Wikipedia:Reference desk/Archives/Science/2012 January 17

= January 17 =

Order of chemical reaction
It is said by many references (e.g., http://en.wikipedia.org/wiki/Reaction_order) that the rate at which a chemical reaction of reactants A and B to give products X, Y, ... occurs is given (approximately) by: r = [A]^n [B]^m k where r = reaction rate, [A] & [B] are the reaction concentrations, n & m are "reactant" orders, and k = the arrhenious factor, modified arrhenius factor, or eyring factor to taste. The sum of n amd m is known as the reaction order and is generally 2 or 3. I gather that depending on the actual reactants, n & m may or may not be equal, and are usually but not always are equal to 1 or 2 (may be fractional vaules]. Chemical databases often seem to give only the reation order, and not the individual n & m values. What I would like to know is this:  Q(a) how do you apportion the order between [A} and [B]?  Q(b) The Wikipaedia at http://en.wikipedia.org/wiki/Reaction_order implies that if the reaction is elementary, the reaction order is related to the stoichiometry.  What is the relation?  Some simple reactions I have checked e.g., O+C>CO (monatomic O & C) are order 3 if there is one product, and order 2 if there 2 products (eg O2>O+O or O2+C>CO+O) - is this the rule?  None of this makes much sense to me - one would think that doubling the concentration of of either reactant would double the rate of particle collisions & therefore the probability of a molecular change. So for O2>O+O (which would actually be O2+O2>O+O+O+O i.e., two oxygen molecules colliding) and ought to be proprtional to the square of the O2 concentration, ie order 2 - but this is apparently wrong! Why? — Preceding unsigned comment added by 124.182.32.40 (talk) 03:05, 17 January 2012 (UTC)
 * Can you give an example of a chemical database that only gives the overall order? Often times, a reaction will be described as "2nd order in "A", or something of that nature, meaning that [A] has an exponent of 2.  As for why a reaction is a particular order, it has to do with the reaction mechanism.  You cannot simply look at the overall reaction and predict the order (i.e. your "rules" for one or two products will not hold up in general).  This is not a subject that is easily learned from an encyclopedia such as Wikipedia - it is best to learn from a competent teacher, or at least a textbook.  Even many chemists do not really fully learn about why reaction orders are the way they are (though many do - it depends on the person and the sub-field) Buddy431 (talk) 03:34, 17 January 2012 (UTC)
 * One major flaw in your (IP 124.) thinking is the "therefore..." of "doubling the concentration of of either reactant would double the rate of particle collisions & therefore the probability of a molecular change". The actual change from reactants to products might involve several discrete steps (the "reaction mechanism" Buddy431 mentions), not just a single unified change as suggested by the equation. One key is to look at the rate limiting step and the identities and numbers of molecules involved in it, definitely not the collisions of reactant molecules for the net reaction. DMacks (talk) 03:45, 17 January 2012 (UTC)

Buddy431 may be correct, but weren't the examples given elementary (single hump) reactions? The database at http://kinetics.nist.gov/kinetics/index.jsp is an exapmple that gives the overall order only. So unless there is a way of working out how to allocate the reaction order, this database would seem to be near useless. Surely, if it is not just a matter of particle collision rates, the additional factor(s) can be at least named, if not described? I have some chemistry textbooks of undergraduate & graduate level, and the most intelligent thing they say on the matter is "the order can be determined by experiment". Is this because the subject is outside the syllabus, or because it is too complex, or because it is not understood by even experts? I would suspect the first. Meanwhile, I obtained data for all the reactions involving monatomic and molecular C & O and H & O, and the Inquirer's "rule" holds up in every case. Keit124.178.159.199 (talk) 04:25, 17 January 2012 (UTC)


 * In an elementary reaction, with no intermediates, the reactant order for liquids and gases is generally the same as the stoichiometry. For example n A + m B -> C is generally governed by a reaction rate like $$r = k [A]^n [B]^m$$, where the exponents are the same as the reduced stoichiometric coefficients.  There are lots of exceptions however.  For example, reaction involving solids (or solid catalysts) are generally zeroth order.  In addition, many reaction rates are controlled by non-obvious intermediate phases that may have unexpected reaction orders.  So in general one usually has to look up the reaction rate (or at least the reaction mechanism) to know for sure what the rate is.  Dragons flight (talk) 06:02, 17 January 2012 (UTC)


