Wikipedia:Reference desk/Archives/Science/2006 December 29

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December 29

Relativity of light

I was reading about the Theory of Special Relativity in The Elegant Universe, and Greene said that if you chase a beam of light at light speed, the beam of light will continue to get farther away from you at the speed of light. This made my head hurt. Is it just one of those strange quirks of physics that you just have accept because there is proof for it, no matter how weird it is; or is there a real qualitative reason behind this phenomenon? Interesting stuff, this special and general relativity. :-) Imaninjapirate^{talk to me} 01:33, 29 December 2006 (UTC)

The fact that the speed of light is exactly the same in all inertial reference frames is an axiom (that is, an assumption) of the theory of relativity. It does not seem to have been intuitively obvious to anyone except Einstein himself, but it does turn out to result in theories that are correct to very high precision. -- SCZenz 01:55, 29 December 2006 (UTC)

This axiom comes directly from Maxwell's equations. They are what say that the speed of light is the same in all reference frames. —The preceding unsigned comment was added by 86.128.118.78 (talk) 21:49, 1 January 2006 (UTC).

Before you can understand that example, you need to understand that time and speed are linked. The faster you move, the slower time moves (relative to other viewers). If you travel away from Earth at the speed of light for a thousand Earth years and then traveled back to Earth for another thousand Earth years, you wouldn't have aged a second, but it'd be 4006 on Earth. Because of this 'distortion' (lack of a better word) of time, it's hard to grasp some of these examples where the viewer is asked to imagine what it's like to move at the speed of light. I'm probably getting this all wrong since I'm only an arm-chair physicist (read: computer scientist).  :-) I'm sure any errors will be corrected by the smarties on this page.

It's also true that nothing with mass can move at the speed of light, correct? In which case the example is only a rhetorical device. -Quasipalm 04:31, 29 December 2006 (UTC)

At the speeds we are used to, we can just add velocities. The error is so small that we do not notice it. If there is a long train travelling at a speed of exactly 50 km per hour, and on board the train there is a car travelling exactly 50 km per hour (with respect to the train) in the same direction – the extremely accurate speedometer of the car indicates 50.000000000000000 km/h – then its speed with respect to an observer standing next to the track is 100 km/h. But according to the velocity-addition formula of special relativity, it is slightly less, namely 99.999999999999785 km/h. Of course you wouldn't be able to tell the difference. If we were used, in everyday life, to speeds in the same ballpark as the speed of light, then we would grow up being familiar with the fact that you cannot just add speeds like that, and it would not appear strange. Suppose no-one had ever told you that the Earth is round, and you grow up thinking it is flat. Then if you hear that people at your antipodes, say the Kerguelen Islands, are hanging upside down from the Earth without falling of, you would find that mighty strange. And if you try to understand that that is normal to them, and that to them you are hanging upside down, your head might start to hurt.  --Lambiam^Talk 09:38, 29 December 2006 (UTC)

Are you saying tht the train is going 50 km/h and the car 49.999999999999785 km/h or that both are going 49.9999999999998925 km/h? 71.100.6.152 20:19, 29 December 2006 (UTC)

Nope. He's saying that for someone standing on the ground, the ground is standing still, the train is going 50 and the car is going 99.999...785. For a person on the train, the train is standing still, the ground is receding at 50 and the car is going forward at 50. And for someone in the car, the car is standing still, the train is receding at 50 and the ground is receding at 99.999...785. That's the point of relativity: you can't really just say "this thing is travelling at this speed", you have to specify that it's travelling at a certain speed relative to a particular frame of reference, and by changing the frame of reference you change what all the measured velocities are. Confusing Manifestation 12:29, 30 December 2006 (UTC)

DAD in hydrology

for what specific reasons other than designing of hydraulic structures (dams, weirs,etc.) "rainfall depth-area-duration relation (DAD)" is used in hydrology? thanks... —The preceding unsigned comment was added by 85.107.167.247 (talk) 03:42, 29 December 2006 (UTC).

