In the quantum world, uncertainty reigns – or is it all in the mind? | 91av

Physics

In the quantum world, uncertainty reigns – or is it all in the mind?

Schrödinger's dead-and-alive cat embodies the uncertainty of the quantum world. But whether parallel realities truly exist is a question less of science than belief

By Daniel Cossins

11 December 2019

cat

HEISENBERG is speeding down the autobahn when a police officer pulls him over. “Do you know how fast you were going? 150 kilometres an hour!” the cop tells him. “Great,” Heisenberg replies, “now I’m completely lost.”

The joke is a pretty fair summary of Werner Heisenberg’s notorious quantum uncertainty principle. This says that certain pairs of properties in the quantum world can never be known simultaneously with perfect accuracy – the speed (or momentum) of something and its position, say. Measure one precisely and you know nothing about the other.

The uncertainty principle is more than a curiosity. Among other things, it allows a latitude in particles’ states and positions that explains the radioactivity that ultimately powers the sun and the fluctuations that gave rise to all matter in the early universe (“Big bang retold: The weird twists in the story of the universe’s birth”).

What it isn’t, however, is a principle: a fundamental truth on which other theories can be based. “It is actually a consequence of something more fundamental, and that something is quantum mechanics,” says at Griffith University in Brisbane, Australia.

Quantum mechanics neatly explains how atoms and particles work. But it isn’t like the classical mechanics that governs our macroscopic world, where one thing definitely leads to another and objects have set properties. Quantum objects are described by probabilistic mathematical entities known as wave functions that only give you the odds on what you might find when you make a measurement. At this point, a definite answer for a particle’s position or momentum, for instance, “collapses” out from its wave function. This odd behaviour explains how Schrödinger’s quantum cat can be simultaneously dead and alive before you measure its state – at least according to the mathematics.

Whether that means quantum cats are really dead and alive is a matter of taste, and what philosophical interpretation of quantum mechanics you subscribe to. The “many worlds” interpretation, for instance, says all feasible quantum states are objectively real: the possibilities encoded in the wave function do exist, just in different universes that split off every time a measurement is made.

The Copenhagen interpretation – often called the “shut up and calculate” approach – says on the other hand that uncertainty is inherent, and you simply can’t know anything about quantum realities before you measure them. “We can be agnostic about the quantum world independent of our interactions with it,” says Wiseman.

Quantum Bayesianism, or QBism, takes things further: wave function collapse is down to observers updating their beliefs about what is behind the quantum curtain. As such, quantum theory isn’t about objective reality, just our subjective estimation of it. “I would say QBism views quantum theory as much more deeply indeterministic than any other interpretation,” says at the University of Massachusetts, an originator of this variant.

“In quantum theory, the only certain thing is uncertainty”

If all that leaves you scratching your head, you aren’t alone. Feel free to come up with your own interpretation: when it comes to quantum mechanics, the only thing that is certain is uncertainty itself.

Cutting-edge science throws up all sorts of controversial, nebulous and mind-bending concepts. Here’s your guide to how to think about some of the fiddliest of them:

Topics: Quantum mechanics

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