Wave packet
In quantum mechanics, for particles in a region of space where no forces act, Schrodinger's Equation admits solutions known as wave packets. By virture not being constrained to boundary conditions, the energy of these particles is not quantized.
In the image, the direction of motion is along the green strip. The vertical shows the real part of Psi(x,y).
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See an animation of two interfering electron wave-packets.
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