Wikipedia:Reference desk/Archives/Science/2011 May 1

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May 1[edit]

Are good looks natural or a product of good care?[edit]

Assuming one is symetrical and has no deformities, are human good looks natural or a product of good grooming? Clear skin, attractive hair, a healthy body etc are all attainable through effort, but does that really make a difference? How much effect does one's skeletal structure have upon attractiveness?-- (talk) 00:16, 1 May 2011 (UTC)

Very much, I guess. Think of a male and a female, both with the same clear skin, hair etc. They will differ heavily in skeletal structure. Depending on feeling yourself as a male or female, you will feel very much attracted to one but not the other; not in the same way. Aside of the skeletal structure, there are other characteristics influencing that. (talk) 00:32, 1 May 2011 (UTC)
As with many questions of this type, the answer is that there is a complex melange of factors which make someone "beautiful" and we can't pick one of those factors out and random and say "Aha! It's all bone structure" or "Aha! It's all grooming" or "Aha! It's purely a cultural constuct" or "Aha! Its purely genetics." The truth is more likely that there's a combination of factors, cultural, biological, and otherwise, that determine when one person finds another person attractive, and while there certainly have been studies which attempt to parse out some general trends, see Physical attractiveness, there's no universal formula for describing a singularly beautiful person with universal acceptance. --Jayron32 01:40, 1 May 2011 (UTC)
As they say, "beauty is in the eye of the beholder". What one person finds beautiful isn't necessarily what other people will. In particular, people's feelings about a person can affect whether they think they are beautiful. --Tango (talk) 16:53, 1 May 2011 (UTC)
I saw a great doco a while ago which showed the courting rituals of some African tribe where the males on a particular day of the year doll them selves up with feathers and make up and do these crazy dances (based on eagle behavior, from memory) in order to vie for the affections of the limited number of eligible females. While watching it, it was hard to suppress how ridiculous it seems, but fascinating realizing that the females of the tribe must actually find this display attractive; They'd be looking at this display thinking some of those guys are hot stuff! Vespine (talk) 02:03, 2 May 2011 (UTC)
I'll just add of course OUR cultural courting practices must seem equally as ridiculous to them. The relevant article being Cultural relativism. Vespine (talk) 03:47, 2 May 2011 (UTC)

The stroke of midnight[edit]

What time (UTC) is "the stroke of midnight"? If you have a reference, that would be greatly appreciated. Bielle (talk) 02:16, 1 May 2011 (UTC)

00:00:00 (or 24:00:00). Stroke of midnight = midnight. Clarityfiend (talk) 02:32, 1 May 2011 (UTC)

On a related thing, when Big Ben chimes midnight, is the first of the twelve strokes midnight, or the last of the twelve strokes? I've always wondered. --TammyMoet (talk) 08:03, 1 May 2011 (UTC)

It's the first of the twelve for Big Ben, but for the Greenwich time signal, it's the leading edge of the sixth (last) "pip". Transmissions are significantly delayed if you hear either signal via a satellite receiver, so a terrestrial transmission is needed if you are concerned about split-second accuracy. Dbfirs 08:13, 1 May 2011 (UTC)
Does not the length of the final pip vary, implying that the final end of the last pip would be the exact time? (talk) 11:53, 1 May 2011 (UTC)
Our article – which Dbfirs linked – describes the lengths of the pips in some detail, and notes that the start of each pip corresponds to the start of each second. TenOfAllTrades(talk) 14:17, 1 May 2011 (UTC)

Thank you all, especially for the references. I am surprised that 24:00 and 00:00 denote the same time. Any "24" notation would seem to belong to the day ending, while "00" seems to belong to the day beginning. The newest member of our family was born, according to the recording clerk, at 00:00 on Saturday April 23. If the time had been written as 24:00, I might have assumed the relevant date to have been Friday, April 22. Bielle (talk) 16:07, 1 May 2011 (UTC)

