Wikipedia:Reference desk/Archives/Science/2007 March 28

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March 28

Spitting and wiggling

So I was making that noise you can make my putting your tongue between your lips and forcing air out, often called spitting, while I was looking at a clock with a digital readout, and I found the numbers were wiggling, jiggling just above the chamber door. So I asked why I perceive the numbers as wiggling, jiggling just above the chamber door, if they have to do with my eyeballs actually moving, but Leonore only quoth "nevermore." Thanks for the big help Lenore. What do you say? [Mαc Δαvιs] (How's my driving?) ❖ 00:10, 28 March 2007 (UTC)

(Lenore is your raven?)

(And don't you mean a raspberry?)

The digits on many clocks and other digital displays are multiplexed -- they're actually lit in sequence, one at a time. (This ends up reducing the wiring.) But the sequencing happens fast enough that, normally, you don't notice it; the persistence of vision means they seem to all be on all the time. You can see the actual sequencing effect with a display on a device small enough to hold in your hand and wave around: if you can move it fast enough in a direction perpendicular to the row of digits, you can often get them to spread out, like this:

12:34:56   (stationary)

1          (moving

 2          quickly

   3        downward

    4       .

      5     .

       6    . )

Now, if you jiggle your head in certain ways, it somehow causes a stop-action effect with your eyes. (It's as if they can't or don't blend smooth motion as they normally do.) For example, many people have noticed that if they watch a television or an (older) CRT monitor at a distance while eating something crunchy, strange horizontal black lines or bands appear on the screen. These are in fact the unlit portions of the raster, that you normally don't see, because even though only one raster is being scanned at a time, your eye usually fuses them all together.

So what I believe you're seeing -- and I believe I've noticed the same sort of thing myself -- is those individual digits in your clock lighting up at slightly different times and while the vibration of your head is making them appear to be in slightly different positions.

—Steve Summit (talk) 00:38, 28 March 2007 (UTC)

[P.S. I was going to link to our article on multiplexed display for the explanation of why digital display digits are typically lit one at a time, but the article doesn't didn't seem to exist yet. Here's the answer: suppose the digits are standard seven segment ones, and suppose there are six of them. Suppose further that the clock contains a single IC containing most of the circuitry. Now, how many wires will we need between the chip and the display (or, more importantly, how many pins will the chip have to have for display output)? If we used the obvious, naïve approach, we'd need 7 × 6 = 42 wires. But if instead we have 7 outputs for the 7 segments on any digit, and 6 outputs to say which digit to light up just now, we can do it with only 7 + 6 = 13 wires. The circuitry on the chip has to be a bit more complicated to do the multiplexing, and the display potentially has these subtle flicker effects, but it's usually considered to be well worth it for the pin savings. —Steve Summit (talk) 00:51, 28 March 2007 (UTC)]

Holy crap! Awesome answer! I've wondered about that for many years when putting -- ahem -- vibrating objects on my chin and looking at ssd clocks. Nice work. --TotoBaggins 01:50, 28 March 2007 (UTC)

(Multiplexed displays now exists, but feel free to help improve it!)

Atlant 12:29, 28 March 2007 (UTC)

You can also make an oscilloscope trace seem to jump around by chewing carrots or celery. Edison 05:02, 28 March 2007 (UTC)

Also, you can play Tetris on the oscillosope by pressing the two buttons next to the knob. Titoxd^{(?!? - cool stuff)} 05:41, 28 March 2007 (UTC)

WP:AGF. WP:AGF. WP:AGF. MAC DAVIS, WHY DID YOU POST THIS NONSENSE? Nimur 16:38, 29 March 2007 (UTC)

WP styling mystery

[prolly belongs in Computing, but bear with me...]

So this is strange. The little wave-the-clock illustration above is supposed to look like:

12:34:56   (stationary)

1          (moving
 2          quickly
   3        downward
    4       .
      5     .
       6    . )

But the  's I carefully formatted it with, that show explicitly in the wikisource and that display fine when I preview, are turned into ordinary spaces when the page is rendered for real. Yet this happens only here on the RD, not (say) on my sandbox page when I experimented with it just now to see if there might be a workaround. What gives? —Steve Summit (talk) 03:52, 28 March 2007 (UTC)

It is the <code> tag. Titoxd^{(?!? - cool stuff)} 05:41, 28 March 2007 (UTC)

What, the <code> tag turns &nbsp; into plain space? What kind of sense is that? Let's try it without the <code>:

12:34:56   (stationary)

1          (moving

 2          quickly

   3        downward

    4       .

      5     .

       6    . )

Nope. Same problem. —Steve Summit (talk) 12:41, 28 March 2007 (UTC)

Huh. Now the original one has fixed itself (though the one without the <code>'s is still broken). I wonder if the archiving made a difference? —Steve Summit (talk) 00:12, 31 March 2007 (UTC)

hormone therapy capillary breakdown

does there exist a link between hormone therapy in later life for menopause symptoms and capillary bleed out shown on the face?

Multimillion dollar idea?

Shortly after purchasing a new Atari 512 personal microcomputer which sported a new built-in 3½ inch floppy disk drive to replace the TRS-80 Model II business computer I was using the thought occurred to me: wouldn’t it be nice if you could take this little diskette that fits so nicely in a shirt breast pocket with you to any retail store and have the receipt (in addition to the printed paper receipt) recorded on it for use as data input for your own personal computer at home. Since then most businesses have upgraded from virtually indestructible receipts printed on real paper using real ink to thermal receipts which degrade faster than the store return policy deadline. Updating this idea by replacing the 3½ inch floppy diskette (although they are still in use for various reasons) with a magnetic strip card, smart card or even a ram disk my question is: wouldn't this be a really great idea for some little startup venture company in Silly-Conehead Valley to make the founder rich, rich, rich? 71.100.175.98 05:18, 28 March 2007 (UTC)

Thanks for the idea. I'll remember you while drinking my pina colada. Well, they'd need to have some way to make sure people don't forge data or change data too. --Wirbelwindヴィルヴェルヴィント (talk) 05:39, 28 March 2007 (UTC)

Well, I would hope that you would remember me like mom and dad with a big fat check every year! Actually such a receipt could have an encrypted duplicate for official purposes but the true purpose of my intent was simply assist me in keeping home inventory by handling order management so that all I had to worry about was digging out the printer from beneith all the other computer stuff whenever the old hardware said it was time to go to the store. 71.100.175.98 05:51, 28 March 2007 (UTC)

Mmm, then a completely electronic version would be nice. Such as, you would be able to get the data off the store's site or something. Because a hardcopy and an electronic copy is probably going to slow down the line in most cases. And with my luck, I'll be stuck behind the ones who write checks that take 5 minutes to process and then digs all over their purse trying to find their little memory stick for their receipt. --Wirbelwindヴィルヴェルヴィント (talk) 05:55, 28 March 2007 (UTC)

Well actually the thin "temporary" magnetic cards such as Wal-Mart and Home Depot and other stores use for refunds without a receipt would most certainly be sufficient for the purposes mentioned above. 71.100.175.98 06:21, 28 March 2007 (UTC)

