Wikipedia:Reference desk/Archives/Science/2021 April 15

= April 15 =

Weak charge
I saw the following file: https://commons.wikimedia.org/wiki/File:QCD.svg I made a three-dimensional version of it in geogebra.org: https://www.geogebra.org/classic/wzjdbwhs where the original image is a blue plane. It occurred to me to make a similar scheme for the weak interaction: https://www.geogebra.org/m/cqgjvzes where v and w are generators of the group SU(2). Question: is it correct that v and w are "weak charges", but T=(w-v)/2 (just as isospin is 1/2 of the difference in the number of quarks)? --YushinSasha (talk) 16:40, 15 April 2021 (UTC)
 * Unfortunately this is a little above my pay grade. I suggest asking at Wikipedia talk:WikiProject Physics. --47.155.96.47 (talk) 01:44, 21 April 2021 (UTC)
 * No -- the numbers of up and down quarks aren't generators for flavor SU(2). The Lie algebra for SU(2) has three generators, not two; these are the three components of isospin ("strong isospin" for flavor SU(2), or "weak isospin" for electroweak SU(2)). --Amble (talk) 05:12, 21 April 2021 (UTC)
 * Looking at the diagrams... I don't think you can quite make the kind of diagram you're trying to make for SU(2). The two-dimensional QCD diagram works because you can pick two generators of SU(3) that commute, in this case, λ3 and λ8 in Special_unitary_group, or the charges g3 and g8 in the original QCD diagram. To put it another way, SU(3) has a two-dimensional abelian subgroup U(1)xU(1), so we can place the gluons and quarks at definite locations (definite color charges) on a two-dimensional SU(3) diagram, but not a three-dimensional one. But for SU(2) no two generators commute, so you can only assign a definite charge along one dimension of isospin (T3), and that means you can only make a one-dimensional diagram for SU(2). --Amble (talk) 18:00, 21 April 2021 (UTC)
 * To step back a bit: you might think that the QCD diagram should "really" be eight-dimensional because SU(3) has eight generators, and that we have a two-dimensional diagram just to squash it into something that fits on a page or a screen. But that's not the case. The QCD diagram has to be two-dimensional because of the structure of SU(3). The SU(2) diagram is one-dimensional for similar reasons. The nonabelian gauge groups don't allow a higher-dimensional diagram to be made, even when you have a more capable visualization tool. --Amble (talk) 18:09, 21 April 2021 (UTC)
 * You can see some examples of one-dimensional weight diagrams for representations of SU(2) in Fig. 13 and 14 of arXiv:1401.3968v1 (hep-ph). --Amble (talk) 18:50, 21 April 2021 (UTC)
 * See also arXiv:1009.0437 (math-ph) Fig. 1, Fig. 2, and Sec. VI. --Amble (talk) 23:19, 21 April 2021 (UTC)

Where do birds get fur for nesting materials?
I watched a YouTube video where a bird plucked fur from a sleepy fox. My impression was that it didn't hurt the fox enough to keep it from falling asleep, but I can't imagine it helping the fox--despite commenters claiming that the fox would need to shed fur to change seasons--since pulling out hair damages hair follicles. But how common is this behavior and is it the main source of fur in bird nests? Thank you. Imagine Reason (talk) 19:24, 15 April 2021 (UTC)
 * Animals shed fur. ←Baseball Bugs What's up, Doc? carrots→ 21:01, 15 April 2021 (UTC)
 * That's what I thought, but do birds really go around looking at the ground for fur? Would the fur stand out from the other stuff like vegetation?
 * Animals typically rub against tree trucks and leave fur behind. Abductive  (reasoning) 22:51, 15 April 2021 (UTC)
 * Carcases. Greglocock (talk) 08:14, 16 April 2021 (UTC)
 * They also use dust bunnies which can form in natural surroundings - not just under your sofa. 41.165.67.114 (talk) 10:26, 16 April 2021 (UTC)
 * Thus proving that Wikipedia has an article on pretty much everything and that requiring admin candidates to find topics and create articles about them instead of improving exsisting articles is stupid. --Guy Macon (talk) 17:30, 17 April 2021 (UTC)
 * Was the bird a crow/raven/magpie? They are known for plucking fur/feathers from the tails and backsides of predators. I think it's more to piss them off and make them leave than to gather nesting material in any significant amount. Sometimes you'll get two or three of them working together. One plucks, the predator turns around and chases the crow, another crow then uses the opportunity to scoot up behind and pluck. Predator chases the second crow, third bird scoots up behind and plucks. Repeat until the predator has had enough. Lesson in why you don't try to hunt prey that's smarter than you? --Iloveparrots (talk) 13:10, 16 April 2021 (UTC)


 * Nope, it was a titmouse. Imagine what bigger birds would be willing to do. Imagine Reason (talk) 22:49, 16 April 2021 (UTC)
 * Seems to be fairly common.  And this one is definitely not mutually beneficial.  Imagine Reason (talk) 12:11, 18 April 2021 (UTC)
 * Pulling out hair doesn't usually damage hair follicles, as far as I am aware. If it did, hair removal would be a lot easier. This is a kind of commensalism. --47.155.96.47 (talk) 01:51, 21 April 2021 (UTC)
 * Makes sense. Imagine Reason (talk) 12:29, 21 April 2021 (UTC)