Wikipedia:Reference desk/Science

Welcome to the science section
of the Wikipedia reference desk.

skip to bottom

Select a section:

• Computing
• Entertainment
• Humanities
• Language
• Mathematics
• Science
• Miscellaneous
• Archives

Want a faster answer?

Main page: Help searching Wikipedia

   

How can I get my question answered?

• Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
• Post your question to only one section, providing a short header that gives the topic of your question.
• Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
• Don't post personal contact information – it will be removed. Any answers will be provided here.
• Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
• Note:
  • We don't answer (and may remove) questions that require medical diagnosis or legal advice.
  • We don't answer requests for opinions, predictions or debate.
  • We don't do your homework for you, though we'll help you past the stuck point.
  • We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.

Ready? Ask a new question!

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

• The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.

See also:

• Teahouse
• Help desk
• Village pump
• Help manual

July 24

How many megapixels equivalent does our eyes have?

How can you measure the amount of megapixels of a human eye?--YX-1000A (talk) 02:04, 24 July 2015 (UTC)

I keep hearing the claim "about a megapixel" over the years, and after looking into it again, I still think that's the case. First I should get out of the way the issue of measuring the eyeball's resolution to begin with: the human eye is not like a camera. You didn't ask how the human eye is not like a camera, so I won't get into it, but there are ways to measure things like resolution. For instance, if looking at a grated material, how close do the grates have to be (while viewing from a given distance) to tell them apart. Using techniques like this and citing lots of sources that you may also be interested in, this website does a somewhat detailed derivation of the resolution of human vision. They come to a rather ludicrous value in the hundreds of megapixels, but they asking a rather particular question, "how detailed does an image need to be to recreate what you would see while standing in that scene." Since most of our photoreceptors are located in our fovea centralis, your eye is only focused on a very small region of your field of view at a given time. If you take the angular width of the fovea to be only 3 degrees, the result on that source falls down to 1 megapixel. That is, at any given moment, the image you are seeing in your mind could be represented by a 1 megapixel image. Someguy1221 (talk) 03:09, 24 July 2015 (UTC)

Modern research places the count of photoactive cells in the retina at over 100 million: Facts and Figures Concerning the Human Retina, published by NIH. The authors have made their entire book available at no cost: WebVision: Anatomy and Physiology or the Retina, from the University of Utah.

Are photoactive retinal cells "equivalent" to pixels? That depends on who you ask!

I work with image sensors as part of my day job (when I'm not too busy reading encyclopedias)... so I know a little bit about "pixels..." To be honest, I still don't know how to count pixels on digital cameras, let alone in biological systems with cognitive perception! Today, reputable sensor-manufacturers sell things called "effective pixels" - what are those!? (Here's how Exmor technology works, at least in cartoon form: there's an "intelligent pixel-by-pixel algorithm" baked right into the silicon! But hey, when I learned about them in school, I don't remember photodiodes running algorithms!) Here's another vendor whose latest and greatest toys will also resample the pixels: 2x2, 3x3... you can get as many pixels as you want from today's camera sensors! And here's another vendor who will reshape your square pixels into half as many rectangles! And all these are consumer toys! If only we could see what pixel weirdness they're surely cooking up in the confidential image sensor technology divisions of BAE or Lockheed - they don't put the details on their websites, because we don't need to see what they can see!

The real answer is, whether you're talking about human eyes or exotic cameras, you should probably stop counting pixels, and start looking at other metrics that meaningfully represent image information content - like the optical transfer function and the signal to noise ratio and the dynamic range.

Nimur (talk) 03:42, 24 July 2015 (UTC)

The vast majority of those 100+ million cells are rods, which don't contribute to daytime vision. There are only about 6 million cones, according to that page. It's also worth noting that there are only about 1 million axons in the optic nerve. -- BenRG (talk) 07:34, 24 July 2015 (UTC)

Do you have a reference for rods being inactive in daytime vision? I know that rods are what are responsible for night vision, and that they're almost entirely lacking from the fovea, where sharp-focused vision takes place, but I was unaware that rods didn't contribute to daytime peripheral vision. - By the way, the Fovea centralis article has a little table talking about pixel densities of displays in matching the cone density of the fovea (the whole "retina display" thing), which is another way of looking at the resolution of the eye. -- 160.129.138.186 (talk) 15:42, 25 July 2015 (UTC)

This contributes some interesting data to the discussion, based on resolving power.--Phil Holmes (talk) 11:42, 24 July 2015 (UTC)

The question is deeply problematic as phrased as the eye is not designed in any sense to function as a camera. It's doesn't capture an image in the sense that a camera does, and if it did, it wouldn't be very ideal to the manner in which the brain processes and assimilates visual phenomena. I'll get into the actual physiology of the eye tomorrow, and try to make some rough correlates between the biomechanics of the eye and the physics of a camera, but first one has to understand the complexities of what human vision actually is. Intuitively, we tend to assume that the eye is taking a recording that is being projected into the brain, but that's not really what is going on. Rather for visual cognition, the brain creates a narrative (that is disturbingly illusory in many ways) from rather limited data. Well, here, no one explains this kind of stuff better than Susan Blackmore, so... Snow ^{let's rap} 10:38, 25 July 2015 (UTC)

The references posted by nimur and holmes seem to be valid and address the context of the OP's inquiry. I am dubious of Snow's link... Susan Blackmore seems to be full of New age nonsense like "consciousness is an illusion". The link from Blackmore's video lecture is not scientific, the links posted already are scientific, and I'm border line sure that Blackmore is full of ill-informed, unsubstantiated, unfounded claptrap! Void burn (talk) 16:00, 25 July 2015 (UTC)

