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

Schrdinger’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

RooM the Agency / Alamy Stock Photo

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 (How to think about… THE BIG BANG).

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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 Howard Wiseman 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 …