Wikipedia:Reference desk/Archives/Science/2013 June 17

= June 17 =

NIR and static
Hello

I had a question about Near Infra Red Spectroscopy. If we were conducting the measurements on a powder sample, is there any potential for the electrosatic forces within the powder to affect the readings in anyway?

Many thanks

114.77.39.141 (talk) 12:44, 17 June 2013 (UTC)


 * Yes - in science, almost everything has some effect on almost everything else. I would not consider the presence of an electrostatic field to be a primary contributor to the infrared spectrum for most materials.  But, it is detectable - at least in some materials, with some very specialized equipment.  For example, a near-field optical microscope can detect Raman scattering, and the spectrum of the Raman-scattered light may be affected by an applied electric field.  In fact, I know of a specific professor who specializes in this sort of thing, and you can read his publications-list; and here is Electric field gradient effects in Raman spectroscopy (Physical Review Letters, 2000).  I don't think powdered substrates are very good candidates for an NSOM microscope experiment; but in principle, the application of an electric field does have a small and measurable effect on the scattering spectrum.  For most cases, in ordinary spectroscopy using ordinary equipment, the DC electric field has no significant effect on the infrared spectrum of a sample.  Nimur (talk) 17:41, 17 June 2013 (UTC)

Dinosaur's genders
There was a line in the "Jurassic Park" film that said that dinosaurs could change their gender, or even have offspring by themselves when in a time of need. Just to confirm it: that was a huge artistic license, and in the real world dinosaur's reproduction did not work that way, right? Cambalachero (talk) 14:28, 17 June 2013 (UTC)


 * Did they claim that the dinosaurs of old did that? I don't recall them saying it.  What I do recall is them saying that the dinosaurs in Jurassic Park had that trait because the bits of DNA that were missing from the archeological samples were replaced from frogs who do, in the present day, show this ability in times of need.  Dismas |(talk) 14:35, 17 June 2013 (UTC)


 * Of course there's much artistic license there, but it does not seem impossible. Our knowledge about dinosaurs is limited, but the process you describe, which is called Parthenogenesis has been observed in some modern-day reptiles (I'm talking about having offspring by themselves, not changing their gender). - Lindert (talk) 14:37, 17 June 2013 (UTC)


 * (EC) Well, we can't really be sure if dinosaurs could change sex or not. As I recall, the book had some "rationale" for this plot device. The idea was the partial genomes recovered were supplemented with frog DNA. And it is true that some frogs (and fishes, and a few other things) can change their sex, even after sexual maturity. The common example of this is the Common Reed Frog, you can find other examples by googling things like /frog fish sex change/. I don't think the book or movie had the dinos selfing, but I could be mistaken. Lastly, not to nitpick too much, but recall that gender is a social identity in humans (and perhaps a few other social animals), while sex is the biological concept. SemanticMantis (talk) 14:38, 17 June 2013 (UTC)


 * Temperature-dependent sex determination occurs in reptiles but not birds. It would probably be hard to figure out if it happens dinosaurs, which are generally agreed to occupy the "space" between reptiles and birds. Roger (Dodger67) (talk) 17:23, 17 June 2013 (UTC)


 * More specifically, dinosaurs lie between birds and crocodilians, not the much more distant lizards and turtle. Crocodilian eggs are known to beinfluenced in sex by external temperature, but I am unaware of sex change in adults.  That usually occurs in species where the female is the larger sex (if the one large female is removed from a group of smaller males, the largest male will turn female), but this is not the case in crocodilians. μηδείς (talk) 18:16, 17 June 2013 (UTC)
 * Put another way, birds are dinosaurs, a subgroup of the theropods :) SemanticMantis (talk) 18:32, 17 June 2013 (UTC)
 * Yes, but all that proves is that some branches of the dinosaurs can't/couldn't do it. We still don't know whether some could.  Actually, birds can kinda/sorta change sex.  This article and others indicate that about one in 10,000 female birds will spontaneously change into males towards the end of their lives.  Female chickens will grow rooster tails, start crowing and become quite defensive about males trying to muscle in on their hareem.  However, they don't completely finish the job - they may change plumage and behavior and in most outward ways appear to be male - but they don't grow primary sexual organs to match - so they can't reproduce as males.  There is no particular reason to assume that dinosaurs were unable to do that - so perhaps they did.  However, that doesn't fit with the plot point in Jurassic park which was that somehow the dino's were breeding even though they'd all been created as females...so we're left with some very dubious handwaving about frog DNA, yadda, yadda, yadda. SteveBaker (talk) 18:47, 17 June 2013 (UTC)

