Wikipedia:Reference desk/Archives/Science/2013 March 12

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March 12

Why light does not hurt us

Light travels at, well, the speed of light. If it were possible to accellerate a tennis ball to the speed of light and the tennis ball were to hit me in the chest, the results would be disasterous. So, why do light particles not hurt us when then hit us? Even though they are very small, there is a lot of them . . . 99.250.103.117 (talk) 01:49, 12 March 2013 (UTC)

Their mass is very small. How small? Well, consider what a small fraction of the mass of a lump of plutonium can do to a city. E=mc² Wnt (talk) 02:05, 12 March 2013 (UTC)

Light at ultraviolet frequencies and above does hurt us. It's ionizing radiation. Photons at lower frequencies don't individually have enough energy to break chemical bonds, and the aggregate energy doesn't matter (see photoelectric effect) unless it's enough to heat body tissue to the point of damage (which is why you shouldn't put your cat in the microwave). -- BenRG (talk) 02:15, 12 March 2013 (UTC)

Light is made of photons and (barring semantic pedants), unlike tennis balls, photons are massless particles. They do not have mass, not even very small mass, they have momentum. Vespine (talk) 02:23, 12 March 2013 (UTC)

Yes, but photons have energy, which is the exact same thing as mass, at least for the point of this discussion. The fact that photons are in motion and have relativistic mass means that photons can, say, move electrons (that's the idea behind the Bohr model and all more modern models of how light interacts with electrons) and photons can even move objects, see Radiation pressure and solar sail. Light, all by itself, can move an object. Photons do not have rest mass, but they have energy, and there's no functional difference between mass and energy for the point of calculating the effect of a photon striking an object. One low energy photon is not going to move a massive object, but a whole shitload of high energy photons absolutely can move objects. --Jayron32 02:51, 12 March 2013 (UTC)

Most skin cancer is caused by (sun)light. HiLo48 (talk) 02:25, 12 March 2013 (UTC)

Nope, it's caused by UV rays, not visible light. 24.23.196.85 (talk) 04:29, 12 March 2013 (UTC)

I would argue that most of us mean "sunlight" to include all wavelengths bombarding our bodies that are present on a sunny day. Beach drifter (talk) 04:38, 12 March 2013 (UTC)

• Even infrared radiation can bake us to death, and radio waves can induce incredible, killing heat in certain metal conductors. FYI. μηδείς (talk) 02:55, 12 March 2013 (UTC)

A tennis ball, moving at literally the speed of light would have infinite mass and would rapidly become a black hole that would (at the very least) swallow the entire universe. Photons have a "rest mass" (if they were stationary) of zero. They only have mass at all because they are travelling at the speed of light. A photon with enough energy could be extremely damaging - but a low energy photon is utterly unnoticable. Stick your hand in front of a 200 watt visible light laser beam and it'll burn your skin off, stare at the sun for any significant amount of time and you'll blind yourself - so even visible light can most certainly hurt you if it has enough energy. SteveBaker (talk) 03:05, 12 March 2013 (UTC)

Thanks all. Clearly, I was referring to the kind of damage a tennis ball could effect at relativistic speeds (IF that were possible). I am well aware that light can harm us as outlined above (i.e. sunburns, etc.). 99.250.103.117 (talk) 03:48, 12 March 2013 (UTC)

The direct answer to your question is simple: An arbitrarily fast-moving tennis ball has arbitrarily high energy. Close to the speed of light, getting hit by it would be like getting hit by a nuclear detonation (if you are interested in precisely what would happen, the best source i can recommend is actually this xkcd). But a photon does not necessarily have such high energy. Since a photon has zero rest-mass, it does not have infinite energy at the speed of light. So that's the answer - it's not a matter of speed, it's a matter of energy. And a relativistic tennis ball has a hell of a lot more energy than a square meter of sunlight. Someguy1221 (talk) 03:59, 12 March 2013 (UTC)

Energy of a photon is Plancks constant times the frequency: E = ħ ⋅ f. Einstein won his Nobel prize in 1921 for physics related to the frequency and energy of photons in a 1905 paper related to the minimum frequency to emit an electron,see Photoelectric effect (like dunking a basketball, if you can't reach the rim, no amount of jumping will help). More precisely, accelerating a tennis ball to the speed of light would require all the mass of the universe. Using relativity, arbitrary speeds and the corresponding energy/mass can be calculated as mass and energy are equivalent. --DHeyward (talk) 08:15, 12 March 2013 (UTC)

Why is milk homogenised?

When I was young, milk came in bottles with a layer of cream on top, something most consumers seemed to enjoy, and others my age reminisce about. Then it started to arrive in cartons, and it had been homogenised. The cream was no longer floating on the top. I know there's many choices now, but the most common product is still sold that way, homegenised. Why? NOTE: I'm talking about Australia, and customs elsewhere will obviously vary, but the basic question is, why is most milk homogenised these days (when I'd prefer it wasn't)? HiLo48 (talk) 02:44, 12 March 2013 (UTC)

Why would you prefer it that way? If you want skim milk, you can buy it. If you want cream, you can buy it. Why buy something that makes unnecessary work for you regardless of what you do with it? Looie496 (talk) 02:53, 12 March 2013 (UTC)

Per Looie496, milk is homogenized because people didn't want to have to shake the heck out of the milk every time they wanted to drink it. I'm not sure why that is hard to understand; that you don't mind shaking it is irrelevent; others found it easier not to have to shake it every time, where it became economically more viable to homogenize milk before it was sold to the public than to make the public have to do it every time they wanted to drink it. --Jayron32 02:55, 12 March 2013 (UTC)

Looie - where is the evidence that "people didn't want to have to shake the heck out of the milk every time they wanted to drink it", and that this was really the reason for the change? I can remember lots of people asking, why the change, at the time. I can't recall anyone saying thank you. HiLo48 (talk) 03:06, 12 March 2013 (UTC)

