Once in every while I have to delve into the esoteric world of quantum mechanics. So, you will have to forgive me.
Since it was formalized in the mid-1920s QM has been extremely successful at describing the behavior of systems at the atomic scale. Two giants of the field — Niels Bohr and Werner Heisenberg — devised the intricate mathematics behind QM in 1927. Since then it has become known as the Copenhagen Interpretation, and has been widely and accurately used to predict and describe the workings of elementary particles and forces between them.
Yet recent theoretical stirrings in the field threaten to turn this widely held and accepted framework on its head. The Copenhagen Interpretation holds that particles do not have definitive locations until they are observed. Rather, their positions and movements are defined by a wave function that describes a spectrum of probabilities, but no certainties.
Rather understandably, this probabilistic description of our microscopic world tends to unnerve those who seek a more solid view of what we actually observe. Enter Bohmian mechanics, or more correctly, the De Broglie–Bohm theory of quantum mechanics. An increasing number of present day researchers and theorists are revisiting this theory, which may yet hold some promise.
Of the many counterintuitive features of quantum mechanics, perhaps the most challenging to our notions of common sense is that particles do not have locations until they are observed. This is exactly what the standard view of quantum mechanics, often called the Copenhagen interpretation, asks us to believe.
But there’s another view—one that’s been around for almost a century—in which particles really do have precise positions at all times. This alternative view, known as pilot-wave theory or Bohmian mechanics, never became as popular as the Copenhagen view, in part because Bohmian mechanics implies that the world must be strange in other ways. In particular, a 1992 study claimed to crystalize certain bizarre consequences of Bohmian mechanics and in doing so deal it a fatal conceptual blow. The authors of that paper concluded that a particle following the laws of Bohmian mechanics would end up taking a trajectory that was so unphysical—even by the warped standards of quantum theory—that they described it as “surreal.”
Nearly a quarter-century later, a group of scientists has carried out an experiment in a Toronto laboratory that aims to test this idea. And if their results, first reported earlier this year, hold up to scrutiny, the Bohmian view of quantum mechanics—less fuzzy but in some ways more strange than the traditional view—may be poised for a comeback.
As with the Copenhagen view, there’s a wave function governed by the Schrödinger equation. In addition, every particle has an actual, definite location, even when it’s not being observed. Changes in the positions of the particles are given by another equation, known as the “pilot wave” equation (or “guiding equation”). The theory is fully deterministic; if you know the initial state of a system, and you’ve got the wave function, you can calculate where each particle will end up.
That may sound like a throwback to classical mechanics, but there’s a crucial difference. Classical mechanics is purely “local”—stuff can affect other stuff only if it is adjacent to it (or via the influence of some kind of field, like an electric field, which can send impulses no faster than the speed of light). Quantum mechanics, in contrast, is inherently nonlocal. The best-known example of a nonlocal effect—one that Einstein himself considered, back in the 1930s—is when a pair of particles are connected in such a way that a measurement of one particle appears to affect the state of another, distant particle. The idea was ridiculed by Einstein as “spooky action at a distance.” But hundreds of experiments, beginning in the 1980s, have confirmed that this spooky action is a very real characteristic of our universe.
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Image: Schrödinger’s time-dependent equation. Courtesy: Wikipedia.