Uh, friend, you need to do your research (or at least watch the video which you are judging) before making such hyperbolic, unsubstaniated statements. Susan Blackmore is a leading scientific authority in these matters. There's nothing that is any sense "new age" about the notion that consciousness is a deeply-problematic concept that has troubled any and every researcher who has tried to quantify it in an empirical manner. This is a well-known issue in cognitive science, known very specifically as the "hard problem of consciousness" and the fact that a certain amount of philosophy inevitably creeps in when we try to tackle such a complex and frustratingly elusive concept of what consciousness really is does not change the scientific rigor with which researchers come at the question. And Blackmore doesn't make any strong or remotely controversial claims in that lecture, she just presents the confusing problems, inconsistencies and logical paradoxes that arise from trying to apply the classical intuitive and/or dualistic notions of a mind or soul, and nothing she says is the least bit new to cognitive science -- these are all conundrums that are decades (or in some cases thousands of years) old in the record of inquiry.

I strongly suspect that what happened here is that you watched a particularly vague thirty seconds of that video and then read our article on Blackmore, saw the word "parapsychology" and decided she was a kook. Quite the opposite in fact; here's the story on her checkered history as a hard-science researchers -- in her youth, Blackmore had an "out of body" experience and was so convinced that it was real that she spent years trying to test notions likes telepathy and astral projection, using hard scientific methods. However, having the outlook of a scientist, she was able to see the writing on the wall and eventually accepted that the evidence was not there for the phenomena she had set out to validate, and then changed course and started studying the mind, and in the decades since has become a respected and leading authority in the field of consciousness studies, even penning the definitive introductory text on the topic, that is now used widely in psychology courses. I admit our article on her does a terrible job of contextualizing the two phases of her activity as a researcher (I've been meaning to put together the sources to fix it for ages), but I think that kind of intellectual honesty should be applauded and her early forays into projection (which she did after-all approach with hard scientific methods, hence the lack of confirmation) should not be held against her. The irony of your comments dismissing her stances on consciousness (aside from the fact that they represent the central, non-controversial territory of this field of inquiry) is that the video in question comes from a talk she was giving to the Skeptics Society, an ideological organization devoted to deconstructing pseudoscientific notions (and her introduction makes clear that she was invited to speak in part because she is a stellar example of a scientist who started out studying dubious phenomena and then moved to topics of real significance).

But if you don't like to accept the existence of the hard problem and what modern research says about it from her lips, perhaps you will be more receptive to David Chalmers, Daniel Dennett, and Donald Hoffman. How about John Searle [1]? I'm just using youtube as a shorthand here so as not to swamp the thread with a tangential discussion. There's a stack of research on this topic taller than Everest converging with the issues Blackmore raises in that video (and again, I reiterate that she doesn't even make any novel strong claims therein, she just points out how questionable intuitive models of consciousness are, and that doesn't even begin to be controversial in the hard study of the mind). But if you feel that you can prove, beyond a shadow of a doubt, that consciousness is an absolute and quantifiable phenomena and can define it in a clear and scientific fashion, I have great news for you -- you're perhaps the greatest thinker in the entire history of philosophy, and you've revolutionized both cognitive science and our fundamental understanding of reality in ways relevant to every last field of human inquiry. But you really need to share that understating, because the rest of us are really struggling with this topic!

All of which is a massive distraction from the OP's question. I didn't link that original video to discuss the notion of consciousness in general; I linked it for (and a put a time parameter in the link to direct to) Blackmore's discussion of visual cognition and how the mind constructs a visual narrative based on the information it receives from the eye (including some mechanics of how the eye itself operates). That is all deeply and essentially relevant to answering the OP's question. You can't understand the eye's "resolution" until you understand the nature of the phenomena of visual cognition and how the brain processes that stimuli. If you have a specific scientific rejection of something she says about human visual cognition in that segment of the lecture which you think might be relevant to the OP's understanding of this topic, by all means, let's discuss it in more detail. But as I see it, you made an exceedingly vague and knee-jerk assessment of a broader topic that was not being addressed, without saying a single thing to address the specific mechanics of vision which we were discussing. I'll let the OP and others here watch the content I linked and judge the academic and empirical rigor of Blackmore's observations (and their relevance to that actual question we're discussing here) for themselves. But with regard to the red herring you've raised here and referenced as "new age nonsense" (specifically, the possibility that consciousness might not conform to our basic intuitions about its fundamental nature), I have to tell you that this is considered an open and inescapable possibility arising from the hardest science you can find in the study of the nature of the mind; this speculation is nowhere near new and hardly considered fringe thinking in consciousness studies. Point in fact, it the opposite view (consciousness is a definite and undeniable phenomena, the existence of which we can test for and verify experimentally) that is presently considered untenable.

But again, I specifically posted that video (and its time parameter) for the content on how the brain contextualizes visual stimuli -- and there's no subfield of perception studies that has more robust data than that of visual cognition, including for the specific phenomena Blackmore mentions (inattention blindness, the biomechanical limitations of saccades, the mapping of photoreceptors on the retina/fovea), none of which are controversial and all of which are necessary in answering the OP's question with any degree of depth and accuracy. Snow ^{let's rap} 19:50, 25 July 2015 (UTC)

I agree that counting the retinal cells doesn't get you a good answer. For example, our eyes continually vibrate at around 80Hz in Ocular tremor - which allows each cell to successively sample a small area around the point where it's nominally pointing - this produces an effective resolution that's considerably higher than you'd get by counting the cells. When you get tired, this motion decreases and your vision gets noticeably more blurry as a result. But your eyes don't transmit "pixels" to your brain anyway - they send information about higher level concepts like the orientation and motion of high-contrast edges. The number of rod/cone cells give some kind of a lower bound of a couple of megapixels...but if I take a photo of a page from a book with a two megapixel camera and examine the image it produces, it's very obvious that my eyesight is much better than that. SteveBaker (talk) 18:17, 25 July 2015 (UTC)

If you post your original vision research here, please tag it as "OR" or something so that people realize that it's not supported by the peer-reviewed literature.