 * So, taking C+O>CO as an example, DragonsFlight's n and m are both 1 and thus the reaction order is 2. Isn't there no intermediate phase?  But this reaction is listed (http://kinetics.nist.gov/kinetics/index.jsp ) as Order 3, not 2.  Why?  Keit124.178.159.199 (talk) 07:34, 17 January 2012 (UTC)


 * Monoatomic oxygen isn't stable. 2 C + O2 -> 2 CO is a third order reaction that would seem more natural.  The intermediate in this case would then be 2 O -> O2 via fast reaction.  Dragons flight (talk) 07:42, 17 January 2012 (UTC)
 * Yup, again there is an assumption that the given net equation is actual reaction (atomic/molecular species as written, single mechanistic step, no other catalytic components, etc.). The reaction proposed in the ref given in the database is:
 * C + O + M → CO + M
 * all taking taking place in the gas phase as part of a rapid high-energy sequence of reactions (so single-atom oxygen is correct--actually they studied the dissociation of CO and extracted this reverse-reaction as part of a complex series of reactions under steady-state or equilibrium conditions). But notice "M"--an unidentified "collision partner" involved in the reaction! If I understand the details correctly, it could just be a sink for excess energy or balance the electronic states or something like that. So its chemical identity is not changed or specifically identified, so a "net equation" analysis might ignore it on both sides. But there it is, now that the actual reaction is given for the specific mechanistic details, rather than the net chemical one--a third chemical entity to correlate with it being third-order. DMacks (talk) 08:05, 17 January 2012 (UTC)
 * Yep, that's a correct interpretation for "M"; it's an energy sink. You'll quite often see something similar whenever you have two monatomic species coming together to form a diatomic product in the gas phase.  (Consider what happens when, for example, the independent C and O atoms meet.  They start out as free atoms, then when they get together, they form a chemical bond...which releases energy...which has to go somewhere...and the only place that energy can go is back into the kinetic energy of the two atoms, which just pulls the bond apart again.  The only way that the bond can be stable is if a third atom or molecule is present right when the C and O get together; one or both atoms can then collisionally transfer some energy to the third party, leaving the newly formed CO without enough internal energy to break itself apart again.) TenOfAllTrades(talk) 15:33, 17 January 2012 (UTC)


 * Heterogeneous catalyst anyone? Gas-phase reactions involving three reactants with no intermediates don't really happen in real life. "2 C" would simply be part of a solid carbon substrate, oh hey like they form syngas.
 * and the only place that energy can go is back into the kinetic energy of the two atoms, which just pulls the bond apart again.
 * Not necessarily. See rabi frequency. I wonder if it is possible to do stimulated emission with photochemical reactions; it would be so cool. 137.54.17.9 (talk) 20:23, 17 January 2012 (UTC)
 * It's not clear they are proposing exactly "no intermediates" for the C+O→ CO (with M involved), just that the concentrations of all three are involved in the rate-limiting part of the pathway. However, they do propose and then argue against:
 * C + O → CO*
 * followed later by a discrete reaction of decay of that excited state via collision with M using data from their experiment. DMacks (talk) 20:38, 17 January 2012 (UTC)

I had thought that when two particles collide, then there is a probability of forming a new complex, depending on whether their kinetic energy and their orientation (steric factor) permits it. The new complex then spontaneously decays back into new particles,which may or may not be the same molecules as what collided (and they could be of the same molecular formula but different atoms paired/joined up. In the case of two atoms such as C & O coliding (as distinct from two molecules colliding) there are no bonds to break, but the resulting CO holds less (bond) energy - this just means that the CO complex will form at lower kinetic energies than otherwise.  In other words, TenOfAllTrades is incorrect - the resulting CO molecule will carry on with kinetic energy greater than the summed kinetic energy of the reactant C & O - hey - this means a temperture rise, just what we get!  Is this wrong?  I read the wikipedia article on rabi frequency - I'm none the wiser for it.  Keit

