Wikipedia:Reference desk/Archives/Science/2012 February 14

= February 14 =

The Biohazard Symbol

 * Japanese Design Motifs by Fumie Adachi
 * Page 195 - Chinese Characters

It was actually created back in ancient times, not in 1966 by the Dow Chemical Company. The thing is, did it originate in Japan or China? --Arima (talk) 01:32, 14 February 2012 (UTC)


 * That's interesting. What does it say next to it? (Whether a similar symbol was used in ancient times is somewhat irrelevant if the Dow company independently re-invented it; it certainly wasn't used for biohazard back then...) --Mr.98 (talk) 01:36, 14 February 2012 (UTC)
 * Thanks Mr.98. I would say these symbols originated in Japan, but that's just a guess. Each symbol is a kanji, or combination of kanji (generally repetition of the same one three or four times), arranged in a circle. The writing to the right hand side is the description or name of each one. For example, second from the left, on the bottom row (the biohazard symbol), is called 三ツ大の字丸, which just means 'three circular 大 (big, great) characters', or bottom right is 丸道角字, which means 'square character of Tao in a circle', and so on. Hope this helps.  KägeTorä - (影虎)  ( TALK )  01:51, 14 February 2012 (UTC)
 * The symbol isn't quite the same either. The bottom-row second-from-left has the inner circle-segments centered on the stem to the outer sprouts (a contiguous color), whereas File:Biohazard symbol.svg has those arcs centered in the spaces between them (disjoint from the sprout). DMacks (talk) 01:58, 14 February 2012 (UTC)
 * I have just read an article on the Harvard website (New York Times article), saying that when they were choosing the symbol, they used existing symbols, as well as newly designed ones. Maybe this was just based on this original Japanese one?  KägeTorä - (影虎)  ( TALK )  02:25, 14 February 2012 (UTC)
 * It's possible; the thing is, it was an advertising agency that chose the symbols originally for the pool. Thanks for the translation. --Mr.98 (talk) 03:19, 14 February 2012 (UTC)

Flourescent lights
How can they be dimmed? Is there a special dimmer unit available?--92.25.98.82 (talk) 12:01, 14 February 2012 (UTC)


 * There's some information at Fluorescent lamp (note spelling), in particular that "CFLs are available that work in conjunction with a suitable dimmer" - see Compact_fluorescent_lamp for slightly more. AndrewWTaylor (talk) 12:53, 14 February 2012 (UTC)
 * I have seen small commercial fluorescent tubes powered from high voltage DC  (circa 300 volts) current in optical experiments, and their brightness could be varied substantially by adjusting the current (circa 80 mA). No ballast as such was used. Basically, a variable resistor could adjust the brightness somewhat. Edison (talk) 21:53, 14 February 2012 (UTC)


 * An alternative to dimming is to use multiple bulbs with a switch that controls which ones light. If you 3 bulbs, equivalent to 40W, 75W, and 100W (each of which you can get for under $1 in the US), this would give you the ability to have 40, 75, 100, 115, 140, 175, or 215 watts.  I'd like to find lamps that can do just that. StuRat (talk) 22:09, 14 February 2012 (UTC)
 * Three-way bulbs (hmmm...redlink?) are a standard incandescent type found in lamps. Two filaments in the glass envelope, each connected to a separate contact on the end of the Edison screw. The switched socket can then either power filament 1, or filament 2, or both, giving, for example, 50W, 100W, or [50+100=150]W all from a single housing. And there are now CFLs that have the same contact arrangement and give comparable results--several separate spirals or loops attached to a single base with several contacts to control which tube(s) are powered. DMacks (talk) 02:53, 15 February 2012 (UTC)


 * Right, but I expect they cost a lot more than the two single wattage CFLs would cost, and, when either bulb goes out, you then have to decide if you toss the whole thing or use it "at half mast". You would also need to maintain stocks of those as well as single wattage CFLs, for your single sockets, versus with my plan you only need single wattage CFLs.  Also, if you want to up the wattage, with my system you could put in one or more higher wattage single CFLs.  So, my system is cheaper and more flexible, although, admittedly, if the fixture can only handle one bulb, my idea won't work. StuRat (talk) 03:23, 15 February 2012 (UTC)


 * I'm seeing three-way CFLs starting at around US$8, so it would be a loss of good-with-bad, but still teeny (especially given how long CFLs are supposed to last). As to your separate-bulb approach, that seems like old-hat too. Lots of lamps have several individually-switched sockets. I do have some where a single "three-way" switch controls A/B/AB among two separate Edison sockets (rather than separate contacts in a single socket), and some with even more complex switching patterns among three sockets. DMacks (talk) 03:33, 15 February 2012 (UTC)


