In Our Time - The Speed of Light

Episode Date: November 30, 2006

Melvyn Bragg and guests discuss the speed of light. Scientists and thinkers have been fascinated with the speed of light for millennia. Aristotle wrongly contended that the speed of light was infinite, but it was the 17th Century before serious attempts were made to measure its actual velocity – we now know that it’s 186,000 miles per second. Then in 1905 Einstein’s Special Theory of Relativity predicted that nothing can travel faster than the speed of light. This then has dramatic effects on the nature of space and time. It’s been thought the speed of light is a constant in Nature, a kind of cosmic speed limit, now the scientists aren’t so sure. With John Barrow, Professor of Mathematical Sciences and Gresham Professor of Astronomy at Cambridge University; Iwan Morus, Senior Lecturer in the History of Science at The University of Wales, Aberystwyth; Jocelyn Bell Burnell, Visiting Professor of Astrophysics at Oxford University.

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or wherever you get your pods. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash Radio 4. I hope you enjoy the program. Hello, this week we're discussing the speed of light. The medium most of you are listening to through radio waves travels at the speed of light. Those of you closer to the radio transmitter will hear in our time

fractionally before someone further away. Scientists and philosophers have been fascinated. with light for millennia. Aristotle wrongly contended that the speed of light was infinite. It was the 17th century before serious attempts were made to measure its actual velocity. We now know it's about 186,000 miles per second. Then in 1905, Einstein's special theory of relativity predicted that nothing can travel faster than the speed of light. This had dramatic effects on the study of the nature of space and time. It's also been thought that the speed of light is a constant in nature, a kind of cosmic speed limit.

now the scientists aren't so sure. Join me to discuss this is John Barrow, Professor of Mathematical Sciences and Gresham Professor of Astronomy at Cambridge University, Jocelyn Belnell, visiting professor of astrophysics at Oxford University and Ewan Morris, senior lecturer in the history of science at the University of Wales, Aberystwyth.

John Barrow, first of all, what is light? What's it made of? Well, it's always a bad question to ask scientists what something is. We tend not really to know what things really are, but just what they do and what their effects are. And we look upon light

as a way in which energy is transmitted from one place to another in the universe in a wave-like fashion. We're familiar with light having many possible wavelengths now, not just the visible forms of light that we've known about

ever since there are human beings with eyes, but we know that light spectrum stretches into the infrared and and far beyond the ultraviolet. So light we see as a way in which the forces of electricity and of magnetism propagate their influences from place to place in the universe. And once upon a time physicists 300 years ago even thought that this happened instantaneously, that in effect the speed was infinite.

But we now understand that the speed is finite but extraordinarily large. When you're going willing to say what it is But as I understand it can be either photons or light waves Can you explain that to listeners Why can be one or the other Yes it's a useful way of thinking of light Either as being rather like little Bullets moving from one place to another

But these bullets have no mass at all And so they move at the fastest possible speed That it's possible for anything to move at And that's what we call the speed of light But there's always been a strange dichotomy about the behaviour of light. In some experiments you can make light behave as though it is a collection of tiny little microscopic billiard balls. But in other experiments, it doesn't behave like that at all. It behaves like a wave. And when you add two waves together,

if you put them out of phase so that troughs coincide with peaks of the other wave, you can produce a net effect of darkness in one place and extra brightness in the lightness in another. So in some experiments and some physical phenomena, light behaves as though it's a wave. And for many hundreds of years, this was something of a paradox. Was it really a wave as some scientists like Hoygens argued, or was it really a particle, as others argued? In the 20th century, quantum theory has allowed us to square this circle, as it were, and understand how it is that you can have these two aspects to this phenomenon. And I was like to think. And I was like to think of the wave quality of light, not really like a water wave at all. It's more like a crime wave.

So a crime wave is a wave of information. So if a crime wave hits your neighbourhood, it means that a crime is more likely to be committed in your neighbourhood. So it's a wave of information. And so if a photon wave passes through your laboratory, you're more likely to detect a photon in your laboratory than if the photon wave is not there. and in quantum mechanics what this wave tells you is that probability, the chance that you'll detect something like light affecting your detector. Thank you, Ewan Morris, why do you think people have been fascinated by the study of light? Can you give us some of your reasons why that's the case? In the first instance, I think at any rate that, I mean, light itself is intrinsically fascinating. I don't think that anybody, or at least anybody with any imagination,

can look at a rainbow or go out. side and look at the night sky without wondering how and why. I mean, more historically specifically, though. When we're looking at the night sky? We're looking at light that could have started off many, many years ago, aren't we? Millions, tens of millions of years ago. I mean, more historically specifically, and certainly in terms of the interest of some of the people in the 18th and 19th century we're going to be discussing now.