It would be cool to use it for retaining warranty information and such too. The issue of forgery could be dealt with using encryption. The store could encode the data both in clear-text for you to read and in encrypted form. When time came to prove that the data was real, a public-key decrypt of that data would prove that the item number, price, store number, date and storage-media serial number all match up. A standard magnetic stripe card won't work though - a credit-card type of card can only store about 1000 bits of information. That's only about 150 characters - just barely enough to store one purchase. If you went to a supermarket and bought 50 items, the card wouldn't have enough capacity for just that one visit - for an entire year's worth of purchase you'd need much more than that. Even a floppy disk at 1.44Mbytes would be marginal. A 'smart-card' could contain comparable amounts to a floppy disk - half megabyte smart cards are common - multi-megabyte cards are in use in cellphones and such. SteveBaker 15:43, 28 March 2007 (UTC)

I certainly find such an idea to be technically feasible, now the issue is the business case for it. I would expect it would be most valuable for businesses which sell to other businesses, such as office supply stores. If a secretary is sent out to buy office supplies, it would be far more efficient to have an electronic receipt for accounting than many paper receipts which would have to be manually entered into the system, possibly introducing errors in the process. This convenience, in turn, might make business customers more willing to shop at stores which offer this service. This increase in customers might make up for the added expense of the office supply store maintaining a dual receipt system for those customers who still want paper receipts. I would expect there to be less of a business case for those companies which sell directly to the public, and not to other businesses. For example, how many people would want an electronic receipt for a movie ticket ? There are also some disadvantages, like the inability of the customer to verify that the receipt is correct when they receive it (unless they have reading equipment right there). For example, I often find mistakes on my grocery receipt, and have them make the corrections immediately. If I didn't find the mistakes until I got home, it wouldn't be worth the effort of returning to the store to make the correction. StuRat 17:00, 28 March 2007 (UTC)

The way to market such a thing would be to make a gadget that is plug-compatible with the little thermal printers in cash registers that has a USB port or something on the side. You could fit it inside a cash register and make it copy everything that goes to the thermal printer off the USB port. Rather than have some special card reader - just use a standard memory stick. Give those gadgets away for free to the right kinds of business supply stores and sell them at cost to manufacturers of cash registers. Then make your money selling software for the customer's PC's to read the (encrypted) files from a USB drive and put it into the right format for all of the common business/accounting software. I think that's a viable buainess model. SteveBaker 17:41, 28 March 2007 (UTC)

Limit?

A transmission electron microscope.

Is there a limit to how many photons can fit in a prticular space? (I guess that would be proportation.) And is there a limit to how many photons can be emited from a (any?) particular source? What I'm trying to ask is the concept of intensity, but I don't really know how to describe it. Hope my last sentence helps. Thanks.100110100 06:29, 28 March 2007 (UTC)

At the same time? 71.100.2.150 06:38, 28 March 2007 (UTC)

Yes, how many photons can fit in a specfic space at the same time. We are presuming the photons are not moving.100110100 06:42, 28 March 2007 (UTC)

I think you'll have a problem there, photons always move at the speed of light, because they have no mass. If they have no mass and they aren't moving, they have no energy, and thus they don't exist, so your question is "how many nothings can you fit in a particular space?" which is division by 0 and undefined!? Capuchin 06:50, 28 March 2007 (UTC)

Heh, there's a number of things wrong with your logic Capuchin. :) For the original question, perhaps it would be something involving... an infinite amount. Perhaps? At first I thought the answer would have to do with the wavelength of the photon, but that's irrelevant since it is 3D space we're looking at. [Mαc Δαvιs] (How's my driving?) ❖ 06:57, 28 March 2007 (UTC)

Yeah I realise that, I just like making people think a bit :p Capuchin 06:58, 28 March 2007 (UTC)

Yes, I knew that photons are always moving. Ok, let my pose my question in a different way. How many photons can hit a specific point in space at one time? Thanks.100110100 07:00, 28 March 2007 (UTC)

Photons are [bosons] so infinitely many can fit into one quantum state, so choosing a quantum state that fills your space (the photons could be moving by bouncing in a box say, not stationary) you could have infinitely many. Although of course this would give you infinite energy, pretty hard to contain!137.138.46.155 07:47, 28 March 2007 (UTC)

Then again, we've not only had bosonic condensate, but fermionic condensate. [Mαc Δαvιs] (How's my driving?) ❖ 18:34, 28 March 2007 (UTC)

If you call it a Singularity then you might not also be able to call it space but probably where an infinite number of photons are located. 71.100.2.150 08:46, 28 March 2007 (UTC)

Thank you. Is photon energy?100110100 07:48, 28 March 2007 (UTC)

Energy can be infinite in any point in space?100110100 08:19, 28 March L2007 (UTC)

I'd just like to comment that Capuchin's first comment was correct. There is no such thing as stationary light. Light "stops existing" when it stops moving. —LestatdeLioncourt 13:24, 28 March 2007 (UTC)

Yes, but his application of that fact to the question was in error. The fact that photons can't be stationary does not prevent an infinite number of photons from existing in one space at one time. Whether anything else prevents this, such as some type of quantum interference, I don't know. StuRat 16:46, 28 March 2007 (UTC)

So, where we stand now, it seems to be you can fit as many photons as you have into even one particular point in space, simply because they are bosons? [Mαc Δαvιs] (How's my driving?) ❖ 18:34, 28 March 2007 (UTC)

Wait a second... there's this thing called the Bekenstein bound. Bekenstein got a bound on the entropy of a black hole, and according to his work he got it to work also, possibly for any space. A question is “Is there a finite amount of information in a spacetime unit?” Some people say yes, some people say no, and this is what we are discussing aren't we? If we call a photon a small collection of information as we would any particle, say, particle x, we would write down the quantum spin, location, energy, ect. According to the HUP, the precision of information you want to detect is proportional in some way to the amount of energy it takes to record it (think atom smashers, HRTEM). So, I bet there is a bound between how much information you can get, because the more precise you get, the more information you get, the more energy you spend, and there is a bound on energy density, something like the Schwarzschild radius, because if you pass the bound on energy density, spacetime collapses, and you get a black hole.

$r_{S}={\frac {2Gm}{c^{2}}},$

r_S is the Schwarzschild radius, G is the universal gravitational constant, m is the mass of the gravitating object, and c is the speed of light in a vacuum.

Gerard 't Hooft, from the Netherlands made a very general principle called the holographic principle. It is very controversial and vague, and postulates that any physical system, has a finite amount of discrete information, of 0s and 1s, bits, and the amount grows, proportional to the surface area of the spacetime unit grows (not as the volume, interestingly, just like "black hole physics"). So it looks like a holographic projection onto a plane, of a three-dimensional system, and it is usually explained in popular science by saying that looks like it is saying something like that things are two dimensional instead of three dimensional, which is a relation in saying that things might be instead of four dimensional, more, and we are actually only living in a hologram of a larger reality. Kind of like in the Matrix, how there is a larger reality that is much out of our sight, that the reality normally seen is only a projection.

A non-rotating black hole's photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. If it is zero-thickness, then that means photons would pile on top of each other, kind of, overlapping each other. So? I guess the answer is still "I don't know" Sorry for probably a confusing and maybe speculative reply. [Mαc Δαvιs] (How's my driving?) ❖ 18:50, 28 March 2007 (UTC)

But I'm trying to use relativity to explain the quantum. Maybe it's more simple. Photons are bosons, so the answer is yes. "As many as you want." [Mαc Δαvιs] (How's my driving?) ❖ 17:36, 29 March 2007 (UTC)

See Energy density. Nimur 16:40, 29 March 2007 (UTC)

Photosynthesis.

