Wikipedia:Reference desk/Archives/Science/2010 December 22

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December 22

glowing embers on the road

For the past few months, whenever I drive home on the highway during the evening, I see the same strange thing on that little bit of road between the median and the fast lane: There, on the ground, is a small pile of what appears to be glowing coal embers - a pile of about ten roundish egg-sized things (in a small cluster the size of a bird's nest) emanating orange light. The first time I saw them, I thought they were actually coal embers. But because I've seen them in the same spot night after night, that can't be the case. Anybody have any idea? --Vectorflux (talk) 02:19, 22 December 2010 (UTC)

Where in the world? HiLo48 (talk) 02:49, 22 December 2010 (UTC)

Simplest explanation is a broken orange corner reflector. An idea: Make a careful note of where it is, then drive it during the day with a passenger with a camera. Have them try to take a high speed picture of the spot (make sure to set the exposure very low/fast to avoid blurriness, and physically pan the camera as well). Ariel. (talk) 05:29, 22 December 2010 (UTC)

Edit: Even more likely: Cat's eye reflector. Maybe a construction crew dumped some they didn't need? Or lost them? Ariel. (talk) 05:34, 22 December 2010 (UTC)

Due to the location on the road I suspect the objects are something that fell from a vehicle. The article Automotive lighting provides candidates such as turn indicators, brake lamps, sidemarker lights or reflectors, emergency lights and others. Someone might have lost their Christmas lights or possibly fluorerscent strip lights that served the 90's fad for under-car lighting. But could not the OP just go back and look? Cuddlyable3 (talk) 10:11, 22 December 2010 (UTC)

I, too, would venture to say it's a cat's eye reflector. There are varying designs; some are just one chunk of mirror whilst others have several small, circular reflectors and that sounds like what the OP saw. The best way to tell is to go back in daytime. Regards, --—Cyclonenim | Chat  18:36, 22 December 2010 (UTC)

Normal human body surface area = 1.73 m^2

How was 1.73 m^2 chosen as the "normal" human body surface area? If women typically have a BSA of 1.6 m^2 and men 1.9 m^2, 1.75 m^2 would seem a more likely choice unless there was a reason for choosing a less "round" number. Note that this normal BSA enters into the choice of units for renal function where the BSA corrected glomerular filtration rate (GFR) is given in mL/min/1.73 m^2. -- 119.31.126.66 (talk) 03:03, 22 December 2010 (UTC)

The origin of the 1·73-m² body surface area normalization: problems and implications by James G. Heaf in Clinical Physiology and Functional Imaging, Volume 27, Issue 3, pages 135–137, May 2007, sounds like it would tell me what I want to know. Now if only I can get access somewhere. -- 119.31.126.66 (talk) 03:27, 22 December 2010 (UTC)

Here's a relevant excerpt from Heaf: "In these evidence-based times, it is perhaps surprising to read how little documentation was required for the derivation of this figure, which first appeared in a study by McIntosh et al. (1928) (Fig. 2). On the basis of clearance studies of eight children and seven adults, the paper states: ‘Our experience confirms that of Addis and his colleagues. More constant normal values are obtained if one substitutes A (= surface area) in place of W (= weight) in the clearance formulae. We have found it convenient to use as a unit the surface area 1·73 square meters which is the mean of the areas of men and women of 25, estimated from the adjusted medico-actuarial tables of Baldwin and Wood published by Fiske and Crawford’. The Du Bois formula was used. The tables described young US citizens applying for life insurance. They were not referenced directly, but an adjusted version was published by Fisk & Crawford (1927). While these individuals had been examined fully clothed, every effort was made to reach as accurate an estimate as possible: the figures for men were ‘adjusted from the Medico-Actuarial Tables by subtracting 1 inch from height in shoes, and 5 pounds from weight in clothes’, and for women by subtracting 1·5 inches and 4 pounds, respectively, thereby giving us an insight into American clothing standards in the 1920s. There seems to be a minor error in the calculation: the correct figure is 1·72 m2."

References are McIntosh: PMID 16693840; Fisk: Fisk EL, Crawford JR. How to Make the Periodic Health Examination (1927) p. 345. Macmillan, New York.; Dubois: Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Med (1916); 17: 863–871.

