Quantum physics describes the behaviour of the world on very small scales, the scale of atoms and molecules and below. The feature of the quantum world that gives quantum physics its name is that at this level physical physical processes are discontinuous and occur in quantum leaps. But these discontinuous jumps are very small because Plank’s constant is very small, so the enormous number of quantum leaps going on at this sub-microscopic level add up to give us the illusion of a world in which change is smooth and continuous.
There are other peculiar features of the quantum world. It works in accordance with the laws of chance, so that a quantum entity selects from the options available to it at random- this discovery was much to the disgust of Albert Einstein, who made this famous comment that he could not believe that ‘God plays dice with the universe’, but the randomness of the quantum world has been borne out in countless experiments. (quantum world is also affected by non-locality. Two quantum entities that have once interacted with one another (or been part of the same system, such as two photons ejected simultaneously from an atom) remain somehow entangled with one another for ever, and ‘aware’ of each other’s state and any change in the partner’s state . And there is nothing in the equations of quantum physics to distinguish the past from the future and indicate an arrow of time in the Universe – but this last feature it shares with classical mechanics.
Although quantum physics may be weird, it works. It isn’t just a rather abstract and abstruse collection of theories and hypotheses that physicists can play with to keep themselves amused, but a highly successful, practical package which underpins many aspects of modern society that we take for granted. Here are a few examples.
One of the most common examples of quantum physics at work in a practical application in the home is the laser. Many people own a CD player, which works by scanning the disc on which information stored (words, music, pictures or whatever) with a laser beam. Like its cousin, the maser, the laser operates on fundamental quantum principles, involving the stimulated emission of radiation from atoms. Albert Einstein investigating spontaneous emission and laying the statistical ground rules for quantum theory in 1916 (the same statistical rules that he was later to find so abhorrent) was the first person to appreciate that an excited atom might be triggered into releasing a quantum of energy (a photon) and failing back into its ground state by receiving a nudge from another photon with the same wavelength as the one the atom is primed to release. Rather like the cascade of neutrons that is involved in a chain reaction, the laser process produces a cascade of photons from an array of excited atoms.
There are many variations on the theme, but they all use the same principle. Incoherent energy (radiation in which all the photons are jumbled up, so that the wave from one photon is partially cancelled and partially reinforced by the waves from the photons next door) is used to excite the atoms, and the laser (or maser; the same thing at longer wavelengths) mechanism releases that energy in a coherent beam in which all of the photons march in step, so that the energy from each wave reinforces the energy from all the other waves in constructive interference. Some lasers provide the ultimate ‘straight edge’ used in surveying work; others produce short-lived, powerful pulses of energy that can be used to drill holes in hard objects. Laser cutting tools are used applications as diverse as microsurgery and bulk cutting of cloth in the garment industry. And you also see lasers at work in the supermarket, reading the bar codes on the products. It is all quantum physics.
Nuclear energy, in the form of both nuclear weapons and power generation, is also a practical application of quantum physics. Whether we are dealing with fission or fusion, quantum processes such as tunnelling are Crucially important, along with (in the case, for example, of fission-powered electricity-generating stations) the statistics of how nuclei decay, either spontaneously or under the influence of an impact from outside.
Then there are computers. Leaving aside the exotic possibility of developing true quantum computers, all modern computers, from the chip in your washing machine to a home PC to the biggest electronic superbrain in the world, depend on the properties of semiconductors, in which the properties of the conduction electrons (as in all conductors) depend on Fermi-Dirac statistics. A promising possibility for future generations of computers is to make use of another quantum phenomenon, superconductivity.
And what about life? Quantum chemistry is the branch of quantum physics that deals with interactions between atoms and molecules, and that is what life is all about. It is no surprise that in the middle of the 20th century many of the great advances in the understanding of the life molecules, DNA, and the genetic code were made by scientists who had training in physics. Erwin Schrodinger wrote an influential book called What is life?; Francis Crick, who ( with James Watson) discovered the structure of DNA, was a physicist who turned to molecular biology largely under the influence of that book; Linus Pauling was a quantum chemist who also turned to molecular biology; and George Gamow, who made many contributions to quantum physics, also played a part in the cracking of the genetic code. Such later developments as genetic engineering and cloning have also been achieved because of the understanding of how DNA works that has been achieved directly through the application of quantum physics to biology. But I know of no biologist who switched to quantum physics and made a mark in the field. This is not just because biology is easier than physics (although, according to biologist John Maynard Smith, who trained as an aeronautical engineer, it is), but because quantum physics is more fundamental than biology. Indeed, quantum physics is as fundamental as science gets.
But all of this, it is worth emphasizing, has been achieved through Quantum cookery. Computer technology, lasers nuclear power, genetic engineering and more besides use quantum physics, to be sure; but they use it in recipe-book fashion, without any need to worry about what it all means. The average quantum mechanic is no more worried about philosophical implications and the interpretations of quantum physics than the average car driver worries about what is going on inside the engineer of the car. As long as it works, their attitude seems to be, why worry about how it works? But if you do want to know what is going on inside the engine, take a look at the entry on the double-slit experiment and follow your nose from there.


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