Wikipedia:Reference desk/Archives/Science/2009 November 12

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November 12

Distance

I would like to know what the distance is from the Congo to New Orleans? 00:06, 12 November 2009 (UTC)67.48.17.69 (talk)

According to indo.com, the distance from Kinshasa to New Orleans is 7294 miles. Of course it depends on what point on the Congo you are asking about. Looie496 (talk) 00:33, 12 November 2009 (UTC)

Also see this (unless you mean Congo Square :) hydnjo (talk) 00:56, 12 November 2009 (UTC)

Laser apperance

What causes that strange grainy look that the light produced by most commercial lasers has? Horselover Frost (talk) 00:17, 12 November 2009 (UTC)

Laser light is normally invisible, as it is coherent light that's being aimed somewhere other than your eyeball. If there's dust or fog in the air, the tiny particles of dust or fog scatter the light so that some of it enters your eyeball and you can see it. That grainy look is because the person operating the laser has tossed some dust in the air so that you can see the laser beam. Comet Tuttle (talk) 00:20, 12 November 2009 (UTC)

I should have been more specific. I'm aware laser light in the air is invisible (I'm the one shining the laser). I was referring to the light reflected off a surface. I.e. I take a laser and shine it at a wall, the spot it makes has a grainy appearance, as opposed to, say, shining a flashlight on the same wall. Horselover Frost (talk) 00:25, 12 November 2009 (UTC)

I would guess (and this is just a guess) that it is because the wall is uneven. Different parts of it reflect differently, resulting in the grainy appearance. When you shine a flashlight at a wall each bit of the wall is illuminated from different angles from different bits of the flashlight. That means what you see is an average of how that bit of wall reflects from different angles. That results in a more even appearance. --Tango (talk) 00:40, 12 November 2009 (UTC)

See speckle pattern. TenOfAllTrades(talk) 00:49, 12 November 2009 (UTC)

I have always observed a laser spot striking a surface to look different from a simple focussed light beam of the same size, hue and brightness striking the same surface. The interference of the "speckle pattern" article seems like a plausible explanation. Edison (talk) 00:45, 13 November 2009 (UTC)

A-A bonds are weaker than A-B bonds, which are in turn weaker than B-B bonds

So would a small amount of A be weakly soluble in B, before a separate phase of pure A starts accumulating?

Basically I want to also connect this to the idea of polar covalent bonding versus Van der Waals. Thus, are polar compounds always slightly soluble in nonpolar compounds, but not the other way round? That is, if you had a phase diagram of say, oil and water, you might find an oil+water single phase at maybe 0.1% water by volume, but after that, water starts accumulating as a separate phase. However, oil can't break hydrogen bonding, so oil never dissolves in water, not even at 0.001% levels.

Say you had a eutectic solution of alpha + beta phases, which are solutions of A and B. Could this arise if A-A bonds were weaker than A-B bonds, which were in turn weaker than B-B bonds? John Riemann Soong (talk) 05:43, 12 November 2009 (UTC)

why are eutectic isotherms, well, isoterms?

I don't understand how to get this result from free energy curves... John Riemann Soong (talk) 10:46, 12 November 2009 (UTC)

I don't know this topic, but this seems to address it? Rckrone (talk) 01:47, 13 November 2009 (UTC)

Hollow core conccrete slabs

How are hollow core concrete slabs manufactured? —Preceding unsigned comment added by 113.199.150.180 (talk) 14:26, 12 November 2009 (UTC)

See Hollow-core slab.--Shantavira|^{feed me} 15:38, 12 November 2009 (UTC)

There is no answer of my question. —Preceding unsigned comment added by 113.199.138.211 (talk) 16:53, 12 November 2009 (UTC)

Really? What's wrong with:

Slabs are usually produced in lengths of about 100 meters. The process involves extruding wet concrete along with the steel rods from a moving mold. The continuous slab is then cut according to the lengths required on the construction site. Factory production provides the obvious advantages of reduced time, labor and training.

