The Vacuum in Quantum Mechanics

In quantum mechanics, there is no such thing as complete stillness. The Heisenberg Uncertainty Principle tells us that if we try to localize a particle at the very bottom of the pendulum, then the uncertainty in its momentum and its velocity needs to be large. So you can already see that this notion of stillness, of zero energy, of like absolutely nothing is happening, doesn't quite make sense. There is some minimum amount of energy that it is possible to reduce the energy of a quantum system down to, and the number is not zero.