Interestingly, here's a description (from Heaf) of the method of Dubois: "The most commonly used formula for determining BSA is the Du Bois formula from 1916 (Du Bois & Du Bois, 1916). BSA was determined by tightly covering the patients with manila paper moulds. The moulds were then removed, opened and placed flat on photographic film. The film was subsequently exposed to light. Finally, the unexposed film was cut out and weighed. The BSA was derived from the weight by dividing by the average density of the photographic area." Hope this helps. -- Scray (talk) 05:09, 22 December 2010 (UTC)

Thank you! I am surprised that the original figure was actually expressed in square meters as opposed to square feet or square inches since the tables mentioned were apparently of US citizens described in pounds and inches. I had not realized that the medical / physiological community in the US had adopted the metric system so early. -- 119.31.121.84 (talk) 05:32, 22 December 2010 (UTC)

Based on the description of the gross approximations in the determination, it is hard to justify more than 2 significant figures. Edison (talk) 16:49, 22 December 2010 (UTC)

Arsenite color

Are they colorless? --Chemicalinterest (talk) 16:30, 22 December 2010 (UTC)

Yes, as long as the cation isn't coloured. So sodium arsenite is colourless, while copper arsenite is a well-known green pigment. The colour isn't the normal blue colour of copper(II), but the principle is the same. Physchim62 (talk) 16:34, 22 December 2010 (UTC)

Thanks. I needed the information for my work. --Chemicalinterest (talk) 17:24, 22 December 2010 (UTC)

Arsenite solubility

Are arsenites soluble? Sodium and potassium vs. copper arsenite. --Chemicalinterest (talk) 17:26, 22 December 2010 (UTC)Never mind. --Chemicalinterest (talk) 19:23, 22 December 2010 (UTC)

hear sine wave vs square wave

Is there an easy way that I can hear the difference in a sine wave and a square wave? For instance, is there a website or sound files that demonstrate the two, so I can listen to them? Bubba73 ^{You talkin' to me?} 16:41, 22 December 2010 (UTC)

A sine, square, and sawtooth wave at 440 Hz

Our article on waveforms actually has an example sound file; I've copied the link here. The addition of harmonics to the square wave gives it a much 'harsher' sound that the pure, single-frequency sine wave tone. TenOfAllTrades(talk) 16:54, 22 December 2010 (UTC)

Thank you, that was pretty much what I was looking for. It is a bit short and the square wave starts while the sine wave is still going on. Bubba73 ^{You talkin' to me?} 18:19, 22 December 2010 (UTC)

Puts on professor hat The square and the sawtooth waves are made by adding harmonics to the sine wave, which means the sine wave is actually present through the entire sequence. If one filters away their harmonics, the square and sawtooth waves turn into sine waves. ("Harmonics" means other sinewaves with frequencies that are whole multiples of the fundamental sinewave.) A proper mathematical sinewave keeps on going forever so any demonstration has to be cut short to be practical. Cuddlyable3 (talk) 21:34, 22 December 2010 (UTC)

For a more mathematically rigorous take on the matter, you can read the article Fourier series. It turns out that any periodic function can be treated as a superposition of multiple (perhaps infinite) sine and cosine functions. Buddy431 (talk) 21:50, 22 December 2010 (UTC)

OK, I understand that. To me, though, it sounds like two things - the sine plus something else, rather than one tone. Bubba73 ^{You talkin' to me?} 21:54, 22 December 2010 (UTC)

A square wave is $y=\sin x+{\frac {\sin 3x}{3}}+{\frac {\sin 5x}{5}}+{\frac {\sin 7x}{7}}+\cdots$

Power-of-2 harmonics step up the octaves and thus sound pleasant. Other harmonics can sound, well, unharmonic. Wolfram-Alpha square wave graph. CS Miller (talk) 23:07, 22 December 2010 (UTC)

And the triangular wave is $y=\sin x-{\frac {\sin 3x}{3^{2}}}+{\frac {\sin 5x}{5^{2}}}\cdots$    CS Miller (talk) 23:21, 22 December 2010 (UTC)

I guess it is those non-power-of-2 harmonics that makes it sound like two things. Musical instruments have a lot of harmonics, but not like the square wave. Musical instruments still sound like one thing whereas the square wave sounds like two. Bubba73 ^{You talkin' to me?} 00:41, 23 December 2010 (UTC)

Try this (lets you do sine and square at the same time) and this from [1] Ariel. (talk) 00:46, 23 December 2010 (UTC)

That is even more interesting. I started wondering about this because of my memory of the sound on my old Apple II. I thought it was probably a square wave. My memory is that it sounded like these square waves. Bubba73 ^{You talkin' to me?} 01:12, 23 December 2010 (UTC)

The cochlea has a profoundly different frequency response to waveforms with infinite first derivatives. Ginger Conspiracy (talk) 03:32, 23 December 2010 (UTC)

