Wikipedia:Reference desk/Archives/Science/2006 October 20

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1 October 20

[edit] October 20

[edit] The fourth dimension?

I've read the article and I still don't understand it. Is it actually physically possible for the human brain to directly visualize a 4-dimensional object, or is it one of those things like directly visualizing the concept of literal 'nothingness' that is beyond our capabilities? --Kurt Shaped Box 00:55, 20 October 2006 (UTC)

It sounds pretty much beyond our capabilities to me, and I'd be pretty suspicious of anyone who claimed they could visualize otherwise (though, of course, it would be impossible to really know if they were telling the truth). We have brains hard-wired for three spatial dimensions + a time dimension. Anything beyond that is not really in our "intuitive" hardware, I don't think. We can create analogies and simulations and calculations and etc. but all of that are poor substitutes to any sort of real way to directly visualize a fourth spatial dimension. --Fastfission 01:10, 20 October 2006 (UTC)

No, it's not possible because we live in a 3-dimensional world. See Flatland and Spaceland (book). -THB

The theory goes, you can't see past your own dimension. Like, a point can't see a line, a line can't see a 3D shape, 3D (us) can't see 4D. --Wirbelwind 01:28, 20 October 2006 (UTC)

Or like somebody who is completely colorblind would find it impossible to visualize what color looks like, even though they could understand it theoretically as light at certain wavelengths. StuRat 01:35, 20 October 2006 (UTC)

I've read somewhere that blind mathematicians actually had a pretty good intuition of 4th dimension and topology. If I recall correctly, they mentioned Bernard Morin among others. ☢ Ҡi∊ff⌇↯ 01:38, 20 October 2006 (UTC)

But can you even say they even know what the third dimension is supposed to be like, much less the 4th? --Wirbelwind 02:24, 20 October 2006 (UTC)

Since when is the third dimension dependant of vision? The point here is that they have a more developed spatial intuition because they have to build 3-D objects in their minds. They already visualize (aw crap) the third dimension in a more abstract way than we do, so perhaps they are better prepared to visualize higher dimensions. ☢ Ҡi∊ff⌇↯ 02:30, 20 October 2006 (UTC)

What I mean is how do you know they visualize 3D correctly, because if they don't, they probably didn't visualize 4D correctly. Their way of seeing 4D is probably similar to understanding 4 dimensional arrays in programming, which is hardly a visual. --Wirbelwind 02:38, 20 October 2006 (UTC)

In the end, I find it somewhat questionable about whether we really see in 3-D ourselves. I mean, our eyes individually give 2-D images of the world which are compiled into a 3-D understanding (sometimes quite a wrong one, as optical illusions point out) of the world. But perhaps we'd be dropping down a rabbit hole here. (Honestly seeing in 1-D makes almost no sense to me. 2-D makes a lot of sense, because you open up a plane. 3-D makes conceptual sense but I haven't seen any way to visualize it that didn't involve building it up from composite 3-D images. You can, however, perceive the 3-D directly if you open up perception to include non-visual sources, I suppose. But by this point I think I'm falling down that rabbit hole I mentioned...) --Fastfission 02:54, 20 October 2006 (UTC)

Actually, that a very good point. That reminded me of some religious argument where they said we live in 3D so we can see below us, or 2D and 1D. God or whoever is in 4D and thus can see 3D, or everything in the physical world. But that really is another subject, and one I'm much less interested in =P --Wirbelwind 02:59, 20 October 2006 (UTC)

Eh, that is assumed for sure that a four dimensional "being" would have great more power over us. Imagine a three dimensional being seeing over a two dimensional world. No safe or prision is safe, because it is just a square. If you get what I mean. The possibilities are endless, since the 4D being could see and manipulate anything physical in the third dimension without restraint. It's tough to explain. Hyperspace does it well. — X [Mac Davis] (SUPERDESK|Help me improve)04:55, 20 October 2006 (UTC)

Wirbelwind, what did you mean by "visualising 3D correctly"? What would be the correct perception of it, and who gets to decide that? Also, I don't think it's possible to be categorical about the dimensionality of God. In fact, I'm sure it's not. JackofOz 08:17, 20 October 2006 (UTC)

Some people claim to be able to visualise a closed universe. I simply can't do that, because the concept of space/time/space-time coming to an end somewhere/when, offends my notion of an all-ness that is somehow not all. But that's not to say others can't conceptualise these things. They say they can, so who am I to deny that? Same with a 4D object. For me, it's just too strange to understand visually, but I'm sure some people can do it, or they know what it would "look" like if they could, which amounts to the same thing. JackofOz 08:17, 20 October 2006 (UTC)

I've always believed that in reality we simply won't be able to interact with a 4 dimensional being. It can't affect us and we can't affect it. Nil Einne 12:26, 20 October 2006 (UTC)

If the n dimensions of a being intersect with the m dimensions of a different being and they have l dimensions in common, they should be able to interact in those l dimensions..... Paul venter 15:31, 26 October 2006 (UTC)

While it's difficult to say what the human mind is capable of, I would say no assuming you're talking about a fourth spatial dimension. However I would say humans are capable of visualising four dimensions in terms of the 3 'spatial' dimensions and 'time' as in Eistein's theory or relativity... Nil Einne 12:12, 20 October 2006 (UTC)

I actually saw The Fifth Dimension , close up and in person.Edison 23:30, 20 October 2006 (UTC)

I think it's possible. 'Visualize' as in 'imagine' and not 'visualize' as in 'see'. The concept of "see" in itself is a 3D concept - our eyes and the way our mind processes visual information is in a 3D 'template.' But i think it's possible to imagine what a fourth dimention would be like. I think shows it nicely. You can't really imagine how the 4D cube would look like - because the way we percieve objects is competely 3D. But if you look at the 4D cube carefully, it's easy to understand it. Each point has four perpendicular lines coming off it, one for each spatial dimention. you can't 'see' it, but you could 'imagine' how it would exist if the rest of the world was also 4D.