I believe it allows calculation of surface run-off, which is important in other cases besides hydraulic structures. Flood prediction and such. Peter Grey 04:56, 29 December 2006 (UTC)

Running muscle

The only thing that troubles me now when I go running for a long distance, is the long muscle next to my shins. I've attempted to stretch them out, but it's difficult, and I'm not even sure if im doing it correctly.

What's the name of that muscle? Why is it causing me pain in long distance runs? (should be typically easy to figure out) And how do I prevent it from hurting during a run? PitchBlack 03:58, 29 December 2006 (UTC)

It's probably your tibialis anterior, and you may be suffering from shin splints. See the aforementioned wiki on shin splints for prevention tips and the like. --Chickenflicker---♣ 04:32, 29 December 2006 (UTC)

thanks! you got exactly what i was looking for. and i see you like A clockwork Orange... i saw it again last night. Fuckin horrorshow! =p PitchBlack 05:04, 29 December 2006 (UTC)

Have you considered slowing down your runs to make them relaxed, pleasant walks? I do not think jogging is nearly as healthy as one might assume. Some call jogging aerobic exercise, I call it stressful.

You may feel wonderful having a hot shower after a long run, but what about the hour when you had to discipline yourself to keep going? Vranak 20:57, 29 December 2006 (UTC)

what seems cheaper, $9.99 or $10

This is a soft science question, but I'm setting prices for services in a small biz setting. I'm wondering if I should do what so many other businesses do and subtract one cent from the prices to make them seem like more of a deal. Have any publicly available studies been done on the subject? Should I follow the general practice of ending in .99 assuming that so many other retailers wouldn't do it if it didn't work? Or maybe this is a case of group think? Thanks for any input. -Quasipalm 04:13, 29 December 2006 (UTC)

I found one link about it, but it doesn't offer up too much info. -Quasipalm 04:17, 29 December 2006 (UTC)

There seems to be some controversy - comments and further links under Psychological pricing and Price ending. ---Sluzzelin 04:18, 29 December 2006 (UTC)

Great find with the articles. Thanks!! Any opinions (since you can't states those on the articles themselves) are welcome as well. -Quasipalm 04:33, 29 December 2006 (UTC)

Typically, people perceive $9.99 as much less than $10.00, but a lot of the 'advantage' is lost because it's so common. But pricing should also consider other factors, like making it easy for customers to give exact change, or anticipating the mark-up of value-added/sales taxes, etc., and the obvious thinking about what your target market will be. Maybe the novelty of round numbers would even attract customers! Also people looking closely at prices are likely to override their intuition and think objectively about costs. Peter Grey 05:03, 29 December 2006 (UTC)

I believe it probably also depends on the industry and the type of goods or services you are pricing and how the prices are presented and advertised to the customer (tags, quotes, billboards, ads, catalogs, by request etc). ---Sluzzelin 05:10, 29 December 2006 (UTC)

A further issue is whether you are in a place where displayed prices will have sales taxes added (as is customary in most of the US and Canada) or not (as in most other countries I've been to -- if there's sales tax it's included). I'm not very price-sensitive when it comes to things around $10, but I sure notice the convenience when the total amount I'm expected to pay is in whole dollars, simply because most places here don't do that. (Merchants who do display tax-inclusive prices in a place where it isn't customary may have to comply with laws requiring them to disclose the amount of tax.) However, I don't know how many other people react the way I do. --Anonymous, December 29, 05:44 (UTC)>

This study by Cornell University on the very issue will probably be useful: [1]. JDoorjam JDiscourse 06:08, 29 December 2006 (UTC)

That's a great article, JDoorjam -- thanks! -Quasipalm 16:06, 29 December 2006 (UTC)

It's not just about psychology. Traders discourage customers from tendering the exact money so that the sales assistant has to ring it up, open the till, and give change. Otherwise it would often be easy for the assistant to pocket the cash without registering it.--Shantavira 09:33, 29 December 2006 (UTC)

I personally find retailers who refuse to round prices up to be highly annoying, particularly when I say "I want to buy the $100 dollar item you have on sale" and they feel the need to say "Sir, we don't have a $100 item, perhaps you mean the $99.99 item ?". StuRat 13:48, 29 December 2006 (UTC)

