Talk:Resonance

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Recent changes to lead[edit]

Recently the lead has been changed from

In physics, resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others.

to

In physics, resonance is a phenomenon that consists of a given system being driven by another vibrating system or by external forces to oscillate with greater amplitude at some preferential frequencies.

I think the new wording is not as good, for two reasons: (1) The term resonance is not applied only to driven systems, but is also used for free oscillations excited by initial conditions. (2) The wording gives the misleading impression that resonance is something imposed on one system by another, and fails to communicate the idea that the frequencies of greater oscillation amplitude are an inherent property of the oscillating system. I'm not saying it couldn't be improved, but I felt the original wording was a good, simple introduction which conveyed the idea well to general readers. Anyone else have an opinion? --ChetvornoTALK 19:34, 7 April 2015 (UTC)

But neither this sentence nor the dominant article content reflects that[edit]

"Resonance also occurs in quantum mechanics"

Sure, but does this description apply to quantum systems without misleading? I think it would require an explicit qualification as a "semiclassical heuristic" Furthermore, only one subsection of the article addresses quantum systems, and the rest of it isn't now qualified by the "classical" restriction. Layzeeboi (talk) 18:31, 10 February 2017 (UTC)

But isn't it far more misleading to say that resonance is a phenomenon of classical physics and doesn't occur in quantum mechanics, as your wording does? The difference between classical and quantum mechanical resonance is far too complicated a topic to go into in the introduction, and I don't see any need to mention it. The qualification "semiclassical heuristic", will just confuse general readers. The introduction doesn't go into the differences between mechanical and electromagnetic resonance, either. --ChetvornoTALK 19:14, 10 February 2017 (UTC)
I expect that you may agree that false statements are more serious than possibly misleading statements. I think that the present statement as you have reverted it is false:
In physics, resonance is a phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency.
This sentence purports to apply to all of physics, which is untrue. Consider the figure and last sentence in this article section. Those peaks at low energy in the figure are neutron resonances. There is no modern model of these resonances in which a vibrating something is driving another system. Even in this older book about the theory of such resonances, this concept does not appear.
My proposed change " In classical physics, resonance is…" results in a sentence that is at least correct. You worry that it may seem to imply that the concept of resonance does not appear in other branches of physics. Any such implication would be a fault of the entire lede, similar to the fault that I was trying to mitigate. I think it is necessary to mention (briefly) the different meaning in quantum physics, which dominates much, or maybe even most, of modern physics. For example:
In classical physics, resonance is a phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency.
In quantum physics, resonance refers to an unstable (short-lived) state of a system that is more readily excited by some particular process than other states with nearby energies. 
Layzeeboi (talk) 22:39, 11 February 2017 (UTC)
If the two meanings are not really the same, they should not be defined as the same. If the article covers both, they should both be mentioned at the very beginning—either defined universally for the parent subject area or individually and specifically for each sub-area. Is there some unifying aspect that can be mentioned first, followed by the separate aspects? DMacks (talk) 22:46, 11 February 2017 (UTC)
I agree with the principles that you state. I think attempting to include a unifying aspect could result in less clarity; the application of this term for quantum systems reflects the personal thought processes of some pioneer(s) in this area maybe 80 years ago, who didn't worry about accessibility for a wider audience. Neutron excitation is more analogous to hitting a classical bell with a hammer. Although the hammer itself doesn't vibrate, the bell responds at its resonant frequency, but I doubt that this response to an impulse is what we usually mean by a resonance, which is presently captured by the lede. For example, the bell would also respond to an audible tone close to that frequency. Layzeeboi (talk) 22:59, 11 February 2017 (UTC)
It's all the same. In a linear, or nearly linear, system, the ringing due to an impulse is one way to look at resoance, and the preferential response to coupling near a certain frequency is another. But these are not different things, even in the QM world. The QM guys mostly just don't talk about it that way. Dicklyon (talk) 23:07, 11 February 2017 (UTC)
I agree. I understand that WP should reflect how most folks "talk about it". Layzeeboi (talk) 23:18, 11 February 2017 (UTC)
I forgot another important distinction between quantum and classical resonances. In the quantum context, "resonance" may be applied to any short-lived excitation with significant spread in energy, in any of several modes, not only vibrational but also collective excitationss such as rotational, scissors, or single-particle (nucleon), two-particle,… . Layzeeboi (talk) 00:35, 12 February 2017 (UTC)
Classical resonance can also be in any mode, such as rotational, electrical, scissors, waves on a string, orbital, all kinds of things. Dicklyon (talk) 02:16, 12 February 2017 (UTC)
I agree that the statements above are probably too narrow; and also that the distinction between classical and quantum should not be a top-level distinction. Dicklyon (talk) 02:17, 12 February 2017 (UTC)
I see that my list of examples may have weakened my case. That list of quantum modes usually all refer to theory. All that one usually observes is a peak in an energy spectrum that has a spectral linewidth, indicating that the system prefers to absorb and re-emit energy quanta similar to certain amounts. That's all that we really know, unless we observe the characteristics of the decay products. So a quantum resonance is defined in terms of (imperfect) energy quantization. Classically, one can observe that the system prefers to (continuously) store or release energy at certain frequencies (and one has the luxury of inspecting the system microscopically to identify modes.) Hence I think that we need a top-level distinction that classical resonance is identified by characteristic frequencies, while quantum resonance is identified by characteristic energies. Time and energy are closely related in quantum mechanics. Layzeeboi (talk) 03:58, 12 February 2017 (UTC)
But in QM, frequency is energy, so they're very much the same. Time and frequency are related by Fourier transform, inducing an uncertainty principle. It's just math. Dicklyon (talk) 06:36, 12 February 2017 (UTC)
This comment seems facile to me. (Are you just teasing us?) I've never known a physicist who thinks quantum mechanics is "just math". It introduced a profoundly different view of physics. It is probably considered the most enigmatic established theory. Feynman said very publicly "I think I can safely say that nobody understands quantum mechanics". Here is the evolving perspective[1] of Weinberg after a long career working with the tools of quantum mechanics. Layzeeboi (talk) 08:49, 12 February 2017 (UTC)
Maybe I misunderstood you. What do you propose that "in QM, frequency is energy" implies for the article?
I can assure you that physicists working with quantum systems don't usually think of time or frequency when they discuss a resonance in an energy spectrum. The classical and quantum communities think and speak about resonances differently. Why shouldn't they share the lede? Layzeeboi (talk) 09:49, 12 February 2017 (UTC)
Then why do they use the same word, resonance? I agree with Dicklyon. For me, I think the point is that the underlying mathematics is the same. Resonance is a phenomenon that occurs in systems that are governed by underdamped linear differential equations. In mechanical resonance its Newton's law, in electrical resonance its Maxwell's equations, in quantum mechanical resonance its the Schrodinger equation. The same mathematical techniques, and the same (or equivalent) terminology, are used in both classical and quantum resonance: resonant frequencies, damping, time constant (only in QM it's called "lifetime"), resonance width, Q factor, frequency spectrum, superposition, eigenvalues and eigenvectors. In this encyclopedia article aimed at general readers, I think it would be misleading to say in the intro that the fundamental idea "resonance" is different in quantum mechanics. --ChetvornoTALK 15:25, 12 February 2017 (UTC)
They talk of energy. But they do also realize that in a quantum wave function, energy and frequency are the same thing, related my Planck's constant. Dicklyon (talk) 15:44, 12 February 2017 (UTC)
And I didn't say QM is just math; but the uncertainty principle is; it's a relationship on things related by a Fourier transform (or roughly equivalent transform in a different space). Energy is Planck's constant times the frequency of the wave function; not just for photons; and time is the variable related to that via the transform; no difference between classical and QM there. When physicists talk about the width of a resonance using a Cauchy–Lorentz distribution, that's not different from the "universal resonance curve" of Frederick Emmons Terman; you can label the axes in whatever units you prefer. Dicklyon (talk) 04:36, 13 February 2017 (UTC)

