Owen Maroney worries that physicists have spent the better part of a century engaging in fraud.
Ever since they invented quantum theory in the early 1900s, explains Maroney, who is himself a physicist at the University of Oxford, UK, they have been talking about how strange it is — how it allows particles and atoms to move in many directions at once, for example, or to spin clockwise and anticlockwise simultaneously. But talk is not proof, says Maroney. “If we tell the public that quantum theory is weird, we better go out and test that's actually true,” he says. “Otherwise we're not doing science, we're just explaining some funny squiggles on a blackboard.”
It is this sentiment that has led Maroney and others to develop a new series of experiments to uncover the nature of the wavefunction — the mysterious entity that lies at the heart of quantum weirdness. On paper, the wavefunction is simply a mathematical object that physicists denote with the Greek letter psi (Ψ) — one of Maroney's funny squiggles — and use to describe a particle's quantum behaviour. Depending on the experiment, the wavefunction allows them to calculate the probability of observing an electron at any particular location, or the chances that its spin is oriented up or down. But the mathematics shed no light on what a wavefunction truly is. Is it a physical thing? Or just a calculating tool for handling an observer's ignorance about the world?
The tests being used to work that out are extremely subtle, and have yet to produce a definitive answer. But researchers are optimistic that a resolution is close. If so, they will finally be able to answer questions that have lingered for decades. Can a particle really be in many places at the same time? Is the Universe continually dividing itself into parallel worlds, each with an alternative version of ourselves? Is there such a thing as an objective reality at all?
“These are the kinds of questions that everybody has asked at some point,” says Alessandro Fedrizzi, a physicist at the University of Queensland in Brisbane, Australia. “What is it that is really real?”
Debates over the nature of reality go back to physicists' realization in the early days of quantum theory that particles and waves are two sides of the same coin. A classic example is the double-slit experiment, in which individual electrons are fired at a barrier with two openings: the electron seems to pass through both slits in exactly the same way that a light wave does, creating a banded interference pattern on the other side (see 'Wave–particle weirdness'). In 1926, the Austrian physicist Erwin Schrödinger invented the wavefunction to describe such behaviour, and devised an equation that allowed physicists to calculate it in any given situation1. But neither he nor anyone else could say anything about the wavefunction's nature.
Ignorance is bliss
From a practical perspective, its nature does not matter. The textbook Copenhagen interpretation of quantum theory, developed in the 1920s mainly by physicists Niels Bohr and Werner Heisenberg, treats the wavefunction as nothing more than a tool for predicting the results of observations, and cautions physicists not to concern themselves with what reality looks like underneath. “You can't blame most physicists for following this 'shut up and calculate' ethos because it has led to tremendous developments in nuclear physics, atomic physics, solid-state physics and particle physics,” says Jean Bricmont, a statistical physicist at the Catholic University of Louvain in Belgium. “So people say, let's not worry about the big questions.”
But some physicists worried anyway. By the 1930s, Albert Einstein had rejected the Copenhagen interpretation — not least because it allowed two particles to entangle their wavefunctions, producing a situation in which measurements on one could instantaneously determine the state of the other even if the particles were separated by vast distances. Rather than accept such “spooky action at a distance”, Einstein preferred to believe that the particles' wavefunctions were incomplete. Perhaps, he suggested, the particles have some kind of 'hidden variables' that determine the outcome of the measurement, but that quantum theories do not capture.
Experiments since then have shown that this spooky action at a distance is quite real, which rules out the particular version of hidden variables that Einstein advocated. But that has not stopped other physicists from coming up with interpretations of their own. These interpretations fall into two broad camps. There are those that agree with Einstein that the wavefunction represents our ignorance — what philosophers call psi-epistemic models. And there are those that view the wavefunction as a real entity — psi-ontic models.
To appreciate the difference, consider a thought experiment that Schrödinger described in a 1935 letter to Einstein. Imagine that a cat is enclosed in a steel box. And imagine that the box also contains a sample of radioactive material that has a 50% probability of emitting a decay product in one hour, along with an apparatus that will poison the cat if it detects such a decay. Because radioactive decay is a quantum event, wrote Schrödinger, the rules of quantum theory state that, at the end of the hour, the wavefunction for the box's interior must be an equal mixture of live cat and dead cat.
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Quantum physics: What is really real? : Nature News & Comment
