Wikipedia:Reference desk/Archives/Science/2007 July 11

=July 11=

Big red insect
I was having dinner when I suddenly heard a buzzing noise from the kitchen. I went to inspect, and it turned out to be this large insect struggling to fly through the pane of glass in the kitchen window. I helped it out, and it flew straight for the dew covered rose bush, presumably to drink water. I rushed to get the camera, and I was able to take this picture. I did not have a ruler handy, but it might have been as large as 4 cm in length, which is exceptional for Norway. Does anyone have an idea what species this is? -GSchjetne 00:33, 11 July 2007 (UTC)
 * It could be an Elephant Hawk-moth (Deilephila elpenor). ---Sluzzelin talk  05:48, 11 July 2007 (UTC)

I concurr, definitely looks like a hawkmoth. Species unknown??? Rana sylvatica 02:37, 13 July 2007 (UTC)Rana sylvatica

Reactivity of Chrome plating to Hydrocarbon fuels
I was wondering if anyone knew how reactive hydrocarbon fuels (ranging from jet fuel all the way to mogas) reacts with chrome plating?
 * the surface may catalyse a reaction (depending on temperature) - you could check up about reactions catalytised by chromium and chromium oxides, you'd also have to take into account other substances present such as oxygen or water vapour.

history of hygiene
Hi - not sure if this is a humanities or science question. In the articles on hygiene, pasteurisation, etc, there is mention that medieval europeans bathed, but no mention why - did they believe dirt carried disease? If so, I'd love it if someone could point me to a medieval text where this is set down.

Thanks,

Adambrowne666 01:58, 11 July 2007 (UTC)


 * I don't know the answer to this, but I hope that at least some medieval and ancient persons valued not smelling bad. Someguy1221 02:22, 11 July 2007 (UTC)
 * Hello, Adam! Indeed, I recall reading that sweet fragrances were appreciated in courtly love. Bathing was also a social act, fun, and felt (and feels) good. Here are two links that might help you a bit: Tubd-a-Scrubd, bathing and cleanliness in the Middle Ages. ---Sluzzelin  talk  06:00, 11 July 2007 (UTC)


 * Our article on hygiene has an, albeit short, section on history. So short, in fact, that it fails to mention, Ignaz Semmelweis, the Austrian physician who revolutionized hospitals by showing how dramatically survival rates in hospital raise if staff adheres to rules of hygiene. Before him, your chances of catching a deadly disease in a hospital and dying there may have been higher than getting cured from your original problem. Simon A. 07:14, 11 July 2007 (UTC)

Thanks, all, for your generous answers. As I often do, I posed the question in completely the wrong way. The thing is: I'm writing a bit set in 1650 France, where a badly wounded guy is about to be treated by a surgeon who wants (for reasons we need not go into here) to plaster his wounds with filth. I want the wounded guy to resist the whole idea - but would he have? Did people have any idea that dirt + wounds = problems? (In my desperation, and knowing people in the renaissance were less interested in bathing than in medieval times, I thought I'd have my character hark back to some medieval medical text that commends cleanliness for open wounds, and get around it that way - hence my previous question.) Sorry about this - hope I'm making sense here.

Adambrowne666 07:37, 11 July 2007 (UTC)


 * Hippocrates, like the Sumerians before him, had the right idea, advocating avoidance of suppuration, and treatment by cleansing with wine, applying a bandage, and then pouring wine on the bandage. It was Galen who screwed up, and pontificated that the formation of pus was essential for wound healing.  I'd suggest looking at Theodoric Borgognoni of Cervia (1205–1298), who opposed Galen's idea of laudable pus, and proposed instead attention to control of bleeding, removal of contaminated or necrotic material, avoidance of dead space, and careful application of the wound dressing. Perhaps your wounded guy could have studied him, or harkened back to Hippocrates. But Galen was orthodoxy, and his (erroneous) principles governed treatment generally for fifteen centuries: until the late 19th century. Other names to look at would be Henry of Mondeville, Casare Magati, and Paracelsus - though Paracelsus was in his advocacy lucky rather than perspicacious. - Nunh-huh 08:17, 11 July 2007 (UTC)

Wonderful answer, thanks so much! - I love 'laudable pus'; might even end up as the chapter titleAdambrowne666 11:16, 11 July 2007 (UTC)

Anaconda migrations
The movie Anacondas: Hunt for the Blood Orchid is about a group of giant anacondas (from South America) congregating in Borneo. Ignoring the plausibility of an anaconda deciding to swim across the Pacific as far as Borneo, would it be possible for the animal to make such a journey? 69.77.205.206 02:29, 11 July 2007 (UTC)


 * I doubt it - anacondas are river snakes, and probably would not survive prolonged immersion in salt water. They would be continuously exposed to direct sunlight, and would likely be unable to feed for the duration of the voyage. While snakes can go weeks (months?) between meals, they typically do not exert themselves continuously during this period. -Eldereft 05:22, 11 July 2007 (UTC)


 * Moreover, what would they drink? Batmanand | Talk 08:17, 11 July 2007 (UTC)


 * Fish like Rainbow Trout are Euryhalines, which means they can live in salt and fresh water. They actually go through a metabolic change to be able to do so.  As far as I know, River Anacondas are not Euryhalines, so they'd die as soon as they entered salt water. --  JSBillings  11:34, 11 July 2007 (UTC)


