Wikipedia:Reference desk/Archives/Science/2011 November 11

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November 11

Looking for simple General Relativity formula

Ahoy. Back in 1997 or 1998, Nova did a special on Einstein and his theories, where I was first introduced to the beauty of General Relativity. In the special, Nova used a field equation identical to the one illustrated at the top of this web page to describe General Relativity, and that equation has been in my head ever since as representing General Relativity the way that the much sexier E=mc^2 describes Special Relativity. Now perusing our many wonderful pages on Relativity, I see there is no one equation that says "General Relativity." The equation on the aforementioned web page is described as an "elegant symbolic formulation" of General Relativity - on a t-shirt or as a tattoo, would a person knowledgeable in physics see that as "General Relativity" and would it be accurate to describe that formula as such when asked about it? Thanks. --Jeffrey O. Gustafson - Shazaam! - <*> 00:50, 11 November 2011 (UTC)

No equation is meaningful unless the terms are defined. We can't even say it's correct unless we know what "G" and "T" and the various Greek letters are intended to mean. Fortunately, this equation is the "standard" notation commonly used to describe this bit of physical equivalence. It would probably be recognized by anyone who has taken an advanced physics course in the western world. I don't think the equation is very profound; the physics is really in the definition of what "G" and "T" mean, and how they correspond to physical, observable reality. You may enjoy reading Feynman's story about the difference between knowing the name of an equation, and knowing what it means. In the same way that a textbook can blather about energy without teaching anything: you can write $G_{\mu \nu }=8\pi \,T_{\mu \nu }$ all you like... and we can say "Wakalixes = $G_{\mu \nu }$." It would not be correct, from this perspective, to say that a physicist "sees" the formula as "General Relativity." The formula is just a shorthand way we write conceptual relations. General relativity is the bold statement that these concepts apply to our physical world, and to understand what that means requires a much greater depth than the formula, on its own, can possibly convey. Nimur (talk) 01:09, 11 November 2011 (UTC)

And once all the caveats that Nimur talks about are taken into consideration, that is indeed the best choice of equation to represent the physics of general relativity (if you know what the symbols mean). Dauto (talk) 01:14, 11 November 2011 (UTC)

I think you have to add a lot of extra ingredients to $G_{\mu \nu }=8\pi \,T_{\mu \nu }$ before you can appreciate its physical meaning. Mathematically, all $G_{\mu \nu }=8\pi \,T_{\mu \nu }$ says is that one tensor is proportional to another tensor throughout spacetime. So, in addition to this, you also have to know:

1. The left hand side of the equation, $G_{\mu \nu }$ (the Einstein tensor) actually represents a complex non-linear expression in the metric tensor and its first and second derivatives.
2. Once we know the metric tensor, we can determine the paths of geodesics in spacetime.
3. In the absence of external forces, both matter in free fall and photons follow spacetime geodesics.
4. So, if we can ignore electromagnetic forces etc., the distribution of matter and radiation throughout spacetime is completely determined by the metric tensor (plus some given boundary conditions).
5. The distribution of matter and radiation in spacetime in turn determines the right hand side of the equation, $T_{\mu \nu }$ (the stress–energy tensor).

So to find the metric tensor you need to know $T_{\mu \nu }$, but to find $T_{\mu \nu }$ you need to know the metric tensor ! Gandalf61 (talk) 09:57, 11 November 2011 (UTC)

Note that t-shirts with the Einstein field equations are available. Looie496 (talk) 16:25, 11 November 2011 (UTC)

The Lagrangian formulation is simpler. The Lagrangian is:

${\mathcal {L}}=\left({1 \over 2\kappa }\,R+{\mathcal {L}}_{\mathrm {M} }\right){\sqrt {-g}}$

See here

Count Iblis (talk) 16:49, 11 November 2011 (UTC)

I fail to see how the Lagrangian formulation is any harder or simpler. Dauto (talk) 17:00, 11 November 2011 (UTC)

Consider that ${\sqrt {-g}}$ is an artifact of the coordinate system that should properly be absorbed into the measure. --Itinerant1 (talk) 12:52, 15 November 2011 (UTC)

Burning with fluorine

Can materials "burn" in an atmosphere of a highly reactive gas other than oxygen (fluorine for instance)? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 01:51, 11 November 2011 (UTC)