For what it's worth, I once read an article about a brand of children's books (Little Golden Books?) were found to sell better when the price was raised from 25 cents to 29 cents. Edison 15:19, 30 December 2006 (UTC)

Color Changing Glass

recently i saw in 24 (the series) that the guy pressed a buuton on remote control and the glass walls in his office turned opaque black. what was that? is that available in market? if yes then where? and is that related to the color changing optical glasses? —The preceding unsigned comment was added by 66.206.34.250 (talk)

Sounds like one of the common Applications of liquid crystals. DMacks 05:00, 29 December 2006 (UTC)

Take a look at smart glass. No piccies unfortunately. BenC7 10:59, 29 December 2006 (UTC)

Indentation marks

I seem to have the kind of indentation marks you get if say, you wear a watch tightly, or skin is pressed against something for a very long time. They are horizontal tiger stripe-ish and are in the middle. They've been there for over two weeks (since somebody else noticed them), and I wonder what they could be. Anybody? X [Mac Davis] (DESK|How's my driving?) 05:57, 29 December 2006 (UTC)

WIKIPEDIA IS NOT FOR MEDICAL ADVICE. ASK A DOCTOR IF YOU WANT MEDICAL ADVICE.

Ok, now we've got that over with :), Any answers? X [Mac Davis] (DESK|How's my driving?) 05:57, 29 December 2006 (UTC)

In lieu of a photo, one would need an accurate description of the placement, extent and appearance of what is being asked. Dermatologists have developed an amazingly accurate technical terminology for describing skin changes, but the rest of us struggle with inadequate common words. --Seejyb 08:57, 29 December 2006 (UTC)

Ah! Do you have help in me learning these words, or am I going to have to do it on my own? X [Mac Davis] (DESK|How's my driving?) 09:37, 29 December 2006 (UTC)

Your pants are too tight.--Shantavira 09:35, 29 December 2006 (UTC)

Hahahaha! Halfway up my back? :) X [Mac Davis] (DESK|How's my driving?) 09:37, 29 December 2006 (UTC)

You're an old man, and your pants are on too tight. :P --liquidGhoul 14:04, 29 December 2006 (UTC)

Are they there all the time or just when you get up in the morning?--Light current 02:52, 30 December 2006 (UTC)

Some type of stretch marks perhaps? Any recent changes in weight, etc? --jjron 04:44, 30 December 2006 (UTC)

WATER GOING BANG!

hello,

Why doesn't water combust?; hydrogen is explosive and oxygen is what is needed for combustion so why when heat is applied to water doesn't it go bang?

thanks, --84.69.39.119 13:42, 29 December 2006 (UTC)

Water is the product of the BANG between 2H₂ and O₂. After the reaction the water is stable, as the Hydrogen and Oxygen have already reacted. ɪŋglɪʃnɜː(r)d(Suggestion?|wanna chat?) 13:54, 29 December 2006 (UTC)

The general rule of thumb is that if the reactants (Oxygen and Hydrogen in this case) are highly reactive, the product (water) is unreactive. The recatants have lots of pent up energy. The "bang" is the releasing of this energy into the universe, and therefore the product does not contain the energy. --liquidGhoul 14:02, 29 December 2006 (UTC)

Setting light to an explosive mixture of oxygen and hydrogen was one of my science teacher's favourite tricks. He would then show us the resulting droplets of water left in the container.--Shantavira 14:38, 29 December 2006 (UTC)

Yep, my chem teacher did it to me. We seperated water, and put the gas into detergent water, so it would create bubbles. He told me to ignite it, and I was expecting something similar to the pop test. The first time it fizzled, as there was too much oxygen already in the pipe. But the second time it was a perfect ratio of oxygen to hydrogen, and it was extremely loud. Scared the crap out of me. Then we got people from the quad (who hadn't seen it) to do the same, good fun! --liquidGhoul 04:32, 30 December 2006 (UTC)

query (Fullform of Rx ?)

what is the exact fullform of Rx in medical science?—Preceding unsigned comment added by 221.134.57.251 (talk • contribs)

