Bubble chamber tracks from Fermilab, revealing the charge, mass, energy and momentum of the particles created. If a newly created particle is not stable to arbitrary lifetimes, it will have an inherent uncertainty to its mass. (FNAL/NSF/DOE)

In A Quantum Universe, Even Mass Is Uncertain

In the quantum world of the unstable, even identical particles don’t have identical masses.

In the microscopic world of the quantum particle, there are certain rules that are wholly unfamiliar to us on a macroscopic scale. If you measure a particle’s position and ask “where are you,” the more accurately you learn the answer, you’ll fundamentally know its motion, or its momentum, less well. Other properties, however, like electric charge, remain perfectly well-known at all times, regardless of what else you measure. For purely stable particles, whether elementary or composite (including electrons and protons), mass is one of those perfectly-known properties. If you know the mass of one electron under one set of conditions, you know it for all electrons everywhere in the Universe. But this isn’t the case for all the particles we know of. The shorter-lived an unstable particle is, the more uncertain its mass is. This isn’t just a hypothesized effect, but rather one that’s been experimentally observed and verified for decades.

The quantum nature of the Universe tells us that certain quantities have an inherent uncertainty built into them, and that pairs of quantities have their uncertainties related to one another.(NASA/CXC/M.Weiss)

From a theoretical standpoint, quantum uncertainty ought to play a role wherever two…