Sure. Here's a reaction of sodium metal in chlorine gas: [1]. Looks pretty burny to me. Fluorine should be equally as spectacular. --Jayron32 02:57, 11 November 2011 (UTC)

Absolutely and emphatically yes. In the 1960's, the use of liquid fluorine (and some fluorine compounds) was contemplated as the oxidizer for rocket engines. Despite fluorine's incredible potency, this application was scuppered by insurmountable handling issues and by the extraordinary toxicity of the combustion products (including hydrofluoric acid). TenOfAllTrades(talk) 04:22, 11 November 2011 (UTC)

Undoubtedly there will be an energetic reaction. From a terminology point of view though, I have sometimes encountered people who define "burn" to exclusively mean a type of reaction with oxygen, while other people seem to be more liberal. So whether or not the reaction qualifies as "burning" or "fire", etc., may depend on who you happen to ask. Dragons flight (talk) 06:16, 11 November 2011 (UTC)

I object to this use of the word liberal -- I am an American nationalist and I define burning to include reactions with halogens, perchlorate, etc. (and I vote ). 67.169.177.176 (talk) 06:27, 11 November 2011 (UTC)

What about an exothermic, self-sustaining redox reaction, where the reaction causes the reductant to be thermolysed and dispersed, increasing reaction rate? Plasmic Physics (talk) 06:40, 11 November 2011 (UTC)

Except burning in oxygen is unique. Remember that oxygen is a stabilised diradical (under normal conditions) and doesn't form its radical form (singlet oxygen) as its dominant form until above a certain temperature. Even after forming its radical form and reacting, substrates generally form peroxides first, (which then cleave and get further oxidised, forming ketones, aldehydes and the like). Once carbonyl and alcohol groups on a carbon chain get close enough, retroaldol decomposition can proceed.

I can't help that "burning" in fluorine occurs by a different mechanism and this results in different characteristics from the familiar "flame". For example, burning an alkane in fluorine may not result in breakdown of the carbon chain, because fluorine cannot "expel" other fluorines (compare the tetrahedral intermediate involving oxygen). You might just get a haloalkane instead, and a whole lot of aersolised haloalkane soot. If you burn organic compounds in fluorine, you may produce a lot of HF, but you won't produce a lot of carbon tetrafluoride. elle _{vécut heureuse} ^{à jamais} (be free) 15:25, 11 November 2011 (UTC)

Some textbooks show silicon burning in flourine gas. Polypipe Wrangler (talk) 22:49, 14 November 2011 (UTC)

Silicon indeed reacts violently with fluorine to form SiF4. But could you please explain what is "flourine" -- is it some new substance derived from flour? If so, I haven't heard of it, and have trouble understanding how such a substance can ignite silicon.  :-) 67.169.177.176 (talk) 01:20, 15 November 2011 (UTC)

Electric field=flux density?

from gauss law: ∮E.ds=φ or, E=φ/S now flux density(D)=Φ/S SO, can it be written D=E? — Preceding unsigned comment added by Intr199 (talk • contribs) 15:21, 11 November 2011 (UTC)

The Electric displacement field D AKA electric flux density is related to but not identical to the electric field E. Read the linked article to see how they relate to each other. In very brief terms, D takes into account only external charges (Charges that were not present in the medium to begin with) while E also takes into account induced charges (Charges that were part of the medium but became displaced from their equilibrium position when the medium was polarized by the presence of the electric field). -- Dauto (talk) 16:53, 11 November 2011 (UTC)

Is acne becoming less prevalent?

Some pure OR here. A colleague and I, both in our 60s, and working in high schools, were discussing the above question. Our answer, based purely on our own observations in south eastern Australia, was yes. To us it seems much less severe than 20 years ago or earlier. I've never read anything about such a possibility. The Acne vulgaris article doesn't help. What do others think? HiLo48 (talk) 16:46, 11 November 2011 (UTC)

It would be interesting to see if medical treatments have increased in prevalence. When I was in high school (now almost two decades ago), Accutane was an option that many parents/kids were pursuing, but it was expensive and not covered by insurance providers. I see the patents have since expired and generics are available. I wonder if that hasn't anything to do with it? Nothing else on the list of mitigation seems to have changed much since that time. --Mr.98 (talk) 16:57, 11 November 2011 (UTC)