Blurred vision is caused by poor focus, not by a lack of eye motion.

If the eye completely ceases to move, the result is complete loss of vision due to neural adaptation, not blurred vision.

Curcio et al say "Foveal cone spacing is commonly assumed to be the limiting factor of visual resolving power. Resolution of gratings consisting of alternating light and dark bars requires that at least one row of unstimulated cones lie between rows of stimulated cones", so as of 1990 vision researchers had not heard of the idea that resolving power is increased by ocular microtremors.

The resolution of the image on the retina is limited by diffraction and lens aberration, and the cone density at the center of the fovea roughly matches that limit. A higher sampling density would not increase resolving power by much, if at all. -- BenRG (talk) 01:57, 26 July 2015 (UTC)

Nuclear Fission

What is the equation describes the evolution of the energy (w.r.t time) in nuclear fission? 11:34, 24 July 2015 (UTC) — Preceding unsigned comment added by 5.29.9.245 (talk) 11:34, 24 July 2015‎

At it's most basic, it's an exponential decay, depending on the half-life of the material under question. That will do just fine if you're talking about a simple uncontrolled reaction (eg. in a radioisotope thermoelectric generator as used on spacecraft). More complex reactions (for example, a nuclear chain reaction, or a decay chain with several steps) have more complex equations (although still based on exponential decay). Our article on Nuclear reactor physics has equations showing how neutron production changes over time in a reactor - that's a good proxy to energy (since each fission event releases the same amount of energy), but the variables depend not just on the physical properties of the fuel, but also on the design of the reactor. Smurrayinchester 13:35, 24 July 2015 (UTC)

Thank you! :) 5.29.9.245 (talk) 14:05, 24 July 2015 (UTC)

Temperate zones in the Pleistocene

Roughly where were the boundaries of the permafrost regions during the Pleistocene epoch, and did any parts of the world stay warm all the way through? With snow at the equator, surely all the crocodiles would have frozen to death? 213.205.198.236 (talk) 16:49, 24 July 2015 (UTC)

There's some information in our Pleistocene article. The glaciers pushed as far as the 40th parallel, with the permafrost extending a few hundred kilometres in North America, and several hundred in Eurasia. Rojomoke (talk) 17:01, 24 July 2015 (UTC)

Here's a paper that dicusses this, with a map (Figure 1) that matches pretty well with Rojomoke's information above. Mikenorton (talk) 19:52, 24 July 2015 (UTC)

And another that looks at conditions in equatorial parts of Africa and South America, with estimates of about 5°C reduction in average temperatures compared to the present day. Mikenorton (talk) 20:01, 24 July 2015 (UTC)

5 C would be a larger than typical change for the tropics (though potentially accurate for some locations). Most of the tropics are more typically estimated as having a 2-3 C cooling [2]. We also have articles on Last Glacial Maximum and CLIMAP which may be interesting. To answer the original question, no there was no snow at the sea level equator. The all-time record low at a place like Guayaquil, Ecuador (a coastal equatorial location) is 15 C. [3] It would take much more extreme cooling than hypothesized for the recent sequence of glaciations before it would be cold enough to snow there. Dragons flight (talk) 13:20, 25 July 2015 (UTC)

One of the most pronounced differences in the tropics is the hypothesized "permanent El Niño" state during much of the Plasticene. But as far as I know the tropics stayed pretty warm overall, albeit cooler than present at least in some places. I'd be curious to know your source for "snow at the equator" (other than at high elevations). Short Brigade Harvester Boris (talk) 03:59, 25 July 2015 (UTC)

I think you're getting confused - the plasticene era was the time when Wallace and Gromit walked the Earth. Richerman (talk) 08:34, 25 July 2015 (UTC)

See plasticine to understand the above comment. StuRat (talk) 16:17, 25 July 2015 (UTC)

July 25

Mountain building before plate tectonics

How did scientists explain mountain building and earthquakes before the theory of plate tectonics became widely accepted?

Plate tectonics is indeed relatively recent. But erosion and Deposition_(geology), sedimentation, etc. were worked out long before that. See Uniformitarianism#18th_century for some background. We also have fairly good articles on history of geology and timeline of geology. SemanticMantis (talk) 06:08, 25 July 2015 (UTC)

See also Geosyncline. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:09, 25 July 2015 (UTC)

The emphasis was originally on vertical tectonics - unsurprising considering most geoscientists thought that the continents were fixed. The evidence for lateral shortening, however, in the Alps, the Scottish and Scandinavian Caledonides and the Rockies convinced many that the earth was shrinking as it cooled, causing mountain ranges to rise - see Diastrophism#Historical development of the concept and Timeline of the development of tectonophysics (before 1954) (the latter has issues, but it has some useful information). Mikenorton (talk) 11:31, 25 July 2015 (UTC)

Mountain building can also be explained by vulcanism. Of course, not all mountains are built that way, but some early geologists might have thought they all were. StuRat (talk) 15:40, 25 July 2015 (UTC)

That was the argument of the plutonists, such as James Hutton, who explained everything in terms of vertical movement due to magma emplacement, see also here. Mikenorton (talk) 16:18, 25 July 2015 (UTC)

Has anyone researched levee failure due to earthquake ground motion?

