Wikipedia:Reference desk/Archives/Science/2008 August 9

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August 9

Mosquitos

If a mosquito can spread malaria and West Nile virus, why can't it spread AIDS? 99.245.92.47 (talk) 00:52, 9 August 2008 (UTC)

Because the HIV is digested by the mosquito. Unlike the other viruses and mosquito-borne parasites, HIV hasn't evolved the mechanism to avoid the insect's stomach. See here.--Lenticel ^(talk) 02:23, 9 August 2008 (UTC)

Botany(project topic)

Good day Pls I want to know growth of garlic (Allium sativum) is affected by temperature variables of 28 degree and 37 degree.When grown in Bauchi,North West of Nigeria;West Africa and also, method of observing this changes Thanks —Preceding unsigned comment added by Rukima (talk • contribs) 01:07, 9 August 2008 (UTC)

This sound like an interesting topic. I assume that you intend to do long-term growth experiment. This will be feasible for you only if you can create two environments (such as small greenhouses) that are identical except for temperature, and you can control the temperature. Equipment to control the temperature at 28 and 37 degrees in Bauchi for enough time to grow your plants may be difficult, especially since you will need to control the relative humidity to avoid a second independent variable. According to the internet, the temperature in Bauchi often rises above 27, so you will need a cooling system: this raises the expense of the experiment beyond the amateur level. (A heating system is a lot cheaper.) Alternatively, you might try to do a respiration experiment whereby you somehow grow the plants for an hour inside two small glass jars and measure the oxygen content inside the jar, at two different temperatures. Good luck! -Arch dude (talk) 14:07, 10 August 2008 (UTC)

Obstetricians

what classes do i need to take in high school to give me a head start to become an obstertrician? —Preceding unsigned comment added by Goals101 (talk • contribs) 02:53, 9 August 2008 (UTC)

Obstetrics is a specialty within medicine. Medical students don't choose their specialties until they've already had a few years of med school, so if by "head start" you mean for getting into an obstetrics program, then you should focus on the same classes you would to get into a good college with a strong science program. First college, then med school, then obstetrics. If you are looking to actually start learning about obstetrics, then I would suggest taking sexual education classes and perhaps even getting EMT-B (or better yet EMT-P) certified. You may also be able to volunteer at an obstetrics ward at a local hospital if you're lucky. --Shaggorama (talk) 04:31, 9 August 2008 (UTC)

Different medical schools have different requirements, so be sure to check the website of the college you intend to enter. Generally, high-school courses in science (especially biology), math, and other analytical subjects are more useful for a medical career than art-related courses. --Bowlhover (talk) 07:00, 10 August 2008 (UTC)

Conservation of linear/angular momentum

How is it that someone is able to walk? I mean, when you push against the surface of the earth, you're really applying a torque, and thus giving the earth angular momentum rather than linear momentum. So where does a person's linear momentum come from? I've given this some thought, and the explanation I came up with is that we don't gain linear momentum, we gain angular momentum; basically, we push against the earth with one foot, and the rest of our body rotates around our ankle, which makes some sense. But this explanation doesn't seem to work in all cases. Consider a car, how is its momentum conserved? Well, I would say that when the wheels of a car push against the earth, the angular momentum lost by the earth is transfered to the car's wheels. Everything seems to make sense, until the body of the car is considered: where does it's linear momentum come from? Finally, let's say someone jumps against a wall, while the earth gains angular momentum, the person in question gains linear momentum. How is this possible? Thanks in advance. —Preceding unsigned comment added by 76.68.246.7 (talk • contribs) 04:28, 9 August 2008 UTC

Good question.

What you have to remember is that a body has angular momentum when it is revolving around an external point, as well as when it is rotating around its own center. For example, the Moon has angular momentum due to its rotation around the Earth. Well, if you press horizontally against the Earth with your foot, or you press horizontally against a wall, or a car presses horizontally against the Earth with its tires, you or the car are putting yourself/itself into motion around the Earth. You are revolving around the Earth's center and that gives you the angular momentum to offset the angular momentum that your torque applied to the Earth.

If you jump vertically off the Earth, then you are in a non-rotary motion and you are acquiring linear momentum, and you are giving the Earth the same momentum in the direction that is downward for you. And real-life movements are a combination of the two cases, as a car rolls over hills and valleys and irregularities in the road, and your body moves up and down with each step.

