Science desk
< October 12	<< Sep \| October \| Nov >>	October 14 >

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.

October 13

The irregular dark spots on sidewalks

What are they and how are they formed? Thanks. Imagine Reason (talk) 00:55, 13 October 2009 (UTC)[reply]

Very very old gum, actually. This is what you're talking about, right? Someguy1221 (talk) 01:15, 13 October 2009 (UTC)[reply]

I thought it might be an urban legend. That's gotta be a lot of gum, though, coz most sidewalk blocks have quite a few of them each! 66.65.140.116 (talk) 19:59, 13 October 2009 (UTC)[reply]

They could also be the imprints of fallen leaves: I've noticed that sometimes (perhaps when the cement of the sidewalk was recently poured) a fallen leaf will stain the sidewalk brown. Sometimes the outline of the leaf is unmistakably clear. (I think there's a sidewalk near me that displays this; perhaps I can take a picture tomorrow.) —Steve Summit (talk) 01:45, 13 October 2009 (UTC)[reply]

Such leaf stains would be caused by tannins - effectively permanently dyeing the concrete. Even after the leaf blows or erodes away, the tannins can remain, staining the sidewalk. Nimur (talk) 04:19, 13 October 2009 (UTC)[reply]

An interesting opposite effect I've seen is bleaching of the sidewalk by acids in pine needles. --Sean 13:42, 13 October 2009 (UTC)[reply]

Does this staining and bleaching occur on cement that's dried, too? 66.65.140.116 (talk) 20:00, 13 October 2009 (UTC)[reply]

To tell whether your sidewalk spots ("pavement" spots for Brits) are from gum or leaves, here are some suggestions: Are they under a tree (then probably leaves) or where people gather, like bus stops, benches, corners, entrances to buildings, etc. (then probably gum). Gum spots tend to be black and relatively smooth at the edges, while leaf stains can be brown or other colors and have rough edges. StuRat (talk) 14:32, 13 October 2009 (UTC)[reply]

volume change in an isothermal process versus an adiabatic process

I'm trying to figure out which is more in magnitude ... and not being very successful at it? I'm using the ideal gas equation. Help! I get as far as -Int[nRT/V dV] = change in U and stuff, but just need some direction here. John Riemann Soong (talk) 06:15, 13 October 2009 (UTC)[reply]

It's going to depend on what you consider to be "equivalent" adiabatic and isothermal processes. Comparing the volume change for an equivalent change in U doesn't make sense since U doesn't change in an isothermal process. We can compare them for an equivalent amount of mechanical work W done. At a given state, dW/dV = nRT/V regardless of what kind of process it is, so the infinitesimal change is the same. If we regard two processes where the low volume states are set equal, the adiabatic one will have to undergo a larger volume change, since it'll be cooler than the isothermal gas, and dW/dV will be lower. If the high volume states are set equal then the opposite is true.

The work formula you have there

W=\int nRT/VdV

is good for the isothermal case since T is constant. For adiabatic, the constant is PV^γ = c, so

W=\int cV^{-\gamma }dV

. Rckrone (talk) 21:36, 13 October 2009 (UTC)[reply]

If you're comparing the change in volume for an equivalent change in pressure, then the isothermal case requires the bigger change. As the volume increases, the adiabatic gas looses internal energy doing work so the pressure drops by more than for the isothermal case. Rckrone (talk) 22:10, 13 October 2009 (UTC)[reply]

symbols of restraint in women's fashion

Hi - I'm interested in the idea that some clothes and accessories worn by women symbolise helplessness and/or restraint. Some examples might be tight skirts, stilettos, chokers - and at the more extreme end of the scale, footbinding and neck rings. I'm also interested in the idea, once ascribed to Catherine MacKinnon, that all sex is rape -- of course this isn't literally true, and is it turns out, it was never made as a serious quote, but I do wonder about the fact that the females of so many mammals, with some exceptions, are statistically smaller and weaker than the males. Are the two things related? Were we more likely to breed if a male was able to run down and restrain a female? Did this result in weaker females being selected for? Is this why we have 'restraint-fashion'?