I've tried looking online for scenarios with models that calculate probabilistic levee failure in Vancouver Canada due to earthquake ground motion, without any success. Does anyone know of such scenarios? I found a scenario with a model calculating probabilistic levee failure due to tsunamis, but not one due to earthquake ground motion.Philly underwater (talk) 13:38, 25 July 2015 (UTC)

If you want people to reply, make the title of the thread more informative - researched what. Oh, and please sign your post.DrChrissy ^(talk) 13:22, 25 July 2015 (UTC)

University of British Columbia at Vancouver hosts one of the world's top geology, geosciences, and geological engineering programs in the world. Surely there have been hundreds of research projects regarding their local geology and civil infrastructure! Maybe a good place to start is the UBC Geological Engineering program website, and in related departments.

For example, I found Applied Science 170, an introductory undergraduate course they offer (at the Okanagan campus) in case studied of engineered systems failures. Here is their additional bibliography, which includes studies of dam and levee failures in the Pacific coast region. Some of those resources point to American (rather than Canadian) studies. For example, from the Teton Dam article, (whose failure was not due to earthquakes - but spurred a major study of dam failures in the entire Western United States), I found the Pacific Northwest Region Safety of Dams program, an ongoing research program of the United States Bureau of Reclamation - the other major government agency that manages water projects and dams in the U.S. Between this website and the websites of the U.S. Army Corps of Engineers, you can find just massive quantities of research and data on dam and levee safety. Here's an entire book: State of the Art for Assessing Earthquake Hazards in the United States (1995).

Nimur (talk) 14:35, 25 July 2015 (UTC)

But none for canada24.207.79.50 (talk) 17:22, 25 July 2015 (UTC) I was thinking more of scenarios written by companies that do catastrophe modeling24.207.79.50 (talk) 17:34, 25 July 2015 (UTC)

I think I should ask again, but in different wording.24.207.79.50 (talk) 21:18, 25 July 2015 (UTC)

Are Canadian earthquakes really that different from everyone else's tremors? The first half of Nimur's post is all about Canadian sources. Roger (Dodger67) (talk) 09:24, 27 July 2015 (UTC)

Sphere formation

What is the name for the phenomenon when a celestial body is massive enough so that it becomes a sphere? Th4n3r (talk) 16:07, 25 July 2015 (UTC)

Hydrostatic equilibrium apparently. Mikenorton (talk) 16:12, 25 July 2015 (UTC)

Yes, as mentioned in List of gravitationally rounded objects of the Solar System. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:14, 25 July 2015 (UTC)

1 2 3 Void burn (talk) 16:14, 25 July 2015 (UTC)

Dextrocycloversion and Laevocycloversion

In answering an earlier question, I was reading Eye movement - where it describes our ability to rotate our eyeballs not just left and right, up and down - but also to rotate them clockwise or anticlockwise about an anterior/posterior axis! (So that the top of your eye moves towards your nose while the bottom moves away from it).

I was totally blown away to discover this because it's not obvious that we can do it...and I can't think of any reason why we'd evolve to be able to do it. But This article shows the pair of muscles on each eye that are responsible for doing rotating our eyes that way (they are the "Superior oblique" and "Inferior oblique" muscles).

The motion is called Dextrocycloversion and Laevocycloversion - but web searches on those terms don't produce anything other than dictionary definitions of those terms that describe (often poorly) what this motion is and what muscles produce it.

Clearly we don't have conscious control over this motion as we do with the left/right and up/down axes. Standing in front of a mirror, tilting my head to one side or the other doesn't seem to result in any such motion. You'd think that doing it would result in the horizon spinning clockwise and anticlockwise in front of you!

I really find it hard to believe that this is true - but the muscles clearly exist - and the name for the motion is in every medical dictionary!

So - the obvious questions:

1. Am I totally misunderstanding these terms?
2. Why do we need this kind of motion?
3. Is there a way to produce the motion so we could observe it happening?
4. By how many degrees can the eyeball rotate in such circumstances? (I'm guessing it's a tiny number...but who knows?)

SteveBaker (talk) 18:34, 25 July 2015 (UTC)

The chief function of the two oblique muscles is to rotate the eye on its own principal axis to keep the horizontal level in certain head movements, but since the eyeball is radially symmetrical this movement is not easily observable.

Wendell J. S. Krieg, Functional Neuroanatomy (Philadelphia: Blakiston, 1942),pg. 68.—eric 21:05, 25 July 2015 (UTC)

Interesting. When walking with a normal video camera, the video is rather painful to watch, as it bounces about (hence the invention of the steadicam). We don't observe this when walking ourselves, though. Presumably the brain does a lot of image stabilization, but, since it's not enough to fix the video, there must also be some other mechanism to keep the image from bobbing about. I suspect that this is where those muscles come in. They might even predict your next step and act accordingly, as you make it, to avoid a delay that would wiggle the image.

Also, I suspect that some birds lack these muscles, since they keep their head steady as they walk, then suddenly move it to a new spot, then hold it steady again. A much cruder way to stabilize the image. StuRat (talk) 21:14, 25 July 2015 (UTC)

:The terminology section in Eye movement and the table in Extraocular muscles look useful.—eric 21:20, 25 July 2015 (UTC)

This abstract may help in answering #4. Cyclotorsion seems the best search term.—eric 21:31, 25 July 2015 (UTC)

See the last part of the "Torsion" section of this article for #3 and #4.—eric 22:16, 25 July 2015 (UTC)

Searching for Ocular countertorsion i think answers everything.—eric 01:01, 26 July 2015 (UTC)

How long does it take for a new habitat to change an organism's DNA?

