Wikipedia:Reference desk/Archives/Science/2013 December 4

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.

December 4

Lagrange's equilateral triangle solution

I've tried googling but with no luck- what are the conditions on the initial velocities / masses for Lagrange's equilateral triangle solution to the three body problem? 68.0.144.214 (talk) 00:05, 4 December 2013 (UTC)

Lagrange points L4 and L5 - those which are described by an equilateral triangle whose base is the line between n₁ and n₂ - have no constraint on mass of n₃. In the rotating frame of reference, there is no net velocity; equivalently; the velocity of the L4 and L5 points is the orbital velocity of n₂ about n₁. If you're looking for Lagrange's actual mathematics, you can find the link in our article on Lagrangian point: Essai sur le Problème des Trois Corps. Lagrange's notation is actually almost identical to the sort of modern differential equations I used in university - so his work is a lot easier to read than, say, that of Isaac Newton - except, of course, that you need to be fluent in French. He sets up the equations for constant angular offset in Article XLII ("Page 308" in the PDF file); and also in Article XXV: Ainsi les Corps B et C ne feront que tourner autour du Corps A avec vitesse angulaire constante et égale à ${\sqrt {\frac {A+B+C}{r^{3}}}}$. Granted, this is a little bit more obtuse than stating "L4" and "L5" outright, as you will find in a modern physics book; but if you really get the points, you probably don't need to enumerate the points. Nimur (talk) 01:33, 4 December 2013 (UTC)

Awesome answer, thanks Nimur! SemanticMantis (talk) 02:25, 4 December 2013 (UTC)

Hm, ok. Are there configurations with non-zero velocity that maintain the equilateral triangle while the distance between the three bodies changes? 68.0.144.214 (talk) 03:41, 4 December 2013 (UTC)

Not quite, but the horseshoe orbit is an almost stable orbit around L4 or L5 that has a non-zero relative angular velocity between the second and third bodies: as the angular distance varies, the leg lengths of the triangle also vary and are not equilateral. It is not possible to thus maintain an equilateral triangle: varying the velocity changes the distance between n₂ and n₃ - that implies that you are varying all the leg lengths; that would change the orbital radius of the second body, but not in a stable way. The radial period can not be resonantly locked with the orbital period of n₂ and n₃: as you set up your scenario, they have different velocities! The resulting behavior would be a non-stable orbit and proceed in a chaotic way. The system would rapidly decay into unstable motion. You can develop an intuition about the metastability of these orbits; there are actually very few stable 3-body configurations, even in an ideal system. Nimur (talk) 04:03, 4 December 2013 (UTC)

Here is a page with equilateral examples where the distance changes. Case 7 shows that the bodies can maintain an equilateral triangle while moving in elliptical orbits when the masses are different. The last paragraph focuses on your question and has a journal article citation that sounds like it will get into the math. Katie R (talk) 12:47, 6 December 2013 (UTC)

Twins

Do identical twins (or triplets, etc.) have DNA that is exactly identical? Or is it somehow different? Also, do identical twins (or triplets, etc.) have distinct fingerprints or identical fingerprints? Thanks. Joseph A. Spadaro (talk) 05:38, 4 December 2013 (UTC)

Identical DNA. Identical fingerprints. HiLo48 (talk) 06:25, 4 December 2013 (UTC)

No, they do not. The process that creates fingerprints is not determined by genetics. Even identical twins have distinguishable fingerprints. See http://www.straightdope.com/columns/read/1277/do-identical-twins-have-different-fingerprints for reference. Someguy1221 (talk) 06:30, 4 December 2013 (UTC)

No they don't. See Fingerprint. It's rather disturbing that a reference desk answer with no citation would contradict a WP article. Identical twins do not have identical fingerprints. If HiLo48 has a source that contradicts the article s/he should update it. If s/he doesn't, s/he shouldn't guess. --DHeyward (talk) 06:36, 4 December 2013 (UTC)

Sorry. No source. Just mistaken confidence in my knowledge. HiLo48 (talk) 07:12, 4 December 2013 (UTC)

