Wikipedia:Reference desk/Archives/Science/2007 May 24

= May 24 =

Two photons
If two photons are moving in the same direction, one in front of the other, it seems like the gravity from the one in the front would increase the energy in the one in the back, but the gravity from the one in the back could only travel at the speed of light and would never effect the one in the front. This seems to violate the law of conservation of energy. What am I missing? — Daniel 01:13, 24 May 2007 (UTC)


 * Photons have zero mass, and thus generate no gravity? That's what I'd guess, though our article on photon says that "Since photons contribute to the stress-energy tensor, they exert a gravitational attraction on other objects", but that's out of my league. --24.147.86.187 03:17, 24 May 2007 (UTC)


 * While I haven't heard of this particular thought experiment, it is a fact of some annoyance to those working with general relativity that in certain circumstances conservation of energy breaks down. Usually, though, (if I remember correctly), it tends to be more of a case of energy "disappearing" than "appearing", so to speak, so I suspect there's something else involved that my addled brain can't come up with. Confusing Manifestation 03:44, 24 May 2007 (UTC)


 * Response to 24.147.86.187: photons have no absolute mass, that is, from their point of reference they don't have mass. Since you can't possible move as fast as a photon and share its point of reference, this is pretty meaningless. They have relative mass, meaning from your point of reference they have mass. Usually, when talking about mass, people mean absolute mass, however, gravity is from relative mass. Response to ConMan: this applies to special relativity, so if it's just general that has that problem then this still is answerable. By the way, are there only a few people using the reference desk? ConMan's link is still purple from when I clicked on it days ago. — Daniel 04:11, 24 May 2007 (UTC)
 * Er, so? The purpleness of links is determined by your browser (typically, its history duration), not by Wikipedia or anyone else's use of it. --Tardis 16:11, 24 May 2007 (UTC)


 * Simply and semi-classically put, the photons never change their separation (SR postulate 2), so gravity can do no work on them (think of each as "orbiting" the other at a constant distance). In GR, it's harder to say things unambiguously, because the gravitational field can carry energy and momentum.  But it's probably safe to say that the geodesic followed by the trailing photon is in a space curved by the presence of the leader, but never reaches a point of greater curvature (again, it's an "orbit"), so it looks straight (and acceleration/energy-gain-less) in an external frame.  This may seem unsatisfactory (why doesn't it reach more curvature?), but remember that in GR gravity is not a force and so hasn't the capacity to "give energy" to anything in the first place.  It is only in an accelerated frame (e.g., standing on the surface of the Earth, being accelerated upwards by the ground) that photons are blueshifted anyway; in an inertial frame the photon has constant energy and it is the ground observer who has slow clocks, short rulers, and light test masses (note the antiparallel with SR there) with which to measure that photon.  --Tardis 15:43, 24 May 2007 (UTC)


 * Gravitational interactions of light are not so simple (as you can probably guess from the diversity of answers here). There are quite a few counterintuitive results, like beams of light traveling in opposite directions having very different effects from beams of light traveling parallel to each other.  If you have a chance, you might look for an article by Tolman, Ehrenfest, and Podolsky, "On the gravitational field produced by light," Phys. Rev. 37, 602 - 615 (1931).  You should be able to get the full article here if you're at a university.  --Reuben 16:20, 24 May 2007 (UTC)
 * That's an excellent article; thanks. Would I be proper, though, in concluding from it that in the weak-field limit it is appropriate to consider momentum as having gravity in that it is the optimal momentum-energy ratio posessed by photons that endows them with their doubled effective gravitational mass?  Normal, non-relativistic matter cannot hope to possess $$p_0\gtrsim\rho_{00}$$ because $$\rho_0$$ is so large, and so its momentum's contribution to the energy-momentum tensor is negligible; my question is whether it is fair to state that light's equivalent contributions to both aspects of that tensor can be considered to be on equal footing with regards to generating gravity.  --Tardis 18:19, 24 May 2007 (UTC)
 * In special relativity, wouldn't the gravity still increase the energy of the photons by increasing their frequency? If not, couldn't you build an entirely different perpetual motion machine by converting massive energy to massless energy e.g. making light with nuclear power, sending the massless energy strait up, turning it back into matter, than dropping it? With nuclear power, this would require using the newly powered matter to lift the slightly lighter recently used matter, and would probably convert more energy than it's making to heat, but it would be making energy nonetheless. I can't read the Reuben cited because I'm still in high school. Everything I know about relativity I learned from Wikipedia. What it taught on special relativity seemed to be quite complete, but I didn't get general relativity. Tardis, what I meant when I said the link was purple was that I click on it looking through the reference desk before. I would expect that if a sizable portion of Wikipedia was using these desks, finding the same person twice would be extremely unlikely. — Daniel 23:34, 24 May 2007 (UTC)
 * Gravity does do that if it manages to ever actually bring the photon closer to the gravitating object (or, in some time dependent systems, bring it into a gravitational wave, ergosphere, etc.). And the reverse happens if you manage to climb out of a well despite gravity.  So no, you cannot get free energy by "teleporting" it against a gravitational field while it's massless.  As far as the RD goes, realize that each desk has maybe half a dozen of us (different across desks) who obsessively answer questions.  Maybe we should have a list.  --Tardis 19:10, 25 May 2007 (UTC)

Rossby waves
More atmospheric physics for you! Our article on Rossby waves says that the wave speed is given by


 * $$ c = u - \frac{\beta}{k^2+l^2}$$

where c is the wave speed, u is the mean westerly flow, $$\beta$$ is the Rossby parameter, and k and l are the longitudinal and latitudinal wavenumbers. The Rossby parameter is given as


 * $$\beta = \frac{2\omega cos\phi}{a}$$

where $$\phi$$ is the latitude, $$\omega$$ is the angular speed of the Earth's rotation, and a is the mean radius of the Earth.