Just another thought - if we can accurately use a 2D plane to draw a convincing representation of a 3D object, shouldn't it be very possible to use a 3D plane to construct a convincing representation of a 4D object? --`/aksha 13:01, 21 October 2006 (UTC)

It is a bit more complicated than that. We being able to draw a convincing 3D object on a 2D plane is a form of optical illusion; one made possible by our way of perceiving 3 dimensions outside of that 2D plain. Seeing as we are only able to see 3 dimensions through shading, itself a kind of optical illusion, that would extremely difficult, if not impossible, to do. We would have to create an optical illusion inside of an optical illusion. It may not be impossible, but... T H L 02:27, 22 October 2006 (UTC)

Just like a cube's 'shadow' on paper is two squares with the corners connected, a third dimensional 'shadow' of a hypercube would be two cubes with their corners connected. A nice visualisation-aid is Image:Hcube fold.gif DirkvdM 08:56, 22 October 2006 (UTC)

Yes, but the line would have to be connected at a 90 degree angle to the others; which we would not be able to perceive. Representing one is possible, as the image you linked to and the one above shows, but actually creating a convincing recreation of one in 3D is another story. T H L 15:51, 22 October 2006 (UTC)

What, build a real model of a hypercube in 3D, complete with right angles? That would of course be as impossible as drawing a 2D version of a cube, with all right angles. DirkvdM 19:11, 22 October 2006 (UTC)

Sorry, I lost myself for a second. I hate it when I do that. Yes, that could be done; I'm not sure how convincing it would be, but it could be done with supplies as simple as straws and marshmallows. T H L 21:22, 22 October 2006 (UTC)

[edit] ossification and calcification

what is the relationship between the process of ossification and calcification?

See ossification and calcification. And sign your posts!!! -THB 02:46, 20 October 2006 (UTC)

[edit] Combustion of BIO Gas...(Didn't get an answer yet!!)

I'm working on an anaerobic digester design and i found that the CH4:CO2 ratio is between 1:1 and 3:2. In addition it may contain Hydrogen Sulphide, Nitrogen, Amonia and Hydrogen in small amounts.

I'm planning to utilize this gas to produce CO2 to be used in another industry. So i would like to know further information about following issues.

1. will it be effective to combust methane+CO2+other trace gasses mixture to produce CO2 (will the excess CO2 affect the complete combustion?)

Do you mean you want to combust methane and O2 ? StuRat 03:49, 20 October 2006 (UTC)

2. What will be the products after combustion of the above gas mixture?

3. what will happen to NH3 and H2S after combustion?

It is planned to combust the mixture in lower temperature (Less than 200 C)

Appreciate any suggestion to success my effort and any links regarding this process will be really helpful to me...!! Thank you. --192.248.8.100 03:41, 20 October 2006 (UTC)Sithara, SL

[edit] rechargeable batteries

Why are the voltages lower on rechargeable batteries than on non-rechargeables? ie, AA alkaline is 1.5 volts, while AA NiH rechargeable is 1.2 volts

Please sign your posts. Have you tried reading our page on rechargable batteries? I don't know if it answers your question but it's a start. However this is a very simple and basic topic in chemistry. I suggest you start at Electrochemical cell. Unfortunately, it doesn't look like we cover this that well at the moment but any basic chemistry book will cover this and there must be a ton of websites as well Nil Einne 12:15, 20 October 2006 (UTC)

Cell voltage depends entirely on the particular chemical reactions that the battery designer chooses. And while the NiCd and NiMH reactions are pretty low voltage, this isn't true of all rechargeable battery chemistries. Lead acid cells tend to run about 2.0 V and lithium cells tend to run about 3.6 V.

Atlant 18:45, 23 October 2006 (UTC)

[edit] The Sun Core

Hi, the question is how many years does it take for heat generated in the Sun's core to reach its outer layers, the photosphere? Thank you.

--202.184.216.70 07:46, 20 October 2006 (UTC)

Our article on the Sun covers it. ☢ Ҡi∊ff⌇↯ 07:59, 20 October 2006 (UTC)

[edit] Puzzling physics question

This is for a student I am helping.

An iron cube floats on mercury at 0°C. The temperature is increased to 30°C. a) Does the cube float higher or lower in the mercury? b) What is the percentage change in the volume of submerged iron?

I assume that the iron would sink somewhat, intuitively I suppose. But I'm not sure how to calculate the % change in submurged volume. I assume that you would use, in part, the change-in-volume equation, i.e., ΔV = β V₀ ΔT. The β values for iron and mercury are known. BenC7 10:18, 20 October 2006 (UTC)

THe upthrust equals the weight of fluid displaced (Archies principle)
THe weight of liquid displaced is the depth times csa of cube times its density.
does the density of mercury change between 0 and 30 celscius?

OTOH, is this a surface tension problem?--Light current 10:39, 20 October 2006 (UTC)

No. It is part of the physics curriculum dealing with heat. And the mass and size of both metals are unknown. BenC7 10:42, 20 October 2006 (UTC)

Ah, so its just the temp of the cube thats raised (not the mercury)?--Light current 10:45, 20 October 2006 (UTC)

Remember that both the iron cube and the mercury will expand, and that will change their densities. The expansion rates will be enough to tell if it will float more or sink when heated up. ☢ Ҡi∊ff⌇↯ 10:46, 20 October 2006 (UTC)

The question isn't terribly clear on whether just the mercury, or the whole system changes in temperature. I assume that it will be the whole system. OK, so let's say, for argument's sake, that the β value for iron is 3 times as much as that for mercury. So it will come up higher in the mercury. What does this mean for the percentage change? Is it possible to work the question out without knowing the density? BenC7 10:51, 20 October 2006 (UTC)

Yes, I think it is. If you find out the iron expands 3 times more than the mercury, its density will drop proportionally more than the mercury, therefore it will float even better than before. You can use the variation of the proportion of densities to work the variation of the percentage of immersion. ☢ Ҡi∊ff⌇↯ 10:54, 20 October 2006 (UTC)

OK - the β for mercury is 20; for iron it is 4. What is the answer, then? BenC7 10:59, 20 October 2006 (UTC)

Consider that an object with density

D 1

is immersed in a liquid with density

D 2

, floating with a certain N% of it submerged. If I halve

D 1

, N will also be halved (it floats twice as much). If I make the liquid twice as dense (double

D 2

) N will also be halved. Therefore, we have a multiplier for N given by $\frac{D_1}{D_2}$ .

