## In A Quantum Universe, Even Mass Is Uncertain

“It’s one of the most remarkable and counterintuitive results of the quantum Universe, that every unstable particle that you make has an inherent uncertainty to the most seemingly fundamental property of all: mass. You can know what the average mass of a typical particle of any particular type, and you can measure its width, which is directly related to its mean lifetime through the Heisenberg uncertainty principle. But every time you create one new particle, there’s no way to know what its actual mass will be; all you can do is calculate the probabilities of having a varieties of masses. In order to know for sure, all you can do is measure what comes out and reconstruct what actually existed. Quantum uncertainty, first seen for position and momentum, can now be convincingly stated to extend all the way to the rest energy of a fundamental particle. In a quantum Universe, even mass itself isn’t set in stone.”

There are a few properties you can say intrinsically belong to a particle: things like mass, spin, electric charge, and certain other quantum numbers. If your particle is completely stable for all eternity, there’s no reason to question any of this. But if a particle you create, even a fundamental one, has an inherent instability and can decay, all of a sudden Heisenberg comes in to mess everything up! Suddenly, the fact that you have an uncertain lifetime means you have that pesky energy-time uncertainty, and the energy of your particle is intrinsically uncertain, too. Because *E = mc^2*, that means your mass is uncertain, too. And the shorter-lived your particle is, on average, the more uncertain your mass is. This means when you make a top quark, for example, it could have a mass of 165 GeV, 170 GeV, 175 GeV, 180 GeV, or anywhere in between those values. (Including some values outside of that range!)

In a quantum Universe, even mass is uncertain. Here’s the fundamental physics story of how that came to be, both theoretically and experimentally.