One of the most puzzling aspects of quantum physics is how thoroughly it defies our intuition. If you take any stable quantum particle, like an electron, you'll find that it shares a certain set of properties in common with all the particles that are like it. Every electron, for example, has:

  • the same mass, of 511 keV/c2,

  • the same electric charge, of -1.6 × 10-19 C,

  • the same quantum spin, of ±ℏ/2,

along with other intrinsic properties like electron magnetic moment, its adherence to the Pauli exclusion principle, and is the matter counterpart of the antiparticle known as a positron. These properties are completely certain, even in a quantum Universe, unlike quantities like position and momentum, or spin in multiple different directions, where measuring one to a certain accuracy means you know the other one less accurately.

But not all particles are like the electron. For some of them, even their mass is inescapably uncertain.

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