Incidentally, I think this is Wikipedia Reference Desk at it's best. Right or wrong in theory, each contributor, and my own effort in explaining it, has improved my undertsanding better than hours of pooring over textbooks. Please keep it up! Keit121.221.234.228 (talk) 02:51, 18 January 2012 (UTC)

Energy density greater than diesel petrol
Are there any liquids that have an energy density that is greater than diesel petrol? ScienceApe (talk) 10:41, 17 January 2012 (UTC)
 * I think some thicker fuel oils are higher energy density than diesel. This slightly dubious chart claims that #6 oil has significantly more energy than diesel. (However, #6 fuel oil is not something you would put in your car.) APL (talk) 11:35, 17 January 2012 (UTC)
 * Would #6 fuel oil be useful as a napalm ingredient? ScienceApe (talk) 12:38, 19 January 2012 (UTC)

The list cited by APL gives energy density per volume. The energy in a liquid fuel comes from the hydrogen and carbon atoms. It follows that liquids with a higher density (mass per unit volume) have proportionally higher energy value, and meassured on a energy per unit mass basis, all hydrocarbon fuels have the same energy value, within an insignificant variation range. But measured on a volume basis, the energy content can vary a little bit, as carbon is a heavier atom than hydrogen, so an increased carbon conent leads to greater mass per volume. Keit124.178.159.199 (talk) 12:23, 17 January 2012 (UTC)


 * Doesn't the energy come from the hydrogen and carbon bonds? (Also, what is "diesel petrol"? "diesel" and "petrol" are two different things.) 86.181.206.2 (talk) 12:51, 17 January 2012 (UTC)


 * According to diesel fuel diesel is any fuel that can be used in a diesel engine. Diesel petrol is just a way to be more specific about the petroleum derived substance. ScienceApe (talk) 12:59, 17 January 2012 (UTC)


 * Perhaps this is a regional thing. Are you American? In the UK, "diesel" and "petrol" are two different types of fuel. What we call "petrol" over here is called "gasoline" in the US, so perhaps "diesel petrol" would not sound so odd there. 86.181.206.2 (talk) 13:55, 17 January 2012 (UTC)
 * No, it still sounds pretty odd. We mostly don't use the word petrol at all; most people probably recognize it, as the British way of saying "gasoline".  To the best of my recollection I have never heard anyone refer to diesel petrol. --Trovatore (talk) 17:08, 17 January 2012 (UTC)
 * That's because no one does. I got the name wrong. According to the article, it should be called petrodiesel. ScienceApe (talk) 19:43, 17 January 2012 (UTC)
 * Oh, I see &mdash; sounds like a back-formation, like snow skiing, motivated by the advent of "biodiesel". --Trovatore (talk) 20:44, 17 January 2012 (UTC)
 * In USA, it's relatively common to hear people refer to "diesel gas", even though that's technically incorrect. (I guess on the idea that anything you get at a gas station and then put in your car must be a type of gas.) I assumed that the commonwealth speakers did the same with their words. APL (talk) 21:20, 17 January 2012 (UTC)
 * The chart in the Energy density article is very relevant. It visualizes the distinction that Keit above emphasize. From the chart you can see that diesel beats liquid hydrogen on density per volume, while it beats aluminium and anthracite on density per mass. Considering fuel for transportation, it's appaling to see how bad lithium ion batteries are doing. The chart also seem to indicate that laptops (where volume is key) would not run significantly longer if they were fuelled by hydrogen fuel cells. EverGreg (talk) 12:56, 17 January 2012 (UTC)
 * Could I burn "aluminium" in an aluminum engine block? What does it cost per gallon? Edison (talk) 14:42, 17 January 2012 (UTC)
 * A mixture of water and molten aluminium is highly explosive and has been theorized to have played a part in the WTC collapse . Aluminium powder can be used as Thermite, which produces extremely high temperatures. So the energy is there, but nobody has built an engine that runs on it. Yet.. EverGreg (talk) 14:58, 17 January 2012 (UTC)
 * One of the early internal combustion engines could run on coal dust. It burns at a significantly lower temperature than aluminum, though.
 * Actually, Space Shuttle Solid Rocket Boosters ran on aluminum. It's a pretty standard material for solid rockets. --Itinerant1 (talk) 20:30, 17 January 2012 (UTC)

