Wikipedia:Reference desk/Archives/Science/2007 June 23

= June 23 =

microorganisms
Explain how humans can harbor many microorganisms on and inside their bodies without experiencing symptoms of infectionbAspen0212 01:24, 23 June 2007 (UTC)
 * I will if you explain why you have the impression that most microorganisms would be expected to cause infections! :) The normal flora of humans include organisms that seldom if ever cause infection (unless the human host is immunocompromised), and those that cause infection only in special circumstances (breaks in the skin). While it's possible (and usual) for there to be no microorganisms in internal organs, it would be impossible to maintain a sterile environment at the places where the human body and environment meet, because bacteria are ubiquitous. Various mechanisms (mechanical barriers, the human immune system, and bacterial adaptation) allow the microorganisms to exist there without causing disease.  Staph bacteria, for example, will be found on nearly every human surface (skin & mucosa).  When microorganisms are present but do not cause illness, this is called colonization rather than infection. Some organisms are normal flora in one place, and cause infection when moved elsewhere (for example, Enterococcus faecalis is normal flora in the bowel, but would be pathogenic in the bloodstream; E. coli doesn't cause problems in the bowels, but is a frequent cause of urinary tract infections.) And some microorganisms are actually beneficial; for example, the flora found in the intestines are and are the source of nutrients required for human growth and functioning (Vitamin K particularly); and the normal flora generally discourage the growth of pathogens. There's additional information on the mechanisms bacteria use to colonize the human body on this page. - Nunh-huh 02:37, 23 June 2007 (UTC)


 * There are also fungi, such as Candida albicans, which may cause thrush, but not in all the 80% of people that carry it around in their gut. Even some parasites are very common in people without causing illness, e.g. Toxoplasma gondii. Bendž|Ť 07:59, 23 June 2007 (UTC)


 * Think about it from the bacterium's perspective: You get a nice home - it's warm, free from predators, you get fed regularly - and nobody clobbers you with antibiotics or evolves a means to flush you out providing that you don't make a nuisance of yourself!  So these critters have evolved not to do bad things to us - and in some cases to be beneficial to us in order to survive and do what they do best.  It's very much NOT in the bacterium's interest to kill off it's host - and the really nasty bugs are generally the ones that have only recently (in evolutionary terms) managed to cross over into our species and have not yet evolved a way to not be quite so harmful.  It's well known that the lethality of various bacterial strains declines over time as the bacteria evolve.  From the point of view of the bacterium, there is no benefit to causing your host to die - leaving you buried 6' underground in a nice wooden box or cremated!   The bacteria (and virus strains) that DO cause symptoms - generally do so for a reason (eg when you sneeze because you have a cold - that's helping to spread the common cold virus - so that symptom is in the interests of the virus).  It's just an evolved behavior. SteveBaker 13:44, 23 June 2007 (UTC)

pH - 2 little questions
Hi all, Could you please explain my two little things about acids? Would be great - it's for an exam on monday...

1) Which formulae do you use to calculate the proportion of the protonated form of the acid when you have the pKa of the acid and the pH of the environment?

2) Which formulae do you use to know the pH of the mix KH2PO4 0.01 M + K2HPO4 0.01 M ?

Thanks very much in advance!!!


 * See Henderson-Hasselbalch equation. Icek 14:32, 23 June 2007 (UTC)

Seeing what isn't there.
Can someone give me the name of the effect, where you see something, like the virgin mary, that's not really there?--Honeymane Heghlu meH QaQ jajvam 08:14, 23 June 2007 (UTC)


 * I assume you're talking about the situation where you look at something with a complex random texture, like a cloud or a hillside or a piece of food, and the texture seems to form a shape: a sort of optical illusion. I didn't find this one in the list of cognitive biases (the nearest thing there is the clustering illusion), so I looked at the page for a specific example, the Face on Mars, and there was a link to the answer: pareidolia.  (If I'd thought of looking under perceptions of religious imagery in natural phenomena, it would have let me to pareidolia straight away.)


