Wikipedia:Reference desk/Archives/Science/2013 March 9

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March 9

Why do ceiling and walls get darker except for the corners

My room used to be white but darkened because of smoke, etc. Except for the very corners that seem to be unaffected. A professional painter tells me this is very common. How would smoke even know it's in a corner? Joepnl (talk) 00:00, 9 March 2013 (UTC)

Just a WAG, but my guess is that it has something to do with the way that air currents distribute the smoke around the house; the corners may present perturbations to air flow that prevent as much smoke from getting into the corners as along the surface. --Jayron32 00:05, 9 March 2013 (UTC)

In that case I'd expect a big white spot in the "very 3D-corners", where there are three angles at the same time giving reason to the smoke to not stick to the wall. The white stripe seeminglingy has the same width everywhere. I was thinking that it might have to do with the way the paint was applied (using a paint roller for the large areas and a brush for the corners), but the white stripe also appears next to lights that were installed after the painting was done. Joepnl (talk) 01:19, 9 March 2013 (UTC)

Doesn't laminar flow decrease exponentially with proximity to a surface? Wouldn't that imply airflow in room corners is minimal, and that wsmoke would more likely escape a room before getting into its extreme corners? I am reminded of H. P. Lovecraft. μηδείς (talk) 02:03, 9 March 2013 (UTC)

It's an effect called ambient occlusion. SteveBaker (talk) 05:00, 9 March 2013 (UTC)

An optical illusion? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:29, 9 March 2013 (UTC)

No, neither of these. I've observed exactly the same effect and it is real and independent of paint and light (it happens to wallpaper, too). Jayron's and Medeis' answer of airflow must be correct one, though there will sometimes be a bit of turbulence. I'm not sure how airflow behaves at the intersection of room right-angles, but presumably one could make some deductions from the lighter areas. The pattern might depend on the temperature difference between the room air and the walls, thus creating constant convection currents unless the walls are really well insulated. Dbfirs 15:50, 9 March 2013 (UTC)

• This phenomena is certainly not Ambient occlusion. As the professional painter informed Joepnl, it is is very common. It becomes apparent in a room by the temperature differential on the surfaces. If Joepnl looks around s/he will notice it is less worse in the outer corners of the house which loss the least heat if the roof if the roof is uninsulated but worst if it is. A quick, off the top of my head, physics explanation, is that the colder places, the Brownian motion of the dust particles, loose their energy fastest here and thus are more likely to get stuck on those surfaces (the surface may also change its electrostatic charge in the process – in comparison to the rest of the room- leading to even more dirt to be deposited). It is no optical illusion or anything like that. As Joepnl can problem come back and testify – it is a very noticeable and up-close inspection discoloration. Particulate are more likely to stick to colder surfaces -period. No mystery.--Aspro (talk) 21:45, 10 March 2013 (UTC)--Aspro (talk) 21:45, 10 March 2013 (UTC)

I can't vouch for this unequivocally, as I am not in the mood to think it trough properly (unless someone wants to pay me loads of dollars). Yet this appears to support my thinking as well. www.google.com/url?q=http://www.nbslgt.com/designbuild/pdf/18Ghosting.pdf&sa=U&ei=dfc8UZrYN8vA7Ab814CgCw&ved=0CBgQFjAA&usg=AFQjCNEbn0bL19bdERgriI-4vrmsAxhv7A

--Aspro (talk) 22:17, 10 March 2013 (UTC)

There is no apparent difference between the outer wall (on the right on the picture) and an inner wall (to the left) which I would expect if it has to do with a temperature difference. (It's not due to removing spider webs either, it's the same at the top and bottom). But Brownian motion might still be the solution. If the particles were huge (like tennis balls) and shot them randomly at the wall they wouldn't even be able to reach to corners. Making them gradually smaller still would make it harder to get to the exact corner. All the way to the smallest particle. But then again, I thought Brownian movement would only be visible under a microscope, but the stripes are about a centimeter in width. Joepnl (talk) 23:30, 12 March 2013 (UTC)

• Indeed, it is not ambient occlusion. As our images at Cornell box or Radiosity (computer graphics) show, the way the light falls tends to make edges and corners appear darker, as it is more difficult for light to bounce into them off a wall. But the contributor here is observing that actually the edges and corners are appearing lighter, with less dirt apparently settling there. Jheald (talk) 23:32, 10 March 2013 (UTC)

science writing

Why is science writing usually done in the third person, passive, past tense? — Preceding unsigned comment added by 99.146.124.35 (talk) 02:52, 9 March 2013 (UTC)