 * Apparently some amazingly far animal transplants have occurred when critters have ended up on floating mats of vegetation that took them across the oceans. An anthropologist speculates here that Homo floresiensis might have ended up on Flores that way, so presumably a snake could as well.  This isn't the sort of deliberate migration you're talking about, though.  --TotoBaggins 16:38, 11 July 2007 (UTC)


 * That's the way new species get to islands - it doesn't seem too impossible given enough time. Anaconda is sitting sunning himself on a tree limb (think HUGE tree!) - on the opposite limb of the same tree is the mate of a rival anaconda.  There is a storm - the tree snaps and falls into a river - it floats downstream into the ocean with two anaconda's (one already pregnant) - it takes a month or two for the current to wash it over to Borneo - meanwhile maybe they eat seabirds - but a snake can live that long without food or water.  They wash up on the coast of Borneo - momma-snake gives birth, then mates with the male - now you don't have a whole lot of genetic diversity - but your growing anaconda population maybe won't be too inbred by the time another anaconda washes up on the beach 50 years later.  It could certainly happen...and it this kind of story must have happened tens of thousands of times in history in order for remote islands to have their own odd-ball species. SteveBaker 02:20, 12 July 2007 (UTC)


 * But, Borneo could have been populated by animals from neighboring landmasses. South America is very, very far.  I don't know if it's plausible for a snake to float on a branch across the largest ocean on the planet... Nimur 02:27, 12 July 2007 (UTC)


 * Sure - Borneo is going to have a lot more animals from close-by places than it's gotten from South America. So I think the creation of a viable colony of anaconda's on the island is possible - plausible even - it's just very, very unlikely.  But one assumes that anacondas have been around for a good few million years - there might have been a hundred instances per year of tree+anaconda ending up in a river - so even if the odds of one making it across the pacific is only one in a million, it'll have happened a hundred times throughout history.  But this is a movie - it doesn't have to be a common occurrance - it just has to be borderline-possibility.  The deeper question is why the movie script writers felt compelled to set a story about anacondas in a place where there aren't any! SteveBaker 14:24, 12 July 2007 (UTC)


 * I would think it's because they didn't do any research and/or confused anacondas with pythons. Vultur 15:49, 12 July 2007 (UTC)

Cloning
What is the difference between Cloning and Artificial Insemination ?


 * See Cloning and Artificial insemination, basically cloning is recreating an organism using its DNA, while artificial insemination is simply combining gametes artificially. Artificial insemination will produce similar offspring to ones that are produced normally while cloning will produce identical copies of the parent organism. --antilivedT 06:56, 11 July 2007 (UTC)


 * Artificial insemination requires a male and a female parent, while cloning only requires a single genetic "parent." Nimur 07:23, 11 July 2007 (UTC)

Whoa!
I was just reading a post on my work's forums about how it doesnt automatically correct for DST, and I was thinking "hmmm, wouldnt it be easy to add such a function to automatically correct for it like other forums do?". And POW, featured article is daylight savings time. More than a little creepy.

What's the proper term for this? Confirmation bias? Capuchin 07:56, 11 July 2007 (UTC)

"The root of all superstition is that men observe when a thing hits, but not when it misses."

- Francis Bacon Bendž|Ť 10:12, 11 July 2007 (UTC)


 * Synchronicity? Or just coincidence?--Shantavira|feed me 13:29, 11 July 2007 (UTC)


 * Similar to Bendzh's note, coincidence is to be expected. Amazement should be at a lack of coincidence. -- Kainaw (what?) 13:31, 11 July 2007 (UTC)


 * I'd say that rather than "confirmation bias", a closer match would be observer bias (the same reason that people believe that they can psychically control streetlamps by walking under them); it would only be creepy if you'd spent yesterday chatting about building of the World Trade Center, and the day before that dicussing the Ebionites... Laïka  22:32, 11 July 2007 (UTC)
 * Yes, i'm completely aware that this is perfectly explainable by statistics, don't worry about that. Thanks for the links :) Capuchin 07:56, 12 July 2007 (UTC)

how hard
am not sure if its a medical or not question but how hard does a penis have to be to know ur ok.the other part is hard but the head is a bit soft.is it ok.before u have sex.am 24 and am not sure if am impotent or just beoing stupid.its hard but i tried sleeping withing a prostitute and it kept bending.


 * I think the quality of your erection is something that you should discuss with your doctor. Splintercellguy 09:01, 11 July 2007 (UTC)


 * If she displays no resistance to a second round, then your impedance is nothing to worry about. Oh sorry, wrong question!  --TotoBaggins 16:16, 11 July 2007 (UTC)

Coconut Husks and Water absorption
My son science project is to establish whether coconut husk absorbs water or not ? Anyone has the reference ? Scientific explanation, absorb or not ? Thanks. —Preceding unsigned comment added by 60.54.53.151 (talk • contribs)


 * The articles Coconut and Coir might be helpful. DuncanHill 11:59, 11 July 2007 (UTC)


