Wikipedia:Reference desk/Archives/Science/2012 February 10

February 10Edit

Meter definitionEdit

i read that the definition of metre is "The metre is the length of the path traveled by light in vacuum during a time interval of 1⁄299792458 of a second." Why not define metre as 1/300,000,000 of a second so that speed of light will be exactly 300,000,000m/s? MahAdik usap 01:05, 10 February 2012 (UTC)

Because, as Metre#Meridional definition states, it was not defined in terms of the speed of light originally, but rather as "one ten-millionth of the length of the Earth's meridian along a quadrant". It's a little too late to go changing it now. Clarityfiend (talk) 01:13, 10 February 2012 (UTC)
(ec) Because the meter (and the second, and the gram) already had a widely-implemented physical meaning prior to defining it in terms of the speed of light. Switching its size capriciously would create untold havoc and inconvenience for very little gain. Instead, the speed of light definition ties an existing value to a physical constant. — Lomn 01:14, 10 February 2012 (UTC)
The key issue is that, while 300 000 000 m/s and 299 792 458 m/s only differ by less than one part in a thousand, when the definition of a meter was set to the 1⁄299792458 of a second definition in 1983, people were already using the meter, and were able to accurately measure distances with much greater accuracy than one part in a thousand. (Which isn't actually that hard, as the difference between the two meters is about 0.7 (current) mm - a difference that's visible to the naked eye.) While ponderously slow about it, the international metrology community is perfectly willing to redefine units (see New SI definitions) however, the key consideration in all such changes is that the old and new units must match up to within the error for even the most accurate measurements. That is, if something measured 10245.6±0.3 mm prior to the redefinition, it should measure 10245.6±0.3 mm after the definition. To do otherwise invites confusion as you'd constantly have to check the publication date of books, references, etc. to see if they were measuring things with the old meter or the new meter. -- (talk) 17:03, 10 February 2012 (UTC)


Crossword Clue : Anaconda Answer Given. : Boma.

Nowhere can I find any mention of, or reference to, the word BOMA in any capacity much less to connect with Anacona. Can you help me - I hate words I cannot explain. My, hopeful, thanks - J. Mumvonston (talk) 02:40, 10 February 2012 (UTC)

Googling for those two terms finds many hits. The first two are "boma" in a Scrabble dictionary and a work by Richard Price (American anthropologist) about explorers in Suriname. Each one gives them as synonymous or a cross-breed or more specific type or something like that. DMacks (talk) 02:48, 10 February 2012 (UTC)
The OED has it: bom or boma - "The name in Congo, W. Africa, of ‘a huge non-poisonous snake swallowing deer, etc.’ (see Merolla, Vocab.; Proyart; Cavazzi Congo, Matamba, & Angola; Magyar Süd-Afrika). Apparently carried by the Portuguese from Congo to Brazil (Roquete has bom bôma ‘serpent d'Angola et du Brésil’), and there applied to the largest boas, in which sense it appears in some English works. (The history has been traced for us by Dr. E. B. Tylor.)" (Oops, forgot to sign.)--Rallette (talk) 08:31, 10 February 2012 (UTC)

Electronic color codeEdit

May I know how to distinguish colour coding for 5 bands described by this article from the same colour coding used by some commercial vendors such as [1].--Almuhammedi (talk) 08:35, 10 February 2012 (UTC)

As far as I can tell, that coding schemes are completely compatible. The second one simply lists extra colors for the fifth band that aren't even used in military applications. (Unless the US military color-coding chart I found is simply incomplete.) Someguy1221 (talk) 10:09, 10 February 2012 (UTC)
It's been a long time since I worked on any Defence Work but I think I have found an inaccuracy in the WP article Electronic color code ( apart from they don't know how to spell colour of course) which doesn’t ring true. It states: ”Resistors manufactured for military use may also include a fifth band which indicates component failure rate (reliability); refer to MIL-HDBK-199 for further details.” Reliability? This sounds like nonsense. If one wants the resistor to last and not fail too soon, one simply chooses a resistor with a high wattage rating. Doubling it is a good rule of thumb but one need to consider the ambient temperature also –so in the case of (say) a missile, where air friction is going to heat the interior up, one might want to calculate a high value still -there are handy graphs provided by the manufacturer to avoid doing the sums). As for 5 band resistor colour codes (and the military certainly do uses them), this chart explains it well. [2]
So 15K Ω example in the above chart put simply:
1 (brown)
5 (green)
0 (black)
00 (green)
=15000 Ω
@ 1 % tolerance (brown)
As a certain meerkat might say : Simples! --Aspro (talk) 23:14, 11 February 2012 (UTC)


