This Week in Science: Relativity and Quantum Mechanics

From the New York Times: Sons of Atom by Peter Galison

The first quarter of the 20th century produced two theories, Relativity and Quantum Mechanics, that are still changing our universe.

With special relativity, Albert Einstein upended the long-understood meaning of time, space and simultaneity. With general relativity, he swapped Newton’s law of gravity based on force for curved space­time, and cosmology became a science. Just after World War I, relativity made front-page news when astronomers saw the Sun bend starlight. Overnight, Einstein became famous as no physical scientist before or since, his theory the subject of poetry, painting and architecture.

Then, with the development of quantum mechanics in the 1920s, physics got ­really interesting. Quantum physics was a theory so powerful — and so powerfully weird — that nearly a century later, we’re still arguing about how to reconcile it with Einsteinian relativity and debating what it tells us about causality, locality and realism.

Relativity leads to a world far from every day intuition. But relativity was still classical physics: classical in the sense that it was as causal, maybe even more so, as the physics of Newton. The relativist could defend the view that we could refine our local specification of the state of things now — that we could spell out what every last particle was up to — and then predict the future, as accurately as wanted. Back in the Enlightenment, Pierre-Simon de Laplace imagined a machine that could calculate the future. He didn’t know relativity, of course, but you could imagine a Laplace 2.0 (with relativity) that kept his predictive dream alive.

Quantum mechanics shattered that Laplacian vision. From 1925 to 1927, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Max Born and many others made the theory into a toolkit that could be used to calculate how copper conducted electricity, how nuclei fissioned, how transistors worked. Quantum mechanics was easy to use, but hard to understand. For example, two particles that interacted might subsequently fly to opposite sides of the solar system, and still act as if they were dependent. Measuring one near Pluto affected measurements as the other zipped by Mercury. Einstein viewed this inseparability, now known as “entanglement,” as the fatal mark of the incompleteness of quantum mechanics: he sought a successor theory that would be local, realist and therefore complete.

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