 * I'm no expert - but I strongly doubt it because:
 * A coconut contains 'coconut milk' that would leak out and evaporate if the husk could soak it up.
 * The reason the coconut tree has evolved this massively protected seed case is so that coconuts can float on the ocean from one island to another and thereby spread. If they soaked up water, they wouldn't last for long before sinking.
 * SteveBaker 14:15, 11 July 2007 (UTC)
 * I don't see how finding the answer online wil help your son with his project... I suggest your son buys a coconut, slices it such that only the husk is remaining and puts it in a measured amount of water for a certain amount of time (being sure to prevent any evaporation of water). Then after a certain time, remove the coconut husk and measure the amount of water remaining. Any difference will show any net absorption (or desorption). He can also weigh the husks before and after to confirm the result.
 * Steve, I'm thinking that perhaps the husk is protected from the milk by the flesh inside the coconut (that seems to be where the milk is located when you open one up, interior to the flesh)? Similarly this would protect them from water ingression when floating, still leaving a hollow inside and keeping them afloat? I don't know whether the husk does absorb water but your reasoning isn't solid enough to rule it out. Capuchin 14:59, 11 July 2007 (UTC)


 * A good starting place would be to decide on a precise definition of the word "absorption. Rubber is not known as absorbent, but a mesh bag full of shredded rubber would certainly pick up some water in Capuchin's experiment, as would a rubber sponge, of course.  It's hard to imagine the fibers on a coconut husk not doing the same.  --TotoBaggins 16:28, 11 July 2007 (UTC)


 * I have several large pots filled with coconut husk fibre which I am growing tomatoes in. They were sold in dry compressed bricks as peat for gardening. They did expand several times when soaked in water, and also seem to retain water well. So yes, they do absorb it. As you may know, coconuts in their wild state are enveloped in a large husk which is several times bigger than the nut in the middle. I do not know if the husk has an outer layer which stops them absorbing water, but inside the nut is air as well as coconut milk, so this must help them float. I suggest you son buys a coconut, scrapes whats left of the fibre off it, and compares the weight and (visually) the volume before and after soaking it in water. He probably wont have enough to weigh, but he should I expect be able to see an increase in volume which implies absorbtion of water. Another point is we only see the old dried coconuts for sale - apprantly the green ones are much more delicious and could have diffferent properties when it comes to flotaing from island to island. 80.0.106.3 14:06, 14 July 2007 (UTC)

Electrical
what is the difference between resistance and impedance?


 * The articles Electrical resistance and Electrical impedance may help, but they are written in rather a technical manner. DuncanHill 12:46, 11 July 2007 (UTC)


 * Sometimes a dictionary is more helpful than an encyclopedia. My Oxford Concise defines impedance as "the total effective resistance of an electric circuit etc to alternating current, arising from ohmic resistance and reactance".--Shantavira|feed me 13:35, 11 July 2007 (UTC)


 * Yes, that's basically it. Resistance describes how strongly a circuit resists the flow of direct current whereas impedance describes how strongly a circuit resists the flow of alternating current at a specified frequency. Note that for "pure resistances" (imaginary devices that have no capacitance or inductance, just resistance), their impedance at any frequency is the same as their dc resistance.


 * Atlant 16:18, 11 July 2007 (UTC)

Re:Black Hole(s) Vs. Black Hole(s)?
What would happen if a black hole with a force and density of x were to meet another black hole which is far more powerful then it? Will they annihilate each other? Or will the stronger one suck in x? And what will happen if black hole x meets another that is more or less x's power? -- Zachary crimsonwolf  13:24, 11 July 2007 (UTC)


 * It is all theoretical at this point in time. It is my understanding that the theory of black holes merging is accepted by more people - which doesn't necessarily mean it is correct.  I googled for a paper I read a while back.  I found this link to a news article about it.  I don't know if the article is good or not. -- Kainaw (what?) 13:35, 11 July 2007 (UTC)


 * The two will merge, and outside the immediate vicinity of either original black hole (where the local gravity of one dominates that of the other), there won't be any change from a gravitational standpoint (neither the total mass nor center of mass changes appreciably). There's nothing special about "force" or "density" to this question; it's just two objects colliding.  The interactions of their accretion discs (if present), however, would likely be spectacular. &mdash; Lomn 13:41, 11 July 2007 (UTC)

The Black hole article states that black holes are a result of individual stars collapsing. So, if, if, a pair (or more) of supergiant stars collapse and form black holes around an area, will they merge as well? -- Zachary crimsonwolf  13:56, 11 July 2007 (UTC)


 * Sure - the two would approach each other and pretty swiftly spiral around faster and faster until they smoosh together ("smoosh" - it's a technical term - right?!) ...subject to all of the weird relativistic stuff about event horizons and time dilation that make it tough to describe what happens when a black hole devours anything. I suppose it's possible that they might somehow get into stable orbits around each other - so you'd get a kind of 'binary black hole' - but I doubt that could be stable for very long.  The black hole article correctly states that a hole initially forms from the collapse of a single star - but it can and will absorb all sorts of other stuff in order to grow - so it will ultimately have 'eaten' many more stars, planets, gas clouds, photons, other black holes...you name it.  SteveBaker 14:11, 11 July 2007 (UTC)