Welcome to Wikipedia. You can easily look up this topic yourself. Please see Rx. For future questions, try using the search box at the top left of the screen. It's much quicker, and you will probably find a clearer answer. If you still don't understand, add a further question below by clicking the "edit" button to the right of your question title. --Shantavira 15:31, 29 December 2006 (UTC)

If you're going to just redirect him, I suggest you actually take the time to check the link. This provides a more detailed explaination: Medical Prescription, Format and definition --Russoc4 15:45, 29 December 2006 (UTC)

Unknown reptile eggs

Dear Volunteer at wikipedia

I have discovered some unkown reptile eggs in an outdoor light case. However only two of the eggs are fertilized, colour beign pink. Since they did fall on the floor i do not know if my incubation attempts will be successful, i am however still making the effort. I would just like to know if u have way of identifying the eggs, they are grouped in twos with a shared fluid around the eggs. Meaning to say that the shell covering the two eggs is fused together. The egg shells also have a hard cover. I would greatly appreciate it if you could help me identify these eggs. Thank You Charmaine —The preceding unsigned comment was added by 198.54.202.250 (talk) 17:06, 29 December 2006 (UTC).

I'm no reptile expert, but I think a photo would greatly improve your chances of getting a positive ID. Do you have, or can you borrow, a digital camera? —Keenan Pepper 19:13, 29 December 2006 (UTC)

Also, I thought reptile egg shells were leathery, not hard like bird eggs. StuRat 21:00, 29 December 2006 (UTC)

Depends on the reptile, skinks eggs are leathery but are still hard. Not as hard as birds, but they aren't malleable. Also, a location will hugely increase the chances of identification. Thanks. --liquidGhoul 04:28, 30 December 2006 (UTC)

buses

i'm always told buses are "greener" than cars and i can accept that if the bus is full of people, but if its only got one person on then it cant be greener cus of all the extra mass to has to lug round. so i'm asking how many on average would need to travel on a bus for it to be "greener" than using cars? --86.29.50.214 19:00, 29 December 2006 (UTC)

• I would assume this applies mostly within a mass transit system, where you tend not to find an empty bus. If you're inside a city with 10 or 11 million people, and only a fraction of them drive cars, A) buses will always be full of people, B) that's a few million cars not on the road--74.66.242.190 19:24, 29 December 2006 (UTC)

Depends on the type of bus. I don't know if a diesel bus full of people is much better than cars. A gasoline bus isn't quite as bad, and an electric bus is much better. StuRat 19:22, 29 December 2006 (UTC)

That depends on a lot of factors. For raw comparisons of fuel burned per kilometer (or per mile), Fuel_efficiency#Fuel_efficiency_in_transportation is a good place to start. From that article, one finds that diesel buses typically operate at five to ten miles per gallon. In comparison, typical SUVs travel about fifteen miles per gallon (mpg) of fuel; North American midsize automobiles about 25 mpg; highly fuel efficient subcompact diesels (like the Smart Fortwo) can brush 70 mpg.

In other words, a bus carrying at least two or three people (on average) burns the same fuel per passenger-mile as a single-occupant SUV. A bus carrying (again, on average) at least ten passengers is more fuel efficient than any single-occupant vehicle, and compares favourably with most two-occupant vehicles.

In addition to the question of fuel economy, there are a lot of other factors in the 'green' question. Fewer automobiles on the road means fewer roads and parking lots and less traffic congestion. There may be economies and environmental benefits to manufacturing and maintaining a few buses rather than dozens of automobiles, as well. Buses in many places also act as test beds for new, environmentally friendly technology that may not yet be cost-effective for the general public. (Public transportation fleet buses in many cities incorporate natural gas, hybrid electric, or even hydrogen fuel cell technologies to reduce air pollution.) TenOfAllTrades(talk) 19:26, 29 December 2006 (UTC)

There are also lots of inefficiencies that come with buses, like idling at stops (when ahead of schedule) and people taking several buses, and a much longer route, than they would take with a car, since no bus routes goes quite where they need to go. StuRat 20:39, 29 December 2006 (UTC)

