# What happens if you try to apply Maxwell's Equations to this quantum mechanical system?

In another post, we discussed the oscillating charge in a hydrogen atom and the weight of opinion seemed to be that there is indeed an oscillating charge when you consider the superposition of the 1s and 2p states. One of the correspondents (freecharly) went a little farther and said that Schroedinger believed this oscillating charge to be the source of radiation. I wonder if the actual calculation bears this out? Specifically, in the case of the hydrogen atom in this particular superposition, do you get the correct decay times for the superposition of states if you apply Maxwell's equations to the oscillating charge and assume that as the system loses energy by radiation, the "probability" flows from the 2p to the 1s state in accordance with the amount of energy remaining in the system?

EDIT: Some people are objecting in different ways to the basic premise of the question, so let me make it a little more specific: I am not asking if hydrogen atoms ACTUALLY EXIST in a particular superposition of these states. (I may ask that in another question.) What I am asking here is IF you take (just to be specific) a 50-50 superposition of the 1s and 2p states, and apply Maxwell's equations to the oscillating charge, AND you assume that as the atom radiates the probability drains from the excited state to the ground state in such a way as to maintain conservation of energy...IF you do all those things, do you get a result that is consistent with standard QM?

• You've got another problem - if you just initialize on the $2p$ state, which should give you the strongest emission, there's no charge oscillation and therefore no radiation. (Similarly, if you initialize in, say, $\sqrt{0.01}|1s⟩+\sqrt{0.99}|2p⟩$, the oscillation amplitude will be very small, and it will take a long while to get up to speed.) That said, this question would improve a fair bit if you specify the mechanism you have in mind for "as the system loses energy the probability flows from the 2p to the 2s state". Are you just assuming Schrödinger dynamics? Or something else? Nov 19, 2016 at 17:56
• The moot point Marty is that the hydrogen atom isn't really in a superposition of 1s and 2p states, it's in neither state. It's akin to a 1s state moving back and forth with 10.2ev worth of energy, but it isn't actually a 1s state. In similar vein it isn't actually a 2p state either. Nov 19, 2016 at 18:28
• You are posing a question ("I wonder if...") without any effort to research an answer. For someone of such high rep, that is a bad example to newcomers. Moreover, this is not a discussion site. Nov 19, 2016 at 18:30
• I am not concerned about the example I set for newcomers. Nov 19, 2016 at 20:26
• @freecharly I am glad you find my question interesting. I had hoped my intention was clear that the calculation ought to be carried out by assuming the ACTUAL energy of the superposition should be taken as |c1|2*E1 + |c2|2 *E2, with c1 growing at the expense of c2 as the radiation carries energy away from the system. I think it's a fairly straightforward calculation and it's odd to me that, assuming you are knowledgeable in this territory, you aren't aware of anyone having published a calculation of this type. Nov 20, 2016 at 18:00

I suspect the results would be correct (at least, approximately), as Barut developed his "self-field electrodynamics" (see, e.g. http://phys.lsu.edu/~jdowling/publications/Barut89b.pdf) and claimed results very close to those of QED. In self-field electrodynamics, radiation is created by charge density related to the wave function in a standard way (for the Dirac field).

I am disappointed that no one in this discussion group has been able to post a definitive answer as to whether the semi-classical calculation, applying Maxwell's equations to the quantum-mechanically oscillating charge, gives the correct result for the emission of radiation from an excited hydrogen atom. I appreciate akhmeteli's reference to a related publication but it does not directly address this question. So I am going to have to answer this question myself to the best of my ability by demonstrating a "back-of-the-envelope" type calculation.

I said I wanted to consider the 50-50 superposition of the 1s and 2p states. So first we need to know the maximum dipole moment of the superposition. I found the result on this University of Texas website by Prof. Richard Fitpatrick. I think I am interpreting it correctly when I say that the maximum charge displacement is 0.4 angstroms (about 75% of the standard radius of the ground state).

Then we need the frequency of the oscillation. Of course, this is the difference frequency corresponding to the 10.5 eV energy difference of the states, or 1.6 x 10^16 rad/sec.

Now we can calculate the acceleration. The easiest way to do this is to pretend it is uniform circular motion and use w^2*r. I get an acceleration of 10^22/m-sec^2. (Since it is actually harmonic motion and not circular, this will give us an error factor of 2 in the final result.)

Now I just plug this acceleration into the Larmour formula. You can find the Larmour formula anywhere on the internet, but I have simply converted all the physical constants into numerical values, and it comes to

   **Radiated Power  =   6 x 10^-54 a^2**


You can see that when I plug my value for acceleration into this formula I get a total radiate power of 6 x 10^-10 watts. This we divide by 2 to account for the harmonic motion vs circular.

Is this the correct power? We have to convert to "transition time" to find out. The total energy of the excited state is 10.5 (call it 10) eV which comes to 1.6 x 10^-18 Joules. Dividing the energy by the power, we get the lifetime of the excited state as just about 5 nanoseconds. Or maybe I'm wrongt about the energy and I should be taking it as half (because of the superposition) which would then give me a lifetime of 2.5 nanoseconds. Something like that.

This may not be exact but I think it's pretty much in the ballpark.

• Hard to believe how quickly the downvotes come in on this answer. Is there something wrong with my physics? Nov 21, 2016 at 14:36
• You should not forget that the "lifetime", which is also given in the linked website to be 1.6ns for this transition, is actually the time until a large number $N_0$ of excited H atoms has decayed to $N_0/e$. This lifetime, which assumes a sudden quantum jump for the transition, is not the same as the one you are calculating with the emitted energy consideration. Nov 21, 2016 at 18:38
• Actually, it should be possible to experimentally differentiate between these models. If the classical radiation model were correct, then all atoms should decay with the same time. Quantum mechanically some decay much earlier and some much later. You also see this in radioactive decay. While the half-life gives you the decay of a large number of atoms to a half, there are many atoms that decay only after an orders of magnitude longer time. Nov 21, 2016 at 19:08
• This answer shows that you get answers in the ballpark order of magnitude, but that is not as unexpected as you think - both are (relatively) simple theories with few dimensional constants, being used in relatively similar ways, so there isn't all that much room for them to disagree. In any case, you're missing the important part: for the model to be of any use at all, it needs to directly predict an exponential decay, and it's not clear that it can (because the oscillation amplitude of the dipole decreases as the superposition changes). Nov 22, 2016 at 21:24
• I also don't know where your disappointment comes from. The question as posed is interesting but it asks for a fairly substantial calculation, which is barely explored in the literature (and for good reasons), so answering this takes time. Nov 22, 2016 at 21:26