And particularly in Britain, there's a very specific. interest in light and the properties of light, because of the kind of philosophical ideas that people have about how we get to know about the world around us. 18th and 19th century philosophers in the empiricist tradition tend to think of the eye as the main conduit through which we get to know about the world around us. David Brewster, for example, famous,

I mean, he describes the eye as guarding the portal between matter and spirit. It's the medium to which things actually get into our heads. And of course, light is what strikes the eye. So all of these natural philosophy, if we look at the sorts of natural philosophers of scientists during the 18th and 19th century, who are interested in the properties of light,

they're also very often, people like David Brewster, Thomas Young, James Clark Maxwell himself. They're also often very interested in the physiology of vision, the physiology of the eye as well. They're interested in this combination, between the properties of light matter because by understanding the properties of light you get to understand something about the way

that we as human beings get to know the world around us. Rather unusually in an area of science, there isn't much that comes, as I understand it, please go. There isn't much that comes from the Greeks. Aristotle thought the speed of light was infinite. People seemed to have accepted that or not been very interested in pursuing it until it was challenged by Galileo.

Was there any more around? Can you explain what Aristotle meant by that? Was there any more around at that time? And why did it take so long before Galileo got to grips with it? I mean, it's not really clear what precisely Aristotle said or meant. It seems that he simply took light to be more or less instantaneous and doesn't really go about it in one way or the other. And, I mean, until really the emergence of what my monoeuvre,

might call the new sciences during the 16th, 17th century. Aristotle is more or less taken as the authority. And then you get people like Galileo coming along. I mean, Galileo in the discourse on the new sciences, argues really in passing that no, light isn't instantaneous. Light travels at a finite speed. And it suggests some what he called experiments. that one might perform to try and find out what the speed of light is.

I mean, essentially, he visualises two men carrying lanterns on distant hills. One of them opens a lantern, shines a light, as soon as the other one sees the light, he opens his lantern and shines it back, and thereby one might be able to find out at what speed light travels. I mean, of course, I so often with Galileo's experiments, it's not clear whether or not Galileo actually performed this experiment. I mean, certainly when he's writing the discourse and the two new sciences, he's under house arrest.

So maybe might not be in a position to do so. But Aristotle challenged by Galileo, along where it gets going this investigation of the speed of light, Jotlin Medo Nguyen in the 17th century with Ole Romo's work in Paris. Can you tell us about his Jupiter experiments and what they demonstrated? Yeah, this was one of the first indications that the speed of light was not infinite. Circa 1676, Romer was observing the planet Jupiter, which has several moons. And as the moons go round Jupiter, they go into eclipse. The sunlight gets cut off.

You don't see them. And Romer was timing the eclipses and found that there was something slightly wrong with the timings. they fluctuated a bit. And he came to realize that the problem wasn't with his clock or his measurements. The problem was that he was assuming that when he saw an event, it was the same time as when it happened. Now, Jupiter is some way out in the solar system, and it takes time for the light to get to us from it.

And it takes time for the light from the moon of Jupiter to get to us. but the moon of Jupiter's moving around and so that time lag changes and what he was actually measuring was the time it took for the light to get from the moon to us, Jupiter's moon to us and that changed and so he was demonstrating

that the speed of light was not infinite. This was remarkable for the time how was it received by other scientists around Europe? I don't know but I would guess it's well probably the same way we react to some things. You know, initially, oh, mad scientist can't do proper experiments, and then gradually getting accepted and not only accepted but built on by other scientists. Because as I say, Newton is soon onto it, is worked out for what the distance of light is between ourselves and the moon, hasn't it,

but the Earth and the Moon. And there's a whole string of scientists working on measuring the speed of light with more and more ingenious methods as time goes on for the next 200, 300 years. But at this stage we're talking about measuring the speed of light out of curiosity. It's a wonderful example of science for the sake of science. There's no end product. They're not going to do things

