COVER STORY
Reality? It’s what you make it An audacious new take on quantum theory suggests alternative facts are a fact of nature, says Philip Ball
OES reality exist without us? Albert Einstein appeared to be in no doubt: surely the moon doesn’t vanish when we aren’t looking, he once asked incredulously. He had been provoked by the proposition, from quantum theory, that things only become real when we observe them. But it is not such a daft idea, and even Einstein kept an open mind. “It is basic for physics that one assumes a real world existing independently from any act of perception,” he wrote in a 1955 letter. “But this we do not know.” In the decades since, physicists have found it maddeningly difficult to write the observer out of quantum theory. Now some are contemplating a mind-boggling alternative: that a coherent description of reality, with all its quantum quirks, can arise from nothing more than random subjective experiences. It looks like the “perspective of a madman”, says the author of this bold new theory, because it compels us to abandon any notion of fundamental physical laws. But if it stands up, it would not only resolve some deep puzzles about quantum mechanics, it would turn our deepest preconceptions about reality itself inside out. When it comes to forecasting how the world
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will behave, quantum theory is unsurpassed: its every prediction, no matter how counterintuitive, is borne out by experiment. Electrons, for instance, can sometimes display behaviour characteristic of waves, even though they seem in other circumstances to behave like particles.
Wave of confusion Before observation, such quantum objects are said to be in a superposition of all possible observable outcomes. This doesn’t mean they exist in many states at once, rather that we can only say that all the allowed outcomes of measurement remain possible. This potential is represented in the quantum wave function, a mathematical expression that encodes all outcomes and their relative probabilities. But it isn’t at all obvious what, if anything, the wave function can tell you about the nature of a quantum system before we make a measurement. That act reduces all those possible outcomes to one, dubbed the collapse of the wave function – but no one really knows what that means either. Some researchers think it might be a real physical process, like radioactive decay. Those who subscribe to the
many-worlds interpretation think it is an illusion conjured by a splitting of the universe into each of the possible outcomes. Others still say that there is no point in trying to explain it – and besides, who cares? The maths works, so just shut up and calculate. Whatever the case, wave function collapse seems to hinge on intervention or observation, throwing up some huge problems, not least about the role of consciousness in the whole process. This is the measurement problem, arguably the biggest headache in quantum theory. “It is very hard,” says Kelvin McQueen, a philosopher at Chapman University in California. “More interpretations are being thrown up every day, but all of them have problems.” The most popular is known as the Copenhagen interpretation after the home city of one of quantum theory’s pioneers, Niels Bohr. He argued that quantum mechanics tells us only what we should expect when we make a measurement, not what causes that outcome. The theory can’t tell us what a quantum system is like before we observe it; all we can ever ask of it is the probabilities of different possible outcomes. Such a perspective seems to back you into > 11 November 2017 | NewScientist | 29