Yes, 24:00 on the 22nd is the same time as 00:00 on the 23rd. I would guess that your latest family member could have either date chosen to celebrate as their birthday, but once it's down on a birth certificate it becomes fixed. (Most people regard the new day as starting the instant that the clock clicks over from 23:59:59 to 00:00:00, but different cultures have different conventions, and sometimes the day starts with dawn.) Dbfirs 16:30, 1 May 2011 (UTC)
I wonder what an astrologer will make of it. Bielle (talk) 16:41, 1 May 2011 (UTC)
I expect they'll make something up! Richard Avery (talk) 17:53, 1 May 2011 (UTC)
"00:00" means "00:00:00 to 00:00:59", so means they were born after midnight. Midnight itself is an instantaneous moment, so the probability of anything happening at exactly that moment is zero. It's only that moment itself that is ambiguous about what day it is part of. --Tango (talk) 17:03, 1 May 2011 (UTC)
Gosh. We don't appear to have a category yet for People born on the stroke of midnight. That's a bit of an oversight huh!--Aspro (talk) 18:07, 1 May 2011 (UTC)
There's a complication here. Many births take more than a minute. Which part of the process defines the "moment" of birth? Dbfirs 20:50, 1 May 2011 (UTC)
The moment of birth is when the nurse looks at her watch and enters the time to the certificate the hospital issues. – b_jonas 08:35, 2 May 2011 (UTC)
That's assuming that the birth takes place in a hospital. I see that only 6 in every 1000 Americans are born at home, and only half this rate in Australia, but it's more common elsewhere. There's also the issue of the nurse's watch. When was it last synchronised with local standard time, and does it have an analogue or a digital display? I wonder how many people celebrate their birthday on (technically) the wrong day. World-wide, of course, there are many people who don't know the actual date of their birth. Dbfirs 08:43, 5 May 2011 (UTC)

Differential pitch perception?[edit]

Yesterday, I was riding my bicycle toward a railroad crossing as a train was approaching. The crossings here in Southern California have a loud clanging bell that starts sounding at about twice a second just before the crossing arms come down, and continue while the arms are down.

Being a more-or-less railfan, I rode my bike right up to the crossing arm to get a better look as the train passed. A few seconds passed before I realized that the clanging bell was VERY loud, almost painful. The bell was located about 6 feet (2 meters) off to my right, so I scooted back about 10 feet. It was still painfully loud, so I put my hands to my ears (I still wanted to be close to the train), and that muffled the sound. I then decided that, since the sound was coming from my right, maybe all I had to do was cover my right ear. For some reason, I dropped both hands, then just covered my right ear.

Immediately, I noticed that the pitch of the bell seemed to drop about (just guessing here) half a semi-tone. It startled me enough that I repeated that procedure many times, and each time I covered my right ear, the perceived pitch seemed to drop around the same amount.

Even though it was a slow freight (goods) train, it wasn't until after it passed that I even thought of turning around and testing the other ear, and I wasn't going to wait around for another train to test it.

Not living too near a railroad crossing, it'll be several days before I can conduct further original research, but the question is; is this a recogni(s/z)ed phenomenon?

Before anyone screams "medical question", I'm not asking if anything is wrong, only if others have noticed this.

Thanks for any help y'all can give me. Bunthorne (talk) 03:23, 1 May 2011 (UTC)

Well, there's no plausible way you could be shifting the frequency by doing that. I can think of two possibilities: (1) The clang is a mixture of frequencies, and you are differentially filtering out higher ones. (2) The sound is so loud that the response of the cochlea to it is distorted. Looie496 (talk) 03:45, 1 May 2011 (UTC)
I'm not sure what the bell sounds like, but in general, a high pitched sound should move more directly than a low pitched sound due to diffraction. Wnt (talk) 04:18, 1 May 2011 (UTC)
I agree with Looie496's suggestion (1). The hand will preferentially suppress higher frequencies, resulting in the tone appearing lower although the pitch of the base tone doesn't actually change. The article spectral glide should be relevant. Note that you can modulate the tone by bulging your hand outwards. This indicates that the space between the hand and the ear acts as a resonator with varying frequency characteristics. I remember doing that during a concert by My Bloody Valentine in the 90's, while they were doing their 15-minute pure noise workout in the middle of You Made Me Realise, thus putting some welcome structure into that noise. --Wrongfilter (talk) 06:47, 1 May 2011 (UTC)

Name this garden flower?[edit]