Dear Sirs My colleague Rex Palmer and I are preparing a 5th edition of "Structure Determination by X-ray Crystallography". We would be grateful for permission to reproduce the top right-hand two Wikipedia figures that appear under "Penrose Tiling!. Yours sincerely Mark Ladd & pp Rex Palmer — Preceding unsigned comment added by Kramdaal (talkcontribs) 10:13, 10 February 2012 (UTC)

The article in question is Penrose tiling. Do you mean the two pictures - File:Penrose Tiling (Rhombi).svg and File:RogerPenroseTileTAMU2010.jpg? Wikipedia doesn't own those pictures, and so can't grant you anything. The diagram is in the public domain, but the photo was uploaded by Solarflare100. That was Solarflare100's only contribution, so I know of no way to contact them. -- Finlay McWalterTalk 10:25, 10 February 2012 (UTC)
However Wikipedia:Reusing Wikipedia content and Commons:Commons:Reusing content outside Wikimedia should provide you some idea how you may be able to reuse the content without further permission of the copyright holder. Bear in mind this is not intended to be legal advice and we cannot guarantee the copyright situation of either image is as suggested in the file description. Nil Einne (talk) 15:49, 10 February 2012 (UTC)
You should probably discuss with your publisher about what licensing rules they have. The licenses on the two images in question are clearly marked: this one was released into the public domain by its copyright holder, and this one is licensed for free re-use under the Creative Commons "Attribution 3.0 Unported (CC BY 3.0)" license. Your publisher will probably know whether such content is acceptable for their uses. Nimur (talk) 19:19, 10 February 2012 (UTC)

UV light from sunEdit

Our article on vitamin D says that humans only make vitamin D when the UV index is above 3. Assuming someone standing directly in the sun with no cloud cover, roughly what angle of the sun with the horizon is that equivalent to? And is there a website that you can stick your location or even just latitude in, and it tells you what hours and days the sun is above that angle? I assume such things must be possible, because our articles seem to talk about different places having strong enough sunlight for given percentages of the year. (talk) 14:06, 10 February 2012 (UTC)

You do know that you can get vitamin D from vitamin pills or foods and beverages supplemented with it, right ? Considering the damage UV causes to the skin, this is the safer approach. StuRat (talk) 19:13, 10 February 2012 (UTC)
Apparently sunscreen or other sunlight denial has not proved itself as an unmixed blessing. (See PMID 8475009) It should be very surprising to me if humans truly suffer from an appropriate level of exposure to the Earth's sun. Wnt (talk) 20:45, 10 February 2012 (UTC)
A couple questions about that study:
1) Did they eliminate vitamin D deficiency by comparing those with sunlight exposure with those taking vitamin D ?
2) People who get more sunlight exposure may also get more exercise (the outdoorsy type of people). Did they control for this variable ? StuRat (talk) 20:55, 10 February 2012 (UTC)

I'm not especially interested in what various unqualified people think about appropriate sources of vitamin D (this, and the dangers of sunlight, attracts a lot of crank views for some reason, and I don't really want to play that game). I'm really interested in being able to look at when there is strong enough sunlight to make vitamin D, in different parts of the world. I assume such a thing is achievable, and given the various interests it seem like something that would be a widget on a website somewhere. Is this so? (talk) 23:06, 10 February 2012 (UTC)