 * Compact binary systems are never completely stable because they emit energy in the form of gravitational waves, causing them to spiral in to each other at an increasing rate. I'm doing a summer internship for LIGO and we just had a talk about these things, which are likely to be the first recognizable signals that LIGO hears. There are actually three kinds of binary inspirals being searched for: neutron star-neutron star, neutron star-black hole, and black hole-black hole. The inspiral phase is relatively easy to model, because you can use Post-Newtonian expansion, but when the two objects get close enough to be in the nonlinear strong-field regime of general relativity, different numerical models give very different results. We know exactly what the GR equations are, but we don't know how to solve them correctly for this complex and violent situation. After the two black holes merge, the resulting larger black hole is distorted from the stable spherical shape, so it emits more gravitational waves in a "ringdown" phase.
 * In 5-10 years, Enhanced or Advanced LIGO will probably hear the gravitational waves from one of these things, and then we'll be able to say in much more detail what actually happens. Won't that be exciting? —Keenan Pepper 18:49, 11 July 2007 (UTC)
 * I'm rooting for LISA. If they ever build it, that is...  Someguy1221 22:08, 11 July 2007 (UTC)
 * This page has an animation of two neutron stars merging and forming a black hole. Sorry I couldn't find an animation of two black holes merging. —Keenan Pepper 18:57, 11 July 2007 (UTC)

Ooo...okay. Nice stuff, black holes. Anyone can recommend nice snapshots of them? Thanks, Steve, Lomn, and Kainaw. Cheers!!! -- Zachary crimsonwolf  14:16, 11 July 2007 (UTC)


 * Snapshots? They're kind of... you know... black. Capuchin 14:26, 11 July 2007 (UTC)


 * I don't think we have good enough telescopes to image black holes - partly because none of them (as far as we know) are nearby and partly because they are...black...kinda like space is black! What you'd see would be a very small black circular region with some wild distortion of the background stars in the region just outside of that...as in the artistic impression over here =====>
 * The huge secondary energies that the black hole is emitting in gamma and X-rays as it swallows stars, etc might in turn cause nearby gas clouds to glow in visible light - so there might be some noticable effects because of that. There are lots of nice artist impressions out there - but no decent photos as far as I'm aware. SteveBaker 14:33, 11 July 2007 (UTC)


 * Just looking at that picture, would there really be a black circular region? surely light would be lensed to this area of the detector. Capuchin 14:52, 11 July 2007 (UTC)


 * Once again, artistic license in an astronomical photo... I don't think I need to rant about it this time. Nimur 15:20, 11 July 2007 (UTC)


 * Okay, I can't resist. My biggest complaint is not with the speculative physics, nor the gross misrepresentation of gravitational lensing... (I have seen several images of actual black hole gravitational lensing and none are this ... "gigantic circle of black in the middle.")  But what irks me most about this photo is the total absence of CCD noise in the black hole region.  Throughout the rest of the photo, the starry background is punctuated by a sort of fuzzy background noise, which is almost certainly due to the thermal effect of the camera sensor.  So are we to believe that the black hole is so powerful that it sucks the thermal noise out of the center of the camera?  I think I'll just conclude by saying that taking artistic license with a scientific subject is walking a fine line.  Nimur 15:25, 11 July 2007 (UTC)


 * Just so you know I'm not blowing smoke, here is an actual scientifc photograph of gravitational lensing from UC Davis Cosmology Group. It may be less dramatic, but its scientific accuracy is far more inspiring than the other image.  Nimur 15:30, 11 July 2007 (UTC)


 * The image description says that this is a technical simulation too, but under the implausible observing conditions of a 10 solar mass black hole at only 600 km distance from the camera. At that distance, I suppose a hole may well look hole-like, and the lensing effects be severe.  Dragons flight


 * Agree, that image is pretty useless. Why is it at the top of black hole?  &mdash; Laura Scudder &#9742; 16:49, 11 July 2007 (UTC)


 * I think that the black disc (the hole itself) is realistic, whatever the image's other flaws. I can't see a mechanism for a spherically-symmetric hole to cause a light ray to leave it in such a direction as to appear to come from the body of the hole, since any such ray in the other direction would fall in.  --Tardis 18:32, 11 July 2007 (UTC)


 * You have to take lensing into account. Imagine light rays going around the black hole on all sides and being bent, so that they eventually come together again (i.e. focus) at your eye.  They can go around, not through.  By the way, there are also counterintuitive effects that can't be understood in terms of tracing rays of light.  If you block the sun with a circular disc, you can get a bright Arago spot right in the center of the shadow! --Reuben 18:53, 11 July 2007 (UTC)


 * I'm aware of lensing, thanks. But for light to appear to come from within the black hole's "disc" (just the section of your sky the event horizon subtends) and yet not come from the hole (obviously impossible), it must start on some other course that intersects a point between you and the hole (but does not intersect the hole!) and then be somehow bent away from the hole onto the line from that point to your eye.  I don't see how the hole can do that.  As for diffraction effects, I tend to believe that the spatial distortions near the hole would decohere the starlight, but I could be wrong.  --Tardis 23:16, 11 July 2007 (UTC)


 * You're right. Sorry, I hadn't thought through your point clearly.  I suppose that leaves only Hawking radiation and any diffraction effects (probably not significant here).  --Reuben 14:59, 12 July 2007 (UTC)


 * Some links: a video of gravitational fields during a black hole merger (done by actual scientists); an article about the video; an article about using the violence of black hole collisions to detect gravitational waves. --TotoBaggins 16:13, 11 July 2007 (UTC)


 * The video seems to be a bit different from the image above, doesn't it? I mean, in the movie, there is no apparent bending of light around the holes. —Bromskloss 16:30, 11 July 2007 (UTC)
 * I believe the stuff in the video is supposed to represent gravitational fields, rather than bent light from behind the objects. --TotoBaggins 17:56, 11 July 2007 (UTC)