As you're in Australia, I guess one explanation (if this is a confirmed observation) is that increased levels of UV may be helping. I say this because treatment with UV for severe acne used to be common in the UK. --TammyMoet (talk) 06:15, 12 November 2011 (UTC)

This explanation is unlikely, as 20 (40, 60) years ago, non-indigenous Australians were pasty North West Europeans who regularly burnt themselves to a crisp out of some kind of anti-dermal slash and burn campaign, whereas today many more Australians have more UV resistant skin types, and most young Australians have been inculcated into a culture of skin cancer avoidance. Fifelfoo (talk) 06:18, 12 November 2011 (UTC)

Acne is hormone related - hence the prevalence of acne in children undergoing puberty, women starting on some types of the Pill, and occaisonally women in menopause. Australian children these days are generally significantly heavier than previous generations, due to popularity of fast food, reduced emphasis on sport/exercise in schools, and displacement of outdoor "play" by computer use. Heavier weight, particularly fat, tends to damp out hormonal changes and thus reduce the incidence of acne. This is about the only advantage of being fat. ......Kieth — Preceding unsigned comment added by 60.230.200.222 (talk) 11:50, 12 November 2011 (UTC)

It's an interesting question, and I had been thinking much the same thing myself recently. I probably would even say more the last ten or so years rather than the last twenty. I had put it down to possibly better medications (both prescribed and OTC topical treatments (maybe Katy and Avril are right and Proactiv really does work...)), along with the increased likelihood of people seeking medical treatments. Back say twenty years and more ago most people, especially those not suffering from it, just regarded it as 'a part of growing up'. But these days we seem to be increasingly prone to seeking medical solutions for all problems, and people also seem to look at acne in broader terms, such as it's impacts on self-esteem, employability, etc, and thus seek solutions rather than just dismissing it. Interested to hear other ideas. --jjron (talk) 14:33, 12 November 2011 (UTC)

Yes, the higher average weight of children was one possibility I had thought of. It's the only one that really seems to fit the situation I see. I cannot imagine the subset of kids that I mostly see being heavy users of anti-acne drugs. You have to have a condition before you would decide to take drugs to lessen the impact. These kids don't even seem to be encountering the condition. HiLo48 (talk) 16:48, 12 November 2011 (UTC)

Hmm, there could be something in that perhaps. I had also noticed that the kids that do still seem most acne prone (from a few zits here and there to more serious stuff) do "on average" seem to be the more active fitter kids who are in better shape and often have reasonably good diets; much individual variability of course. Well there's a real incentive for looking after yourself! --jjron (talk) 02:15, 13 November 2011 (UTC)

"Lone Tree" on rock (Copper Canyon, Mexico) (done)

Greetings! Two botany questions:

• (1) Can the following tree species (Mexico, Copper Canyon) be identified ?
• (2) Is it possible to exclude certain species? There can be found: 23 species of pine, 200 species of oak, mex. Douglasia (up to 2400 m), Alnus acuminata, poplar,ficus and palm trees.

Here the photos: Photo 1, Photo 2, Photo 3, Photo 4, Photo 5. Thanks for your help! Grey Geezer 17:47, 11 November 2011 (UTC) — Preceding unsigned comment added by Grey Geezer (talk • contribs)

With regard to (2), it appears to be a broadleaf tree, so it's none of the species of pine. It's also not a palm (those, I think, grow only at the bottom of the canyon, anyway). It certainly doesn't look like a poplar. It's too bad the tree in the right foreground of photo 4 is so out of focus, as it looks like it might be the same species as your lone tree. Deor (talk) 00:38, 12 November 2011 (UTC)

If you have any (even non-photographic quality) pictures that show a closer view of the tree (leaves, flowers, fruits, etc.), that would be a far better picture to use for identification. If the original photo of this was high resolution, you can crop a small closeup area as well.