I realise that the equation for photosynthesis is given both in our article and in most textbooks as $6CO_{2}+12H_{2}O\rightarrow C_{6}H_{12}O_{6}+6O_{2}+6H_{2}O$ Obviously 6 H2O on each side can could be cancelled. Why isn't it? What role do the extra water have? Capuchin 06:53, 28 March 2007 (UTC)

No, the 6 H2O cannot be cancelled. That's as far as I can help you with it. The rest has to do with chemical balancing, & I'm sure other people could help you with the, more complicated, pedology.100110100 07:05, 28 March 2007 (UTC)

Think of the equation as already in lowest terms.100110100 07:06, 28 March 2007 (UTC)

I'm aware of this, hence this question, but I don't know why those extra water are there :), maybe a small edit to the original question. Thanks anyway :) Capuchin 07:13, 28 March 2007 (UTC)

Your welcome. Maybe I am also at fault. On the left side you have 6*2+12 oxygens. That means you must have the same on the other side. If you cancel out the 6 H20s on both sides, the equation will not balance.100110100 07:25, 28 March 2007 (UTC)

I could restate the question as "Why is $6CO_{2}+6H_{2}O\rightarrow C_{6}H_{12}O_{6}+6O_{2}$ wrong?" Capuchin 07:35, 28 March 2007 (UTC)

Hhhhmmm, ok you might be right, maybe I was thinking of somthing else, sorry to cause you trouble.100110100 07:44, 28 March 2007 (UTC)

It's okay, I still love you, now, anyone know why those extra water are there? Capuchin 07:46, 28 March 2007 (UTC)

uhm total amount of H's is the same there buddy (12H2 = 24 H) —The preceding unsigned comment was added by 145.24.133.60 (talk) 09:11, 28 March 2007 (UTC).

It's the same in both, isnt it? 24 in the first one and 12 in the second. Capuchin 09:14, 28 March 2007 (UTC)

OK, you can cancel the waters out if you are only interested in the net results of the reaction. However, in reality, the waters on the left are split to make the oxygens on the right. The waters on the right come from the carbon dioxide on the left. This was unexpected as it was easier to imagine the oxygen coming from the carbon dioxide. The general reaction is CO₂ + H₂A -> CH₂O + 2A + H₂O. A can be oxygen OR it can be S in some green photosynthetic bacteria. An experiment was done with ferricyanide too, and eventually radiolabelled water proved the source of the oxygen was water not carbon dioxide. Have a look at a biochemistry text book if you happen to have one lying around - most of these details were from Voet and Voet. Good question, btw! Aaadddaaammm 09:42, 28 March 2007 (UTC)

Deindented. Thanks, this is exactly what I suspected but didnt know the specifics of it. Thank you! Capuchin 09:49, 28 March 2007 (UTC)

Energy = Photon?

Is energy photon? Is photon energy? Thanks.100110100 07:49, 28 March 2007 (UTC)

Energy is a bit more of an abstract concept than that — photons have energy (radiant energy) and are an extremely basic form of energy transfer but they are not, in and of themselves, the definition of energy. See the "forms of energy" section of the energy article for some more detail. --24.147.86.187 09:46, 28 March 2007 (UTC)

A photon is a quantum of energy, so I guess that you can say that it is energy. This issue once came up in class, when a friend of mine asked what happens to a photon after its energy is absorbed by an electron (we were studying the photoelectric effect and Bohr's model of the atom). I guess this must be due to a common mistake in phrasing: when we say "a photon's energy", it's as if we're implying that photon has energy apart from other things. A photon is really nothing but energy. —LestatdeLioncourt 13:20, 28 March 2007 (UTC)

A photon is really nothing but energy. As is all other matter. -- mattb @ 2007-03-28T16:09Z

While all matter can be converted to energy, I wouldn't go so far as to say that it currently is energy. That would be like saying that a pile of construction material is currently a house. StuRat 16:40, 28 March 2007 (UTC)

You can easily be confused by it, but matter is not energy. As StuRat said, matter can be entirely converted to energy, but it is not energy, and vice versa. The amount of energy you can theoretically get is where E=mc² comes from. [Mαc Δαvιs] (How's my driving?) ❖ 18:29, 28 March 2007 (UTC)

Okay, granted, I should've said "there is a matter and energy equivalence". I only meant to hint that there's a reason photons (and all quanta) have a special name rather than simply being called "energy". -- mattb @ 2007-03-28T18:55Z

Whether matter is a form of energy, or is only something that can be converted to and from energy, isn't a well-defined question, because the term matter isn't well-defined. It isn't possible in practice to say precisely how much of a closed system is matter, vs. how much is energy. "Mass" is well-defined, "energy" is well-defined, but "matter" is an old-fashioned, vague word that only really works well as a concept distinct from energy when dealing with non-relativistic phenomena. MrRedact 21:56, 28 March 2007 (UTC)

I didn't read matt's answer, and it was probably the best. Down at the quantum level, no body really cares what is energy or what is mass—it's the same, measured in various denominations of eV. I suggest reading our article, mass in special relativity. It is difficult to explain, but the invariant mass (more often known as "rest mass" is different from "relativistic mass." Perhaps Baez would be better[1]. Still kind of complicated. This answer is more comprehensive[2] [Mαc Δαvιs] (How's my driving?) ❖ 22:20, 28 March 2007 (UTC)

Energy

Energy can be infinite in any point in space?100110100 08:19, 28 March 2007 (UTC)

There is a finite amount of energy in the universe and it is conserved. Except for potential quantum weirdness that I don't know about, the answer should be no. --24.147.86.187 09:49, 28 March 2007 (UTC)

There would be infinite energy density at a gravitational singularity (a finite amount of energy, but in an infinitesimally small volume). However, this may be only a theoretical concept - I don't think a naked singularity has ever been observed. Gandalf61 12:57, 28 March 2007 (UTC)

According to the laws of physics at current there is a finite amount of energy. At gravitational singularities, the mathematics and laws of physics break down, so we aren't too sure about a lot of things involving that. [Mαc Δαvιs] (How's my driving?) ❖ 18:28, 28 March 2007 (UTC)

It is better to say, "current theory and observation supports a finite energy universe." We may be very, very wrong. Nimur 16:41, 29 March 2007 (UTC)

Chaos Theory

What is the Chaos Theory and who propounded it? —The preceding unsigned comment was added by Bhavikmehta2685 (talk • contribs) 08:19, 28 March 2007 (UTC).

You know, this place works somewhat like an encyclopedia. Capuchin 08:22, 28 March 2007 (UTC)

We appreciate it if you try to look it up yourself, and if you can't understand it ask for help in understanding it. Next time you can just type in "Chaos theory" in the search box to the left at the top of the page and click "Go." [Mαc Δαvιs] (How's my driving?) ❖ 19:10, 28 March 2007 (UTC)

Linear beam theory

(Also cross posted to the Mathematics desk - sorry I don't know which one is appropriate since this is mostly an Engineering question.)

I've been trying to find references on the dynamics of a cantilever beam (assume Euler-Bernoulli beam equation) coupled to a forced mass-spring-damper system, but unfortunately I haven't found any that use a partial differential equation formulation rather than a Lagrangian approach. I'm trying to model this as $m{\ddot {u}}(x,t)+EIu''''(x,t)=0$ with the BCs $u(0,t)=0$, $u'(0,t)=0$ (fixed beam BCs) and $u''(1,t)=0$ (zero bending moment) and $M{\ddot {u}}(1,t)+2\zeta {\dot {u}}(1,t)+\kappa ^{2}u(1,t)+EIu'''(1,t)=A\sin(\omega t)$.