Milk is also homogenized so that consumers can know what they are buying. Without homogenization, milk can vary greatly in fat content, taste and consistency. I, probably like you, don't have a problem with natural variation, but it's the way market forces and regulation have progressed. Vespine (talk) 03:01, 12 March 2013 (UTC)

Surely fat content, taste and consistency can be managed just as easily without homogenisation? HiLo48 (talk) 03:06, 12 March 2013 (UTC)

No because the cream at the top is most of the fat and taste. Rmhermen (talk) 03:35, 12 March 2013 (UTC)

In fact, I have seen one person who was so used to homogenized milk look at a cup of unhomogenized milk, and believe that it had gone bad. Someguy1221 (talk) 04:04, 12 March 2013 (UTC)

Is there a science question in here somewhere? This seems like a discussion of preferences and business practices. -- Scray (talk) 03:45, 12 March 2013 (UTC)

Well, "when I was a kid, the world was better. Damn innovation!" That isn't a science question? --Jayron32 04:11, 12 March 2013 (UTC)

• But, if we want to really answer with a reliable source, the basic question, the "purpose of homogenization is to make more permanent and uniform suspension of the fat by reducing the fat globules to a very small diameter" and "Homogenization is used for making milk more stable against creaming and for giving the product a richer mouthfeel due to a slight increase in viscosity" per the Ohio State University. I don't know that we need any better answer than that. That should satisfactorily answer the question "Why is milk homogenised?" As to the implied commentary which is couched as the subtext of the question, I'm not sure that needs any more response than has already been given. --Jayron32 04:15, 12 March 2013 (UTC)

Jayron - I take your point about this perhaps not being a Science Desk question. I did ponder it before I posted. Obviously the answer is going to be part scientific and part something else. (Marketing?) I'm happy to post it elsewhere (Miscellaneous?) as well, but expected that if I posted there first someone would be bound to tell me to take it to the Science Desk. And if I posted in two places.... Well, we all know what THAT leads to. HiLo48 (talk) 04:25, 12 March 2013 (UTC)

If you want the marketing answer, then Vespine gives the best answer above, which is "consistency" of product. Food marketing and distribution over the past umpteen years has been focused on providing a "consistent product". It is the ubiquitous mantra of the factory-food industry, the entire system is geared to producing a product which is universal for the customer. If I buy a hamburger at, say, a McDonalds anywhere in the world, McDonalds wants it to taste exactly the same. That's how mass produced food tries to work. Milk is certainly no different than that. Homogenization of milk means that every gallon of milk from the same dairy should be exactly the same taste every time. Homogenization helps do that. It is also undoubtedly more convenient, as I noted above, it's what the market seems to want: convenient and consistent. If you doubt that the food industry has those as its primary goals, you haven't been paying attention. That you don't value those things as an individual is irrelevent: the market does, and the evidence that the market does is that food companies aren't in the business of making less money. Milk companies wouldn't homogenize their milk if it meant they sold less milk doing so. I'm sorry if this is unsatisfactory, because you don't hold the same values that the preponderance of the milk buying public does. If the milk-buying public only bought milk that wasn't homogenized, that's what would be produced. That's the answer as to why some types of products are sold instead of others. The answer is that companies try to maximize profit, and will produce the products that do that, while at the same time also creating that market for the product in the first place. To some extent, dairy companies may have convinced people that not shaking their milk was something they wanted out of their milk product, and the milk-buying public agreed with them (or was convinced to agree with them). --Jayron32 04:39, 12 March 2013 (UTC)

Here in the UK, I bought non-homogenised milk for as long as it was available because I preferred the taste, and I knew that there was a difference in digestion of finer globules of fat. That option is no longer available, and milk is now also "standardised" (that's how they achieve "consistency", by mixing lots of batches to bring the total fat content to a standard level). I'm not sure whether I've gradually adapted, but modern homogenised milk doesn't seem to taste so much of "machinery" as it used to. Homogenisation and standardisation are different processes, but they seem to go together. I've never seen one without the other. Our article Homogenization (chemistry) is was in error. Dbfirs 09:39, 12 March 2013 (UTC)

My milkman still delivers pasteurised milk in bottles, which has a layer of cream floating on top. So it's still done. (UK) --TammyMoet (talk) 10:23, 12 March 2013 (UTC)

I can remember when my milkman delivered unpasteurised milk (green-top), but, sadly, we have no local milkman now. I'm glad to hear that "silver-top" (with cream) is still available. I expect I could still obtain it if I travelled to the appropriate farm or dairy. Can you still get "gold-top" (with extra cream from Jersey and Guernsey cows)? Dbfirs 16:58, 12 March 2013 (UTC)

Here in NZ, you don't need a local milkman for unhomoginised milk, it's available at many supermarkets and even some dairys. I believe it's the same in Australia where the OP is from although they use gold tops instead but I believe their milk fat concentration is closed to here in NZ (see also Milk bottle top). Of course the price is a lot higher but I don't believe this is the only reason why you'll find far fewer people purchasing these then the blue and light blue tops. Nil Einne (talk) 17:59, 12 March 2013 (UTC)

Thanks for the link. (I knew that top colours were different elsewhere, but hadn't realised there was such a variety.) What do they feed their cows in India to produce milk with 6% fat? (... or is it just that their cows produce a much lower volume?) Dbfirs 18:50, 12 March 2013 (UTC)

My question was just as much a historical one. Way back in the 1960s we got non-homogenised milk in bottles. Then, fairly suddenly, the ONLY product believable was homogenised milk in cartons. We didn't have the acres of choice on supermarket shelves. There had been no public clamour for homogenised milk in cartons. It just happened, without the public having seemed to have asked for it. I was pretty confident it was marketing to support cost cutting in handling. But I was interested to see if there was more to it. Nothing here has convinced me there was. HiLo48 (talk) 20:17, 12 March 2013 (UTC)

