forbidden transition

physics

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June 26, 2026

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In physics, a transition refers to the alteration of a system from one state to another. In atomic and particle physics, these transitions are governed by selection rules, which determine their likelihood.123

Allowed transitions have a high probability of occurring and happen quickly, often seen in radioactive decay.14
Forbidden transitions have a minimal or zero probability of occurring. This doesn't mean they are impossible, but rather highly improbable. They may be forbidden by fundamental conservation laws, such as the conservation of angular momentum or electric charge.125

In astronomical spectroscopy, "forbidden lines" are emission lines observed in nebulae that are extremely weak or unobservable in laboratory settings because the required transition takes a very long time.5

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spectroscopy
Atomic transitions
…classified as either allowed or forbidden, depending on the probability of their occurrence. In some instances, as, for example, when both the initial and final states have a total angular momentum equal to zero, there can be no single photon transition between states of any kind. The allowed transitions obey…
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…classified as either allowed or forbidden, depending on the probability of their occurrence. In some instances, as, for example, when both the initial and final states have a total angular momentum equal to zero, there can be no single photon transition between states of any kind. The allowed transitions obey…
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selection rule
…high probability of occurring, or “forbidden transitions,” those that have minimal or no probability of occurring. Selection rules usually are stated as sets of changes in one or more quantum numbers that characterize properties changed by the transition in question.
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…high probability of occurring, or “forbidden transitions,” those that have minimal or no probability of occurring. Selection rules usually are stated as sets of changes in one or more quantum numbers that characterize properties changed by the transition in question.
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transition
Forbidden transitions, on the other hand, are those that have a high probability of not occurring. A strictly forbidden transition is one that cannot occur at all.
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Forbidden transitions, on the other hand, are those that have a high probability of not occurring. A strictly forbidden transition is one that cannot occur at all.
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