~ Amory (u • t • c) 17:13, 12 November 2009 (UTC)

(after ec) The article says "Slabs are usually produced in lengths of about 100 meters. The process involves extruding wet concrete along with the steel rods from a moving mold. The continuous slab is then cut according to the lengths required on the construction site." Alternatively, the slabs can be manuafactured with a machine called a slipformer. There is a comparison of the two manufacturing methods here [1]. Gandalf61 (talk) 17:16, 12 November 2009 (UTC)

Social memory

Anybody out there that can give a proper definition of social memory (in psychology)? I've checked Wikipedia, Wiktionary, and about fifteen Psychology glossaries; I've even checked articles with the term social memory as part of the title - yet nowhere a clear definition of what is meant. As far as I understand, social recognition (defined as the process by which an individual recognizes another individual as familiar based on social cues) is part of social memory, but what is social memory?? Lova Falk (talk) 14:54, 12 November 2009 (UTC)

My understanding is that it is the same thing as collective memory. --Mr.98 (talk) 16:06, 12 November 2009 (UTC)

Thank you. However, that may be true in sociology, but it is not in psychology. My question is about a definition of social memory within the individual. This is an example of a short article on social memory. I even checked the article it refers to but even though the authors freely tell that certain mice have intact social memory, whereas others fail to develop social memory, they don't define social memory, apart from an operant definition. Lova Falk (talk) 16:19, 12 November 2009 (UTC)

Please see episodic memory.--Gilisa (talk) 16:57, 12 November 2009 (UTC)

I did, but there is no definition of social memory in episodic memory (or an explanation of what exactly it is.) Lova Falk (talk) 17:12, 12 November 2009 (UTC)

Well, now I may be more helpful after I read the link you provided. I never heard about social memory in psychological context (a side for pseudoscinetific theories such this of Yong)-but in neurobiology reaserch it have a totaly different meaning -in this study they specifically looked at the recollection of social information (and as being done with mice, through smell). It have great meaning to us humans as in different disorders or mental illnesses (such as Autism and Schizophrenia) social and emotional cognition is impaired and so social secens/information (e.g., face expressions) may be stored less efficiently than instrumental information. So, all in all, for the research purposes and to have operative measure, they defined these aspects as "social memory" (and it's a good term, I think)--Gilisa (talk) 17:17, 12 November 2009 (UTC)

• BTW, recent study I'm somewhat familiar with, found higher levels of emapthy in schizophrenic patients who recived oxytocin oraly in a double blind experimental set. So it make perfect sense that they were meaning to memory of social scenes (which are included in episodic memory). --Gilisa (talk) 17:29, 12 November 2009 (UTC)

Thank you!Lova Falk (talk) 19:03, 12 November 2009 (UTC)

The more commonly used term for this is bonding. Looie496 (talk) 19:16, 12 November 2009 (UTC)

No Looie, bonding is not the same as social memory. However, I suppose bonding would be impossible without social memory. Lova Falk (talk) 19:48, 12 November 2009 (UTC)

Indeed, another ongoing study that I'm somewhat familiar with focus on the effects of oxytocin on postpartum depression and many studies have shown that bonding is much dependent on oxytocin. Bonding may be achieved through classical conditioning (in mice cubs, by smell) and social memory as I understand it is more about remembering social hierarchy, acquaintance and etc. Studies in humans have repeatedly found that memory for categories of living things is different from this of items (actually in different brain deficit, different supercategories may be stricken) -memory of items is generally more connected to procedural memory, while memory for living things may have much more implications for the organism and include much more complex demands of detaild memory. For example, returning to mice, there are studies on learned cooperation between a pair of mouse who are locked at the same cage ( with divider between them) and they can get tasty reward only by each of them pressing pedal at the same time with the other-there were also much more social connections between those mice because of the cooperation (it's measured by the amount of subsonic signals they sound)-so this is a classic case for social memory: It was found that entering other mice, instead of one of the two, to the cage ruined the conditioned action of running to the pedal when the light is on: the mice just rememberd who was his original partner and that he was not only an instrument.--Gilisa (talk) 20:21, 12 November 2009 (UTC)

Very interesting! I was actually reading an article on oxytocin when this concept of social memory came up several times, and I wanted to know exactly what was meant. Lova Falk (talk) 21:01, 12 November 2009 (UTC)