Does this look reasonable to anyone? Or should there be a $m{\ddot {u}}(1,t)$ term in the BCs as well as the $M{\ddot {u}}(1,t)$ one?

Also, if anyone has any suggestions on analytical solutions to this that would be great!

Thanks Davidbarton 09:13, 28 March 2007 (UTC)

That looks vaguely like a cantilever with a harmonic damper hanging off the tip. Off hand I don't think it has an analytical solution, in general mixing discrete and continuous systems results in an unsolvable equation. I'd use Rayleigh-Ritz method or FE, or look up the answer in Blevins. Another approach is the theory of receptances, which describes how to mate two simple systems, but I have forgotten how to do that. Neither Rao or Thomson discuss this case, that I can see. Greglocock 05:59, 29 March 2007 (UTC)

ethylamine

which reactant when added to bromine and sodium hydroxide give ethylamine?

X+Br2+NaOH = CH3CH2NH2

Technically this is not homework, it was an exam question and I can't seem to find it.Bastard Soap 11:14, 28 March 2007 (UTC)

Nothing obvious, especially since amines react with halogens to form N-X compounds, see chloramine. Cacycle 12:56, 28 March 2007 (UTC)

Hofmann rearrangement? DMacks 16:12, 28 March 2007 (UTC)

Yep, that's the one, Hofmann is my man. Any of you guys know the mechanism involved?Bastard Soap 23:37, 28 March 2007 (UTC)

Should be in any undergrad orgo text. The reagent soup forms electrophilic bromine, which halogenates the amide nitrogen (so Cacycle was actually exactly right:). You're under basic conditions, so deprotonate the N to get an anion, which rearranges to form an isocyanate: carbon attached to carbonyl of amide migrates to the N, halide is lost. From there, it's the same as in the Curtius rearrangement and Schmidt reaction (pages which have more mechanistic details about that rearrangement step, albeit in slightly different structures): hydrolysis and loss of carbon dioxide. DMacks 01:05, 29 March 2007 (UTC)

I deleted his reply by mistakeBastard Soap 10:58, 29 March 2007 (UTC)

Fire preservation

My school aged son is researching the properties of fire, and one area of interest is the techniques used in the past to "preserve" fire - such as the use of slow burning torches, carrying embers from home to home in the event a family's fire went out, etc. Any idea where we can find a discussion regarding of how fire was "preserved" in the past? Thanks —The preceding unsigned comment was added by Hedaku (talk • contribs) 12:31, 28 March 2007 (UTC).

There is some mention of this in our article Making fire. I've looked around for more details, but to no avail. --Zeizmic 14:33, 28 March 2007 (UTC)

I remember a fictional account of this in one of Jean Auel's books; I think it was The Valley of Horses. ike9898 19:18, 28 March 2007 (UTC)

Check a Boy Scout Handbook, and check the Foxfire series of books, and check Mother Earth News. I found re :banking th coals: [3]. Also [4] from "Plimouth Plantation . As a safety note, don't do this, since the fire can also blaze up unexpectedly and start a conflagration, expecially with campfires or a fireplace or firering which would allow flames or sparks to reach combustible substances. There were ways of carrying an ember from house to house, but I could not find a reference. Edison 23:02, 28 March 2007 (UTC)

See specifically "The Foxfire Book" edited by Eliot Wigginton, Anchor Books, NY, 1972, ISBN 0-385-07353-4.The chapter "Cooking on a fireplace, Dutch oven and wood stove, says that (p 160) before modern furnaces were in homes, fires were kept burning all day in the winter to heat the house. After an hour or so there would be a bed of hot coals from the partially burned wood, which could be used to roast potatyoes and othr foods such as nuts and onions, or even to bake bread. At night the coals were covered with a thich layer of ashes. In the morning the coals were exposed and fresh wood was added and the fire would spring up. In the summer, ashes were raked over the coals between meals when cooking was done. P162 says coals from the fireplace could be used to start a fire on the wood burning iron stove. My own grandmother grew up living this way. Edison 16:51, 29 March 2007 (UTC)

airborne radio direction finder

explain the principle of operation of airborne radio direction finding system with block diagram? —The preceding unsigned comment was added by Ambuj0542 (talk • contribs) 12:36, 28 March 2007 (UTC).

We have several articles about this topic; they may help you with your homework.

Atlant 12:37, 28 March 2007 (UTC)

Well fancy this... Direction finding -- mattb @ 2007-03-28T14:23Z

Blood pressure during sickness

I woke up today rather ill indeed, and while this is of course something I should ask a doctor about rather than you lot, I was wondering if it is common to experience higher blood pressure and a higher pulse during illness similar to flu. 81.93.102.185 16:00, 28 March 2007 (UTC)

Do not request medical or legal advice. Ask a doctor or lawyer instead. --TotoBaggins 17:09, 28 March 2007 (UTC)

I know I definitely do. It's terrible, I can feel it in my eyes too. [Mαc Δαvιs] (How's my driving?) ❖ 18:25, 28 March 2007 (UTC)

Thank you, Mac Davis. I was only worried if this was an additional thing gone wrong. I will in any case make contact with a doctor within a day, I need notification for school anyway... ;) 81.93.102.185 18:55, 28 March 2007 (UTC)

what the ****

I have been blocked, what happened? No, seriously I have been. My computer signs out whenever I go into a reference desk, so I have to sign back in again each time, and occasionally I forget and go to answer a question. But today it has suddenly started coming up saying I have been blocked whenever I do this. But it goes away when I sign in again.

Also what is up with all the alien related questions the reference desks have sprouted recently, did I miss something important?

HS7 19:12, 28 March 2007 (UTC)

I see no reason to believe you've been blocked. -- mattb @ 2007-03-28T19:45Z

Unless of course someone applied an anon only block to your IP for some reason, sharedip? public computer? anything like that?--VectorPotential^Talk 19:48, 28 March 2007 (UTC)

What alien questions? I can only think of one or two farther up the page. Now the seagull questions... now that's another story! xD [Mαc Δαvιs] (How's my driving?) ❖ 19:51, 28 March 2007 (UTC)

One question frequently spawns many similar questions. I've even been caught spawning, from time to time. StuRat 20:37, 28 March 2007 (UTC)

Are teeth techically considered bones?

