19 Oct ‘Quantum tunnelling’ (JIM AL-KHALILI & JOHNJOE MCFADDEN)
Take a straw poll today among scientists asking them what they think is the most successful, far-reaching and important theory in the whole of science and the answer will likely depend on whether you are asking someone working in the physical or the life sciences. Most biologists regard Darwin’s theory of evolution by natural selection as the most profound idea ever conceived. However, a physicist is likely to argue that quantum mechanics should have pride of place – after all, it is the foundation on which much of physics and chemistry are built and gives us a remarkably complete picture of the building blocks of the entire universe. Indeed, without its explanatory power, much of our current understanding of how the world works disappears.
Almost everyone will have heard of ‘quantum mechanics’, and the idea that this is a baffling and difficult area of science understood only by a tiny, very smart minority of humans is very much part of popular culture. Yet the truth is that quantum mechanics has been part of all our lives since the early twentieth century. The science was developed as a mathematical theory in the mid-1920s to account for the world of the very small (the microworld, as it’s called), which is to say the behaviour of the atoms that make up everything we see around us and the properties of the even tinier particles that make up those atoms. For example, in describing the rules obeyed by electrons and how they arrange themselves within atoms, quantum mechanics underpins the whole of chemistry, material science and even electronics. Despite its strangeness, its mathematical rules lie at the very heart of most of the technological advances of the past half-century. Without quantum mechanics’ explanation of how electrons move through materials, we would not have understood the behaviour of the semiconductors that are the foundation of modern electronics, and without an understanding of semiconductors we would not have developed the silicon transistor and, later, the microchip and the modern computer. The list goes on: without the advances in our knowledge thanks to quantum mechanics there would be no lasers and so no CD, DVD or blu-ray players; without quantum mechanics we would not have smartphones, satellite navigation or MRI scanners.
In fact, it has been estimated that over one-third of the gross domestic product of the developed world depends on applications that would simply not exist without our understanding of the mechanics of the quantum world. And this is just the beginning. We can look forward to a quantum future – in all likelihood within our own lifetimes – in which near-limitless electric power may become available from laser-driven nuclear fusion; when artificial molecular machines will be carrying out a vast array of tasks in the fields of engineering, biochemistry and medicine; when quantum computers will be providing artificial intelligence; and when potentially even the sci-fi technology of teleportation will be routinely used to transmit information. The twentieth century’s quantum revolution is picking up pace in the twenty-first century and will transform our lives in unimaginable ways.
But what exactly is quantum mechanics? This is a question we will be exploring throughout this book; for a taster, we will start here with a few examples of the hidden quantum reality that underpins our lives.
Our first example illustrates one of the strange features of the quantum world, arguably its defining feature: wave–particle duality. We are familiar with the fact that we and all the things around us are composed of lots of tiny, discrete particles such as atoms, electrons, protons and neutrons. You may also be aware that energy, such as light or sound, comes as waves, rather than particles. Waves are spread out, rather than particulate; and they flow through space as – well, waves, with peaks and troughs like the waves of the sea. Quantum mechanics was born when it was discovered in the early years of the twentieth century that subatomic particles can behave like waves; and light waves can behave like particles.
Although wave–particle duality is not something you need to consider every day, it is the basis of lots of very important machines, such as the electron microscopes that allow doctors and scientists to see, identify and study tiny objects too small to show up under traditional optical microscopes, such as the viruses that cause AIDS or the common cold. The electron microscope was inspired by the discovery that electrons have wave-like properties. The German scientists Max Knoll and Ernst Ruska realized that, since the wavelength (the distance between successive peaks or troughs of any wave) associated with electrons was much shorter than the wavelength of visible light, a microscope based on electron imaging should be able to pick out much finer detail than an optical microscope. This is because any tiny object or detail that has dimensions smaller than the wave falling on it will not influence or affect the wave. Think of ocean waves with wavelengths of several metres washing up against pebbles on the beach. You would not be able to learn anything about the shape or size of an individual pebble by studying the waves. You would need much shorter wavelengths, such as those produced in a ripple tank, of the type everyone encounters in school science lessons, to ‘see’ a pebble by the way that waves bounce off it or diffract around it. So, in 1931, Knoll and Ruska built the world’s first electron microscope and used it to take the first ever pictures of viruses, for which Ernst Ruska was awarded the Nobel Prize, perhaps rather belatedly, in 1986 (two years before he died).
Our second example is even more fundamental. Why does the sun shine? Most people are probably aware that the sun is essentially a nuclear fusion reactor that burns hydrogen gas to release the heat and sunlight that sustain all life on earth; but fewer people know that it wouldn’t shine at all were it not for a remarkable quantum property that allows particles to ‘walk through walls’. The sun, and indeed all stars in the universe, is able to emit these vast amounts of energy because nuclei of hydrogen atoms, each composed of just a single positively charged particle called a proton, are able to fuse, and as a result to release energy in the form of the electromagnetic radiation that we call sunlight. Two hydrogen nuclei have to be able to get very close in order to fuse; but the closer they get, the stronger the repulsive force between them becomes, as each carries a positive electric charge and ‘like’ charges repel. In fact, for them to get close enough to fuse, the particles have to be able to get through the subatomic equivalent of a brick wall: an apparently impenetrable energy barrier. Classical physics – built upon Isaac Newton’s laws of motion, mechanics and gravity, which describe very well the everyday world of balls, springs, steam engines (and even planets) – would predict that this shouldn’t happen; particles should not be able to pass through walls and therefore the sun shouldn’t shine.
But particles that obey the rules of quantum mechanics, such as atomic nuclei, have a neat trick up their sleeve: they can easily pass through such barriers via a process called ‘quantum tunnelling’. And it is essentially their wave–particle duality that enables them to do this. Just as waves can flow around objects, like the pebbles on the seashore, they can also flow through objects, like the sound waves that pass through your walls when you hear your neighbour’s TV. Of course, the air that carries sound waves doesn’t actually pass through the walls itself: it’s the vibrations in the air – sound – that cause your common wall to vibrate and push on the air in your room to transmit the same sound waves to your ear. But if you could behave like an atomic nucleus then you would sometimes be able to pass, ghost-like, straight through a solid wall. A hydrogen nucleus in the interior of the sun manages to do precisely this: it can spread itself out and ‘leak’ through the energy barrier like a phantom, to get close enough to its partner on the other side of the wall to fuse. So when you are next sunning yourself on the beach, watching the waves lapping on the seashore, spare a thought for the spooky wave-like motions of quantum particles that not only allow you to enjoy the sunshine but make all life on our planet possible.
Life on the Edge: The Coming of Age of Quantum Biology
Jim Al-Khalili & Johnjoe McFadden