 * Coincidentally, I just had a 3-way incandescent bulb burn out one of it's filaments today. Being a cheap bastard, I will continue to use it until the last filament dies.  (I only use incandescent and halogen bulbs in winter, when the excess heat is appreciated.) StuRat (talk) 22:35, 15 February 2012 (UTC)

Arachnids vs Opiliones and Body Segments
The wikipedia article on Arachnids says that all Arachnids have two body segments. But the one on Opiliones (a subset of Arachnids) seems to say that they only have one body segment. Am I missing something here? Do the Opiliones have some internal structure that allows biologists to still say they have two body segments or is something else going on? --217.41.234.13 (talk) 15:02, 14 February 2012 (UTC)
 * Did you read the first paragraph of Opiliones? Deor (talk) 15:10, 14 February 2012 (UTC)


 * Also note that tagmata are not "segments" per se. They are groupings of segments (which are known as somites). All arthropods follow the same basic body plan - ringlike segments repeated and attached end to end, each with appendages on both sides. The way these segments are fused, modified, or lost (as well as that of their appendages) is an invaluable tool in tracing back the evolution of the different arthropod groups. For example, what is commonly perceived as the "head segment" of malacostracans ("higher" crustaceans, including crabs) are actually composed of five fused segments, with their appendages highly modified (becoming the mouthparts, antennae, etc.) -- O BSIDIAN  †  S OUL  15:44, 14 February 2012 (UTC)


 * I've changed this in that figure legend in the article. To me the most recognizable feature of true segments or somites in animals is that each one has a single unit of excretory tissue - see nephridium and nephrostome, though in vertebrates the pronephros regresses.  Also see engrailed (gene) for the developmental specification of compartments within each segment.  Segments are a trace of the fundamental way animals have been put together since the first bilaterians, whereas tagmata are of relatively recent origin, and reflect functional design for specific lifestyles. Wnt (talk) 05:04, 15 February 2012 (UTC)

Snellen vision
If someone has 20/20 vision on the Snellen vision test how would you fill out this chart right eye...... Left eye.......... Total............

See Snellen chart - we aren't going to do your homework for you. AndyTheGrump (talk) 17:29, 14 February 2012 (UTC)

Speed of gravity waves to an observer at relativisitic speeds
I understand how light is seen as travelling at c no matter your reference frame. I assume the same must be true for gravity waves (assuming you could "see" them). Is this correct? Do gravity waves have a frequency like light does? Goodbye Galaxy (talk) 17:13, 14 February 2012 (UTC)


 * I think you probably actually mean gravitational wave. Yes, according to theory, small amplitude gravitational waves travel at c, and have a frequency whose value depends on the observer's frame of reference, just like light.  However, so far LIGO has been unable to detect a single gravitational wave after a decade of trying, which suggests that it might be possible that there's some kind of problem with physicists' understanding of gravitational waves.


 * In the unlikely event that you really did mean gravity wave, then no, they travel slower than c in any frame of reference, but they do of course have a frequency. Red Act (talk) 18:09, 14 February 2012 (UTC)
 * Thanks! (Yes, I did mean gravitational wave) Goodbye Galaxy (talk) 20:16, 14 February 2012 (UTC)
 * One of the problems with gravity, from a theoretical physics point of view, is that as a fundemental force it lacks any known means of transmission. If it is a force, then there should be a force carrier which actually transmits it, something like what a photon does for the electromagnetic force and light.  This complete lack of any actual means of transmission; the idea that gravity seems to work without any known mechanism, is fundemental to the general relativity perspective on gravity, in the sense that gravity is not a force, but a pseudoforce; the effect of gravity is created not by a force but by perterbations in spacetime created by objects with mass and energy.  The inability to work GR-definitions of gravity in with things like quantum mechanics and the standard model is the central stumbling block in creating a so-called theory of everything.  -- Jayron  32  18:44, 14 February 2012 (UTC)
 * Funnily enough, we do actually have an article called speed of gravity, which may be useful. Smurrayinchester 21:26, 14 February 2012 (UTC)

Effect of an EMP on a positive current
Lets say you have some positive current, whether it's protons, or ions, running through a circuit. What would the effect of an EMP be on it? Would a positive current be immune to an EMP? ScienceApe (talk) 20:16, 14 February 2012 (UTC)
 * It is either going to add or take away from it, or both. So for ions they should be accelerated some way not necessarily in the direction they were going. Immune would mean not damaged or affected by it, so the current itself it not really an "item" tat can be damaged, but what supplies the ions or surrounds them may be affected. Equipment that prodces protons or free space ions is probably built for high voltages and will not be hurt by a pulse of a few thousand volts, but check your circuits. Graeme Bartlett (talk) 20:43, 14 February 2012 (UTC)
 * You're saying positive currents naturally have high voltages? ScienceApe (talk) 02:11, 15 February 2012 (UTC)