Height of a bed

Does it really matter if a mattress lies on the floor or on a bed? Usually it seems the height of the support matters, but bunk beds don't feel different from regular beds. Imagine Reason (talk) 18:21, 12 November 2009 (UTC)

A bed will be sprung (with flexible wooden slats, or something), the floor isn't. That means a mattress on a bed will be softer than a mattress on the floor. The difference is minimal, though, I'd say. Most of the difference is psychological, I expect. --Tango (talk) 18:39, 12 November 2009 (UTC)

It really depends on the mattress, in my experience. I've been on some mattresses that don't seem to let anything through, but I've definitely had my fair shares of sore backs from a mattress on the floor. As a child I never really new the difference until I lost the bunk bed and got an amazing new spring/mattress. ~ Amory (u • t • c) 18:48, 12 November 2009 (UTC)

A bed is important for airing the mattress - and the mattress needs airing to get rid of surplus warmth, and most importantly, for not getting humid. A mattress without a bed or without good airing can start to mould. Lova Falk (talk) 19:08, 12 November 2009 (UTC)

You'll also notice that that are a wider-range of different bed-types, some higher than others, some lower. I would suspect that a huge amount of 'choice' in terms of 'beds' is style/look. I like beds with frames, some people like Divan beds with nothing, some like them with a Headboard, some like Four poster bed and so on. Ultimately whilst there may be reasons of hygiene as per Lova's comment, a large part of it is personal preference. ny156uk (talk) 19:18, 12 November 2009 (UTC)

A bed also gives you a decent amount of extra storage space, which I at least have always found useful. Algebraist 23:27, 12 November 2009 (UTC)

Having a bed at the proper height can minimize the work you have to do lifting or lowering your center of mass while getting in and out of bed, which can be important feeble people. Also beds make sex easier. -Craig Pemberton 01:16, 13 November 2009 (UTC)

"Also beds make sex easier." Yes but at what height? --Jmeden2000 (talk) 17:31, 13 November 2009 (UTC)

As high as they come. Cuddlyable3 (talk) 22:15, 13 November 2009 (UTC)

Emission spectra (II)

Thanks for your help before. Based on the CLOUDY link, I found this which indicates calculation of H and He is possible. Indeed, I then discovered this covering C, N and O. Two points here, firstly it uses the "Case B" exception (a few small assumptions, given usuable range) and secondly it uses "Recombination-line" spectra, which according to this is a type of emission spectra (OK). Can I just confirm that this is, given those points, a calculation of emission line intensities? Secondly, is there any way for me to interpret the data here [2]? Presumably one set of axes (rows/columns) demonstrates the wavelength/frequency and the other some other condition of the emission. If anyone could identify these two things are, I'd appreciate it. Thanks. - Jarry1250 ^{[Humorous? Discuss.]} 19:10, 12 November 2009 (UTC)

question on strength

I am interested if a pipe is equally strong in both compression and tension. For example, If I take a standard straight piece of pipe maybe PVC or something that is part of a much larger system, and can have an internal pressure down to -5 psig before it ruptures, does that mean that it could also hold an internal pressure of 5 psig before rupturing? (For simplicity, ignore everything except that straight piece of pipe. Things like welds or whatever you use to join PVC to other pipe sections, etc). Would the same be true of a metal pipe? Googlemeister (talk) 20:19, 12 November 2009 (UTC)

It doesn't seem obvious that they should be identical...some materials are much stronger in compression than tension - I doubt PVC is is a rare example of something that has precisely the same tensile as compressive strength. Metals certainly don't have that property. I can't imagine any reason for the two to be the same - so unless your pipe is made of some rare, exotic material - then it'll rupture at very different positive and negative pressures. SteveBaker (talk) 20:53, 12 November 2009 (UTC)

Completely different for geometrical reasons as well. But I guess as the diameter of the pipe becomes very large they must converge? --BozMo talk 22:14, 12 November 2009 (UTC)

Why would they converge when it gets large? Think of a balloon - it's strong in tension (pressure), but has zero strength in compression. And conversely you can have something strong under vacuum, but would blow apart under pressure. For example split a pipe lengthwise, and put some rubber (but not glue) on the seam. Now put a vacuum on it - the two sections will be drawn together, and be strong, but would instantly fail under pressure. Ariel. (talk)