I imported the sound file of 1 second of sine, 1 second of square, and 1 second of sawtooth into a sound editor - and it is what it says (although there are some squiggles on the square wave). Bubba73 ^{You talkin' to me?} 04:12, 23 December 2010 (UTC)

The "squiggles" might be due to the Gibbs phenomenon. —Bkell (talk) 05:10, 23 December 2010 (UTC)

Looks like they are. Bubba73 ^{You talkin' to me?} 06:26, 23 December 2010 (UTC)

Space between particles

Considering the amount of space between subatomic and all other particles, what percent of the human body is empty space? — Preceding unsigned comment added by Daselter (talk • contribs) 19:30, 22 December 2010 (UTC)

If you only consider the gaps between atoms as empty space, you will get one figure, if you add the gaps between the nuclei and electrons, you will get a much higher percentage of empty space, and if you add in the space between quarks, you will get a higher figure still. It may well be that the answer is 100%, as no fundamental particle exists which is completely solid. (I don't believe the strings in string theory have a volume, although they do have a length.) StuRat (talk) 19:47, 22 December 2010 (UTC)

It should come up to 100% empty space. This is fundamentally the same as measuring infinite distance along a shoreline by using a smaller and smaller unit of measurement. -- kainaw™ 19:52, 22 December 2010 (UTC)

Or if you consider everywhere that a molecular orbital has non-zero probability as "occupied", then you get approximately 0% empty space (not counting the actual open spaces in the lungs and bowel, etc.). The question hinges as much on the semantic meaning of "empty space" as it does on the physical properties of the body. Dragons flight (talk) 20:07, 22 December 2010 (UTC)

Alas, we mortals live from Cantor dust to Cantor dust, the peculiar states of being nowhere dense. Cuddlyable3 (talk) 21:39, 22 December 2010 (UTC)

Let me explain why we can be 100% empty space, and yet still physically interact. When you touch an object with your finger, the atoms of your finger don't actually touch the atoms in the object. Instead the electromagnetic force of the atoms in your finger repels the corresponding electromagnetic force of the atoms in the object. It's just like two magnets that repel, and yet have empty space in between them. This is true when two atoms interact, it's true for an electron around a nucleus (it's all empty space, yet they interact), it's true for the nucleons in the atoms (except they also use the strong force), and it's true for the quarks inside each nucleon (but they use a different force). As far as anyone can tell electrons and quarks have no physical size (although they do have a wavelength). So where do you want to stop the line? If you draw a box around an atom it certainly has a size - and you can talk about the space between atoms. You can instead draw your box around the nucleons, or you can go larger and draw your box around molecules (like Dragons flight suggested). Ariel. (talk) 21:55, 22 December 2010 (UTC)

Isn't it something like 99.999999%? I forget how many nines. 100% is absurdly wrong, but very close. Ginger Conspiracy (talk) 23:05, 22 December 2010 (UTC)

From Point-like particle: "There is no experimental evidence for any of the elementary particles having spatial extent, and so they are usually considered to be point particles in the more general sense too...." Ariel. (talk) 00:35, 23 December 2010 (UTC)

That seems inconsistent with Planck length and the general idea of surface area corresponding to the force activity regions involved. Ginger Conspiracy (talk) 01:20, 23 December 2010 (UTC)

We don't have the ability to experimentally probe down to the planck length, and that sentence specifies experimental. One day we may find that electrons have internal structure (maybe by probing with neutrinos?) But, right now as far as we know they are point-like. I don't understand the second part of what you wrote. Ariel. (talk) 02:50, 23 December 2010 (UTC)

One analogy I've read, I'm not sure how accurate it is, if the nucleus of an atom was the size of a pea and you put it in the middle of a football field, the electrons would be orbiting around the boundary of the field. But as the above states, even the nucleus isn't really a solid. Vespine (talk) 23:26, 22 December 2010 (UTC)

I agree with Ariel that 100% "empty space" is a valid answer. There is no "solid matter" even at deepest level. It is all "energy". Of course, various other answers are possible, depending (as others have said), on where you draw the "boxes". Dbfirs 00:03, 23 December 2010 (UTC)

It will depend on our definition of solid. We have a variable volume dependent on pressure. In Human experience it means that the material resists human force. In that respect the human body is solid enough, but if the force becomes extreme enough you can make degenerate matter or neutron star matter. If pressure is reduced, then the body will evaporate and fill a much larger volume with water vapour and carbon dioxide! Graeme Bartlett (talk) 00:37, 23 December 2010 (UTC)