Now, given the above coefficients of expansion, we have the relative expanded volumes for 30° C as:

$V_{mercury} = 1 + 30 \times 20 \times 10^{-6} = 1 + 6 \times 10^{-4}$

$V_{iron} = 1 + 30 \times 4 \times 10^{-6} = 1 + 1.2 \times 10^{-4}$

Since densities are inversely proportional to the volumes, their relative densities at 30° C are then:

$D_{mercury} = \frac{1}{V_{mercury}} = \frac{1}{1 + 6 \times 10^{-4}}$

$D_{iron} = \frac{1}{V_{iron}} = \frac{1}{1 + 1.2 \times 10^{-4}}$

(these values are the multipliers that give us the density of each material at 30° when multiplied by their original density at 0° - we don't have that, so we work with proportions)

Throwing that in the first equation, we have:

$N = N_0 \frac{D_{iron}}{D_{mercury}} \approx 1.00047994 N_0$

Which means a 100.048% ~~change in~~ of the submersion amount from before, that is, it will sink (higher submersion than before) by 0.048%.

I'm not too confident about this result, though (I'm a bit rusty with these things, and it's been a bad week... so sorry about that) but that's basically the sort of thing you'll have to do here. Hope it was any help. ☢ Ҡi∊ff⌇↯ 12:09, 20 October 2006 (UTC)

Correction, that means the cube has 100.048% OF the submersion it had before, or a 0.048% change in the amount of submersion from before. StuRat 13:02, 20 October 2006 (UTC)

Good one! Thanks for the correction. I often get confused with some of the use of "from"\"of" "in"\"on" in english. ☢ Ҡi∊ff⌇↯ 13:58, 20 October 2006 (UTC)

No problem, lots of ads seem to get this wrong, too, along with saying "get 10 items ALL for $1" when they really mean "get 10 items EACH for $1". StuRat 14:38, 20 October 2006 (UTC)

Hmm, I accidentally divided the α values by three to get the β values instead of multiplying them. So in actual fact they are 180 x 10^-6 for Hg and 36 x 10^-6 for Fe. I think this makes the answer a 0.4% change. Sounds plausible. Thanks guys. BenC7 05:16, 22 October 2006 (UTC)

[edit] Metabolism etc.

It says in a number of health-related books that "If your energy input exceeds your energy output, you will gain weight. If your energy output exceeds your energy input, you will lose weight." I can understand that in many cases this would be true. But what about for skinny people (like myself), who seem to be able to eat large amounts of food and never put on weight? BenC7 10:46, 20 October 2006 (UTC)

Your enegry output equals your energy input? (Or you have a tapeworm- 8-)-Light current 11:02, 20 October 2006 (UTC)

Try reading a bit on metabolism and stuff. Especially Basal metabolic rate (which coincidently looks like it might need some improvements to make it more encylopaedic) Nil Einne 12:22, 20 October 2006 (UTC)

Some possible mechanisms for being able to eat a large amount without gaining weight:

You expend more energy than others. Some people seem to be always moving. Is this you ?

The food passes through you largely undigested. StuRat 12:56, 20 October 2006 (UTC)

Or: The food is digested just as for most people, but your liver does not convert excess energy into triglycerides that are stored in adipose tissue. --Lambiam ^Talk 14:17, 20 October 2006 (UTC)

"If your energy input exceeds your energy output, you will gain weight. If your energy output exceeds your energy input, you will lose weight." that statement is actually a very simplistic explaination of metabolism and weight. Whether you put on weight or gain weight is VERY complicated. Genetics, age, health, and a lot of other factors which are not directly related to metabolism affects weight. Also, being skinny doesn't mean you have a low energy output. You may want to take a look at Obesity, which does discuss how weight lose/weight gain works. --`/aksha 12:49, 21 October 2006 (UTC)

OK, so if the liver does not convert the excess energy into fats (to simplify), but digestion occurs the same as in most people, where does the excess energy go? BenC7 05:31, 22 October 2006 (UTC)

Down the drain when you flush the toilet. --Lambiam ^Talk 19:08, 22 October 2006 (UTC)

[edit] Beta particles

It's been a while since I've done physics so forgive me if this is a silly question. But someone was claiming that CRT monitors emit beta particles. I know how CRT monitors work and I also think I know what beta particles are. However I don't know if he is correct or not. Are high speed electrons only considered beta particles when emitted by beta decay? If so the answer would be they're not. But what is the difference between beta particles and electrons from an 'electron gun'. My guess is from the CRT, the velocity is very different and the number/concentration is a lot less but I don't know. Don't get me wrong, I'm not worried that I'm going to get cancer or anything I'm just wondering about the technicalities... Nil Einne 12:22, 20 October 2006 (UTC)