How does constructive/destructive interference work in a chorus?
Hi all. When a single voice is singing in a chorus, from a given distance back you hear it at a given volume. When another voice joins in, singing at the same volume, you the listener hear it grow louder. As more voices join in, it becomes louder still. I assume that this is because of interference &mdash; the different waves are contributing to make bigger waves. But why is this always the case? Shouldn't the additional waves be just as likely to cause destructive interference? Shouldn't the net effect of a dozen voices cancel each other out? Why does adding another voice (and note, they could be singing something completely different) always make it louder? Thanks! &mdash; Sam 63.138.152.219 (talk) 14:53, 17 January 2012 (UTC)


 * Great question! I'll give it a first try. :-) Each singer produces a lot of different frequencies, so some will cancel destructively and some constructively with other frequencies from other singers. This also depends on where you and the singers are standing. It can be constructive in one place and destructive in another. But your issue remains. This should even out, so that two singers sound as loud as one on average.
 * That is, if the amplitude of one singer is 2, you get constructive interference with 2+2 = 4 or you get 2-2=0, with the average (4+0)/2 = 2 or one singer. So what's wrong here? Judging from the decibel article, the power is proportional to the square of the amplitude. So an amplitude of 2 results in a volume from the singer of 2*2 = 4. While the amplitude of two singers in constructive interference gives a volume of 4*4 = 16. If you then average constructive and destructive interference you get (16+0)/2 = 8. Then we're back with our initial intuition: Two singers produce a volume twice as high as one.
 * But since decibles involve a logarithm, this isn't quite right, nor is our intuition. The relevant formula, with a handy calculator, can be found here: EverGreg (talk) 15:28, 17 January 2012 (UTC)


 * Very interesting. So is this saying that, while the average amplitude of the waves that hit our ears will still be the same ("2", in your example), the average power will be greater? That's pretty surprising. &mdash; Sam 63.138.152.219 (talk) 16:09, 17 January 2012 (UTC)


 * No—the amplitude does vary from 0 to 4, but the average isn't 2 because it isn't symmetrically distributed. For incoherent superposition the average works out to 8/π ≈ 2.5, but I don't think this is physically meaningful. The square of the amplitude is proportional to the energy of the vibration and simply adds, as you'd expect. -- BenRG (talk) 01:20, 18 January 2012 (UTC)


 * Yes, but on top of that, a doubling in energy does not double the perceived volume, because our ear (and the decibel scale) has a logarithmic relationship between sound energy and volume. EverGreg (talk) 14:21, 19 January 2012 (UTC)


 * Interference only happens when waves are coherent. Sound from two different singers is never coherent, therefore neither destructive nor constructive interference occurs. The intensity of the waves adds linearly: two singers produce twice the intensity of one. In theory, if two singers sang exactly the same frequency to much better precision than the human ear can distinguish, their voices could interfere. This isn't going to happen in practice, though. --Srleffler (talk) 17:48, 17 January 2012 (UTC)
 * That doesn't sound like a correct explanation. Humans can most certainly perceive interference that occurs between two humanly distinguishable pitches.  Humans can hear pitch differences down to about 6 cents, which for example for a 440 Hz pitch (A above middle C) is 2.6 Hz, well below the roughly 15 Hz cutoff below which audible beats can be perceived.  For a pitch difference of considerably more cents (and/or between higher pitches), the interference can instead be perceived as a difference tone, but it's audible interference occurring between distinguishable pitches either way.  Red Act (talk) 19:03, 17 January 2012 (UTC)