That's like saying that my great aunt is changing into a great uncle because she's sporting a mustache :)). Dauto (talk) 14:18, 19 June 2013 (UTC)


 * As I recall, the book made it pretty clear that the sex change was an accident based on the frog DNA, and unexpected by the experimenters. As for real life, I would speculate pretty confidently (uh-oh...) that we can't know, because we're still short on dino sequence, and a fossil is a snapshot, not a video.  (One of the beautiful things about biology is that certain speculations like this that are logically foolproof are often wrong) Wnt (talk) 23:27, 17 June 2013 (UTC)

Related question
If dinosaurs fit between crocodilians and birds in a taxonomic sense, then why would you repair their fragmented DNA with frog DNA? Why not use bird or reptile DNA? 39.214.54.44 (talk) 04:41, 18 June 2013 (UTC)
 * Good question! A crocodile's DNA would be a particularly good match in this case -- MUCH better than frog DNA!  But I guess they needed some kind of plausible explanation for the dinosaurs breeding despite being all of the same sex (a similar kind of handwaving used for the movie "Godzilla", as far as I remember...) 24.23.196.85 (talk) 06:19, 18 June 2013 (UTC)
 * Pardon my ignorance, but if missing DNA portions are filled with that of another species, how do you still get a dinosaur, as opposed to some other, exotic creature? ←Baseball Bugs What's up, Doc? carrots→ 13:49, 18 June 2013 (UTC)
 * The premise what that the majority of the DNA was intact, and the frog DNA was only used to patch up the little bits that were missing. Closely related species will share very similar genes (think of the humans/chimps share 95% of their genome claims), so the hope was that any differences would be minor enough that there was still a viable dinosaur embryo. Technically, the DNA would be different than the actual dinosaur it was based on, but it would be close enough that there isn't a point in calling it something new. Of course, frogs aren't that close to dinosaurs at all, but that was part of the hand-wavy "science" that allowed the dinosaurs to breed in the end. Some highly-conserved genes do stay functionally identical across huge gaps. For example, a human HOX gene for a limb can be used in a fruit fly to cause the fly to grow a fruit fly limb. 209.131.76.183 (talk) 14:40, 18 June 2013 (UTC)
 * The reason is likely to be that Jurassic Park was published in 1990 (and probably written over a couple of years before that). Solid evidence that birds are dinosaurs didn't really start to appear until the early 1990's - and wasn't widely known and accepted until the mid-1990's.  So Crighton (who went to medical school - not to learn archeology) was unlikely to be aware of this new understanding as he wrote.  Frogs are certainly an odd choice as a source of DNA patches - why not lizards or some other reptile?  My best guess is that he wanted to use the "changing gender" thing as a plot point and the claim to have introduced frog DNA was his way to make that seem more plausible.  In the end, it's fiction - and there doesn't have to be an answer to the "WHY?" question. SteveBaker (talk) 15:33, 18 June 2013 (UTC)
 * Obviously, Crighton was aware of the understanding that dinosaurs were closely related to birds, as his characters discuss it a several points during the story. 182.1.217.41 (talk) 23:56, 18 June 2013 (UTC)

