Praise for the previous edition:
"Extraordinary, humorous, interesting, awesome."--New Scientist
Light Years: An Exploration of Mankind's Enduring Fascination with Lightby Brian Clegg
This is the story of the greatest puzzle in our universe: what is light? Light Years is an engaging survey of everything we know of the universe's most enigmatic phenomenon and the remarkable people who have been captivated by it. Light Years looks over the shoulders of the great revolutionaries of light theory--Bacon, Galileo, Newton, Faraday,/i>/i>
This is the story of the greatest puzzle in our universe: what is light? Light Years is an engaging survey of everything we know of the universe's most enigmatic phenomenon and the remarkable people who have been captivated by it. Light Years looks over the shoulders of the great revolutionaries of light theory--Bacon, Galileo, Newton, Faraday, Maxwell and Feynman--and traces the evolution of light-driven devices from the camera to the laser.
In this revised edition, leading popular science author Brian Clegg reveals how twenty-first century scientists have achieved the seemingly impossible in bringing light to a halt, and used the quantum properties of entangled light to produce unbreakable encryption and unbelievable computers. He explains the mind-bending advances that put light at the heart of all matter and which could finally make time travel possible.
Praise for the previous edition:
Read an Excerpt
An Exploration of Mankind's Enduring Fascination with Light
By Brian Clegg
MacmillanCopyright © 2008 Brian Clegg
All rights reserved.
AT THE SPEED OF LIGHT
For now we see through a glass, darkly.
Imagine this. The dawn light is creeping into your room. You get up from your bed and open the curtains. Outside the window, the inferno of an active volcano distorts the air. A river of red-hot lava is streaming down the scarred mountainside. A rain of ash falls near the window, yet you hear nothing, feel nothing.
Quickly, you move to the second window and pull back the curtain. Here, even though it's morning, the sky is black, a crisper black than you have ever seen. The stars stand out, laser sharp. Before you is a rugged, near-white plain, surrounded by impossibly high, needle peaks. And then your eye is caught by something else. Standing out from the blackness is a bright circle of blues and greens with traceries of white. You are seeing the Earth from the surface of the Moon.
Nervously, half-expecting the air to rush out of the room, you open the window, to be struck by a burst of vertigo. Behind the glass is a leaden yellow-grey sky, hanging heavy over the already-bustling city streets 25 floors below. Nothing that you saw through the window glass exists. There is no volcano, no lunar landscape; there are no stars.
A magical tunnel
Close the window again and still the Earth is riding serenely in the sky. It's as if the window's glass were not looking out of the side of the building, but opening instead onto a magical tunnel leading straight onto the surface of the Moon. There are no video screens or electronics involved, just glass with very special properties. This is slow glass, first dreamed up by the 1970s visionary writer Bob Shaw. A special glass that light takes months or even years to pass through.
With such remarkable glass it would only take a good site in front of a beautiful view to create such stunning windows. If light takes a year to get from one side of the glass to the other, then one year after it is put in position, the first glimpse of the landscape will reach the other side. As the light takes a year to pass through, everything that has been happening in front of the glass will be seen a year later behind it. Shift the glass into a building and it carries a year's worth of light with it. You've got a window on an exotic location for as long as it takes the remaining light to make its slow journey through the material.
The ultimate speed
It was only in the late 1990s that technology caught up with the imagination and made slow glass a possibility. The discoveries described in this chapter demonstrate the remarkable power of the new light technology. Later, we will plunge back 2,500 years to follow the story of humanity's fascination with light. In that story, light's immense speed will be a recurring theme. For slow glass it presents a particular problem.
A beam of light travels at around 300,000 kilometres each second in the vacuum of space, a speed that belies our experience of nature. A hummingbird's wings flap 4,200 times in a minute, near invisible to the human eye. Yet in the duration of a single flap of those wings, a beam of light could have crossed the Atlantic Ocean. On 20 July 1969, Apollo 11 landed on the Moon after a journey of four days. If it had, instead, set off for the nearest star, Alpha Centauri, which light takes four years to reach, the Apollo capsule would still be travelling after a thousand millennia.
In glass, light moves a little slower than it does in space, but it would still require a window 5,000,000,000,000 kilometres thick to hold a year's worth of light. If slow glass is to be made, there is an enormous challenge to face. There has to be a way to apply the brakes, to slow light down by a factor of a billion billion or more. Unlikely though this sounds, in the late 1990s a substance was created that can do just that.
Einstein's strange matter
The substance with the amazing effect on light is a strange form of matter called a Bose-Einstein condensate (physicists have to have a particularly good day to come up with a snappy name like 'photon' or 'quark'). We are used to matter coming in three types – solid, liquid and gas. Since the 1920s it has been known that there is a fourth form of matter, generated in the raging nuclear furnace of the Sun – plasma. This is the next stage beyond a gas, where the easily removed electrons have been broken off the atoms and the result is a soup of ions – atoms with some electrons missing – and the electrons themselves.
