|The experiment requires preparing pairs of "entangled" photons, the particles from which light is made|
The Heisenberg uncertainty principle is in part an embodiment of the idea that in the quantum world, the mere act of observing an event changes it.
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.
That could play havoc with "uncrackable codes" of quantum cryptography.
Quantum mechanics has since its very inception raised a great many philosophical and metaphysical debates about the nature of nature itself.
Heisenberg's uncertainty principle, as it came to be known later, started as an assertion that when trying to measure one aspect of a particle precisely, say its position, experimenters would necessarily "blur out" the precision in its speed.
That raised the spectre of a physical world whose nature was, beyond some fundamental level, unknowable.
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."