Have these ideas ever been seriously discussed? If so, I'd appreciate it if someone could let me know who and where.

sorry - I realise these ideas are sort of ugly, but it'd be great if we could discuss them without rancor.

Thanks all,

Adambrowne666 (talk) 06:28, 13 October 2009 (UTC)[reply]

The rules of the RD say that this is not a forum for discussion and if the subject of the question is dry then this rule is usually adhered to; but when sex is the subject of the question the rules go out the door - just watch. Caesar's Daddy (talk) 07:03, 13 October 2009 (UTC)[reply]

Fashions like leggings or mini-skirts are far less restricting than men's trousers for example, and therefore contradict your hypothesis. Currently womens fashion seems to be similar to the fashion that male Cavalier's wore centuries ago (high boots, big belts for example). 92.24.99.195 (talk) 12:10, 13 October 2009 (UTC)[reply]

I find ties to be very restrictive. Heck, they are even named "ties", what could be worse, maybe "chains" ? StuRat (talk) 13:30, 13 October 2009 (UTC)[reply]

In any case, this probably belongs on the Humanities desk since it asks about cultural norms and sociological ideas. Nimur (talk) 13:34, 13 October 2009 (UTC)[reply]

Not the part about natural selection, that belongs right here. StuRat (talk) 13:41, 13 October 2009 (UTC)[reply]

As for the hypothesis that most male animals rape the females, I don't think this matches observations, where most females exhibit considerable choice. Even if the males overpowered the females, the females still sometimes exert their own control by not ovulating or else rejecting (perhaps even eating) the offspring. StuRat (talk) 13:41, 13 October 2009 (UTC)[reply]

A young woman wearing a tight skirt was at the front of the queue when the bus arrived. She struggled to step up but her skirt was too tight. So she reached behind and loosed the zip, and tried again. Still she couldn't reach the step. So she loosened the zip some more to try again. But the impatient man behind her just picked her up bodily and put her in the bus. The woman turn furiously on him with How dare you touch my body when I don't even know you? The man answered I thought we were getting aquainted after you twice unzipped my trousers. Cuddlyable3 (talk) 15:21, 13 October 2009 (UTC) [reply]

I believe the idea was that all sex is rape in a society where woman are marginalized. I don't think Xena would be all that upset by... well... things of this nature.

Also, in The Second Sex, Ms. Beauvoir touches on the idea that all clothing is in fact bondage, a mutilating bridle that chafes the skin and burdens the senses. Vranak (talk) 15:33, 13 October 2009 (UTC)[reply]

I recall reading someplace that the costume designers for the original StarTrek show went to a LOT of trouble to discover what kind of womens' clothing most attracted men. (I suspect I read it in one of Gene Roddenberry's biographies...but I'm not 100% sure.) It turned out that it didn't matter AT ALL how tight/restraining the clothing was - nor (to their surprise) how revealing it was. What attracted men most was how much it appeared that the clothing was likely to fall off at any moment. When you look at the costumes they produced with this in mind - you can see where they were going with that. SteveBaker (talk) 21:33, 13 October 2009 (UTC)[reply]

I don't really believe that, even generally. To me it sounds like something either a feminist or a misogynist would make up, trying to deny or excuse responsibility for human actions. Females choosing who to mate with is commonly observed in many mammal species. A great example I saw recently was lionesses who sneak away from the pride to mate with solitary nomad males, even at risk of physical retribution. This has the obvious biological advantage of mixing the gene pool in groups where the alpha male does the majority of the breeding. Also helps mitigate the impact on the group if the Alpha male just happens to be sterile or have some other genetic abnormality, which could otherwise just about wipe out a whole generation if the alpha is otherwise fit and strong. Vespine (talk) 23:46, 13 October 2009 (UTC)[reply]

Sorry i've kind of missed the point.. To answer the actual question, I think it is the opposite that is true. It isn't "females are weaker because males will mate with them easier" I think it is "males are stronger because females select the strong males to breed with". Vespine (talk) 23:51, 13 October 2009 (UTC)[reply]

