Quantum mechanics (QM) is a set of principles describing the physical reality at the atomic level of matter (molecules and atoms) and the subatomic (electrons, protons, and even smaller particles). These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation ("wave–particle duality"). Quantum Mechanics is a mathematical description of reality, like any scientific model. Some of its predictions and implications go against our "common sense" of how humans see a set of bodies (a system) behave. This isn't necessarily a failure of QM - it's more likely to be a reflection of how we as humans are used to describing things at the scale of metres and days rather than much smaller. QM says that the most complete description we can make of a system is its wavefunction, which is just a number varying between time and place. We can derive things from the wavefunction, such as the position of a particle, or its momentum. Yet the wavefunction describes probabilities, and it turns out that some physical quantities which we usually assume are both fully defined together simultaneously for a system are not simultaneously given definite values in QM. It is not that the experimental equipment we're using to measure is not precise enough - the two quantities in question just really aren't defined at the same time by the Universe. For instance, location and velocity just do not exist simultaneously for a body (this is called the Heisenberg uncertainty principle — see its formula in the box to the right).

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