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Last week I posted this question which was massively downvoted. The downvotes appear to be largely motivated by an observation I made which was not essential to the question, but which seems to have branded me as a quack in the eyes of the downvoters. Here is what I said:

It is common to talk about an atom absorbing a photon, transitioning to a higher energy state, and then re-emitting the photon. But none of this is directly observable. All you can really do is shine light on some atoms and measure the scattered radiation. You never actually observe "an atom/molecule in an excited state".

I still think this is correct. How would you possibly observe a hydrogen atom in a 2p state? You can infer that it must have been in that state when you detect a photon, but that's a little indirect isn't it?

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  • $\begingroup$ physics.aps.org/articles/v8/23 $\endgroup$ – nasu Sep 13 '16 at 13:19
  • $\begingroup$ The point is that you can't observe an excited state because you can't observe a ground state either. You have to shine some light on a ground state so that it transitions to an excited state too, and then you measure that photon's frequency and infer what you have to infer. I thus believe this question is actually deeper and follows a different direction: can you observe states? $\endgroup$ – QuantumBrick Sep 13 '16 at 13:21
  • $\begingroup$ And in the comments you were pointed to papers that you said didn't apply to your original question, yet are directly on topic for this one. Did you actually look at them? So, tell me how creating and manipulating Rydberg states is not observing an atom in an excited state. "Ramsey Interference in Strongly Driven Rydberg Systems", Phys. Rev. Lett 71(16) 2575-2578 (1993). $\endgroup$ – Jon Custer Sep 13 '16 at 13:26
  • $\begingroup$ The study of excited states is common in chemistry. Ultra short laser pulses prepare the state, followed a short time later by ultra short probes. $\endgroup$ – garyp Sep 13 '16 at 13:50
  • $\begingroup$ It is not exactly helpful to say that you observe an excited state by using "an ultra short probe". $\endgroup$ – Marty Green Sep 13 '16 at 15:47
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I still think this is correct. How would you possibly observe a hydrogen atom in a 2p state? You can infer that it must have been in that state when you detect a photon, but that's a little indirect isn't it?

Even in most classical physics situations, one uses the mathematical model to infer the state of what is under study, because it is the mathematics that defines the state. Before Newton and the mathematics of gravitational forces, what were people observing when throwing a stone? Parabolas? The parabola is a solution of the mathematical model which fits the data perfectly in everyday observations, until one needs special and general relativity.

2p states are a solution of a mathematical model which fits the data perfectly and the model is predictive of new data. Thus one infers it was a 2p state if the energy fits a 2p state of the mathematical model. Without the model, it is just light of some frequency and polarization. It is the atomic mathematical model that gives the structure, labels and organizes observations.

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  • $\begingroup$ So in other words I was right all along. $\endgroup$ – Marty Green Sep 13 '16 at 14:31

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