I won't offer an unqualified opinion on the Solar altitude, but will mention that air quality may also be an important factor: even on a cloud-free day, the degree of various pollutants in the air may, I suspect, greatly influence how much UV gets to the ground. {The poster formerly known as} (talk) 00:48, 11 February 2012 (UTC)

Such a widget to compute the U index is undoubtably possible. Its really just the same as what experts have come up with for the photo-voltaic (solar panels) industry to predict power output of solar panels. But I doubt that it would be any practical value. Vitamin research is notoriously shakey, and requirements will vary from race to race and person to person. I've never seen it cited anywhere what area of skin is required to be exposed to maintain a given vitamin level at a given UV index, and that is surely a valid and important question. Keit120.145.158.234 (talk) 02:59, 11 February 2012 (UTC)

Let me first note that the UV index is defined as a measure of UV exposure at noon, so it is not a simple measure of UV exposure as a function of sun angle. With that being said, it seems to be difficult to give an authoritative answer to the question in terms. Calculating the UV index requires having a "radiative transfer model", a mathematical formula for the amount of UV light that reaches the ground at each frequency for a range of parameters, and it seems that each agency that calculates UV indices uses its own idiosyncratic transfer model. According to this paper, vitamin D production in the skin begins when the sun is around 20-30 degrees above the horizon. Looie496 (talk) 03:12, 11 February 2012 (UTC)

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation

From an astronomy/sunlight perspective this on-line calculator may help with what you're looking for. When trying to determine how much UV gets through, the transmittance of the atmosphere to UV is quite low, so you'll need to think about path lengths, attenuation and the Beer–Lambert law. LukeSurl t c 11:10, 11 February 2012 (UTC)


Hello. Can Hemoptysis result from flu, or flu-like conditions? (Before some zealot removes this question, please note that it is just a general medical question, and that I am not asking for a medical diagnosis, and don't want one, either. Thank you). Polisher of Cobwebs (talk) 18:59, 10 February 2012 (UTC)

Any condition which causes a sore throat could cause blood to be coughed up from the esophagus. However, hemoptysis doesn't include that. It does include blood from the trachea, but if the patient has an infected trachea, that might then be classified as pneumonia. StuRat (talk) 19:10, 10 February 2012 (UTC)

Age and intelligenceEdit

At what age do humans reach the maximum capacity for learning ? (Note that this should be earlier than the age at which they attain the maximum amount of knowledge, since there is a substantial delay between obtaining the capacity to learn and absorbing all the information possible.) StuRat (talk) 19:22, 10 February 2012 (UTC)

What makes you think there is a "maximum capacity for learning"? The brain is not a bucket. It is not a hard drive. It is a complicated living organ. Over time, various parts of it have increased ability (e.g. language sparks up in an incredible way when very young), and some parts decline over time, but I'm not sure how one correlates that with a singular notion of a "maximum capacity for learning." I don't know if the processes described on this page are correct or not, but it points out what a more nuanced analysis of age and learning might look at. --Mr.98 (talk) 20:04, 10 February 2012 (UTC)
The Aging brain article might help answer this question. Mitch Ames (talk) 11:31, 11 February 2012 (UTC)
There's a sentence about this in the IQ article, which can be found by searching for the phrase "age 26". You'd still keep gaining knowledge and experience for decades afterwards though. Sagittarian Milky Way (talk) 19:35, 11 February 2012 (UTC)

Relativistic (polar) jet directed in one directionEdit

It seems that the Messier 87's jet is going only in one direction while other jets as seen in Bipolar outflow seem to go in two opposite directions. The article on relativistic jets seems to indicate that jets from black holes can also go in two directions. What causes the jet to either go in one or two directions? ScienceApe (talk) 20:53, 10 February 2012 (UTC)