Seen in a garden in southern england, growing in clumps in a rockery setting. I thought it could be oilseed rape, but the leaves are different. The flowers have four petals. Must be part of the cabbage/mustard family. It has wilted and is flattened under the scanner lid. Thanks. (talk) 12:01, 1 May 2011 (UTC)

Could be Yellow Rocket Barbarea vulgaris. DuncanHill (talk) 12:21, 1 May 2011 (UTC)
Might be Charlock, (Sinapsis arvensis). Richard Avery (talk) 17:50, 1 May 2011 (UTC)

Equations of motion for position/momentum uncertainty[edit]

(This is a homework question; I'm not asking to be given the answer, just for a nudge or two in the right direction). The problem says that, given a particle moving freely in the x direction (no other information is given), I should calculate the permitted values at t=0 for <p2> and <x2>, and find equations of motion for the uncertainty of both position and momentum. I'm not entirely sure how to go about this.

If the particle is "free", that means there is no force acting on it anywhere, right? So V(r) is 0 everywhere? So by Ehrenfest's theorem, momentum is constant, and position varies linearly with time, I think. Using the same principles, I calculated that (d/dt)<x2> is <xp + px>/m, which is the same as <2xp - i*hbar>/m, and a slightly more convoluted form for <p2>.

I'm not sure where to go from here, however, in figuring out the values of <x2> and <p2> that are "initially allowed", or how to find Δx or Δp as a function of time. (Obviously, the initial value of Δx will be sqrt(<x2>) as <x> is initially 0, and likewise for Δp.) -- (talk) 14:05, 1 May 2011 (UTC)

Ehrenfest's theorem gives you the time derivatives of the average of the position and momentum. What you want here are the time derivatives of the square of the momentum and the square of the position. You can derive this easily from:
d/dt<A> = 1/(i hbar) <[A,H]>
if A does not explicitely depend on time. Count Iblis (talk) 15:35, 1 May 2011 (UTC)
And since the particle is moving freely, the Hamiltonian is simply the kinetic energy . Dauto (talk) 15:55, 1 May 2011 (UTC)
So then p2 would commute with H, indicating that the time derivative of p2 is 0. Is that right? Then I have a differential equation that gives <p2> a constant value, but I don't see anything that indicates a set of "permitted" values. -- (talk) 20:27, 1 May 2011 (UTC) (original asker)

Experiments related to the digestive system[edit]

A relative of mine is doing a school project about the digestive system. The previous groups have used various demonstrations for their school projects: putting one hand in a bowl of hot water and the other hand in a bowl of cold water and then putting both hands in a bowl of room temperature water + eye adjustment to darkness (etc) for the sensory system; the patellar reflex for the nervous system; measuring blood sugar and blood pressure for the circulatory system, etc. However, she has no idea what she could do. The teacher apparently takes the projects quite seriously. Any ideas? Surtsicna (talk) 14:39, 1 May 2011 (UTC)

Without getting too scatological, lots of digestive experiments are excluded. But people forget that digestion begins in the mouth -- with both masticatory mechanical digestion and alivary enzyme chemical digestion. Perhaps he/she can construct an experiment comparing stimulated vs. unstimulated salivary secretion. It's a documented and readily available subject, and although it might get a bit icky...hey, that's science. DRosenbach (Talk | Contribs) 15:11, 1 May 2011 (UTC)
An old one I did at school was to get 5 different substances and put them on different parts of the tongue to see how they taste - does something taste different when you taste it with the tip of the tongue, or at the back of the tongue? Whether we can taste something and then like the taste is an important part of digesting something: as DRosenbach has already said, saliva contains digestive enzymes and if we don't salivate as much when eating, digestion is at a disadvantage. --TammyMoet (talk) 15:58, 1 May 2011 (UTC)
One we did when I was in school was to masticate a soda cracker until it began to taste sweet (as a result of the enzymes in saliva breaking down the carbohydrates into sugars, as I recall). Deor (talk) 16:26, 1 May 2011 (UTC)

Perhaps a demostration of palatability difference between cooked and uncooked (potato?). (talk) 16:34, 1 May 2011 (UTC)

