**A Little History Of Science**

**The Game-Changer: Einstein**

Albert Einstein (1879–1955) is famous for his shock of white hair and his theories about matter, energy, space and time. And the equation E = mc2. His ideas might be frighteningly hard to under- stand, but they changed the way we think about the universe. He was once asked what his laboratory looked like. By way of answer, he whipped out his fountain pen from his pocket. This was because Einstein was a thinker, not a doer. He worked at a desk or chalkboard rather than the laboratory bench.

Still, he needed the kind of information that could be gained by experiment, and in particular he would come to rely upon the work of the German physicist Max Planck (1858–1947). Planck was a thinker and an experimenter. He was about forty years old when he made his most fundamental discovery, at the University of Berlin. In the 1890s he started working on light bulbs, to see how he could produce a bulb that gave out the maximum light but used the least electricity. In his experiments he was using the idea of a ‘black body’, a hypothetical object that absorbs all the light shone onto it, reflecting none back. Think how hot you get wearing a black T-shirt in the sunshine, and how much cooler it is to wear a white one: the black clothing has absorbed the energy from the sunlight. So, the energy that comes with the light is absorbed by the black body. But it cannot simply store all this energy, so how does the black body give it back out again?

Planck knew that the amount of energy absorbed depends on the particular wavelength (the frequency) of the light. He took his very careful measurements of the energy and wavelength and put them into the mathematical equation E = hv. The energy (E) is equal to the frequency of the wavelength (v) multiplied by a fixed number (a ‘constant’ – h). In this equation the energy output Planck measured was always a whole number, not a fraction. This was important because being a fixed number meant that the energy came in individual little packets. He called each of these little packets a ‘quantum’, which just meant a quantity. He published his work in 1900, introducing the idea of the quantum to the new century. Physics, and how we understand our world, have never been quite the same since. The fixed number (h) was called ‘Planck’s constant’ in his honour. His equation would prove just as important as Einstein’s more famous E = mc2.

It took some physicists a while to appreciate the real significance of Planck’s experiments. Einstein was one who saw what it meant straight away. In 1905, he was working in the Zurich Patent Office as a clerk, and doing physics in his spare time. That year, he published three papers that made his name. The first, which won him a Nobel Prize in 1921, took Planck’s work to a new level.

Einstein thought more about Planck’s black body radiation, and drew on the still-new quantum approach. After much thought, he showed – by some brilliant calculations – that the light was indeed being transmitted in small packages of energy. These packets moved independently of each other even though together they made up a wave. This was a startling claim, for physicists since Thomas Young a century before had analysed light in many experimental situations as if it were a continuous wave. It certainly generally behaved that way, and here was a young, still obscure, worker in a patent office saying that light could be a particle – a photon, or quantum of light.

Einstein’s next paper from 1905 was equally revolutionary. This was where he introduced his Special Theory of Relativity, which showed that all movement is relative, that is, it can only be measured in relation to something else. It is a very complicated theory, but can be explained quite simply if you use your imagination. (Einstein was a great one for deep thinking about known data and exploring, in his mind, what would happen if . . .?) Imagine a train is moving out of a station. In the middle of one of the carriages there is a light bulb flashing on and off, sending out a flash at exactly the same time forwards and backwards, which is reflected in a mirror at each end of the carriage. If you were standing exactly in the middle of the carriage you would see the light bounce back from both mirrors at exactly the same time. But someone standing on the platform as the train went past would see the flashes one after the other. Although both flashes are still hitting the mirrors simultaneously, the train is moving forwards, so on the platform you would see the flash from the furthest-away mirror (at the front of the carriage) before you saw the flash from the closer mirror (at the back). So, although the speed of light remains the same, when it is seen is different depending on – or rather, relative to – whether the observer is moving or still. Einstein argued (with the help of some complicated equations, of course) that time is a fundamental dimension of reality. From now on physicists would need to think not simply of the three familiar dimensions of space – length, width and height – but of time, too.

Einstein showed that the speed of light is constant, no matter whether it is moving away from or towards us. (The speed of sound is different, which is why a train sounds different depending on whether we hear it approaching or moving away.) So the relativity in the Special Theory of Relativity doesn’t apply to this constant speed of light. Instead the relativity occurs in the observers and in the fact that time needs to be included. Time is not absolute but relative. It changes the faster we travel and so do the clocks that record it for us.

There is an old story about an astronaut travelling near the speed of light and coming back to earth to find that time has moved on. Everyone she knew has grown old and died. She is not much older that when she left, but as her clock has slowed down she is not aware of how long she has been away. (This is just a thought experiment and could happen only in science fiction.) As if that wasn’t enough, Einstein’s famous equation E = mc2 brought together mass (m) and energy (E) in a new way. The c is the velocity of light. In effect, he showed that mass and energy were two aspects of matter. Since the velocity of light is a very big number, and, when squared, an even bigger one, this means that only a very small amount of mass, if completely converted to energy, would be a lot of energy. Even atomic bombs convert only a tiny fraction of the mass to energy. If the mass in your body were totally converted into energy, it would have the force of fifteen large hydrogen bombs. Don’t try the experiment, however.