Relatedly
I don't want to hijack the above question (please feel free to reply to the section above!) but I had a related question that came to mind. When I was a child (well before puberty), I could at will make a kind of high pitched ringing voice in the back of my throat. It was quite loud. I can't quite describe how I did it — it wasn't just screaming or shrieking, but sort of interior whistle. It wasn't just me that could do it, there were a few other boys who could as well. Anyway, if two of us did it at the same time, it would produce a very strange sound, almost electrical sounding in nature (like the sound a rippling electrical transformer would make in movies), or something somewhat similar to what a cicada sounds like. (I'm having trouble describing this, like most qualia.) My real question is what caused this very strange and really quite distinct sound. My guess is that the frequencies being emitted by me and the other boys were more or less fairly close to being pure tones of some sort, and the differences between them were somehow creating some kind of dynamic interference. Does that make any sense? (Is there a name for this kind of sound that young children can produce?) --Mr.98 (talk) 19:23, 17 January 2012 (UTC)
 * If the frequencies were close enough, probably they could produce beats. Have you tried it out as an adult? :P  Lynch 7  19:31, 17 January 2012 (UTC)
 * Agreed that beating of two similar frequencies is a key player. Note that the beats will drift dynamically. I had a very similar experience to Mr.98 as a child, with the exception that my high-pitched sound was not especially different than a normally-voiced falsetto. The phenomenon does sound rather different than say, listening to someone else tune a flute to a tuning fork. I think this is because, if you are producing one of the two voices, then, in addition to the beats in the sound waves carried by the air, your skull is beating too :) SemanticMantis (talk) 20:09, 17 January 2012 (UTC)
 * Good point. Your voice will be sorta interfering with the beats as well, so all in all, quite a weird sound.  Lynch 7  20:39, 17 January 2012 (UTC)


 * Re: "Is there a name for this kind of sound . . .", Overtone singing and its links may be of relevance/interest. {The poster formerly known as 87.81.230.195} 90.197.66.192 (talk) 09:22, 19 January 2012 (UTC)

Gravitational time problem.
On earth I am attracted by a force = G * m1m2/r^2

However what happens if there is say a globular cluster 100 light years away. I did not exist 100 yrs ago so It cannot be the above formula there must be a time element. — Preceding unsigned comment added by 92.30.204.80 (talk) 17:25, 17 January 2012 (UTC)
 * The formula you gave applies only for two masses at a single instant in time. If there is another mass (your cluster) or a mass which is changing over time (you?) or relative movement of the various objects, then things get more complicated. Basically a time element will appear because m1, m2, and r will not be constants but each will be a function of t. Staecker (talk) 17:38, 17 January 2012 (UTC)


 * You're quite correct, there is a time element to gravity. Gravity is most accurately modeled with general relativity, which does take speed-of-light considerations into account.  The simple Newtonian gravity model expressed by the equation above can be viewed as an approximation to general relativity, that only works well under a certain set of circumstances.  Red Act (talk) 17:51, 17 January 2012 (UTC)


 * [edit conflict]You are correct that the formula you give is not appropriate for that situation. You need general relativity to deal with gravitational forces between very distant objects. --Srleffler (talk) 17:52, 17 January 2012 (UTC)
 * If you don't like Einstein tensors, it's possible to use a formulation analogous to the relativistically-correct Liénard–Wiechert potential for electromagnetism. You simply replace time with retarded time in all the relevant places in your equations, and presto - a general, relativistically-correct formulation of gravity.  (Not to be confused with a general-relativistic correct formulation of gravity).  There's no shortage of research on the applicability of this approach: Google found almost 2,000 research papers; and I'm pretty sure I saw this worked out to completion in a physics course-note some time ago.  The moral is, you don't need general relativity, unless you need it.  Nimur (talk) 19:41, 17 January 2012 (UTC)
 * In addition to the Newtonian model, there are dozens of other alternatives to general relativity, some of which are consistent with special relativity, and some of which avoid the use of tensors. One historical example that uses retarded potentials is Whitehead's theory of gravitation (although that one does use tensors).  Like the Newtonian model, the other alternatives all work well in some circumstances, and give wrong answers in other circumstances.  Only general relativity always gives the right answer (although it has problems dealing with quantum mechanics, and it has awkward singularities).  Red Act (talk) 01:34, 18 January 2012 (UTC)
 * I'm not sure if everyone's got what the OP is on about. I think the question is really about how the gravity formula can even make sense, given that there is an attraction between two things that are so far apart, the concept of their simultaneous existence is mucked up. In fact, if I've understood rightly, it's not that complicated. Whatever general relativity involves, it reduces to Newtonian physics in most cases (I don't know how big the corrections are for a distant star). So the gravity that you feel is from the star as it was 100 years ago, and in simple calculations, you don't need to know anything more. If it got destroyed about 50 years ago, that will have no effect on the current gravitational pull. Same for your gravitational pull on the star, ie. at the moment, it doesn't sense any pull from you, since your existence is outside its light cone. IBE (talk) 20:11, 17 January 2012 (UTC)
 * When I was in school ca. the early 2000s, our astronomy professor taught us that it wasn't entirely clear how fast gravitational force propagated. That is, if the Sun blinked out of existence right now, how long until we felt the gravitational effects on Earth? At the time I recall some science writer (Brian Greene?) suggesting it would be instantaneous, while my professor (a serious astronomer) said that this was wrong, that it would propagate at c. I see we have an article on speed of gravity which more or less agrees with my professor. --Mr.98 (talk) 20:33, 17 January 2012 (UTC)