No need to worry. The electrons are completely absorbed by the thick glass in front. The guy your were talking with might have been worrying about X rays. As you might know, these are produced in an X-ray tube by accelerating electrons and smashing them into the anode. The energies involved are comparable, and hence, a CRT tube in fact produces X rays. As the glass of the tube should not only shield you from the cathode ray (i.e. the elctrons) but also from these X rays, it is very thick glass which contains lead. Nevertheless, customers are concerned by it, especially those working a lot in front of a CRT monitor, and hence, manufacturers of CRT monitors like to proudly put a sticker on their product that they comply with the TCO standard. Simon A. 13:43, 20 October 2006 (UTC)

And the term "beta particle" for an electron (or positron) is indeed normally only used in the context of radioactivity in the form of beta decay. The only relevant thing that may be physically different from other electrons is the high energy, which is fully determined by the speed of the particle. --Lambiam ^Talk 14:28, 20 October 2006 (UTC)

[edit] Human Body

Where is the actual centre of gravity to be found in the human body

I would think somewhere around the small of the back, along the centre line, but its giong to depend on the distribution of fat on your body.--Light current 17:10, 20 October 2006 (UTC)

It's somewhere in the lower abdomen, just above the pubic area. If you've seen dancers, ice skaters, and acrobats lifting their partners, you might notice the place where they have to hold their partners to balance them (the CG point), is a bit uncomfortably close to the pubic area. StuRat 17:13, 20 October 2006 (UTC)

Just as is the case in other physical characteristics, the center of mass varies from person to person. In general, the center is higher in the male adult than the female adult. Also, there is also a correlation between higer center of mass and poor health-- see Metabolic syndrome.--Mark Bornfeld DDS 18:11, 20 October 2006 (UTC)

The higher the muscle mass (relatively), the closer the center of gravity is to the chest area. The lower the muscle mass, the closer the CoG is to the abdomen. Note how skinny the figure skaters being held usually are. A popular demonstration of this fact used in biology/anatomy/physics classes (depending on which class the school teaches about the CoG in) is: To have a boy, usually a football player, put his head against a wall bending his back to an 80 degree angle. Then, have the boy grab a box weighing ~30 pounds (~13.6 kg), and try to stand up without moving his feet. He shouldn't be able to do it. After that, have a weak-looking girl try; she should be able to do it with ease. After the class finishes laughing, explain why this happened. T H L 22:50, 20 October 2006 (UTC)

Depends on where those muscles are. Speaking of football players, I know one who is all muscles below the waist and skinny above that.(Then again, you were probably not talking about the original football. :) ) Also, he's Dutch, so the cycling will also help in strong leg muscles. Something similar will be the case with skaters, which would explain why they have to be held up near the groin, as StuRat pointed out. DirkvdM 09:11, 22 October 2006 (UTC)

Right, I forgot that everyone else calls a different sport football ;). In the general population, meaning people who do not do athletics professionally, muscle mass tends to be distributed in such a way that the higher the muscle mass compared to the overall mass of the body, the closer the center of gravity is to the chest, and vice versa. Professional athletes probably aren't the best examples, seeing as many train one area of the body harder than the others; creating more exceptions. If every area of the body is trained equally, and they can lift the same amount pound/kilo for pound/kilo, then the center of gravity will move closer to the chest as the muscle mass increases. The greater amount of testosterone in men tends to cause the center of gravity to be closer to their chest; the exception being men like me who sit at the computer eating Tostitos and drinking Coke all day; so basically American men age 40-death are the exception. Anyway, muscle distribution tends to be equal enough for this to be the norm. T H L 21:45, 22 October 2006 (UTC)

THL: Middle-aged men might feel a little less creeped out by being categorized as "40 and up", as opposed to "age 40-death". :-) StuRat 00:36, 23 October 2006 (UTC)

I'll keep that in mind ;) T H L 01:19, 23 October 2006 (UTC)

[edit] The Past

Does the past exist. If it does is it frozen or always changing. 64.119.104.205 16:13, 20 October 2006 (UTC)bgoldie

Well, if the future exists now, then (as the present also exists-- obviously), it follows that the past also exists. This is becuase the future's past includes the present, and the present is the future as seen from the past! We believe the future can be changed becuase we have not reached it yet and the present is changeable. So to change the past , you would have to make it at least the present by going back there. Do I make myself clear? 8-)--Light current 17:19, 20 October 2006 (UTC)

But the future doesn't exist now. "Exist" is a present tense verb. The past is that which once "existed", but no longer exists. Definitions of existence vary, but in normal usage things "exist" only in the present. --Shantavira 17:28, 20 October 2006 (UTC)

)edcon)

I dont think its proven that the future does not actually exist now. Anyway the future will exist (probably) and at that time the present will be the past. If the future doent exist now, how can we hope to travel there other than at the same old rate?--Light current 17:59, 20 October 2006 (UTC)

(This is one perception of the universe, enabling me to answer your question)

We here touch upon the fourth dimension, that in which objects no longer simply exist as lengths, but (seen from our view) start to move. The past you are looking for is a point in this dimension which has certain qualities, so yes, it has existed. In the fourth dimension, the progress from that point in time to right now, has become a line - you may say that this line has snaked through a landscape, touching upon different places as it went along. Each second you choose to measure between the past (thirty minutes ago) and now (now) will see the line/snake having progressed a bit beyond what it was at. It changed coordinates each time. These coordinates you may understand as data about what the universe and everything else looked like right then. This is because "past" and "future" are the victims to diffuse semantics, and that your question is completely unanswerable to anyone without a proper language for it. I pray to god some alien civilization comes upon us to resolve these things soon.