I'm not sure if my question makes sense. I was wondering if, theoretically speaking; my ancestors and I reside in Norway for the last 300 years or so (light-skinned and blue -eyed) and travelled to a warmer climate like Dominican Republic or Africa, how long and what mechanisms by which a person's DNA is changed to look like the current inhabitants? — Preceding unsigned comment added by 67.142.96.127 (talk • contribs)

• How much sex are you having with the natives? --Jayron32 01:57, 26 July 2015 (UTC)
• The DNA itself doesn't change. What happens is that certain organisms reproduce and others don't, and the recombined DNA of those that reproduce is passed on. Imagine if there's a comet that strikes the earth, and only aquatic creatures and those that live buried in underground dens survive, while those that live above ground are roasted to death. (KT event). Then all the elephants will go extinct in a few days, if not hours. That is a drastic "change" in the organism's DNA. For human evolution, disease (think smallpox in the Americas) is the biggest game changer. Other traits like hair color can change very slowly within a population. What matters is the selection pressure. μηδείς (talk) 02:07, 26 July 2015 (UTC)

(edit conflict × 2) Assuming no interbreeding with the locals (so, assuming a large number of Norwegians accompany you) and ignoring tanning, the skin changes would be the result of Natural selection. One of your descendents could be born with a genetic mutation (not like sci-fi super powers, but a benign mistake in combining the parents' DNA) that causes them to produce more melanin than their ancestors (or their immediate ancestors at any rate). That descendent would be less likely to develop skin cancer and have more resistance to sunburning, so s/he would lead a healthier life and so be more likely to reproduce. A mutation could also occur to make them produce less melanin, which would make them more prone to sunburning and skin cancer, and so make it harder for them to survive long enough to reproduce as much as the darker-skinned descendent. The darker skinned descendent would have their own children, and some of their descendents could have a similar mutation, increasing melanin and so their chances of survivability. It is theoretically possible (though practically impossible) that the change could happen after a single generation (i.e. you have a kid who blends right in with the Khoisan). What would be more likely is that it'd take many generations. I'd guess that 1,000 years would still be theoretically possibly but practically impossible, 10,000 years as possible but remarkably fast, and that it could definitely happen in about 100,000 years. Of course, this is based on reversing how long it took African colonists in Europe to lose their melanin (giving them more vitamin D), a number that might have been thrown off by interbreeding with neanderthals. Ian.thomson (talk) 02:16, 26 July 2015 (UTC)

I don't think it took nearly 100,000 years for white skin to develop when Homo Sapiens Sapiens moved to Europe. According to Europe#Prehistory, modern humans appeared there some 40-43 thousand years ago. Not sure if we know how long it took for skin color to lighten after that. Does the earliest European modern human artwork from which color could be determined show black skin ? If so, when did it change ? StuRat (talk) 04:19, 26 July 2015 (UTC)

Why use artwork when you can use genetics? See Human_skin_color#Light_skin, Light_skin#Evolution. It's also not just time, one hypothesis is that there was relatively little selection towards lighter skin in Europe until about 6-10k years ago, roughly when certain behavioral and agricultural changes occurred. It seems from the research linked there that there were few if any light-skinned Europeans before ~20k years ago. SemanticMantis (talk) 14:19, 26 July 2015 (UTC)

July 26

Scar and hair biology

If someone has a small scar on the chin that makes a beard clearing... Can he transplant hair on the scar? If there are some Dermatologists that claim that they can transplant hair on this scar with about 60% success, are they talking the truth from a biological perspective? Can hair really grow on scars?... Maybe they mean that only some areas in the scar are living skin? What is your final conclusion on this? Thank you!!! Ben-Yeudith (talk) 04:42, 26 July 2015 (UTC)

If you are talking about hair transplant in scar area then answer is "yes". but one should have enough blood supply at that particular area..turbo 05:20, 26 July 2015 (UTC)

Turbo, Could you please elaborate even more? Can you give more details of what of the sub-questions the "yes" applies? Thank you. Ben-Yeudith (talk) 13:31, 26 July 2015 (UTC)

Can we have speed greater than c ?

In Michelson-Morley experiment, we saw that the speed of light in space is equal in all inertial frames and in relativity, light speed is defined to be the maximum. Is it really impossible to get speed slightly greater but equivalent to light speed such that if we calculate that speed as light speed, we won't be wrong?Sayan19ghosh99 (talk) 07:52, 26 July 2015 (UTC)

Are you asking how sure we are that nothing travels slightly faster than c? Experimentally I think all we can do is measure approximate masses. The mass of the photon is known to be very close to zero, but I suppose that leaves open the possibility that it's a tiny imaginary number (which would make it a tachyon). Theoretically, in quantum field theory, tachyonic particles make the vacuum unstable, which is a very strong reason to think they don't exist. (See Tachyonic field#Interpretation.) -- BenRG (talk) 08:57, 26 July 2015 (UTC)

My (non-expert) understanding is that the true c, from the viewpoint of relativity, is the velocity that can be attained only by truly massless particles. If photons had a tiny bit of (non-imaginary) mass, they would necessarily have to travel a bit slower than the relativistic c. However, this would also mean that c_photon, unlike the true c, would no longer be a constant in all frames, which would be quite easy to detect: experiments that measure this are thus experiments which can put the masslessness of the photon to the test.