Why do some algae make PUFA (poly unsaturated fatty acids)?
Some species of algae make a large amount of PUFA. This leads me to ask the question of, why? What are the functions of PUFA for single celled organisms? As I get it, one of the reasons is to maintain membrane fluidity under a variety of temperature and salinity conditions. But, why do different algal species make chains of varying lengths? For instance, some make C20:5 others C22:6. What's the added advantage of an extra double bond? Especially considering that these organisms tend to inhabit the same ecological space (thus enviromental effects are the same). If anyone has any ideas, or can refer me to papers published on the subject, I'd be eternaly grateful. 137.224.252.10 (talk) 14:53, 17 June 2013 (UTC)


 * As I've commented recently, it is hard to say "why" evolution does something. But I should point out that every organism has a cell membrane, made up of phosopholipids, typically 16-20 carbons long according to the article (which sounds right).  Even in humans, which you would think would do more to regulate such things internally, polyunsaturated fatty acids incorporate into membranes and change their mechanical properties concerning "rafts" of lipid-modified proteins, cholesterol etc.   If you can name the species you have in mind it would be easier to try to research further about the specific system, but tracking exactly why two extra carbons are used by one rather than the other can't be easy, and might even be chance. Wnt (talk) 17:01, 17 June 2013 (UTC)

Race (biology)
The article Race (biology) is kind of incomplete. I still want to know how do biologists put into words the difference between humans who belong to what's socially considered different races. I understand that the socially relevant definition might be biologically inaccurate, but how do you express that? OsmanRF34 (talk) 17:04, 17 June 2013 (UTC)


 * Existing human races aren't different enough for scientists to consider them subspecies, which would lead to terms like Homo sapiens causcasicus. Individual traits which are considered racially salient include the cephalic index, objective descriptions of human skin color, eye color, and human hair.  There are other traits like Steatopygia and the epicanthic fold, blood types, limb length, tooth shape, lactose tolerance, and many other things that can vary along with what is perceived as race. μηδείς (talk) 17:38, 17 June 2013 (UTC)


 * The problem is that many of these indicators vary more within a "race" than between races. That is the source of the statement that "race has no biological basis in humans". --Stephan Schulz (talk)


 * Right, your quote is a good answer to Osman's final question. It's not that the social notion of race is biologically inaccurate, it's that it is biologically unfounded, or perhaps ill-defined. SemanticMantis (talk) 18:36, 17 June 2013 (UTC)


 * That's a rather facile and unhelpful, but typical denial. Something like "naive social notions of race are rarely rigorously defined or helpful biologically" would be much more accurate.  The desire to remove the term race from all scientific discussion is a political one, not a scientific one. μηδείς (talk) 19:01, 17 June 2013 (UTC)


 * I agree with μηδείς. Unless you believe that skin color varies more within a race (defining by a society) than between races, you have to admit that there is something biological there, and that some societies consider these differences to group people. It's not all chaos when it comes to the social identity. OsmanRF34 (talk) 22:44, 17 June 2013 (UTC)


 * The problem even with this trait is you are referring to, skin colour, is that if you go solely by it you will often be highly mislead. Someome with dark skin could easily be more closely related to someone with light skin. This shouldn't exactly be a surprise. A chimpanzee (either species) is far more closely related to a a human than a New World monkey even though both the monkey and chimpanzee have a lot more fur. And Opossums are quite distantly related to primates despite some having shared features like opposable thumbs and a prehensile tail. Nil Einne (talk) 00:17, 18 June 2013 (UTC)


 * To be fair to Stephan, if you define the traditional three old-world races, then compare the nominally Caucasian Saami people and Tamil people, you are going to find a far greater difference in skin color than you will betweeen any various populations of Sub-Saharan blacks. But that is taking a very specific and limited definition of race as if it were the only way that four-letter word beginning with arr could be used.  Obviously it is quite reasonable to talk about the racial differences between Madagascans and Tanganyikans, or between Ainu and Japanese, as opposed to the linguistic or religious differences between such peoples.  Arguing that the word has no use, or that there are no scientific correlates for some of the word's various senses, is simple sophistry. μηδείς (talk) 01:16, 18 June 2013 (UTC)