Also, never underestimate the ability of marketing to sell you something that helps the business as "for your convenience" (as in the frequently seen by me around here at least, "for your convenience we are now open 9-5 instead of 8-6"). In this case, imagine how much more convenient it must be for the dairies if big tanks of the stuff didn't have to be stirred up vigorously to keep it from separating before it was removed? Might be not too much of a problem or even might be fun to have to shake a quart before pouring out a glass, but 2,000 quarts, not so much. Gzuckier (talk) 17:42, 12 March 2013 (UTC)

Incidentally, if you prefer your milk non-homogenized, try freezing it, as this seems to cause separation of the cream. (Be sure to pour some out of the container first, to allow room for expansion when it freezes.) StuRat (talk) 19:01, 12 March 2013 (UTC)

• I think people are missing what HiLo48 is asking. I too noticed the difference when homogenised milk was introduced. It a had a different mouth-feel (as best as I can put it)( and NO homogenisation DOES NOT give a better mouth-feel -as Im sure that HiLo will testify, as anyone else that hast drunk un- homogenised milk). The explanation I was given (because I too, did lot welcome the new taste) was that was to gave the milk a longer shelf-life. Un- homogenised, the milk would not only separate into milk and cream but after a few days, but even in a fridge, a layer of cloudy water would form at the bottom. WP editors of a 'certain age' may also claim witness to this. So, to answer the OP's question: Homogenization is simply to prolong the self-life of milk. In the old days, milk was brought from farms by the overnight milk-train and then got drunk within a couple of days. Now, homogenised milk it can comes from all over Europe and get purchased in a supermarket a considerable time after it came out of the cows teats. Probable the same situation for Aussie.--Aspro (talk) 23:02, 12 March 2013 (UTC)

Re: Relativistic Baseball

Up above, the thread on tennis balls at light speed pointed me to this page. Particle accelerators accelerate particles until they're close to the speed of light; why don't we see this effect? Is it simply that the particles they accelerate are just as tiny as the air particles around them, and thus unable to pick up molecules in the air (or leave them behind) in the way that molecules in the 0.9c baseball would be doing? Nyttend (talk) 05:09, 12 March 2013 (UTC)

Particle accelerators are evacuated very close to vacuum before anything is accelerated in them. otherwise, yes, I suspect the accelerated particles would just be smacking into air molecules all the time. Vespine (talk) 05:24, 12 March 2013 (UTC)

But surely they are still hitting a fair amount of "air" molecules. Any data on that? Probably boring to the scientist at the facility but maybe interesting to us normal folks. Beach drifter (talk) 05:54, 12 March 2013 (UTC)

The entire purpose of a particle accelerator is to smack small particles into each other at relativistic speeds (or sometimes a small particle into a large, stationary object). To make the collisions occur in a controlled manner, the air is necessarily evacuated from the tunnel. But in essence, nearly all particle accelerator data is the result of particles colliding with at least one moving near the speed of light. Someguy1221 (talk) 07:16, 12 March 2013 (UTC)

See Anatoli Bugorski for a real-life example. Tevildo (talk) 20:09, 12 March 2013 (UTC)

From this article the proton beams inside the LHC travel through pipes in what CERN calls an "ultra-high vacuum." The reason for creating such a vacuum is to avoid introducing particles the protons could collide with before they reach the proper collision points. Even a single molecule of gas could cause an experiment to fail. Also our own Large Hadron Collider article outlines how several of the problems encountered during construction were related to the vacuum pipe. Also, cerns own page has some info on their specific vacuum configuration.Vespine (talk) 02:30, 13 March 2013 (UTC)

why do optics use IR and not UV?

why do optics (remote control, laser mice, whatever), Microsoft Kinect etc prefer to work with IR if you dont want to bother with distracting people with whatever dots or lines or whatever you're doing - and not with UltraViolet which wouldn't it have the same effect?

Also range sensors etc. Lots of stuff is Infrared. Why? 91.120.48.242 (talk) 09:42, 12 March 2013 (UTC)

Infrared LEDs are cheap and invisible to the human eye, and plastic covers that look black to us but are transparent to IR are easy to make.

Ultraviolet LEDs are expensive and the human eye can see a violet glow, and plastic covers that look black to us but are transparent to UV are hard to make. --Guy Macon (talk) 09:55, 12 March 2013 (UTC)

This just deepens the mystery. Are you saying there is no usage benefit, dangerousness benefit, to being outside visible range via IR but simply is a matter of price and appearance? This completely does not explain why I wouldn't see it in very expensive vision research setups. It's just not used. Would it work if you changed everything that was currently IR to UV instead or would it not work for some other theretical reason? (Other than appearance of the LED's).

Secondly, following up on just your statements: just why are these LED's or cameras more expensive? Is there something about UV that makes it harder? 91.120.48.242 (talk) 12:13, 12 March 2013 (UTC)

Also: why do UV lights cause a violet glow but IR lights don't cause a red glow? If the wavelength is outside human sight, shouldn't we see nothing in both cases? 91.120.48.242 (talk) 12:14, 12 March 2013 (UTC)

UV lights, if they only emit ultraviolet radiation, are not visible to the human eye. Any violet glow from UV-lights is the result of it emitting a range of wavelengths, including visible (violet) light. Infrared LEDs and lasers are not only cheaper, but also safer. Ultraviolet light is Ionizing radiation and can therefore be both destructive to equipment and harmful to people. One might of course argue that low-intensity UV-radiation has negligible effects, but even then the the mere possibility of harmful effects and any resulting negative publicity is enough to deter manufacturers from implementing UV optics, especially when IR works just as well and is cheaper. - Lindert (talk) 13:10, 12 March 2013 (UTC)

does everything glow precisely because uv is ionizing, so that it will always have a relatively wide-spectrum glow? (Meaning it is not a good source for a signal as random things will glow under uv light as well, causing reflection and noise)? I find the 'negative publicity' aspect of your argument very unconvincing as we use lasers that are literally dangerous in all kinds of CD readers. researchers use things that are literally non-safe. people solder with lead. so if it's (in actual reality) not dangerous, then researchers won't care and people such as myself don't know anything about this subject anyway. I have no idea what's in a tv remote. so, there must be some other reason it simply doesn't work for the same applications. — Preceding unsigned comment added by 91.120.48.242 (talk) 15:51, 12 March 2013 (UTC)