It's also not true that twins have identical DNA. In fact, due to mutation, a random pair of twins probably don't. See [1], which found that the average pair of twins have 359 genetic differences that occurred early in development. (Mutations that occur late in development would only be shared by a small fraction of the twin's cells.) --Bowlhover (talk) 07:43, 4 December 2013 (UTC)

However, that's a very small portion of the total, so, if your evil identical twin kills somebody, and they compare his DNA from the murder scene with your own, they might very well declare it a match, unless you can prove you have an identical twin and they then do more detailed DNA testing. StuRat (talk) 10:18, 4 December 2013 (UTC)

Identical twins are considered to be natural clones of each other, but as noted above, during gestation their genetics diverge a bit. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:22, 4 December 2013 (UTC)

stingless bees finding their hive

Note this curious detail in an article today on our local stingless bees in the Sydney Morning Herald"

"They can find their way back to the hive if it is moved less than a metre or more than a kilometre, but anything in between means some bees will never find their way home."

http://www.smh.com.au/environment/animals/no-sting-in-this-tale-of-homely-bees-20131203-2yoq7.html

I am trying to figure out how that could possibly be. (Note, downside of our indigenous stingless bees - a hive produces half a kilo of honey each year.)

From same article:

"For most Australians the only connection they have with insects is from the wrong end of a spray can."

Is this arse-about? If there is a "right end" of a spray can, surely it is at the end where you push the button, not the end which where the target of the spray is.

banned user
The following discussion has been closed. Please do not modify it.
Whether it is arse-about depends on your point of view. But the article seems to be rubbish anyway. Given the methods bees use to get back to the hive (they sense the polarisation of light from the sky to get general direction, and then use landmarks to localise on the hive), it is reasonable to expect bees to be tricked by moving the hive perhaps a few metres, but unreasonable to expect them to somehow recover their ability to find the hive if it is moved a larger distance. I have kept bees myself, and have found that moving the hives a few metres is not a problem - however I had 4 hives in a group in an open area at least 20 metres form any tree or structure. So they had to 1) find their way back to the clear area, and 2) pick which of the 4 hives is theirs. Quite a few Australians have more contact with insects than as something to spray at. Red Back spiders, very poisonous, infest toilets, garden sheds, and other structures that have a fair bit of ventilation to outside air. Squirting them with flyspray can make them struggle about as though drunk, and then they recover. I squirt them with CRC - that imobilises and suffocates them. And CRC is nowhere near as harmful to humans as poisons designed for spiders. Many Australians live in old suburbs in old houses built with wooden floors (houses built in the last 40 years or so mostly are brick with concrete floors and are easy to keep cockroach-free). These tend to be infested with cockroaches, which require treatment by a pest exterminating company (while you stay in a hotel for a few nights, or get it done before you move in) to really get rid of. Then there are wasps that crawl into your can of beer or soft drink while you are not looking. In areas close to fresh water, mosquitoes are a nuisance - anit-mosquito burners & candles are best for them. 1.122.59.94 (talk) 06:59, 4 December 2013 (UTC)

Without commenting on whether the statement is correct, I could propose a reason why that might not be able to find the hive after a moderate move. It could be that they have two distinct detection methods, one short-range and one long-range. The short-range method would be used normally, while the long-range method might be used to find the hive if it needed to be moved in an emergency (perhaps a fire), where presumably they would move it a good distance. What could this long-range location system be ? Perhaps smell ? In this case, if they were too close to the old hive, it's smell might overpower the scent of the new hive, since it seems more familiar, so they would need to move outside the range where they could smell the old hive to detect the new one. StuRat (talk) 10:14, 4 December 2013 (UTC)

The only interpretation I can think of that seems to make sense is that if you move a hive, with the bees inside(!), to another location nearby, once a bee comes across a place she "knows" she will take the route home she remembers best, which is the one to the old location. That can't happen if you move the hive far enough so the old and new foraging areas don't overlap; the old memories can't override the new route she learned. I guess for small displacements the hive could be found by accident and the bees would take the new route eventually, but the number of bees lost would increase with distance; and likewise when the areas overlap only slightly, you'd only lose the ones who happen to wander into the old territory. Ssscienccce (talk) 12:36, 4 December 2013 (UTC)

