Since 1927, when Werner Heisenberg formulated the uncertainty principle, it has stood as one of the cornerstones of quantum mechanics. In a simplified form, the uncertainty principle states that it is impossible to measure anything without disturbing it. For example, any attempt to measure a particle’s position must randomly change its speed.

For clarity’s sake, one should be aware that the uncertainty principle is not the same as the observer effect, which states that the act of observing a phenomenon will change the phenomenon itself. The uncertainty principle is more about how precisely something can be measured in two dimensions such as position and momentum, simultaneously. In common parlance, the two theories are often conflated.

This principle has driven quantum physicists crazy for nearly a century. That is, it drove them crazy until recently, when researchers at the University of Toronto demonstrated the ability to directly measure the disturbance and confirm that Heisenberg was too pessimistic.

“We designed an apparatus to measure a property – the polarization – of a single photon. We then needed to measure how much that apparatus disturbed that photon,” says Lee Rozema, a Ph.D. candidate in Professor Aephraim Steinberg’s quantum optics research group at U of T.