The case of a single charged particle with potential energy is considered when the classical electromagnetic field is zero. The vector and scalar potentials describing this situation are not necessarily zero. When the conventional approach to radiation problems using the interaction Ap plus pA plus A2 plus A0 (constants omitted) is used, time‐dependent probabilities are predicted if the vector potential A=−∇Λ≠0 even though the electromagnetic field is zero. The reason for this unphysical behavior is that eigenfunctions of the unperturbedHamiltonian, which is not gauge invariant, are used to calculate the probability amplitudes. When a manifestly gauge‐invariant procedure is applied to this case the probabilities are constant in all gauges. The reason that physical results are obtained is that eigenstates of the energyoperator are used to calculate the probability amplitudes.

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