Much of the physics world, and millions in funding, is spent on the ceaseless hunt for a theory of everything which unites general relativity and quantum mechanics. Quantum gravity – a description of gravity that obeys the principles of quantum mechanics – is held up as the holy grail that will lead our discovery, however Isaac Layton argues this search is misguided.
In 1905, whilst working in a patent office, Albert Einstein published four ground-breaking papers, setting the stage for half a century of rapid progress in theoretical physics.
Two of the papers laid the basis for what is now known as general relativity. Completed a decade later in 1915, general relativity provided a new understanding of gravity, explaining the apparent force we feel from massive objects in terms of the bending of space and time. To this day, general relativity provides our best theory of gravity, and has been extensively tested, most recently with the detections of gravitational waves.
Although general relativity provided a radical conceptual shift from the earlier work of Newton, it is still what physicists would call a classical theory. Classical theories are those which build on the basic intuitions that we all have about the natural world: that objects have definite positions in space, and that these don’t change when we look at them.
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On the other hand, Einstein’s first paper of 1905 helped push scientific progress in a completely different direction – to that of quantum theories.
In contrast to the classical theories, quantum theories throw away the notion that everything has a definite state of existence. The most famous example of this is the now infamous Schrodinger’s cat, which stays in a state of simultaneously being alive and dead inside its box, until the box is opened and the state of the cat is measured.
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This collapse is random: when Schrodinger’s cat is measured, 50% of the time it instantly goes to the state of being alive, while 50% of the time it becomes dead.
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The strangest property of quantum theories compared to classical theories is that every measurement must be accounted for in the theory explicitly – like if the weather forecast for tomorrow depended on whether or not you looked up at the sky today. In quantum theory this appears as the so-called “measurement postulate”, which says that every time a measurement is made of something quantum, the quantum system instantly “collapses” into a definite state of being. This collapse is random: when Schrodinger’s cat is measured, 50% of the time it instantly goes to the state of being alive, while 50% of the time it becomes dead.
Replacing the classical notions of determinism and objectivity, quantum mechanics provided a broad new framework with which to understand the world. The development led to a new rush to understand reality through a quantum lens, and "upgrade" existing classical theories to quantum ones. Technically known to physicists as “quantisation”, the quest to upgrade the classical theory of electromagnetism to a quantum one took the efforts of many of the best physicists of the century, and was ultimately successful. The resulting theory of quantum electrodynamics provided a new viewpoint on electromagnetism; while it may look regular and classical on the surface, the underlying theory is fundamentally unexpected, with subatomic particles continually popping in and out of existence. This theory was subsequently followed by the successful unification of electromagnetism with the strong and weak forces into a single combined quantum theory.
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