That's explained in the article Relativistic_beaming (found via M87#Jet...) --Wrongfilter (talk) 20:59, 10 February 2012 (UTC)
As a physicist, I would simply say "this behavior is obviously caused by an asymmetry in the physical system." Isn't it obvious? The interesting follow-up question is, of course: is this a fundamental asymmetry of the process (which has universal cosmological implications), or is it simply due to initial inhomogeneity (i.e., did the accreted dust-cloud have a net initial angular momentum)? Here's a nice web-page from UCLA, on cosmological anisotropy and inhomogeneity, and here's an online text from CalTech, specifically discussing velocity inhomogeneity in M87. As an engineer, I'd just say, "... there's probably a second jet we just can't see from Earth. Either way, the experts have chimed in with their opinions; the M87 radio jet is quite famous. Nimur (talk) 22:08, 10 February 2012 (UTC)
Obvious? What's obvious about it? @_@ I hope that was a joke.. Not all of us are physicists, I'm just a layman. ScienceApe (talk) 23:57, 10 February 2012 (UTC)
(It was a physicist joke, sorry if it was lost on the uninitiated). Nimur (talk) 00:26, 11 February 2012 (UTC)

Related question, if the single jet is just an optical illusion, and there's really two jets, is it possible for there to be just one jet? ScienceApe (talk) 00:02, 11 February 2012 (UTC)

This is, essentially, the crux the links I posted. The answer is, "it's hotly debated, because the observational evidence is still fuzzy enough that either answer makes sense." We don't really know whether the other jet exists (and is invisible); or if it doesn't exist, and there's some interesting physics happening. Nimur (talk) 00:29, 11 February 2012 (UTC)
If there was only one jet, wouldn't it cause the galaxy to be accelerated with equal momentum in the opposite direction like a rocket? Whoop whoop pull up Bitching Betty | Averted crashes 23:16, 15 February 2012 (UTC)

How much processing power would be needed to emulate a 4D visual cortex?Edit

That would be as good in 4D space as a human visual cortex is in 3D space. How many rods and cones would 3D retinas need to get similar resolution and field of view? 1 trillion? (100 million3/2) How many kilograms would it weigh if built out of neurons (in 3D space)? (and presumably would need to be fed Matrix-style to run as the requisite 4D eyeball is not in existance). Since I read that even the current neuron-based paradigm is close to limits of intelligence (tradeoff of wiring thinness vs. transmission quickness and energy usage) then this number might be ridiculouly high or maybe impossible. On the other hand, how well would a current visual cortex do if rewired for 4D space? Not very well I suppose? Sagittarian Milky Way (talk) 22:35, 10 February 2012 (UTC)

I guess from teh way you worded it that you did not consider time as the 4th dimension, as is sometimes done. This question can be answered in two ways:-

1) The question is not a valid question because systems of greater than 3 dimensions are mathematical constructs useful in certain fields and do not represent any physical reality.

2) No increase in retina sensors and no increase in brain neurons would be necessary. This may surprise you, but there are good reasons as follows: We don't see like a televion camera - most of the information from our retinas is thrown away on the path to conscious vision. The brain selectively, depending on the merits of the scene and what you are looking for, "focusses" on only a limitted subset of visual information, and reconstructs what you consciously see or think you see. This is easily demonstrated with optical illusions. So if you get 4D eyes, the brain can still throw most of the info away and reconstruct. We don't actually see in full 3D, its more like enhanced 2D, as demonstrated by the long success of 2D representations in printed matter and movies, and perhaps the success of 3D enhancemnt to movies shot in 2D (eg Titanic). Also, human vison is essentially with 3 primary colours - red, green, and blue. Most birds insects have 4 primary colours - red, green, blue, and ultravioet, and dogs sort of 2 and a bit. An extra primary colour is potentially adding information just as an extra dimension would, but in fact its just a re-allocation of a portion of rods and cones or sensor cells to a different colour. But researches have in certain animals manipulated the genetic expression of retinal cells to add a primary colour. They fould that the standard brain adapted and coped just fine. So it should also cope with an extra dimension. As I recall, this was covered in a recent Scientific American article. Keit120.145.158.234 (talk) 02:35, 11 February 2012 (UTC)