Some of your ideas sound great! Unfortunatly, experiments related to tasting food have already been done by the team whose project was the sensory system. Surtsicna (talk) 17:14, 1 May 2011 (UTC)

If your relative is old enough (I was doing this sort of thing when I was 12-13 in the UK, don't know how restrictive health and safety would be nowadays, not that it's really dangerous if supervised), and has access to basic chemistry lab chemicals and equipment she could show that saliva contains α-amylase by applying it to starch (corn or potato starch in suspension in water), leaving for say 10-20 minutes (ideally at body temperature 37°C, but that's not essential), filtering and showing the presence of reducing sugar with benedict's reagent. If she wanted to be really clever she could show that what's doing the digestion is an enzyme by showing that it is inactivated by heating (e.g. dip a test tube of saliva in boiling water for maybe 5 minutes before the experiment). If she wanted to be really really clever she could show that the enzyme only works in the mouth since it is inactivated by stomach acid (spike some dilute hydrochloric acid into the saliva before running the experiment). She could even show that the enzyme needs metal ions to work if the biology lab has any EDTA around. This is cool because it gives you clues as to the type of enzyme it is (a metalloenzyme), and its a test that real biochemists often do. Remember that you would need good controls for these experiments. As a negative control you could do the experiment without adding starch (to check that the test isn't positive because the saliva donor has been eating sweets just before and there is sugar in their sample), and a a positive control you could do the test with something with glucose in it (like gatorade) to show the benedict's reagent is working properly. IMO this would be a cool demonstration because it is relevant to the process of digestion - showing the first chemical step of the process and it's an actual experiment with controls and an hypothesis (e.g. that Deor's soda cracker starts to taste sweet because an enzyme is digesting the starch to sugar) and therefore proper science. Of course she'd need to play around with times and concentrations beforehand, but after all three quarters of the work of science is optimising your experiments! Equisetum (talk | email | contributions) 18:00, 1 May 2011 (UTC)

I'm afraid those experiments are a bit too complex given the time limit (45 minutes, during which she and her team have to do a presentation of the entire human digestive system and illnesses related to it) and lack of chemicals. Surtsicna (talk) 21:13, 1 May 2011 (UTC)
Sorry, probably got a bit carried away and didn't realise the time constraints. Hmm, digestive system is quite a tricky one then. I like the idea of the plants though, although if the talk is on the human digestive system may not be quite what you're looking for. Anything I can think of takes too long (digestion is quite a slow process). The best one so far would be Deor's soda cracker (which could be handed round for everyone to try). I know there are people doing demonstrations on tasting food, but that one really isn't about taste. It's not about it tasting sweeter it's about it being sweeter - the carbohydrates are being digested to sugar. Taste is just the way we happen to detect the sugar. Equisetum (talk | email | contributions) 00:41, 2 May 2011 (UTC)
I agree with you, Deor's suggestion is very good. She could explain why something salty suddenly becomes sweet very simply. I've also searched for more info about DRosenbach's suggestion and found this link, which suggests chewing on a piece of paraffin to stimulate salivation; can anything else be used instead of paraffin? Surtsicna (talk) 08:33, 2 May 2011 (UTC)
I would have though chewing gum would work - it just has to be something you can chew. I guess the reason paraffin was used is so that the effects of flavour don't influence the measurement (which studies the effect of chewing). Actually, as this is a reference desk here is a reference which concludes that the effects of chewing gum and paraffin are similar. Chewing gum is also recommended for xerostomia (dry mouth), so it meshes nicely with the section on digestive illnesses. Equisetum (talk | email | contributions) 15:49, 2 May 2011 (UTC)

Based on the other experiments mentioned, that sounds a little advanced for the age group. Does it have to be an actual experiment or can it be maybe something like a short sissertation aided by diagrams to show the difference between ruminent digestive systems and others (carnivore, omnivore etc). this would demonstrate an ability to research associated variations. sounds like she was given a acomparatively tough assignment. (talk) 19:37, 1 May 2011 (UTC)

She plans to do things like that but something experimental is needed as well. By "experimental", I don't mean neccessarily things involving tubes and chemicals - simple things can do (eg. the students whose assignment was the circulatory system measured blood sugar and blood pressure). Surtsicna (talk) 21:13, 1 May 2011 (UTC)

