Quantum physics is our basic theory of how particles and the forces that act on them work. It is the foundation of the hugely successful standard model of particle physics – the most exhaustively tested theory ever. But quantum theory is also notoriously inscrutable: to get it to work, you must first assume some very basic, rather counter-intuitive things about the way nature works on its smallest scales. Complementing our grand tour of the universe in Reality guide: The essential laws of cosmology, here we explore the laws of the quantum world.
LAW 4: Quantisation
Things come in bite-size chunks
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The origin of quantum theory was, quite literally, a light-bulb moment. In 1900, Max Planck was trying to describe mathematically the energy output of light bulbs, and so make better ones. Existing theories failed to match reality. After a few false starts, Planck found he could bridge the gap by making a radical assumption: the electromagnetic energy given out by a radiating body was emitted not continuously, but in indivisible packets.
Planck initially thought these “quanta” were a limitation of the theory, not a description of reality. But in 1905, Einstein showed that the way some metals expel electrons when light shines on them – the photoelectric effect – could also be explained by assuming that light is made of discrete particle-like quanta, which he called photons. This was just the beginning. As quantum theory developed, it became clear that not just energy, but many other properties such as electric charge and spin, come in units of a minimum size. Why that should be, no one knows.
LAW 5: Uncertainty
There’s a limit to how much any of us can know
If you kick a football, knowing where it is doesn’t stop you knowing where it’s going. Not so with a subatomic particle. The more precisely you know its position, the less precisely you know its momentum, and vice versa.
This is the quantum uncertainty principle, devised by the Werner Heisenberg in the mid-1920s. It connects not just position and momentum, but energy and time and a whole host of other pairs of quantities. Uncertainty doesn’t come from the accuracy of our measuring devices: it is apparently a fundamental limit on how much we can know about the world.
Uncertainty shapes our world in unsuspected ways. It allows particles to “tunnel” through otherwise insurmountable energy barriers to initiate nuclear fusion in the sun, for example. It also enables them to pop up out of a seemingly empty vacuum for short periods – an ability that’s crucial for explaining how the quantum forces that shape reality operate.
Reality guide: How everything fits together
The six principles that rule the universe… and the six big problems we still can’t crack
LAW 6: Wave-particle duality
Quantum objects exist in many different guises at once
The discovery in the early 20th century that light comes divided into discrete, localised chunks – particles – created a puzzle. Light also interferes with itself, diffracts and otherwise acts as if it is a non-localised wave.
In 1924, Louis de Broglie proposed that this behaviour was universal and worked both ways: if wave-like light can act like a particle, electrons and other matter particles can also act like waves.
In this dual wave-particle picture, a quantum object exists in a wave-like “superposition” of all its possible positions or states, only settling in one state on measurement. Erwin Schrödinger lampooned this idea in his thought experiment about a cat that is simultaneously alive and dead. But experiments since have made single particles as large as buckyballs – molecules made of 60 carbon atoms – diffract and interfere at two slits as if they were a wave, and superposition is one basis of the much-touted, enhanced information-processing power of future quantum computers.
QUANTUM MECHANICS
If general relativity is the theory of the universe at large, on small scales quantum mechanics calls the shots. Derived from the principles of quantisation, uncertainty and wave-particle duality, it is a peerless predictor of the workings of subatomic particles – although the principles behind it often remain counter-intuitive and mysterious. Like general relativity, quantum mechanics is just a framework. Before it can be applied to the workings of real particles that often move at close to the speed of light, it must be married in some way with special relativity.
Entanglement
Einstein didn’t think much of this consequence of quantum mechanics when he proposed it with two other physicists in 1935. The states of two once-correlated quantum particles can remain correlated, even when they are separated by a long distance. Interfering with one appears to have a spontaneous effect on the other. Einstein dismissed this as “spooky action as a distance”, insisting that some unseen physical influence must regulate entanglement. This means quantum mechanics must be incomplete.
Many experiments have since shown no trace of that physical influence. If it does exist, it must travel at least 10,000 times faster than light, in flagrant defiance of Einstein’s own special relativity. The current distance record for entanglement stands at 143 kilometres between photons on two different Canary Islands – with plans to extend tests into outer space.
Quantum field theories
Special relativity and quantum mechanics get hitched
Mass and energy are interchangeable, so says special relativity. Particles can pop out of nowhere, says quantum theory. Quantum field theory marries those ideas to depict all particles as “excitations” that arise in underlying fields. The British physicist Paul Dirac started the ball rolling in the late 1920s with his equation describing how relativistic electrons – and with it most other matter particles – behave.
The Dirac equation had a sting in its tail: it predicted the existence of a particle identical to the electron in every way, apart from the opposite electric charge. The positron, the first antimatter particle, was duly discovered in cosmic rays a few years later. It was the first of a whole new menagerie of particles that theorists proposed as quantum field theories evolved – and that later popped up in reality.
THE STANDARD MODEL OF PARTICLE PHYSICS
The product of many decades of theoretical work, meticulously confirmed by experiment, the standard model of particle physics covers the workings of three of the four forces of nature. It describes the interactions of force-carrying boson particles with matter-making fermions, and two quantum field theories lie at its heart. Quantum electrodynamics (QED) is the unified “electroweak” theory of electromagnetism and the weak nuclear force. Quantum chromodynamics (QCD) is the theory of the strong nuclear force. The crowning glory of the standard model came in 2012, with the discovery of the Higgs boson, predicted almost five decades earlier.
Electromagnetic force
Transmitted by massless photons of light, this is the most familiar of the quantum forces, governing all electric and magnetic phenomena.
Weak force
The weak force governs nuclear processes such as radioactive beta decays that are crucial, for example, in how the sun burns its fuel. It is transmitted by massive particles known as W and Z bosons.
Strong force
Transmitted by bosons called gluons, this strong, very short-range force binds quarks together to make particles such as protons and neutrons.
The Higgs boson
Mass is the most solid property of matter, and the mass of a fundamental particle is determined by its degree of interaction with the Higgs boson. According to a theory first proposed in 1964, the molasses-like field associated with the Higgs provides a drag that varies according to particle type.
The discovery of the Higgs boson means the standard model is complete – yet also strangely incomplete. To find out how, see Reality guide: Six problems physics can’t explain