Visualizing point dataEdit

Does there exist software which would allow me to view point clouds (at least XYZ, possibly XYZ+colour) and measure distances between points? e.g. so I can measure the approximate distance between two walls in a model. Thanks! (talk) 22:44, 10 February 2012 (UTC)

Certainly, pretty much any 3D CAD system will do that, although it's quite difficult to visualize 3D points. Some type of motion (rotation or Z-clipping as you move through the point field) or connections drawn between them (like constellations) can help. (Z-clipping is where the parts of the object closer to you are hidden so you can see the parts farther away.) Otherwise, the usual cues for telling which objects are closer and which are farther don't work with points. I suppose you could have them be little spheres or cubes, in a perspective view, so they do get larger as you get closer. You could also use color, where points farther away become darker and bluer colors, like distant mountains. StuRat (talk) 22:48, 10 February 2012 (UTC)
For a free, open source option, Octave can do that. It has MATLAB style syntax, and uses gnuplot for a variety of visualization techniques. SemanticMantis (talk) 23:19, 10 February 2012 (UTC)
Thanks. Octave might do the trick, although I was hoping for more of a 'prepackaged' solution - perhaps from the GIS field. I shall have a look! (talk) 03:45, 11 February 2012 (UTC)

Astronomy stuffsEdit

For number 18, the answer is ultra light. I want to know the relationship between temperature and wavelength range. And a brief description of wavelength for each of the answer.

For number 19, the answer is 4 times more. I don't really understand how they got it. So basically if what is the relationship between angular diameter and amount of energy it radiates and its wavelength. So if i was given 2 out of 3 i can figure out the last one. And don't ask me why i didn't ask my teacher. This is not part of my classes. Thanks.Pendragon5 (talk) 23:21, 10 February 2012 (UTC)

Wien's displacement law will give you a relation between peak blackbody-wavelength and temperature. The Stefan–Boltzmann law will give you a relation between temperature and the rate of energy radiated (per area or per solid angle, depending on the formula you choose to use). By stringing these two equations together with a bit of algebra, you can calculate the total energy radiated, if you know the size of the objects. Whether these laws are a good approximation for your object will depend on how accurately you need to calculate: there are loads of empirical adjustment "fudge factors" - like "grey body radiation;" and my personal favorite astrophysical fudge factor - whether the radiant antenna temperature of the universe is 0 or 4 kelvins. (Properly, you should modify Stefan Boltzmann equation to account for this, but in practice it makes almost no difference when you are considering stellar radiation). And for the true astrophysical pedant, 4 kelvin isn't accurate enough; you should actually calculate the impedance-matching for every single emitted photon wavelength to the color temperature of the universe, lest you calculate a miniscule error on the radiated power that is far below any realistic noise-floor. (As an addendum, I'll just comment that our cosmic microwave background radiation article cites recent COBE measurements of the blackbody temperature as "2.725 K" with error-bars too small to mention. I still always use 4 K, and it's never interfered with any of my work so far). Nimur (talk) 00:49, 11 February 2012 (UTC)
2.8977685(51)×10−3 What is the (51) mean? And when i plug it in my calculator. For the temperature, do i put 8,500 or 8,500,000? Pendragon5 (talk) 01:06, 11 February 2012 (UTC)
Alright never mind. Base off from the answer i got, it all makes sense now. Let me put it out here and can you tell me if i did it correctly ok? So let put 2.8977685 X 10^-3/8,500= 3 X 10^-7. Ultra violet range is from 1 X 10^-8 to 1 X 10^-7. So 3 X 10^-7 is in that range so that's why the answer is ultra violet right?Pendragon5 (talk) 01:20, 11 February 2012 (UTC)
Almost, except 3x10^-7 is greater than 1x10^-7, so it isn't actually in that range. The first sentence of ultraviolet says it goes up to 4x10^-7, though, so it in the range, the range is just a little bigger than you thought. --Tango (talk) 01:27, 11 February 2012 (UTC)
Oh yea ops. I meant 4 X 10^-7.Pendragon5 (talk) 20:27, 11 February 2012 (UTC)
The (51) is a short way of saying +/- 0.0000051, ie. it's the uncertainty in the last two digits of the main number. K means Kelvin, not thousand. --Tango (talk) 01:22, 11 February 2012 (UTC)
Oh wow pretty useless then. I'm looking for the rough estimate is good enough. On number 19 i'm still lost on how to do it. How am i going to calculate the energy of Star F and E then compare them? Thanks.Pendragon5 (talk) 01:31, 11 February 2012 (UTC)
Never mind. I have figured out how. Thanks for pointing out directions for me in the first place :D.Pendragon5 (talk) 04:09, 11 February 2012 (UTC)