There are also more sensitive tests available: for more on these, see John Baez' page here, and Photon#Experimental checks on photon mass -- The Anome (talk) 13:38, 26 July 2015 (UTC)

I might be wrong, but I think the OP is saying something like "What if we accelerate something to light speed, and keep on accelerating it ... even if it is still calculated as moving at the speed of light, couldn't it be going faster?" And the answer there is that it takes an infinite amount of energy to accelerate any object with mass to lightspeed, and it would have infinite relativistic mass; if you could apply two infinities of energy it would have two infinities of relativistic mass, but that doesn't really mean anything, and it doesn't ever happen, because if it happened even once we'd have all been sucked away by the infinite gravity of the object, no matter where it is in the observable universe.

That said, photons experience a phenomenon that is sort of like deceleration, in the red shift that occurs as space expands. After all, a galaxy that is coming at us at a slower speed might seem to decelerate and eventually turn back and move away from us as billions of years pass. The photons can't go slower than light speed, but they do lose energy. You might compare this to the difference in overall energy in them going from 0 * infinity to 0 * infinity / 2 (or whatever the redshift is at their source), but that of course isn't really proper mathematics. Wnt (talk) 14:59, 26 July 2015 (UTC)

Just to be clear, you CAN have 'speed' grater than C, quite easily depending how you define "speed". What you can't have is information faster than C. Our article Faster-than-light which no one has linked yet, is quite good! Vespine (talk) 22:40, 26 July 2015 (UTC)

A shadow can travel faster than C. But of course a shadow is not a thing. Void burn (talk) 23:36, 27 July 2015 (UTC)

Shadows are indeed mentioned in the article Vespine linked. Along with many other classic examples. Someguy1221 (talk) 00:01, 28 July 2015 (UTC)

Why in Bohr's stationary orbits an electron completes integer wavelengths?

Why mvr=nh/2π and 2πr=nλ ?Sayan19ghosh99 (talk) 07:53, 26 July 2015 (UTC)

In the Bohr model it was just assumed to be that way. The only justification for it was that it led to a correct formula for the spectral lines of hydrogen.

In real quantum mechanics with the Schrödinger equation, it's for more or less the same reason that musical instruments have discrete harmonics. -- BenRG (talk) 08:36, 26 July 2015 (UTC)

To continue a bit with the above, look at the Schrödinger equation article itself, specifically the first time-dependent formula: iħ multiplied by the partial derivative over time of the wave function is equal to the Hamiltonian of the wave function. Solutions to that equation always (???) seem to take the form e^{iħ [phase]}, so the wave function ends up as a three-dimensional loop in space, generally with a positive lobe and a negative lobe, as shown. I'll admit that one of the little mysteries I missed is where the negative end of an 1s orbital is, but I think it must be tucked in there somewhere... anyway, the point is that the wave function has to go "all the way round" in a way that ends up quantizing its angular momentum in units of ħ. Wnt (talk) 10:55, 26 July 2015 (UTC)

How much can ice reduce the temperature in a room.

If you put two liters of ice in 20 cm3 room, how much degrees will the temperature drop, when the ice melts?--Scicurious (talk) 17:03, 26 July 2015 (UTC)

20 cm3 is a very small room. You couldn't even fit 2 L of ice in there.

Assuming you meant 20 m3, you can work it out easily. Density of ice is about 0.92 kg/L, latent heat of fusion is about 335 J/kg, density of air under typical conditions is 1.2 kg m-3, specific heat of air is 1004 J/(kg K). The rest is just arithmetic.

If the ice is colder than 0 C you need to take account of the heat absorbed by warming up the ice to the freezing point. Short Brigade Harvester Boris (talk) 17:17, 26 July 2015 (UTC)

However, the adjustment for the ice being below 0 C is not large, because the latent heat of fusion is large compared to the heat capacity of ice. Robert McClenon (talk) 17:52, 26 July 2015 (UTC)

Generally true, which gives me the opportunity to correct my units: latent heat of fusion is about 335 J/g, not 335 J/kg. Short Brigade Harvester Boris (talk) 21:53, 26 July 2015 (UTC)

How did we get images of asteroid 2011 UW 158?

A week or two ago it came screaming by the Earth and the radio telescope in Puerto Rico took some pictures. Cool, radio pictures of screaming asteroids, but how was it done? One report said they used a 20 tw signal. How do you generate 20 tw of anything, much less a microwave? So they generate this signal, and point it at the asteroid, and the signal bounces back, but how do you make an image out of returns? Are they scanning the asteroid, pixel by pixel? Or are they somehow able to focus the return signal and get an image all at once, like a camera? 50.43.33.62 (talk) 19:14, 26 July 2015 (UTC)

Generating 20TW of (peak) power is actually quite easy: you really only need a very short pulse. When this pulse is scattered its temporal profile and spectrum change. From these you can reconstruct the shape of the surface. Ruslik_Zero 20:07, 26 July 2015 (UTC)

The technique of synthetic aperture radar is used to image objects. Twenty Terra Watts is not really a lot of energy, since a watt is defined by joules per second. I.E. If one was to release (transmit) one joule in a nano-second that would would equate to a 100,000,000 watts.. should the transmit power last for a full second.--Aspro (talk) 20:48, 26 July 2015 (UTC)

Easy? Sure it is, but with what? (Watt?) I can imagine all kinds of equipment that could be used for this, like capacitors and batteries and transformers, but what are they actually using? Tin cans? Diesel generators?

I can see how the temporal profile can tell you that some parts are farther away, and spectrum change can tell you if something is moving closer or away, but how do you turn that kind of info into an image? 50.43.33.62 (talk) 02:22, 27 July 2015 (UTC)

What report quoted twenty terawatts? Are you certain they didn't mean effective radiated power? Power is a complicated parameter for RADAR! Start by reading RADAR signal characteristics.