It looks like an oak though (genus Quercus). But seeing that there are probably dozens of species in that region, you can't really be sure as to what species without a closer look. It looks similar to Quercus sideroxyla, Quercus chrysolepis, Quercus grisea, Quercus coccolobifolia, Quercus toumeyi, etc. just to name a few. All the previously mentioned can be found in the Southern US and Northern Mexico region.-- Obsidi♠n Soul 01:29, 12 November 2011 (UTC)

Thanks very much for the comment! My argument against oak was the hight (to my knowledge, oaks only grow up to 1900 m - and the place looks high up, although I do not know, HOW high up it is) and the position. Is it known that (any kind of) oak can grow on bare rock? I am sorry that there are no better close up pictures. This Tree+Rock must be close to a hiking route, as many pictures of it are found in the Web. Any comment? Grey Geezer 08:33, 12 November 2011 (UTC) — Preceding unsigned comment added by Grey Geezer (talk • contribs)

Altitude of the Sierra Tarahumara is between 1600 to 2400. And yes, oaks can grow to those heights. In fact, pine-oak forests are the defining ecology of the upper slopes of the Sierra Madres, in this case, Sierra Tarahumara contains part of the Sierra Madre Occidental pine-oak forests. The oak populations of the region are the dominant broadleaf trees and are also quite well-studied as they form a multispecies hybrid zone. And yes, oaks can grow on rocky surfaces, often becoming stunted in the process, some growing to no larger than a shrub ("natural" bonsais). However, again, it's impossible to be certain without a closer look at the leaves, flowers, and fruit.-- Obsidi♠n Soul 11:13, 12 November 2011 (UTC)

Thanks. Concerning arguments, we seem to have reached the end of the branch. I learnt something! Cheers Grey Geezer 11:59, 12 November 2011 (UTC)

chemistry

what is the difference between charge and ion? — Preceding unsigned comment added by Bhaskarandpm (talk • contribs) 17:55, 11 November 2011 (UTC)

Electric charge is a physical property of matter that (for normal matter) is determined by how many protons the matter has, minus the number of electrons the matter has. An ion is an atom or molecule in which the number of electrons isn't equal to the number of protons, so that it has a non-zero charge. All ions have a nonzero charge, but not everything that has a nonzero charge is an ion. For example, a single electron all by itself has a nonzero charge, but it doesn't count as an ion because it doesn't have any atomic nuclei. All ions have at least one atomic nucleus. Red Act (talk) 18:21, 11 November 2011 (UTC)

Ions are always charged molecules, while charge is a property of matter that can apply to any kind of particle. For instance, a charged capacitor does not (usually) contain ions. The electrons in the Atoms are concentrated to one side, so the density of electrons between the two sides of each atom differ, resulting in a localized charge, even though the atoms are still electrically neutral. Phebus333 (talk) 15:19, 12 November 2011 (UTC)

help ....plz (best physics and maths book)

which is the best physcis and maths book for claring concept and problems and tell the website also — Preceding unsigned comment added by Bhaskarandpm (talk • contribs) 18:14, 11 November 2011 (UTC)

These web sites might be helpful: http://en.wikipedia.org/wiki/Physics http://en.wikipedia.org/wiki/Mathematics Mitch Ames (talk) 23:00, 11 November 2011 (UTC)

Large French spider

Anyone have any ideas what spider this could be: [2]? I found it in a well in southern France. Not a lot to go on, but I had to hold the door open (bushes pressing against it) and wasn't about to get any closer! The color seemed black, not brown, but it was pretty early in the morning too, so not a lot of light. Just curious, thanks!Reflectionsinglass (talk) 18:17, 11 November 2011 (UTC)

I guess I should add that it was probably about three or four inches from rear leg to front leg. To me, it's a big spider. Reflectionsinglass (talk) 18:23, 11 November 2011 (UTC)

Sounds like a cave spider? --TammyMoet (talk) 21:23, 11 November 2011 (UTC)

Tegenaria domestica, the domestic house spider

Cave spiders are too small (maximum leg span is around 2 in, with a 0.5 in body) and are orb-weavers.

Given the estimated size by OP and the location, it's likelier to be the common (in Europe) house spiders of the genera Tegenaria or Malthonica. The giant house spider (Tegenaria duellica), for example, regularly reaches 4 inches in legspan.

While most members of those genera do not have venom that is medically significant to humans, be very careful. It could also be the hobo spider (Tegenaria agrestis, which has a legspan of 2 inches or more), which unlike the former, is considered dangerous to humans. Though this is controversial, as some studies claim that bites attributed to hobo spiders were in actuality from brown recluse spiders. Still, it's better to err on the safe side - don't handle it.-- Obsidi♠n Soul 21:57, 11 November 2011 (UTC)

The article I linked to claims that the cave spider is "amongst the largest spiders found in the UK". From experience, it is easily possible to overestimate the size of a spider, particularly if you're not anticipating finding one! I suppose it could also have been a raft spider, which does live in wet conditions, but unlikely. It does look like a normal house spider, though - they seem to have been extra large this year. --TammyMoet (talk) 06:11, 12 November 2011 (UTC)

Heh, agree. I have a friend who is mildly arachnophobic and regularly screams about "giant" spiders that turn out to be quite tiny. :P -- Obsidi♠n Soul 11:31, 12 November 2011 (UTC)

Propagation of oxalis triangularis plant.