Another thought! Pitcher plants. honeydew plants, venus fly trap. All variations on digestion. (talk) 20:49, 1 May 2011 (UTC)

Something along these [1] lines? Aaadddaaammm (talk) 13:40, 4 May 2011 (UTC)

Electricity generation and conservation of energy[edit]

If I use something similar to a wet/dry fishtank filter to create a waterfall and use the potential energy of the raised water to run a watermill generator, is it possible to compensate for all of the energy expended in running the filter? If that's not too difficult, couldn't we run the contrived waterfall through 2 watermills to create a net gain of energy? Theretically, then, could we lift the water high enough to generate enough electricity with n number of watermills to make this entire thing worthwhile? If so, we wouldn't realy need a river -- we could do this in an artificial lake? And this idea can be stretched to work with steam turbines, etc. -- but as far as I know, this is not done, so where's the catch that makes this idea not cost-effective (cost referring here to both time, money and energy)? DRosenbach (Talk | Contribs) 15:07, 1 May 2011 (UTC)

Great idea, except that it doesn't work. Dauto (talk) 15:42, 1 May 2011 (UTC)
In particular, a certain amount of water falling a certain distance gives you less energy than it would take to reraise that same amount to that same height. DMacks (talk) 16:36, 1 May 2011 (UTC)
Put another way: the reason large hydroelectricity dams work is because the Sun does most of the work for us. The water cycle represents a huge investment of solar energy to heat and evaporate water, and a huge return of that energy in the form of rain. The rain has gravitational potential energy (and thermal energy, and other forms of stored energy); all that energy was originally supplied by the Sun via solar heating. When humans build a dam and hydroelectric turbine, they're just tapping off a small percentage of that solar energy. If you wanted to pump water to higher altitude to fill a reservoir, you would have to supply the energy. Nimur (talk) 16:45, 1 May 2011 (UTC)
How huge is "huge"? Weather operates on incredibly huge scales compared to normal human thermodynamic processes. A few winters back, I got stranded in a small snowstorm for a few days, and consequently had a lot of time and very little electricity. I ran some back-of-the-envelope calculations on paper, and determined that the snowfall from this blizzard released more energy than the entire human fossil-fuel energy-production in a year, in gravitational energy alone, not even to count the thermal effect of snow-melt and latent heat of fusion, and so on. For comparison, the scant rainfall in the Colorado Desert basin provides enough energy to power most of the Los Angeles metro area, Las Vegas, and most of the rest of Southern California. Nimur (talk) 16:50, 1 May 2011 (UTC)
Can you please elaborate on this? I know the energy is unrecoverable in the OP's scheme. However, your statement makes it seem as though gravitational potential energy displays some kind of hysteresis, which seems bizarre to me. Does not the energy required to lift a 1 Kg mass 1 M equal the gain in potential energy of that mass? SemanticMantis (talk) 17:16, 1 May 2011 (UTC)
Yes, it does. The energy lost is due to imperfect engineering solutions. Dauto (talk) 17:18, 1 May 2011 (UTC)
...imperfect engineering solutions and fundamental behaviors of the universe, as described by the laws of thermodynamics. For example, the water cycle loses energy to turbulence in atmospheric-scale flows; rivers also exhibit turbulence; energy is also lost to surface-tension as rainwater flows along the ground toward a river channel. All of these are examples of entropy-increasing processes that represent unrecoverable heat. Add on top of this any engineering imperfections, like friction in the turbines, and so on.
To elaborate further: solar energy comes in; it has a cyclic process here on earth; and some of the energy is irreversibly lost because of inefficiency in the process. Where does the unrecoverable heat go (from atmospheric turbulence, and so on)? There are exactly three options: the planet either warms (a process we call "global warming"); or the planet finds a way to convert this heat energy into some other energy-storage system (e.g., via an atmospheric chemical change); or the planet radiates this heat energy back into space. Nimur (talk) 17:50, 1 May 2011 (UTC)
Nimur, the 2nd law of thermodynamics allows for entropy preserving processes which are in practice impossible to implement because of imperfect engineering solutions as I stated. Dauto (talk) 18:00, 1 May 2011 (UTC)