Your point along this line of time, or rather just your line in the fourth dimension, will keep progressing. Because of that it is correct to say - at least I would claim so - that the past is frozen. This demands that the universe is as intuitive as I want it to be. Don't count on it. 81.93.102.3 17:56, 20 October 2006 (UTC)

NOW lasts forever, but NOW has just passed. :) Dave 205.188.116.74 18:06, 20 October 2006 (UTC)

Seriously though, Stephen Hawking declares that a very tiny fraction of a second, after the Big Bang, The future was 'carved in stone', so to speak...if one could possibly know the position and momentum of every particle, one could conceivably calculate every future event. I`m paraphrasing, of course. If one could suppose that the Universe was 'pre-determined' to be a 'quantum' Universe, even quantum mechanics would be 'covered'. So, if this is all true, then the future already "is"...one cannot change the future! Dave 205.188.116.74 18:23, 20 October 2006 (UTC)

In some civilisations, the past is in front of us, as we can imagine that we see it clearly ; future stays behinds ... -- DLL ^{.. T} 18:47, 20 October 2006 (UTC)

What if the past exists and is constantly changing. We're not aware of it because it already happened. The future also exists and is constantly changing. The future has an equal effect on the present as the past. Time, if we could observe it as a 4 dimensional being would be like a guitar string stretching from the big bang till the end of time. It would be constantly changing and vibrating. Einstein said time was an illusion. The relationship between cause and effect is not as we perceive it. bgoldie

I think that is a very interesting theory that bgoldie has just outlined. It tends to make sense to me. I dont know why it should, but it does--Light current 20:16, 20 October 2006 (UTC)

I have 'proof' that the future already exists. I just made a post AFTER the page was last modified. : ) Dave 205.188.116.74 20:12, 20 October 2006 (UTC)

If the past is changing, does it have a future? And is that future also changing? Is it perhaps the past of our changing future? Will the past of the future that will have been changed be part of an everchanging present, or is the present frozen? And if the present is changing, when is it changing? Is it changing now or in the future? Can we visualize these changes as the vibrations of the guitar strings being strung/having been strung/willing be strung from the big bang till the gnab gib? Should these vibrations be quantized and unified with string theory? I hope this makes sense to Light current. --Lambiam ^Talk 23:04, 20 October 2006 (UTC)

Per Philip Gosse's Omphalos (book) we can have no assurance that the world was not created 1 minute ago, so the past might be fake.Edison 23:37, 20 October 2006 (UTC)

But is it possible that the world has not yet been created and that what we are experiencing is the changing past of that future creation? Also, if the Omphalos (book) was written in the past and not created just one minute ago, it may be changing in the past and have said something very different. We can have no assurance that the world will end in 1 minute. The future may be real. If the past is fake, can it still change or is it frozen? Also, I wonder, does time always flow in the same direction? How do we know it does not occasionally flow backwards or sideways. Or stops flowing. Does the time in the past also flow, or only in the present? Will the questions change when the answers change? These are just a few of the questions I have. --Lambiam ^Talk 00:56, 21 October 2006 (UTC)

It doesn't matter to us if time stops or not, because we wouldn't notice. It wouldn't matter if it went backwards or "to the side" if possible, or time went faster or slower. Time is the reference, and the only way to know or observe any change in time would have to be from "outside" of time, wouldn't it?X [Mac Davis] (SUPERDESK|Help me improve) 22:33, 21 October 2006 (UTC)

Subatomic particles have been observed to disappear and then reappear. I believe that these particles are travelling to the past or the future. They become part of another atom in a different time. They are linked (entangled) with this particle. From the point of view of the particle the two times are the same. It is our illusion of "The Present" that makes us think that they are seperate. bgoldie

[edit] Tough pool leak...

I know this isn`t 'rocket' science but I`m really at a loss here. I have a particular type of pool. We`re not supposed to use copyright names so let`s just call it a "not very difficult at all" pool. You know, the type that has a ballon, which I`ll refer to here on in as, the bladder, which, once filled with air, simply fill pool with water, and the bladder 'floats' the walls up. Well, my bladder has developed several leaks. With the pool-repair kits that are sold won`t work...when I place a patch on a leak, the internal pressure pushes the patch away. I cannot deflate the bladder as that would empty the pool. i.e. walls would 'fall'. Can anyone here suggest a method that might work, without having to completely deflate bladder? I really don`t want to empty pool. Thanks, Dave 205.188.116.74 17:03, 20 October 2006 (UTC)

First, the rule about not using copyrighted names doesn't apply to the Ref Desk. Second, I think you will indeed need to deflate the bladders and put the patches on the inside, so the pressure holds them in place. You will then need to let them fully dry and cure before putting any air or water pressure on them. StuRat 17:08, 20 October 2006 (UTC)

I was afraid of that. Thanks for the heads-up about copyright. Dave 205.188.116.74 17:36, 20 October 2006 (UTC)

Dave,I think I have a Potential Solution to your delmna.Why not try to saturate Q-Tip with glue.Poke Q-tip through hole.Inside pressure should hold in place,repairingleak.That way you won't cut up bladder,creating another hole to patch.Andrea216.218.126.83 17:51, 20 October 2006 (UTC)

Just so people aren't confused: you can't copyright names. You trademark them. (See intellectual property for a discussion of this confusion.) And it's actually relatively hard to infringe on a trademark, particularly accidentally. (The most common case is treating a trademark as a common noun, a la "Legos" as opposed to "Lego bricks" or even "Lego brand bricks".) See the manual of style; as far as I know that's the only trademark policy on Wikipedia, and it doesn't even dissuade their use. --Tardis 18:08, 20 October 2006 (UTC)

Oh My! Andrea...I tried your trick. I just went and checked...it WORKED! What a brilliant solution. You`ve saved me a HUGE headache. Thank you so very much. Dave 205.188.116.74 18:31, 20 October 2006 (UTC)

[edit] Nasal mucus

Having a bad cold ATM, this is a question that has just come to mind. What determines the color of nasal mucus. I mean sometimes its clearish, sometimes greyish, and just recently greenish.--Light current 17:08, 20 October 2006 (UTC)

My understanding is that it is, at least in part, colored by bacterial colonization. I'm not 100% sure of this: a physician would probably be better prepared than I to answer this fully. – Clockwork Soul 19:00, 20 October 2006 (UTC)

Also Im getting different colors between nostrils. Is that normal?