I meant the geometrical effect. Anyway, perhaps you are right and when there is a linear failure mode like the one you suggest you are trivially right (I think pipes normally failure in a nonlinear mode via bulging but it must depend on the wall thickness and on the material). I thought Steve was possibly confusing the state of the material (tension or compression) versus the state of gas, or he read the OP as standing on a pipe rather than inflating it? Anyway a flat thin wall fails under pressure by bulging the same amount either way whether the overpressure is left or right. --BozMo talk 23:07, 12 November 2009 (UTC)

Just to nitpick a little, if you had a flat infinite wall with different pressure on either side there would be no stress on it at all. Every part of the wall would be subject to the same force and it would simply accelerate in that direction. It's only when some curvature is introduced that there's stress, since now the forces on various parts of the surface are pointed in different directions. Various parts have relative forces pushing them together or pulling them apart. Rckrone (talk) 06:04, 13 November 2009 (UTC)

Ok, then an anchored flat wall if you must. Which of course when flat is a solution to the stress equations but nonlinearnly unstable if the pressure diff is enough. --BozMo talk 06:41, 13 November 2009 (UTC)

To the OP, pipes are given ratings for vacuum, so you should be able to check. The maximum negative pressure you can have (on the surface of the earth) is about -15psi so it's not hard to find a strong enough pipe. Ariel. (talk) 22:38, 12 November 2009 (UTC)

Whereas of course gas pipes under deep ocean have massive crushing pressure on them. Can you tell us more about what pipe what pressure? I know you are into ships are you after underwater air supply?--BozMo talk 23:16, 12 November 2009 (UTC)

This is not for a practical application. I am just trying to know whether a standard pipe would be more susceptible to failure due to pressure in which direction, overpressure on inside, or overpressure on outside. Googlemeister (talk) 14:31, 13 November 2009 (UTC)

A water. gas, steam, or oil pipe would normally only have to withstand atmospheric pressure of 65.5 newtons (14.7 lb), even with the largest possible vacuum inside, but pipes commonly carry far higher water, oil, or steam pressure inside pressing outwards. Our Water pipe article discusses pressure pipes having thicker walls, but does not answer this question. Sounds like a question for a mechanical engineer, mechanics of materials expert, boilermaker or submarine expert. Sewer pipe is designed to resist external loading. This ref is nearly illegible, but discusses PVC, ductile iron and high density polyethylene as "flexible pipe" which can deflect without failing, compared to clay or concrete which are rigid and fail if they flex. The flexible pipes get an internal and external pressure rating. I can't figure out from the table if they are the same or different. I found a 1913 pipe reference [3] which listed experimental results and empirical formulas for the strength of then used steel pipe subjected to bursting or collapsing forces, but my eyes glazed over reading it before I found comparable pressures for identical geometries, and the results might not apply to modern pipe (and certainly not to PVC). Edison (talk) 21:12, 13 November 2009 (UTC)

Found a modern ref A312 stainless steel pipe. In every case, across diameters and schedules (pressure ratings), the Bursting (internal)pressure was much greater than the collapsing (external) pressure. As the diameters get larger, the ratio of bursting pressure to collapsing pressure gets even larger. Edison (talk) 21:39, 13 November 2009 (UTC)

An online forum I found has engineers sniping at each other in as funny a way as any Wikipedia WP:AFD squabble or edit war, about how to calculate the crush depth of 20" PVC with air pressure inside submerged in water. Their calculations run over a 10 to one range from each other. If the pipe is out of round, it will fail at a much lower external pressure than if it is perfectly round. If it sags between supports, that will hasten collapse. No comparison of burst.collapse pressure for PVC there. Edison (talk) 21:53, 13 November 2009 (UTC)

Farsightness

is Farsightness done by any vision acuity like 20/x or what way is farsightness measur by. Is 20/5 vision a farsightness. Could people be 20/1 vision?--209.129.85.4 (talk) 21:11, 12 November 2009 (UTC)

The definition of "20/x" allows it, and 20/10 has a valid interpretation. And, I've seen one or two (but not all) charts which have 20/15 and 20/10 lines.