 * Yes, there seemed to be some fields there. What I was thinking of, however, was that the still image and the movie must have used different methods to project the three-dimensional space onto the screen. The still apparently follows light rays, which seems natural enough. The movie must use some other method (perhaps equally natural). It would be nice if someone could sort these things out. —Bromskloss 18:05, 11 July 2007 (UTC)


 * The two visualizations treat space differently because they are trying to illustrate very different things. The still image is specifically illustrating how light rays are bent by the gravitational field, by distorting a photograph of the sky. The video simulation is trying to illustrate gravitational waves. It would be extremely confusing, and thus less useful, if it tried to distort the view of those waves like the still image does. Instead that movie is apparently (I'm guessing here, but it is a pretty obvious guess) produced using a purely spacelike projection. This is what you would see if the speed of light were somehow made infinite for purposes of your vision, so that your vision follows spacelike paths instead of lightlike ones, and you were at rest with respect to the center of mass of the black hole pair. You can see a less compressed version of the movie and read about it here. Returning to the still image, you can read more about it, and see other stages of approaching a black hole, here, and read about the ray paths involved here. Note that the calculation (for the still images) was made for observers that were at rest with respect to the black hole, which would require extreme amounts of rocket power when this close to the event horizon, for a black hole as small as they assumed. An observer that was free-falling into the hole, from having been at rest at great distance, would see a different view (he would be traveling at a significant fraction of the speed of light at that point, so the apparent angle from which photons hit him would be different because of the different reference frame, just like runnning in the rain makes the raindrops hit your front more than if you stand still). --mglg(talk) 19:53, 11 July 2007 (UTC)


 * Wouldn't the light travel along the same paths even if its speed changed? —Bromskloss 20:11, 11 July 2007 (UTC)


 * Yeah, that was not a very useful thing for me to say, since the impossibility of faster-than-light light makes it hard to be sure what I meant. Let me formulate my guess this way instead: I think they
 * Picked the distant-observer space-time coordinate system that coincides with the standard, non-relativistic, Euclidean one at large distance, and in which the center of mass of the black holes is at rest. (For a single black hole, such a coordinate system can be extended to the event horizon, where it breaks down because the radial direction becomes timelike. I imagine it works similarly with two well-separated black holes, but what happens at near-merger I have no idea).
 * Then treated that coordinate system as a normal, non-relativistic, Euclidean space in which vision occurs along straight lines.
 * Which is just a complicated way of saying that they ignored the bending of light by gravity. --mglg(talk) 20:46, 11 July 2007 (UTC)


 * I sort of understand what you say, it sounds good, but I can't follow it quite all the way. What other "straight lines" are there than the geodesics, which light follows? —Bromskloss 13:11, 12 July 2007 (UTC)


 * I mean treating the coordinate system as Euclidean, so that there is no need for curved-manifold concept like "geodesics", and a straight line is just x=x0+a dx. --mglg(talk) 19:42, 12 July 2007 (UTC)


 * OK, it seems my problem is that I don't know how I would introduce a Euclidean coordinate system that extends into curved regions of spacetime, but this is probably because of my insufficient knowledge of curved spaces and I won't bother you more with it. Thanks! —Bromskloss 20:23, 12 July 2007 (UTC)


 * Of course you are familiar with treating a curved manifold like a flat one: ever seen a world map? If you draw a straight line on a world map using a ruler, it will not in general represent a geodesic on the real Earth. --mglg(talk) 20:40, 12 July 2007 (UTC)


 * I get that, but the result would depend on what projection was used to draw the map, i.e., what coordinate system was used to map the Earth. Actually, even in a small region (where curvature could be neglected), I don't know how I would introduce a Euclidean coordinate system without using geodesics. —Bromskloss 20:50, 12 July 2007 (UTC)


 * Because the spacetime curvature goes to zero at large distances, I thought that my clumsy expression the distant-observer space-time coordinate system that coincides with the standard, non-relativistic, Euclidean one at large distance would more or less specify which coordinate system I meant. The space part of the actual curved manifold can be thought of as something like Flamm's_paraboloid. Look at the diagram at Flamm's_paraboloid, which uses a fictitious w dimension to embed the surface. I imagine plotting such a curved surface with w as a function of x,y,and z, and then discarding w. That's all. No, it is not a "correct" coordinate system; it does not locally correspond to local euclidean coordinates, and it will not produce gravitational lensing, for example. But if we just want to illustrate where some things actually are located, and not how their image gets distorted by gravitational lensing, then lensing would be a terrible distraction. This is like if we are making a diagram of where some fish are located inside a fish tank, as seen from some diagonal angle: the diagram will be much clearer if we ignore refraction at the water/glass/air interfaces, and draw the tank simply as if light traveled in straight lines (or the glass and water had refractive index one), even though that is not how the scene would look in a real photograph. --mglg(talk) 22:33, 12 July 2007 (UTC)

Someone said way up above that the mass doesn't change substantially in a collision between two black holes. I don't have a reference but I don't think that's true. As I recall, a very substantial portion of the invariant mass of the combined system is released in the form of energy, as gravitational waves. --Trovatore 22:13, 11 July 2007 (UTC)
 * I attendent a lecture from Kip Thorne a couple years ago, and he stated a predicted 10% mass loss in a collision between two large black holes. Don't ask me for a paper to cite or anything, though.  Someguy1221 22:23, 11 July 2007 (UTC)