This was asked recently. The answer depends 100% on what you mean by "empty space". This term has no definition in modern physics, unless you define it as the absence of any field, in which case 0% of your body is empty space. It's not true that the fundamental particles are Planck-sized. Planck units are obtained by dimensional analysis and their physical significance, if any, isn't clear. -- BenRG (talk) 07:39, 23 December 2010 (UTC)

The "empty space" inside each atom is perhaps 99.9999999999% or more of the "volume" of the atom (based on the atomic radius being at least 10,000 times the nuclear radius), but the space can hardly be described as "empty" because other atoms cannot normally enter it, and only certain particles can normally gain access. This analysis is not at all scientific, and has little basis in "reality", because each constituent particle in the atom has a calculable probability of being found at any point in this "empty space". In fact, each electron has a probability of about 1/3 of being found somewhere "inside" the Bohr radius. Dbfirs 12:53, 23 December 2010 (UTC)

They aren't particles, they're more like waves. There's some amplitude everywhere. As resuch, it's 100%. I think pretty much any particle has a chance of being anywhere in the universe. — DanielLC 22:37, 25 December 2010 (UTC)

Has anyone told electrons that they are not particles? Perhaps they need to change their behaviour? If all particles are waves and exist everywhere with a finite probability, then you meant 0% empty space, as discussed above. (I wasn't actually disagreeing with the statistical analysis, just trying to answer the question from the point of view of the Bohr model which seemed to be what the questioner wanted.) Dbfirs 08:34, 26 December 2010 (UTC)

Arsenic trisulfide's NFPA health rating

Why does arsenic trisulfide, one of the less toxic arsenic compounds, have "4" for its NFPA health rating while arsenic trioxide, an extremely toxic arsenic compound, has "3"? --Chemicalinterest (talk) 21:40, 22 December 2010 (UTC)

This may be a MSDS error. Thank you for bringing it to our attention. Please forgive the research delay necessary to answer. Ginger Conspiracy (talk) 23:10, 22 December 2010 (UTC)

One of my purposes here is to find and report errors in chemistry articles. I hope you all don't get annoyed at my often posting here. In the course of my work at Simple English Wikipedia (using this Wikipedia as a source) I come across errors that I fix or ask clarification. --Chemicalinterest (talk) 23:50, 22 December 2010 (UTC)

One of the difficulties of the NFPA system appears to be that for many compounds the classification is left up to individual suppliers or users, and may vary somewhat depending on quantities in use or other application-specific conditions.

• Stanford synchrotron: Health: 3, Fire: 1, Reactivity: 0
• Temarry Recycling of Mexico: Health: 2, Fire: 0, Reactivity: 0
• HazMat data: for first response, transportation, storage, and security by R.P. Pohanish: Health: 3, Fire: 0, Reactivity: 0
• NOAA Cameo Chemicals database: Health: 3, Fire: 0, Reactivity: 0
• Fisher Scientific: Health: 3, Fire: 1, Reactivity: 0
• our own article (needs citation?): Health: 4, Fire: 0, Reactivity: 2

I suspect that our article is overstating the hazard. Ideally, someone should check the relevant NFPA guideline (NFPA 704) and report back to us. TenOfAllTrades(talk) 01:24, 23 December 2010 (UTC)

If there is a conflict in reliable sources, my opinion is that wikipedia should list the most stringent (and link to it), but also include links to sources with lower numbers, perhaps with a footnote mentioning the discrepancy. Deciding on a number ourself is reasonable to avoid sources that are probably errors, but would otherwise be a violation of wikipedia policy. We should only do that if we can't find any good sources. Ariel. (talk) 02:55, 23 December 2010 (UTC)

I think it should list the one that makes sense, not necessarily the one that is most stringent. You can always rely on these people to make up something really scary-sounding. --Chemicalinterest (talk) 13:46, 24 December 2010 (UTC)

3-0-0 is a reasonable mid-point of the values listed above. 4-0-2 is ridiculous for this compound, and is clearly an outlier when compared with other sources. Physchim62 (talk) 09:51, 23 December 2010 (UTC)

If I may have an opinion, 3-1-0 would seem right as it is poisonous when left in air, burns when heated to around 300C, and does not explode or make other substances ignite. --Chemicalinterest (talk) 12:11, 23 December 2010 (UTC)

Does it actually burn? You can certainly roast it in air, but I can't find any reference to a self-sustaining reaction: in other words, as far as I can tell, if you take the heat away, the reaction stops. That would be a zero for flammability. Physchim62 (talk) 18:46, 23 December 2010 (UTC)