The Quantum Universe_ Everything That Can Happen Does Happen - Brian Cox [79]
Figure 9.7. An ‘OR’ gate built using water pipes and two valves (left) or a pair of transistors (right).
Figure 9.7 illustrates a different logic gate. This time water will flow out of the bottom if either valve is open and only if both are closed will it not flow. This is called an ‘OR’ gate and, using the same notation as before, ‘1 OR 1 = 1’, ‘1 OR 0 = 1’, ‘0 OR 1 = 1’ and ‘0 OR 0 = 0’. The corresponding transistor circuit is also illustrated in the figure and now a current will flow in all cases except when both transistors are switched off.
Logic gates like these are the secret behind the power of digital electronic devices. Starting from these modest building blocks one can assemble combinations of logic gates in order to implement arbitrarily sophisticated algorithms. We can imagine specifying a list of inputs into some logical circuits (a series of ‘0’s and ‘1’s), sending these inputs through some sophisticated configuration of transistors that spits out a list of outputs (again a series of ‘0’s and ‘1’s). In that way we can build circuits to perform complicated mathematical calculations, or to make decisions based on which keys are pressed on a keyboard, and feed that information to a unit which then displays the corresponding characters on a screen, or to trigger an alarm if an intruder breaks into a house, or to send a stream of text characters down a fibre optic cable (encoded as a series of binary digits) to the other side of the world, or … in fact, anything you can think of, because virtually every electrical device you possess is crammed full of transistors.
The potential is limitless, and we have already exploited the transistor to change the world enormously. It is probably not overstating things to say that the transistor is the most important invention of the last 100 years – the modern world is built on and shaped by semiconductor technologies. On a practical level, these technologies have saved millions of lives – we might point in particular to the applications of computing devices in hospitals, the benefits of rapid, reliable and global communication systems and the uses of computers in scientific research and in controlling complex industrial processes.
William B. Shockley, John Bardeen and Walter H. Brattain were awarded the Nobel Prize in Physics in 1956 ‘for their researches on semiconductors and their discovery of the transistor effect’. There has probably never been a Nobel Prize awarded for work that directly touches so many people’s lives.
10. Interaction
In the opening chapters we set up the framework to explain how tiny particles move around. They hop around, exploring the vastness of space without any prejudice, metaphorically carrying their tiny clocks with them as they go. When we add together the multitude of clocks corresponding to the different ways that a particle can arrive at some particular point in space, we obtain one definitive clock whose size informs us of the chance of finding the particle ‘there’. From this wild, anarchic display of quantum leaping emerges the more familiar properties of everyday objects. In a sense, every electron, every proton and every neutron in your body is constantly exploring the Universe at large, and only when the sum total of all those explorations is computed do we arrive at a world in which the atoms in your body, fortunately, tend to stay in a reasonably stable arrangement – at least for a century or so. What we have not yet addressed in any detail is the nature of the interactions between particles. We have managed to make a lot of progress without being specific about how particles actually talk to each other, in particular by exploiting the concept of a potential. But what is a potential? If the world is made up solely of particles, then surely we should be able to replace the vague notion that particles move ‘in the potential’ created by other particles, and speak instead about how the particles move and interact with each other.