Wikipedia states that propagation of this plant is from bulbs, yet I have cut down long leaves, rooted them in water and planted them when roots appeared. Not only did they grow but they sent out additional shoots of leaves with flowers popping up later. How come this is not mentioned as an alternative to propagation? — Preceding unsigned comment added by 108.27.62.56 (talk) 18:40, 11 November 2011 (UTC)

As stated by the sentence, dividing the bulbs is the easiest and the usual way for propagating Oxalis triangularis, as it results in higher survivability. This does not preclude other methods of course. Notice that the article also does not mention propagation by seeds.-- Obsidi♠n Soul 19:22, 11 November 2011 (UTC)

Electomagnetism, quantum mechanics, and the strong nuclear force vs. black holes

What would happen in the following situations? Just curious.

1) The nucleus of a hydrogen-1 atom exists such that a tangent plane to it would be the same plane as a tangent plane to the event horizon of a black hole. Its electron is located at some point on the black hole side of the tangent plane. What happens to everything in the next instant?

2) The same nucleus is now located however much distance below, and the two up quarks (charged +2/3) are above the horizon while the down quark (charged -1/3) is located below the horizon. The tangent plane for the black hole is also now the tangent plane between the most upper point on the down quark. What happens to everything in the next instant?

Bonus points: What if an incredibly strong negatively charged object was nearby the atom described in these questions? — Preceding unsigned comment added by Shrug-shrug (talk • contribs)

Except for micro black holes, in general the tidal force at the event horizon of a black hole is much smaller than intra-atomic forces. So almost certainly, in each case the whole atom is going to get sucked into the black hole, along with the negatively charged object mentioned in the bonus section. From the point of view of an infalling object, nothing particularly noteworthy happens locally as the object crosses the event horizon of a black hole. The "almost certainly" was thrown in because nothing's ever precisely 100% certain at the quantum level. Red Act (talk) 20:09, 11 November 2011 (UTC)

Yes, I know that the entire atom is going to fall into the event horizon given time. I'm talking on the order of plank times. If planck times are too short for anything whatsoever to happen, go to whatever time it would take for something to happen. Actually, I reconsidered the question and my thought process concerning it, and realized the negatively-charged object was only put in so as to put more resistance into entering the black hole, thus making it more possible to conceive a time between the parts falling in. Sorry I worded that awkwardly. Shrug-shrug (talk) 20:50, 11 November 2011 (UTC)

As seen from the atom's point of view, nothing unusual happens over even macroscopic time scales as it crosses the event horizon. In a proper frame attached to the atom's center of mass, from say a millisecond before the atom crosses the event horizon to a millisecond after it crosses the event horizon, the atom evolves with time almost exactly as if the atom were deep in interstellar space. As expressed in Schwarzschild coordinates, the shit hits the fan when the atom reaches the event horizon, but that's just due to a coordinate singularity that reflects a poor choice of coordinates for observing the atom in near that event, and is not due to anything that's actually physically unusual going on near that event. That's in contrast to the gravitational singularity at the center of the black hole, where something physically unusual really is going on (and which isn't well-understood). Red Act (talk) 22:00, 11 November 2011 (UTC)

fiberglass insulation

I read that below a certain temperature. I think it's 20°. Fiberglass insulation looses 50% of its effectiveness. Can someone tell me why this is true and how it happens? — Preceding unsigned comment added by 208.86.2.206 (talk) 19:03, 11 November 2011 (UTC)

Read where? 20° what? How are you measuring effectiveness? Dauto (talk) 19:29, 11 November 2011 (UTC)