Actually, there really doesn't have to be some other reason. IR works. UV wouldn't intrinsically overcome IR's limitations nor would it be cheaper (the first LEDs were IR, and creating LEDs in new frequency ranges tends to be expensive). These drawbacks don't say that UV wouldn't work, but more relevant is that they also don't present any reason why the established technology should be replaced. See also reinventing the wheel. — Lomn 16:09, 12 March 2013 (UTC)

doesn't answer why things were as they were before the Current Era, but as far as I know, modern image sensors are naturally sensitive to IR as well as visible, so that IR filters need to be installed to make them perform like the eye/film, right? so that kind of constitutes a solid bias towards IR instead of UV. Gzuckier (talk) 17:46, 12 March 2013 (UTC)

Our Light-emitting diode article gives a lot of detail on what went into making the various colors work. This may come as a shock, but the ones made of sapphire and diamond cost more than the ones made out of dirt. --Guy Macon (talk) 20:01, 12 March 2013 (UTC)

Another disadvantage of UV is that it is absorbed in glass. Optics made from quartz or fluorite may let it through, but cost more. In addition to the sources of UV giving out visible light there are plenty of fluorescent things around, so you will get fluorescent glows coming from various illuminated things, not so good if you want to be undetected with your night vision equipment. Graeme Bartlett (talk) 20:48, 12 March 2013 (UTC)

Species Identifcation..

2 Butterflies for identifcation:

Species identification desired so these can be re-titled approrpriately prior to Commons transfer. Sfan00 IMG (talk) 12:06, 12 March 2013 (UTC)

Can't immediately contribute (being at work), but it might help if you narrowed down the location from "India" to a particular locality - the State, say (not trying to dig out your personal details!) {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 15:06, 12 March 2013 (UTC)

Bihar, india according to the uploaders detail on another upload. Sfan00 IMG (talk) 16:57, 12 March 2013 (UTC)

These both look like Nymphalidae to me by the shape of the wings (looking at the legs would be helpful...). Searching "nymphalid" and "india" on Google I turned up "common sailor butterfly" Neptis hylas varmona. Switching to Bing I quickly got "Gray Pansy" Junonia atlites though the search result had somewhat more strength to its eyespots. But definitely it looks like a Junonia. Now, at this point this is still very tentative - butterflies are notorious for mimicry (also hybridization of related species) that makes using the wing colors hazardous. I feel like I'd be afraid to illustrate an article on this basis without an actual identification on the ground, rather than by looking at a picture. (Though you can run down mimicry rings and host plants and geographic range if you feel like it to be more confident) Wnt (talk) 22:09, 12 March 2013 (UTC)

How much nucleus/nuclei are present in the eggs of birds such as hen? What is the structure of those big cells?

I have heard that the egg of ostrich is the biggest cell in the world.So, if eggs are so big, how much nuclei are present in it.I know nucleus is present as a white dot in the yolk of an egg. So how much nuclei are present in an egg(which is a cell)? I think the egg contains only one nucleus. But I want to make sure that. Also are there any kind of cell that contains two or more nuclei?(other than during the process of reproduction) I think it only happens during the process of reproduction in micro-organisms. If I am wrong, please correct me.

The other thing I want to know is that If a single cell can be that much big, what is the outer shell made of(i.e what is it really)? Is it the cell membrane or plasma membrane? What is the yolk made up of? Do the eggs contain cytoplasm,vacoules, lysosomes, etc.., which are found normally in an animal cell?

If the yolk is not made up of cells, what is it made up of? Ganesh Mohan T (talk) 14:39, 12 March 2013 (UTC)

A ostrich egg is not a single cell, though at some stage it might contain a single cell that then develops into an embryo, then an ostrich chick. You might like to read the article on Egg (biology), but it includes other senses of "egg" that are single cells. The article Ovum explains this sense. The article Egg (food) explains a little bit about the make-up of yolk, white and shell. Dbfirs 17:19, 12 March 2013 (UTC)

Maybe we should separate fertilized eggs from unfertilized eggs. An egg containing an embryo surely contains multiple cells. But what about an unfertilized egg? What other cell is present other than the ovum? SemanticMantis (talk) 19:14, 12 March 2013 (UTC)

The egg yolk and germinal disc make up a single cell (which like many eggs develops with the help of many little cells surrounding it - see [1]) The germinal disc reaches a size of 2-3 mm in the mature ovum. At the time of ovulation, the oocyte nucleus (germinal vesicle) is 0.5 mm in size [2]. I didn't run down this size difference as of yet. Wnt (talk) 21:22, 12 March 2013 (UTC)

... so it would be accurate to claim that the yolk of an unfertilised ostrich egg is the biggest cell currently known to exist? Dbfirs 21:33, 12 March 2013 (UTC)

I'm afraid to claim that. I keep running candidates through my mind - slime mold plasmodium (life cycle), fungal syncytium, blue whale skeletal muscle ... I haven't thought of anything I can say for sure beats it. But nature is more creative than I am. Wnt (talk) 21:57, 12 March 2013 (UTC)

I was just about to suggest the slime mould as a candidate, though it is multinucleate. I agree that it would be risky to make an absolute claim. Dbfirs 22:02, 12 March 2013 (UTC)

Thanks. So, is this correct? In a freshly laid chicken egg, the yolk and germinal disc compose one macroscopic cell. The albumen and shell are separate, and some helper cells are also at play. The culinary (unfertilized) "egg" then holds many cells, though eventually the helper cells die off as the embryo develops. SemanticMantis (talk) 02:11, 13 March 2013 (UTC)

Thanks, So many say that an ostrich egg is the largest cell on earth? So it maybe wrong, Isn't it? But in an unfertilized egg, Is the egg of ostrich a single cell?Ganesh Mohan T (talk) 08:26, 13 March 2013 (UTC)