I dunno, the type of crayon you recently shoved up your nose? What I really wanted to say is that I think ish is a great word for nasal mucus: "I've got some terrible green ish today." Perhaps I should wipe this on the Language Reference Desk. MeltBanana 18:59, 20 October 2006 (UTC)

Yeah you could wipe some over there. See who bites the monkey. Actually the correct term is liquid snot. but I wanted to be polite! 8-)--Light current 00:17, 21 October 2006 (UTC)

If requesting medical or legal advice, please consider asking a doctor or lawyer instead. Doctors sometimes ask what color it is to help determine if it is a common cold or a bacterial infection.Edison 23:39, 20 October 2006 (UTC)

Doc never asked me that. They just said try some of this, try some of that! None has worked yet 8-(--Light current 00:15, 21 October 2006 (UTC)

When I had bronchitis, I was told by my doctor that producing green-yellow mucus was a sign that it was bacterial, not viral, but according to the article bronchitis, the colour has no bearing on this... La ï ka 10:29, 21 October 2006 (UTC)

colored snot means there's an infection. It's like if you cut yourself really bad, and the wound gets infected, and you don't treat it...you'll end up with colored crap oozing out of the wound. Same deal - greyish --> yellow --> green snot means some kind of infection, in that order too (as in green = worst).

To clarify, i'm talking about colored snot, not colored mucus. Which is what you're asking, since you can't really tell the color of the mucus. i believe the thing that contributes to the yellow/green factor in snot is actually pus, not mucus. Colored snot (not colored mucus) is an indication of bacterial infection - since pus is produced by the body during inflammatory responses against bacteria. --`/aksha 12:37, 21 October 2006 (UTC)

Sorry to hijack the thread, but somehow the fact that Wikipedia has an article on snot just seems hilarious to me this morning. 192.168.1.1 10:00, 21 Rocktober 2006 (PST)

That s'not funny! 8-)--Light current 17:07, 21 October 2006 (UTC)

Wikipedia has everything. (since when did RD questions become threads anyway...) --`/aksha 02:23, 22 October 2006 (UTC)

[edit] Questions about cholesterol and saturated fat

A couple questions that I am interested in mainly from the point of view of my personal health, fully aware of all the caveats about Wikipedia not offering medical advice. Here goes... (If you choose to inline your replies, please use at least two colons of indentation.)

Question 1: If you switch from a diet high in saturates to a diet which is low in fat and in particular low in saturates, what is a typical timescale for blood (LDL) cholesterol levels to come down?

Motivation for question 1. Until three months ago, I had a diet which was moderately high in saturates (dairy fat and hydrogenated vegetable fats), and also my overall calorie intake was high. In mid-July I decided to address this, and increased my fruit and veg intake while reducing fat intake, particularly saturates, controlling overall calories, and also getting more exercise. The health benefits are fairly obvious in that my weight has already dropped from 97kg to a more appropriate 82kg (I am 1.83m tall), but I was wondering specifically about cholesterol. My cholesterol was never previously measured but I can only assume that it was on the high side. Should I bother to get it measured? Or by what stage can I start to assume that it is unlikely still to be high? (I am male, and in my mid-thirties.)

I wouldn't bother measuring blood cholesterol since, as you've said, you have no base reading to compare with. Also, blood cholesterol levels are a measure of the rate of change of your body cholesterol level more than they measure the amount of stored cholesterol. As a result, blood cholesterol may be high when your body cholesterol is either rapidly increasing or decreasing. StuRat 18:29, 20 October 2006 (UTC)

Question 2: If you consume saturated fat but burn off the calories, can it still raise your cholesterol?

Motivation for question 2. Although it has become very rare for me to eat chocolate and cookies during my normal routine, there can be days when I am out hiking all day, when I need the extra energy. Such foods are a very portable source of extra calories. Is it reasonable on those days to supplement an otherwise balanced diet with some of these sorts of food for the extra energy, and not worry about cholesterol? Or is it still better to avoid them, and instead carry extra bread, nuts etc? (I am aware there may be dental issues too.)

You should still have a healthy diet, no matter how much you exercise. Athletes who eat huge quantities of bacon and eggs are headed for health problems some day. StuRat 18:22, 20 October 2006 (UTC)

Many thanks. Slarey 17:14, 20 October 2006 (UTC)

Can't answer the first question exactly, but your cholesterol level should respond (if it does respond) within a month or so. Keep in mind that cardiovascular risk is correlated with several different blood lipids, as well as other measurable blood constituents. Not all of these parameters respond to dietary or other behavioral means of control; some are genetically determined. If you are unfortunate enough to have a heritable hyperlipidemia, dietary control will not be all that effective. Why should you assume your cholesterol is on the high side? In any case, a blood lipid profile is a routine test, and would certainly be part of a typical diagnostic exam for a male in his mid-thirties. As for the occasional consumption of high-fat foods-- for the average person, this would not cause a lasting change in the blood lipids (although it might cause a short-term dramatic rise in the chylomicron fraction of your triglycerides), as long as it is done in the context of an overall prudent diet. Man does not live by bread alone...--Mark Bornfeld DDS 18:31, 20 October 2006 (UTC)

Many thanks for your help. To answer your question, I simply assumed that cholesterol would have been high because of all the cheese, cookies etc I was eating. Anyway, I will ask for a blood lipid profile to be done at some stage. (I didn't actually realize the test measured more than just cholesterol, or that it was something considered routine in one's mid-30s.) I am sure at the time there will be appropriate advice given when discussing the results, but based on what you say I will probably assume that provided that levels are generally okay I don't need to worry too much about what forms of high-energy snacks I use on the odd occasions when I need them e.g. hill walking. Slarey 18:09, 23 October 2006 (UTC)