with it. No, I think it was probably they were doing measurements because it gave better understanding of God's universe and it was all to the greater glory of God. And that was the way it was operating at the time. That's the way it was couched, yes. So how did it spread through the science community? I mean, we're going to go back

to France soon. but how did it spread to the crisis in the rest of Europe, this discovery, this was a form of measurement? Most of the spread of information around about that time was word of mouth with a few very precious books being carried around given to people. So it was a good deal slower than here. You and Morris, in the 19th century in France there are two scientists in particular, Hippellate Vizot and Leon Foucault making great progress,

but they seem more interested in finding out about ether. Can you tell us about what they did briskly and how accurate their findings were? It's around about the middle of the 19th century. Both Fizzo and Foucault, who are disciples of the grand old man of French physics Aragon, have certainly been set the task of measuring the speed of light. Fizzo uses an apparatus whereby you shine light through the gap between a toothed wheel,

and you get the wheel to rotate and when the wheel's rotating at the right speed you don't get the light bouncing back at you from a mirror beyond the wheel and that gives you a measure of the speed of light Fouca uses a

revolving mirror the shine light on the rolling mirror it bounced off onto a mirror bounces back in that time the revolving mirror has revolved a bit so you try and measure the angle and that tells you something How close did they get to it?

They're reasonably near. I can't remember the exact figures off the top of my head. But I mean, these are pretty accurate experiments. And then much later, Albert Michelson, although much later in America, Polish-American picked up Fouca's work and repeated his experiments with much more accurate results. Jocelyn Bell by now.

Yeah, that was fun to read about that one. Ewan was talking earlier about Galileo's experiment, about two guys with lanterns, open a shutter on one lantern. When the other guy receives the light, he opens the shutter on his lantern, and back goes the light pulse. Michelson's method involved mountain tops.

The mountain tops were 22 miles apart from Mount Wilson to Lookout Mountain. And they had a revolving mirror. A bit like one of those big disco balls with mirrors all around it. And you shine a light onto one mirror. It goes off to the far mountain. it bounces back and the ball has turned

and if the ball has turned just the right amount you get the returning beam of light bouncing off the same mirror and into the detector. The problems were huge. The US Geodetic Survey measured the distance between the two mountain tops. 22 miles. They measured it with metal tapes,

albeit invar tapes, and they claimed to be accurate to a quarter of an inch. But there was a big earthquake in the middle of the measurements and that may have upset their baseline. And there were forest fires as Michelson was sending his beam out, so the air was shimmering. So Michelson was never terribly satisfied with this and kept working on this project the whole of his life.

He was quite a character. I think looked in some ways a bit like Einstein with hair all over the place. So he devoted his life to shining light beams from one spot to another to get the space. He came remarkably close to what's the right. His value was really very good, but he was a superb experimentalist. And going on at the same time to round up this opening phase, Jan Barra,

there's work going on in the field of electromagnetism, particularly with Maxwell. Can you tell us why that's so significant in relation to the speed of life? Well, Maxwell was in some ways mathematically, rather ahead of his time. He created a set of equations that most electrical engineers and physicists use on a daily basis even now, which tells us everything we want to know about how electricity and magnetism behave. And hidden in these equations was the feature that the effects of electricity and magnetism propagate as waves at the speed of light. And at some stage, a curious coincidence was noticed from these equations that if you took two of the properties of free space for the propagation of electricity and of magnetism, and you multiplied them together, the result had the dimensions of a velocity squared.

And remarkably when you did that, the answer you got was exactly equal to what was believed to be the square of the speed of light. And physicists thought this was really too much of a coincidence to be merely a coincidence. And so after that, you see gradually the emergence of a deeper understanding of the link between light and electricity and mass. What you have to remember is that if you look back 150 years ago and earlier, the speed of light was nothing more than the speed of light. It doesn't have a special status like it does in physics today. It's just the speed of something and something happens to be light, like the speed of sound. But today we're in a situation where for physicists, the speed of light has all sorts of other deep features.