--Anonymous, 07:00 UTC, August 9, 2008.

Angular momentum of an object can be seen as the sum of the linear momenta of all its particles. It's just a handy way of describing thing going around in circles, but angular motions of systems can always be seen as linear motions of particles. (Until one starts talking about quantum "spins" and such, and nice people don't do that.)

Hitting a wall is a force to the brick but a torque to the earth.

Torque (twisting force) is a linear force applied at a (linear) distance. Pistons push on the crankshaft (a "circular" lever) which twists more circular levers until the last ones, the wheels, push on the ground. It's all linear if you look at it closely.

How do you walk? First you have to start falling over, then move your feet under you to keep you from doing it. Saintrain (talk) 13:41, 9 August 2008 (UTC)

Triceratops vs Tyrannosaurus Rex

With their features and body parts, which do you think will win in a fight between a Triceratops and Tyrannosaurus Rex?MachaLeague (talk) 09:31, 9 August 2008 (UTC)

You say "will," which implies future, so I'll go out on a limb: a tie, since they're both extinct. For another opinion, see The Truth About Killer Dinosaurs. (They think it'd be a tie, too.) OtherDave (talk) 11:29, 9 August 2008 (UTC)

I'd probably say Tyrannosaurus as a 'trained' predator, whereas Triceratops were herbivores. That said, Triceratops may have had experience in defending itself when confronted by a predator. I don't really know. —Cyclonenim^T@lk? 12:16, 9 August 2008 (UTC)

God, you guys looovvvee being literal-minded, don't you? --98.217.8.46 (talk) 13:30, 9 August 2008 (UTC)

I can't speak for anyone else. I was having fun with the future tense, but I did post a link to help answer the actual question. And I signed it, too. OtherDave (talk) 17:07, 9 August 2008 (UTC)

I'd point out that this is not purely a theoretical discussion. Both the Triceratops and Tyrannosaurus lived in the Late Cretaceous (68 to 65 million years ago), with overlapping ranges (e.g. both were found in what is now Montana). If we operate under the assumption that the Tyrannosaurus was an active hunter (as opposed to being a scavenger - see Tyrannosaurus#Feeding strategies), one can conclude that there likely has been numerous clashes between the two. Indeed, the Tyrannosaurus article notes "A Triceratops was found in Mexico found with bite marks on its ilium. These were also inflicted by a tyrannosaur and they too appear healed, indicating active predation by the tyrannosaur." (And survival of the incident by the Triceratops, I might add.) This is also mentioned extensively in the Triceratops article. One might surmise that such "fights" generally went the way of most predator/prey encounters. A fraction of the time the predator will win the "fight", bringing down the prey. However, the majority of the time either the prey escapes, or the predator decides it isn't worth the hassle and gives up. Usually the predator will only go for the young, the elderly and the weak. Normally you wouldn't see a predator take on a healthy adult prey - it isn't worth it, as there will probably be opportunities for an easier kill later. I seriously doubt that most encounters would be the epic showdowns normally depicted in fiction - it's not worth the risk to the Tyrannosaurus. (Be honest, if your steak fought back you'd probably just order the fish, even if the resulting clash would have made for good cinema.) -- 128.104.112.147 (talk) 20:40, 9 August 2008 (UTC)

Movies and TV aside, "fights" almost never happen between different species. The closest you'll get is a back-and-forth encounter like this: http://au.youtube.com/watch?v=LU8DDYz68kM

Predators and prey both have a vested interest in avoiding fights. Out and out fighting to the death is "stupid" (i.e. not evolutionary advantageous) behavior. If the predator even gets slightly injured, it's chances of getting more prey and competing against healthy predators drops to almost nothing, and it will probably die later. In other words, as the IP above said, predators only attack prey that they think will not fight back. If they prey starts to fight back, the predator will get out of there (as demonstrated in the video--the lions get chased around by the adult buffalo, while trying to avoid the adults and get the the calf, which is not putting up a fight). In the example of the Triceratops with healed bite marks, the fact that the bite is on the hip bone is telling. All the defenses are on the head, so the Triceratops was most likely running away when it was bitten, not fighting. Dinoguy2 (talk) 23:48, 10 August 2008 (UTC)