Given the above, I cannot see how c is anything other than tiny. Yet it is not.

For example, if I put in u = 3, k = 5, l = 3 and calculate $$\omega$$ for a latitude of 60 degrees, I get an answer of order $$10^{-13}$$, which is far too small.

I am surely misunderstanding something. Can someone check through this and see where I am going wrong? Many thanks, →Ollie (talk • contribs) 01:22, 24 May 2007 (UTC)


 * I agree with your assessment. Something must be awry!  My understanding of the units in the problem suggest that $$\beta$$ must be in units of velocity (meters/sec).  However, because it is defined as $$\omega / a = sec^{-1} / meters$$, this does not work out.  So, perhaps it ought to be defined as:
 * $$\beta = 2a\omega cos\phi$$
 * This would make beta quite a bit larger (~1012x, which solves our issue quite nicely). I will try to find an alternative source for this equation which may verify my belief.  Nimur 15:08, 24 May 2007 (UTC)


 * This lecture note seems to corroborate my belief that earth-radius should be multiplied, not divided. I will edit the articles in question.  Nimur 15:16, 24 May 2007 (UTC)


 * Excellent, well spotted. Never trust anything you read on Wikipedia! Although I am certain I have seen the same definition elsewhere. Many thanks. →Ollie (talk • contribs) 19:40, 24 May 2007 (UTC)


 * Hey, c'mon. Only one error out of billions of facts isn't half bad. :)  JackofOz 22:04, 24 May 2007 (UTC)

Wasp identification


I found a wasp. As far as my expertise goes, it is the best kind of wasp - a dead one! Can anyone identify the species? J     Are you green?  01:59, 24 May 2007 (UTC)

Your user page says you're from the northern hemisphere, but that doesn't really narrow down enough where you're from. But I can't help much, since my insect book has very few wasps, and none of the ones in it are brown. --Wirbelwind ヴィルヴェルヴィント (talk) 03:50, 24 May 2007 (UTC)


 * Oh! I always forget... Mid-eastern United States is the location. It probably has a nest inside the house judging from the wasps' activity, but I cannot say for sure. J      Are you green?  04:00, 24 May 2007 (UTC)


 * The bug-nerds at whatsthatbug.com know their stuff. You might try asking there.  FYI, most wasps are gentle.  --TotoBaggins 14:14, 24 May 2007 (UTC)


 * You can tell that to the seven yellow jackets that stung me after I mistakenly got too close to their nest! J      Are you green?  19:59, 24 May 2007 (UTC)


 * Haha, that's exactly the beastie I was thinking of when I said "most"! I once was holding my infant daughter (in the summer, so she was lightly dressed), and a yellow jacket actually *chased* me while I ran away and eventually zapped me a few times.  I agree that the only good yellow jacket's a dead one, or else far away.  --TotoBaggins 23:11, 24 May 2007 (UTC)

It could be a small type of hornet but i'm not sure


 * Looks more like a paper wasp to me. Paper wasps are closely related to hornets BTW. Cheers, Dr_Dima.
 * More precisely, it could be Polistes carolina. Dr_Dima.


 * Yeah, we've always called them "red wasps" colloquially, and their nests are paper-thin. They're quite common in Texas, at least in the countryside. This page suggests their normal range is southeast to south central US.  They tend to like overhangs that are out of the sun, or if you are less lucky, wooden swings!  I've always considered them beneficial, so unless their nest is in some place where I will bother them quite often, I just leave them alone. Root4(one) 21:27, 26 May 2007 (UTC)

Megapixel


Based on the article, it sounds like a (for instance) 6-megapixel camera will produce, in terms of display on a computer screen, something like 2,000 x 1,000 actual pixels. Is that about right? I'm buying a camera soon, and I'd like to understand the terminology. If that's right, is it measured by what resolution it's accurate to, or just whatever resolution is captured? I mean, will it be blurry at the resolution advertized or crisp? Black Carrot 02:34, 24 May 2007 (UTC)


 * Whether it will be blurry or crisp should have nothing to do with the megapixels, but with the lens and whether it is focused or not. If it is a true 6 MP camera, then each one will come out crisp when focused. If it doesn't really have that many MP and must extrapolate (using hardware or software), then it might not be crisp. --24.147.86.187 03:15, 24 May 2007 (UTC)


