An experiment conducted at the University of Toronto has thrown a fundamental aspect of the Heisenberg uncertainty principle into doubt. For decades, we've believed that you can't measure a quantum state without inducing uncertainty — but now, we're not so sure about that.

While this new discovery doesn't overturn the uncertainty principle as a whole, it does indicate that certain aspects of Heisenberg's conjecture may need to be revised.

Heisenberg's uncertainty principle is an integral component of quantum physics. At the quantum scale, standard physics starts to fall apart, replaced by a fuzzy, nebulous set of phenomena. Among all the weirdness observed at this microscopic scale, Heisenberg famously observed that the position and momentum of a particle cannot be simultaneously measured, with any meaningful degree of precision. This led him to posit the uncertainty principle, the declaration that there's only so much we can know about a quantum system, namely a particle's momentum and position.

Now, by definition, the uncertainty principle describes a two-pronged process. First, there's the precision of a measurement that needs to be considered, and second, the degree of uncertainty, or disturbance, that it must create. It's this second aspect that quantum physicists refer to as the "measurement-disturbance relationship," and it's an area that scientists have not sufficiently explored or proven.

Up until this point, quantum physicists have been fairly confident in their ability to both predict and measure the degree of disturbances caused by a measurement. Conventional thinking is that a measurement will always cause a predictable and consistent disturbance — but as the study from Toronto suggests, this is not always the case. Not all measurements, it would seem, will cause the effect predicted by Heisenberg and the tidy equations that have followed his theory. Moreover, the resultant ambiguity is not always caused by the measurement itself.

The researchers, a team led by Lee Rozema and Aephraim Steinberg, experimentally observed a clear-cut violation of Heisenberg's measurement-disturbance relationship. They did this by applying what they called a "weak measurement" to define a quantum system before and after it interacted with their measurement tools — not enough to disturb it, but enough to get a basic sense of a photon's orientation.

Then, by establishing measurement deltas, and then applying stronger, more disruptive measurements, the team was able to determine that they were not disturbing the quantum system to the degree that the uncertainty principle predicted. And in fact, the disturbances were half of what would normally be expected.

But as noted, this quirk in quantum physics does not overturn the basic tenet of the uncertainty principle; we still cannot know a quantum system with precise certainty.

But that said, as Steinberg told *Nature News*, "[T]he experiment shows that the act of measurement isn't always what causes the uncertainty. If there's already a lot of uncertainty in the system, then there doesn't need to be any noise from the measurement at all."

You can check out the entire study at *Physical Review Letters*.

*H/t to Geoff Brumfiel @ Nature.** Image via shutterstock/optimarc. Inset image **via**.*

## DISCUSSION

So does this mean the cat is alive or dead?