Actually, a study exists, done by the Oak Ridge National Laboratory [3] that shows poor performance for loose-fill (not necessarily fiberglass batts) fiberglass at high temperature differentials due to diffusion of air of different densities through the insulation, setting up a form of convection. This is opposite to other insulations, who generally do worse at a small temperature differential. Acroterion (talk) 21:22, 11 November 2011 (UTC)

is there anyway you can explain that in layman's terms? And in my understanding I read that other types of insulation performs better in colder temperatures is that correct? — Preceding unsigned comment added by 208.86.2.204 (talk) 22:02, 11 November 2011 (UTC)

You're right. The problem that's always existed with fiberglass is that air can move through it. In the case of blown-in, the cold-dense air can settle at the bottom of attic insulation, which should normally be warmer at the bottom and colder at the top. This makes it less effective in attic installations, which is normally where you see blown-in insulation. Acroterion (talk) 22:56, 11 November 2011 (UTC)

In other words, some types of fiberglass insulation can leak air through the insulating layer, thus reducing its effectiveness. This does not apply to all types of fiberglass insulation. 67.169.177.176 (talk) 22:56, 11 November 2011 (UTC)

The point he's making is that if it is cold outside, there will be a large difference of temperature between inside and outside, and that causes convection (air circulation). Convection works as a conveyor belt removing heat from one side of the wall and delivering it to the other side. That defeats the purpose of the insulation which is to prevent heat from crossing the wall. Dauto (talk) 22:57, 11 November 2011 (UTC)

It's not so much of a "leak" as diffusion of colder, denser air to the bottom of the insulation, which should be at room temperature where it touches the ceiling, but is not in the case of blown-in fiberglass: the ceiling will be colder than it should otherwise be. A denser insulation or one with closed cells would grade from room temperature at the bottom to outside air temperature at the top. Acroterion (talk) 01:03, 12 November 2011 (UTC)

radiation

it was reported in the news today that there is radioactive iodine in the air over Europe. What is the likely cause of this? — Preceding unsigned comment added by 208.86.2.206 (talk) 19:05, 11 November 2011 (UTC)

For reference, the report is from the IAEA, of "very low levels of iodine-131" being detected. The Reuters report says they don't know the cause, and that it's probably not Fukushima. -- Finlay McWalterჷTalk 19:27, 11 November 2011 (UTC)

I-131 is made in nuclear reactors (either deliberately for medical applications, or as part of the power generation process). It has a pretty short (8 day) half life. I believe they're rejecting the Fukushima Daiichi nuclear disaster because the accident was 245 days ago, which is 30 I-131 halflives; so only about a billionth of the I-131 released then is still I-131 now. The IAEA will have a pretty good idea of the gross volume of material released into the air then, so they'll know how much I-131 they'd expect to be seeing now. That would suggest it's a small release from a reactor or from a medical user of I-131 - it's not from a reprocessing facility like THORP, because they don't reprocess fuel hot from the reactor, but wait until stuff like I-131 is gone. It sounds like no-one is admitting (and maybe the leakers don't know) whence it came; we may never know. -- Finlay McWalterჷTalk 19:55, 11 November 2011 (UTC)

At many medical radiation facilities it's not uncommon to vent certain nasty things to the atmosphere. In a well-regulated facility this sort of thing is done at consistently safe levels and not with things like I-131 in it. I would not be surprised if this was some kind of venting accident or error. I suspect whomever did it knows they did it, because such places (at least in Europe and the US) are required to keep pretty strict records of their emissions, and something like that is going jump out. Another possibility is a venting from some sort of more shoddy Eastern Bloc facility wafting over but I don't know how likely that is weather-wise. I do know people who have been to various Russian medical facilities where it is a known fact that they vent enough radioactive material weekly to set off radiation detectors designed to detect smuggled nuclear material (which is not only disturbing from a health perspective, but disturbing from a safeguards perspective, because they ignore the alarms that go off simultaneously with the venting). --Mr.98 (talk) 16:37, 12 November 2011 (UTC)

Terraformed Mars map

Where's a map of what Mars would look like if it were terraformed? --70.250.212.95 (talk) 20:43, 11 November 2011 (UTC)

Terraforming of Mars has pictures. -- Finlay McWalterჷTalk 20:46, 11 November 2011 (UTC)

I meant a 2d map, not a 3d globe. --70.250.212.95 (talk) 21:34, 11 November 2011 (UTC)

I don't think cartographers can really predict that accurately. All such maps are highly subjective, depending on what kind of terraformation, how wide a scale, or how long it's been going on. That's why 3d renderings of terraformed Mars are labeled artist's depictions, they are guesses. Just putting an image to a vague idea.-- Obsidi♠n Soul 23:44, 11 November 2011 (UTC)

Take a look at this site. You can find more with this Google images search. SpinningSpark 16:05, 13 November 2011 (UTC)

Smallest circle with no perceptible curvature?