The four states of matter – solid, liquid, gas and plasma – have a startling parallel in a theory developed over 2,000 years ago. The Greek philosopher Empedocles thought that everything was made up of four elements – earth, water, air and fire – each equivalent to one of the modern states. Some of the ancients thought there should be a fifth element, the substance from which the heavens were constructed, called the quintessence. This handily corresponds with a then-hypothetical fifth state of matter that Einstein dreamed up. The idea also dates back to the 1920s, when a young Indian physicist called Satyendra Bose wrote to the world-famous scientist describing his ideas. Einstein would have received many letters from scientific hopefuls, but this one caught his attention. Bose had found a totally new way to describe light.
Thanks to Einstein's theories, light had begun to be thought of as photons – tiny, insubstantial particles that shot through space like bullets from a gun. Bose experimented with describing light mathematically as if these photons were a collection of particles that was already well understood – a gas. Einstein helped Bose firm up the maths, but was also inspired to imagine a fifth state of matter. By applying intense cold or pressure to a material he believed that it would eventually reach a state where it would no longer be an ordinary substance; instead it would share some of the characteristics of light itself. Such a state of matter is a Bose-Einstein condensate, the material that could provide the key to producing slow glass.
Nearly 80 years after the theory was developed, a Danish scientist has used a Bose-Einstein condensate to drag the speed of light back to a crawl. Her name is Lene Vestergaard Hau, one of the few women to take an active part in the history of light. In 1998, Hau's team set up an experiment where two lasers were blasted through the centre of a vessel containing sodium atoms that had been cooled to form a Bose-Einstein condensate. Normally the condensate would be totally opaque, but the first laser creates a sort of ladder through the condensate that the second light beam can claw its way along – at vastly reduced speeds. Initially light was measured travelling at around 17 metres per second – 20 million times slower than normal. Within a year, Hau and her team, working at Edwin Land's Rowland Institute for Science at Harvard University, had pushed the speed down to below a metre per second – and more was to follow, as we will discover later.
Hau's material is not quite slow glass. There is one more problem to overcome. Imagine you had a piece of special glass one centimetre thick that took a year for light to get through. It would live up to expectations if you were looking straight through the glass. But things would be different when looking at the edges of the scene. Now the light is arriving at an angle, passing through more of the glass before it gets to you. It could easily travel through half as much glass again, and so take half as long again to get through. With an ordinary window the difference isn't noticeable, but light that hits slow glass at an angle would take months longer to arrive than the light that arrived straight on. Views from every direction would appear at different times, combining images to produce a nightmare confusion.
To overcome this effect, a slow glass window has to do more than just let light through. It needs to capture the whole image at the surface of the window, whatever the angle the light has arrived from. That total view then must pass through the window as a piece, rather than as masses of uncoordinated rays heading in all directions. This requirement isn't as impossible as it sounds. It is very similar to the way in which a hologram is produced, combining the rays of light from different directions to make a unified picture. In the hologram, this gives a three-dimensional view that changes as you move, built into a flat, two-dimensional photograph. It is such an image, with three dimensions compressed into two, that would have to be sent through the window. The combination of holographic techniques and a very slow material would deliver true slow glass.
While the technology required at the moment to have such control on light's speed is formidable, the mere fact of its existence gives some hope that in the future slow glass may move from fiction to practical reality. The first lasers, after all, were heavy-duty, complex devices requiring conditions that were inconceivable outside the laboratory – yet some modern lasers can fit on a pinhead and are happy to function in the unprotected environment of a consumer product like a portable CD player.
Breaking the light barrier
If the possibilities of slow glass, bringing light to a virtual standstill, are fascinating, then taking the opposite tack, pushing light above its normal speed, has even more remarkable consequences. As we will explore in detail in Chapter 8, Einstein's special theory of relativity showed that light was the fastest thing in existence. Nothing, he argued, could exceed that 300,000 kilometres per second. According to the special theory of relativity, any solid object approaching the speed of light would get heavier and heavier until its mass was infinite. Even the speed of an insubstantial snippet of information should never get past the 300,000 kilometres per second barrier, because the peculiarities of relativity mean that a faster than light signal would travel backwards in time. If it were possible to broadcast a message fast enough, we could use light to say hello to our ancestors.
Such technology would transform human existence. If a signal could be sent back even a tiny fraction of a second it would make it possible to build computers that worked thousands of times faster than current machines, limited as they are by the speed of internal communication. With information sent even further back, disasters could be averted by broadcasting warnings. All gambling based on prediction, from the roulette wheel to the stock market, would be destroyed. There is hardly an aspect of life that would not be fundamentally changed. Yet this is not the most dramatic implication of sending a message back in time.
The very foundations of reality would come under threat. Being able to send a message into the past would shatter the rigid connection of cause and effect. For most scientists this is enough to prove that getting a message past light speed is impossible. It's not that they have any objection to getting a sneak preview of lottery results this way, but rather that bewildering paradoxes emerge when information is sent backwards through time.