 * I could explain to you the intricacies of resolution, but I will let someone else tackle that. For purposes of simplication, the resolution will not produce a blurry imagine UNLESS you want to use photoshop and "zoom in" on an object. Imagine a face in a crowd in a stadium, a 1mega pixel with no zoom it will look like a mass of people. With 5 mega pixels you may be able to use digital enlargement in photoshop and recognize a face. With say 10 mega pixels you might be able to let say read the watch on someone's wrist WITHOUT any optical zoom just by looking at it in photoshop. If you tried to zoom in on a face with 1 mega pixel, you would only see a pixelated image because it has low resolution. —Preceding unsigned comment added by 69.241.225.43 (talk • contribs) 20:20, 23 May 2007


 * The advertised number of megapixels counts red, green, and blue pixels separately. There are usually two green pixels for every one red and blue, so each physical, monochromatic pixel corresponds to somewhere between 1/4 and 1/2 of a full-color pixel on your screen.  Whether or not it looks good at full resolution depends less on the CCD and more on the optics of the camera.  Be careful of the difference between optical zoom and digital zoom: optical zoom means that it has a real zoom lens like a film camera, and digital zoom is the same thing as blowing up the image file on your computer.  --Reuben 03:37, 24 May 2007 (UTC)


 * Agreed, digital zoom is less than useless, it's of negative value. StuRat 03:43, 24 May 2007 (UTC)


 * Resolution isn't measured as megapixels. Megapixels are just a count of how many sampling points a digital image is constructed of. Instead, look for resolution measured as lines per X. 69.241.225.43 is not precisely correct. If I have a ten megapixel camera that captures 200 lines per picture height (and I'm just making these numbers up; that would give horrid image quality) and a two megapixel camera that captures 200 lines per picture height, both have the same resolution in terms of detail. I could upsample the two megapixel image (assuming the only variable is pixels) and get the same image as the ten megapixel image. For an example, take the 5 MP Sony F-717 (that's what I use). DPreview records 1300 lines per height. Now take the 7.2 megapixel Olympus SP-550UZ. Did you think that you were going to get 1870 (=7.2*1300/5) lines? Wrong. Adding 2.2 megapixels only added 100 more lines of resolution. Oh, and don't forget megapixels = pic height * pic length. Therefore, megapixels are proportional to the square of its dimensions - 1000 x 1000 = 1 MP, 2000 x 2000 = 4 MP. 6 Megapixels means nothing. Be sure to look at all the numbers before making a decision. J      Are you green?  04:27, 24 May 2007 (UTC)


 * I have to say i dont quite understand the statement "6 megapixels means nothing"213.48.15.234 08:11, 24 May 2007 (UTC)


 * What I mean is that the number of megapixels alone is useless. I can have a 3 megapixel camera that, upsampled to 6 megapixels, produces better photographs than a 6 megapixel camera. What matters in resolution is detail, not pixels. J      Are you green?  19:57, 24 May 2007 (UTC)


 * Anyone else thinks that the word "megapixel" is awkward? I feel it's like saying about a parking facility with 1000 cars that it contains a "kilocar". No, wait, it sounds quite funny as I think of it. :-) (Especially in Swedish.) —Bromskloss 09:57, 24 May 2007 (UTC)


 * No, wait again. That would be mixing things up. The number of cars is a natural number, which is no different from other natural numbers that have nothing to do with cars. When describing the number, we should therefore not mention cars. In fact, it would be no more correct to say that the number of cars is "1 kilocar" than it would be to say that it is "1 kilobicycle". —Bromskloss 10:09, 24 May 2007 (UTC)
 * Would that be 1 kilobiHertz, or 2 kHz? Nimur 14:08, 24 May 2007 (UTC)


 * Still, I have seen image clarity increase with increases from 700k pixels to 2.2 meg and from 2.2 meg to 6 meg. The number of pixels is not a "useless" index. Similarly, a 4x5 camera can take a sharper image than a 35mm camera. Edison 21:09, 24 May 2007 (UTC)


 * The number of pixels is meaningful, but it's not enough information on its own to know how the final image will look. In order to get a sharp image at a given resolution, you must have both a large enough imaging array (megapixels) and sufficiently good optics.  If a 6 megapixel camera can't take sharper photos than a 3 megapixel camera at high resolution, that just means the 6 megapixel camera has low-quality optics, and the extra CCD elements are going to waste.  --Reuben 21:24, 24 May 2007 (UTC)


 * Exactly! Stuffing 10 megapixels into a 1/2.5" sensor doesn't mean more detail than a 6 megapixel dSLR. J      Are you green?  00:48, 25 May 2007 (UTC)


 * (As an example after Edison's comments) Thought you might be interested, so I uploaded two identical crops of different image sizes. The 1.2 MP was upsampled to 5 MP using the Bi-cubic setting on Adobe Photoshop. Both have been subject to a slight amount of unsharp masking to emphasise the amount of detail that is present in each crop. I think that you'll find that the 5 MP crop is crisper, but not that much crisper. Think about it... a 4x size reduction. Is there really 4x the detail in the 5 MP image?  J      Are you green?  22:16, 24 May 2007 (UTC)


 * And, just a minor point, 2000 x 1000 pixels is 2 MP, not 6. 6 megapixels will probably somewhere around 3000 x 2000 (3:2 aspect ratio) or 2828 by 2121 (4:3 aspect ratio). J      Are you green?  01:32, 25 May 2007 (UTC)