(undent)@SemanticMantis: Think about it this way: A 1 kg mass hanging 1 meter in the air has a certain, calculatable, potential energy relative to its position relative to the earth. However, this is not to say that the act of lifting that mass to that position expends exactly that amount of potential energy. Any real act of lifting the mass will involve losses which mean that the energy expended in lifting will always be greater than the potential energy gained by the lifting itself. Consider even the simple problem of friction with the air as the mass moves upwards; that friction heats the air and the mass, and that heating causes the lifter to expend an equivalent amount of extra energy which is nto translated into potential energy. That's just one example, there are plenty of other places where losses will occur. --Jayron32 18:09, 1 May 2011 (UTC)

Thanks all, I get it now. I was confused because I was trying to apply DMacks' comment to simple frictionless Newtonion systems. SemanticMantis (talk) 18:58, 1 May 2011 (UTC)

It is possible to use the energy of a waterfall to pump water to a reservoir higher than the waterfall. Cuddlyable3 (talk) 20:17, 1 May 2011 (UTC)

Sure, but you can't pump as much water to the higher reservoir. Energy still needs to be conserved. On a related note, Heron's fountain makes for an interesting demonstration. TenOfAllTrades(talk) 22:54, 1 May 2011 (UTC)
Hydraulic ram is another (perhaps more practical) application of "some water falling raises other water up higher". DMacks (talk) 06:28, 2 May 2011 (UTC)

You may be interested in reading about Pumped-storage hydroelectricity schemes, such as Dinorwig power station. These are methods of storage, though, not perpetual motion machines. Tonywalton Talk 23:30, 1 May 2011 (UTC)

Measuring noise levels[edit]

Does anybody have experience with measuring noise levels, preferably with regards to environmental noise pollution? I was just looking to buy a [|decibel meter] to do a study for my access course (a UK course for people who are going back to uni) and discovered that there's not a simple method of measuring noise pollution. What sort of equipment do councils, HSE, etc use to measure decibels? Any links regarding how measurements are taken and converted into perceived noise would be much appreciated. I'm guessing that a basic decibel meter for about £20 would be reasonably accurate as well? Tx, Mike --19:38, 1 May 2011 (UTC) (talk)

When I was at university in the dim, distant past, the local council came along with an orange ball which they put on top of the amplifier stack during one of our discos. If the noise level went above a certain stage the amps cut out. The ball disappeared, needless to say. Back to the present: you may find thisof use. --TammyMoet (talk) 08:40, 2 May 2011 (UTC)
See sound level meter for a start. Try eBay. Make sure to buy one that has been calibrated recently. (talk) 12:34, 2 May 2011 (UTC)

(OP here) Thanks for the links but I was ideally looking for something a bit more in-depth. I've had a look at buying something similar to Tammy's link (is something that cheap considered accurate?) and my own link to 'decibel meter' is actually the same page that 92 suggested but I appreciate all replies, thank you. I'm not sure how decibel ratings are taken with regards to noise pollution. It seems fairly straightforward when you're measuring specific noise levels and direct damage to hearing but with noise pollution I assume that a low level noise 24 hours a day would be considered as annoying as a loud noise 1 hour per day. Is there some common way that this is quantified? This is mainly for the psychology part of my course and partly for human physiology if that helps! Tx again, Mike -- (talk) 16:11, 2 May 2011 (UTC)

root root[edit]

okay, two questions i feel i should know, but don't;

  1. do plant cells respire directly? more specifically, do cells in the roots of plants get their oxygen from whatever air is around them, underground? given that their circulatory system is pretty different from blood... that might explain why most plants don't do well in continually soggy ground, aside from the problems of infection.
  2. is a specific piece of root of a tree coupled to a specific branch? If that section of root is cut, will that result in the death of a specific branch of the tree, or does the contribution of the roots get pooled among all the above ground structures?