Pollution and cycling

Have there been studies of the impact on the respiratory system of riding a bike in the city? Thanks. 190.244.186.234 (talk) 14:23, 9 August 2008 (UTC)

Any particular city in mind? I don't think it matters what form of exercise you're doing, just how much of it. With the Beijing Olympics, the authorities are saying the pollution is only a risk for events lasting over an hour (the marathon, for example), so if you're in a city with similar pollution levels to Beijing, just ride your bike for less than an hour, and you should be fine. --Tango (talk) 15:21, 9 August 2008 (UTC)

Found this but not much more.

"even when account is taken of effort (a cyclist breathes on average two to three times as much as a motorist), the cyclist emerges as the victor of this comparison" (quoted in Cycling: the way ahead for cities and towns). (1996)

The thing I'm more worried about is all the fine particles. I sometimes ride behind buses in their thick black smoke. Fourty-five minutes twice a day. The article has got some real scare stories too and it talks about Australia and Europe that are not nearly as polluted as South American capitals. Any more links? 190.244.186.234 (talk) 19:38, 9 August 2008 (UTC)

Supertime

What would it mean to physicists if a second dimension of time were discovered ? [1] 69.157.227.243 (talk) —Preceding undated comment was added at 15:59, 9 August 2008 (UTC)

The first dimension of time as I understand it is the interval between one event and another or speed as we know it. As I understand it the change in this interval between repeated occurrences of events is the second dimension of time called acceleration. But then I've never slept in a Holiday Inn. —Preceding unsigned comment added by 71.100.162.249 (talk) 16:18, 9 August 2008 (UTC)

You do know that's garbled nonsense, right? 79.66.38.215 (talk) 17:12, 9 August 2008 (UTC)

It could have some interesting effects. I'm not sure exactly, but I think it would make time travel possible. The same as you can use a second space-like dimension to change your direction along a first space-like dimension (without having a slow down and stop momentarily), the same is probably true of time. I'd have to dig out my GR notes to work it out properly. --Tango (talk) 17:32, 9 August 2008 (UTC)

In one dimension there are two discrete directions which you can identify with past and future. In two or more dimensions there's a continuum of directions and you can't make that distinction any more. It's hard to see how such a universe could have a second law of thermodynamics or anything associated with the second law, like biological evolution. The one-dimensionality of time is almost its defining characteristic. -- BenRG (talk) 18:37, 9 August 2008 (UTC)

The other time dimension could be compact, like the extra dimensions in (some versions of) string theory, so it's hardly noticeable on an everyday scale. I'm really not sure of the details of how that would work. I'm not sure what the 2nd law of thermodynamics has to do with evolution, but that's not really important, an extra time dimension would mess with causality, and that's going to mess with everything. Time machines and temporal wormholes and things usually work in such a way that you at least have local causality, it's just globally that things get mess up. This would mess it up locally as well, which is asking for trouble! Could be interesting as a purely intellectual excercise, though. --Tango (talk) 19:11, 9 August 2008 (UTC)

It's not a new idea, there's been some real research into it. I haven't read much of it yet, but it's there. --Tango (talk) 19:14, 9 August 2008 (UTC)

Centrifugal Sorting of Saturn's Rings

The rings around Saturn are believed to consist mainly of pieces of water ice and a small amount of other substances. The material varies in size from dust-like particles to lumps the size of a small automobile. (See "Saturn" in Wikipedia.) There are probably individual atoms and molecules in the rings as well.

The rings vary in the brightness of the light they reflect from the sun. Each ring has the same brightness all the way around it and across its width. There are also gaps between some of the rings. Various theories have been advanced to account for the differences in brightness and the gaps.

The brightness variation suggests that the various rings are comprised of different material. My theory, which I have not seen elsewhere, is that the material in the rings has been sorted by centrifugal separation. (Think of a centrifuge.) Impurities in the water ice would give pieces of it a different density and color.

I conjecture that the densest material appears in the outermost rings, with decreasing density of the material in rings progressively closer to the planet.

Some of the gaps are caused by moons and moonlets clearing their orbit of small-size material by gravitational attraction. Other gaps, perhaps, are due to the fact that there is no material with a density that will place it in the gap.

