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The integration of quantum mechanics with spacetime, treating quantum phenomena as occurring within the four-dimensional fabric of relativity. In spacetime quantum mechanics, particles are not point-like objects moving through time but four-dimensional worldlines with quantum properties—they exist in superpositions across spacetime, entangle across distances without signal, and pop in and out of existence in ways that respect relativistic causality. This framework is the foundation of quantum field theory, where particles are excitations of fields that permeate spacetime, and where the vacuum itself is alive with virtual particles. Spacetime quantum mechanics explains why empty space isn't really empty, why particles can appear from nowhere (briefly), and why the universe at its smallest scales is a frothing, probabilistic mess.
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