Gzuckier (talk) 21:28, 1 May 2011 (UTC)

Although oxidation plays a part in plant growth they don't use oxygen as animals do. They use CO2 and expel oxygen as a by product of photosynthesis, And their (circulatory system) is more like a one way system. See plant respiration.Phalcor (talk) 22:14, 1 May 2011 (UTC)

If a plant's circulation is a one way system, how do you figure the roots get access to sugars produced at the leaves? Dauto (talk) 16:54, 2 May 2011 (UTC)
Plants respire just like animals. That link redirects to a general article on cellular respiration. Photosynthesis creates sugar which is then used in respiration to get energy. --Tango (talk) 22:45, 1 May 2011 (UTC)
To clarify, during the day, plants photosynthesise more than they respire. During the night, they respire more than they photosynthesise. On average, they photosynthesise more than they respire. CS Miller (talk) 23:43, 1 May 2011 (UTC)

For question 2: there is no strict direct link between individual roots and shoots, though there are trends in the flow of nutrients through the phloem. Consider a few examples: if we cut off one root segment, we cannot know which (if any) branches may die back. However, if you girdle half the vascular cambium of a tree, we would suspect that branches on the same side and immediately above the cut would be the first to die. SemanticMantis (talk) 14:11, 2 May 2011 (UTC)

Atmospheric pressure vs sea level[edit]

I can't help asking this stupid question. If all the polar ice melted and the sea level became 400 ft. higher or whatever the experts calculated it would be, then would the atmospheric pressure at the new sea level be the same as it is now because the air has the same volume, or would the pressure change to the new altitude level because the ice at the poles is no longer displacing the air. It's understood that any difference would be minute. but it's a real scientific question and I can't sleep at night for worrying about it.Phalcor (talk) 23:17, 1 May 2011 (UTC)

Polar ice melting won't increase sea levels at all. Polar ice is floating, so it is already displacing an amount of water equal to the amount of water it will add when it melts, so there is no change. It's ice on land (such as glaciers) melting that will increase sea levels. That shouldn't have any effect on air pressure, though, since the amount of atmosphere will be the same, it's just moved about a bit. --Tango (talk) 23:27, 1 May 2011 (UTC)
You might want to reconsider that statement. If you are picky, you might be able to squeeze out Greenland as "non-polar". But there is plenty of non-floating ice over at the south pole. --Stephan Schulz (talk) 23:34, 1 May 2011 (UTC)
@Tango: Polar ice at the North Pole is floating on the Arctic Ocean, but polar ice at the South Pole sits on top of the continent of Antarctica. See Antarctic ice sheet. Dolphin (t) 00:21, 2 May 2011 (UTC)
There is quite a large quantity of land-based ice on Greenland.file:Greenland_ice_sheet_AMSL_thickness_map-en.pngGzuckier (talk) 02:36, 2 May 2011 (UTC)
And some more in northern Canada and various islands in the Arctic Sea.thx1138 (talk) 18:24, 4 May 2011 (UTC)
It is a sound question. I can't be assertive with my answer, but this is how I think about it. Consider a column of air above the sea. The cross sectional area of this column is one square metre, and the pressure at sea level is standard pressure - 101325 Pa, so the weight of this column is 101325 Newtons and its mass is approximately 10300 kg because the effective acceleration due to gravity is about 9.8 m.s-2. After the polar ice cap has melted the mass of air in the column is unchanged. However, the center of mass of the column has risen by 400 feet and as a result the effective acceleration due to gravity is a little less than it was before. Seeing the effective acceleration due to gravity is slightly less, the weight of air in the column is slightly less and the standard pressure at sea level is slightly less than the current value. Dolphin (t) 23:32, 1 May 2011 (UTC)
You think the atmosphere will get shifted upwards? Won't it just get shifted sideways to wherever the ice used to be? There may be a slight difference due to the differing densities of ice and liquid water, but that's it. --Tango (talk) 23:45, 1 May 2011 (UTC)
Your comment would be relevant if the question was If the polar ice cap melted, would the atmosphere rise or be shifted to lower latitudes? However, that is not the question posed by Phalcor. His question is If sea level became 400 feet higher ... Dolphin (t) 23:56, 1 May 2011 (UTC)

I read that much of the ice on the southern continent fills the vallies between the mountains to altitudes of three kilometres or more which means that the air mass displacement at higher altitudes would be less than at lower levels. (talk) 00:14, 2 May 2011 (UTC)