Can we come to that rather more slowly? And what we're going to look at, I think. Yeah, we are, just in a moment. So we've got that. Now, Einstein comes onto the scene. Einstein's greatest hero is Newton, but he also was very influenced by Maxwell. And he had conversation with Michelson,

whom Jocelyn was talking about, the Polish man who worked in America, and in 1905, he presents his paper on the special theory of relativity. Can you explain, John Barrow, the significance of this work as regards to the speed of light? Well, it was predicated really upon a belief that

the speed of light had a special status in physics that whoever you were, however you were moving in the universe, if you made a measurement of the speed of light, you should get the same answer. So there's no special observer who has a specially moving rocket to, if he makes his or her observations from this rocket, will get a different answer. Why is that so significant? Well, Einstein's belief,

was that physics shouldn't, and the laws of physics, should not look simpler to some observers. It's a sort of extrapolation of the Copernican principle, you know, that there's not a special place in the universe. But he didn't think there should be special observers, privileged positions, privileged viewpoints. But how is that significant in physics? I mean, that sounds almost like a political, democratic... Yes, it sounds as though it has nothing to do with physics. But it turns out that, I mean, take Newton's famous simple laws of motion, you know, that force is... equal to mass times acceleration or a body acted upon by no forces remains at rest.

This is not something that would be seen by every observer. If you're in a spinning rocket and you look out of the window, you see the stars spinning past you in the opposite direction. And so you see them accelerating, even though there's no forces acted upon them. It's only very special observers who are not spinning and not accelerating, who see those simple laws of Newton. So Einstein began this quest to try to produce a form of the laws which had this democratic feature. And one of the consequences is that Newton's simple laws of motion change their form as you start to approach the speed of light and the changes become very large.

And all of a sudden the speed of light has a very special status in your view of the world. It's no longer just the speed at which light moves. it turns out it's the maximum speed that any information can be transmitted at and that anything can move at. Can you give us, Jocelyn Belbinow, I don't know whether you, can you give us some examples of one consequence of this, which was time dilation? Can you talk about, say, the clock on the jumbo jet or the twin paradox? The twin paradox, I think, is easier to grasp and more vivid.

Yes, we're actually moving beyond special relativity here to Einstein's next relativity theory. called general relativity. Special relativity doesn't allow any accelerations or forces, but the real world has lots of forces and mass of bodies and gravity and things like that. So you need the general relativity. And clocks can behave differently depending how strong the local gravity is in effect. If you use GPS, as many people now do, your GPS result will involve a correction, a general relativistic correction, because the GPS satellites up above the earth are in lower gravity than you are down here on the surface of the earth. It's a very small effect. It gets more striking if you go somewhere with stronger gravity like a neutron star or maybe

even black holes where, for instance, if I go to a neutron star with a great big clock and you stay here safely on Earth with a big telescope so you can read my clock, you'll see that when I'm at a neutron star, the clock's doing one tick every couple of seconds instead of one tick every second. I don't notice anything's wrong because my heart has also slowed similarly and my metabolism and everything. But it's actually compared with your clock, mine's going slow. And the twin paradox is even more vivid, isn't it? The twin paradox, yes, that's basically saying that if something is moving, it goes slow. So the twin paradox is you have twins, preferably identical twins.

One stays on Earth, the other goes on a rocket trip, which goes a long way out into space at high speed and returns. And the returning twin finds they're younger than the remaining twin. It's perfectly correct. I've given a rather simple version of it, ignored acceleration and all sorts of things like that. But basically, yeah, that's what Einstein says and that's the case. And even dramatically younger. It can be.

It depends how far he goes or how fast. So we have this groundbreaking idea, Ewan Morris. It came out, just for a moment, talking about where the tradition Einstein came out of, which was a different tradition from the British empiricists and even from the French. Can you explain that to us?

Yes, actually, I mean, when Einstein was first read in England, when the electrodynamics of moving bodies was first read in 1905 in Britain, it was actually lumped in. with British physicists. Einstein is just like, sort of people like Oliver Lodge and Joseph Lammel working in the Maxwellian

electromagnetic ether tradition. But, you know, to an untutored eye, maybe it looks like that. But, I mean, in fact, something quite different is going on in Einstein's physics. British physics at the end of the 19th century, if you feel like, is an engineer's physics. It's all about the ether being this construct

of some pullies and gears and wheels. It's, you know, they're fascinating. The Holy Grail of British physics is the, is, is the mechanical structure of the ether. People from, you know, somebody from Einstein's background, you know, doesn't, it doesn't really give a hoot about the ether, if you like. He's coming from a, from a German physicist and philosophical tradition that, you know, that emphasizes the abstract. They think of physics as a physics of the, of the appearances of things. I mean, it's, you know, it's still, you know, an industrial modern physics, you know,

