Puzzling out the mass-superselection rule

Understand

Mass-superselection rule (MSR) states that in the non-relativistic quantum theory superpositions of states with different masses are unphysical.

• While MSR features even in textbooks, its validity, physical content and consequences remain debated.
• Its original derivation is known to be inconsistent, while a consistent approach does not yield a superselection rule.
• Yet, superpositions of masses do not seem to be present in Newtonian physics.