The centrifugal separation would have taken place regardless of how the rings were formed.

The rings are comprised of an estimated 93 percent water ice. But the ice would not be pure. There would be many kinds and strengths of impurity. This would account for the large overall width of all the rings together

The differences in density of the impure ice particles would probably be small; but over millions of years, sorting would take place.

Material that contains no water would take its place in the rings. These rings would probably be at the outer and inner edges of the ring structure because the density of the material might be considerably different from that of impure water. —Preceding unsigned comment added by 67.150.190.122 (talk) 17:11, 9 August 2008 (UTC)

The centrifugal sorting may have worked together with other causes that have been suggested for the formation of the rings.

AndMe2

67.150.190.122 (talk) 16:30, 9 August 2008 (UTC)

It's an interesting hypothesis! Are the planets in our solar system in order of mass or density? DMacks (talk) 16:51, 9 August 2008 (UTC)

No. See Table of the largest objects in the Solar System. --Tango (talk) 17:38, 9 August 2008 (UTC)

Er yeah, I'm aware of that. It was a continuation of poster's thought experiment, giving him an opportunity to do some actual science and test his idea (make hypothesis, use it to make predictions, check if those predicions agree with reality). If centrifugal sorting worked on "dust particles orbiting a planet", it would likely also be evident in "planets (really big particles) orbiting the sun" (that latter having been taking place for eons longer that Saturn's rings). So if not, it means hypothesis isn't true (or needs to be adjusted to account for this new data). DMacks (talk) 05:33, 10 August 2008 (UTC)

I don't think that's likely. An object's orbit depends only on it's velocity, position and the mass of the central object (assuming there is a big difference in the masses, as there is in the case of dust orbiting Saturn), density doesn't come into it. A centrifuge is very different from orbiting dust - particles of dust don't interact with each other much, the various things in a centrifuge do, and that's why density becomes a factor. The gaps are caused by resonance with the moons, I believe. --Tango (talk) 17:38, 9 August 2008 (UTC)

What if the rings formed in a gas-rich environment? Then they would be particles in a fluid. Would there be a centrifugal effect in that case? Just asking as I have no idea. Franamax (talk) 19:58, 9 August 2008 (UTC)

I'm just guessing now, but I expect if there was enough gas to make a difference, the orbits would just decay and the rings would crash into the planet. Even if they didn't, I think it would only the difference in density between the gas and the ring matter would be significant, so you would get one ring at the bottom of the gas made up of heavier stuff and one at the top made up of lighter stuff. It's an extremely hypothetical situation, though, I don't think you would ever have things orbiting in dense enough gas for it to be significant. --Tango (talk) 21:28, 9 August 2008 (UTC)

Well presumably the gas would be orbiting with similar velocity, so I'm not sure I see the mechanism to crash the rings, since there would be no significant drag in that case. Given a sufficient gas component though, there would be a mechanism to transfer momentum between the solid particles similar in a way to actual collisions between the particles themselves? If they're building a new planet anytime soon, I guess we should try that out. :) Franamax (talk) 01:30, 10 August 2008 (UTC)

Hmmm... I hadn't thought of the possibility of the gas being in orbit, I was thinking of it being like an atmosphere. I honestly have no idea what would happen in that case... --Tango (talk) 02:55, 10 August 2008 (UTC)

This is very interesting and suggests to me that the rings around Uranus may be similarly governed. —Preceding unsigned comment added by 79.76.242.64 (talk) 23:03, 9 August 2008 (UTC)

What STD can be transmitted through oral sex?

Which is the most common? Mr.K. (talk) 18:39, 9 August 2008 (UTC)

Well firstly particularly for the 'most common' bit, it'll be helpful if you specify what & who you're referring. The partner performing oral sex on the genitals of a female partner? The partner performing oral sex on the genitals of a male partner? A male receiving genital-oral sex from a partner? A female receiving genital-oral sex from a partner? (The sex of the performing partner doesn't tend to matter for obvious reasons.) Something else? The rates tend to vary a bit depending on what and who. But to answer the first part of your question simplisticly, I don't know of any STD that can't be transmitted via oral sex. Logically there's no reason an STD won't be transmitted via oral sex unless the STD doesn't infect and/or can't be passed via the mouth/oral region. Of course, STDs which only infect a local area will only generally infect the area they are passed to be it the oral region or the genitalia region. According to this [2] HPV is perhaps the only one which isn't that much of a concern when it comes to oral sex (but it can be transmitted in rare instances) and herpes is the most common (but that probably has a lot to do with do with how common it is in the general population and the fact it tends to be lifelong with occasional flareups). Nil Einne (talk) 19:27, 9 August 2008 (UTC)