The problem is, there are two terms clashing here. That's why the shell and its contents are usually called an "egg", while the biological "egg" is often call an "egg cell". Especially as the term "egg" was around a long, long time before anyone knew anything about egg cells. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:15, 13 March 2013 (UTC)

Use of laxatives in Anorexia/Bulimia

I am writing a paper on Anorexia/bulimia and I'm trying to understand use of laxatives. Wikipedia's articles say they are a method of getting rid of the food from the system just as vomiting is, but they don't go one farther and say how that works. My lack of understanding is that, whereas with vomiting, it is obvious that undigested food is purged, it is not obvious to me that the same would be true with laxative use. I would guess, actually, that the body would digest the food just the same as it normally would in the small intestine, but that the end product would be expelled more quickly from the large intestibe and in the form of (sorry) diarrhea, but it must have some actual effect on lack of digestion in the small intestine. So what actually happens with laxatives? Sorry if this question is a gross out. I want to get an A and want to describe the scientific matters in detail, with good sources.--108.27.62.131 (talk) 14:58, 12 March 2013 (UTC)

Some laxatives have the power to empty the entire digestive tract, not just the large intestine. Powerful laxatives used prior to surgery often do just that. So, yes, the partially digested food comes out as loose stool. Of course, the problem is that having an empty digestive system also makes you hungry, so this leads to binging on food, followed by more laxatives. This is all quite dangerous, of course, as the lack of nutrients, such as potassium, can cause health problems, such as heart failure. StuRat (talk) 19:05, 12 March 2013 (UTC)

While more powerful laxatives can purge the entire stomach and gut of everything, including undigested food, frequent use of ordinary over-the-counter laxatives simply keeps the weight down by a few ounces or a pound by keeping the bowels emptier. Diuretics that force elimination of some extra fluid from the kidneys do the same thing. The weight difference is small and not cumulative; but part of the illness of an eating disorder is often an obsession with small weight changes. Frequent use of either method can lead to potassium depletion and other problems. alteripse (talk) 09:57, 13 March 2013 (UTC)

Laxatives work to facilitate the movement of material through the intestines. Bulk-forming laxatives such as Metamucil use fibre to draw fluid into the intestines to soften the stools and increase their bulk. Osmotic laxatives are a different technique to draw fluid into the intestines. Whenever fluid is being artificially drawn into the intestines, there is a risk of dehydration and electrolyte imbalance, which can trigger various health problems including cardiac arrhythmias . Stimulant laxatives directly stimulate the colonic muscle to increase peristalsis, and also fluid into the intestine. This results in the same risk of electrolyte imbalance and dehydration as other laxatives, plus the added risk of bowel dependence. There are some other types of laxatives, but the common theme is that none of them are intended for long-term use. Ashleyleia (talk) 16:14, 14 March 2013 (UTC)

Scissors VS Paper

Why do sharp objects cut through things? 203.112.82.129 (talk) 14:59, 12 March 2013 (UTC)

In the instance of scissors, consider shear action. Sfan00 IMG (talk) 15:04, 12 March 2013 (UTC)

We have an article on cutting that covers the basics. The short answer is that cutting "occurs only when the total stress generated by the cutting implement exceeds the ultimate strength of the material of the object being cut". — Lomn 15:06, 12 March 2013 (UTC)

Yeah, and metallic bonding in the scissor blades trumps the hydrogen bonding in the paper. Plasmic Physics (talk) 23:27, 12 March 2013 (UTC)

Note that scissors are basically a lever, converting a large motion (exerting a small pressure) to a small motion (exerting a large pressure). Scissors and paper cutters cut one point on the paper at a time, so they can concentrate their force on that point, versus over the entire area to be cut at once. StuRat (talk) 04:28, 13 March 2013 (UTC)

The thickness of the paper also counts, try cutting card with a small pair of medical scissors. Plasmic Physics (talk) 07:47, 13 March 2013 (UTC)

I suggest you don't; you will ruin the scissors.--Shantavira|^{feed me} 10:36, 13 March 2013 (UTC)

Also the sharpness of the scissors reduces the surface area on which the force is being exerted, and so increasing stress on the object being cut. Alansplodge (talk) 20:41, 13 March 2013 (UTC)

Energy

Hi Noosphere, If the universe is infinite, the quantity of available energy is thus infinite? Thank --YanikB (talk) 15:12, 12 March 2013 (UTC)

Of course. --PlanetEditor (talk) 15:17, 12 March 2013 (UTC)

Hmm. Depends on what you mean by "available". I would say that only energy within the observable universe can be considered "available", even in the most theoretical sense. And the observable universe is always finite. Gandalf61 (talk) 15:21, 12 March 2013 (UTC)

If the energy density (Energy per unit volume) of the universe is zero, than its total energy might be zero even if the universe is infinite. Dauto (talk) 19:00, 12 March 2013 (UTC)

@Dauto. If E=mc² then if m = ∞ then E = ∞.--YanikB (talk) 22:25, 12 March 2013 (UTC)

@YanikB. You misread my post. Dauto (talk) 13:57, 13 March 2013 (UTC)

If the energy density of an infinite universe is zero, the total energy could be any value: see Indeterminate form. --Carnildo (talk) 00:14, 13 March 2013 (UTC)

Yes, any value including zero. Dauto (talk) 13:58, 13 March 2013 (UTC)

... yes, see Zero-energy universe for a "fringe" theory. Dbfirs 18:39, 13 March 2013 (UTC)

Black hole

What black hole are made up of? For example, the composition of neutron stars are known. I want to know what is the composition of a black hole? What is the internal structure of a black hole? --PlanetEditor (talk) 15:22, 12 March 2013 (UTC)

Current theory believes that black holes have only 3 meaningful physical properties: mass, charge, and angular momentum. In terms of your question, then, there is no meaningful answer. The black hole information paradox further discusses ongoing efforts to resolve what happens to our knowledge about stuff that enters a black hole. — Lomn 16:02, 12 March 2013 (UTC)