after all, Einstein works in a patent office. But, you know, if British physics is an engineer's, is about an engineer's universe, I mean, Einstein's physics is about a bureaucrat or a factory managers universe. It's about, you know, it's about clocks and timetables and making sure that, you know, things fit in with each other on paper. So it's a very different kind of tradition

from the sort of, you know, I think, so the messy sort of oil and green. tradition of British physics, their vision of the universe. Can you just go back a little, John Marron, tell us what you think, some of the examples that you can give the listeners about

the consequences of Einstein's discovery of the speed of light, what he discovered about it? Well, the time dilation and twin paradox that Jocelyn just mentioned, I mean, one way to rephrase this is to say that time travel

is possible, but just time travel into the future. You see if the one twin goes off on a spaceship and feels forces and gravity and comes back home, then is younger. In some sense, that twin has travelled into the future of the other people. And time travel to the future is uncontroversial. And we see these effects of time travel to the future, the twin paradox almost on a daily basis in particle accelerators. So if you accelerate elementary particles, their lifetimes will change compared with the lifetimes of particles that sit still and are not accelerated. Also, at this moment, there are cosmic ray muons passing through us at quite high speeds

and hitting the earth everywhere. Passing through us? If Einstein's picture of space and time relativity were not true, we would not see any of these particles. They're made high up in the top of the Earth's atmosphere when cosmic rays hit the top of the atmosphere and they move very, very close to the speed of light

but they only live for a microsecond or so. So they should decay long before they ever reach the Earth's surface. They should only travel about 600 metres but they managed to travel 6,000 meters and reach the Earth's surface and the reason is because their clocks go slow.

They live longer because they're moving at such high speed, or equivalently the distance that they're traveling from the atmosphere to the surface is contracted by relativistic effects. So these transformations of length and of time that you need in order to keep their ratio at speed of light the same for everybody are things that you see on a regular and routine basis. Justine Balbanon, what implications does Einstein's work have for cosmology broadly in our understanding of the universe? It's everywhere. You can't study a lot of the things in the universe

without being very conscious of Einstein and Einstein's work. For example, one of the predictions Einstein made was that a heavy, massive body would bend light rays and radio waves and x-rays and the rest of it. And we see that. We actually see it as a double image. Was this the thing that Eddington proved in 1910? 19. Yes, that was what Eddington. That's right. Yes, yes. Eddington played a key role in many ways in Einstein's work. That's another very interesting story. Yes, so the bending of light was one of those. There's actually a temporal equivalent of the bending of light. There's a delay. And if you're studying pulsars, you see that. Another great prediction of Einstein's was the existence of a new kind of radiation called gravitational radiation.

which also goes at the speed of light, this same speed apparently. And Einstein said that if anybody's accelerated, it'll send out these gravitational waves. So, for example, if you've got a pair of black holes merging, maybe it's a galaxy forming. The two original proto-galaxies had black holes.

As these black holes merge and spiral around each other, they send out gravity waves, which will increase the rate they... two things spiral in and cause their ultimate merger. And there's evidence from some of the pulsar work from the last 30 years, very good evidence that these gravitational waves exist. There's a big campaign now to detect them directly. Let's talk about black holes now, John Barrow.

How did his Einstein's work informer ideas about black holes? Well, since the latter part of the 18th century, there were a couple of people, one in English, England, John Mitchell and Pierre Le Place in France, who had started to think about how and whether light could act on, or gravity could act on light. And those two people both had the idea that perhaps you could have an object that was so dense, whose gravitational pull was so great,

that light couldn't escape from its surface. So we're familiar with that idea on Earth, if you want to launch a rocket from Cape Canaveral that escapes the Earth's gravitational pull, there's a critical speed that you have to achieve. It's about 11 kilometres per second. And if you do that, then the rocket won't fall back to Earth like when you throw a stone or a cricket ball in the air,

but it'll go off into space and eventually get captured by somebody else's gravity. And it turned out that you could conceive of objects that were so dense that light wouldn't leave them and travel far away. So if you were a distant astronomer, you might ask the question, can you see these objects? Well, you can't see them in the normal way, light can't bounce off them and reach your telescope. But maybe you could see the effects of things moving past them or moving around them in orbit, even though they're orbiting around something that's invisible.

