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... but that doesn't mean that a lowest energy state doesn't exist, just that it's not always zero and not always constant.



Yes ,It is what I meant, but according to classic Physics and common sense "Vacuum" must be alwayes zero and alwayes constant, otherwise it is not vacuum ,not at least in the dayly meaning of this term
Not only , due to the equivalence mass\ energy it has been proved that "vacuum" can "create" massive particles from "nothing" , even though their lifespan is extremely low
This is one of the most amazing discoveries of the modern physics , in my opinion