Why wouldn't a bus go where you need it to? They just go in straight lines up and down avenues and streets, you can't get much more direct than that. A bus for every street, and always in striaght lines. The idling when ahead of schedule I'll give you that--74.66.242.190 21:33, 29 December 2006 (UTC)

Apparently you live in an area of the world where towns have avenues and streets that go in straight lines. In most of the world, that is not the case. Also, a bus for every street may be completely unaffordable, and in any case is not very economical.  --Lambiam^Talk 22:19, 29 December 2006 (UTC)

Also, buses carry people who can't afford other transportation. "Green" is a label that gets more funding than "welfare" so it is used. There are other social legislation that use "green" the same way to fund it. To wit, giving poor nations carbon credits to sell to rich nations is an instant exportable commodity. --Tbeatty 06:42, 30 December 2006 (UTC)

Yes, one criticism of the Kyoto Protocol is that it's a massive wellfare system for nonindustrialized nations. StuRat 13:31, 30 December 2006 (UTC)

2 questions

1) My whole life, I've gotten nosebleeds that occured without any known reason. Not like...picking my nose, hitting it with a car door, none of that. They would all come up during sleep, or right after sleep. Why? (if more info is needed, ask)

This can be a sign of a serious medical condition, or could just mean you have something like varicose veins inside your nose. I suggest you ask a doctor to check it out. StuRat 19:24, 29 December 2006 (UTC)

2) There was a night, where my eye itched like hell. I scratched it, but it did'nt feel better. So I did what I do all the time when I have an itchy eye.. go to sleep. The next day, i wake up with that eye bruised. I had no idea why, and I went to school with everyone thinking I got in a fight. Can scratching really cause bruises? PitchBlack 19:16, 29 December 2006 (UTC)

I get random nosebleeds as well. I went to the doctor a couple years ago and got some nasal spray that made it stop for a period of time. But they have since reoccurred. --Proficient 21:14, 29 December 2006 (UTC)

My dad gets lots of random nosebleeds (and has done for about the last 20 years). His doctor told him that he needed to have some weak blood vessels cauterized. He doesn't like the idea of someone inserting hot metal up his nose, so he's decided to leave things as they are. --Kurt Shaped Box 23:40, 29 December 2006 (UTC)

You're telling us you get a lot of random nosebleeds and bruise easily? I'd go to a doctor, or at least a pharmacist, and tell them about it. After all, you've nothing to lose except a source of worry. Get it checked out. Skittle 01:17, 30 December 2006 (UTC)

1. You might have a blood vessel exposed inside your nose. You can go to a doctor and get it cauterized. It happened to me, and never bothered me again. -- Sturgeonman 23:20, 1 January 2006 (UTC)

time interval

How long after the Big Bang were hydrogen atoms created, how long after that were oxygen atoms formed and how long after that did they react to form water? 71.100.6.152 20:24, 29 December 2006 (UTC)

Some of these questions are answered in Timeline of the Big Bang and the articles it links to, while some information is given on some of the others.  --Lambiam^Talk 22:15, 29 December 2006 (UTC)

Focusing the sun's rays through a giant orbital lens?

How practical would this be as a weapon for use against ground-based targets? Just a concept I remember reading about somewhere - a large magnifying lens is placed into low-earth orbit and when required, is used to focus the sun's rays onto a small area of the earth's surface, incinerating everything in the vicinity - much like a kid with his magnifying glass destroying an anthill. On paper it actually sounds like a very simple system (I guess that some sort of shutter would be required to prevent accidents and damage to the lens from space debris). Any thoughts? --Kurt Shaped Box 23:55, 29 December 2006 (UTC)

It would be easy to defend against, just go underground during the day and attack at night. A system of mirrors could be set up to send light from an area not in the Earth's shadow, I suppose. The reality is that such an idea would cost far more than equal destructive power delivered by other means, and would be highly vulnerable to anti-satellite weapons, so it's not a very practical idea. Also, wouldn't such a weapon kill any seagulls in the sky at the time ? :-) StuRat 01:01, 30 December 2006 (UTC)