It is easy to feel the impact of the paradox by considering a simple time transmitter that could send a radio message back just a few seconds. This transmitter is fitted with a radio control, so it can be switched on and off remotely. At noon precisely, the transmitter is used to send a message back in time. This message is the signal to the transmitter's own radio control. When the message is received at five seconds before noon, it switches the transmitter off. Now, when noon arrives, the transmitter is switched off. So how could the message have been sent? But if the message wasn't sent, the transmitter would still be switched on.
Rather than deal with such mind-bending possibilities, physicists resort to the 'Causal Ordering Postulate', sometimes known as the time COP. This sounds impressive, but amounts to little more than saying that effect never can come before cause. (It's actually a little more sophisticated, allowing the effect to precede the cause if there's no way the effect can influence the cause, but the result is the same.) It follows that anything that would endanger the relationship of cause and effect, like sending a message back in time, is impossible. Professor Raymond Chiao of the University of California, a leading exponent of superluminal physics – the science of faster than light motion – believes there is no way to send a message back through time. But Chiao's own experiments in the late 1990s opened a loophole in the light speed barrier.
At the sub-microscopic level of photons, the minuscule particles that make up a beam of light, the everyday expectations of the world fall apart. The familiar, predictable behaviour of objects disappear, leaving only probability and uncertainty. This is the world of quantum physics, discovered by Max Planck and Albert Einstein around a hundred years ago. Thanks to the bizarre nature of reality at the quantum level, individual photons of light have a small but real chance of jumping through solid objects and appearing on the other side in a process known as tunnelling.
Quantum short cut
Tunnelling emerges from the bizarre statistical view that quantum mechanics takes. Generally speaking, quantum mechanics expects, just as we would in the normal world, that when a car drives into a wall it bounces back. Every now and then, though, quantum theory says it should pass straight through. The probability is incredibly low – far less than winning a lottery week after week after week – but it exists. In a beam of light there are many, many photons, and the chance that a single photon will cross an apparently impenetrable barrier is much higher than that of a whole car jumping through a wall. This phenomenon, tunnelling, has been widely observed. In fact, if it weren't for tunnelling, there would be no life on Earth.
The light of the Sun that heats the Earth and triggers the release of oxygen through photosynthesis is produced by a deceptively simple process. In the intense furnace of the core of a star (like the Sun), charged particles of the most basic element, hydrogen, combine to make helium, the next element up the chain. In this process energy is released. The reaction can only happen if hydrogen particles come into close contact, but each particle is positively charged. These charges repel each other, like magnets when the same poles are brought together. Even in the Sun's heart, the particles can't combine – just as well, or there would be an immense explosion, burning out the Sun in a second as all the hydrogen was converted. The repelling force forms a barrier that has to be overcome to form helium, just as we have to fight against gravity to jump over a physical barrier like a fence. It is the strange reality of quantum physics that makes this possible. Some hydrogen particles jump through the barrier to fuse together – they have tunnelled.
To give an accurate picture of what is happening, we should really drop the term 'tunnelling'. It implies slowly grinding your way through an obstacle. What really happens is much more startling. At one moment a particle is one side of the barrier, the next it is on the other. It jumps rather than tunnels, but instead of flying over a physical barrier, it actually passes from one position to the other without moving through the points in between. This instant jump means that any photons travelling along a path that includes a barrier to tunnel through manage to get along that path at faster than the speed of light.
Chiao and his team demonstrated this strange phenomenon, measuring light travelling at 1.7 times the normal speed. If this light beam could be made to carry a signal, that message would, according to relativity, be shifted backwards in time. But Professor Chiao wasn't worried about destroying the fabric of reality. His experiment relied on generating individual photons, and the mechanism that made this possible provided no way of controlling when a photon would emerge. Without such control, the photons could not carry a message. Equally, there was no way of deciding which photons would get through the barrier – most don't – and so it seemed impossible to keep a signal flowing. Without the ability to send a message there would be no chance of disrupting causality.
At the time, Professor Chiao was unaware of developments in another laboratory in Cologne, Germany. The refined tones of Mozart's 40th Symphony, clearly a message, were about to be transmitted at four times the speed of light. The stakes for reality were about to be raised.
But before exploring the nature of these faster than light experiments and how they could pose a threat to existence itself, we need to do some time travelling of our own, taking a 2500 year trip back to a time when the very existence of light seemed as close to magic as it did to science.
Excerpted from Light Years by Brian Clegg. Copyright © 2008 Brian Clegg. Excerpted by permission of Macmillan.
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Meet the Author
Brian Clegg studied Natural Sciences at Cambridge University and Operational Research at Lancaster University, and is a fellow of the Royal Society of Arts.
He has written five popular science books, including A Brief History of Infinity and The God Effect. He lives in Wiltshire, England.
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