 * Remember that many images are compressed in camera into a lossy JPEG format and that level of detail is all you can get. Sometimes with the more expensive cameras you can set the camera to record image in a raw format which will save every detail (including normal noise) possibly in a format of higher precision than you can get with JPEG with 256 (or is it only 254?) RGB color levels..  If your 6 MP camera is intrinsically bad at filtering out noise or just has inaccurate color sensors, it may not do as well as a higher quality 3 MP camera. Root4(one) 21:42, 26 May 2007 (UTC)

Forensic anthropology
I read this in my biology textbook. It is about how a forensic anthropologist can supposedly identify skeletons: "Using a variety of techniques, he can tell the age at death, sex, stature, ethnic group, right versus left-handedness, general body build, and other distinguishing characteristics of individuals." I can see how it would be possible to find out most of those, but can right or left handedness really be determined from just a skeleton? How would this be done? Thank you. 68.231.151.161 04:07, 24 May 2007 (UTC)


 * Presumably the bones of the right arm in a right-handed indivudual will have suffered more micro-stress fractures than the left arm bones, and vice-versa. 04:14, 24 May 2007 (UTC)


 * Apparently if you score a few variables of the intertubercular sulcus, specifically the angle of medial wall, angle of lateral wall and presence of the supratubercular Meyer, then you can correctly classify handedness with 89% accuracy by discriminant function analysis. This according to Selvara et al in Forensic Science International Volume 98, 1998, Pages 101-108. More generally, in individuals that spend their life doing manual labour, the humerus may often be slightly longer on the arm they favour due increased mechanical strain and the surface of the clavicle may vary due to an increase in work for the ligaments on the favoured side. Rockpock  e  t  08:21, 24 May 2007 (UTC)


 * For an extreme example, English longbow notes that the effort of drawing a longbow is such that skeletons are noticably deformed. We don't seem to have pictures, unfortunately. Algebraist 12:26, 24 May 2007 (UTC)

Central European Time
The Wikipedia article Central European Time does not explain why countries like France, Spain, Netherlands or Belgium use a different time zone (UTC +1) than the United Kingdom (UTC) although they are mostly on the same longitude. Wouldn't it make sense in a strictly geographical view if they were in the same time zone? Thuresson 11:49, 24 May 2007 (UTC)


 * Geographically, it would make most sense if everyone used a different time zone, corrected to the second, depending on exact distance west of Greenwich. This would be amazingly stupid, however. In fact, time zones are determined by political and social factors, as noted in the CET article. The article also notes that Spain makes up for using UTC+1 by doing everything an hour later than everyone else, so the anomalies are less than they appear. Algebraist 12:21, 24 May 2007 (UTC)


 * I believe that system was in use prior to the time zone system, but wrecked havoc with railroad timetables, so had to go. StuRat 14:24, 24 May 2007 (UTC)


 * oo, an eggcorn! &mdash;Tamfang 05:37, 27 May 2007 (UTC)

The political unions may be more important than geography in determining time zones. Since England is/was in a political union with Ireland, it made more sense for those countries to be in the same time zone, while Spain was often closer to France in terms of trade and politics, so it made more sense for them to share a time zone with each other. StuRat 14:24, 24 May 2007 (UTC)

railways
why pantographs are used to collect current from wires in railways? what are the difficulties otherwise?125.63.107.134 12:04, 24 May 2007 (UTC)


 * What else would you use? Our pantograph article explains its use. You might also be interested to read up on the third rail system, which is the alternative to pantograph collection.--Shantavira|feed me 12:13, 24 May 2007 (UTC)


 * Earlier railcars and trams sometimes used trolley poles. These are usually cheaper than pantographs and make it easier to separate the live wire from the ground. However, they are an absolute pain, as any trolleybus driver can tell you: they tend to come loose (dewire) very easily, points and junctions become very complex, and you need to get out and move the entire trolley by hand if you want to turn around. Some early railcars, such as those on the Snaefell Mountain Railway, use bow collectors - these are essentially fixed pantographs; they have all the advantages of a pantograph, except that they can't be raised or lowered to adapt to changing wire heights. Laïka  14:18, 24 May 2007 (UTC)

cng
isn't there any drawback in using cng(compressed natural gas)as fuel, in terms of power or else?125.63.107.134 12:07, 24 May 2007 (UTC)
 * Help us improve our boilerplate at the top! Why wouldn't you look up Compressed natural gas first, since the main purpose here is to be an encyclopedia.  Are we missing something? --Zeizmic 12:29, 24 May 2007 (UTC)


 * i have seen the article, i could not find any such drawback, plz tell me.125.63.107.133 11:54, 25 May 2007 (UTC)


 * Low energy density (by volume) is one drawback. And right now, there's not much refuelling infrastructure developed; you can just roll up to any of a dozen neighborhood CNG stations and say "fill 'er up!" And, of course, the internal combustion engine needs slight modifications to run on CNG (or flexible fuels).


