The Big Questions: Physics (22 page)
Read The Big Questions: Physics Online
Authors: Michael Brooks
The corpuscular theory reigned for 150 years, but it didn’t get an easy ride. Newton’s great rival Robert Hooke had
produced a competing wave theory (the wave theories of this time assumed the existence of an ‘ether’ in which the light created vibrations), and so had the Dutch mathematician and astronomer Christian Huygens. Both ideas passed muster in experiments; it was really only Newton’s reputation that gave corpuscles their sticking power. Then, in 1803, Thomas Young performed the definitive demonstration of the wave nature of light.
Young’s demonstration centred on the fact that the interaction of two water waves produces predictable geometric patterns (see
What Happened to Schrödinger’s Cat?
). Where the crests of the waves meet, there is a ‘constructive interference’: a crest that is double the size. Where two troughs meet, ‘destructive interference’ makes the trough twice as deep. Where a crest meets a trough, flat water results. Knowing the speed and direction of travel of the water waves, and their initial separation, it was possible to predict the resulting wave patterns in the water. If light is a wave, the same phenomenon should arise when light is passed through a double slit. The two interacting light waves should produce an ‘interference pattern’.
Young’s double slit experiment, now a staple of the high school science laboratory, worked beautifully. It killed the corpuscular theory outright: light was unquestionably wave-like. Only one question remained: if light is a wave, what does the wave move through? The original answer was similar to Descartes’s plenum: the ether, a ghostly substance that filled space and time and provided the medium through which electricity, light and magnetism were transmitted. However, an experiment performed at the end of the 19th century showed the ether did not exist, at least not in any way that enabled the transmission of light.
In 1887, Albert Michelson and Edward Morley set out to show the ether
did
exist. Their interferometer experiment comprised a rotating table that would measure light’s speed in varying directions. The idea was that, if an ‘ether wind’ was blowing, light would move at different speeds in different directions. This
difference would show up in the interferometer, shifting the interference pattern around.
‘You were probably told about light behaving like waves. I’m telling you the way it does behave – like particles.’
RICHARD FEYNMAN
The experiment failed to detect an ether, a fact that stunned physicists at the time. Even though it was clear that light could travel through a vacuum, and so was fundamentally different from sound, it was assumed that it still needed to travel in something. Light showed wave characteristics, but if there was no ether for it to travel through, light was not the kind of wave anyone had encountered before. We have arrived at a conundrum that remains unexplained. Yes, light is a wave. But it is unlike any other wave. And some of the best minds in physics insist that it is not a wave at all.
Perhaps Richard Feynman put it most forcefully. ‘I want to emphasize that light comes in this form – particles,’ he once told his students. ‘It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told about light behaving like waves. I’m telling you the way it does behave – like particles.’
If Feynman’s insistence is not enough reason for you to give up on the wave picture, consider this. Albert Einstein proved that light comes in particle form. His experiment, which was published in 1905, was called the ‘photoelectric effect’, and its physics lies behind the workings of solar power. It had been known for some time that light hitting the surface of a metal could release electrons from the metal. No one understood, however, why the flow of released electrons seemed to increase as the frequency of the light moved towards the high-frequency, ultraviolet end of the spectrum of light. Common sense – as dictated by Maxwell’s electromagnetic theory – said that the current should increase with the intensity of the light, not its frequency.
Einstein solved the puzzle with the notion of a photon: a packet of energy that was the quantum particle of light. In
Einstein’s prediction, the number of electrons released from the metal would depend on the energy of the photon – which is proportional to the frequency of the light ‘wave’. Only photons with a certain minimum of energy would be able to free an electron. Photons hitting the metal with energy above that threshold would not only free an electron, but would impart their extra energy to it. Experiments that measured the kinetic energy of emitted electrons showed this to be the case, and Einstein was awarded the 1921 Nobel Prize in physics.
It is perhaps unfortunate that the greatest physicist of the 20th century, the creator of general and special relativity, should get his Nobel Prize for the discovery of the photon. Despite the Nobel Prize and despite Feynman’s insistence, the idea of particles of light remains one of the shakiest concepts in physics. It is easy to think of photons as particles in the same way that we think of electrons or protons as particles. But photons are much less particle-like than that. They don’t have mass, for instance.
The physicist Willis Lamb, who made many important discoveries during a stellar career at the famous Bell Labs, went so far as to claim the word ‘photon’ should be banned from physics. At the very best, he said, the word should be licensed – with Lamb giving licenses out only in cases where he felt there was a real need to move away from the wave-like picture of light.
If we can’t quite put our finger on the fundamentals of the nature of light, there is still a lot we can say about the primacy of light’s role in our descriptions of the universe. Most important of all is the fact that light is the fastest thing in the cosmos. There was a time when physicists believed light travelled infinitely fast, the
light from distant stars or planets appearing instantaneously at our eyes whenever we looked up at the heavens. By the end of the 17th century this idea had died, as experiments showed that a finite speed of light could explain anomalies such as the irregular orbit of Io, Jupiter’s innermost moon. The idea that nothing could travel faster than light, though, seemed at first like it was plucked from nowhere.