 * Biologists usually talk about differences in terms of ancestor groups and geographical regions. For example a paper might refer to people of sub-Saharan African descent, or people of Polynesian descent, etc. Looie496 (talk) 18:39, 17 June 2013 (UTC)


 * There are biological terms for groups of a species which vary by less than those of two subspecies. There's a population, a breed (as in cats or dogs), and a color phase, to start.  Those seem closer to what we call "race", in humans.  Of course, it's complicated because there's so much interbreeding between historically distinct populations in humans.  So, we're all "mixed breeds", if you want to put it in those terms. StuRat (talk) 19:46, 17 June 2013 (UTC)
 * Presumably the races arose due to lengthy isolations of groups from each other in prehistoric times. With that isolation having been significantly compromised in relatively recent times, and with more and more cross-race children being produced, the notion of race is getting increasingly murkier. We've got a ways to go, though. ←Baseball Bugs What's up, Doc? carrots→ 23:42, 17 June 2013 (UTC)


 * See Haplogroup. The way people have been classifying race is sort of like a dog pound that gets lots of French poodles and chihuahuas.  Something is either a French poodle, a chihuahua, or "sort of like one or the other".  Or, of course, it could be Spanish-speaking, in which case it is opted out of the classification scheme entirely.  ;)  Though what's sort of clear is that by and large, genetic variation beyond a certain point is called "black". Wnt (talk) 20:03, 17 June 2013 (UTC)
 * Spanish-speaking chihuahuas? I didn't know they really existed! 24.23.196.85 (talk) 20:15, 17 June 2013 (UTC)


 * Were you expecting it to speak Norwegian, perhaps, hmmm? Bielle (talk) 01:22, 18 June 2013 (UTC)

Conservation of momentum in time dimension
Is momentum something that applies and is conserved in all dimensions? So if I push something back in time, I can be accelerated through time? Or is that nonsense? --92.19.68.88 (talk) 23:20, 17 June 2013 (UTC)
 * First of all, there's no way to "push something back in time" -- this would in itself violate the laws of physics. 24.23.196.85 (talk) 23:37, 17 June 2013 (UTC)
 * I don't understand. Could a mass A travelling through time at a greater rate than another mass B, collide elastically with mass B and transfer some of its energy, increasing the rate of passage through time of B? Or do those laws apply specifically to three dimensions of which time is not one? --92.19.68.88 (talk) 23:48, 17 June 2013 (UTC)
 * You can interpet a particle moving back in time as its anti-particle moving forward in time. So, pushing an electron to move back in time is equivalent to that electron getting annihilated by a positron that it collides with. Two photons will be created, you do obviously have conservation of energy and momentum in the conventional sense. Count Iblis (talk) 23:54, 17 June 2013 (UTC)


 * If the OP would be so kind as to define acceleration "through time..." we might possibly answer the question. Acceleration is the rate of change of the change of position per unit of time.  (In other words, the second derivative of position with respect to time).  Is the question supposing to ask about the rate of change in the change in time per unit time?  That quantity makes very little physical sense, whether it is expressed in plain English or expressed mathematically.  Things don't travel through time; they change with respect to time.  In other words, the latter part of the OP's question is nonsense.  Nimur (talk) 00:18, 18 June 2013 (UTC)


