that's the speed that light travels at, let's say, between stars. it's very close to the speed that light travels in air but things like lenses which focus light beams rely on the fact that when light travels through glass it travels more slowly. So the speed of light depends on the substance the light is travelling through but it's actually even more complicated than that because when it comes to light there's actually two different kinds of speed you need to consider.

21:31If I may use an analogy, when you see a bunch of cyclists, then the group travels with one velocity, but maybe individual cyclists within that group might be going slightly faster or slightly slower. Similar things happen in light. Remember earlier we said that light is a wave, a bit like a wiggle? Well, one way of thinking about these two kinds of speed is the difference between how fast the beam itself of light travels versus how fast the waves inside the beam are wiggling, if that makes sense. In Miles's analogy, the beam of light is like the group of cyclists and the individual cyclists are like the individual wiggles in the beam.

22:15You can almost think of it in this way.

22:20If I was to sort of make a stack of partly silvered mirrors, you can imagine the light comes in, it hits the first mirror, some of it gets reflected, but some of it goes through and it hits the second mirror, but then some of it gets reflected back to the first mirror and then gets reflected back again and then maybe goes to the third mirror, but then it might get reflected back and back and back and then forward. So as the light comes through it sort of almost rattles around moving backs and forwards and so it's all the time it's going quite quickly but it still just takes a long time to come out the other end.

23:03So you're saying that photons they're still going to be like kind of rattling around at their own speed But let's say that you could only ever take two steps forward, one step back, and you were trying to walk in a straight line. Your individual footsteps might be quite fast, but your progress to get to the finish line is going to be slow. So the beam of light is slow, even though the photons in the beam are moving quite quickly. Exactly that. The two steps forward, one step back analogy is a very good one indeed. speed and so it can take me a long time to get a message to you in that sense because the information is actually much much less than what we would typically refer to as the speed of light.

23:45I mean how far miles can you push this then I mean if you can get it to walking speed could you make it stop entirely could you slow it down so much that it wasn't moving? Whether it's stopped as in stop stopped or just going incredibly slowly is a slightly different question, but effectively yes. Miles says that there are certain exotic states of matter, as well as crystals like rubies, which can slow light down in this way. We heard earlier that although light may get converted to another form of energy, like matter, energy itself lasts forever. But what about keeping light as light? Could trapping it, like Miles is describing, allow us to hold on to an individual photon for as long as we like, potentially forever, or one day in the future, release it again?

24:36Of course, if we knew what a photon was, it might be easier to answer this question. The particle nature of light as described by photons is obviously very complicated. And And really, often it's not helpful to think about individual photons. Strictly speaking, a photon is what happens when we measure something. Although it's tempting to think of a light beam comprising all of these little ping pong balls traveling along side by side, some of them going faster and some of them going slower. strictly speaking the photons are more i'm going to say delocalized when people say how big is a photon well in the case of a laser beam the photon has the width of the laser beam and its length might be many many meters so i haven't answered your question in terms of you know what are the individual photons doing i'm really trying to convey it's it's much more subtle than that so let me let me ask a different question if i had a skipping rope and you held one end and i held the other end and i wiggled my end

25:56then out the other end you would feel the wiggle you you know the wiggles would come along and you'd feel it. So the energy has travelled along the skipping rope. Is it the same energy? I'm not sure what same energy means in that sense. I put energy in, it travelled along as a wave, and you've got it out the other side.