Wikipedia:Reference desk/Archives/Science/2024 March 16

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March 16

Weld; Syren

This 1878 report makes a passing reference to "Weld's Sound Experiment". What was that, and who was Weld?

It also mentiones "Syren and Galton's Whistle"; the latter is Francis Galton, but who was Syren? Andy Mabbett (Pigsonthewing); Talk to Andy; Andy's edits 21:28, 16 March 2024 (UTC)

My guess is a siren whistle. --Wrongfilter (talk) 21:49, 16 March 2024 (UTC)

Galton's whistle is similar to a boy scout's whistle . It differs in that it has a piston provided with a screw and a milled head... (it's an ultrasonic dog whistle).

From Oscillations and Waves p. 297. Alansplodge (talk) 21:56, 16 March 2024 (UTC)

The most promising Weld is Alfred Weld, who was a bit of a scientist, but I fail to find any indication that he may have dealt with sound. --Wrongfilter (talk) 22:02, 16 March 2024 (UTC)

And but nay. In duo together with Ernst Chladni, Alfred will make too much of a youngster, for those classicals. --Askedonty (talk) 01:33, 17 March 2024 (UTC)

What classicals? This is about the first annual meeting by the Midland Union of Natural History Societies. Also, they had no scruples looking at a toddler like Galton's whistle, which had been invented a mere two years before. But don't hesitate to suggest another Weld if you've got one. --Wrongfilter (talk) 07:29, 17 March 2024 (UTC)

Wiktionary:syren#Noun: "Obsolete form of siren". Alansplodge (talk) 22:03, 16 March 2024 (UTC)

For the meaning of "Galton's Whistle", see Dog whistle.  --Lambiam 00:53, 17 March 2024 (UTC)

Isaac Weld.^[1]  --Lambiam 10:24, 17 March 2024 (UTC)

Resolved

 – Thank you. all. Andy Mabbett (Pigsonthewing); Talk to Andy; Andy's edits 10:50, 17 March 2024 (UTC)

Length of a photon

A couple of questions about photons.

Firstly, am I right to feel slightly uncomfortable about the following passage? From the article photon:

However, experiments confirm that the photon is not a short pulse of electromagnetic radiation; a photon's Maxwell waves will diffract, but photon energy does not spread out as it propagates, nor does this energy divide when it encounters a beam splitter. Rather, the received photon acts like a point-like particle since it is absorbed or emitted as a whole by arbitrarily small systems, including systems much smaller than its wavelength, such as an atomic nucleus (≈10⁻¹⁵ m across) or even the point-like electron.

It's trying, I think, to make a point that a quantum-mechanical wavefunction is different from a classical wave, because it can suffer wavefunction collapse; and because it always represents a single whole particle (and also that a photon is a point particle -- which I understand to mean that its overall (extended) wavepacket can be evolved by integrating it forward via point to point propagators).

But am I right to feel uncomfortable with the argument about it not being distributed because it can interact with objects that have a very small scale? I seem to remember that in the classical treatment of radio waves and antennas, an antenna might be essentially just a single vertical pole; but may interact with a radio wave that may have a wavelength of hundreds of metres, if the antenna is part of a circuit that has the right resonant frequency. If I remember correctly, the radio wave induces a current in the circuit, which in turn can be thought of as exciting an induced radio wave of its own that has an opposite phase in the far field, thus effectively partially cancelling the original wave; and so, despite what may be its small physical size, the antenna can have an effective area ("aperture") that can be very considerably larger. (Did I remember that right? It's a long time ago since those lectures on EM ...)

Now I'm not saying that that is directly equivalent to quantum measurement, because measuring the location of a photon absorbs all of it, not just part of it (give or take whatever discussion we might have about decoherence...) But am I right that the antenna does maybe suggest that we should perhaps be just a little more careful before making a blanket statement that an object cannot be distributed if it can interact with a system much smaller than its own wavelength?

The remark that "the photon energy does not spread out as it propagates" also makes me feel a bit uncomfortable. If one thinks of gravitational lensing, surely it's reasonable to imagine a photon travelling both sides of the gravity well while still retaining self-coherence, with a corresponding energy flow associated with both sides?

Beyond that, the second thing I wanted to ask about (which was what led me to look at the photon article in the first place, to see if it anywhere touched on the subject) was this: I seem to remember, way back at school level, being told that a typical wavepacket of a photon of visible light (eg perhaps from a sodium vapour lamp) had a longitudinal length of the order of about a metre (maybe give or take an order of magnitude either way). Does that seem plausible / viable / about right ?

One constraint would seem to be the time <-> frequency uncertainty relation. The overall linewidth of the source would seem to set a limit to the maximum range of frequencies that could be associated with such a photon, and therefore a minimum to the length of time (and so therefore spatial length) that the wavepacket must persist. Presumably there is also a maximum length of time one pictures the emission event taking -- perhaps even measurable by trying to measure the coherence time of such a source using an interferometer at lower and lower light levels ? Is this sort of thinking at all on the right track?

Thanks in advance, Jheald (talk) 22:05, 16 March 2024 (UTC)

The following, unfortunately unsourced, passage is copied from Point particle § In quantum mechanics:

Nevertheless, there is good reason that an elementary particle is often called a point particle. Even if an elementary particle has a delocalized wavepacket, the wavepacket can be represented as a quantum superposition of quantum states wherein the particle is exactly localized. Moreover, the interactions of the particle can be represented as a superposition of interactions of individual states which are localized.

Does this help to ease your uncomfortability?  --Lambiam 00:48, 17 March 2024 (UTC)

Thanks @Lambiam:. No problem with what you've quoted, so far as it goes.

But I do think it can be useful (and meaningful) to try to give a sense of what the whole overall superposition - the whole wavefunction - may look like, when this is possible (for a typical environment or observational set-up).