 * On the other hand, if you mean the sort of "seeing things" that has to do with your brain malfunctioning, like you look at what everyone else sees as a blank wall and you see (or hear) the Virgin Mary standing there talking to you, that's a hallucination. --Anonymous, June 23, 2007, 09:20 (UTC).


 * We might also mention the Rorschach inkblot test. (Oh, wait, never mind, the aforementioned pareidolia article already does.) —Steve Summit (talk) 18:54, 23 June 2007 (UTC)

I'm also reminded of this little bit of computer doggerel:


 * If you can see it and it's there, it's real.
 * If you can see it and it's not there, it's virtual.
 * If you can't see it but it is there, it's transparent.
 * If you can't see it and it's not there, it's imaginary.

—Steve Summit (talk) 20:48, 23 June 2007 (UTC)
 * (One "not" rotated 90° from real to imaginary.) --Anonymous, June 24, 00:24 (UTC).

Boeing 747 and Airbus
Could you please tell me the advantage and disadvantage of Boeing 747 and Airbus? Which one is the best? Why? I will be gladful if you could answer this question as soon as possible. ^ ^


 * They can fly really fast, but make a nasty kaboom when they crash? Splintercellguy 10:33, 23 June 2007 (UTC)


 * One is made in US, one is made in Europe. Airbus is currently experiencing some problems with their A380 and will take a while to recover while Boeing seems to have snapped out of their technological disadvantage and now on track with their new 787. --antilivedT 10:41, 23 June 2007 (UTC)


 * First of all, I should clarify that Airbus is the company, not the aircraft. Presumably you're referring to the Airbus A380 which is the closest competitor to the 747. In real terms (disregarding production delays with the A380), neither aircraft is "better" than the other and a direct comparison isn't especially helpful. The Boeing 747 is an old design, having first flown in 1969. However it's been continuously upgraded and the latest version, the 747-8, is intended to be significantly more efficient (and larger) than previous incarnations. You might consider that Boeing has the benefit of huge experience with this aircraft, while the A380, being new, is having some teething difficulties. On the other hand, the A380 is brand-new, larger than the 747 and with significant potential for enlargement (whereas the 747-8 is pretty much as big as the 747 can get AFAIK, there are plans for A380s to carry up to ~1000 passengers). Really, which one is the "best" for a particular airline will depend very heavily on what they intend to do with it - mostly this comes down to finding the lowest-cost way of serving their intended routes and customer groups. That in turn will depend on what sort of deal they can work out with the manufacturer - some of the early A380 adopters will have got excellent prices for their aircraft because Airbus want to have prestigious airlines using their planes to get the order book rolling. If you're in the position to need to know which of the two is "best", I'd hope you'd have enough appreciation of the airline industry to realise that it's much, much more complicated than your question suggests. There isn't a simple answer, I'm afraid. --YFB ¿  11:03, 23 June 2007 (UTC)


 * What do you mean, there "isn't a simple answer"? I mean, obviously Boeing is better than Airbus, just like Ford is better than Chevy, Boston is better than New York, emacs is better than vi, Mac and Linux are better than Windows, C is better than C++ and Java, and Bass Ale is infinitely better than Miller Lite. —Steve Summit (talk) 12:47, 23 June 2007 (UTC)


 * Wow! Excellent answer - except for one teeny-tiny typo: emacs is better than vi - you obviously mean "worse"...oh - and see, you made the same mistake with C and C++. (Oh - and Bass is not merely infinitely better than Miller - we actually need to use one of the higher Aleph numbers to express the infinity we're referring to!) SteveBaker 13:35, 23 June 2007 (UTC)


 * FreeBSD might be better then Linux and Linux might be better then Windows but having used Windows and Mac OS X, I can safely say that POS is in no way shape or form better then Windows (which is saying a lot considering Windows is pretty crap). Nil Einne 21:42, 23 June 2007 (UTC)
 * I will not rise to this bait. I will not rise to this bait.  I will not rise to this bait... :-) —Steve Summit (talk) 22:20, 23 June 2007 (UTC)


