ONE snowy day last year, I trekked out of Vienna, Austria, winding my way to one of the city’s graveyards to visit the final resting place of a giant of 19th-century physics. Ludwig Boltzmann’s tombstone features an imposing bust of the man, frowning down with a severe expression. And there above him, in gold lettering, is his formula for entropy. It must be one of only a few tombstones in the world adorned with an equation.
I had come for a spot of contemplation because I think Boltzmann’s century-old ideas could help solve one of the trickiest problems in physics right now: how quantum particles, which exist in a fuzzy cloud of possible states, give rise to the solid, well-defined world of snow, leaves, tombstones and everything else around us.
There have been many attempts to explain this over the years, including the outlandish idea that the other quantum possibilities play out in many other parallel universes or that they simply vanish. But my colleagues and I suspect the answer could lie with Boltzmann.
The theory he worked, called thermodynamics, is centred on entropy, a measure of how disordered things are. It explains how things cool down, power up and, crucially for our purposes, mix. It covers everyday goings-on, like milk mixing into coffee. But if we are right, its powers also extend to the quantum realm. We think those quantum possibilities are never lost at all: instead, they are just mixed so thoroughly into the cracks of reality that we can’t see them. It this is…
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