[edit] link between: diabetes, CHF, renal insufficiency, amputation

Hi, Im trying to understand how each of these diseases effect each other. If a person has diabetes, how would this influence CHF and renal insufficiency? Any information that would better my understanding woud be much appreciated. Thank you in advance

Jodie

I'll try to be short-winded: Diabetes -> high blood glucose -> atherosclerosis and vascular damage -> tissue damage. Since the kidneys and retinae have blind arteries (without anastomoses) they usually go first, but the tissue of the heart will be damaged too, leading to CHF. Decreased circulation in distal vessels, such as those in the limbs, causes ischemia, which increases the risk of infection, which necessitates amputation in extreme cases. Note that while Diabetes can lead to CHF, limb ischemia and renal insufficiency, these conditions will not cause diabetes. In a patient with CHF and RI from other causes, diabetes will further exacerbate the problems. Tuckerekcut 19:25, 20 October 2006 (UTC)

[edit] Patched conics & Lunar Orbit Insertion

Hi guys, an astrodynamics question for you all:

I'm doing a research project (final year of Aeronautical Engineering MEng) which involves determining the maximum payload I can get to the moon's surface, for the minimum cost/kg, from a starting point of 20 tonnes in a Low Earth Orbit (say 200 km, conveniently at the same inclination as the moon's orbit). One of the big factors influencing this is the selection of a suitable transfer orbit, but I'm really struggling to get my head around the orbital mechanics involved. I've got a stack of orbital mechanics, mission design and astrodynamics textbooks but I can't find a simple method for calculating the required delta V's to go from LEO to LLO (Low Lunar Orbit). I understand Hohmann transfers, and I think I understand the principle of patched conics, but all the descriptions of the latter method deal with transfers to other planets, which is different from going to the moon because you leave the Earth's sphere of influence. Basically what I'm asking is, what should the calculations look like for a patched-conic determination of an LEO-LLO transfer? Sorry if this is too obscure... generally seems like there's always someone here who knows the answers to things! Thanks --YFB ¿ 20:05, 20 October 2006 (UTC)

NASA is (or should be) your friend! 8-)?--Light current 20:09, 20 October 2006 (UTC)

Well, yes and no... where do I go to ask them a (technical) question?

Try here first Jet Propulsion Laboratory. THen follow the links all the way. Youll find a phone number! 8-)--Light current 20:53, 20 October 2006 (UTC)

Also have you tried the links in Celestial mechanics--Light current 21:15, 20 October 2006 (UTC)

Also has your reseasch unearthed Astrodynamics#The patched conic approximation ? If not, you just aint been looking 8-)--Light current 21:17, 20 October 2006 (UTC)

Then of course you have [1] on orbital mechanics.--Light current 21:19, 20 October 2006 (UTC)

Cheers, Light current; I've tried all the latter three and come up dry - as I said, I've ploughed through a stack of textbooks (Chobotov's Orbital Mechanics; Battin's Introduction to the Mathematics & Methods of Astrodynamics; Interplanetary Mission Analysis & Design etc. etc.) and done a fairly extensive bit of googling, but not found a decent (worked) explanation for a lunar case anywhere. I think I'm having a bad night, I can't seem to figure out what you mean by 'follow the links all the way' on the JPL website - which ones?! Could you paste a URL for me? Cheers, --YFB ¿ 21:30, 20 October 2006 (UTC)

Go to JPL official site [[2]] then click on public sevices. You should find a contact number there. You could try calling it. 8-)--Light current 21:59, 20 October 2006 (UTC)

Well, that sort of worked :-) - I didn't get anywhere with that number but I ended up speaking to the Education Outreach office and they passed me over to someone else, who gave me an email address for another someone, who should be able to help but if not I'm welcome to call back... phew! In the meantime, if anyone else understands my question, I'm still open to answers (please!) --YFB ¿ 22:30, 20 October 2006 (UTC)

Aha! some success then? Thats what research is all about. Asking people who know the answers! 8-)--Light current 22:37, 20 October 2006 (UTC)

If you want a Hohmann transfer orbit, then the formula for delta-v is on that page. There may be a more energy efficient way, however. Richard B 23:55, 20 October 2006 (UTC)

Thanks, but as I said in my question, I understand Hohmann transfers already. The problem is that a Hohmann transfer only gives the delta-v to transfer from a circular low-earth orbit to another circular earth orbit with the same 'altitude' as the moon (mean 384400 km). What I need is a method to calculate the delta-v to go from LEO to a circular orbit about the moon (i.e. to be captured by the moon and start orbiting it instead of directly orbiting the earth). To approximate that manoeuvre you need to do a 'patched conics' calculation, which is what I'm struggling with. Out of interest, I've obtained copies of that and other papers on low-energy transfers, but they're extremely difficult to model quickly as they depend on chaotic dynamics for which you need a searching algorithm and numerical back-integration. Also, they work by going a very, very long way away from Earth and then coming back again, which means that they tend to take a very long time (10+ times as long as a Hohmann transfer). Cheers though, --YFB ¿ 00:34, 21 October 2006 (UTC)

OK, well the patched conic approximation just looks at the most dominant body - rather than taking into account others. So how about we try an orbit with the following;

A transfer orbit to take us from LEO to some point in the vicinity of the Moon - give the object some Delta-V in LEO - assume that only the Earth and object exists in this calculation.
Give the object a Delta-V whilst in the vicinity of the Moon, to effectively place the object into a parabolic trajectory around the Moon, with closest point to the Moon equal to the desired circular orbit radius - calculate this as though only the Moon and the object exists. A parabolic trajectory has V = escape velocity at that radius, so the necessary velocity should be straightforward to calculate.
Once at the closest point to the Moon, give the object a Delta-V to bring it into a circular orbit around the Moon - again, assume that only the Moon and object exists in the approximation.