 * If a celebrity endorsement will help you decide, John Travolta likes the Airbus. But he owns a Boeing. Anchoress 15:25, 23 June 2007 (UTC)


 * So far, only one person has bought a A380 for personal usage. However more are expected to follow. Personally, I recommend people stop skimping. There's no reason why you can't own a A380, and a 747-8 and a 787 and the A350 when it's ready Nil Einne 21:27, 23 June 2007 (UTC)

The Airbus may be too big. It requires airport modifications to accommodate it, in many cases, and very few routes can support a plane that large. For example, if you wanted to fly from Albany, New York to San Diego, California, using the A380, you would need to fly from Albany to New York City on a smaller plane, then from NYC to Los Angeles on the big A380, then from LA to San Diego on a smaller plane. Who would want to do that ? StuRat 16:34, 23 June 2007 (UTC)


 * If you're as rich as the poster is, either Albany and San Diego fix their bloody airports or they can forget about you ever visiting. BTW, according to the articA380 le, the compatibility requirements don't appear to be that much greater than the 747. I read in Talk:Airbus A380/Archive2 that most airport upgrades that are being carried out aren't strictly essential but stuff like upgrading the bridges to off/load passenger more quickly. This won't matter to the poster of course, since you only need on bridge from him/her. In any case, I was reading in the from the archives of New Scientist an article from when the 747 was being introduced and how no airports supported it... I'm not sure what you mean about routes that can support it but if you mean not many routes have that many passengers, well since the poster appears to be considering it for his or her own personal use (why else would it matter which one is better?), I don't think that is relevant. Nil Einne 21:40, 23 June 2007 (UTC)


 * I don't see the original poster saying he wanted it for personal use. And, yes, I mean that most routes couldn't fill a plane that large, unless they only flew one flight a day, and people want to be able to fly whenever they want, so that won't work. StuRat 02:12, 25 June 2007 (UTC)


 * Well, obviously a plane like this wouldn't be used by "most routes"; it would be used only on those routes that demand that kind of sustained capacity. The 747 isn't suitable for "most routes", either.  (Matter of fact, I can't remember the last time I flew on one. :- —Steve Summit (talk) 12:15, 25 June 2007 (UTC)


 * Yes, but the A380, being larger, is suitable for even fewer routes than the 747. From the POV of Airbus, this is a problem, as they will need to sell a certain number of planes to recoup the development costs.  Ultimately, economics is what limits the size of commercial aircraft, and the A380 might have hit that limit. StuRat 21:11, 25 June 2007 (UTC)

differential gear(battle tank, wheel chair)
for turning, in four wheelers we have differential gear,in rikshaw (3wheeler,pulled by a man)we have a wheel free.what arrangement is there in battle tanks and wheel chairs?59.92.5.237 11:08, 23 June 2007 (UTC)


 * In tanks and other tracked vehicles, there is no need for a differential - the vehicle is steered by disengaging a clutch on one side or other of the vehicle - so one track stops moving and the tank skids around. With electric wheelchairs, each of the two rear drive wheels has a separate electric motor - and they steer by slowing down one or other of the motors - so again, no differential is required. SteveBaker 13:31, 23 June 2007 (UTC)


 * A tank could turn by powering only one track, but this would result in it travelling in a circle around the center of the nonpowered tread. Modern tanks such as the Abrams actually power the treads in oposite directions, allowing them to spin in a circle with the center of the tank as the center of rotation. This would be a big advantage in combat, if they needed to turn around to face an enemy behind them (there is usually thicker armor on the front of the tank)in a street too narrow for the other wider turn. Edison 14:59, 23 June 2007 (UTC)

won't the belt over wheels slide and come out? what about the non-motored wheelchairs?