"The Radio Telescope" mentioned in the original question probably refers to the Arecibo Observatory. Arecibo usually operates passively, but for some experiments it can transmit as a true RADAR, as well: read about how the transmitter works; when it does so, it typically uses about 150 kilowatts of average power - that is how much power the electrical generator is providing. (That website is a bit old, citing an operations manual from year 2001, and there have been many facility upgrades at Arecibo since then - so use all of these numbers with caution)! The transmitter is often used to send a pulsed RADAR signal; so at any given fraction of a second, either zero watts or 2.5 megawatts are being transmitted.

That peak pulse number provides true instantaneous power. To compute peak ERP, you multiply the power with the antenna gain to derive a (fictitious) equivalent as if the energy was transmitted in all directions. This allows us to use simplfications in the RADAR equation. In actual fact, the energy is directed by the RADAR antenna: it is not a spherically-isotropic source. Radio astronomers like ERP because it's the amount of power that an equivalent spherical cow would emit.

In specific, you can read about the high priority NEO survey conducted by Arecibo to image 2011 UW 158: Current NEO surveys from the Solar System Sudies group at Arecibo Observatory. Their nominal operational power was scheduled to operate at 900 kilowatts for the survey.

For big antenna facilities like Arecibo, antenna gain is enormous - so you can get very strange values when you look at "effective" radiated power. The physical quantity of energy per unit time, on average, is still only the amount supplied by the electricity source, and is not "terawatts."

Nimur (talk) 14:29, 27 July 2015 (UTC)

Excellent reply from Nimur, as always. To answer the OP's specific question, the transmitter device is a klystron. Tevildo (talk) 22:12, 27 July 2015 (UTC)

One way to boost the power in a pulse, but keep the same energy, is to generate a sweeping frequency. Then the signal is put through dispersion so that the earlier frequency comes out at the same time as the latter frequency, and all the power piles up into a very short pulse. Graeme Bartlett (talk) 22:57, 27 July 2015 (UTC)

July 27

What chemical properties make soap useful for cleaning?

Why is soap good at cleaning things? What chemical processes underlie the effectiveness of all soaps and detergents? I remember pondering this question during organic chemistry in college, and I vaguely recall coming up with something about "micelles" being especially good at isolating dirt and making it capable of being scrubbed away by physical force or washed away by water, but I feel like there is more to it than that. Why does soap create useful micelles when other compounds with hydrophobic epitopes don't? Is it perhaps related to the reason why soap makes long-lived bubbles with proper agitation?

Also, how is a soap's effectiveness increased or decreased by time and temperature? Does hot water really make certain soaps and detergents more effective than if they were used with cold water? Is there an ideal amount of time for which a given dirty item should be exposed to soap's chemistry, for maximum cleaning power? Thanks for your help! PJsg1011 (talk) 06:39, 27 July 2015 (UTC)

Regarding the chemical processes, this is due to the fact that one end of the molecule is hydrophobic, whereas the other is hydrophilic -- so the hydrophobic end sticks to the greasy dirt, whereas the hydrophilic end is attracted to water and pulls the grease away from the surface being cleaned. In effect, this makes the surface of the grease particles hydrophilic as well (by coating it with hydrophilic molecules), which makes them miscible with water when normally they're immiscible. One end hydrophobic, the other hydrophilic -- THAT is the key. And yes, higher temperatures usually increase soap's effectiveness -- but this is due to the normal increase in solubility with temperature. 2601:646:8E01:9089:F88D:DE34:7772:8E5B (talk) 08:58, 27 July 2015 (UTC)

Surfactants make the water grap nonsolvable stuff. Cheap kitchen cleaners use lactic acid to remove fat and oil. Surfactants are more expensive and some you do not taste or smell them. For that reason such cleaners have added parfume or substances we can detect by smalling them. Whe getting enging oils on your hands and use soap, the oils part is removed by water and soap but your hands feel like sill put into vinegar which is not beeing removed by the soap. Then the soap contains some glycerol which is a part of soap production, the ester can solve in water. For that reason shower gels or shampoo might be more effective aginst engine oil on the hands. Todays full synthetic engine oils are based on hydrcracked substances, some very stable compared to mineral oil are esters. Calcite is being removed by citric acid oder formic acid. This show you the major difference between bath and kitchen cleaners. --Hans Haase (有问题吗) 09:14, 27 July 2015 (UTC)

ESV

What rescue equipment and capabilities does a modern oilfield ESV (Emergency Support Vessel) like the Iolair have? 2601:646:8E01:9089:F88D:DE34:7772:8E5B (talk) 08:50, 27 July 2015 (UTC)

End of domino impulse

Is it possible to estimate how many dominoes it would take to completely stop the impulse imparted by finger or hand to the first domino? This states that 3,847,295 dominoes is still insufficient to completely exhaust the impulse, resembling a perpetual motion. By impulse I mean standard force to provide watchable collapse speed, not too strong and not too slow. Brandmeister^talk 14:45, 27 July 2015 (UTC)

Each domino contains gravitational potential energy. That means that it releases energy when it falls down. That energy goes into knocking down the next domino. You can even have each domino get slightly larger than the previous one, since the gravitational potential energy of the smaller one is more than the force required to tip over the larger one. So, there is no theoretically limit to the number of dominoes that can be knocked down. (There is a practical limit, though, as the more you have the greater the chance of them being knocked down prematurely or being misaligned so they don't all fall down.) StuRat (talk) 15:07, 27 July 2015 (UTC)

I saw a this video of a chain of dominoes of increasing size just the other day. AndrewWTaylor (talk) 16:34, 27 July 2015 (UTC)