I think you're more or less right. See [2] which while not an RS, says something similar. If you move more than a metre but less than 500 metres or so, their homing instinct will bring them back to the old location so they will never find the hive (during previous foraging, they would have learnt the old routes to return). If you move more than 500 metres or whatever, this is further than they ever normally forage so they will I guess learn the new foraging routes (having no old routes as reference) and generally return to the new location. This must be referring to bees inside the hive, bees outside will still I presume return to the old location. Nil Einne (talk) 19:44, 4 December 2013 (UTC)

At the risk of being accused of being pedantic, Red Back spiders are not one of the insects come into contact with. Bazza (talk) 13:09, 4 December 2013 (UTC)

I don't understand that comment, but in any case let me give a pointer to our Redback spider article. Looie496 (talk) 15:46, 4 December 2013 (UTC)

I think despite the extremely confusing indenting and location, Bazza is replying to our resident banned Perth engineer with many names, and pointing out that Red back spiders are arthropods but not generally considered insects. Therefore not one of the insects Australians or anyone else ever come in to contact with, whether on the wrong/right end of fly spray or in some other fashion. Nil Einne (talk) 19:48, 4 December 2013 (UTC)

OP myles325a back live. Well, if Bazza is being a pedant about what constitutes an insect, he has shot himself in the nuts by then expressing this in English that is as meaningless as a spilt bowl of alphabet soup.

I am also a little mystified as to why one of the most impressive answers here is by someone who has been banned for some reason. He SOUNDS sensible, he SOUNDS learned, he SOUNDS like a responsible and proactive member of the Bunyip aristocracy, and yet he is persona non grata. Hmmmm....

But thanks to all respondents. Yes, the answer appears to be along the lines that moving the hive a small distance may lead to the bees following their usual paths and getting lost. Moved 500 metres or so (the article cited suggests) will obviate this problem. Myles325a (talk) 07:47, 5 December 2013 (UTC)

Skin wearing thin during writing

I'm left handed and when I write for an extended period of time, the skin of my middle finger which interfaces with the pen seems to wear thin and it becomes uncomfortable to write. Does this happen to everyone or do most people get calluses? Is it related to left-handedness (I guess I'm pushing the pen rather than pulling it)? It's not really a medical problem so I couldn't justify wasting my (NHS) GP's time but I might ask next time I'm there, not that I'd expect he could prescribe anything or even have anything insightful to say. I remember it bothered me at school but I just assumed everyone was the same. ----Seans Potato Business 12:20, 4 December 2013 (UTC)

My suggestion would be a pen with a soft grip. This can be more comfortable. Also, I think larger diameter pens might be better. Perhaps the larger diameter pen distributes pressure over a larger area of interface between pen and fingers resulting in lower pressure per square unit of contact. Bus stop (talk) 12:32, 4 December 2013 (UTC)

Those would be my suggestions as well. I know the phenomenon, although it was a long time ago. Also, adding some kind of anti-slip material would likely lower the amount of force you're using to hold the pen: This study suggests that a lower-level neural mechanism adapts the force used by each finger to the surface condition under that finger, a sort of anti-slip strategy. When lifting an object by a handle that had a different surface for every digit, each finger applied the amount of pressure and force that assured a stable grip on that specific surface. So it's likely that people unconsciously apply more pressure when holding a pen that is smooth and relatively "slippery". Ssscienccce (talk) 13:55, 4 December 2013 (UTC)

Don't know if there's a medical term for it, people get calluses from writing and blisters from more intensive friction/irritation. I think it's somewhere in between the two, too much friction/rubbing/pressure/irritation for a callus, to little for a blister... Ssscienccce (talk) 14:14, 4 December 2013 (UTC)

As another data point, I'm left-handed, but I've never had this problem - so it's evidently not a universal thing. SteveBaker (talk) 00:30, 5 December 2013 (UTC)

• I used to get a blister on the last knuckle of my middle finger from grade school to freshman year college every september after summer vacation was over. The solution was growing an inelegant callus there by halloween. I still have the ghost of a callus, although I have been typing now since the 80's. (μηδείς (talk) 05:06, 5 December 2013 (UTC))