The atmosphere of gas giants (a hypothetical question)
Would it be theoretically possible for a gas giant to have a liveable atmosphere similar to Earth's? That is to say, oxygen/nitrogen, active hydrosphere, nice fluffy white clouds and so on? 58.7.181.129 17:15, 11 July 2007 (UTC)


 * No. Oxygen and nitrogen are both denser than the hydrogen and helium that make up the body of the planet, so they would sink down to the core and stay there. Keep in mind that gas giants have no definite surfaces. —Keenan Pepper 18:31, 11 July 2007 (UTC)


 * Maybe the question could be rephrased - is it plausible to have a gas giant composed of nitrogen and oxygen (instead of hydrogen and helium?) The answer is, no again, because hydrogen and helium are the only gases that exist in sufficient quantities to form a gas giant.  See planetary formation.  Nimur 18:53, 11 July 2007 (UTC)


 * In other words, once a planet reaches a size large enough that it's escape_velocity is high enough to prevent hydrogen and helium from escaping, very quickly hydogen would accumulate until H2 was the predominant element in the atmosphere. (Because hydrogen is the predominant element in the universe) -Czmtzc 19:23, 11 July 2007 (UTC)


 * What if, by some freak chance, there was no hydrogen or helium in this particular sector of the universe? Or if the planet was specifically engineered to be composed of oxygen and nitrogen by an alien race or something? Would it still operate in basically the same way as a regular gas giant? 58.7.181.129 10:02, 12 July 2007 (UTC)
 * We're now entirely outside the realm of what could naturally exist, but yes -- an appropriate mass of oxygen could form a gas giant, and at some altitude, the pressure is likely to be livable. Extending an active hydrosphere is yet another unsupportable-but-vaguely-plausible possibility, so why not.  You may be interested in checking out Larry Niven's The Integral Trees. &mdash; Lomn 14:22, 12 July 2007 (UTC)
 * ...and the sequel The Smoke Ring - both great mind-stretching Sci-Fi by the way! The premise of those books is a toroidal gas cloud surrounding a neutron star (IIRC) with some other planets orbiting nearby to keep the ring together.  The gas cloud is made of breathable air - and it contains plants and animals - plus some humans who have been stranded there for a few dozen generations.  Forests take the form of gigantic puff-balls of vegetation, there is water in the form of large, spherical 'ponds' that contain fish that float freely around the toroidal 'world'.  It's fun to read because these people are living in an essentially zero-g environment.  The life cycle of the 'integral trees' is fascinating.  I think it's pretty sure that this couldn't work in reality - but the story is good enough that it doesn't really matter.  Someone should make a movie based on these two books - it would be AWESOME. SteveBaker 16:42, 12 July 2007 (UTC)

Vacuum blimp
Would it be theoretically possible to make a high-lift blimp type device by enclosing a relatively perfect vacuum in some sort of sealed aerogel? I'm thinking that the total displacement would be pretty significant compared to the weight of the vacuum (negligible) combined with that of the aerogel (roughly eqivalent to an equal volume of air) and the sealant (maybe about the weight of a normal dirigible skin). The only questions really would be whether or not the aerogel would be strong enough to withstand the pressure while sustaining a vacuum (obviously not alone, as it is porous), and whether the difference in lift between it and helium would be worth the cost of manufacturing the aerogel/evacuating the chamber of air. 65.96.237.245 20:46, 11 July 2007 (UTC) Theonewhoclimbs28


 * It would be theoretically possible, but aerogel is both expensive to make, and somewhat brittle. -Czmtzc 20:47, 11 July 2007 (UTC)


 * The least-dense aerogels reported so far have a density (after the air is evacuated from them) of about 1 mg/cm3. (The density of a 'functional' aerogel that could be used in the application you propose would be higher; you correctly note that the aerogel would have to be sealed against vacuum, adding weight.  In addition, I suspect that these record-setting aerogels lack the mechanical strength to hold up against a full atmosphere of pressure.)
 * The density of helium is a shade less than 0.18 mg/cm3 at 0 &deg;C, and even lower at higher temperatures. So for now, even a state-of-the-art vacuum aerogel would have less lifting capacity than an equivalent volume of helium.  Indeed, given that the sea-level density of air is about 1.2 mg/cm3, it may not be possible to create a sealed, mechanically stable vacuum aerogel with a positive buoyancy at all. TenOfAllTrades(talk) 21:02, 11 July 2007 (UTC)


 * How about a vacuum bubble constructed by covering a loose aluminum frame with lightweight plastic? Building some sort of vacuum blimp feels possible to me, though obviously a substantial technical challenge with little practical advantage.  Dragons flight 21:10, 11 July 2007 (UTC)


 * For purposes of light-than air travel, "helium is almost vacuum"! The buoyancy of a balloon or blimp is proportional to the density difference between the air outside and the gas (or vacuum) inside. Since the density of helium (molecular weight 4) is already a small fraction of that of air (molecular weight about 29), buoyancy would not change very much if the helium were replaced by vacuum. (To be explicit, it would increase by a factor of 29/(29-4), or about 16%.) The negative effect on buoyancy from the added weight of any conceivable vacuum vessel would almost certainly overwhelm the small improvement gained from replacing helium with vacuum. And most (3/4) half of that same buoyancy improvement can be achieved in a much simpler way: by replacing helium with hydrogen, which is "even closer to vacuum" (molecular weight 1 2), and also much cheaper than helium. That would not require adding any weight to the balloon, except for a large No Smoking sign... --mglg(talk) 21:34, 11 July 2007 (UTC)