So this general idea, although little known, was around. Einstein's theory allowed the prediction and very detailed description of objects like this, which we now call black hulls. That name was only invented in the early 70s. And the idea is just like that of Michel and Laplace. You have a region of space and time within which so much matter has accumulated, pull of gravity is so strong

that light can't pass out through the boundary of this surface and reach us far away. So this is the most dramatic example of light bending, if you like. It's light bending with a vengeance. The light doesn't assess. escape at all. It's trapped inside this region. And I say region because the popular

image is that these are stupendously dense objects, great lumps of stuff. But that's not necessarily the case at all. A black hole of the sort that we suspect sits at the centre of most galaxies including our own would

be a billion times heavier than our sun. And its average density is just like that of air. So just like in your living room. And you could be passing across that surface of no return at this moment, and you wouldn't notice anything odd at all. It's only if you try to retrace your steps and go back to base on your distant planet that you would find you were stubbornly trapped inside this surface. So it's only when you reach the very centre of the black hole that forces start to tear you apart. But big black

holes are really quite benign. Can I go back a little at you and Morris? Um, John's mentioned the Reverend John Mitchell and Pierre Laplace. It'd be interesting to know what they thought, how they were getting towards the idea of the Black House, in a little more detail. I mean, both Mitchell and Laplace are coming from the perspective of a Capuscularian theory of light. That is to say that it's standard throughout the 18th century,

pretty much, following Newton, to think of, you know, by far in a way the dominant model of light is this kind of speeding bullet, little particles being, travelling through space very quickly. You know, that's what they think that light is. And in both cases, really, it's, I mean, it's if you like, an off-the-cuff remark almost. I mean, John Mitchell, for example,

suggests that, I mean, he's giving a paper to the Royal Society in 1783 on double stars, as it happens, and he just simply points out in passing that if you had a star, that there's the same density as the sun, but I think it's 50 times as big, then he says light wouldn't be able to escape that body's surface,

and you would therefore quite literally not be able to see that body. So, I mean, he's working in, in a particular Newtonian tradition, he's actually, on the whole, rather interested in other things, and nobody really picks up on this notion. It's the kind of amusing conceit almost that

18th century natural philosophers sometimes like to make drawing interesting and possibly paradoxical conclusions from their speculations. Can we briefly John Barron and then I want to turn to Jocelyn about something everything's

based on the presumption that the top speed of light is about 186,000 miles per second in a vacuum. But it can move faster in water I understand. How do we explain this? Can we slower in water? Sorry, slower in water. It's not saying, I mean, we use this number 186,000 miles per second. I mean, even if we were using metric units like we should,

physicists measure and determine this quantity to fantastic accuracy. It's one of the most accurately determined numbers that we have, and we use it for defining standards of length and so forth. But when we've been talking about the speed of light, and when Einstein talks about the speed of light being a cosmic speed limit, technically what that is, it's the speed of light in a vacuum. But when light moves in a medium, in glass, or in water, it will move more slowly. And we see evidence of that.

I'm looking at a bottle of water in front of me now, and if you're looking at one where you are, you'll see a refraction effect as the label on the other side gets bent. Or if you put a straw in a glass, you're used to this dislocation where the straw gets bent. This is a manifestation of the fact that light travels at a different speed in the water than it does in the air. And the ratio of the two speeds is something that we call the refractive index of the glass.

So we're quite familiar with the fact that we can manipulate the speed at which light travels in different media. Jocelyn Bell-Bern-Urne-L, will you say, please, you want to come in? Can I come in? Yes. Because what you said is both right and wrong, Melvin. We've talked up to now about the speed of light. In fact, if you do physics experiments, you learn to distinguish between two different speeds, what's called the group velocity and the phase velocity.

The information carrying speed is the one that can't be faster than the speed of light, and that's slightly slower in water. But there are circumstances where you can see this other velocity, this phase velocity and it produces some interesting phenomena as well and provided the group velocity

multiplied by the phase velocity those two are less than the speed of light squared then you're okay so if one goes under the speed of light you can put the other over but you can't send information ever faster from the speed of light

but where I got confused because there is something in some of the notes I read which said There is something that can move faster than the speed of light. You've just said it can't. Right. Are they called tachions?

There's tachions that we need to talk about, yes. There's also instances where a beam of very high-speed particles go into a medium like water, and you get a faint blue light called Cherenkov light. So that's going faster? And that's one of the velocities is faster than the speed of light, yes. So there is some speeding up in water sometimes. Yes, one of the two.