 * Atlant 23:23, 25 May 2007 (UTC)

Super Boy
I remember reading an article a few years ago about a toddler in Germany who had a genetic disposition for Muscle_hypertrophy. He was only a couple of years old and he had huge muscles without working out, just through natural activities. I figure he may be somewhere from 4-6 years old by now, and I was curious to know if there is any more information about him. (google Muscle hypertrophy boy returns articles from 2004.--Czmtzc 13:16, 24 May 2007 (UTC)


 * I don't know anything about this particular child, but the symptom and the fact that he's German suggest Myotonia congenita, a channelopathy resulting in tonic contraction of skeletal muscles (which constantly "exercises" them). tucker/ rekcut 14:41, 24 May 2007 (UTC)


 * If you read the Article I linked above, the growth is caused by a defective myostatin gene as mentined on the mystatin page. This causes the muscles to grow much larger thatn normal.  As the article says, mice who have had their myostatin genes disabled produce twice the muscle mass of a normal mouse.  I am still interested to know what has happened to this boy. --Czmtzc 17:24, 24 May 2007 (UTC)


 * I think you'll find it hard to find anything recent on him. Even at the time, I seem to recall the articles didn't disclose who the boy was; it was just reported in scientific journals as being of interest, then a few news sources picked it up. I think this is a case of the public being interested, but it not being in the public interest :-) He's probably trying his best to live a normal life. Skittle 14:27, 25 May 2007 (UTC)

Diseased Vikings ?
After Columbus and friends "discovered" the Americas, they spread diseases among the native Americans. Did this also happen earlier when the Vikings visited ? I would think most European diseases would have existed at that time, with the possible exception of bubonic plague. StuRat 13:24, 24 May 2007 (UTC)


 * I recall that syphilis has a very modern history. Our article mentions some disagreement on its source; but I think this disease plays a larger role than is often emphasized.  It might not be discussed in some historical treatments of the New World epidemics, because venereal disease still carries something of a taboo.  In any case, it is unlikely that Vikings had syphilis, because it was not widely documented until the post-Columbian era.  Nimur 14:13, 24 May 2007 (UTC)


 * It's been a while since I researched the Viking landings in Newfoundland, but I don't recall anything about disease transmission. That's not too surprising though, since the natives were pre-literate and the Vikings wouldn't have noticed unless there was a really spectacular die-off.  It was not a friendly encounter.  That being said, I wouldn't be all that shocked if there really wasn't much in the way of disease transfer - most diseases prefer large masses of people; isolated Viking settlements around the North Atlantic wouldn't have been ideal breeding grounds.  On top of that, the voyages were done in mostly open craft around and about the arctic circle - not the kind of place a sick man would last long.  Matt Deres 16:40, 24 May 2007 (UTC)

Native Americans (American Indian) - Grey Eyes
I am looking for information on a tribe that had grey eyes about 500 years ago due to Nordic people arriving at the Americas. Thank you! --Juliet 13:59, 24 May 2007 (UTC)


 * I know nothing about this tribe, but I noticed a parallel with the above question that seems interesting. Nimur 14:18, 24 May 2007 (UTC)


 * Nudge, nudge, wink, wink. Article Virginia Dare has some info about native americans with gray eyes. Thuresson 16:09, 24 May 2007 (UTC)


 * Grey Owl might have had grey eyes. Not that he was a native. JackofOz 22:00, 24 May 2007 (UTC)


 * How about Iron Eyes Cody, the "crying Indian" in the "Keep America Beautiful" commrcial? Unoxidized iron is gray. No, wait- tears would cause rust, and the article says he was really Sicilian. Never mind. Edison 04:18, 25 May 2007 (UTC)

Pig-related death
According to this Shark pamphlet, sharks are less deadly than pigs. Where can I find statistics on pig-related human fatalities? I'm having a tough time with Google on this one. Nimur 17:06, 24 May 2007 (UTC)


 * There are two problems with the above statement. First, the absolute numbers are a very poor indicator as far as that kind of statistics is concerned. There are many more car passengers than spacecraft passengers killed; is spaceflight safer than a drive home from work? Second, "pig-related" is too broad. Pigs harbor a variety of human pathogens - do these count? People get killed in "pig-related" car accidents - do these count? Pamphlet simply says "... even pigs kill more people than do sharks." Still, yes, pigs do kill people. Wild pigs/boars are fiercely protective of their young, and turn aggressive when threatened/cornered/injured. Domestic pigs may attack children (a famous poem by Nekrasov gives a horrifying example). So I would not be surprized if, indeed, "pigs kill more people than do sharks". And, no, I could't find any statistical data either. Sorry. Dr_Dima.


 * Yes, it's amazing what one can do with statistics. Almost anything kills more people than sharks. I heard that more people die after playing contract bridge than any other game (because they tend to be elderly) and a large proportion of the population die in hospitals, so avoid those too!--Shantavira|feed me 18:39, 24 May 2007 (UTC)


 * I believe trees cause many more deaths than one would expect, as well, if you include climbers falling out of them, trees falling on people, cars, and houses during storms, trees conducting lightning to people sheltering from a storm underneath them, cars that run into trees, etc. But before you yell "Save the children, cut down all those killer trees !", beware that people trying to cut down trees are often killed, as well (especially chain-saw newbies).  I guess we just need to learn to live with this evil menace. :-) StuRat 18:49, 25 May 2007 (UTC)


 * My searching turned up no reliable statistics, and I have doubts that the shark-pamphlet satisfies WP:Reliable sources. I did find this article about a woman who was savaged by a pig.  Nimur 21:21, 24 May 2007 (UTC)