(No idea who posted the above unsigned) I write with my right hand and also use to get a callus/blister on my middle finger when I was in school. IIRC it was mildly uncomfortable sometimes but it didn't really bother me much, more just something I noticed. Nil Einne (talk) 04:13, 5 December 2013 (UTC)

T'was me. Or t'were I. In any case, the callus was never a problem until one year I tore it. That's godawful. μηδείς (talk) 05:06, 5 December 2013 (UTC)

PS, I was lectured that I got the callus because I held the stylus rubbing against the knuckle, rather than between the knuckle and nail. It all seemed like barely veiled sadism from my elders at the time. μηδείς (talk) 05:10, 5 December 2013 (UTC)

Given I have the ghost of a callus between the knuckle and the nail, it also seems like complete rubbish. I get the same problem as the OP on the few occasions I have to write a large amount by hand now, but it tends to work itself out eventually, and the slight arthritis in my fingers tends to kick in before it gets uncofotable in any single session. I'm right handed. MChesterMC (talk) 09:40, 5 December 2013 (UTC)

BTW, why do you need to write so much by hand ? Can't some of this writing be done on computer ? Are you somebody famous who has to sign a lot of autographs ? StuRat (talk) 07:59, 5 December 2013 (UTC)

I consider my writer's callus a mark of honor! There are plenty of situations other than autographs where people need or want to hand write things, for example I hand-write all my notes in seminars, meetings etc. (then scan them into evernote) since I haven't been able to find a software package or markup language that allows me the flexibility I have with my own notetaking on paper. This involves custom symbols, sometimes invented on the fly, sections of mind-maps interspersed with lists, diagrams and longer sections of prose and extensive use of arrows to link concepts together, with the shape of the arrow and the style of the arrowhead conveying various meanings. There's still plenty of reason to work with pen and paper (besides, a pen never runs out of battery). Equisetum (talk | contributions) 11:40, 5 December 2013 (UTC)

I still occasionally write things by hand, such as keeping score during card games, making a grocery or "To Do" list, etc. It's just doing so much manual writing that it damages the hand which seems odd to me. When I first felt any pain, I'd switch to computer. StuRat (talk) 09:01, 6 December 2013 (UTC)

Well, as a left-hander who used to have a terrible hand position when writing (because I was taught, badly, by right-handers who didn't take the time to understand the fundamental asymmetry between right handed and left handed writing) I can attest to the fact that it doesn't take very much manual writing at all to cause pain - half a page would do it for me, admittedly not because of a wearing down of the skin, but certainly the pressure of the pen on my middle finger and the position of my wrist would both be markedly uncomfortable after that point. This caused problems both with the legibility and speed of my writing, which was a bit of an issue for exams in high school. Fortunately this was entirely correctable (at least in my case) by adopting a relaxed, neutral wrist position and switching to a pen which requires very little pressure to write with. It took my a couple of years to fully change my writing position, but I've had no issues since then. Curiously the thing that helped most was learning to write with a fountain pen, which I was always told was impossible or at least very difficult for a left-hander (this notion is complete bollocks). I also made a mockery of the practice of handwriting analysis, since the change of hand position radically changed my handwriting without, presumably, completely changing my personality (unless the proponents of such analysis have cause and effect mixed up, hmm.. that reminds me of retrophrenology).. Equisetum (talk | contributions) 10:26, 6 December 2013 (UTC)

I'm a PhD student and we have to keep paper logbooks. It's an institute-wide policy and there's no option for using a computer except perhaps if I type everything up and print and stick it into the book, signing the corners. 78.148.106.99 (talk) 10:34, 6 December 2013 (UTC)

For the OP - what sort of pen do you use, and how do you hold your hand when you write? I am also left-handed and found I had much the same problem when I was using a ballpoint pen due to the pressure required to write with one. Once I changed my hand position to be below the line I am writing (to alleviate smudging issues), and switched to using rollerball or fountain pens which require much less pressure, the problem more or less resolved itself (I still have a small residual callus, but it is never painful). Equisetum (talk | contributions) 11:40, 5 December 2013 (UTC)

