Wikipedia:Reference desk/Archives/Science/2022 September 14

= September 14 =

Non-avian dinosaurs
As I understand things, 66 million years ago a meteor or asteroid hit the earth and wiped out all non-avian dinosaurs. How did the avian dinosaurs (leading to modern birds) survive but not the terrestrial or marine dinosaurs? 142.51.201.3 (talk) 20:53, 14 September 2022 (UTC)
 * Our article on the K-T extinction event does discuss extinction patters in some detail. DuncanHill (talk) 20:59, 14 September 2022 (UTC)
 * You also have to remember that the "avian" vs "non-avian" distinction only really makes sense in the aftermath of the extinction. "Avian dinosaurs" are simply the ones that did survive. Before the K-T event, there were (or at least had been) many dinosaurs that weren't quite "birds" in the modern sense, but were probably more like birds than they were like other "non-avian dinosaurs". And even more unlike what we normally think of as "reptiles". If a different group of species had survived, the modern idea of "bird" might have been more broadly conceived. Or the idea that birds were just a group of particular "reptiles" might have been accepted much earlier. Or the classic "dinosaur" discoveries might have been immediately classified with "birds" as opposed to with "reptiles". --User:Khajidha (talk) (contributions) 01:28, 15 September 2022 (UTC)
 * There is also likely a single last common ancestor for all birds, and it likely post-dates the extinction event; so all "avian dinosaurs" are descended not just from dinosaurs in general, but from a single species of ground-dwelling therapod who likely lived at the time of the meteor strike that was the likely precipitating event for the mass extinction 66 mya. See evolution of birds and origin of birds.   Archaeopteryx was long considered a candidate for such an LCA, but some recent studies have proposed that it may have been on a parallel branch of the tree of life, and that modern birds may descend from another branch.  -- Jayron 32 15:28, 15 September 2022 (UTC)
 * I disagree that the avian last common ancestor "likely post-dates the extinction event." Birds proper, as well as their 'near-bird' relatives, had been around and had been diversifying for the thick end of 100 million years by then: one common definition of birds is 'all descendents of the last common ancestor of Archaeopteryx (which dates to around 150 mya) and Passer domesticus (the common House sparrow).'
 * Two other major branches of birds proper (Enantiornithes and Hesperornithes) apparently didn't survive the KT event, but the article Evolution of birds you link itself states in the lede "Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago". There would likely already have been multiple species in each of these lineages. {The poster formerly known as 87.81.230.195} 90.193.131.160 (talk) 16:22, 16 September 2022 (UTC)
 * Good catch. Thanks for the correction!  -- Jayron 32 11:16, 19 September 2022 (UTC)

Effect of an electric field on electromagnetic radiation
With the Faraday or Zeeman effects, the polarization of electromagnetic radiation is modified, which shows its magnetic nature. Are there also effects of an electric field on electromagnetic radiation, thus showing its second electric nature? Malypaet (talk) 22:45, 14 September 2022 (UTC)
 * See Kerr effect. In none of these effects does the static field act directly on the electromagnetic radiation; some sort of material is required. catslash (talk) 22:55, 14 September 2022 (UTC)
 * Sorry, but for Faraday effects there's an example in astronomy where a galaxy magnetic field polarise the light, from max planck institute: https://www.mpifr-bonn.mpg.de/pressreleases/2017/7]. For Kerr effect, what I understand, it is due to electric field induced by the magnetic field of the light itself, so that's not so clear. Malypaet (talk) 13:01, 16 September 2022 (UTC)
 * See Faraday effect catslash (talk) 20:34, 16 September 2022 (UTC)
 * Electromagnetic fields obeying Maxwell's equations in free-space exhibit superposition, because Maxwell's equations in the absence of charge-carriers are linear. Superposition means that electromagnetic waves pass through each other, or through static fields without interaction - they are entirely transparent to each other.
 * The introduction of electrons (or other charge-carriers), whether as a constituent of a material, or as free particles changes the situation. Electrons have charge and magnetic moment and interact with both static fields and electromagnetic waves, giving rise to the effects in question. catslash (talk) 22:40, 16 September 2022 (UTC)
 * But on my example the effect is induced by the strong magnetic field of the foreground galaxy, like the earth magnetic field created by a dynamo made of circulating electron. Malypaet (talk) 19:36, 17 September 2022 (UTC)
 * It's not clear what your point is there. The rotation occurs in the interstellar medium of the lensing galaxy.  From the second paragraph of the abstract of the paper from the linked press-release: "To quantify the appearance of the Faraday depth spectra and to extract physical properties of the magnetized gas in the lensing galaxy, we performed a direct fit to the observed fractional Stokes parameters Q/I and U/I as a function of frequency of the two images independently across 1–8 GHz."
 * (my italics) catslash (talk) 22:13, 20 September 2022 (UTC)