I don't think that they are, but I didn't see the question clearly answered in either article (although I may have missed it, I was skimming). What is the technical name for the type of tissue that makes up teeth? Thanks ike9898 19:15, 28 March 2007 (UTC)

I think the outside bit is called enamal, which is almost bone :) No idea what the rest is called, but I suspect all of my whatever it is have rotted away, so they can't be that important :( HS7 19:18, 28 March 2007 (UTC)

Actually, enamel isn't "almost bone". It's really not even a tissue, since tissue contains cells and enamel is acellular. Moreover, enamel is much harder than bone; in terms of hardness, dentin (which comprises a good hunk of a tooth's volume) is between bone and enamel. --David Iberri (talk) 21:37, 28 March 2007 (UTC)

Look at the article teeth about a third of the way down a diagram shows what a tooth is 'made of'. Look up each respective 'item' and you can probably make a judgement. bone will help as it defines a little more 'openly' what consistutes being called a bone. ny156uk 19:37, 28 March 2007 (UTC)

Thanks for the help but I am really looking for an answer from the perspective of histology. Histologists seem to have a heirarchical classification for tissue types and I would like to know how bone and tooth relate to each other in this sense, i.e. are they both members of some larger class? What is that class called? ike9898 20:40, 28 March 2007 (UTC)

Well, you probably won't like the answer :). But the study of the microscopic structure of teeth and bones is called "hard tissue histology". Sorry it couldn't have been something fascinatingly Latin.... - Nunh-huh 20:58, 28 March 2007 (UTC)

I guess I'm not being clear. I'm not interested in the name of the study of teeth and bones. My real question is at the title of this section. Are teeth technically considered bones or not? Assuming they are not, can someone point me in the direction of something that explains the relationship between teeth and bones? ike9898 21:06, 28 March 2007 (UTC)

No, teeth are not bones. They are histologically distinct. Teeth and bones are both classified as "hard tissue". - Nunh-huh 21:11, 28 March 2007 (UTC)

The differences between the two are pretty great. The most salient similarity to me is that both are made largely of hydroxyapatite (calcium and phosphate crystals). In teeth, these crystals comprise much of the dentin; in bone, they form the inorganic component of bone matrix. Cheers, David Iberri (talk) 21:34, 28 March 2007 (UTC)

Just to reiterate the above: no, teeth are not bones. (They're often included in skeletons probably because they, like bone, contain large amounts of mineralized matrix that survives decomposition.) — Knowledge Seeker দ 06:24, 29 March 2007 (UTC)

On the other limb, caries affects both teeth and bones!

Photons

Will enough photons in a singularity collapse into a black hole?100110100 19:42, 28 March 2007 (UTC)

Great questions :) I suggest you should ask them on physicsforum's general physics discussion[5]—you can get better answers there from actual physicists and professors, then here. See Geon (physics). [Mαc Δαvιs] (How's my driving?) ❖ 19:43, 28 March 2007 (UTC)

I'm going to venture a guess: no, since photons have no rest mass, so I don't believe they themselves distort spacetime. This is nothing but a wild guess by someone who is totally unqualified to answer, though. -- mattb @ 2007-03-28T19:47Z

The Einstein field equations say oppositely I believe, and the article I suggested does too. [Mαc Δαvιs] (How's my driving?) ❖ 20:00, 28 March 2007 (UTC)

Yes, I believe that once the energy density of a small region becomes high enough, a singularity should form. There's nothing that says it needs to be in the form of (rest) mass; but it'd take a hell of a lot of photons directed simultaneously at a single point. Penrose-Hawking singularity theorems briefly touches on the notion, too. Spiral Wave 00:46, 29 March 2007 (UTC)

Bug

Can anyone help the Test Match Special commentary team and identify this insect? It's in Guyana, if that's any help. Thanks! →Ollie (talk • contribs) 20:18, 28 March 2007 (UTC)

I'm not sure, but I sure wouldn't want to be on the soccer team which has to play against it ! StuRat 20:34, 28 March 2007 (UTC)

But that wouldn't be cricket. Skittle 22:18, 28 March 2007 (UTC)

Below Freezing Temperatures & Pipe lines

How does a city keep the water and seawge pipes running when the temperature is below 0 C? --Shines8 21:04, 28 March 2007 (UTC)

The pipes are buried beneath the frost line. --Steve Summit (talk) 21:27, 28 March 2007 (UTC)

And note that even under the frost line water can turn to ice in city pipes in cold enough weather, such as described in this article from New Jersey. Pipes in homes can also freeze, and one way to help prevent pipes from freezing is to keep your water running a bit. I wouldn't be surprised if the fact that water is probably running almost constantly in large city pipes further reduces the risk of those pipes freezing when underused or unused pipes at similar depths might (just a guess). --Dugwiki 21:48, 28 March 2007 (UTC)

And occasionally a pipeline runs above the frost line (crossing a bridge, for example), but in that case, the diversity factor helps ensure that water is always flowing in the pipeline, even if it's hard to believe that people are flushing their toilets that late in the cold night. --Atlant 22:39, 28 March 2007 (UTC)

Thanks for the answer. I understand that having the pipes below frost line, has an effect of insulating it from cold. I was just wondering, in my city, it sometimes goes to to -25 C, and that times when I trun on the hot water tap there is so much pressure (its like it wants to come out), whereas the cold water tap seems to have no pressure (it doesnot want to come out). Do you know why this happens? The hot water boiler is at the basement of my apartment building. --Shines8 00:33, 29 March 2007 (UTC)

In the kind of system in my house, the pressure of the incoming water supplies pressure to all the pipes, both hot and cold. So if I found the cold water pressure lower at the tap, it would indicate a partially blocked pipe. However, I know nothing about apartment systems. Possibly the hot water system does not pass the pressure through from the input pipe and has a separate pump, but that's just a guess. Why not ask the building manager? --Anonymous, March 29, 2007, 01:26, edited 06:00 (UTC).

A brief pulse of high pressure happens because the cold water heated in the tank expands and a valve in the cold water inlet tap prevents it getting out.Polypipe Wrangler 01:50, 8 April 2007 (UTC)

Possible explanations for the hexagonal storm on Saturn?

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• 

I read an article in the paper today about the hexagonal storm at Saturn's north pole, but I haven't found any freely accessible articles, either here or elsewhere, that offer possible explanations for the weird shape (why isn't it elliptical or circular?) Can anyone post some reasonable natural scenarios which would produce a hexagonal stormfront? Just curious, thanks. Dugwiki 21:15, 28 March 2007 (UTC)

It most likely has to do with this[6]. I do not believe this is natural on Earth, but that does not make it void for Saturn, the wind velocities are much higher there. The pentagon shown looks a lot like the hexagon on Saturn. I'm sure we'll have a picture uploaded soon. [Mαc Δαvιs] (How's my driving?) ❖ 21:51, 28 March 2007 (UTC)

Thanks, very interesting article and experiment. That definitely sounds to me like a likely explanation. Dugwiki 22:28, 28 March 2007 (UTC)

It's not really an explanation, just an observation that we are able to create the same effect in a bucket. While fascinating, we don't have any model that would predict exactly which polygons should occur under which conditions. It's still a mystery. StuRat 00:19, 29 March 2007 (UTC)

How bizarre, I've never seen anything like that before. Could it be the result of some resonant (m=6) mode, perhaps? That Nature article doesn't really suggest anything, but perhaps someone else has attempted the problem since then. Spiral Wave 00:35, 29 March 2007 (UTC)

Do we know why certain crystals, such as snowflakes, are hexagonal (or form other polygon shapes) ? If so, perhaps a similar cause underlies this phenomenon. StuRat 00:38, 29 March 2007 (UTC)

Snow#Geometry offers a couple of explanations, neither definitive, but I don't think the process can be the same - that's a bottom-up process, this looks more like the collapse of a larger (non-)structure. The Saturn article suggests it may or may not(!) be a standing wave, which would also have been my first guess. But that still doesn't explain how it formed, of course. Spiral Wave 00:53, 29 March 2007 (UTC)

