Quantum physics | New Scientist

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What is quantum physics? Put simply, it’s physics that explains how everything works: the best description we have of the nature of the particles that make up matter and the forces with which they interact.

Quantum physics underlies how atoms work, and therefore why chemistry and biology work the way they do. You, me, and the pole – on one level at least, we’re all dancing to quantum air. If you want to explain how electrons move through a computer chip, how photons of light turn into electric current in a solar panel or amplify in a laser, or even just how the sun keeps burning, you will need to use quantum physics. .

The difficulty – and, for physicists, the fun – begins here. For starters, there is no single quantum theory. There’s quantum mechanics, the basic mathematical framework that underlies everything, which was first developed in the 1920s by Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and others. It characterizes simple things such as how the position or momentum of a single particle or a group of a few particles changes over time.

But to understand how things work in the real world, quantum mechanics must be combined with other elements of physics – primarily, Albert Einstein’s special theory of relativity, which explains what happens when things move. very quickly – to create what are called quantum field theories. .

Three different quantum field theories deal with three of the four fundamental forces by which matter interacts: electromagnetism, which explains how atoms hold together; the strong nuclear force, which explains the stability of the nucleus at the heart of the atom; and weak nuclear force, which explains why some atoms undergo radioactive decay.

Over the past five decades or so, these three theories have been brought together in a crumbling coalition known as the “Standard Model” of particle physics. Despite all the impression that this model is lightly held with tape, it is the most accurately tested image of basic material operation that has ever been conceived. Its crowning achievement came in 2012 with the discovery of the Higgs boson, the particle that gives mass to all other fundamental particles, whose existence has been predicted on the basis of quantum field theories as early as 1964.

Conventional quantum field theories work well to describe the results of experiments on high-energy particle crushers such as CERN’s Large Hadron Collider, where the Higgs was discovered, which probes matter at its smallest scales. But if you want to understand how things work in a lot of less esoteric situations – how electrons move or don’t move through solid material and thus make a material a metal, an insulator, or a semiconductor, for example. example – things get even more complex.

The billions upon billions of interactions in these crowded environments require the development of “effective field theories” that obscure some of the gory details. The difficulty of building such theories is the reason why many important questions in solid state physics remain unanswered – for example, why at low temperatures some materials are superconductors that allow current without electrical resistance, and why we cannot operate this trick at room temperature. .

But beneath all of these practical problems lies a huge quantum mystery. At a basic level, quantum physics predicts some very weird things about how matter works that are completely at odds with the way things seem to work in the real world. Quantum particles can behave like particles, located in one place; or they can act like waves, spread all over the space or in several places at once. Their appearance seems to depend on how we choose to measure them, and before measuring them they seem to have no defined properties, which leads us to a fundamental conundrum about the nature of basic reality.

This vagueness leads to apparent paradoxes such as Schrödinger’s cat, in which, thanks to an uncertain quantum process, a cat is left both dead and alive. But that’s not all. Quantum particles also appear to be able to affect each other instantly even when they are far from each other. This truly deceptive phenomenon is known as entanglement or, to use a phrase coined by Einstein (a great critic of quantum theory), “spooky action at a distance”. Such quantum powers are completely foreign to us, but are the basis of emerging technologies such as ultra-secure quantum cryptography and ultra-powerful quantum computing.

But as to what all this means, no one knows. Some people think that we just have to accept that quantum physics explains the material world in terms that we find impossible to reconcile with our experience in the larger “classical” world. Others think there must be a better, more intuitive theory that we haven’t yet discovered.

In all of this there are several elephants in the room. To begin with, there is a fourth fundamental force in nature that quantum theory has not been able to explain until now. Gravity remains the territory of Einstein’s general theory of relativity, a decidedly non-quantum theory that doesn’t even involve particles. The intensive decades-long efforts to put gravity under the quantum umbrella and thus explain all of fundamental physics within a “theory of everything” have come to naught.

Meanwhile, cosmological measurements indicate that over 95% of the universe is made up of dark matter and dark energy, elements for which we currently have no explanation in the Standard Model, and puzzles such as the he extent of the role of quantum physics in the disorderly functioning of life remains unexplained. The world is at some quantum level – but whether quantum physics is the last word on the world remains an open question.

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