I'm hoping here to hit "reset". I realize that we may have drifted off topic. There is no need to discuss our own understanding of "resonance". We need a consensus on only what cited reliable sources indicate about the modern understanding and usage of the term. I think that there is clear evidence that different branches of physics use this term differently. I think that WP articles should reflect that. The historical reasons for this difference may be relevant only for possible "History" subsections. I offer evidence about the meaning of "resonance" in nuclear and particle physics, for which academic sources are most relevant. Please examine the article Resonance (particle physics) and this one that it links: Relativistic Breit–Wigner distribution. Neither article mentions "frequency", and I think the lede to this article fails to do justice to those articles. [The treatment in nuclear physics is similar but there are differences — I think we need a new article "Resonance (nuclear physics)".] Furthermore, the field of particle physics is very fortunate in having an online library summarizing everything that is known about particle physics, each piece of information accompanied by an independent estimate of its reliability, all continuously maintained by teams of world experts. The accepted approach and terminology of the global community of particle physics to the topic of resonances is given here (which we need to cite). Do you think the lede to this article does justice to that? Layzeeboi (talk) 09:17, 16 February 2017 (UTC)

In the first place, I think you are confusing a specific use of the word resonance in particle physics, a "resonance particle", for the general concept resonance. The WP article Resonance (particle physics) should probably be renamed Resonance particle, a clearer usage in physics literature [2], [3]. Resonance particles are the subject of the PDG article you referenced. There are many examples of resonance in QM that are not "peaks" in "differential cross sections of scattering experiments... associated with subatomic particles" [4]. When a molecule of ammonia absorbs a microwave photon in an ammonia maser, the result is a "flipped" ammonia molecule, not a totally new particle. And the frequency of the microwaves is certainly relevant. Another similar example of a specialized usage of the word is Resonance (chemistry) in chemistry, which refers to delocalized electrons in molecules like benzine rings, another use in QM. The reason the word "resonance" is used for all these examples is that (as mentioned in the article Relativistic Breit–Wigner distribution you referenced) the general resonance equation for a damped harmonic oscillator from classical physics applies to all of them. That is what I think this article should emphasize. I wouldn't mind a hatnote distinguishing the usage of "resonance" for a particle, but I don't see that WP:RSs support saying that the concept of resonance is different in quantum mechanics than in classical physics.
Here are some examples of the definition of resonance from general physics textbooks and websites [5] p.425, [6], [7] p.425, [8], [9], [10]. None of them distinguish resonance in QM as different. Finally, here's the chapter on resonance in one of the most famous physics texts, the Feynman Lectures on Physics by a nuclear physicist, Richard Feynman. The purpose of the chapter is to show that resonance in all areas of physics, including QM, has the same characteristics. --ChetvornoTALK 18:41, 16 February 2017 (UTC)