 * I found a site that purports to track worldwide reports of animal attacks on people. The most recent death from swine attack I saw looking through them was in 2001, but there had been a couple of attacks by pigs since. There had been far more deaths from attacks by sharks (and gators, spiders,  snakes(none on planes), and lion and tigers and bears.)Edison 21:34, 24 May 2007 (UTC)


 * There has to be a reporting bias inherent with that. People who have been attacked by sharks are usually treated with an element of respect, whereas I suspect being attacked by a pig would be more likely to draw laughter from the average punter. Hence there may well be plenty of swine related injuries that are explained by the old "I fell down the stairs" chestnut. Mind you, the if the plot of Hannibal is anything to go by, wild pigs can be as dangerous as your average shark. Rockpock  e  t  01:49, 25 May 2007 (UTC)


 * The 2001 killer hog was in fact named "Hannibal" because of an earlier attack on a human. The two more recent attacks were by little pink piggies(painful bites, but nonfatal). The site seemed to be attempting to be comprehensive. Edison 04:13, 25 May 2007 (UTC)


 * Shark attacks however are very well documented and recorded. Considering every year about 5 people in the whole world are killed by sharks, and considering millions of people would visit beaches, it's not hard to come up with things that are more deadly then sharks. I suppose the reason why the danger of sharks is so blown out of proportion is because we are so comfortable being at the top of the food chain that being the hunted not the hunter is so extremely terrifying. Vespine 23:01, 24 May 2007 (UTC)

new moon
If there was another moon permanently above Brazil, what would it look like? How would it change through the day or the year and what effect would it have on the earth under it? JS

Probably it would go through all the moon phases every day :) And I expect all the mountains around there would be a bit higher because of the extra gravity :) And there might be an odd effect obn the tides, but I'm not totally sure :( HS7 18:06, 24 May 2007 (UTC)
 * Correct and correct. As for the tides, tidal characteristics (such as low- and high-water marks) would be different due to the gravity, but they'd stay fixed in that new pattern rather than becoming chaotic.  This would apply, in fact, to all effects on the earth.  Since the mass is fixed relative to Earth, it can be treated as any other gravity anomaly on the geoid. &mdash; Lomn 18:20, 24 May 2007 (UTC)
 * I think what you're talking about is a Geosynchronous orbit. That article may help.  Also, if you want to understand phases of the moon we already have, see Lunar phase. Friday (talk) 18:10, 24 May 2007 (UTC)


 * Just in case,

Michel M Verstraete 19:56, 24 May 2007 (UTC)
 * It's important to realize that this question only (barely) makes sense because Brazil straddles the Equator. It would be impossible to locate a satellite (natural or artificial) above any point NOT on the Equator.
 * Also, locating an object as large as the Moon at such a short distance of Earth (about 36,000 km from the mean sea level) would be virtually impossible to achieve and probably highly unstable.


 * An object anywhere near the size and mass of the size of the moon anywhere that close to the earth would cause earthquakes and bad times in general. There would be hurricanes blowing, and rivers overflowing due to tidal forces. You would be in for nasty weather. I would give no credence to such a possibility. Edison 21:04, 24 May 2007 (UTC)


 * To be stationary over some point on the equator you need to be at 36,000km's above the earth's surface - the moon is usually at 360,000km - so it would be ten times closer than it is now. It would look ten times bigger - which is pretty huge.  But tidal forces increase with the square of the distance - so our tides would be 100x more intense.  But with the moon not moving relative to the surface of the earth, the entire planet is going to end up becoming somewhat egg-shaped.  But this would be a fixed, permenant change.  Not like normal tides that come in and out twice a day.  Brazil and Australia would be pulled outwards - the Pacific and Africa pushed inwards.

In all likelyhood, Brazil and Australia would be submerged under an awful lot of water - the Pacific would be come a shallower ocean - Africa would be standing high and dry. If this happened suddenly, the earthquakes and such would be devastating - but once everything's settled down, that ought to stop. The moon would also undergo some serious forces on it (the earth exerts huge tides on the moon - that effect would also get a lot bigger). The hardest problem (I think) is that this isn't a stable situation. Lower altitude orbits are much more susceptible to being disrupted by small movements...pretty soon the Moon will start to shift either in or out - then the disruptions to our planet would be continuous, unending and devastating. SteveBaker 01:29, 25 May 2007 (UTC)


 * Correction: tidal forces increase with the cube of the distance. Remember, the tide arises from the gravity gradient and the first derivative of 1/r&sup2; has magnitude proportional to 1/r&sup3;.  The tidal forces would be be 1000 times more intense than we now have, increasing the effects Steve describes accordingly.  --Anonymous, May 27, 2007, 05:55 (UTC).
 * Oooh! Yikes! You're right. But still, the idea that this new moon is in geosynchronous orbit means that those are static forces - not dynamic.  Over time, the earth and moon would both undergo permenant distortions to their shapes - and once that's over and done with, the tidal forces are no problem whatever.  Both bodies would be kinda oval - but that wouldn't be noticable to inhabitants. SteveBaker 06:22, 27 May 2007 (UTC)