Now you've got 3 different phases of the trajectory to calculate. How about we work backwards - start by taking the desirable orbit around the Moon, and work out the circular orbit velocity. The last delta-V - from the parabolic trajectory to circular orbit will have a delta-V of (sqrt(2)-1) times the circular orbit velocity. Then project the parabolic trajectory back to a point where the Moon just dominates over the Earth. Then look at what transfer orbit you'll need to minimise the delta-V in LEO, and at the point between the transfer orbit and parabolic trajectory. The transfer orbit needn't be the same as the Hohmann transfer orbit to take you to a circular orbit around the Earth at the Moon's distance. Richard B 18:32, 21 October 2006 (UTC)

You are talking about "calculating the delta V's". To me, that sounds like you will apply them as short impulses and already know how many they are and when they will take place. Is that how I should read it? (Sorry if I'm asking stupid questions – I just want to learn.) I mean, isn't it possible that the best trajectory results from thrusting in the right way during the whole trip? —Bromskloss 09:54, 21 October 2006 (UTC)

As far as I know (I'm not claiming to be an expert) that's not the most efficient solution. Taking the 'most direct route' (i.e. a one-burn tangent) is actually less efficient, because effectively you are fighting against your own orbital inertia as well as the gravitational attraction of the central body (here the Earth). The Hohmann transfer is the most efficient 'two-burn' trajectory, because you use one initial burn to move from your initial orbit to an ellipse with its apoapsis at the target altitude, and then a second burn to turn this elliptical orbit into a new circular orbit at apoapsis. By doing that you minimise the energy you expend in overcoming the inertia of your initial orbit. 'Impulsive' (i.e. infinitesimally short) burns are the ideal, because the shorter the burn, the greater the component of thrust that goes in the intended direction. If you make a long burn, you will gradually shift into the desired orbit rather than 'snapping' into it. During the transistion, some of your thrust will be going in a direction other than the intended one, and is wasted.

For my problem, I know that a 'classical transfer' (i.e. not a low-energy transfer based on chaotic dynamics) consists of an initial burn at LEO to enter the transfer trajectory, followed by a further burn (or burns) to enter a circular orbit about the moon. What I'm having trouble with is working out what is required to make the latter insertion to the moon's orbit. Hope that makes a bit of sense, --YFB ¿ 14:02, 21 October 2006 (UTC)

Looks like you may have to ask your Prof then! 8-)--Light current 17:16, 21 October 2006 (UTC)

[edit] What is the total number of known chemical elements?

I want to know the total number of known chemical elements. I mean the discovered ones. Thanks.

--196.202.91.65 22:15, 20 October 2006 (UTC)

Your answer is in the second sentence of the very article you've linked to.- Nunh-huh 22:18, 20 October 2006 (UTC)

Or, if you don't want to go there, the answer is 117: everything from 1 to 118 with the exception of Ununseptium or 117.Phsource 22:23, 20 October 2006 (UTC)

You'd be better served by actually going there and reading the details. - Nunh-huh 22:40, 20 October 2006 (UTC)

Thanks to every one who answered my question. I, of course, read the article before posting the question, but the version I read was old and didn't contain the number of articles.

--196.202.92.94 07:04, 21 October 2006 (UTC)

gee...that must have been a real old version you read. Because the answer to your question has been in the article since June 2004. --`/aksha 12:37, 21 October 2006 (UTC)

Isn't the number of discovered ones 92? The ones above that are manmade. DirkvdM 09:19, 22 October 2006 (UTC)

Because of this mention of "discovered", I cannot resist quoting the end of Tom Lehrer's The Elements song:

These [102] are the only ones of which the news has come to Harvard,

and there may be many others but they haven't been discarvard.

That song was from 1959. Even by about the late 1980s the number had only grown to 103 (this is just when I personally happened to first hear the Lehrer song; I remember checking, and all it was missing was lawrencium), so much of the increase since the song is relatively recent. Arbitrary username 18:41, 23 October 2006 (UTC)

[edit] Known compounds

What is the total number of Known chemical compounds? (to the nearest 1000) 8-)--Light current 22:28, 20 October 2006 (UTC)

By known, are you including all hypothesised compounds as well (such as UuoO₃)? La ï ka 22:54, 20 October 2006 (UTC)

Actually, on a more useful note, currently there are about 30,100,000 compounds registered under the CAS registry number system, but the number is growing constantly; the current number can be seen here. La ï ka 22:58, 20 October 2006 (UTC)

I suppose I was asking how many are naturally occuring of have been synthesised. Not how many were theoretically possible. 8-|--Light current 00:08, 21 October 2006 (UTC)

When I last looked, the CAS site said: 11,962,499 Commercially available chemicals. I guess these have been isolated or synthesized, but perhaps this works like just-in-time synthesis-on-demand :) --Lambiam ^Talk 00:27, 21 October 2006 (UTC)

I believe all 30,000,000 at CAS are real ( have been synthesized or found). Theorectical possibilites would be many times greater. They say 4,000 new ones a day. --GangofOne 05:58, 21 October 2006 (UTC)

[edit] is this a shockwave affect or just accelerated air?

http://en.wikipedia.org/wiki/Vortex_ring_gun

I asked this before but never got an answer. does the effect used in this [the version that just uses the air in the cylinder anyways] count as an over pressure shockwave, or not? robin

A shockwave is just a pulse of high pressure air, isnt it?--Light current 00:44, 21 October 2006 (UTC)

I'm pretty sure you did get an answer. Quoting: "I think this is just a more powerful Airzooka. … There is no supersonic air motion, and no shockwave." —Ilmari Karonen (talk) 12:57, 21 October 2006 (UTC)

oooh i didnt come up with that, stupid browser , cheers and thanks Robin

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