The thing that freaks me out most is that whilst the experiment in the spinning bucket elegantly demonstrates how this hexagon came to be as a perfectly natural phenomenon - it goes nowhere in explaining why Saturn has this at only one of it's two poles. If it were a natural result of fluid dynamics at a particular speed and with a particular type of fluid - why isn't it there at both poles? SteveBaker 18:05, 29 March 2007 (UTC)

A complete wild guess, but it could be seasonal, and shift from pole to pole between winter and summer. Saturn is in roughly the same place in its orbit now as it was when the Voyager images were taken of it 30(-ish) years ago, and we haven't been observing it very closely in the intervening time. Saturn's wind systems aren't perfectly symmetrical from hemisphere to hemisphere either, so vanishing from one hemisphere needn't imply it should appear at the other. Perhaps we'll be able to rule that one out in about 15 years! Spiral Wave 23:31, 29 March 2007 (UTC)

Observing drug resistance developing by mutation

Hello. I am a layman interested in various evolution issues. Is there a clear example of a bacteria population adapting to antibiotics or other drugs, where the trait that gives drug resistance clearly came from a mutation and NOT from recombination, transposons, or some form of DNA transmission? Actually, any example of a population that has clearly adapted to its environment due to mutation, rather than recombination, will do. Also, how do we prove that it really is mutation and one of those other factors? I asked a similar question before, I hope no one minds. 69.223.174.232 22:05, 28 March 2007 (UTC)

Mutation is relatively rare, so one would have to observe thousands, millions, billions, or even trillions of cells to actually observe a mutation taking place, at least under normal circumstances. However, if the cells are exposed to mutagens, the odds of detecting a mutation can be significantly improved. StuRat 22:16, 28 March 2007 (UTC)

One common cause of resistance to the antibiotic vancomycin is a single changed amino acid. DMacks 22:44, 28 March 2007 (UTC)

Yes, there are many examples. It's pretty easy to comprehend, too. Since many antibiotics work by interfering with an enzyme, a mutation causing a change in the structure of that protein could cause resistance. With enough infections and enough antibiotic use, the chance become quite high. That being said, if you mean a case where we actually observed a bacterium developing the original mutation, I'd say that's quite unlikely. — Knowledge Seeker দ 06:29, 29 March 2007 (UTC)

Mutation can be induced fairly easily in bacteria. If you cultivate bacteria on a petri dish with a low concentration of antibiotic, they will develop resistance to the drug as shown by the colonies that survive. Bacteria do not go through meiotic recombination, and a colony of "clones" can be easily isolated removing the chance of DNA transmission from outside. This leaves transposons, and outright mutation of genes as the source of resistance. Since transposons are genes that move from one place to another within the bacterial genome, resitance can not come directly from a transposition, unless an error (or mutation) occurs in the transcription process. -Czmtzc 16:10, 29 March 2007 (UTC).

These guys are irradiating rice to induce mutation, and then selecting the ones with desirable qualities for further breeding. They've released three strains so far. --TotoBaggins 05:01, 29 March 2007 (UTC)

There are lots of examples of this out there - it's a truly proven thing. But my favorite example is Lactose tolerance in humans. Evidently in the times of the early Egyptians (a few thousand BC) all adults were lactose-intolerant because in times prior to widespread farming practices, there was evolutionary benefits to babies being tolerant of lactose in their mother's milk only for as long as it took them to wean onto solid foods. Beyond that point lactose intolerance would kick in and the growing child would be unable to drink the milk that younger siblings might need. Lactose intolerance is "normal" in most mammalian adults for this reason. When we started keeping (and being able to milk) cows, goats and sheep - the inability of adults to tolerate lactose became a negative thing because in times of famine we'd have to slaughter cattle for food rather than living on milk and cheese. Over the past 4,000 or so years, we have evolved lactose tolerance in adults. The process clearly isn't quite complete because lactose intolerant adults are still to be found. Interestingly this 'problem' of lactose intolerance is most common in more civilised societies where famines are essentially unknown and there is no longer evolutionary pressure to complete the process. The genetic difference is clearly that the ancient mechanism that used to turn off our ability to digest lactose at around two years of age must have malfunctioned (perhaps in just one individual) as a result of some kind of mutation - descendents of that person could survive and reproduce when others died (or at least were too sick to breed) because the only option left was to drink milk and eat cheese or slaughter your livestock for food causing worse problems on the following year. We tend to think of evolution as something that happened millions of years ago and to other species than us - but it's very clear from this example that it's still an ongoing process in humans. SteveBaker 18:00, 29 March 2007 (UTC)

Interesting stuff, but our map of lactose intolerance seems to tell the opposite story to what you're saying in the second half of your post. The US and Europe may or may not be more "civilized" than sub-Saharan Africa and East Asia, but they can hardly be said to to have suffered more famines. --TotoBaggins 21:43, 29 March 2007 (UTC)

As I recall, it's somewhat complicated by the fact that the domestication of milk-producing animals started later in places where 'hunter/gatherer' lifestyles prevailed - so the native populations in those places have had less time to evolve. I need to go back and find where I read about this...I recall an article pointed at by something that was in turn pointed at by Slashdot a couple of months ago. I know that this hardly counts as an adequate reference! Also, some less advanced societies (such as the Kenyan Maasai) have come up with ways to 'fix' the problem of lactose intolerance by mixing cows blood and urine with the milk before adults drink it. One presumes that this does something to the chemistry to make the milk more digestible - I have no clue what. SteveBaker 05:05, 30 March 2007 (UTC)

I seem to recall that they mixed cow's blood and milk and drank it, but not urine. The urine they used as a "disinfectant" on wounds and such. They didn't kill the cows to get blood, either, but cut them and took an amount that wasn't too much for the cows to handle. StuRat 00:30, 31 March 2007 (UTC)

Iapetus (moon of Saturn) as a Death Star-style artificial alien world/spacecraft thing...

This crackpot-sounding theory used to be mentioned in the Iapetus (moon) article but all reference to it seems to have been deleted now. Anyone able to remember the main gist of it? It was an entertaining, thought provoking theory - even if it was 99.99999% net.crank BS. --Kurt Shaped Box 22:10, 28 March 2007 (UTC)

See Mimas (moon)#Mimas_in_fiction_and_film. StuRat 22:19, 28 March 2007 (UTC)

Is this what you are talking about? →Ollie (talk • contribs) 22:26, 28 March 2007 (UTC)

Na, it was definitely Iapetus - the 'great wall' around the equator was mentioned as being something to do with the 'framework' of the thing. As far as I can remember, it was postulated that what we see now is the result of millions of years of erosion stripping away the surface layers of the 'hull'. --Kurt Shaped Box 22:26, 28 March 2007 (UTC)

Yes Ollie! That's exactly what I was thinking of! Thanks very much! --Kurt Shaped Box 22:27, 28 March 2007 (UTC)

I always liked the way Iapetus has the same circular indentation in its northern hemisphere. I thought the resemblance was striking long before anyone pointed it out to me! Spiral Wave 00:27, 29 March 2007 (UTC)

Maybe that was the exhaust? ;) --Kurt Shaped Box 00:42, 29 March 2007 (UTC)

As a matter of interest, how much energy would be required for a spacecraft this size (1.805635±0.000375×10²¹ kg according to the article - presumably heavier when spaceworthy) to reach the speeds necessary (say, a significant %age of the speed of light) for interstellar travel? --Kurt Shaped Box 00:54, 29 March 2007 (UTC)

Ignoring relativity (because it's too much effort) a quick calculation shows that getting that mass to about 10% light speed would require about 8×10³⁵J, or approximately 200 yottatons of TNT. Equivalently, (and again ignoring relativity) to get any object to 10% light speed you'd need to annihilate 0.5% of the object's mass and somehow harness all that as kinetic energy.

