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HOME > PAST ISSUE > July-August 1999 > Article Detail

MARGINALIA

Pulse, Pump & Probe

Roald Hoffmann

Useful Uncertainty

Heisenberg's uncertainty principle places an inherent limitation on the simultaneous determination of certain pairs of physical observables—momentum and position, energy and time. Might the principle constrain our "visualization" of the location in space of, say, moving atoms? Actually, it doesn't; the localization in time allowed by coherent, ultrashort laser pulses indeed leads to a substantial indeterminacy in the energy. This in turn is related to an inability to define well the momentum of the particles in motion, which actually leads to a good coherent localization in space. I oversimplify a complex and interesting quantum-mechanical situation. A language of matter wave packets is necessary, but in that language we can actually think of atoms, which are very much quantum objects, moving as roughly classical particles.




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