Instead of "there is no meaningful answer", a more accurate description is "we don't know". There is no theory of quantum gravity that can describe what happens at the singularity; that's why there's a singularity at all. --140.180.249.27 (talk) 16:15, 12 March 2013 (UTC)

No, my understanding of the matter (which is admittedly amateur) is that, from the perspective of an observer external to the black hole, there really is no meaningful answer, and that we know (within the limits of present understanding) that that is the case. Even in the case of the holographic principle, which I understand to be the best current presentation for preserving information about what goes into a black hole, that information is believed to be encoded at the event horizon rather than within it. — Lomn 18:08, 12 March 2013 (UTC)

Or, another way to rephrase Lomn's explanation is that if we know more than those three properties (mass, charge, and angular momentum), then we are not talking about a black hole. The use of the term is definitional - "black hole" refers to an entity that has no other describable properties besides these defining characteristics. Long before we ever observed any such object, physicists and mathematicians were able to describe a coherent, cogent mathematical framework in which the existence of such an object is consistent with our generally accepted assumptions. The fact that some physical phenomenon does exist and meets these criteria is purely an observational result - and over the last few decades, we have observed them - or at least, we have observational evidence that strongly match the predicted behavior. Nimur (talk) 18:22, 12 March 2013 (UTC)

That's not how black holes are ordinarily defined; they are defined by the existence of an event horizon. The no-hair theorem is not simply a matter of definition. In fact, it fails in higher dimensions (see No-hair theorem#Counterexamples). -- BenRG (talk) 06:27, 13 March 2013 (UTC)

And what is an event horizon? Isn't it simply the geometric representation of the surface beyond which we know there's some mass, charge, and angular momentum, and beyond which we know nothing else? Our article cites a 1958 paper as the original use of the term; that paper uses a symmetry argument, but essentially it boils down to this: the event horizon surrounds a region in which any property other than mass, charge, angular momentum, cannot have any causal influence on anything outside it. That's the definition of an event horizon as it applies to a black hole. Of course, the Stanford Encyclopedia of Philosophy entry on black hole definitionalism is worth a read: "To begin, we were confronted with the question of the definition and significance of singularities. Should they be defined in terms of incomplete paths, missing points, or curvature pathology? Should we even think that there is a single correct answer to this question?" Nimur (talk) 02:27, 14 March 2013 (UTC)

The event horizon (of a black hole) is the boundary of the region from which you can't escape to infinity. The fact that it's a one-way surface isn't the distinguishing feature. There are one-way surfaces literally everywhere in spacetime (Rindler horizons) and they look just like black hole horizons, complete with gravitational attraction and Hawking radiation (except that it's called Unruh radiation).

Nothing inside the event horizon can get out, not even mass, charge, or angular momentum. They are stored in the gravitational and electromagnetic fields outside the event horizon. Spacetime could just end at the horizon, and it wouldn't matter.

The singularities in GR are mathematical singularities. They're just situations where the mathematical theory of GR doesn't make any physical prediction, and quantum gravity presumably should. You can raise the same objections about any singularity in mathematics—does f(x) = 1/x have a singularity at 0? How can it "have" anything at a point that's not in its domain? The answers to these questions can sometimes matter but I don't think they're very deep. -- BenRG (talk) 05:09, 14 March 2013 (UTC)

Kind of like asking, what color is darkness, and is it striped or plaid? Gzuckier (talk) 18:47, 12 March 2013 (UTC)

Those questions seem to have perfectly meaningful answers, namely "black" and "no", respectively. --Trovatore (talk) 18:50, 12 March 2013 (UTC)

Fuzzball (string theory) gives a hypothesis. Wnt (talk) 20:04, 12 March 2013 (UTC)

@Lomn: You're confusing what we can measure from the outside to what actually exists. In principle, there's nothing preventing you from jumping into a black hole and approaching arbitrarily close to the singularity, if there is a singularity at all. If the black hole has any internal structure, you'd be able to measure it, although not report it to the outside world. --140.180.249.27 (talk) 22:36, 12 March 2013 (UTC)

Black hole complementarity is worth looking at. This is the very interesting idea that falling through an event horizon and getting converted into Hawking radiation at the event horizon are distinct futures of the same matter, i.e., that the future bifurcates in much the same way as Everett worlds. It sounds crazy, but at this point I'm pretty sure that it's true, because it works too well to be an accident (just like Hawking radiation, which no one seriously considers discarding even though it's never been seen and it leads to the information loss paradox). I think it was this paper that ultimately convinced me. It also convinced me that the universe has finitely many degrees of freedom—specifically, the Bekenstein-Hawking entropy of the future cosmological horizon of ΛCDM, which is about 10¹²² bits. That's a natural guess in any case, but without black hole complementarity it doesn't make any sense, because there's obviously all that other stuff out there beyond the cosmological horizon, just like there's obviously stuff behind the event horizon of a black hole. And so there is, but not in a way that involves additional degrees of freedom. -- BenRG (talk) 06:27, 13 March 2013 (UTC)

Speaking of black hole internal structure, I notice that our Black hole illustration is a plain black disc amid distorted spacetime - which is sensible enough, since light can't come out of one. But Hawking radiation can. So suppose the black hole in the illustration was a really really small one, or it was being viewed with a super long radio telescope, so that the Hawking radiation glowed white-hot, in accordance with it having a white-hot "temperature" based on its mass.

