The uncertainty principle proposed by Werner Heisenberg states that the more precisely one property of a particle is measured, like position, the less precisely another related property, like momentum, becomes known. Debate continues over whether this means measurement actively changes the measured property or if particles do not have precisely defined values. This debate is important for emerging fields like quantum computing and cryptography that aim to use the uncertainty principle for security by detecting any disruption from eavesdropping.

1. Eighty-six years after Werner Heisenberg first described his eponymous uncertainty principle,
experts are still arguing over what, exactly, the infamous inequality really means. Briefly, of
course, the principle says that the product of the uncertainties of position and momentum will
always be greater than a constant —though a very, very tiny constant (*see note below). The
more tightly you tie one factor down, the more the other swings.
But does it mean that the act of measuring position changes the momentum, and vice-versa—the
observer effect? Or does it mean that the particle simply doesn’t have precisely defined
momentum and position to measure?
The uncertainty principal became a matter of urgent practical debate with the first glimmerings
of quantum computing. In particular, some schemes for quantum encryption derive much of their
promised security from the uncertainty principle’s assurance that any attempt at eavesdropping
will disrupt the information and betray the interloper’s presence. As Ron Cowen pointed out
recently in Nature, “Quantifying by how much a measuring device can disturb the properties of a
quantum system is crucial to the burgeoning field of quantum cryptography and computing.”