"Better" than 20/10 would be a pretty significant distortion of the lens, though. --DaHorsesMouth (talk) 21:45, 12 November 2009 (UTC)

No, 20/x for x<20 means you have better visual acuity than a typical person (my visual acuity is 6/5, or about 20/16, that means that I can see things clearly at 6 metres away that a typical person would need to be no more than 5 metres away to see clearly). That doesn't correspond to farsightedness. Farsightedness will reduce visual acuity, just as nearsightedness does. However, they reduce visual acuity at different distances. What people usually mean by the "20/x" notation is distant visual acuity, but the same notation is used for near visual acuity (although you don't actually measure at 6 metres, you measure at 40cm, so some reason they still call nominal vision "6/6" or "20/20" and scale the numbers appropriately). My optician doesn't write anything down under "near visual acuity" on my prescription, so I guess they only fill it in if you are farsighted. --Tango (talk) 21:58, 12 November 2009 (UTC)

The "far" in farsighted does not mean you can see far away. It means that the lens on the eye focuses too far away from the lens - overshooting the retina. A nearsighted person's lens focuses too near the lens. Regardless of how far away something is that a person is trying to see, not having the lens focus on the retina makes objects appear unfocused. See hyperopia for more info and a clear graphic showing how this is a focusing problem. -- kainaw™ 01:48, 13 November 2009 (UTC)

Kainaw is correct. Farsighted people do not have "better" distance vision (in general) than regular people. Rather, they have worse close vision. Axl ¤ [Talk] 07:32, 13 November 2009 (UTC)

Color vision

Do everyone's color vision work the same or some people's color vision could work differntly? Is this possible people with bad blue sensor can see pink as another color, could some people's human eye be less sensitive for another. Because from my personal observance, when I look at green object for a long time I still able to see green just fine, and white stays white, I see nothing differnce. Is this possible some people when they go to visit planets see colors exactly same as they do on earth, this is what I think I would, or this works for some humans but not the others? —Preceding unsigned comment added by 209.129.85.4 (talk) 21:14, 12 November 2009 (UTC)

is color blindness measur by any numbers like 30 or 100 degres. If people is yellow blind could they see blue as green? Could alot of people have color blindness?--209.129.85.4 (talk) 21:17, 12 November 2009 (UTC)

Have you read color blindness? --Tango (talk) 22:00, 12 November 2009 (UTC)

Not all of us perceive colour the same in the red/orange range,[4] and some women may perceive more colour gradations due to having more than one type of red/green cones; the same thing causes colour blindness in men, because it is X-linked.[5] Fences&Windows 00:51, 13 November 2009 (UTC)

Tetrachromacy is also pretty amazing stuff. SteveBaker (talk) 03:10, 13 November 2009 (UTC)

Does anyone know which materials are used for shielding against infrasound and ultrasound?

Does anyone know which materials are used for shielding against infrasound and ultrasound? Wikipedia articles about sonic weaponry, ultrasound and infrasound unfortunately doesn't say anything about it.

Very grateful for your help

TimmoJTimbboy (talk) 21:30, 12 November 2009 (UTC)

I guess it is the same kind of stuff as is used for other sound proofing. --Tango (talk) 22:01, 12 November 2009 (UTC)

AFAIK Tango is right but it depends how far into fantasy you get. If you mean shielding from sonic weaponry you either need something incredibly strong and rigid (?Chobham armour) or for ultrasound through shock waves something which is flexible (wood, like for construction to survive earthquakes). Generally it needs to be strong in both compression and tension (so steel not brick). --BozMo talk 22:08, 12 November 2009 (UTC)

In case of infrasound, I'm looking for a material, that would be rather easily obtainable, not like Chobham armour. And in the article about sonic weaponry they say about infrasound that armor and concrete walls and other common building (does it include steel, isn't it also a common building material?) materials allow sound waves to pass through, providing little defense.

Thank you!