If for example humans evolve w/o the sense of sight, can we come up with theories about light? MahAdik usap 23:53, 10 February 2012 (UTC)

I suppose so. After all, we are able to come up with theories to explain X-rays, and we can't see those. Sound is not as similar as X-rays, but still somewhat similar to light, so, once we explained that, we would have a head start towards explaining light. StuRat (talk) 00:00, 11 February 2012 (UTC)
Can you think of a scenario where we will be curious about light? MahAdik usap 00:32, 11 February 2012 (UTC)
Even a blind man finds relief in the shade. -- (talk) 04:17, 11 February 2012 (UTC)
It is a weird question but i suppose pretty interesting. We can't see anything then we must have developed some special kinds of functions that don't require the sense of sight to do all the things humans have accomplished until to this day. If we don't have any special function to interchange with the sense of light then i suppose we probably are still stay at same as cavemen. And by the way, you don't need to see or hear or sense something to understand it. What makes us special is our conscious and imagination, which are far more important than knowledge. Knowledge is already there, you just have to learn it. But to invent, motivate, something new... like a theory or new technology... A lot of people can learn but not many have rich imagination such as Albert Einstein. As smart as Einstein has said once: "Imagination is more important than knowledge", which is really true when i think about it.Pendragon5 (talk) 00:42, 11 February 2012 (UTC)
And in human history, there are A LOT of predictions and theories through out human history about something that we don't even understand it yet but with our imagination we can come out with something that makes sense. I'm really astonished that people thousands years ago already have richer imagination than me. Imagination is something that has no limit and infinite of possibilities. And later on, when we are advanced enough then we can confirm it. Not all of them were correct though, just some of them. Predictions and theories came first then the confirmation comes later, it may take few years up to hundreds or thousands of years to confirm them.Pendragon5 (talk) 00:48, 11 February 2012 (UTC)
(edit conflict) Einstein is what im thinking when i asked this question, i just thought that if we cannot percieve light, then we wont have much understanding about it, then we wont be able to formulate a good explanation for gravity, im just thinking that if its true, then maybe their some forces that we cant percieve right now that might help us explain the universe and i hope i am making sense MahAdik usap 01:06, 11 February 2012 (UTC)
We better try to predict those forces now and let wait until we are advanced enough to find them out. But as always the first step is to predict and choose the correct direction.Pendragon5 (talk) 01:13, 11 February 2012 (UTC)
Regardless of the environment, some method of creating heat would eventually be riddled out (mechanical friction, combustion, something). The transmission of heat by thermal radiation in the infrared would then be hard to miss except in the most exotic environments. Once infrared is known, curiosity would eventually follow it up to wavelengths we regard as visible. Wnt (talk) 07:57, 11 February 2012 (UTC)
If one is conceptualising a physical phenomenon for which humans have no ability to naturally sense directly or even indirectly, one may consider neutrinos. One a practical level we have no interactions with these particles. For much of human history we were simply unaware of their existence, and our theories of the universe didn't include them. However eventually as our understanding of the universe grew, Wolfgang Pauli saw a need for them in our theories, and in 1956 they were detected. There are other concepts in physics which have been theorised but not yet observed, and I'm sure there are things that exist that we haven't even thought of yet. That said, a conceptualised phenomenon that would have absolutely no interaction with us or anything we can observe, and thus be completely undetectable, is essentially indistinguishable from fiction. LukeSurl t c 11:27, 11 February 2012 (UTC)