But we want to talk about tachions. They're much more fun. Yet something else in the wonderful world of physics that we can't see. Yeah, we don't anything about it. But they're very important. Well, I'm not sure they're very important, but they're immense fun. Well, actually, the interesting thing about the speed of light over the last few years is that the people who have pursued it have done it for immense fun. And now it turns out to be terrifically important.

Yeah, yes, yeah. We've said up till now that nothing can go faster than the speed of light. That's true if we're talking about real objects with real mass. But if you're a mathematician, you'll be aware of complex numbers which have got real parts and imaginary parts. And these tachyons maybe do not have a real mass, but have an imaginary mass.

Now, this is getting into Alice and Wonderland world thoroughly, and it's quite hard to explain. But just follow me through the argument for a moment. moment. If tachions have imaginary mass, they can go faster than the speed of light. They frequently do go faster than the speed of light. And if you want to slow them to the speed of light, you actually have to put energy in. These are things that give out energy as they speed up, in contrast to real mass, where you work really hard to make something speed up. Tacions are the exact opposite. So you've got this whole possible category

kind of in another space of particles called tachions which go faster than the speed of light. Is it anything to do with thought or imagination going faster than the speed of light? Could you prove that, Melvin? No, I'm just asking you. Look, no, the proofs are you.

That's just a mild question from out of your space. Yeah, yeah. I mean, there's another whole series of questions where people often twins sense that their fellow twin is in trouble, how quickly does that signal travel? Don't know. And not even established what the signal is.

So we have these unknown, unseen, but imagine tachgons, which may be, may go fast and speed of light. When you nail them, the world will change again, just like. The way they're looking for them is for this Cherenkov light, this blue light that you get when a shire of particles passes through a medium like water. but they haven't found him. Talking about the...

Kimmy kind of... that it might not be a constant, John Barrow. Has the speed of light changed since the beginning of the universe? Yes, in cosmology, we haven't really talked about cosmology yet. I mean, light plays a crucial role in our understanding of what the universe is and what it's doing.

And the expansion of the universe, the fact that distant clusters of galaxies are fleeing away. from one another at high speed was something that was discovered by examining the light that comes towards us from the stars in those distant galaxies

and you could imagine what's happening here that as light comes towards us from a source that's going away its wavelength of oscillation gets stretched out it's as though the one end of the wave is being pulled away by the expansion

and the wavelength gets stretched out and longer wavelength means redder in colour if it's optical and hence we have this expression the red shift. So the effect of receding galaxies on the light that's coming towards us reveals to us the expansion of the universe. And most of what we know about the universe just comes from collecting light from far away. So we can start to think about could there be things going on in the universe that affects that light en route to us or if we can start to think about could there be things going on route to us

or if we look at the way in which the lights formed at its source and compare it with what we think to be the same features of physics here in the lab, could we learn something about what the world was like when the light left us? And so one way of seeing whether the speed of light, for example, has really been constant, whether other aspects of the physics of light have always been the same, is to compare light from very distant objects like quasars with the same sort of light.

here in the laboratory on Earth and see if they're intrinsically the same in particular ways. Very briefly, Ewan, can you give us some idea and then Jocelyn, can you give us some idea of what the developments at the moment are? What is being found new in this area? As a historian, this is rapidly

sort of getting beyond my expertise. I mean, I think that we live in very interesting times, shall we say. Well, there are new projects as the NASA project and then there's the planks of our project coming out. Jocelyn, you want to talk about that then? As we move out of you in space into the future. Yes, but I need to go right back 13.7 billion years to the Big Bang.

We've got about a minute to do this. You've got a minute, right, Big Bang, explosion, heat, radiation, thins and cools. Some of the radiation still there, few degrees above absolute zero. and these satellites are out looking for that. Well, they've found it. They're looking at it now in detail, studying how the early universe formed, how the galaxies clumped, and that kind of thing.

And finally... Yeah, so this light from the beginnings of the universe carries with it a sort of footprint of the universe it's travelled through. It tells us when and how galaxies formed, and it might tell us some of the goings-on in the first instance of the universe. after it first began expanding.

So light has a way of allowing us to look back virtually to the beginnings of the expansion of our universe. Well, that's a resounding, I think. Thank you very much for that. Thanks very much, Sean Barrow, Jacqueline Bell Bonell, and Ewan-Morris. And next week we'll be talking about anarchism. Thanks for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.

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