Over the next few years, Einstein extended his thinking, and in 1916 he came up with a more general framework for the universe. This was his General Theory of Relativity. It introduced his ideas on the relationship between gravity and acceleration, and the structure of space. He showed that gravity and acceleration were actually equivalent. Imagine you are standing in a lift, and you drop an apple from your hand: it will fall to the floor of the lift.

Now, if you let go of the apple at exactly the same moment that someone cuts the cord of the lift, you will fall along with the apple. It won’t actually move, relative to you, as you both fall together. At any time, you can simply reach out and take hold of the apple. It will never reach the floor so long as the lift (and you) continue to fall. This is of course what happens in space, where there is no gravity. Astronauts and their spacecraft are essentially in free fall.

Einstein’s General Theory of Relativity demonstrated that space, or rather space-time, is curved. It made predictions about several puzzling things that physicists had had difficulty in explaining. It suggested that light would be slightly bent when it passed near a large body. This was because light (made up of photons) has mass, and the larger body would exert a gravitational pull on the smaller mass of light. Measurements during an eclipse of the sun showed that this actually happens. Einstein’s theory also explained curious features of the orbit of Mars around the sun, which Newton’s less complex laws of gravity could not do.

Einstein had worked with the very small (the tiny photons of light) and the very large (the universe itself). He offered a compel- ling new way of bringing them together. In this he contributed to quantum theory as well as introducing his own ideas of relativity.

These ideas, and the mathematics behind them, helped define the way physicists thought about both the large and the small. But Einstein did not approve of many of the new directions that physics was taking. He never lost his belief that the universe (with its atoms, electrons and other particles) is locked in a system of cause and effect. He famously said, ‘God does not play at dice.’ He meant that things always happen in regular, predictable patterns.

Not everyone agreed, and other physicists who took on Planck’s quantum ideas came to different conclusions. The electron was central to much of the other early quantum work. Chapter 30 explained Niels Bohr’s model of the quantum atom, in 1913. He had the electrons in fixed orbits with definite energies whizzing around the central nucleus. A lot of work was done trying to explain these relationships mathematically. Ordinary maths didn’t work. To solve this problem, physicists turned to matrix mathematics. In ordinary maths, 2 × 3 is the same as 3 × 2.

In matrix maths, this isn’t so, and these special tools allowed an Austrian physicist, Erwin Schrödinger (1887–1961), to develop new equations in 1926. His wave equations described the behaviour of the electrons in the outer orbits of the atom. This was the beginning of quantum mechanics. It did for the very small what Newton had done for the very large. Like many of the physicists who changed the way we think about the world in the early twentieth century, Schrödinger had to flee the Nazis, and spent the war years in Dublin. Einstein, as we know, went to United States.

Schrödinger’s wave equations brought some order into the picture. Then Werner Heisenberg (1901–76) came up with the ‘uncertainty principle’ in 1927. The principle was part philosophy, part experiment. Heisenberg said that the very act of experimenting with electrons changes them. This places limits on what we can know. We could know an electron’s momentum (its mass multiplied by its speed), or its position, but not both. Measuring one affected the other. Einstein (among others) was appalled by this idea, and set about disproving Heisenberg’s uncertainty principle. He couldn’t. Einstein admitted defeat. So far the principle remains intact: there are simply limits to our knowledge of the very small. The electron was also crucial to Paul Dirac (1902–84). This complex Englishman was considered to be almost another Einstein.

His book on quantum mechanics led the field for three decades. His own equations about the quantum activities of atoms and sub-atomic particles were little short of brilliant. The trouble was, his equations required a strange particle – a positively charged electron – in order to work. This was like saying that there was both matter and anti- matter. The whole idea of ‘anti-matter’ was bizarre, since matter is the solid stuff of the universe. Within a few years, the search for such a particle was successful and the positron was discovered. This twin to the electron had a single positive charge. It combined with an electron, produced a burst of energy and then both particles disappeared. Matter and anti-matter could annihilate each other in less than the blink of an eye.

The positron showed physicists that atoms were composed of more than protons, electrons and neutrons. We look at some of these profound discoveries later, after physicists produced ever- higher energies to examine their atoms and particles. ‘Examine’ is not quite the right word. When working with high energy, physicists cannot actually see directly what is going on in their experiments. What they see instead are spots on a computer screen, or changes in the magnetism or energy of their experimental set-up. But atomic bombs, atomic energy and even the possibility of quantum computing all testify to nature’s power and mystery – even if we cannot see it.

Max Planck’s packet, or quantum, of energy, and Albert Einstein’s realisation that mass and energy are merely two aspects of the same thing: these discoveries changed forever the way that the universe could be understood. Mass and energy; wave and particle; time and space: nature has revealed herself to be ‘both . . . and . . .’ , not ‘either . . . or . . .’ . And while all this helped explain the structure of atoms and the creation of the universe, it also helps you get home at night. Satellites are so far above the earth that Satnav must include special relativity. If it weren’t factored in, you could soon get lost.