And before someone argues with me, I do mean "select" in the broad "natural selection" meaning, not the "females making individual personal choice" meaning. "Strong" males will tend to be fitter, healthier, better able to provide and protect, therefore make a better choice for mate. Females on the other hand are not chosen for strength but for fertility and other maternal qualities. Vespine (talk) 23:56, 13 October 2009 (UTC)[reply]

Yes, very good point, Vespine - I didn't think of that. Adambrowne666 (talk) 22:14, 14 October 2009 (UTC)[reply]

I don't believe it either. Think Muslim garb vs. bikini. Imagine Reason (talk) 04:57, 15 October 2009 (UTC)[reply]

Of course, I'm not saying symbolic restraint is the only factor, ImagineReason - attractiveness is multifactorial - it was just the one I wanted to talk about here, and I certainly would say it figures in a lot of fashion Adambrowne666 (talk) 10:06, 16 October 2009 (UTC)[reply]

The Victorian Crinoline put a steel cage around the lower half of a woman that hampered most activities except dancing. Males can find the sight of such an encumbered woman performerpleasurable. Cuddlyable3 (talk) 13:41, 18 October 2009 (UTC)[reply]

I'm not one of those males, as it happens, but thanks, Cuddlyable, good example. Adambrowne666 (talk) 22:25, 18 October 2009 (UTC)[reply]

Physics

is C= 2F in [lens]? its no, but why? use n/v-1/u= n-1/r if n= 1.5 i.e. glass —Preceding unsigned comment added by Fantasticphysics (talk • contribs) 09:53, 13 October 2009 (UTC)[reply]

The second formula is the thin lens formula. See Lens (optics). Depending what is meant by C, the first formula might be to find C = diameter of a spherical mirror. Cuddlyable3 (talk) 13:38, 13 October 2009 (UTC)[reply]

The sexual desire of post-op transexuals

Because they have lost their testes and do not have ovaries, does this mean they have no or a greatly reduced sexual desire? I recall hearing that they cannot orgasm. Perhaps taking female hormones results in desire, but I've heard that it may be normal for them to stop taking it after a while. 92.24.99.195 (talk) 11:40, 13 October 2009 (UTC)[reply]

I believe they need to take hormones for the rest of their life if they expect to look like their chosen gender. If they stop, they will regress to "something in-between", and it's not a pretty sight. StuRat (talk) 13:26, 13 October 2009 (UTC)[reply]

The OP must mean a male-to-female transexual. The article Eunuch cites Wille, Reinhard & Klaus M. Beier (1989), Castration in Germany. Annals of Sex Research, vol. 2, pp. 103–33 Convicted sex offenders who have been castrated are rare; a lack of testosterone and the consequent ability to better control their own libido does result in negligible recidivism. Cuddlyable3 (talk) 13:30, 13 October 2009 (UTC)[reply]

Does transexuals getting female hormones make a difference in this respect? 92.26.174.34 (talk) 19:38, 13 October 2009 (UTC)[reply]

In the best study I can find, transsexual male-to-females had a 34% incidence of decreased libido, which was more prevalent than in true females (23%) but not statistically significant. Libido is (controversially) thought to correlate with serum testosterone levels, even in women. The transsexuals' testosterone levels were lower than true females on average, but their testosterone levels did not appear to correlate with their libido (as it did in true females). This is according to Elaut E; De Cuypere G; De Sutter P; Gijs L; Van Trotsenburg M; Heylens G; Kaufman JM; Rubens R; T'sjoen G. Hypoactive sexual desire in transsexual women: prevalence and association with testosterone levels. European Journal Of Endocrinology 2008 Mar; Vol. 158 (3), pp. 393-9. - Draeco (talk) 17:03, 14 October 2009 (UTC)[reply]

What is TRAIL PCR?