 * Yeah, but a bouyant vacuum bubble would just be cool. ;-)  Dragons flight 21:38, 11 July 2007 (UTC)


 * Minor quibble in an otherwise good analysis: hydrogen molecular weight is 2. DMacks 21:43, 11 July 2007 (UTC)
 * Of course. Thanks. --mglg(talk) 21:45, 11 July 2007 (UTC)


 * Remember to take into consideration that the gases (both Hydrogen and Helium) are pumped at higher pressure than air so the real world buoyancy would be less. --antilivedT 22:35, 11 July 2007 (UTC)


 * This actually amounts to an almost trivially small difference. The internal pressure of a modern airship is at most a few inches of water over the ambient air pressure, resulting in a difference in gas density of less than one percent.  Much more important is the change in gas pressure (and therefore density) with altitude.  In going from sea level to 300 meters (1000 feet), you lose 4% of your pressure.  At 3000 meters (10 000 feet) your pressure is down 30%.  TenOfAllTrades(talk) 01:32, 12 July 2007 (UTC)


 * People definitely under-estimate the power of atmospheric pressure! Ten tonnes on every square meter! Just think of a 1 meter cube acting as a vacuum balloon - think how much structural steel you'd need to prevent the ton of pressure acting on the top - plus the ton pressing up from beneath from crushing it flat!  Could you even consider building something that strong that weighs less than a cubic meter of helium?!  The thing that would maybe make it possible would be to go BIG!  The force you have to withstand from air pressure is proportional to the area of the craft - but the amount of air you displace is proportional to the volume.  If you double the diameter of your 'balloon', you'd quadruple the surface area (quadrupling the pressure) - but you'd octuple the volume - so you'd have eight times the available material with which to withstand that pressure.  So I suppose the answer is to build something GIGANTIC! SteveBaker 00:12, 12 July 2007 (UTC)


 * Unfortunately, Steve, scaling up (or down) doesn't help. Consider two cases:
 * If the buoyant object is supported by something that fills its volume (like a solid block of aerogel, or some framework of braces), that structure has a yield strength with dimension force/area, i.e. pressure, so the ratio of this yield strength to the atmospheric pressure it must support does not change with size. Therefore the required yield strength, and thus density, of the gel or framework does not change with size. The weight of the aerogel or framework is then proportional to volume, just like the buoyancy is, so effective buoyancy per volume unit does not improve with increasing (or decreasing) size.
 * If the buoyant object is instead supported by a rigid shell, the force it needs to support is proportional to area, i.e. to radius squared, but the cross section area of shell that must support it (consider the equatorial cross section) is proportional to radius (all right, circumference) times shell thickness. As we increase size, therefore, shell thickness must increase in proportion to the radius. And the area of the shell is of course proportional to radius squared. Therefore the weight of the shell increases proportionally to radius cubed = volume, as does the buoyancy, so effective buoyancy per volume unit does not improve with increasing or decreasing size in this case either. Sorry.
 * (p.s. This is the same reason why it is hard to find a good way to store pressurized hydrogen gas for automobile use: scaling some storage scheme up or down in size doesn't change the vessel weight required per volume of gas, at a given pressure.) --mglg(talk) 00:45, 12 July 2007 (UTC)
 * Hmm - well argued! Yep - I think you're right.  Oh well - there had to be a reason nobody ever seriously considers this kind of thing.   Thanks for the correction! SteveBaker 02:04, 12 July 2007 (UTC)
 * "Nobody ever seriously considers these sorts of things" is a bit misleading, eh? Somebody had the idea to build a GIGANTIC BAG and fill it with helium, hydrogen, or just plain hot air.  Somebody even went to the trouble of doing the math for it!  And somebody believed it might actually work, and stuck to the crazy idea long enough to build it.  The same train of thought can be said for manned space flight, submarines, and a whole host of other wacky ideas that could have catastrophically failed in ten million ways.  Somehow, an idea gets trivially considered, and sometimes seriously considered, and finally it becomes reality.  Nimur 05:07, 12 July 2007 (UTC)

If you'll permit me to synthesize several recent Reference Desk questions, imagine for a moment a sphere of completely-reflective foil. Within this sphere is a vacuum, but we will introduce an enormously bright pulse of light. The radiation pressure keeps the sphere pressed-out against the air pressure and because the foil is completely reflective, the light keeps bouncing around and the foil is not ablated by the enormous luminous flux.

What do you think? Could it work? ;-)

By the way, I'm pretty sure we've discussed vacuum blimps before on the Reference Desk.

Atlant 12:52, 12 July 2007 (UTC)

As we've explained a couple of times before - the idea of trapping light inside a big reflective sphere doesn't work because no mirror is ever 100% efficient. Even if it's 99.99999% efficient, the light would be absorbed due to that 0.0000001% absorbtion rate - and given the speed of light - that would happen in a tiny fraction of a second. But in any case, for photon pressure to move something (like the foil of your sphere) takes energy - and it would have to be a truly amazingly bright burst of light to have enough energy to 'inflate' your balloon against air pressure - but that energy would very rapidly diminish due to absorbtion by the balloons' reflective inside layer. The idea is interesting - but perhaps light is the wrong thing to use. Maybe you could have a vast array of baseball pitching machines at the center of some giant zepplin-like balloon - they would toss baseballs at the inside of the balloon anytime it looked like collapsing - with some kind of machine to pick up the lost balls and recycle them...but that wouldn't be a very 'smooth' mechanism...oh but wait, maybe something smaller than baseballs...pingpong balls...atoms maybe? ...and then you wouldn't need the pitching machines - you could just use brownian motion...and that brings us right back to filling the balloon with gas - which is exactly what we do. SteveBaker 14:11, 12 July 2007 (UTC)


 * The key word in my suggestion was (after the intro) the first word: "imagine". Here in thought experiment land, we can have perfectly reflective foil, perhaps made from unobtainium ;-).