 * As for phases of the moon. It wouldn't be a simple as going through all of its phases every 24 hours because it would be in Earth's shadow for most of the night - and during most of the day, we'd have long duration - very dark - solar eclipses.  Brazil (being right under the moon) would have dark nighttime conditions for an hour or so every day at noon. SteveBaker 02:00, 25 May 2007 (UTC)

Since you did not specify a size, I think we should assume a much smaller body than the Moon, perhaps something as small as Phobos or Deimos. Alternatively, Pick a moon with the same appaent diameter as the moon. A geosynchronous orbit is not strictly stable, so you will need to add thrusters to keep it over Brazil, but since it will be tidally locked to Earth, mounting the thrusters should be a fairly straightforward megascale engineering project. I envision something on the order of the Death Star. -Arch dude 02:20, 25 May 2007 (UTC)


 * Agreed, and I'd go with the same apparent diameter. Also, it may be necessary to do away with our current Moon, since gravitational perturbations from it would knock the new moon out of orbit. StuRat 18:17, 25 May 2007 (UTC)

If there were a moon there, would you still be able to see it from other places?


 * An object in synchronous orbit is visible from almost half the world. &mdash;Tamfang 21:05, 27 May 2007 (UTC)

AIDS Cure
I know that there is currently a search for AIDS cure however who is doing it? What is the status so far with this search? --Juliet 19:03, 24 May 2007 (UTC)
 * Hi, you signed your name, so I will be nice. What's wrong with our Aids article?  Why doesn't anybody consider this to be an encyclopedia?  Where have we gone wrong? --Zeizmic 20:46, 24 May 2007 (UTC)


 * Our HIV vaccine and AIDS articles are probably the best place to start. --TotoBaggins 20:52, 24 May 2007 (UTC)

I think we should ask all medical researchers to put up websites that answer this kind of question with a moving bar, like the download indicator, where you can watch how close you are to finishing your download. That way, all the people who think like Juliet could check every so often to see something like: "2.7 years until a cure for AIDS" and so forth. alteripse 12:17, 25 May 2007 (UTC)

Thanks to you all...not! Next time I will just ask somewhere else where people try not make others look stupid for being curious! signed my name --Juliet 12:38, 25 May 2007 (UTC)


 * I agree that some of the comments were unfair. Reference Desk regulars are sometimes frustrated at how frequently we get questions that are answered in our articles, but we should not take out that frustration on individual questioners.  --TotoBaggins 14:13, 25 May 2007 (UTC)

These questions aren't answered in the AIDS article, or anywhere else I could find on Wikipedia. Seems people should check before they start with all the "why can't people read the article" junk.82.38.197.184 14:42, 25 May 2007 (UTC)


 * HIV vaccine and AIDS discuss the status and players of AIDS treatments in detail. Please tone down the hostility.  --TotoBaggins 15:57, 25 May 2007 (UTC)

What about this word "cure"? Does medical science cure things? Not really. They mostly treat things. I suppose antibiotics might be considered a "cure" for various bacterial infections; however, to my knowledge, we have never discovered anything that could be called a "cure" for a viral condition, like AIDS. So: There are a number of treatments for AIDS. These are constantly being improved. No doubt significantly better treatments will be available in the coming decades. At some point we will probably find an effective prevention (vaccine or something similar); many people are working on this. However, in my opinion, it is unlikely that a cure for AIDS will be found in the lifetime of anyone reading this. Of course, major advances in medicine can and do occur: antiseptic surgery, antibiotics, etc. But these are essentially impossible to predict. --Tugbug 19:16, 25 May 2007 (UTC)


 * Your point is valid, but I think it's fair to call viral diseases like polio and smallpox "cured" (in the developed world, at least, and almost universally). --TotoBaggins 19:27, 25 May 2007 (UTC)


 * His point is quite valid for many things. There are cures of some infections with antibiotics that were formerly fatal. There are completely effective treatments that erase nearly all the consequences of a life-ruining condition, like thyroid replacement. Surgery can completely cure some kinds of cancer. Obviously Juliet was offended by my attempt to show that her question was completely unanswerable and reflected a deeply flawed understanding of how medical research works, but the answer was intended to teach her more about the topic than some polite version of "we are really, really close, ma'am, but if you could just donate another $20 million... " If you would really like that kind of an answer, please feel free to paste it over the offending but educational one. alteripse 23:17, 25 May 2007 (UTC)


 * There was absolutely nothing wrong with Juliet's question. Perhaps she's not aware of the distinction between cure/treatment/vaccine, but that's besides the point.  The question is still quite valid, and she didn't ask "How many years until AIDS is cured?"  She asked what the progress was and who was working on it.  If you came to me and asked for money for your research, but then started spouting nonsense over the meaning of "Progress," I'd turn you down in a heartbeat.  Someguy1221 23:35, 25 May 2007 (UTC)


 * Insults are completely devoid of pedagogic value. We're not training ninjas here.  --TotoBaggins 02:16, 26 May 2007 (UTC)

Upon reflection, I agree that my initial answer was a bit too curmudgeonly even for me. Sorry. Nevertheless, none of you have offered anything even that close to an answer except "read the article", because her question in fact was unanswerable as asked except with an encyclopedic article on the currenst directions of AIDS research. I plead burnout from too many medical researchers and overly optimistic news reporters describing every little thing as "close to a cure" for far too many decades for far too many diseases. alteripse 04:17, 26 May 2007 (UTC)


 * TotoBaggins: Polio and Smallpox are prevented, not cured. If you get one of them, it will still mess you up pretty badly. The difference is that now you don't get them. --Tugbug 23:15, 26 May 2007 (UTC)

OK, So I'm building a dual carriageway...
...which is going to be very busy. If I put down (say) a 40mph speed limit, cos speed is not as important as capacity. What's the steepest that grade that I could build it so that it goes up 60 metres across a bridge over a ship canal and down the other side? I can't have long approaches.