Short answer: a lot. Algebraist 01:34, 29 March 2007 (UTC)

Could humans build something that big? I don't mean make it fly, just create a structure of that size in space? --84.69.54.85 01:50, 29 March 2007 (UTC)

Not at present, no, but way in the future, why couldn't we even build a Dyson sphere ? StuRat 03:39, 29 March 2007 (UTC)

Whoa. I'd never even heard of the yotta- prefix! Big bird. That's ignoring the energy required to slow it down and stop at the other end too. --Kurt Shaped Box 10:47, 29 March 2007 (UTC)

Yeah - I wouldn't want to try aero-braking on that sucker! SteveBaker 17:12, 29 March 2007 (UTC)

But as to the effort to "build something that big" - I think it depends on what you mean by "build". Think "hollowed out moon" and not "death-star construction". If you started with a suitable "icy moon" of that size - then hollowed out your living quarters, space for the engines - and gradually consumed the rest as fuel in getting it from wherever it was to wherever you want it to be...then it might not be outrageous. Assuming you have the patience to do it over many, many generations, you could use fairly small engines over a L-O-N-G period of time to push it around. Once you get it closer to the sun (which is relatively easy because it's "downhill"), the ice on the surface would start melting, you could sculpt even fairly large features just by laying reflective foil over the areas you want to keep as mile-thick 'solid structure' (made of ice of course) and dumping some carbon-black over the areas you'd like melted into ocean-sized fuel tanks. Pumping the warmer liquid down into the core of the planet would allow you to hollow out big spaces for vast living quarters, etc. The internal volume of even a small moon could be turned into continent-sized living areas. If you were prepared to spend lots of effort and many thousands of years - I think you could maybe do it with present day technologies. We tend not to think much beyond our own lifetimes - so such a project is unlikely - but maybe you'd need to do it because you saw some looming catastrophy that required you to relocate the entire ecosphere someplace. The sun going nova or something nasty like that...It's certainly not impossible. SteveBaker 17:12, 29 March 2007 (UTC)

I disagree that it's easier to go towards the Sun than away. Once in a stable orbit, I believe it takes approximately as much energy to move closer to the object you are orbiting as away from it. An exception is if you are close enough to the object to encounter resistance from gases and dust. StuRat 18:15, 29 March 2007 (UTC)

Um...maybe. Yes - perhaps you're right. But it doesn't really change the argument much. SteveBaker 04:51, 30 March 2007 (UTC)

Heh. I guess that there'd be room for more than two of every animal that way... ;) --Kurt Shaped Box 21:49, 29 March 2007 (UTC)

physics

what the difference between ampere and ampere-hour

—The preceding unsigned comment was added by 202.53.9.25 (talk) 22:15, 28 March 2007 (UTC).

An ampere is a measure of electrical current, and an ampere-hour is a measure of electrical charge. Dimensional analysis is something helpful here, if you want to multiply the current and time. Try reading the articles and wait for a better response, heh. [Mαc Δαvιs] (How's my driving?) ❖ 22:23, 28 March 2007 (UTC)

An ampere is a unit to measure the current, or rate of flow of charge. Amp-hour is a unit to measure the amount of charge the flows over a certain period of time. Ampere-hour is comperable to coulomb. 1 amp-hr = 3600 coulombs. Electricity terminology is sometimes easier to understant if u use water analogies. Think of ampere-hours as liters or gallons of water, while amperes are the rate at which water flows through a pipe (like 2 liters per minute or 5 gallons per hour).

Funny how nicely water and electricity mix. [Mαc Δαvιs] (How's my driving?) ❖ 22:28, 28 March 2007 (UTC)

The farting vegitarian

Why do some foods (beans, peanut butter, eggs..) make you fart? I noticed they're all alternative sources of protein... does that have anything to do with it?

I would suggest reading Flatulence. Has all the answers and more. [Mαc Δαvιs] (How's my driving?) ❖ 22:21, 28 March 2007 (UTC)

In a perfect world, there would beano gas. StuRat 00:22, 29 March 2007 (UTC)

But then we would all asphyxiate. − Twas Now ( talk • contribs • e-mail ) 15:24, 29 March 2007 (UTC)

Yes, that would certainly be flatal. StuRat 18:10, 29 March 2007 (UTC)

I heard that flatulence will get you anywhere. And if it doesnt, you can always make a big stink about it ;-) —The preceding unsigned comment was added by 88.110.24.123 (talk) 18:26, 29 March 2007 (UTC).

Well, that would presumably depend on how much of it one has... —Ilmari Karonen (talk) 23:05, 2 April 2007 (UTC)

Color of light

Why is the color of light usually described as a wavlength (for example 700 nm red) instead of a frequency? Wouldnt it be more apropriate, because frequency does not depend on the medium (air, glass, water) whereas wavelength does? for example that same light, will still be the same red when it passes into a lens or glass of water, but the wavelength will not be anything close to 700 nm.

The equation is this:

$\lambda ={\frac {c}{f}}$

Where lambda is the wavelength, c is the speed of light, and f is frequency. As you can see they are inversely proportional, and as far as I know, it is just convention. We could just as easily say the frequency. The higher the wavelength the lower the frequency, the lower the wavelength, the higher the frequency. [Mαc Δαvιs] (How's my driving?) ❖ 23:26, 28 March 2007 (UTC)

I'm not sure of the actual answer but if you look at all electromagnetic waves you generally find that at low frequencies (DC-RF), frequency is used as the main means of description. At higher frequencies wavelength is used. The dividing line seems to be in the THz band, which historically people have had great difficulty generating or measuring so hasn't been used much. I would guess that at the lower frequencies the radiation is often associated with electronics which operates at an easily measurable rate and that the wavelength is long enough to be practically difficult to measure, meaning that frequency is the obvious choice. This convention has now extended up to RF & microwave frequencies. On the optical side it was historically difficult to count fast enough and you could easily measure wavelength via macroscopic apparatus like Young's slits for example. At smaller wavelengths, like X-rays, you can again relate the wavelength to physical length measurements like atomic spacing, making wavelength the natural choice here. JMiall 23:38, 28 March 2007 (UTC)

Ah yes, that is right. In radio we designate radio frequency regions for different level licenses, high frequency, very high frequency, extremely high frequency. [Mαc Δαvιs] (How's my driving?) ❖ 01:28, 29 March 2007 (UTC)

The "level of the license" and the frequency of operation have nothing to do with each other. Amateur radio bands exist at many frequencies, including a few openings in sub-millimeter waves (normally those frequency ranges are for "space-craft communications" and "military radar").