Because it's always done that way. I'm serious. That's what my high school science teachers told me almost 50 years ago. Oh, there's also the fact that we lost marks if we did it any other way. HiLo48 (talk) 02:59, 9 March 2013 (UTC)

Well, there is actually a reason: to maximize objectivity. First-person writing brings ego into the picture, and when ego comes in, objectivity goes out. Looie496 (talk) 03:10, 9 March 2013 (UTC)

My impression is that most papers are written in the present tense, first person plural (even if there's only one author). Popular books about science are written in the past tense insofar as they're histories, in the third person insofar as the author wasn't involved, and in the first person insofar as he/she was involved. I think the passive voice is used mainly where the subject would otherwise be "scientists in our (sub)discipline" and it would be silly to keep repeating it. You do often see "our results are summarized below" instead of "we summarize our results below", but there's nothing wrote with that. -- BenRG (talk) 04:58, 9 March 2013 (UTC)

This does actually vary a little. A friend told me in psychology, the first person is becoming more common, even (I think) normal or expected in some journals. This is true of qualitative and quantitative research. I do not mean to suggest applies across the board in psychology, and it was just what one psych PhD told me. In IT I was told to use the third person, but the reason given was just because my supervisor had seen things done that way himself, not because he had any ideology about it. I think those that have strong feelings get their opinions heard, and others play along. In science, as might be suggested by Looie's post, more people are strongly against the "I" than strongly in favour. I'm in favour, but wouldn't make a drama over it. Also, past tense for your methods section (this is what you did) and present for the conclusions (where the conclusions apply to a general case, rather than reasoning about specific factors in the experiment). So "Method: 33 subjects were recruited....Discussion: It seems from the data that some of the results may have been confounded by the subjects talking on mobile phones and drinking too much beer ... even so, it seems (present tense) that safety lessons improve driver behaviour... " etc IBE (talk) 02:04, 10 March 2013 (UTC)

About 1970 my high school biology teacher said to write up the lab report in the "third person". He meant "passive voice". So I wrote things like "he made the incision in the frog". "She saw the intestines." To address the original question, I think that it is because it is more about the object and procedures than the person doing them. Bubba73 ^{You talkin' to me?} 04:23, 10 March 2013 (UTC)

Papers in the natural sciences usually have a number of authors. Using "I" would be weird: which of the authors is speaking. And using "we" would also jar, because it was actually just one person minding the experiment in the lab, another one crunching the results on the computer, etc. In the social sciences and humanities most papers have one or two authors, so it sounds more natural to write "I" or "we". Itsmejudith (talk) 10:00, 10 March 2013 (UTC)

Journals do ask authors to avoid this old habit of using the third person, see this style guide, section 9 on page 14 and 15. Count Iblis (talk) 23:58, 10 March 2013 (UTC)

But (2) in that section says "The passive voice is often the most natural way to give prominence to the essential facts." Active voice is almost nonexistent in wikipedia. Bubba73 ^{You talkin' to me?} 02:06, 11 March 2013 (UTC)

Glomerular Pore Radius

Hello. Given the filtration coefficient, how can one find the pore radius in order to calculate the "pore surface area to pore length" ratio? I am referring to the formula: $K_{f}={\frac {r_{0}^{2}S}{8\mu l}}$ where $r_{0}$ is the pore radius and ${\frac {S}{l}}$ is the ratio. A link to a journal article would be appreciated. Thanks in advance. --Mayfare (talk) 03:28, 9 March 2013 (UTC)

Eggs and potatoes

Eggs and meat cook from the outside in, but potatoes and root vegetables cook evenly (if that's the right way to put it). I suppose that it has something to do with the fact that eggs and meat contain more protein, but what is the reason for that difference? Sjö (talk) 08:27, 9 March 2013 (UTC)