Would the hole look like a featureless white disc? Or would it vary in color as you look closer to the center of the disc, or toward the hole's spin axis? For that matter, would it actually look like blackbody radiation from a distance, or some other spectrum profile? (Assuming no infalling matter) Wnt (talk) 20:08, 12 March 2013 (UTC)

Interesting question. Here is a partial answer: The profile is that of a blackbody and the temperature is uniform throughout. I say that is a partial answer because I'm not entirely sure whether a spinning hole would create a color gradient due to Doppler effect, but I think that would be the case. Dauto (talk) 20:58, 12 March 2013 (UTC)

I think (agree) that the nonrotating case will appear as a featureless disc, since a simple symmetry argument shows that the apparent temperature must be constant and I don't think you can have a variation in brightness without a variation in temperature. I'm 98% sure that in the rotating case there will be an apparent temperature gradient. -- BenRG (talk) 06:27, 13 March 2013 (UTC)

Philosophically, the only appropriate answer is that a black hole is made up of whatever collapsed to form it + whatever has fallen into it since. 178.48.114.143 (talk) 22:26, 12 March 2013 (UTC)

That's what went into it. That's not (necessarily) what it's made of. We might analogously note that a star that starts as ca. 78% hydrogen will much later be less than 78% hydrogen and a bunch more of other stuff farther down the periodic table, and that it would not make sense to describe all that carbon, nitrogen, and iron as "hydrogen". And philosophically, the ship of Theseus remains a thought experiment without a universally agreed-upon answer. — Lomn 22:47, 12 March 2013 (UTC)

That philosophical problem works better with a starfish. Plasmic Physics (talk) 22:18, 13 March 2013 (UTC)

I opt for the superstring-yarnball theory. Plasmic Physics (talk) 23:31, 12 March 2013 (UTC)

Note: yarnball theory is the same as the fuzzball theory above. I just forgot what it was called. Plasmic Physics (talk) 01:05, 13 March 2013 (UTC)

No one thinks that fuzzballs are the correct description of black holes. They're just one effective model. -- BenRG (talk) 06:27, 13 March 2013 (UTC)

Can you objectively say that? Then I'd like a reference confirming that no one thinks so. Plasmic Physics (talk) 07:44, 13 March 2013 (UTC)

I'm apparently wrong because arXiv:1108.0302 by Samir Mathur, who's mentioned in Fuzzball (string theory) as one of the originators of the idea, clearly shows that he thinks the classical picture of the black hole interior is completely wrong. It's an odd paper because, although its purpose is to answer objections to that idea, it never mentions the equivalence principle, which is the reason people believe in the classical interior in the first place. If you assume the equivalence principle holds and that low-energy gravity is well approximated by GR, the interior has to exist. In the fuzzball picture it's stopped by an enormous macroscopic low-energy violation of classical GR, and I can't figure out why he thinks this will happen at black hole event horizons and not at any other event horizon. For example the de Sitter horizon of ΛCDM cosmology (which I mentioned above) is just like a black hole event horizon turned inside out. It emits Hawking/Unruh radiation and it has the same entropy as a black hole with the same surface area, so it should be described by the same physics. But that would mean that only one Hubble volume of the universe is real, the rest is an illusion coarsely simulated by physics on the boundary, and at some point we will suddenly go splat against that (invisible) boundary unless we're exactly in the center.

Actually I think the fuzzball picture is probably correct, because black hole complementarity requires the existence of an exterior theory of black holes in which everything does go splat at the horizon, and a similar theory for every Hubble volume. But in neither case is it the only correct description. I guess there are people who think it is, but I think they're nuts. -- BenRG (talk) 05:09, 14 March 2013 (UTC)

Well, then call me a cashew. Technically speaking, a chashew is not even a nut.Plasmic Physics (talk) 08:21, 14 March 2013 (UTC)

Can you tell me what happens in the de Sitter case? Because I'd actually like to know. -- BenRG (talk) 20:39, 14 March 2013 (UTC)

I'm not saying that I'm a genius, or and expert in cosmology for that matter. No theory describing a blackhole's interior is perfect yet, I just like this one because it makes more sense in the case of a blackhole. Plasmic Physics (talk) 21:08, 14 March 2013 (UTC)

A giant atom called Blackholeum.165.212.189.187 (talk) 13:54, 13 March 2013 (UTC)

I think you're joking, but black holes are actually like elementary particles in many ways. See black hole electron and hep-th/9309145 for example. -- BenRG (talk) 05:09, 14 March 2013 (UTC)

Not joking. I want credit for the name though.165.212.189.187 (talk) 14:29, 14 March 2013 (UTC)

Someday, when we can send a probe into a black hole, we might discover that it's filled with the mates of socks and shoes which disappeared over the years. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:12, 14 March 2013 (UTC)

Comparison of lander speeds

The NASA Curiosity Rover had to perform a complex landing procedure involving multiple parts compared to the Apollo Lunar Module. What is the difference in the speeds of these two things just prior to the commencement of their landing procedures? I know other factors contributed to the difference in procedures, but I just want to know what their speeds were when they began their respective landing procedures. 20.137.2.50 (talk) 20:37, 12 March 2013 (UTC)

Curiosity was travelling at about 6 km/s when it initiated descent procedures (the "7 minutes of terror"); Apollo 11 was orbiting at about 1.4 km/s when the Eagle detached. The discrepancy appears to be that Curiosity didn't bother entering Mars orbit first but rather landed directly from the interplanetary trajectory. Of course, it's not the high-speed part of the descent that's tricky; it's the soft landing (whether relying on a computer to do course corrections for a rocket-powered sky crane that can't work in real time from Mission Control or an astronaut who knows whether he can ignore a computer alarm and navigate past some boulders before he runs out of fuel). Neither of those things happen at particularly high speeds. — Lomn 21:13, 12 March 2013 (UTC)

While casually reading Plasma Effects on Reentry Communications (1965), I found a reference to NASA-TM-74738 Trajectory studies for use in determining tracking requirements for Project Apollo, (1964), with orbital simulation results, and discussions about all the various options available. Direct descent is easier and more efficient, from an orbital mechanics standpoint; but it's much harder from an entry and descent standpoint because the entry speed is much higher. Nimur (talk) 02:39, 14 March 2013 (UTC)

Scientific method and primitive inventions

Is it wrong to say that primitive inventions, like the agriculture and the writing, were the product of the scientific method? Or can we call it scientific method only from a specific time, like Galilei Galileo? OsmanRF34 (talk) 23:43, 12 March 2013 (UTC)

I would say scientific method, as we know it today, did not exist before 18th century (before David Hume). A proper scientific method developed only during the 20th century (Karl Popper and Falsifiability).

