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Heisenberg’s uncertainty principle, a foundational tenet of quantum mechanics, is essentially the assertion that when one tries to measure one aspect of a particle precisely, say its position, one necessarily “blurs out” one’s ability to know with any precision its speed– or vice versa.  Indeed, Heisenberg’s original word for the phenomenon translates better as “indeterminacy”–raising the prospect of a physical world whose nature is, beyond some incomplete point, unknowable.

Still, as mysterious as the concept is, it has offered a tantalizingly-concrete prospect:  the “uncrackable” codes of quantum cryptography.  If “listening in” distorts the message, then the eavesdropper is out of luck.

But now, as the BBC reports, researchers at the University of Toronto have raised some serious uncertainty about the Uncertainty Principle itself:

The Heisenberg uncertainty principle is in part an embodiment of the idea that in the quantum world, the mere act of measuring can affect the result.

But the idea had never been put to the test, and a team writing in Physical Review Letters says “weak measurements” prove the rule was never quite right…

This problem with the act of measuring is not confined to the quantum world, explained senior author of the new study, Aephraim Steinberg of the University of Toronto.

“You find a similar thing with all sorts of waves,” he told BBC News. “A more familiar example is sound: if you’ve listened to short clips of audio recordings you realise if they get too short you can’t figure out what sound someone is making, say between a ‘p’ and a ‘b’.

“If I really wanted to say as precisely as possible, ‘when did you make that sound?’, I wouldn’t also be able to ask what sound it was, I’d need to listen to the whole recording.”

The problem with Heisenberg’s theory was that it vastly predated any experimental equipment or approaches that could test it at the quantum level: it had never been proven in the lab.

“Heisenberg had this intiuition about the way things ought to be, but he never really proved anything very strict about the value,” said Prof Steinberg.

“Later on, people came up with the mathematical proof of the exact value.”…

In 2011, they carried out a version of a classic experiment on photons – the smallest indivisible packets of light energy – that plotted out the ways in which they are both wave and particle, something the rules strictly preclude.

This time, they aimed to use so-called weak measurements on pairs of photons, putting into practice an idea first put forward in a 2010 paper in the New Journal of Physics.

Photons can be prepared in pairs which are inextricably tied to one another, in a delicate quantum state called entanglement, and the weak measurement idea is to infer information about them as they pass, before and after carrying out a formal measurement.

What the team found was that the act of measuring did not appreciably “blur out” what could be known about the pairs.

It remains true that there is a fundamental limit of knowability, but it appears that, in this case, just trying to look at nature does not add to that unavoidably hidden world.

Or, as the authors put it: “The quantum world is still full of uncertainty, but at least our attempts to look at it don’t have to add as much uncertainty as we used to think!”…

“There’s actually a lot of technology that relies on quantum uncertainty now, and the main one is quantum cryptography – using quantum systems to convey our information securely – and that mostly boils down to the uncertainty principle.”

A pdf of the University of Toronto group’s paper is here.

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As we reconsider the benefits of entanglement, we might spare a thought for Pieter van Musschenbroek; he died on this date in 1761.  A one-time student of Isaac Newton (who helped transmit Newton’s ideas through Europe), van Musschenbroek was a professor of mathematics, philosophy, astronomy, and medicine. (Those were the days…)  Fascinated by electrostatics, he used what he learned from his father, an accomplished designer and manufacturer of scientific instruments, to build the first capacitor (that’s to say, device that can store an electric charge), the Leyden Jar– named for the city that was home to van Musschenbroek’s university.

Leyden jar construction

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