 * For a dual carriageway, I don't expect you'd want more than about 5-10º of gradient on the approaches. More than that would probably lead to problems with heavy vehicles etc. --YFB ¿  23:17, 24 May 2007 (UTC)
 * You've obviously need to tailor it to suit the local conditions, but there is one near to where I live to give you an idea - the Barton High Level Bridge, across the Manchester Ship Canal has maximum gradients of around 1:25, around an angle of 2.3º. Length: ~739m (centre span ~94m), max height above the canal: ~30/31m. It now forms part of the M60 motorway, with a 70mph speed limit, carrying well in excess of 100,000 vehicles per day. Richard B 00:03, 25 May 2007 (UTC)

Speed is capacity. --Tbeatty 14:31, 25 May 2007 (UTC)


 * I think I know what Tbeatty is getting at; if cars are moving twice as fast, and every other factor is the same, then a given stretch of road could carry twice as many vehicles in a given time period. However, for safety reasons it is important for cars to increase their following distance when they increase their speed, so this can quickly eliminate any capacity gains.  Also, if accidents become more prevalent and/or more severe, this can reduce the average capacity due to lane closures. StuRat 18:27, 25 May 2007 (UTC)


 * Tbeatty is talking rubbish and is obviously not a driver! The more there is traffic you slow down, because slower vehicles limit the speed of others, who have to manouevre to overtake them, but yes, also that at lower speeds you can drive closer to the vehicle in front, so you get more vehicles per.  There are probably some equations and modelling that traffic planners use.


 * Yes, they do. And speed is capacity.  For example, say the space between cars going 30mph is 100ft and the space between cars going 60mph is 200ft.  In a 10 mile stretch of road, the 60mph road only has both cars for about 6 minutes.  The 30 mph cars are on the road 12 minutes.  Only in the limit of being full does the capacity approach the same for both cases.  In all other casees, the 60 mph road has higher capacity even with proportional spacing.   A faster road has less traffic.  Ever seen traffic planners take the curves out of a road to reduce traffic jams?  This is solely to increase speed.   --Tbeatty 06:35, 26 May 2007 (UTC)


 * But you failed to account for the number of vehicles. In a 2000 foot stretch of road you could fit 10 cars spaced 200 feet apart or 20 cars spaced 100 feet apart.  So, to go back to your 10 mile example, it takes 6 minutes to carry 10 cars at 60 mph or 12 minutes to carry 20 cars at 30 mph, that's no difference at all. StuRat 18:20, 26 May 2007 (UTC)
 * I did include the number of vehicles. I said in the limit of being "full"  the capacity is the same.  But all other cases, the faster road has more capacity.  That implies that speed is capacity since it's faster except for the extreme limiting case.  It is a false constraint to limit the train of cars to a fixed length less than the length of road.  In your example, you could fit all 20 cars whether they go 60 or 30 mph.  Only in the case of a 10 mile train of cars (i.e. the number of cars is physically limited by the road and spacing requirements) is the 30 mph road equivalent to the 60 mph (the 60 mph train has exactly half the cars going twice as fast).  In every case where the train of cars is less than 10 miles long, the 60 mph road has more capacity. --Tbeatty 07:23, 27 May 2007 (UTC)
 * No need to speculate about this stuff; traffic flow is a well-studied subject. See our traffic flow and traffic congestion articles and their "See also" links, including in particular the Fundamental Diagram of traffic flow. —Steve Summit (talk) 21:25, 26 May 2007 (UTC)


 * Mathematical calculations are not speculation. However, there are some unknowns, such as how following distance actually varies with speed and how accident prevalence and severity varies with speed, which could benefit from studies.  Another big factor is that roads which move faster will draw more traffic from slower moving roads around them. StuRat 22:50, 26 May 2007 (UTC)


 * Not sure why you're calling these "unknowns" that "could benefit from studies"; all the factors you mentioned have been (and are) the subject of extensive study.
 * (Also -- though this isn't worth getting worked up over -- while pure mathematics may be inherently nonspeculative; applied math can certainly be speculative, for example if the model you're using doesn't conform well with reality. Or if you make inappropriate simplifications, such as assuming that the length of a car is 0. :-) ) —Steve Summit (talk) 03:14, 27 May 2007 (UTC)

Anyway, thinking about it the max climb would be for a Loaded HGV to go from a standing start to a decent (20mph?) crawl, as this would occur only rarely. But otherwise that HGVs would be able to maintain a decent speed up the grade if it has a decent run at it (say 40mph at bottom, 30mph at top). There must be more examples elsewhere surely? Harry of the Yellow Banana 21:00, 25 May 2007 (UTC)