There might be a small difference in thermal conductivity as you suggest, but the main difference is that potatoes and root vegetables are normally cooked for much longer at only 100C, so the difference between inside and outside is not really significant because the whole object has been at a constant temperature for a long time. Eggs are normally cooked for a short time so delays in heat flow are noticeable, with the yolk never reaching 100C in some cases. Meat is normally cooked for longer, but at a higher temperature, and not covered in water, so low thermal conductivity and restricted heat flow becomes very important. Those who like their meat "rare" never allow the inside to reach 100C, even if the outside has reached several hundred degrees. If you try grilling potatoes and root vegetables, you will see that they also cook from the outside in. Dbfirs 09:44, 9 March 2013 (UTC)

I can assure you that potatoes and root vegetables do indeed cook from the outside in, and I really have no idea where you got the notion that the inside of a potato cooks at the same rate as the outside. Do you have any references to that effect? --TammyMoet (talk) 10:53, 9 March 2013 (UTC)

As Dbfirs notes above, while they do cook from the outside in, the difference in cooking time between the outside and inside is small, relative to the overall cooking time. I've cooked potatoes, carrots, etc., for hours, while that would ruin eggs or meat, in many cases. In the case of exceptions, like a pot roast, where meat is cooked for a long time, you don't find a noticeable difference in how well the inside and outside are cooked. StuRat (talk) 15:46, 9 March 2013 (UTC)

... and as Tammy mentioned, if you try boiling a large potato for less than five minutes, you will see that the inside is still starchy and uncooked. Dbfirs 15:57, 9 March 2013 (UTC)

Does vitamin D insufficiency known as a potential Obesity cause?

My Nutritionist told me that there may be a connection. my endocrinologist told me: "take at least 10 drops !!! (off 400 IEA)"). is there a connection between being with bmi like 29 (near obese) to this vitamin? thanks Ben-Natan (talk) 08:49, 9 March 2013 (UTC)

That seems unlikely. New Zealand is pretty close to the ozone hole, and apparently we're the sixth fatest country in the world. Plasmic Physics (talk) 10:38, 9 March 2013 (UTC)

Vitamin D is involved in many more process than bone health. E.g. without enough vitamin D, your muscles will have a more difficult time accessing energy, see here:

"Conclusions: Cholecalciferol therapy augments muscle mitochondrial maximal oxidative phosphorylation after exercise in symptomatic, vitamin D-deficient individuals. This finding suggests that changes in mitochondrial oxidative phosphorylation in skeletal muscle could at least be partly responsible for the fatigue experienced by these patients. For the first time, we demonstrate a link between vitamin D and the mitochondria in human skeletal muscle."

Count Iblis (talk) 13:48, 9 March 2013 (UTC)

Also note that just about any vitamin or mineral deficiency may be linked with obesity, in the Western world, as the most frequent cause of such deficiencies (where food is plentiful) is solely eating junk food, which also causes obesity.

In the case of a vitamin D deficiency, since this vitamin is produced by exposure to sunlight, as well as obtained from nutritional sources, a lack of D also implies a lack of time spent outside, which can mean a lack of exercise, which is also linked to obesity.

So, a vitamin D deficiency may or may not cause obesity, but may also indicate an unhealthy lifestyle which is associated with obesity. StuRat (talk) 16:34, 9 March 2013 (UTC)

The likelihood of finding vit D deficiency in obese adolescents and young adults is >90%. The association of vit D deficiency and obesity is strong. However association does not prove causation, and there not yet strong evidence that vit D deficiency actually causes obesity. alteripse (talk) 16:45, 9 March 2013 (UTC)

Both StuRat and alteripse need to provide some sources, especially as they are in disagreement with each other. Bielle (talk) 17:09, 9 March 2013 (UTC)

Read our posts again. We're both saying that, while there's a correlation, that doesn't necessarily means it's the cause. StuRat (talk) 17:12, 9 March 2013 (UTC)

No disagreement. Just emphasis that association does not prove direct causation. alteripse (talk) 17:24, 9 March 2013 (UTC)

Exactly, we're fat, and we don't have a D deficiency, if anything, have a D surplus. Plasmic Physics (talk) 22:44, 9 March 2013 (UTC)

Unless you take something of the order of 5,000 IU/day of vitamin D or if you happen to live in some African tribe in the wild, you are vitamin D deficient, if not severely vitamin D deficient. See here for some actual measuremnts of vitamin D levels of people who live in the way we all lived until about 10,000 years ago. Count Iblis (talk) 23:33, 9 March 2013 (UTC)