What is TRAIL PCR? My Google-fu is weak today :/ ----Seans Potato Business 12:52, 13 October 2009 (UTC)[reply]

Without some context, there isn't much we can do... Are TRAIL and PCR useful? --Tango (talk) 13:03, 13 October 2009 (UTC)[reply]

Yes, I think Tango is correct, it is likey a reference to Polymerase Chain Reaction (PCR) of Tumor necrosis factor-Related Apoptosis-Inducing Ligand (TRAIL) encoding DNA. Rockpocket 16:43, 13 October 2009 (UTC)[reply]

And going by this, I think the relevant article is RT-PCR. Franamax (talk) 16:45, 13 October 2009 (UTC)[reply]

Medicare catheters

In the US there are TV ads for catheters that say Medicare formerly only provided 4 a month and now provides 200. Why the huge change ? Can't they be sterilized and reinserted ? This seems to run counter to recent attempts to contain exploding medical costs. Not a request for medical advice as I don't know anyone who uses these. StuRat (talk) 15:45, 13 October 2009 (UTC)[reply]

U.S. Medicare policy change in catheter guidelines Improves patient care in home and hospice setting. Apparently it's a convenience, not a necessity, to have a fresh catheter each time. Nimur (talk) 16:06, 13 October 2009 (UTC)[reply]

Cost reduction based on reduced UTIs. The cost of a UTI involves a doctor visit, lab culture, prescription and follow-up lab culture. hydnjo (talk) 18:23, 13 October 2009 (UTC)[reply]

But why would there be more UTIs ? Aren't people willing and able to sterilize a used cath ? StuRat (talk) 13:22, 18 October 2009 (UTC)[reply]

TV ad?

Sorry to sidetrack, but was it a political ad, or do they seriously advertise catheters on TV in the USA? ("Buy now and get 200 for the price of 150!") AlmostReadytoFly (talk) 17:49, 13 October 2009 (UTC)[reply]

Serious ad, yes sir. But it's not a discount price. There are lots of home-medical-care ads, and one of their selling points exactly is that it's a reimbursable or covered cost. Some companies even advertise that they will help with the medicare/insurance paperwork. "Why not buy up to the covered limit? It's not really costing you and you'll appreciate having them" is the message. DMacks (talk) 18:33, 13 October 2009 (UTC)[reply]

And then what? Sell them on eBay? Stockpile medical supplies just in case you ever need them and the local hospital is BYOC? AlmostReadytoFly (talk) 18:42, 13 October 2009 (UTC)[reply]

Why would you expect health insurance to cover a cost that is not related to a diagnosed actual need? Again, it's not a special sale targetted to the general public buying with their own money. DMacks (talk) 18:49, 13 October 2009 (UTC)[reply]

The ads I have seen were catering to people who already needed urinary catheters for at home medical reasons. The point was that Medicare at that time required these patients to sterilize and reuse catheters many times per month, while the advertised company would gladly sell you a month's supply of use-once and discard sterile catheters. Hence it catered to reducing people's fear of infection and increasing their convenience. A niche market certainly, but not at all crazy. Dragons flight (talk) 19:06, 13 October 2009 (UTC)[reply]

Ah, now I understand. Thanks. AlmostReadytoFly (talk) 19:10, 13 October 2009 (UTC)[reply]

Sorry if I came across too snarky. DMacks (talk) 19:39, 13 October 2009 (UTC)[reply]

Not in the slightest. I was too baffled by the basic concept of advertising "all-you-can-claim" medical equipment on TV to work out what to expect. AlmostReadytoFly (talk) 20:43, 13 October 2009 (UTC)[reply]

Very large object

Some years ago, out of idle curiosity, I did a google search for "largest object in the universe", or a similar phrase. What this turned up was a long page about astronomy that covered many topics. Somewhere in the dense text it told me about the object in question, which was referred to by a codename consisting of letters and numbers. I think "object" here refers to things like stars, rather than things like galaxies. Wading through the technical information, I established that this object was:

Very, very far away
Problematically large
Inexplicably toroidal

Rather than doing any more searches, I just left it at that and went away with the pleasing impression that God is a giant doughnut. Today I want to know more about this object. Do we have a page on it? 81.131.7.241 (talk) 18:34, 13 October 2009 (UTC)[reply]