The total volume of the solid plus liquid earth will remain almost* unchanged - some of what was solid will have changed to liquid. Therefore the displacement of the atmosphere from what used to be the first 400 feet above sea level will be just about balanced by the volume formerly occupied by ice that is now air. *However (there's always a "however") given that the process necessarily requires a rise in global average temperature one can safely assume that the total water content of the atmosphere would increase. How that shift in the equilibrium would affect the answer to the OP's question is somewhat above my pay grade... Roger (talk) 09:00, 2 May 2011 (UTC)
I think Roger is saying the space several thousand feet above sea level that used to be occupied by ice will be occupied by air, and the space at sea level that used to be occupied by air wil be occupied by water. The volume of one will be almost identical to the volume of the other, but that doesn't have a bearing on the question. The question postulates that the sea level rises by 400 feet, and asks what affect that will have on air pressure at sea level. Dolphin (t) 11:57, 2 May 2011 (UTC)
I think Roger is agreeing with me and if we're both right then air pressure at the new sea level would be equal to the current air pressure at 400ft above sea level. --Tango (talk) 20:13, 2 May 2011 (UTC)
@Tango: As a first approximation, I agree with your assessment. However, some of the ice assumed to melt in the future is presently at a significant altitude where the air density is less than sea level density. (Much of the ice on the Antarctic continent is presently between 5,000 and 15,000 feet.) When this ice melts it will be replaced by lower-density air, but as the melt water flows down to sea level it will replace higher-density air. The first ice to melt will be close to sea level rather than at altitude, so this will be a second-order effect but it will cause the centre of mass of any column of air to rise slightly, rather than remaining at a fixed geopotential height. Dolphin (t) 23:59, 2 May 2011 (UTC)

Many thanks guys. especially Dolphin. Your assessment seems to fit comfortably with my mental vision.Phalcor (talk) 15:20, 2 May 2011 (UTC)

Physics of the force carriers[edit]

How do we know w and z bosons and photons carry the forces and how do they do it for example how can photon coming from a positive particle also attract a negative particle and repel a positive one Also how can photons transfer energy if e equals mc squared as they are massless they are mass 0 so how can they have energy if they are massless —Preceding unsigned comment added by (talk) 23:50, 1 May 2011 (UTC)

Surprisingly, the article Force carrier is quite accessible, given that many physics articles are written in a language only advanced physcists understand. I suggest reading that article, as it answers a lot of your questions regarding how such particles work. If you want more details, with all the gory math, Static forces and virtual-particle exchange covers it at that level as well. --Jayron32 00:39, 2 May 2011 (UTC)
I'll address your second question. Photons have an energy given by E=hν, where ν is the photon's frequency, and h is Planck's constant. Although photons don't have a rest mass, they do have a nonzero relativistic mass equal to hν/c2 (found by combining E=hν with E=mc2). Red Act (talk) 03:13, 2 May 2011 (UTC)
To clarify a potentially confusing part of my last post, the ν in E=hν is the Greek letter nu, which is different from v (vee) in the Latin alphabet, although the two letters can look very similar, especially in sans-serif fonts. Nu represents frequency, not velocity. Red Act (talk) 03:33, 2 May 2011 (UTC)
That's why I like the math font: , , and . Dauto (talk) 14:03, 2 May 2011 (UTC)
Meh. Nu always looks like a vee in every font. It's not a problem that is going to be solved by fonts; it's just that it looks like vee. Likewise, mu looks a lot like a "u" with a leading tail, and gamma looks like a wye. People who are wholly unfamiliar with the greek alphabet will invariably think these are v, u, and y and not ν, μ, and γ (n, m, and g). No font is really going to help that. --Jayron32 05:18, 3 May 2011 (UTC)
Compare with . They look completely different. Now compare uvy with μνγ They look almost the same. The font makes the difference.Dauto (talk) 13:52, 3 May 2011 (UTC)
If you lie them next to one another, you can tell the difference. For a person who sees and has no knowledge of the greek alphabet, and also doesn't have a "v" nearby to compare it too, its quite likely they will call it "vee", and not think its a symbol for which there should be a different name. --Jayron32 19:16, 3 May 2011 (UTC)