Origin of language was not a result of application of scientific method, it was the result of evolution, just as Evolutionary origin of religions or Evolution of morality. The origin of agriculture was analogous to discovery of copper or iron, or domestication of horse or dog. It did not require a scientific method. --PlanetEditor (talk) 01:09, 13 March 2013 (UTC)

The scientific method is a specific process the identification of which was very long in coming. We know that the Sumerians developed writing by drawing pictures of clay tokens that they used for accounting purposes, and that Egyptian, Chinese, and Native American writing also arose from pictographs. Agriculture might have had a religious component, given various fertility cults. For example, Triple Goddess. We can't say that there weren't isolated geniuses, but a neolithic farmer is not going to up and say "let's do an experiment" when that concept took centuries of civilization to come up with. It seems much more likely that once seeds could be kept year round in pottery, people noticed that grain that was kept over winter and then spilled on wet soil would germinate. See also neolithic revolution, origin of writing. And so, yes, it is wrong. μηδείς (talk) 01:22, 13 March 2013 (UTC)

Besides what Medeis says, writing is not an integral part of the language ability, some communities had it, others not. OsmanRF34 (talk) 12:07, 13 March 2013 (UTC)

Sails were evolutionary, a product of trial-and-error, which approaches being scientific, but isn't quite. Contrast that with the rigorous testing the Wrights did to perfect their airplane before they flew. That's the scientific method. — Preceding unsigned comment added by Baseball Bugs (talk • contribs) 01:23, 13 March 2013 (UTC)

I do think dating the experimental method to Hume and Popper is quite a bit late, certainly people like Jābir ibn Hayyān, Robert Grosseteste, Roger Bacon, etc. were writing early versions of the scientific method. The modern formulations were not created out of whole cloth by Karl Popper, instead it represented a centuries-long evolution of scientific thinking. --Jayron32 01:40, 13 March 2013 (UTC)

The last essential of the scientific method, or maybe there is another essential thing we are still missing, is to check for why a hypothesis might be wrong, not just to find confirmatory cases. Lots of people in the past have made careful and systematic studies and analysed their data and published them for others,, but somehow that little trick has been and still seems to be a difficult one to catch on to. Dmcq (talk) 02:49, 13 March 2013 (UTC)

There is no single scientific method, and what is usually purported to be it certainly isn't how scientists (past or present) go about making discoveries. As for things in the past, it really depends on the invention in question, and what we even know about it. In general I think that certain types of very careful trial and error, or extrapolations from what is already known about how something works, probably can be considered roughly equivalent to how inventions are developed today, even fairly high-tech ones. Complete accidents, obviously not. The trick about modern science isn't that it has a fancy magical method that took centuries to develop that yields perfect results, it's that it became an organized network of information that got spread around in a fixed way and could thus become accumulative. That is, what "primitive" people lacked was a means of reliably sharing data and comparing results with people not in their immediate vicinity, and therein lies the biggest distinction, not the method. An inquisitive method is necessary (and more common than we make out today) but not sufficient for "science." (If you are interested in references, Bruno Latour is the biggest proponent of this view of how science works and worked.) --Mr.98 (talk) 02:50, 13 March 2013 (UTC)

The fact that there is not a rigid step-by-step procedure that is slavish followed by all scientists does not mean that there is not some universal principles that distinguish scientific thinking or the philosophy of science from other means by which people attempt to extract truth from the world. The "scientific method" represents a real paradigm and outlook on looking at extracting knowledge, and the fact that all scientists don't merely follow a specific pattern by rote doesn't mean anything, because that's not what the scientific method is. --Jayron32 13:35, 13 March 2013 (UTC)

What people are normally doing when inventing is engineering, as in what the Wright brothers or Edison did. Medicines are also engineered but they also use science to make sure they actually work and for the reasons they think. With just engineering you get people doing things because it works, they stick some ingredient into the paint because it has always been done and it works and no-one remembers that it was actually done to fix another problem that has long disappeared. Dmcq (talk) 03:09, 13 March 2013 (UTC)

I share skepticism of claims about the modern "scientific method". I've never heard real evidence that scientists really follow the rigid procedures sometimes suggested (which seem to vary a lot from one author to the next), nor that these procedures actually improve productivity. I think any plumber, trying this thing and that to unclog a pipe, uses a thought process very similar to that of a scientist, without any such formal training. I think that the ancients came up with quite effective herbal treatments and surgical procedures, which suggests they could do effective research somehow. I think "trial and error" is a principle so basic it just might be instinctive. But I'll admit, I have no proof of any of this. Wnt (talk) 03:20, 13 March 2013 (UTC)

What you are describing is yet again engineering rather than science. Trial and error is a very effective way of getting results, in fact it can be more effective than scientific research at getting an engineering solution, but it only advances what one might call traditional know how knowledge rather than validated understanding. And traditional know how is full of junk ideas as well like doing things during the full moon or chanting incantations. Do they make the steel worse? no so the blacksmith keeps on doing them just in case. Dmcq (talk) 08:55, 13 March 2013 (UTC)

Trial and error testing is not really what I was taught as an engineer. The goal is starting with principles of physics and material science, and devising designs which satisfy the specifications (after developing those specifications) within the allowed budget. Tolerances must be met. Running around trying one thing after another sounds like an old-school inventor. Edison (talk) 22:19, 13 March 2013 (UTC)

Then existing knowledge is good enough for what you are doing. But even for straightforward things don't you check with others if something is a good design and revise it if there seems to be a problem? You probably don't try out designs you don't think would be acceptable though to find out the essential required features. Dmcq (talk) 00:34, 14 March 2013 (UTC)

Modern science still has its incantations under the full moon - like preparing competent cells with rubidium chloride. How many decades did people do that? Wnt (talk) 00:59, 14 March 2013 (UTC)