Wikipedia:Reference desk/Archives/Science/2019 April 3

= April 3 =

Bird or butterfly?
Hello all, I want to ask about a flying creature I saw in the San Jose area (near the border with Smellpitas) a few days ago. I didn't get a good look at it (I only saw it for a couple of seconds), so I don't even know whether it's a small bird or large butterfly (that's why I'm asking), but here's the approximate description: estimated wingspan is about 3 1/2 inches, wings are black or dark purple with a prominent white patch (or band) at the rear and no hint of tiger-striping (so this cannot be a Papilio garamas or P. ornythion) and are significantly longer than they are broad (which is the case with all birds, but also with some butterflies of the Nymphalidae and Papilionidae families), flight is fast but wobbly and butterfly-like in terms of trajectory. What might it be? (Pictures are OK, black butterflies without tiger stripes don't scare me, and neither do birds of any kind.) 2601:646:8A00:A0B3:4E5:9C5D:6742:BAD6 (talk) 00:55, 3 April 2019 (UTC)
 * Manduca sexta MHNT CUT 2010 0 104 Caranavi, La Paz Bolivia male dorsal.jpg]]Hawkmoth? Wnt (talk) 08:38, 3 April 2019 (UTC)
 * Luna moths are green, but are shockingly large. Maybe there is a related species which is dark colored?  -- Jayron 32 11:16, 3 April 2019 (UTC)
 * My other thought was maybe a grasshopper. Many people don't think about them flying, and they can look awkward and wobbly, but you can see wings very much like what you described here.  -- Jayron 32 12:46, 3 April 2019 (UTC)
 * Surely the flight speed here will make it easier to tell if it was a butterfly or bird. You saying it was fast but wobbly makes me think it was a hummingbird, which are not "wobbly" fliers, per se, but often make very acrobatic maneuvers which might make them look wobbly - especially in wind (they're superb aeronauts, but when you weight a couple of grams, even breezes can push you around). Matt Deres (talk) 13:15, 3 April 2019 (UTC)
 * Hummingbirds can easily be identified by their wings during flight, which are moving so fast as to be just a blur. SinisterLefty (talk) 13:23, 3 April 2019 (UTC)
 * Yes, but if there's confusion regarding whether something is a butterfly or a bird, it was obviously not clearly observed. Also, the OP has previously gone on record to explain that they have a severe phobia regarding some species of butterfly, so again the observation was not under neutral conditions. Matt Deres (talk) 18:54, 3 April 2019 (UTC)
 * What time of day was the creature observed? cygnis insignis
 * It was during the day, and the wings were clearly visible (fluttering fast, but not blurred), so it did not look to me like a hummingbird. Neither could it have been a grasshopper, because it was clearly flying (with wings spread out) and not hopping, and because I saw it about 10 yards away (I was looking out of a 2nd-floor window, and the flying creature was outside a few yards from my home, doing something like a split-S or an English bunt above some flowering bushes).  And there was no phobic reaction on my part (because if it was a butterfly, it was only moderately large and not tiger-striped, and these don't scare me), so the size estimate is most likely accurate (as we had established with my sightings of Limenitis arthemis in the Nashville area previously). 2601:646:8A00:A0B3:4E5:9C5D:6742:BAD6 (talk) 00:54, 4 April 2019 (UTC)
 * Grasshoppers do clearly fly. They don't fly often or over great distances (like miles), but I have seen grasshoppers fly between plants or across an open field.  -- Jayron 32 11:35, 4 April 2019 (UTC)
 * Fairly sure some grasshoppers can fly over long distances hence why you hear about plagues of locusts with their swarm behaviour. Anyway my impression is some grasshoppers fly long distances even without entering into the swarm phase, see e.g. . Nil Einne (talk) 11:56, 4 April 2019 (UTC)
 * That's why I thought of it; I have myself mistaken a grasshopper in flight for a clumsy butterfly. We have local grasshoppers that have brilliant fuschia-colored wings, which are quite striking for a critter that is otherwise a bland olive drab.  I was noting that the OP was mistaken about grasshoppers not flying.  They do fly.  -- Jayron 32 12:40, 4 April 2019 (UTC)
 * I'm going to continue with my guess of hawkmoth - for example, Manduca sexta, which I've added as an image above; note lighter region on wings. Adults at least can be observed at this time in April (though in Virginia) ; according to our article it can have four broods a month.  It and related species are important agricultural pests; it can feed on tomatoes as well as tobacco, also other Solonaceae like datura, with as many as four generations in a year in extreme cases.  They can be seen at day and can hover just like hummingbirds.  They lack the tiger stripes of Papilio though the rear patch is not solid white. Wnt (talk) 17:32, 6 April 2019 (UTC)
 * Could be -- I looked at the image, and it did look similar to what I saw (although the one I saw was slightly smaller than the size given in the article)! But do they fly in central California in late March, and do they fly during the day? 2601:646:8A00:A0B3:78DA:714:B603:E789 (talk) 06:00, 7 April 2019 (UTC)

Regarding selection rules
Selection rules in quantum physics are based on calculating the respective electric dipole matrix element & finding out under what conditions we get non-zero results. However, physically, forbidden transitions aren't strictly zero, but rather of very low probabilty. My question is where does lie the difference between the mathematical zero (vanishing integrals) & the very minute transition probabilty ? Is it a result of the due integrals being only approximate themselves ? בנצי (talk) 13:21, 3 April 2019 (UTC)
 * If I understand your question correctly, you are asking why the probability of forbidden transitions is not strictly zero. It may be so for two reasons.  First, what is not allowed as an electric dipole transition because of for example parity, can still be allowed as an electric quadrupole transition; or generally as some other electric or magnetic multipole transition.  Second, as we already discussed, the Hamiltonian eigenstates (corresponding to the atomic energy levels) are really mixtures of basis states when basis is defined by principal, spin, angular orbital, and/or total orbital momentum. Thus, an electric dipole transition may be forbidden between two basis states, but still allowed (with low probability) between the two Hamiltonian eigenstates because each Hamiltonian eigenstate is actually a mixture of many basis states. Hope this helps. Dr Dima (talk) 21:27, 3 April 2019 (UTC)
 * That's what I had in mind by saying "integrals being only approximate themselves", namely when they're calculated based on, e.g., only the electric dipole moment. Very well. Thank you.
 * Please, pay attention to my message to you - in your 'talk page'. What's recommended for ongoing discussions, since they're archived quite (sometimes too) frequently ?
 * Looking forward, בנצי (talk) 18:02, 6 April 2019 (UTC)
 * Bentzi, the archiving of Reference Desk pages is automated, and there shouldn't be any "sticky" pages or threads on RefDesk. The search function of the RefDesk allows to easily find the archived pages when necessary, which I think is the best approach. This being said, if you have a better idea please let us know. Dr Dima (talk) 22:58, 8 April 2019 (UTC)