 * [[Image:Leclerc_p1040868.jpg|thumb|right|320px|Detail of a tank track]]
 * Well, the tank wheels have a deep groove cut into them - and the plates that make up the track have a central 'spine' that slots into the groove in the wheels (you can see this in the photo to the right - look at the row of bright 'spikes' in the dark part of the photo between the two wheels) - this prevents the track from sliding off as the tank turns and the tracks slide sideways. But there is certainly a lot of sideways slippage.  If you look at a road that tanks drive along often, you'll see the scrape marks at every corner as the tanks slewed around them.  Non-motorized wheelchairs have independent rear wheels - they aren't solidly fixed to a single axle - so again, no differential is needed.  The purpose of a differential is to distribute power from some kind of motor to the two wheels such that when you go around corners, the outer wheel gets enough power to turn faster than the inside wheel.   On a vehicle with no engine, you can simply disconnect the two wheels - problem solved. SteveBaker 17:34, 24 June 2007 (UTC)

THANKS A LOT!

the old relativity pole and barn paradox with a twist
Imagine there is a 10 meter pole on the ground and a 5 meter long barn. Imagine there are two persons, superman and barney the barn manager. Imagine superman runs from left to right going through the barn.

Now superman made barney promised that barney would do the following in the correct sequence.


 * (1) Open both barn doors (left door and right door)
 * (2) Signal to superman to pick up the 10 meter pole and start running.
 * (3) Wait for superman to run
 * (4) Close the right hand side barn door
 * (5) Wait for superman to enter the barn
 * (6) Close the left hand side barn door
 * (7) Take a picture of the barn
 * (8) Open the right hand side barn door
 * (9) Watch superman runs out of the barn with the pole through the right hand side barn door.

So far so good. Imagine that superman ran so fast that according to barney the pole has shrunk from 10 meters to 1 meters. While according to superman the length of the barn has shrunk from 5 meters to 0.5 meters.

The paradox is that barney will ONLY open the right hand side barn door AFTER he has closed the left hand side barn door. And once barney closed the left hand side barn door, he will never ever opens it up again.

So the question is this. According to superman, where is superman at the exact moment barney has closed the left hand side barn door?

210.49.223.231 11:55, 23 June 2007 (UTC)


 * I know he's a busy guy but could we ask Mr Superman nicely if he'd please do it again? But with a 10m ladder instead?  If so, then we have an article explaining the whole thing - it's called 'The Ladder paradox'.  The problem with your description is that the concept of 'simultaneous' does not exist in a relativistic world.  In our normal experience, this doesn't matter - but when superman is moving almost as fast as light, it becomes very important.  Anyway - read the article - it explains the whole thing very well. SteveBaker 13:15, 23 June 2007 (UTC)


 * This works only if Barney has got an assistant. Barney is at the right door, the assistant is at the left door, and they have synchronized their clocks. Then the events of closing the left door and opening the right door are spacelikely separated and the order of the events depends on the system of reference. Icek 14:23, 23 June 2007 (UTC)


 * That is nothing we have to care about – it doesn't alter the problem. Just imagine there was an assistant, if you want. —Bromskloss 17:40, 23 June 2007 (UTC)


 * If he doesn't have an assistant, it can't happen because Barnie needs to travel faster than light to get from one door to the other in less time than Superman does. SteveBaker 17:43, 23 June 2007 (UTC)


 * Of course. But we shouldn't get stuck on that, since it is not the point of the thought experiment. —Bromskloss 17:49, 23 June 2007 (UTC)


 * Even in a thought experiment, such things are sometimes relevent. In this case, the question is "What do we mean when we say that the doors are simultaneously shut?" ...and specifically... "In which frames of reference are they simultaneously shut."  Because the time at which each door is shut has a strong bearing on the outcome - it matters greatly that no signal confirming that both doors are shut can be formed due to the distance between them and the brief duration of the event. SteveBaker 13:32, 24 June 2007 (UTC)