Seagulls are mostly white and would thus reflect the sun's rays. :) Seriously though, the system would only be vulnerable to anti-satellite weapons if the enemy knew about it (catch them unawares and the results would be devastating - imagine hitting all their major cities within minutes of each other) and I'd imagine it would be possible to concentrate the sun's energy into a beam capable of melting several tens of metres of rock - they'd have to be very well-hidden. --Kurt Shaped Box 01:13, 30 December 2006 (UTC)

Also note that a converging lens does not focus sunlight to a geometrical point, but instead would produce an image of the sun at its focus, and this hotspot might be far more dim than one would expect. Since the solar disk is about 1/2 degree wide when viewed from the Earth, a lens orbiting at, say, 300mi altitude would produce a hot spot in perfect focus which is a minimum of *three miles wide*. To produce a hot spot of, say, 1000 times normal sunlight intensity, such a lens would need to be about ninety miles wide, or much larger if it's in a higher orbit. If we replace this giant lens with an array of mirrors, each small mirror still acts like a pinhole camera, producing a distant hot spot on the Earth which is three miles wide. (We'd need a LOT of mirrors to produce a damaging intensity in our 3mi diameter target!) Rather than a lens or a curved mirror, it would be better to use non-imaging optics such as a huge solar-pumped CW laser. The laser diameter need not be very large, yet it could still fry a relatively small target 300mi below. Also, unlike the giant lens, the laser aim can be swept backwards to keep aimed at the desired target as the orbiting laser moves forwards at around 20,000 MPH. You cannot do that with a lens, since tilting the lens just distorts the focused solar image without moving it sideways. --Wjbeaty 01:48, 30 December 2006 (UTC)

How about a more lo-tech approach to mass destruction from space? Huge 'harpoons' of depeleted uranium in orbit, needing only a small 'push' in the right direction at the right time to send them hurtling to the earth on a collison course with the target? Let gravity do all the work. --84.68.15.234 02:05, 30 December 2006 (UTC)

If an object is in low Earth orbit, it must be in a nearly circular orbit. In that case the only way that a "small push" can bring it to ground is if by causing the orbit to drop into the atmosphere so that friction eliminates a large part of its speed. That's fine for a space shuttle, where you want a landing speed far below orbital speed, but for a kinetic-energy weapon the point is to retain the speed. It might be possible for a "small push" to make a weapon move into a high-speed-collision course if it was at the outer end of a highly eccentric orbit, but then it would take a long time to fall, again allowing time for an anti-satellite weapon to respond. If someone was looking, that is, and if such counterweapons were deployed. --Anonymous, December 30, 10:17 (UTC).

Something we've all (including me with my original question!) overlooked here. How the hell would we get all that stuff up there in the first place? Depleted uranium is very heavy - how big would the rocket need to be to lift say (random ballpark figure that sounds big enough) a DU 'harpoon' that weighed 2000 tonnes into orbit, plus all the technology required to monitor and 'fire' the thing? If we were going to put several (tens? hundreds?) of these devices into orbit, perhaps the enemy power would notice all those large rockets going up and start to ask questions, or alternatively launch a pre-emptive strike with everything they had at their disposal? --Kurt Shaped Box 19:55, 30 December 2006 (UTC)

Rule of thumb for rockets is that a re-usable rocket can lift 1% of its mass into orbit; an expendable rocket can lift 2%. So for 2000 tonnes of projectile, you'd need at least a 100,000-tonne rocket. --Carnildo 02:10, 31 December 2006 (UTC)

As a matter of interest, would a 90 mile-wide lens be visible from earth (assuming that the 'lens cap' was in place)? --Kurt Shaped Box 19:57, 30 December 2006 (UTC)

What, from 300 miles away, as per the above response? Extremely visible, if only by passing in front of things. --Anonymous, December 31, still 2006, 01:49 (UTC).

• Didn't Archimedes use this as a weapon already? Titoxd^(?!?) 08:55, 31 December 2006 (UTC)

The International Space Station, when passing overhead, is one of the brightest "stars" in the sky. A 90-mile lens has 5 million times the ISS's area and will appear to be 17 degrees across (about 34 times the diameter of the Sun). I wonder what the enemy will think if something like that passed over their heads. --Bowlhover 10:27, 31 December 2006 (UTC)