 * Atlant 15:18, 12 July 2007 (UTC)


 * Well, I was completely with you on the 'thought experiment'...right up to when you asked: What do you thing? Could it work? ;-). At that moment, you crossed the line from 'thought experiment' to 'would it work' - and the answer is "No" for unobtainium-related reasons! SteveBaker 16:34, 12 July 2007 (UTC)


 * Sure it would work, in thought experiment land. As discussed above, buoyancy improves when switching to lighter particles, so photons, which lack rest mass entirely would be the ultimate choice. (How's that for an article title: Quantum limited ballooning?) To lift a 100 kg person we would need a balloon volume of 83.3 m3; to fill this with photons to one atmosphere of radiation pressure would require a total photon energy of 3*pressure*volume = 25 MJ, over something like 18 ns (=diameter/c), for a pulse power of 1.4 PW. This is within a modest factor of what the National Ignition Facility is supposed to output, so once you have built your unobtainium balloon, the NIF people should be able to fill 'er up for you in a few pulses. Of course, you'd have to to make part of your unobtainium mirror electrically switchable, to get the photons in, but your fictitious-materials dealer should be able to help you with that. The total weight of the photons would be about 275 ng. Happy flying! --169.230.94.28 18:24, 12 July 2007 (UTC)

Forget the Vacuum!

Just build a gigantic geodesic dome (D=1+ miles) Then heat all the air inside by a few degrees (if its sealed and clear the sun will take care of this). Presto chango youve got lift off. And an amazing strength/weight ratio.


 * That won't work, especially if it's sealed! You're confused by the myth that "hot air rises."  Hot air is usually less dense, and less dense air rises.  But if the container is sealed, the density will not change, no matter what temperature...  instead, the gas will simply pressurize.  See ideal gas law.  Nimur 15:18, 13 July 2007 (UTC)

vortex force decline
Hi Does anyone know the rate of decline or force delivered by vortex rings? For example if a vortex ring can deliver 20 kgs per square inch of pressure when launched from 10 meters, how much more would it deliver from 5 meters? that sort of thing

Robin


 * It's certainly characteristic of vortex rings that they lose very little energy as they move away from the source - but it's going to be hard to say how much. I'm pretty sure it would be a highly non-linear thing.  They travel for a ways with almost no losses - then something catastrophic happens and they fall apart very soon after.  I hope we get a good answer - I'd like to know this too! SteveBaker 00:02, 12 July 2007 (UTC)

Pressure Points
Is there a name given to the condition whereby a person does not experience pain, like most people normally would, when having pressure applied to a pressure point? --80.229.152.246 21:09, 11 July 2007 (UTC)


 * One such condition is congenital insensitivity to pain with anhidrosis. The inability to feel pain is analgia; "congenital analgia" is a synonym for congenital insensitivity to pain. - Nunh-huh 21:53, 11 July 2007 (UTC)
 * Sorry, but that's not what I am looking for. Thanks anyway, especially seeing as it shows me I need to be clearer. The person I know whom this does affect does feel pain just as much as anyone else I know, it's just that they don't feel increased pain on pressure points like most people do. I.E. poking him hard on a pressure point is just the same to him as poking him on a normal point. --80.229.152.246 21:58, 11 July 2007 (UTC)


 * After reading the Pressure point article, I have to wonder if maybe the term pressure point isn't well defined. I know when I grew up it was quite popular for kids to press on the point in other kid's collar bone to cause pain, and it simply didn't work for on me.  Maybe this person just has a different body configuration such that the pressure point you are using isn't as effective.  Or maybe, as I suspect, the whole idea of Pressure points is pseudoscience. --  JSBillings  12:15, 12 July 2007 (UTC)


 * Surely someone must mention our article on the Vulcan nerve pinch?


 * Oops, too late! Well, how about Radiculopathy ("pinched nerve") instead?


 * Atlant 12:56, 12 July 2007 (UTC)

Cactus
I have a catus, "Opurtia Cylindrica", a native of Chile i believe. Does anyone possess one? I need to know how to best look after it.


 * I found this cahed page which may be helpful DuncanHill 23:33, 11 July 2007 (UTC)


 * That would be Opuntia. --jpgordon&#8711;&#8710;&#8711;&#8710; 00:33, 12 July 2007 (UTC)


 * 120 glasses of water!? Aaadddaaammm 02:36, 12 July 2007 (UTC)


 * Wha? That's only 7 or 8 gallons (approx 28 L). Geesh. ~ hydnjo talk 03:42, 12 July 2007 (UTC)


 * If you have it in a pot or outside, whenever you need to water it just soak it thoroughly. Exact water measurements are not necessary. Just make sure you don't water it too often or not often enough. I've killed cacti by going both ways. Sifaka   talk  06:55, 12 July 2007 (UTC)