I'm writing a story which involves characters interacting with a circular wall that has a radius so large that it doesn't seem to curve if you're standing close to it (they're inside the wall, if it matters). I can do geometry alright, but what I'm stuck on is I have no idea how much curvature is perceptible to humans. If I'm standing 3 feet from the wall and 10 feet further along it's curved 1 foot in, would I notice that? Any assistance is appreciated, I don't really know where to start on this. 24.39.129.184 (talk) 21:22, 11 November 2011 (UTC)

It depends entirely on how much of the wall they can see. If you look to the left and then to the right and see wall in both directions, you know the wall is curved, even if the parts you see are miles away. On the other hand even a 10 foot circle may not have perceptible curvature if you can only see a few inches of it. Looie496 (talk) 21:29, 11 November 2011 (UTC)

This reminds me of part of a story by A. E. van Vogt (I think it is the middle part of "A Can of Paint" from "Astounding" in 1944) where the hero is inside a structure of triangular cross-section but curved in a large loop with a 120 degree twist (in the style of a Möbius strip) and doesn't notice either the curvature or the changing direction of gravity. (This doesn't answer your question -- sorry.) Dbfirs 23:32, 11 November 2011 (UTC)

It also depends on whether the wall is featureless or has any objects attached to it. Ancient sailors could perceive the curvature of the Earth because ships disappeared beyond the horizon, and the Earth is quite a bit bigger than your proposed wall! --140.180.3.244 (talk) 05:31, 12 November 2011 (UTC)

Mexican plane crash

My question has two parts one. How come most of the time when a plane or helicopter is shot down by a surface-to-air missile the government never admits it and tries to cover it up? This was most recently seen in libya and Northern Ireland. They usually say it's due to mechanical failure or pilot error. The second part my question is, do you think of this

http://www.bbc.co.uk/news/world-latin-america-15702285

was due to a Sam. It seems very suspicious that both man and his predecessor were both killed in plane crashes. And they were both the head of the war on drugs against the cartel. — Preceding unsigned comment added by 208.83.61.117 (talk) 21:52, 11 November 2011 (UTC)

There is indeed a terrible threat that they don't want you to know, a conspiracy that has hidden details of dozens of attacks on aircraft over the last decades. It's evident that these crashes, far from being "accidents", are really the work of Mothra. Wake up, sheeple! 91.125.88.45 (talk) 22:30, 11 November 2011 (UTC)

In diplomacy, pointing fingers indiscriminately can be casus belli.-- Obsidi♠n Soul 22:44, 11 November 2011 (UTC)

Definitely doesn't look like a SAM to me. It appears more likely that the chopper's pilot went off-course, probably because of an incorrectly set gyrocompass, and smashed into a mountain that he couldn't see in the clouds -- a classic CFIT scenario. The moral of the story is: (1) gyrocompasses can drift off-course over time, and should be reset every 10-15 mins; (2) in bad weather conditions, your instruments are your only means of maintaining control and staying on course; and (3) in these conditions, failure to pay attention to the instruments can be fatal. 67.169.177.176 (talk) 22:54, 11 November 2011 (UTC)

(CFIT here refers to Controlled flight into terrain.) -- ToE 00:47, 12 November 2011 (UTC)

Here are 2 examples of airlines plane shot down by missiles Korean Air Lines Flight 007 and 2003 Baghdad DHL attempted shootdown incident--Franssoua (talk) 15:41, 14 November 2011 (UTC)

Global warming

Will Earth end up like Venus if they don't stop using fossil fuels? --70.250.212.95 (talk) 23:14, 11 November 2011 (UTC)

No. Dauto (talk) 23:15, 11 November 2011 (UTC)

But didn't the greenhouse effect cause Venus's current conditions? --70.250.212.95 (talk) 23:40, 11 November 2011 (UTC)

Yes, most likely. But conditions on Venus are very different to begin with - Venus gets about twice the energy per unit area from the sun that Earth does. Just because the same mechanism is active does not mean it has the same outcome. Gravity is pulling on a man with a parachute as hard as on a man without one, but they will go very different levels of SPLAT when they hit the ground... --Stephan Schulz (talk) 01:31, 12 November 2011 (UTC)