This notion arose from considerations of the Maxwell equations that describe electromagnetic phenomena. We know that the motion of an electric charge – electricity – causes a magnetic field to appear in its vicinity. That same magnetic field generates electricity as it grows. And so the cycle repeats. Maxwell found that this resulted in something that moved with the undulating intensity of a wave, and that he could calculate the speed at which it moved forward. It was a well-known value, the same speed that astronomers had found (by measuring the timings of eclipses and the orbits of planets and moons) to be the speed of light. Therefore, Maxwell reasoned, light must be an electromagnetic phenomenon.
That was all very well until physicists came to appreciate that not all electromagnetic phenomena were well behaved. Analyse the radiation being emitted by something moving relative to you, and you’ll find that it doesn’t obey Maxwell’s equations exactly. Einstein fixed this with a radical step. Presuming that the laws of physics should be the same however anyone is moving, he made a new law: that the speed of light is always constant – and nothing can move faster than this.
When a car’s headlamps move past you, the light they are emitting is not speeded up by the motion of the car. What’s more, if the car then slows down, the light does not slow down. It is always travelling at just under 300 million metres per second. This counter-intuitive notion, which lies at the core of Einstein’s special theory of relativity, has enormously strange consequences (see
What is Time?
). But it has been shown to be true in countless
experiments. And, according to Einstein’s relativity, the closer you get to the speed of light, the harder it gets to accelerate further.
This effect restores order to the universe, allowing Maxwell’s equations to describe any situation, no matter how much relative motion there is between the emitter and detector of the radiation. Bringing things right up to date, we now have a quantum version of Maxwell’s work, known as quantum electrodynamics, or QED, and this describes light’s behaviour perfectly. Whatever light is, the realization that it travels at a constant speed irrespective of circumstance has enabled us to map the past, present and future of the cosmos.
When we see light coming from a distant star, we know that it has travelled through time as well as space. Our view of the sun is always as it was eight minutes ago; light from other stars gives a view much deeper into the past. What’s more, by seeing stars in their various stages of development, while knowing how far away they are, we can tell what will happen during a star’s lifetime – information that we can use to predict what will happen in the future. The path our sun will follow, for example, is now well understood: we have about 5 billion years before it begins to die, a process that will see it bloat into a ‘red giant’ and engulf most of the planets – including Earth.
The other great application that has arisen from our deeper grasp of light is the technology that perhaps best defines the 20th century: the laser. In an age of CD players, supermarket checkout scanners, high speed optical phone cables and corrective eye surgery, it is hard to believe the inventors of the laser didn’t know what it might be used for.
Laser is an acronym, standing for Light Amplification by the Stimulated Emission of Radiation. The light from a standard bulb, or even the sun, comes from atoms that release light on an individual basis. The principle behind the laser is to pump energy into a gas of atoms and then release it in a controlled way: the
pulses are ‘coherent’, which means they are locked together to give an intense, powerful beam.
YOU CAN TRAVEL FASTER THAN LIGHT
The idea of moving at light-speed took a strange turn when, in 1999, Lene Hau slowed light down to the speed of a passing bicycle. This is possible through the use of two lasers: one to ‘prepare’ some sodium atoms, and one to provide a pulse of light for ‘slowing’. The energy of the preparation beam is tuned to a value where it pushes the sodium atoms into a state where they cannot absorb the ‘slowing light pulse’. That means the pulse, which would normally be absorbed, travels through the cloud of atoms.
As it travels, though, it gives some of its energy to the sodium atoms. These atoms hold the energy for a moment, then release it into the travelling pulse again. The result is rather like plucking the front carriages off a train, and reattaching them at the back. While the carriages are moving at a normal speed, the progress of the train as a whole is impaired.
So, although light’s speed is reduced, it’s really a sleight of hand. Two years later, Hau went a step further and stunned the world by stopping light in its tracks. To do this, the quantum state of the atoms is altered even further, to the point where they hold the energy for as long as is required. By nudging the atoms out of that state, Hau could release the light.
This can be achieved by priming the atoms with a jolt of energy that kicks one of their electrons into a high energy state. A second jolt knocks that electron down again. It emits a photon of light as it falls, kicking off a chain reaction. Each photon kicks another one out of its higher energy state, stimulating the emission of more photons from other atoms in the gas, which in turn stimulates more emission. The result is a laser beam.
It is easiest to explain the mechanism behind lasers using the photon approach, but the power of the beam is best suited to a wave description. We know that waves, such as water waves, can
add up if they meet ‘in phase’, that is, with their peaks coinciding. The result is an enormously powerful wave, which is, essentially, what laser light is like.
It is not just the power of lasers that makes them so useful. The fact that the light is so tightly controlled, with the photons locked together, makes them a great scientific tool, used in myriad ways from finding the distance to the moon to probing the secrets of the atom. The spin-off applications, such as scanning barcodes, reading information from CDs and making the modern telecommunications industry possible, are just the icing on the laser cake.