TimmoJTimbboy (talk) 11:44, 13 November 2009 (UTC)

Well it is a little more complicated than that. Low frequency sound is conventionally considered pretty harmless and the 7Hz referred to in the only reference for the sonic weapon comments is too low frequency to be considered as sound by most people. It will transmit through solid building materials (e.g. as ground shake), but almost all of it is reflected at the interface between air and a solid brick wall for example so it is not easy to fire at a building through air. There have been some studies done on crowd control and low frequency sound though because they can disturb your gut (especially if its full of beer or similar). Can you say anything more about the situation you are considering? Do you live near a railway line and dislike the shake or something? If so little of it will be going through the air. --BozMo talk 12:40, 13 November 2009 (UTC)

I'm living next to a football station, and there has been a couple football riots here, I heard from some locals that the police is planning to use infrasound for crowd control (although I can't be sure if it's true) if something like this should happen again. So, most of the infrasound will be easily reflected by a simple brick? (hope I understood correctly, sorry, I'm not that strong in physics). But why most of the infrasound is reflected and only why only some of it is passing through? Thank you for your help!JTimbboy (talk) 19:00, 14 November 2009 (UTC)

How do we program ourselves?

This question just popped up in my head when I was having a discussion with a friend. When we make a rule for ourselves (let's say, I will turn my head as soon as possible towards whatever direction I hear a sound from), how does this rule translate into an action? There are several things involved here: First I am putting this rule into the brain in some kind of neural language (whatever way the brain understands them), putting in these programs our auditory action, then turning this data into locomotion of our neck. Of course these programs can be made arbitrarily complex (am I right here?). In some cases, the execution will be slower than in other cases. But my question is how this rule is programmed and executed. I know next to nothing about our neural systems. Can somebody explain this taking the above example of rule or another of your own choice. Thanks - DSachan (talk) 22:34, 12 November 2009 (UTC)

For a start look on this then this and finally this. Also, reflexive shifts of visual, but also auditive attention are being mediated by the Magnocellular system.--Gilisa (talk) 22:57, 12 November 2009 (UTC)

The "rule" described by the OP is a function of the Autonomic nervous system that uses the brain structures of the Limbic system which can be called the "reptilian" part of the brain because of its early evolution. One can vary one's sensitivity to sound location mentally either by concentration or distraction. However nobody has to program themselves to be able to do what every living thing that has a brain, ears and a neck is born equipped to do. Cuddlyable3 (talk) 00:34, 13 November 2009 (UTC)

The particular example is unfortunate because it's something we don't need a plan for (I prefer the word "plan" to "rule"), but lets assume a better example could have been chosen, such as immediately striking at a fly if you spot one. It's a really interesting question, and our understanding of the answer is pretty primitive. It's clear that the formulation of plans heavily involves the prefrontal cortex, and that the execution of a plan involves some sort of triggering in the basal ganglia, but it's hard to get much more specific than that. Looie496 (talk) 05:59, 13 November 2009 (UTC)

As I understand it, he was refering to excution of automatic "plans" (e.g., turning your head when someone tap your shoulder) and the not kind of plans that involve with the prefrontal cortex.--Gilisa (talk) 07:53, 13 November 2009 (UTC)

We can give ourselves verbal instructions. These can be the "executive function" which establishes a task "set."[6] I had to climb a tall ladder once to put a chimney cap on a chimney far above the ground. I instructed myself not to jump back if some animal like a raccoon came out of the chimney when I was 30 feet above the ground, since the consequences of a long fall would be worse than getting scratched or bitten by a frightened animal. "Set" is our preparation to selectively perceive something. Perceptual set influences how we perceive ambiguous stimuli, in a hypothetical top down processing system [7]. Some possible interpretations are preferentially activated compared to others. In selecting motor responses (don't jump off the ledge of the building if a pigeon startles or a bee comes at you, shoot if a pheasant swarms up from the ground, run from first base to second if the batter hits the ball) internal instructions might selectively activate some motor responses. Your internally verbalized instructions might have the same priming effect as someone else's statements. For instance if they say "Watch out for rattlesnakes in this tall grass," and someone happens to catch their foot on a vine which makes the grass suddenly move in front of you, you are more likely to jump than if there were no such instruction. You perceptions and response were primed to perceive and avoid snakes. Edison (talk) 14:39, 13 November 2009 (UTC)