 * A varying magnetic field induces voltage in a conductor, so I can appreciate an EMP affecting a conductor or a circuit, but is a "positive current" a well defined thing? Would a beam of positive ions in vacuum be affected the same by a magnetic field transient as a current of positive ions in a tube of liquid, or hole current in a semiconductor? Edison (talk) 15:28, 15 February 2012 (UTC)


 * I was assuming a current of ions in a vacuum, but in solution, it is pretty likely that the EMP will be absorbed, but will still ahve the same kind of effect, eg boosts and then reverses the current whcih then goes back to the steady state. Positive or negative makes little difference, and in a solution you will have negative ions too. Graeme Bartlett (talk) 02:10, 16 February 2012 (UTC)

System of particles in special relativity
Special relativity question: So the 4-momentum of a particle $$\bar{p}=m\bar{v}$$. I'm trying to extend this to a system of particles, but I'm having some trouble.

In Newtonian mechanics, $$\mathbf{p}=m\mathbf{v}$$ for a particle, where $$\mathbf{p}$$ is the 3-momentum. For a collection of particles, $$\mathbf{p}_{\mathrm{total}}=\sum_{i}\mathbf{p}_i$$. I'm then tempted to do the same thing for relativity and say that $$\bar{p}_{\mathrm{total}}=\sum_{i}\bar{p}_i$$.

Here's the problem: the $$\bar{p}_i$$'s are generally changing with time, which means that an observer will have to pick a time t to make the measurements of the 4-momenta. But simultaneity is relative, so another observer will measure different 4-momenta, and hence the total 4-momentum for system of particles will be different for the two observers, which contradicts what a 4-vector is.

What's the deal? — Preceding unsigned comment added by 74.15.139.132 (talk) 20:49, 14 February 2012 (UTC)

Just in case what I said wasn't explained well, here's a concrete example: http://i.imgur.com/cYqg8.png The diagram shows the worldlines of two particles with mass = 1, and two reference frames, S and S'. They each want to measure the magnitude of the total 4-momentum of the two particle system initially (in their frame).

Obviously, the magnitude of the 4momentum in S frame is 2. But in the S' frame, at t'=0 the magnitude of the 4momentum isn't equal to 2; it's greater than that, because when S' adds the two 4-vectors together, they're no longer parallel, an by the triangle theorem in Minkowski space the norm is greater than the sum of the norms. Hence the magnitude of the 4momentum is not Lorentz invariant. 74.15.139.132 (talk) 22:33, 14 February 2012 (UTC)


 * It's true that the magnitude of the total 4-momentum of the system at a particular time will vary depending on your frame of reference if the system is not closed. This is even true for a single particle if you aren't at the same place as the particle.  If 4-momentum isn't conserved in time, then there's no hope that you can measure at a distance in different reference frames and get the same result.  One way to avoid this is to restrict yourself to measuring total 4-momentum only for closed systems (where total 4-momentum is conserved). Rckrone (talk) 03:17, 15 February 2012 (UTC)
 * So there's no way to talk about the 4-momentum of an open system? 74.15.139.132 (talk) 03:29, 15 February 2012 (UTC)
 * You can, it just depends on the reference frame. Rckrone (talk) 04:22, 15 February 2012 (UTC)
 * Okay thanks. 74.15.139.132 (talk) 04:32, 15 February 2012 (UTC)
 * This is why advanced treatment of relativity invariably results in nasty matrix and tensor formulas. You can't do the math correctly unless you write out the constraint equations: and there are a lot of constraint equations.  It's infeasible to do this unless you use the ugly tensor formulas, and you'll probably have to expand them out, instead of writing them compactly.  (A lot of physicists say that Einstein notation is "elegant."  They're wrong.  It is objectively ugly, and this is a fact, not an opinion.  Not only is it ugly, it's just plain useless for numerical work; at least when equations are written in expanded form, you can easily translate them to computer code).  Even in classical mechanics, a system of n moving particles has dimension O(n), and a system of n interacting particles has dimension O(n2), so you can imagine how horrible this becomes when the interactions are corrected relativistically.  I want to say that Bittencourt has a chapter on high energy plasmas with relativistic correction fudge-factors, but I don't recall a full relativistic treatment from first principles.  Nimur (talk) 20:44, 15 February 2012 (UTC)