OK, so we are deficient, but certainly less so than most other areas. Plasmic Physics (talk) 23:41, 9 March 2013 (UTC)

Actually Count Iblis frequent claims about vitamin D deficiency are usually lacking in good sourcing compliant with WP:MEDRS standards. In this particular case, the very source they linked to says that the measurements do not establish a vitamin D deficiency for people living a more modern lifestyle:

Whether this concentration is optimal under the conditions of the current Western lifestyle is uncertain, and should as a possible target be investigated with concomitant appreciation of other important factors in Ca homeostasis that we have changed since the agricultural revolution.

unless I guess you use a rather odd definition of deficiency where someone with an optimal concentration of vitamin D is 'deficient'

Nil Einne (talk) 18:58, 11 March 2013 (UTC)

You have to see this in the context of the current discussion in the scientific community. A number of scientists in the field have argued that there is no rigorous proof that for optimal health you need to be above 50 nmol/l. The Institute of Medicine has argued this way in their latest report from 2010. Then, this does has the effect of setting the bar a lot higher for claims that you need to have higher levels for optimal health, despite many other scientists disagreeing with some of the basic issues here. They will argue that it is foolish to just assume the null hypothesis that that 50 nmol/l is ok. (because this is what indoor living Westerners can easily achieve) while demanding that this has to be proven wrong in randomly controlled trials where you e.g. show that higher levels significantly reduce the risk of heart disease, cancer etc.

Vitamin D is not some manmade drug designed to cure some specific disease, so you don't know a priori what to look for in a randomly controlled trial. It may e.g. have benefits in an indirect way, e.g. there is some evidence that higher levels help with recuperation from exercise. Then, if this is indeed the case, it would be beneficial for people to supplement to higher levels, because that may help them to stick to some exercise program, stay fit and lower the risk of heart disease. However, it is then possible that there are no benefits for the heart due to the higher calcidiol levels at all, so in an RCT you could well see nothing at all. This (potential) irrelevance of RCTs has been argued about in the literature a lot, but vitamin D is still treated more like a drug than a nutrient where we typically don't impose this standard.

Even within the well established issue of bone health, there is a lot of controversy about the 50 nmol/l level, see e.g. here, note also that the endocrine society advices people to have levels between 100 nmol/l and 150 nmol/l, because they do find sufficient evidence of benefits beyond bone health. Count Iblis (talk) 23:25, 11 March 2013 (UTC)

You're basically making my point for me. You frequently present one view as the correct one without making it clear it's much disputed and even when you do provide sourcing such as here, you're frequently relying on a very selective reading of the sources. Note that I've never taken sides on whether or not people are commonly deficient OR the recommendations you present are correct or the current common recommendation is correct. That's beside the point in a lot of the questions you reply to. Ultimately what we need good scientific analysis and sources that provide that, not personal POVs or half baked theories based on what people 10k years ago got or overtly simplistic analysis based on one possible benefit ignoring the plenty of possible conflicting factors at play both positive and negative. Nil Einne (talk) 16:23, 12 March 2013 (UTC)

what is Nec?

in many scientific words like Necrosis, Adiponectin, and also in such as Necromancer there is the morpheme Nec. what is the meaning? Ben-Natan (talk) 10:15, 9 March 2013 (UTC)

It comes from the Greek prefix necro, meaning death. Don't know whether Adiponectin uses that particular meaning though. --TammyMoet (talk) 10:46, 9 March 2013 (UTC)

The etymology of the nect element in adiponectin is from Latin nectere meaning to connect or to bind. SpinningSpark 11:47, 9 March 2013 (UTC)

And to avoid using the word to define the word, "connect" means "to bind together".[1] ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:29, 10 March 2013 (UTC)

How can we predict properties of an element and its compounds when that element is not available or too unstable to experiment?

