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Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help og a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough $n$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity.

What happens to the atom in this case? Briefly speaking, the atom gets ionized with a sudden perturbation. Sudden perturbations are considered within QM as not changing the state (the initial cloud in your case), but as changing Hamiltonians. The electron in QM feels the Coulomb potential changes instantly, but reacts depending on the work done by the force. Suddenly new Hamiltonian acts somehow on the original state and makes it evolve during some (electron reaction) characteristic times.

EDIT: In QFT and in a classical relativistic field theory all potentials are "retarded", to tell the truth.

Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help of a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough $n$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity.

What happens to the atom in this case? Briefly speaking, the atom gets ionized with a sudden perturbation. Sudden perturbations are considered within QM as not changing the state (the initial cloud in your case), but as changing Hamiltonians. The electron in QM feels the Coulomb potential changes instantly, but reacts depending on the work done by the force. Suddenly new Hamiltonian acts somehow on the original state and makes it evolve during some (electron reaction) characteristic times.

EDIT: In QFT and in a classical relativistic field theory all potentials are "retarded", to tell the truth.

Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help og a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough $n$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity.

What happens to the atom in this case? Briefly speaking, the atom gets ionized with a sudden perturbation. Sudden perturbations are considered within QM as not changing the state (the initial cloud in your case), but as changing Hamiltonians. The electron in QM feels the Coulomb potential changes instantly, but reacts depending on the work done by the force. Suddenly new Hamiltonian acts somehow on the original state and makes it evolve during some (electron reaction) characteristic times.

Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help og a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough $n$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity.

What happens to the atom in this case? Briefly speaking, the atom gets ionized with a sudden perturbation. Sudden perturbations are considered within QM as not changing the state (the initial cloud in your case), but as changing Hamiltonians. The electron in QM feels the Coulomb potential changes instantly, but reacts depending on the work done by the force. Suddenly new Hamiltonian acts somehow on the original state and makes it evolve during some (electron reaction) characteristic times.

EDIT: In QFT and in a classical relativistic field theory all potentials are "retarded", to tell the truth.

Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help on a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough $n$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity.

What happens to the electron in this case? Briefly speaking, the atom gets ionized with a sudden perturbation. Sudden perturbations are considered within QM as not changing the state (the initial cloud in your case), but as changing Hamiltonians. The electron in QM feels the Coulomb potential changes instantly, but reacts depending on the work done by the force. Suddenly new Hamiltonian acts somehow on the original state and makes it evolve during some (electron reaction) characteristic times.

Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help og a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough $n$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity.

What happens to the atom in this case? Briefly speaking, the atom gets ionized with a sudden perturbation. Sudden perturbations are considered within QM as not changing the state (the initial cloud in your case), but as changing Hamiltonians. The electron in QM feels the Coulomb potential changes instantly, but reacts depending on the work done by the force. Suddenly new Hamiltonian acts somehow on the original state and makes it evolve during some (electron reaction) characteristic times.