And there are some even bigger ones here. AndrewWTaylor (talk) 18:13, 27 July 2015 (UTC)

This is an example of a chain reaction. To achieve perpetual motion, each domino would have to bounce back up into its upright state after it had toppled. You could then create a circle of dominos that repeatedly toppled and bounced back up - but this is, of course, impossible. Gandalf61 (talk) 15:14, 27 July 2015 (UTC)

Or just have an infinite number of dominoes. That could take a while to set up, though. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:13, 27 July 2015 (UTC)

Not if you have an infinite number of people to do it! --65.94.50.73 (talk) 19:33, 27 July 2015 (UTC)

Desert chimney

I wonder if this concept has ever been tried. In a desert, where the outside is hot but dry, if you use an inside evaporative cooler, that makes the inside air cool but potentially overly humid. If a chimney was added, wouldn't that let the humidity out of the house, but not the cool air, since cool air sinks ? You would need to arrange the chimney so sand wouldn't blow in and fall down it, and some convolutions would also reduce radiative heating. The flue on the chimney could then be opened or closed, to control the house humidity level. So, is this approach ever used ? (I realize there are also swamp coolers that evaporate outside the house, then circulate the coolant inside the house, but I'm not asking about those.) StuRat (talk) 16:27, 27 July 2015 (UTC)

See qanat, windcatcher ... there may be more useful terms for this tech, which has been used a very long time in the Middle East. Wnt (talk) 18:52, 27 July 2015 (UTC)

That's a bit different, in that they use wind to replace the air, while I'm talking about not replacing the air, but only allowing the humidity in it to diffuse away, even without winds. (In fact, in my design, you might want to close the flue when windy, to prevent replacing cool, inside air with hot, outside air.) StuRat (talk) 18:58, 27 July 2015 (UTC)

Desert termite mounds are humid and cool. They are basically very tall chimneys. The termites make them humid. They don't have swamp coolers in them, but I mention this because they remain humid and cool even though they are of the design you mention. 209.149.113.45 (talk) 19:21, 27 July 2015 (UTC)

Certainly the interior would remain somewhat more humid than outside, and, in a desert, that would be a good thing. The idea is just to keep it from becoming oppressively humid, to the point where water no longer evaporates and evaporative cooling fails to work. StuRat (talk) 19:32, 27 July 2015 (UTC)

Enthalpy_of_vaporization, latent heat, sensible heat, adiabatic cooling, relative humidity, Evaporative_cooler#Physical_principles. Evaporative cooling works until the relative humidity is 100%. You can't let out "humidity" without letting out the cooler air. The air has less sensible heat because it has higher latent heat. That's how evaporative cooling works. SemanticMantis (talk) 23:00, 27 July 2015 (UTC)

Precisely. You can't "unmix" the humidity from the cool air unless you add another processing step (e.g., an air conditioner). Short Brigade Harvester Boris (talk) 00:53, 28 July 2015 (UTC)

This runs contrary to my experience. When it's cold and humid outside, and hot and dry inside, just opening the windows seems to make it hot and humid inside. I believe the water vapor achieves equilibrium much faster than the temperature does, and in this Q's scenario, the "unwillingness" of cold air to rise up the chimney should exaggerate this effect even more. And it's not "unmixing" the air, but really mixing it, with regards to water vapor, more quickly than with regards to temperature. StuRat (talk) 12:01, 28 July 2015 (UTC)

Meant to respond to this earlier, but ran out of time; it's a bit redundant on SM's comments now, but I'll comment briefly anyway. The thing is, unless I have misunderstood you in some way, the system which you describe is basically the standard implementation for a swamp cooler. That is to say, the system is almost always more efficient and effective if you allow the space to vent. A flue is probably an atypical means of achieving this in the context of a small domicile, but a window serves just as well under most circumstances; there might be some minimal advantage to this end-point being at elevation, but usually the cooling unit itself (which is typically set at the other end of the system in a stream-lined setup) employs a powerful fan as part of the evacuation mechanic, so any propensity for hot air to rise is going to have very little total effect on either the total heat or humidity of the system as a whole. There are variations on evaporative cooling (and hybrid approaches which employ air conditioning) in which a closed system would be more optimal, but the system you describe is generally the most energy-efficient and common form, especially in the arid context you stipulate. Snow ^{let's rap} 00:20, 28 July 2015 (UTC)

I think you are picturing the swamp cooler venting directly into the chimney, while I had in mind something like a misting system venting right into the living space, with a separate chimney elsewhere to let the excess humidity out. StuRat (talk) 12:09, 28 July 2015 (UTC)

July 28

Multi-cylinder IC engine

Can the same cylinder design for an IC engine be used in multiple engines, with different number of cylinders? For instance, design a 250cc cylinder, and use it in 1, 2, 3, and 4-cylinder configurations for 250cc, 500cc, 750cc and 1000cc engines? Thanks!

Yes, to some extent. For instance it wasn't unheard of for manufacturers to chop two cylinders off a V8 to make a disgusting V6, and I was involved in a project that took a 4 cylinder and turned it into a 3 cylinder, but that did not go into production. But really you only reuse the con rod and piston and perhaps valves and liner, everything else is redesigned. Greglocock (talk) 09:48, 28 July 2015 (UTC)

According to something I read yesterday somewhere on the BBC website's current coverage of Formula 1, one of the engine manufacturers is currently testing a potential improvement on a single-cylinder setup (presumably) which, if successful, will be incorporated in a forthcoming engine upgrade. This suggests that elements of the OP's scenario are valid. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 12:37, 28 July 2015 (UTC)