 * Assuming that Barney closes the left door immediately as the pole is entirely past it, then the answer is simple: everybody agrees that the rear end of the pole was at the left door when that door closed, as those two events coincide both spatially and temporally. Thus, in any reference frame, Superman was one half pole length (as measured in that frame, and assuming that he holds the pole it its midpoint) ahead of the left door at that time (with simultaneity defined in that frame). Your specific question was about Superman's reference frame, in which the pole has its rest length of 10 meters, so Superman thinks he is 5 meters past the left door, which means that he is 4.5 meters past the right side door of the 0.5 m long barn (and in his reference frame that door was re-opened before this time). --mglg(talk) 23:52, 23 June 2007 (UTC)

heart attack
Where can I find statistical distributions for the times of day, i.e., getting ready for bed, being asleep or waking up and getting out of bed that proceed a heart attack most often? For instance does a heart attack occur more often in the evening before bed or in the morning before waking up or after going to bed or after waking up? I need statistical distributions on anything and everything surrounding a heart attack. 71.100.3.132 14:21, 23 June 2007 (UTC)


 * If you do a Medline search on "myocardial infarction" & "diurnal", you'll find about 125 references. "Onset of acute myocardial infarction (AMI) follows a diurnal periodicity, with a peak incidence between 6:00 a.m. and noon. " (Impact of Ramipril on the Circadian Periodicity of Acute Myocardial Infarction, The American Journal of Cardiology, Volume 98, Issue 6, 15 September 2006, Pages 758-760) - Nunh-huh 15:15, 23 June 2007 (UTC)

car batteries
how to find the amount of lead sulphate formed in the paste experimentally? (for a lead acid battery) .Ruvini 15:09, 23 June 2007 (UTC)

Cloud brightness
Why are some clouds pure white, and others are darker, even black?

See Cloud. Skarioffszky 17:58, 23 June 2007 (UTC)


 * It's just down to how sunlight filters through - or reflects off - the cloud. The angle of the sun in the sky and the thickness of the cloud are what makes the biggest differences in brightness. SteveBaker 13:26, 24 June 2007 (UTC)

Mammary glands
OK So I am asking if you removed the mamory glands would the nipple still stimulate the uterus to contract (like when a mother nurses), or would the nipples loose all sensation? Just wondering... 209.33.215.81 21:41, 23 June 2007 (UTC)
 * I'm not quite sure how you remove the mammary glands without removing the nipples. Also, do you know how the nipples stimulate the uterus to contract? This information is easily available on wikipedia (I know because I didn't know and just found out). Stimulation of the nipples during breastfeeding causes the release of Oxytocin which amongst other things stimulates the uterus to contract. I'm not quite sure how the stimulation of the nipples causes the release of oxytocin. However whether it's nervous or hormonal, the answer would probably be yes provided you didn't damage any part of the pathway. There could very well be some feedback mechanism which the mammary glands are involved in which may affect this. But given the impossibility of removing the mammary glands while presering the nipples, this all seems somewhat irrelevant Nil Einne 22:10, 23 June 2007 (UTC)
 * It is certainly possible to preserve the nipple while removing the underlying mammary tissue in Breast reduction surgery. It is done in treatment of Gynecomastia for instance. It might also be part of a female-to-male sex change operation, but stimulating the uterus should not be a consideration in that case. It can be done so the nipple retains all the sensation it had before surgery. Edison 23:04, 23 June 2007 (UTC)
 * Perhaps I misunderstand but from what I can tell breast reduction surgery only reduces the amount of mammary tissue not removes it completely which is what the original question asker was asking (At least from the way it was phrased). Obviously if one simply reduces the mammary tissue this is unlikely to impair anything completely just reduce it at most. A complete removal of the mammary tissue while preserving the nipples could very well have a different effect. Nil Einne 06:42, 24 June 2007 (UTC)
 * Actually I was wrong, it is possible, according to mastectomy, a subcutaneous mastectomy is what's being referred to here. Nil Einne 06:49, 24 June 2007 (UTC)
 * Although looking further, it appears perhaps I was right all along. According to some mammary tissue is preserved. Nil Einne 06:54, 24 June 2007 (UTC)
 * Please read Gynecomastia, cited above, a male condition, in whicvh the breast tissue is completely removed, while preservine the nipple. A comparable operation could be done for female to male sex change. Edison 12:46, 24 June 2007 (UTC)