 * School in the 2000s? Then you must be fairly young, and I don't know where (or rather when) you cram this knowledge in. I doubt it was Brian Greene, since The Fabric of the Cosmos explains quite carefully the speed of gravity is exactly c (p72, 2004 ed). IBE (talk) 20:50, 17 January 2012 (UTC)
 * By the early 2000s, I think it was pretty well established that gravity didn't act instantaneously. Reference 17 of the article you link to is dated 2001 and describes experimental evidence for gravity travelling at approximately the speed of light. --Tango (talk) 21:10, 17 January 2012 (UTC)
 * It was probably in 2000 or 2001, so that probably would have been pretty fresh stuff at the time. (It was right around when all the evidence for inflation came out — the professor in question had some small role in one of the experiments involved in detecting that.) I think it must have been Greene's Elegant Universe. The preview pages I can see online don't have the right pages in them (I think it is 73 or 74? He talks about time and GR there.) so I can't recall who was right and who was wrong — the positions might have become switched in my mind. (And I've long since given away my copy at a yard sale.) But I remember it being a subject of contention. --Mr.98 (talk) 01:21, 18 January 2012 (UTC)
 * It was controversial once, but I thought it was settled by the observations of PSR B1913+16 in the 1970s. I feel obliged to point out that general relativity requires energy conservation, so you can't extract any prediction about what would happen if the sun blinked out of existence. It has to go somewhere, and whatever causes its sudden departure has to come from somewhere, and whatever that is, it gravitates too, because everything gravitates. That's why the whole thing was controversial: in contrast to the electromagnetic case, you can't reach in with an uncharged manipulator arm and push your charged particles around. Gravity "knows" about everything, and that made it plausible that there might be no need for gravitational waves to correct the long-distance field when the gravitating object's state of motion changed. The other problem was/is figuring out what it even means for a gravitational wave to propagate at c, given that a gravitational wave is a distortion of the spacetime geometry that gives meaning to the speed c in the first place. -- BenRG (talk) 02:27, 18 January 2012 (UTC)

'Blue' Veins in the Heart, elsewhere?
So I understand that blood in the human body is never blue. However, veins do look blue from the skin. The wikipedia article claims that this is a side effect of light absorption through the skin. However why does the human heart have visible blue veins? See this photo for an example. --188.220.46.47 (talk) 19:38, 17 January 2012 (UTC)
 * Veins are not blood, veins are vessels that carry blood. If I have a green hose, it does not make the water inside the hose green.  -- Jayron  32  16:44, 19 January 2012 (UTC)

lens as spatial filters
A focusing lens focuses light focused at infinity into some image 100 cm away.

Does it matter if a diffraction filter is placed before, or after the lens? I mean, a lens is an inverse fourier transform right? And a diffraction screen forms a fourier transform? Do the operations commute, so long as the diffraction screen is placed well before the image plane? 137.54.17.9 (talk) 19:49, 17 January 2012 (UTC)


 * A lens is a lens; it's not a fourier transform. The operation that a lens performs can be modeled by a fourier transform.
 * As far as commutation of optical elements: to first order, some optical elements commute; but you should be aware of the real-world effects that countermand that. All (passive) optical elements attenuate the light somewhat.  Imperfections in the optics are usually unpredictable and therefore irreversible in practice.  Changing the optical path changes the scale sizes you need.  (For example, if you magnify an image, your next optical element needs a larger aperture; in other words, aperture is non-commutative if you have magnifying optics).
 * I don't know how you would place a diffraction grating in the optical path. How would the light go through it?  Nimur (talk) 20:18, 17 January 2012 (UTC)