So eg for an electron, it's a point particle, but it's also useful to be able to talk about the whole atomic or molecular orbital that may be a good representation of the whole wavefunction in particular circumstances.

And similarly, it seems to me, for a photon, it's useful to have an idea of what a whole photon wavefunction might be like in a particular circumstance; and in respect of an interaction, what shape the whole overall superposition of point interactions (as referred to in your quote) might take, to represent eg a whole molecular orbital interacting in typical way with the entirety of a photon wavepacket, and what basic picture that may give for a complete scattering (elastic or inelastic) or emission event. That's something I'd quite appreciate some input on, at least in respect of the second part of my question.

In respect of the first part of my question, while I feel comfortable with what you've just quoted from point particle, it doesn't seem to me to address the things I suggested gave me a bit of discomfort with the bit of text from photon. Do you think it should make me feel more comfortable with that text? Thanks, Jheald (talk) 14:00, 17 March 2024 (UTC)

I think you're quite right not to feel comfortable about its wavefunction. It is possible to get pictures that represent it. But thinking a particle was actually at any of the point when it has not been observed to be there - well ... have a look at Quantum Cheshire cat and marvel at its ... well whatever. NadVolum (talk) 15:51, 18 March 2024 (UTC)

• Just adding a holding message, to stop the thread being auto-archived while I continue to try to read into it. I do think it would be useful to try to establish where mainstream specialist thought is on this, as the variety of answers one can find on generalist sites like Quora, StackExchange, Reddit, PhysicsForums, etc and even preprints are all over the place. Even here on Wiki Reference Deska couple of years ago essentially the same question got shut down as not meaningful or not based on a proper understanding of the physics. (Pinging @Malypaet, PiusImpavidus, Jayron32, Nimur, and Rmhermen: from that discussion, if they'd like to come in).

Yet, despite the issues with setups like the Quantum Cheshire cat that User:NadVolum points to (and which I need to think some more about), or our article Wave packet bluntly making the statement that 'Physicists have concluded that "wave packets would not do as representations of subatomic particles"' without further explanation, it seems to me that a wave packet or evolving wavefunction is often a useful physical picture for a particle, at least until environmental interactions make considerations of decoherence or measurement unavoidable. They help us picture, for example, how single photons can exhibit interference effects from their own reflections, or between paths with different flight times in beam-splitter experiments. I need to read up more on Quantum Optical Coherence (eg [2]) as distinct from classical coherence, but it does seem to me that the coherence length of a source of incoherent photons probably can be reasonably pictured as the length of a photon, eg the 67mm for a low-pressure sodium vapour lamp given at Coherence_length#Other_light_sources, extending to six times this for such lamps cooled to liquid nitrogen temperatures.

Our article atomic electron transition seems once to focus on how short the time for atomic transitions can be (given very heavy environmental 'measurement' / decoherence) -- still a bit longer than zero, because (as I understand it) even then the electron wavefunction needs time to change without moving faster than the speed of light -- but little in that article that I could see as to how long in time the electron transition might be considered to take, in the absence of externally-induced decoherence, with measurement setups designed to maintain rather than destroy coherence. Perhaps there should be more of an investigation of this there ? Jheald (talk) 13:28, 24 March 2024 (UTC)

How long the transition can take instead of how short?, see Quantum Zeno effect ;-) NadVolum (talk) 16:45, 24 March 2024 (UTC)

@NadVolum: In fact pretty much the exact opposite of Quantum Zeno Effect. The Quantum Zeno Effect, as I understand it, arises if you are doing almost continuous measurement of the state of the test atom, destroying any superposition that may be beginning and forcing it into a state where either no transition has occurred or a very quick transition has occurred (and been completed) since the previous observation. The experiments to see how fast a transition could be observed were I believe developments of this set-up. The photons released from such a set-up I think have a very short coherence time (and coherence length), with a correspondingly very wide frequency line-width (like thermal broadening only very much more so), a much greater energy uncertainty made possible by the pronounced coupling of the system with the environment / measuring apparatus, that is used to achieve the decoherence.

In contrast I would like to see the articles make a bit more about how long the coherence times / coherence lengths of the emitted photons can be, corresponding to how long a time the transition superposition may have evolved over. Jheald (talk) 09:21, 25 March 2024 (UTC)

Ok @Jheald:, I'll give my opinion. Considering the orbital trajectory of an electron crossing a flow of electromagnetic radiation, how long will it receive enough radiation pressure energy added in this time to eject itself? If we consider the relation $E=h\nu$ as an energy over a second with $\nu$ not a frequency, but a number of cycles over this second, therefore with $h$ the energy of a cycle whatever the frequency, how many cycles of how many ray crossing this orbit will give their energy to eject the electron? I know, it's too simple to imagine with the analogy of an asteroid field crossing a satellite. It's not science fiction enough to enter the official domain of science. Maybe someday... Malypaet (talk) 22:40, 25 March 2024 (UTC)

@Malypaet: Interesting. But, to the extent that the energy density per metre of a photon can be conceptualised at all, I think I would have to come at it from the other direction, ie that the total energy of the photon is $E=h\nu$ (nb total energy, not energy per second), then the energy density per length would be this divided by the length of the photon -- which would depend, like the probability amplitude per metre, on the coherence length (or equivalently, the coherence time). A photon spread out over a long coherence length would have a lower energy density per metre (if such a concept is meaningful); a tightly bunched short coherence photon would have a higher energy density per metre.

But it would seem to me a secondary quantity, that might be derivable from the photon length; but with no apparent very ready way to estimate it in its own right seems of little use as a route in the other direction, to try to determine a photon length. Jheald (talk) 22:19, 28 March 2024 (UTC)