Incredible Stuff: Hidden variables of reality

Returning to Einstein's nagging doubts about quantum mechanics, Nobel laureate Gerard 't Hooft of Utrecht University has begun to outline a way in which its apparent play of chance might be underpinned by precise physical laws that describe the way the world works.

Despite being physicists' most fundamental theory of the properties of matter and energy, quantum mechanics holds that there are things we just cannot know. For example, it forbids us from knowing everything about a subatomic particle: its exact speed, position, mass and energy. We can only put limits on the probable values of all these things at any instant.

Albert Einstein did not like this idea, and suspected that another theory - another layer of reality - might underlie quantum mechanics, in which everything is spelled out precisely. These deeper properties of objects became known as 'hidden variables'. According to this view, our ignorance about the nature of a quantum object is illusory; we just haven't found the right theory to describe it yet.

Today, most physicists adhere to a different reading of quantum theory, called the Copenhagen Interpretation, as advocated by the Danish nuclear physicist of the 1940s, Niels Bohr. This says that there is no deeper reality, that hidden variables don't exist and that the world is simply probabilistic. It holds that we are not ignorant about quantum objects, it's just that there is nothing further to be known.

Indeed, in the 1980s, the Copenhagen Interpretation was put to an experimental test based on a theorem devised by the Irish physicist John Bell - and it stood up. Hidden variables had to go.

't Hooft is not about to resurrect hidden variables. But neither is he convinced that quantum uncertainty has to be the final word. "Contrary to common belief," he says, "it is not difficult to construct deterministic models where quantum mechanics correctly describes stochastic behaviour, in precise accordance with the Copenhagen doctrine."

Here, stochastic means that things seem governed by fuzzy, rather than precise, probabilities. And deterministic means that one thing leads definitely to another, not simply to a range of other things with various probabilities.

"The key, " says 't Hooft, "is information loss. At the smallest conceivable size scale - the Planck Scale, many trillions of times smaller than the nucleus of an atom - there exists complete information about the world. This information gets lost very quickly. By the time we start trying to probe and measure a system, we are like archaeologists trying to make sense of ancient Babylonia: we have only the scantiest of information to go on. We can say only what the system was probably like."

This might sound like sleight of hand to introduce quantum uncertainty, but 't Hooft has outlined a way to turn it into a predictive mathematical theory. But because the Planck Scale is so far below the resolution limit of any conceivable experiment using current technology, it will be very difficult to put his ideas to the test. Richard Gill, who is also based at Universty of Utrecht suspects that the real answer may turn out to be eternally elusive.