One early example of those might be Mendeleev's prediction for germanium and its compounds, which includes some densities and melting points. How are those kinds of predictions made? Just simple linear regression, and in some cases by theoratically calculating the Van der Waals Force? Do we know how accurate it can be? Can we apply such methods to other elements and their compounds? Is it original research to make such predictions?--Inspector (talk) 10:10, 9 March 2013 (UTC)

For putative properties that we don't measure but derive, this is the realm of disciplines such as computational chemistry. The modern methods are more rigorous (as we have more data and better equations) than Mendeleev used in filling in the holes in his table, but the principles are the same. The prediction of these properties is roughly the field of Ab initio quantum chemistry methods, which basically involves deriving putative properties from purely mathematical principles: You start with what you know of the laws of physics as they apply to all smaller atoms you already know, then you extrapolate that data (it's probably a LOT more complex extrapolation than mere linear regression) and predict the likely properties of the unknown element. --Jayron32 16:14, 9 March 2013 (UTC)

Planck derived units

What is the difference between one Planck length per Planck time; and one Planck length per second? Isn't the latter one meaningful when describing quantum speed? Plasmic Physics (talk) 10:49, 9 March 2013 (UTC)

One Planck length per Planck time is simply another way of saying C: the speed of light in a vacuum. It is the equivalent of measuring speed in light-years per year. --Guy Macon (talk) 14:25, 9 March 2013 (UTC)

So, one is the fastest speed, and the other is the slowest measureable speed? Plasmic Physics (talk) 21:47, 9 March 2013 (UTC)

The latter is not the slowest measurable speed. It can't be, because the second is a human invention, and there's no reason for the universe to depend it. Since momentum is not quantized, there's no such thing as a slowest speed. --140.180.243.114 (talk) 21:53, 9 March 2013 (UTC)

Note, I adjectived 'measureable'. Plasmic Physics (talk) 22:01, 9 March 2013 (UTC)

It's important not to get hung up on the Planck units as imposing some sort of limitation on physical phenomena. The Planck mass is a perfectly ordinary macroscopic mass, well within the measurement capabilities of, if not a set of kitchen scales, certainly a cheap education-grade analytical balance. Nothing special happens at that mass - it's not a lower or upper limit to anything. The Planck length and Planck time are outside the range of (practically) measureable quantities, but they have no special significance otherwise. Tevildo (talk) 22:51, 9 March 2013 (UTC)

Planck Time article says it is a theoretical limit due to effects of quantum mechanics. manya (talk) 03:46, 11 March 2013 (UTC)

That's not true. I removed it. -- BenRG (talk) 05:55, 11 March 2013 (UTC)

The Answer to "Relaxing Voices"

This is from January 2011; my original question was deleted because it was too "medical" sounding, and my follow-up question was therefore poorly explained on my part because I didn't want it removed. Anyway, from this thread: "Why do some voices, e.g., Bob Ross's, cause people to relax? I've read on other forums that people will watch his show to help them fall asleep." I didn't know how else to explain this. well just now, some random video on YouTube was being pushed on me ("featured"?), so finally I watched it and I noticed all these "ASMR" videos that seem to be a thing now. I Googled ASMR and it took me to Autonomous sensory meridian response—that's precisely what I've been looking for for decades (long before people discovered this on the Internet). It's a "dizzy" feeling one experiences when certain individuals talk quietly.Reflectionsinglass (talk) 11:59, 9 March 2013 (UTC)

You should look at Bitching Betty. Shadowjams (talk) 12:33, 9 March 2013 (UTC)

Venus flytrap and meteors

Is it true that following meteor impacts, large numbers of venus flytraps can often be found growing around the craters and no-one is really sure how they got there? And that their main habitat is located near what was once a crater left by a huge meteor strike from millions of years ago? — Preceding unsigned comment added by 87.113.139.192 (talk) 17:41, 9 March 2013 (UTC)

Carnivorous plants typically grow where the soil is missing some needed nutrient, which they then get from whatever they digest. I suppose the area where a meteor has struck might be depleted in some nutrients, if it blew away the topsoil. This would make it difficult for normal plants to grow there, but the Venus flytrap could survive. StuRat (talk) 17:56, 9 March 2013 (UTC)

However, none of that answers the question about venus flytraps being mysteriously co-located with impact craters. The answer to that question is firmly "no". Flytraps are native (only) to the Carolinas, which are not notable impact basins, and are not natively found elsewhere. — Lomn 18:04, 9 March 2013 (UTC)

I think we need to link to Little Shop of Horrors. Rmhermen (talk) 20:18, 9 March 2013 (UTC)

What about a link to Dr Quatermass or even The Night of the Triffids. Oh, I'm not going to sleep tonight. I can see the wife's rubber plant looking at me right now and it's licking its lips.--Aspro (talk) 22:32, 10 March 2013 (UTC)