 * Are we moving along a time dimension like a cart moves along a track in, say, x dimension? Or is that nonsense again? 92.19.68.88 (talk) 00:48, 18 June 2013 (UTC)
 * Obviously we are not moving along time in a cart! So the direct answer to your question is "NO!"  But your question can be answered with a little bit more substance, albeit with a lot of abstraction - if you're willing to think a little bit abstractly.
 * In physics, we use very precise terminology. This helps us avoid logical errors or semantic ambiguity.
 * Matter has position. We say something has moved when its position changes: displacement.  Let's say we have an object with a measurable position.  In classical physics, we define this type of object to be a particle - just a thing with a position.  If we measure a starting position (on the x-axis, for example), and a final position, we have a displacement - let's call it Δx for a displacement along the x-axis.  If we know when we took the starting measurement, and when we took the final measurement, we also have a change in time: let's call it Δt.  The average velocity is therefore, by definition, Δx/Δt.  We can talk about the particle moving on the x axis during the experiment time.  We can say that the object was at a position x1 at time t1.  Equivalently, we can use time as a generalized coordinate and describe a position ( x1, t1'') - but this is just a different way of describing the same event.
 * When we describe motion in this way, we are conducting the science of kinematics - the pure mathematical relationships of motion. When we attribute these motions to observable physical law, we are progressing to the science of dynamics - the study of physical motion.
 * In kinematics, we can easily consider time as just another coordinate, but when we study dynamics - in any of its forms, including the modern style relativistic mechanics, we find equations of motion where the dependence on time is not symmetric with respect to the dependence on position. One very trivial example of this is a simple case of energy conservation.  We find that in many cases, potential energy and kinetic energy are interchangeable, but the dynamics of the real world show us that the dependence on position is a first-order effect; and the dependence on time is a second-order effect.  We call this coupled first- and second- order time-space relationship Hooke's law and we use it to describe simple harmonic oscillation, one of the simplest and most fundamental physical descriptions of motion.  (Some people use this to model the behavior of a "spring").
 * So: we can use mathematical abstraction to describe displacement in space, and displacement in time, using the same language; but when we study even the most simple cases, we quickly find that time- and space- do not behave in the same way. Dynamical systems almost always exhibit different properties - rates of change, stability, and so on - with respect to time and with respect to position.  Nimur (talk) 01:12, 18 June 2013 (UTC)
 * Nimur's very excellent and easy to understand explanation is the inherent reasoning behind Minkowski space, which is the four-dimensional space created to do the geometric calculations behind special relativity. The four dimensions in Minkowski space are not identical.  Three are "space-like" dimensions, which behave like space, and one is a "time-like" dimension, which behaves as time does.  Space and time are all different dimensions, but they do not behave the same way, and cannot be treated equivalently, so one's "motion in time" is not equivalent to one's "motion in space" and one cannot discuss the two concepts as though they were interchangeable.  -- Jayron  32  12:12, 18 June 2013 (UTC)
 * Yipes. The OP has asked a reasonable question, but the answers confuse and miss the point.  An object does not have a position in space or time without specifying a slice through spacetime (e.g. "now"): it has a world-line through spacetime, and acceleration is a change in angle of this worldline: a curve in spacetime.  Objects do not travel through spacetime.  Energy–momentum is conserved in all four dimensions.  — Quondum 01:50, 18 June 2013 (UTC)
 * Last week, the term "now" referred to last week; today it refers to today. Presumably if that change were to happen more quickly, it would be an example of the kind of thing the OP is talking about. Card Zero  (talk) 01:53, 18 June 2013 (UTC)
 * Isn't that a bit like saying if a metre were longer, things'd be further apart? If by "that change" and "quickly" you are referring to some concept of movement through time in terms of elapsed time, you're confusing things. By "now" I simply meant a given instant in time as perceived by some observer.  — Quondum 02:20, 18 June 2013 (UTC)


 * This is a sort of have-your-cake-and-eat-it-too question. There is no meaning in the idea of changing your rate of travel through time.  You always go through time at a rate of exactly one second per second. Looie496 (talk) 05:08, 18 June 2013 (UTC)
 * unless you speed up. -- Jayron  32  12:12, 18 June 2013 (UTC)
 * ... but it's your observation of someone else's time that appears to be "speeded up". Your own time remains always one second per second.    D b f i r s   15:34, 18 June 2013 (UTC)
 * According to Noether's theorem the consevration of (linear) momentum arises from (or is, at least, intimately connected to) the invariance of physical laws under arbitrary translations in space. The equivalent conservation law that arises from invariance under translations in time is conservation of energy. Gandalf61 (talk) 13:09, 18 June 2013 (UTC)