A quick Google search suggests that a giant blob is a good candidate for "largest thing". It fails your "things like stars rather than galaxies" criterion, but I'll note that "star" is wholly incompatible with what modern astronomy considers "very far away". For example, VY Canis Majoris is the largest known star but it's practically on our doorstep, less than 5000 light years distant. If you prefer an intermediate biggest object, the black hole at the center of quasar OJ287 is 18 billion solar masses -- a galaxy, give or take. — Lomn 19:06, 13 October 2009 (UTC)[reply]

OJ287 is an impressive thing and may have been what I read about. Perhaps at the time it seemed inexplicably doughnut shaped, before it was explained and found not to be. 81.131.7.241 (talk) 19:54, 13 October 2009 (UTC)[reply]

See Dunkin' Donuts. Bus stop (talk) 18:42, 13 October 2009 (UTC)[reply]

If you ignore the "star-like and not galaxy-like" definition, and instead focus on an object being any gravitationally-bound collection of matter, then the Sloan Great Wall, a massive collection of galaxies known as a galactic filament, is the largest known "structure" in the universe. If you want the largest identified star in our galaxy, then VY Canis Majoris is the largest, it is about 2000x as massive as our sun. We don't have the ability to resolve the size in other galaxies, though individual stars have been located and identified in other galaxies, such Cepheid variable stars; however I am not sure if these have been accurately sized. The so-called "blob" referred to above is a type of Lyman-alpha blob, a class of galaxy-sized, unorganized diffuse hydrogen clouds, though these are also probably more "galaxy-like" objects than "star-like". --Jayron32 19:23, 13 October 2009 (UTC)[reply]

I only put the "star not galaxy" bit in because, thinking about it, I couldn't see why it would be surprising for a galaxy to be toroidal, or why even an unusually large galaxy should raise questions about how it came to exist. Maybe it wasn't anything like a star. Oh, I remember that it was known to be toroidal by watching how it obscured other things passing behind it. (Or gravitational lensing, maybe?) So I guess it was a dark object, whatever it was. 81.131.7.241 (talk) 19:38, 13 October 2009 (UTC)[reply]

I would say that the largest known structure in the universe is probabily the CMB cold spot, that is if voids are acceptable... Dauto (talk) 20:06, 13 October 2009 (UTC)[reply]

Whoa. Does anyone know if a specific word/phrase has been coined to describe the feeling of awe combined with the sad realization of the utter insignificance of one's own existence that I just felt when reading about the large-scale structure of the cosmos? --Kurt Shaped Box (talk) 20:31, 13 October 2009 (UTC)[reply]

I've had it. Does that count? 90.208.3.84 (talk) 20:39, 13 October 2009 (UTC)[reply]

I don't know about a name for it, but the Total Perspective Vortex is a device to induce this feeling. Rckrone (talk) 20:42, 13 October 2009 (UTC)[reply]

The sadness at realizing our lack of significance in the universe is called existential depression (what, no article ?). See [1]. StuRat (talk) 15:22, 18 October 2009 (UTC)[reply]

The real problem at the heart of this question is the definition of the word "thing" and the definition of "largest". Assuming we really mean "biggest in volume" rather than "most massive", that helps a bit - and we only have to worry about what a "thing" is. There are many possibilities - distinguished only linguistically - and not scientifically:

If a collection of solid objects like (say) a solar system (comprising of a star, some planets, some moons, many asteroids and comets, a bunch of dust and gas and a heck of a lot of vacuum) is a "thing" - then so (presumably) is a galaxy, a galactic cluster, an the entire universe itself. The largest thing in the universe is the universe. You could argue that this doesn't count - but then you're just down to who can invent a noun describing the largest number of somewhat related galaxies that doesn't include the entire universe? It's a meaningless limit...it's just down to what words you want to use.
But if you don't allow collections of objects to be a 'thing' - then you're down to looking at stars, black holes and such like - but what about dust clouds? Are those 'things'? Or are they collections of little bits of dust? If a dust cloud can be a 'thing' but a galaxy cannot - then they are the biggest things around because they are vastly larger than stars.
If you disallow dust clouds then we really are looking at things like stars and black holes. But strictly speaking, a black hole can't be the largest thing in the universe because it's really a zero-sized singularity - making them the smallest things in the universe - not the largest! You have to include the event horizon - but is that a 'thing'? It's not solid - and in some cases, you could cross one without ever being aware of having done so.
So if you rule out black holes - then you're really down to stars. So what is the largest star? VY Canis Majoris appears to be the winner (out of all of the stars we could reasonably measure). There will probably be a mathematical limit on the biggest possible star anyway: If the star is very dense - and large - then it'll collapse into some sort of neutron star or a black hole or something; If the star is very large but not very dense - then it won't be able to sustain fusion, the mutual gravitation will pull it back together - and it won't be very large anymore. So there must be limits - we could calculate that limit and when we find any star that's that big, we can say that we have at least one of the largest 'things' in the universe.
There are other possibilities: You could argue that nothingness is a "thing". After all, we talk about the Grand Canyon as if it were a thing - when it's really a very, very big lack of rock and dirt! So how about the voids between galaxies? If we were to call the void between galaxies by some cute name "The intergalactic rift" - then the intergalactic rift would easily take the prize.

So - how you define the word "thing" is the entire question here. It's not science - it's linguistics. SteveBaker (talk) 21:13, 13 October 2009 (UTC)[reply]

Back to the original question. You say it had a "code name" — almost certainly, the name followed the standard convention of simply giving the coordinates of the object, preceded by some letters specifying the type of object or the catalogue in which it appears. The largest objects that carry such names are clusters of galaxies. I'll try a shot in the dark: How about the dark matter ring in the cluster of galaxies Cl0024+1654? Not really the largest object in the Universe, not problematically large (and quite possibly not even real), but at least it's toroidal. --Wrongfilter (talk) 22:52, 13 October 2009 (UTC)[reply]

These sorts of questions lead me to strange (poss. nonsensical) ponderings. If the universe is indeed infinite - does that mean that it may contain an object (somewhere) that is of infinite size and infinite mass? --Kurt Shaped Box (talk) 22:59, 13 October 2009 (UTC)[reply]

I don't see any reason why the one would imply the other. The universe is thought to have finite mass as far as I know. If that were not the case, then I guess you could describe the universe itself (or certain subsets) as an object of infinite mass. Rckrone (talk) 03:56, 14 October 2009 (UTC)[reply]

It's back down to linguistics again. What is your definition of an "object"? If an object can be all of the stars (but nothing else) - then an infinite universe contains an infinite object (that being all of the stars). But if you're talking about a 'continuous' solid object like a star - then, no, I don't think so because it would have an infinite gravitational field...and that's something we'd probably have noticed by now! SteveBaker (talk) 13:46, 16 October 2009 (UTC)[reply]

Lake at the bottem of the Ocean

i was watching the Discovery channel and there was a show going on where thay where exploring deep uncarted areas of the ocean. in the exploration thay came apon what looked like a lake at at the bottem of the ocean, and after thay disidended to try and desend into it it was so dence that the substance just rippled and was to dence for them to go into. i was wandering what this lake was and if there are any articles on it. --Talk Shugoːː 20:42, 13 October 2009 (UTC)[reply]

Might it have been a Brine pool? --Kurt Shaped Box (talk) 20:54, 13 October 2009 (UTC)[reply]

that sounds like your right, because thay mentioned alot of sea life serounding it. the pool was a black color aswell. thanks that pool had we bumb founded for a long time. --Talk Shugoːː 17:10, 14 October 2009 (UTC)[reply]

exothermic reaction with iron powder

Hello, In an exothermic reaction involving iron powder, cellulose, water, activated carbon, vermiculite and salt (like in a disposable hand warmer), which ingredient would you change to increase the average/max temperature of the reaction? How much or how little would you have to change it per degree of temp change? paddleballnut (talk) 21:38, 13 October 2009 (UTC)[reply]

Well, question 1 you would have to answer would be to find the balanced chemical equation for this reaction. Without that, you cannot figure out the rest. --Jayron32 02:25, 14 October 2009 (UTC)[reply]