I use a ballpoint. I'll try a rollerball. 78.148.106.99 (talk) 10:34, 6 December 2013 (UTC)

As you're a PhD student (me too!) one thing to be careful of is the water/alcohol/organic solvent resistance of the ink in a rollerball, and also its longevity, which isn't great in some brands - some institutes actually mandate the use of a ballpoint because of these issues. One good option would be the Uniball Jetstream range [3] , as these have a supposedly very water, solvent and fade resistant pigment ink which was designed to prevent even deliberate erasure, for example in cheque fraud (they even resist acetone!), and are also very smooth flowing and fast drying. They also look quite like a standard ballpoint on the page, so you should be fine claiming they are a ballpoint if necessary. I don't use them myself anymore, as I have fallen in love with the Pilot G-Tec C4, but I did use them for two years during my undergrad and was very happy with them. Remember to consciously loosen your grip and reduce the pressure you apply to the page with them - it can take a bit of getting used to if you only use ballpoints. Hope this helps. Equisetum (talk | contributions) 13:25, 6 December 2013 (UTC)

Huh, apparently it's not just the Jetstream that has that highly resistant pigment ink - take your pick from any of these [4]. Or do as I would, and buy one of each! - I think I once counted that I had 55 different models of pen in my stationary drawer. Equisetum (talk | contributions) 13:34, 6 December 2013 (UTC)

Will light inhibit dark dependent flowering once it's begun?

I have some poinsettias I deadheaded, grew outside over the summer, and rebloomed from last year by bringing them in as the season darkened, around Oct 1. One of them is in "full bloom" (it doesn't have actual flowers yet, but the bracts have changed color richly.

If I bring the poinsettia into an environment where it's on show, but gets low and irregular artificial light when it's dark outside, will this inhibit further blooming? Thanks. μηδείς (talk) 20:58, 4 December 2013 (UTC)

Don't know about poinsettias, but for the orchids in my experience the answer is usually "no, it won't inhibit the blooming". Many orchid species and hybrids initiate a flower spike in response to a shorter day / longer night. Changing the conditions (temperature, humidity) abruptly may cause them to abort the flower spike, but changing the light/dark periods alone will usually has no effect at that point. Some orchids even tend to bloom right after they've been shipped by mail: 2-3 days in a dark box do the trick. --Dr Dima (talk) 22:11, 4 December 2013 (UTC)

Great. I never had noticed any of the flowers abort under long-light conditions. μηδείς (talk) 22:18, 4 December 2013 (UTC)

I agree with Dr Dima, but I'm not sure if I agree with his reasoning (which is essentially that the plant's bloom trajectory is started, and won't easily be knocked off course). Here's my issue: is bract color changing actually hormonally linked to blooming? Conceivably, the bract-turning could respond to one cue, while the true infloresence responds to others. Despite that, I think you'll be fine, esp. since "normal" levels of indoor evening light are essentially worth zero to most plants, in terms of both photosynthesis and photoperiod. Finally, this is exactly how the plants are often sold commerically, showy bracts, few if any true buds. Congrats on your successful mission! SemanticMantis (talk) 17:53, 5 December 2013 (UTC)

Yes, I am going to move the one I have in mind to the front window out of the unused room I have it in now. It will rarely get light after 6pm anyway. It's a white poinsettia that was actually thrown in the trash during the last freezing weather last March. I rescued it and it survived with a stunted growth habit, and now some very nice bracts. I'll have to see if I can upload a picture. μηδείς (talk) 18:30, 5 December 2013 (UTC)

This rings a bell. I'm more interested with preventing my lettuces bolting at the end of season due the the change in day length and ensuring a good harvest of very late cropping tomatoes around Christmas - than flowers (whereas the wife, is more interested in stopping me bolting down the pub as the evenings draw in). Yet yes, even with flowers, temperature and light is everything. Poinsettia Cultural Characteristics . Although it is not quite vegetable, I'd like to know if anybody knows how I can get some haggis to ripen for the new year – about February 2014 would by nice?--Aspro (talk) 22:58, 4 December 2013 (UTC)