by Josh Richardson; Prevent Disease
Does reality exist without us? 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.
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When it comes to forecasting how the world will behave, quantum theory is unsurpassed: its every prediction, no matter how counter-intuitive, 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 an uncomfortable conclusion: that the very act of our observation calls the outcome into being. Can that be true? It seems the antithesis of what science normally assumes, as Einstein intimated. Yet the idea has some pedigree. Hungarian physicist John von Neumann was the first to entertain it in the early 1930s, and his compatriot Eugene Wigner went deepest with a thought experiment in the 1950s now known as Wigner’s friend.
“What if reality can’t be described without invoking our active involvement?”
Suppose that Wigner is standing outside a windowless room where his friend is about to make some measurement on a particle. Once that’s done, she knows what the observed property of the particle is, but Wigner doesn’t. He can’t meaningfully say that the particle’s wave function has collapsed until his friend tells him the result. Worse still, until she does, quantum theory offers no way for Wigner to think about all the unseen events inside the lab as having fixed outcomes. His friend, her measuring apparatus and the particle remain one big composite superposition.
It is as if we live in a solipsistic world where collapse only occurs when knowledge of the result impinges on a conscious mind. “It follows that the quantum description of objects is influenced by impressions entering my consciousness,” Wigner wrote. “Solipsism may be logically consistent with present quantum mechanics.”
John Wheeler at Princeton University put it differently: it’s not solipsism but a kind of interactive collaboration that brings things into being. We live, Wheeler said, in a “participatory universe” — one that can’t be meaningfully described without invoking our active involvement. “Nothing is more astonishing about quantum mechanics,” he wrote, “than its allowing one to consider seriously…that the universe would be nothing without observership.”
But Wheeler could not escape the thicket of irresolvable questions that the participatory universe raises. For one, Wigner and his friend seem locked in an infinite regress. Is Wigner himself in a superposition of states until he passes on the result to his other friends in the next building? Which observer “decides” when wave function collapse occurs? And what constitutes a conscious observation anyway?
Despite the persistence of such questions, some theorists have recently returned to a form of Wheeler’s vision, what Chris Fuchs at the University of Massachusetts in Boston has called “participatory reality”. That shift is partly for want of a better alternative, but primarily it is because if you take quantum mechanics seriously, some element of observer-dependent subjectivity seems impossible to avoid.
A couple of years ago, theorist Caslav Brukner at the University of Vienna revisited the Wigner’s friend scenario in a slightly altered form proposed by David Deutsch at the University of Oxford. Here the friend makes the measurement — she has collapsed the particle’s wave function, producing either outcome A or B — but tells Wigner only that she sees a definite result, not what it is. In Deutsch’s scenario, Wigner is forced to conclude that his friend, her measuring apparatus and the particle are in a joint superposition, even though he knows a measurement has happened.