 * Wouldn't the collimated beam get diffracted and then get focused later? 137.54.17.9 (talk) 20:52, 17 January 2012 (UTC)

Most classical theory considers infinitely thin apertures. What happens when the spacings have thickness, as in real life? Do you get a 3D fourier transform? Do you get optical aberrations because now different orders of light are diffracted and focused differently? 137.54.17.9 (talk) 20:32, 17 January 2012 (UTC)
 * What you get is "a thing that can't be accurately modeled with the thin lens equation." For this reason, we have optical ray tracing and full wave equation modeling.  Such mathematical models take into account the way that waves interact with sophisticated material properties and complicated geometries.  Nimur (talk) 22:45, 17 January 2012 (UTC)

Searching for medical term (not advice)
From personal experience, when I go to the doctor for a regular check-up, one of the first things they do is check my blood pressure. It is invariably always a little higher when they check it at the office, as opposed to when I check it at home. I attribute this to just being nervous- or a little high strung- about the whole doctor's office experience. Is there a specific medical term for this? Note: I am looking for a specific medical term (if it exists). I realize that "false-positive" or "placeabo effect" might loosely apply...but I imagine there is an actual term found in medical texts. Anyone know? Quinn &#10163;WINDY 21:00, 17 January 2012 (UTC)


 * White coat hypertension. DMacks (talk) 21:01, 17 January 2012 (UTC)


 * Excellent, thank you. The converse term Masked hypertension (found in the article) is also very useful info for my project. Cheers! Quinn &#10163;WINDY  21:10, 17 January 2012 (UTC)


 * My own doctor has told me that blood pressure can vary quite a bit during any given day. What they're looking for is trends. There are various tests your doctor can schedule for you if your blood pressure is consistently elevated or high. ←Baseball Bugs What's up, Doc? carrots→ 00:06, 18 January 2012 (UTC)


 * Well, no matter how many BP tests are scheduled at the doctor's office, if every one makes you nervous, then that approach is no good. Perhaps you might get used to them over time.  If not, home testing would seem to be the only alternative.  No doubt the docs are worried those aren't accurate enough, though.  Some way to do highly accurate home tests is apparently needed for these patients. StuRat (talk) 00:27, 18 January 2012 (UTC)


 * That isn't necessarily true. As our article notes and should be obvious, most trials regarding the risks of high blood pressure are based on blood pressures recorded in clinical settings. So most of the values we have for risks are based on the value recorded in clinical settings, not any lower values you may record at home. Furthermore, if you're having your blood pressure recorded regularly, your still likely to detect changes (they may be slightly masked, although that's unclear to me from the article).
 * Of course since the effect varies between individual, it's likely best (as our article also hints at), for further studies to be done on the risks associated with blood pressure based on values not recorded in a clinical setting. But even this doesn't mean the clinical value is useless once we have such information, and it would only make sense to consider non-clinical values. In fact our article appears to suggest it may provide additional useful information about risks, which isn't that surprising.
 * Nil Einne (talk) 05:46, 19 January 2012 (UTC)


 * Also, the blood pressure meter you are using at home could be faulty in a way that it consistently measures lower than real pressure. &#x2013; b_jonas 16:35, 21 January 2012 (UTC)

Deepest drilling
Apologies for the multiple questions (I'm trying to get through some of my to-ask list before the big black banner at the top of the page reaches the Zero hour), but both this one and the above are actually related to a project I'm working on...I'll spare you the boring details. So, the question is, what is the deepest anyone has drilled into the Earth? To clarify, I am looking for "closest to the core" as opposed to "from sea level". And, if possible, I'd like to know what, if any, technical limitation prohibit someone from drilling deeper. Is it just because there has been no reason to go deeper...or is it temperature...pressure...or perhaps the drill itself become too weak after a certain point? Anyway, any links to articles or outside resources about this are appreciated. Quinn &#10163;WINDY 21:17, 17 January 2012 (UTC)


 * The Mohorovičić discontinuity article lists several attempts (each in turn with their own articles linked) to drill down that deep. -- Finlay McWalterჷTalk 21:20, 17 January 2012 (UTC)


 * Take a look at the articles that are linked to in the Mohorovičić discontinuity article as well. Project Mohole, Kola Superdeep Borehole and Sakhalin I.  However, the Sakhalin I wells are "extended reach boreholes" which means the well path turns underground and goes horizontal.  So, the Sakhalin depth of 12,345 meters (40,502 ft) is actually only about 1000m deep.