Also, whether you measure in wavelength or frequency often depends on the machine/instrument you are using to measure the wave. The two are tightly related, and depending on your needs, you can work in either domain. It's a simple unit conversion detail - almost like "inch/centimeter" conversion: pretty irrelevant as far as the science goes; highly important to make sure you use the right units.Nimur 16:50, 29 March 2007 (UTC)

It's not color until it is received by our cone cells and perceived by our color vision system. So we don't really need to make allowances for traveling through different mediums since all of our eyes are the same. Also while a certain wavelength will be percieved as a certain color (say green), there can be mixtures of wavelenghts (say blue and yellow) that might produce the same color perception. Color is more of a subjective construct than a physical reality. If we want to do physics experiments we can just think of it as radiation and not worry about the color -- Diletante 01:53, 29 March 2007 (UTC)

Even that is an over-simplification. The word "Color" (or "Colour") refers only to our perception of frequency/wavelength. We have a very poor sense of colour vision indeed - although it's good compared to many animals - it's a lot worse than (for example) Goldfish. Our eyes have three kinds of colour sensors which are responsive roughly at the frequency of red, green and blue light. We do NOT sense the frequency of the light directly. Each of the sensor types is most sensitive to light at it's ideal frequency and progressively less sensitive at frequencies above and below that level. Hence, (for example) pure yellow light (such as might be produced by a sodium street lamp) at a frequency roughly midway between 'pure' red and 'pure' green will weakly stimulate the red sensors and weakly stimulate the green ones - and will not stimulate the blue ones hardly at all. As far as the red sensors are concerned, there is no difference between dim pure red light and bright pure yellow light. Our brains have learned to "see" yellow when both red and green sensors report roughly equal amounts of stimulus. In fact, the colour display on your computer can't even generate yellow light (that is to say light at the frequency of a sodium lamp)...when you put a patch of "yellow" up onto your screen, the computer is actually displaying a mixture of pure red and pure green that fools our brains into reporting that there is yellow because the two kinds of sensors are equally stimulated. This is a weird thought...but it's true. It's as if you could play two notes on a piano at the same time and instead of hearing a chord composed of two pure frequencies, you could only hear a kind of 'average' frequency that would be indistinguishable from playing a single key on the piano somewhere between the two you chose! Fortunately for musicians (but perhaps unfortunately for painters!) our ears don't work like our eyes in that respect.

Anyway - to return to the question. It doesn't matter that the speed of light is different in air and water because our retinas are sitting in a big puddle of liquid inside the eyeball - so only the speed of the light through that liquid matters - and that's a constant. So it doesn't matter whether you talk about the wavelength or the frequency - the result is the same because the speed of light through the optical humor is always the same no matter whether your eyes are underwater or not. SteveBaker 16:51, 29 March 2007 (UTC)

Seeing under water

How is it possible? doesnt the cornea lens depend on its refractive index being significantly higher than that of air/vacuum? doesnt it have a refractive index too close to that of water (after all, like other tissue, its mostly water) to be able to focus the light? even if it is slightly higher index, doesnt changing medium surrounding a lens drastically alter its focal length? Also, a related problem, how do cameras work under water.. same problem.. right? (asuming the camera's front lens is in direct contact with the water).

You're absolutely right: if the lens (human or camera) were in direct contact with the water, it wouldn't work. (And if you've ever opened your eyes under water, you've seen this.) Divers can see clearly underwater only by wearing a mask, so that their eyes are in air as expected, and the air/glass and glass/water interfaces are all flat and so merely linearly refractive. Underwater cameras are either encased in a housing which functions similarly, or have an outer lens whose outer surface is flat. —Steve Summit (talk) 03:07, 29 March 2007 (UTC)

Underwater cameras can work without a housing because the refractive index of glass is still sufficiently different from water to produce a lensing effect if the glass is shaped appropriately for underwater use. Many so-called underwater cameras are in fact just normal non-underwater cameras encased in a waterproof housing as Steve Summit says - but that's because it's more convenient to use a conventional camera's lenses than to grind special ones. The problem with our eyes is (as you suggest) that the refractive index is almost identical to water and thus they have little or no focussing power when directly in contact with water. Water has a refractive index of 1.33 and glass 1.5. SteveBaker 16:36, 29 March 2007 (UTC)

I confess I was speculating somewhat when I suggested that an unhoused underwater camera might have to have "an outer lens whose outer surface is flat", but what I was worried about (before suggesting the contrary, i.e. that the glass could instead be "shaped appropriately for underwater use") was that the refractive index of salt water might be significantly different from fresh, and everything else being equal you wouldn't want a camera that could work well in one but not the other. (Was I wrong about the differing refractive indices?) --Steve Summit (talk) 21:57, 29 March 2007 (UTC)

Whoa wait a sec howcome is it that i can see clearly more or less under water? i mean i wear glasses and all and without them im blind however under water i can see clearer, and if i have my glasses on as well i can see even better at times. are my eyes diffrent or does it have to do with the water acting as a sort of corrective lens? Maverick423 20:30, 29 March 2007 (UTC)

Well - from the preceeding argument, you can see that it's definitely possible. The lens in your eye is either bending the light too much to focus it on your retina - or not enough (depending on whether you are short or long sighted - I'm too tired to figure out which it is right now). The amount that the light is bent by the lens depends on the difference between the refractive index of the air or the water and that of the lens in your eye. If your eye bends the light too much when in air, it would bend it quite a lot less in water and - yes - it's possible that by a lucky coincidence this could give you better eyesight underwater than in the air...in fact, your ability to see clearly underwater would be better than someone with 20:20 vision underwater! We've been assuming that the refractive index of the lens in your eye is similar to that of water - but it might be just different enough to let you get a good focus. Yeah - I could easily believe that. SteveBaker 04:47, 30 March 2007 (UTC)

Barometric reading and water-vapor pressure

Hi, I ran an experiment where I mixed HCL with water, and put a strip of Mg metal into the solution. I measured the temperature, pressure, and volume. I used an eudiometer tube and let the chemical reaction take place, then I flipped the tube upside down and put it into a solution to even out the pressures outside and inside the tube. The barometric reading was 769.82 torr, and the water vapor pressure was 23.8 torr.My question is: Why is it necessary for me to make a water-vapor correction of the baraometer reading? Thanks for the help! —The preceding unsigned comment was added by 76.188.176.32 (talk) 23:35, 28 March 2007 (UTC).

Nevermind, I have figured it out on my own. Another question comes to mind.. I have determined the molar volume of the hydrogen, but it was .89% off. (22.2 instead of 22.4). Where did this error come from?

That low level of error could come from anything; inaccuracy in the readings, round-off error, or some combo of both. StuRat 00:12, 29 March 2007 (UTC)

How to reduce the temperature of something

In the article Helium-4 I read: "When helium-4 is cooled to below 2.17 kelvins (–271 °C), it becomes a superfluid, with properties that are very unlike those of an ordinary liquid."

How do they refrigerate something to -271Cº? —The preceding unsigned comment was added by 190.30.12.141 (talk) 23:56, 28 March 2007 (UTC).

Ah, take a gander at Cryogenics. -- mattb @ 2007-03-29T00:00Z

This is just a thought... temperature and volume are directly related (Charles's Law). If the volume of the substance is decreased, then the temperature is also decreased. To reduce the temperature to -271K..try reducing the pressure also. —Preceding unsigned comment added by 76.188.176.32 (talk • contribs)

That effect is used in refrigeration, yes. However, they decrease the pressure of the refrigerant, then cool the target with the refrigerant, as opposed to decreasing the pressure of the target directly. This allows them to repressurize the (now warm) refrigerant, which then becomes hot, then cool the refrigerant using air, depressurize it again to make it cold, and repeat the cooling process indefinitely. StuRat 00:46, 29 March 2007 (UTC)

Speaking of refrigeration, its article is quite useful. --Bowlhover 04:02, 1 April 2007 (UTC)