Science desk
< April 4	<< Mar \| April \| May >>	April 6 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

April 5[edit]

water drops[edit]

Which physical qualities / forces participate in forming the the different shapes of water flow and drops? Etan J. Tal^(talk) 09:29, 5 April 2018 (UTC)[reply]

Surface tension and Van der Waals forces are relevant. {The poster formerly known as 87.81.230.195} 2.218.14.51 (talk) 10:42, 5 April 2018 (UTC)[reply]

We have a surprisingly good article about the Plateau–Rayleigh instability; while short, it does a good job of explaining the thing with as little math as possible. Tigraan^{Click here to contact me} 13:51, 5 April 2018 (UTC)[reply]

What about Cohesion? Is that relevant? Etan J. Tal^(talk) 16:54, 5 April 2018 (UTC)[reply]

Sure. While many general principles are understood, there is still a lot of research into the forces that govern drop formation. See e.g. here [1] for some relevant research on drop formation. We have an article on droplet. Surface_tension is what happens at boundaries where cohesion effects (liquid/liquid) are stronger than adhesion effects (air/liquid). SemanticMantis (talk) 00:56, 6 April 2018 (UTC)[reply]

Thanks everyone! Etan J. Tal^(talk) 11:34, 6 April 2018 (UTC)[reply]

There's a stupendous amount of literature about this, particularly regarding dripping faucets (as a topic in nonlinear dynamics). Typing "dripping faucet" into google scholar will find plenty of material. Here's a sample article about a numerical simulation. It's a complicated subject. 173.228.123.121 (talk) 00:56, 7 April 2018 (UTC)[reply]

Pions in atoms?[edit]

The article Isotopes of lithium has the sentence: "Some lithium-7 has been produced, for a few picoseconds, which contains a lambda particle in its nucleus, whereas an atomic nucleus is generally thought to contain only neutrons, protons, and pions.[8][9]" (emphasis mine.) What is it trying to say about pions? (Did it mean gluons?) RJFJR (talk) 17:44, 5 April 2018 (UTC)[reply]

I removed pions. Ruslik_Zero 20:02, 5 April 2018 (UTC)[reply]

Virtual pions mediate the strong force between nucleons. Double sharp (talk) 00:30, 6 April 2018 (UTC)[reply]

... whereas gluons mediate the strong force between quarks *inside* nucleons (and other hadrons). For more details about exotic nuclei see hypernucleus.Gandalf61 (talk) 11:38, 6 April 2018 (UTC)[reply]

Don't virtual W and Z particles mediate weak force between nucleons? Does that mean we should say that a nucleus contains W and Z particles? Wnt (talk) 00:08, 7 April 2018 (UTC)[reply]

Half-octaves[edit]

What is a "half-octave"? I'd always imagined it was the frequency ratio √2, a.k.a. 600 cents, a.k.a. (in 12TET) an augmented 4th, a.k.a. (again in 12TET) a diminished 5th, etc. (Note "tritone" is not a neutral term for both augmented 4th or diminished 5th some people seem to think it is. It is an augmented 4th. The diminished 5th is not a tritone.) But then Google gave me this: Question and answer on Quora where someone claims 750 Hz is at a half-octave from 500 Hz. Do people who fit hearing aids call a "half-octave" what everybody else calls a (Pythagorean) perfect 5th? And another thing: What is the "half-octave shift in mammalian hearing"? (I saw that in this article but the term is not defined). Thanks. Basemetal 23:55, 5 April 2018 (UTC)[reply]

Quora is not a reliable source. A half-octave is indeed a ratio of √2. The "half octave shift" is a phenomenon in which, if you play sounds so loud that they cause auditory damage, the maximum loss of sensitivity generally does not occur right at the stimulus frequency, but rather about half an octave higher. This is however an approximation, so the precise definition of a half-octave doesn't really matter that much.Looie496 (talk) 00:24, 6 April 2018 (UTC)[reply]