 * As for the problems, everything you mentioned above, is part of the reason. The conventional procedure to drill a well uses a motor on surface to turn the drill bit.  The drill bit is connected to the surface by drill pipe, which comes in about 10m/30ft joints, which can be combined into 30m/90ft stands.  As the bit drills through the earth, fluids called drilling mud are pumped though the drill pipe and come out drill bit.  The mud cools the bit and helps wash cuttings (ground up rock) up hole.  You have to remove the cuttings to keep drilling.  Mud also helps to prevent blowouts, which happen if the pressure of the fluids in the rocks exceed the pressure in the borehole.  Blow outs are bad.  Once you've drilled 30m/90ft, you have to stop drilling for a minute and connect another stand of drill pipe, using the derrick and another motor to lift and move the stand.  Each stand is heavy.  Drill pipe can weigh over 100lb/ft (130kg/m) so a stand can easily weigh as much a couple cars.  So, the deeper you go, the more powerful your drilling motor and derrick motor have to be and the stronger your derrick has to be.  So the first reason is every bit of depth you drill adds a lot of more weight.


 * While you drill the bit will start to wear out. It wears out faster at higher temperatures.  While mud can help with this, there are problems with mud at very deep depths as well.  Once the bit wears out, you have to pull out the drill bit and replace it.  A good crew can pull a stand (move the drill string up by 90ft) about every 5 minutes.  So at 40,502 ft, it takes at least 37hours to pull the bit and 37 hours to run back in.  So if it takes 3 or more days every time you have to change the bit, and you have to change it pretty often that deep (due to temperature and other factors) that really adds up and costs a lot of money.  A deep water drilling rig may cost $2-3 US per second to operate.  So the second reason, bits wear out faster due to temperature and take longer to replace and cost a lot of money.


 * Finally, the mud system gets more complex. You have design muds that won't boil at temperatures over 400F and will be viscous enough at those temperatures to bring up cuttings, but not be too viscous at surface temperature.  Exotic muds are very expensive.  Then you get to the point where it's impossible to get the properties you need with conventional fluids, so you have to go to experimental fluids, which cost even more.  Eventually you get to the point where you can't get a fluid that does everything you need, and have to design much more complex systems.  You also need a lot of fluid moving very fast to cool the bit, so you have to have larger and more powerful mud pumps to get flow rate necessary to cool the bit.  Which adds more cost.  The third reason is the cost and complexity of the drilling mud.


 * Even with an infinite amount of money, conventional drilling eventually hits a point where it becomes technologically impossible to go any deeper due to the temperature, ie steel or titanium melt, fluids break down from temperature and pressure, derricks and motors can't be made any stronger.


 * the ;tl:dr version. The deeper you go the hotter it is, the longer it takes and the more it costs until the equipment just can't do it.  And the cost and time don't increase linearly, but maybe exponentially.   Tobyc75 (talk) 22:01, 17 January 2012 (UTC)
 * What a nice and clear explanation. Thank you. --Mr.98 (talk) 12:58, 19 January 2012 (UTC)
 * I concur. Just got back after the black out, and was pleasantly surprised to find this very informative and detailed answer. Thank you for the time you spent on this! Quinn &#10163;WINDY  16:53, 19 January 2012 (UTC)
 * Thanks, for the kind feedback. Tobyc75 (talk) 22:41, 19 January 2012 (UTC)
 * If you launch unmanned probes in space, you get to show nice close-up photos of Jupiter moons to show to the public. But how would you convince a senator to approve money for a project to drill down deeper than ever?  According to Irregular Webcomic strip 2835, geology has always been the abandoned foster child of science.  &#x2013; b_jonas 16:34, 21 January 2012 (UTC)