No. James Hansen thinks so,[4] but that's a minority viewpoint. A mainstream scientific viewpoint, from the IPPC IPCC, is that "a 'runaway greenhouse effect' -- analogous to Venus -- appears to have virtually no chance of being induced by anthropogenic activities."[5] See also Runaway climate change and Runaway greenhouse effect. Red Act (talk) 01:20, 12 November 2011 (UTC)

If Earth is moved closer to the Sun to make solar power more area-efficient. Nevard (talk) 04:04, 12 November 2011 (UTC)

I think the highest CO2 concentration Earth ever had was after the thawing of Snowball Earth, when concentrations reached 13%. That was due to the fact that an all-white Earth is in a very stable equilibrium that takes a large greenhouse effect to escape from, which is not the case with anthropogenic warming. I also don't know how hot the planet got after Snowball Earth; our article doesn't say. --140.180.3.244 (talk) 05:20, 12 November 2011 (UTC)

@140: If I`m not mistaken, 13% CO2 in the air that we breathe would be pretty toxic for us, so we might not wanna go back there, would we?

@Red Act: if you insert a link you might wanna check whether it actually links where you want it to... just linking to an abbreviation is usually not appropriate here.

And neither is quoting out of context... The context to that quote: "Some thresholds that all would consider dangerous have no support in the literature as having a non-negligible chance of occurring. For instance, a `runaway greenhouse effect´—analogous to Venus-- appears to have virtually no chance of being induced by anthropogenic activities. So our focus will be on those events that the literature suggests have a non-negligible chance of being induced by anthropogenic activities." So basically the IPCC states that this "APPEARS to have virtually no chance" based on the fact that they have not found literature quantifying its chance to occur above "non-negligible" (whatever that is supposed to be). Essentially, what they are saying is: No one has been able to plausibly explain this as probable enough for us to worry about, so we prefer not to deal with it in this analysis.

If you wait for someone to understand a complex system like that on a level that he can actually explain this as having a relevant chance to occur (or not occur for that matter) you can probably wait several billion trillion years, as some systems are just to complex to be understood by an observer who tries to explain an effect by applying reason to his observations (a process we call "science"), as his ability to observe is very limited (cf. Heisenberg uncertainty principle)

In respect to global warming, there is a certain point that everyone should be conscious about: We are all humans, so we are going to make mistakes every now and then. It logically follows that we can never with absolute certainty make any prediction about what is going to happen. Therefore, in terms of what policies we need to prepare us for the future, it is irrelevant whether or not we can deem something as probable or not. The real question is: Which risk would we rather take? The risk of taking action without need, or the risk of not taking action where it would have been needed? Phebus333 (talk) 14:44, 12 November 2011 (UTC)

I had already earlier checked that IPCC redirects to the organization in question, so it would be perfectly appropriate in this case to use an abbreviation as a link. The only problem was that I mistyped the abbreviation as IPPC instead of IPCC. I have fixed the typo above.

I disagree that the quote is inappropriately taken out of context. The quote uses the phrases "appears" and "virtually no chance", so the quote by itself already expresses a lack of exactly 100% certainty, which seems to be the bulk of your complaint. And how the organization uses the information that there is virtually no chance of a Venus-like result on Earth is irrelevant to the question asked. Red Act (talk) 16:24, 12 November 2011 (UTC)

Okay, maybe I should rephrase my criticism: I do not think we can assume that the OP (or someone else who reads this) has the educational qualification not to interpret your earlier statement in a misleading way. In my experience even some people with better education tend to "ignore" conditional or relativizing clauses in the face of formulations like "a mainstream scientific viewpoint". Even I initially interpreted that statement in such a way that there is a scientific consensus regarding that question, which is something your source does not state (and AFAIK is not true). Basically, I would advise you be more careful with how you phrase certain things, because the average human might not understand it in the same way that you do. Phebus333 (talk) 15:40, 14 November 2011 (UTC)

There may be some nasty positive Climate change feedback in particular with the release of methane from the tundra and the ocean floor, which could make the earth an unpleasant place for quite some time and kill a lot of other life off but humans as a species would easily survive it, nothing at all like Venus. Dmcq (talk) 01:31, 13 November 2011 (UTC)