(Meanwhile I'm going to insist on the opposite side that while the diminished 5th might not literally be a tritone, it's generally more useful to consider it as one than not to do so. ^_^) Double sharp (talk) 00:39, 6 April 2018 (UTC)[reply]

If you're using equal temperament, I don't understand the difference between a diminished 5th and an augmented 4th. Is it some Scholastic thing? --Trovatore (talk) 00:43, 6 April 2018 (UTC)[reply]

Probably the number of letters between the notes. Sagittarian Milky Way (talk) 00:51, 6 April 2018 (UTC)[reply]

A diminished 5th should resolve inwards to a 3rd (B/F to C/E or C/E♭); its inversion the augmented 4th should resolve outwards to a 6th (F/B to E/C or E♭/C). Of course, since they are enharmonically equivalent, there is nothing stopping you from approaching a tritone as if it was a diminished 5th and leaving it as if it was an augmented 4th, or vice versa. Double sharp (talk) 01:43, 6 April 2018 (UTC)[reply]

Just a note of appreciation that "Double sharp" is talking about musical notes. DMacks (talk) 04:45, 6 April 2018 (UTC)[reply]

A quick note, however, that the above discussion which states that the diminished 5th and the augmented 4th are the same note only works in equal temperament scales. In just intonation, those notes would differ by a few cents. Tritone#Size in different tuning systems discusses this a bit; this small gap in enharmonic notes is called a comma. --Jayron 32 12:50, 6 April 2018 (UTC)[reply]

In even equal temperaments (such as 12TET or 72TET) not in odd equal temperaments (such as 19TET or 53TET). Note the further away you get from the model scale that underlies the Western theory of intervals, namely the Pythagorean scale (statement that Captain

will no doubt question, just watch) the less clear it becomes what an augmented 4th or a diminished 5th, or even a 4th or a 5th is. Basemetal 15:10, 6 April 2018 (UTC)[reply]

Well, that's quite true; music intervals are infinitessimal in size and infinite in number, they are a continuum. There's always going to be a clash when you attempt to place discrete measurments onto a continuous system, as happens in music where continuous possible variation in tone is confined to some arbitrary number of specific tones. There's an infinite number of ways to do that, and the various systems that exist to do so will often clash. However, the context of this discussion makes it clear we're dealing with the standard, western musical system. --Jayron 32 15:15, 6 April 2018 (UTC)[reply]

@Basemetal: Hmm. I'd argue that the model scale underlying the Western theory of intervals is a 12-TET that is very aware of its Pythagorean heritage. In other words, the scale is twelve tones equally dividing the octave, but it did not come from a stack of semitones but a stack of fifths. I am aware that this sounds like I want to have my cake and eat it too, but otherwise it is difficult to explain both the possibility of all those enharmonic modulations and why there is still a difference between dominant and subdominant directions that can be exploited. Similarly I'd argue that the Pythagorean scale that is 12-TET's heritage is also quite aware of its just-intonation heritage, which helps explain why the minor mode is a dissonance compared to the major mode. So the sequence of events is taking just intonation, restricting it to the 5-limit (setting up the tonic major triad as the king of chords), and then tempering out the syntonic comma and then the Pythagorean comma. I think we've completely left the original topic at this point, but at least I'm satisfying the OP's expectations by doing so! ^_^ Double sharp (talk) 15:45, 6 April 2018 (UTC)[reply]

You're right. Therefore I'll move to your talk and challenge you there tomorrow at dawn. My aside about the tritone has generated more responses than the actual question about the definition of the half-octave and the so called half-octave shift (as well as, incidentally, the reliability of the answers at Quora). I only mentioned the tritone to answer in advance someone who might wonder why I didn't use the term along with the others. Basemetal 17:22, 6 April 2018 (UTC)[reply]

(EC)Tritone would seem to support your view, which is common, as a broad sense, and support OP's view under the "strict" heading of diatonic scales. OP asks an interesting question but the aug4/dim5 stuff smells of WP:SOAP. We may as well opine on the 'correct' end from which to open an egg SemanticMantis (talk) 00:46, 6 April 2018 (UTC)[reply]