I'm not so sure HPV isn't much of a concern. According to this, "A recent study conducted by Dr. Maura Gillison at the Johns Hopkins Oncology Center furthered the premise that HPV is linked with certain types of oral cancer." --71.185.73.240 (talk) 04:18, 10 August 2008 (UTC)

Chlamydia [3]. Franamax (talk) 00:46, 10 August 2008 (UTC)

I remember reading a while back that oral gonorrhea was on the rise amongst U.S. and U.K. teenagers. I can't seem to find any statistics that list STDs by type of infection, however. Horselover Frost (talk) 04:05, 10 August 2008 (UTC)

Educational videos in psychology

Where can I found a source of real case studies in psychology Q! Ĩ am trying to find videos of patients with some sort of mental illness or descriptions of a disorder with plenty of examples. Mr.K. (talk) 18:40, 9 August 2008 (UTC)

That would be cool. *adds name to MOAR list* --mboverload@ 19:09, 9 August 2008 (UTC)

Check out www.learner.org. They have many videos on demand that you can watch for free. The videos they make are often shown on PBS, because I specifically remember watching one of their videos on TV about schizophrenia (sp?). You might have to navigate that site for a while, but I'm sure you'll eventually find their archieved psychology videos.--El aprendelenguas (talk) 22:17, 9 August 2008 (UTC)

You'd be surprised what you can find on youtube. My interests lie more in [neurology], but still I'd suggest you run searches on youtube with the titles of conditions in which you are interested in or experiments you would like to find documentation of. --Shaggorama (talk) 13:59, 10 August 2008 (UTC)

Academic, corporate and free researchers

These three are probably closely entagled with each other. However, I would like to know how much researchers work in an academic, corporate or free environment, where is the money, and, in terms of patents and publications, how productive they are.Mr.K. (talk) 18:42, 9 August 2008 (UTC)

Generally speaking, money and often patents are in corporate, publications are in academia. But that's very generally speaking...it varies among the different areas of science, and a famous academic who becomes head of some huge non-profit research center would make more (esp. in honoraria) than an entry-level BS in some company. DMacks (talk) 20:38, 9 August 2008 (UTC)

This varies quite a lot country-by-country. For example, in 1995, in the USA around 17.5% of all researchers were in higher education (as opposed to government or corporate work), whereas in Japan it was more like 36.1%. However in the same period Japan spent only 75% of what the US spent on higher education R&D. Productivity varies a lot per country as well. Some countries, practically all of their research goes into industry or government sources (like China), whereas some are almost all government (like Russia). The OECD has tons of statistics about this sort of thing. If you only care about the US case, the NSF keeps very detailed statistics about how money is being spent and what comes from it. --98.217.8.46 (talk) 15:31, 10 August 2008 (UTC)

Automobile bumpers

Why do cars don't have a bumper car-type rubber bumpers? Mr.K. (talk) 18:49, 9 August 2008 (UTC)

Because cars need to absorb the energy of a collision, and thus need to bend and break to absorb it. We could make cars that would not be damaged in a collision, but at the expense of damaging the human inside instead. Crumple zone, bumper, Car crash. Any car with a bumper-car style bumper would not even be legal to drive in any first world countries. Any car produced today with no crumplezones and a completely rigid frame and bumper would probably not pass the safety requirements in many countries. --mboverload@ 19:04, 9 August 2008 (UTC)

In simplistic terms, this is achieved by controlled weakening of outer parts of the car while strengthening the inner (passenger cabin) part of the body --mboverload@ 19:15, 9 August 2008 (UTC)

Because SPEED does the matter. With the highest speed of rubber bumper car can't even kill a man, but the speed of car on the road makes much higher impact. Even we can found material in the future to adsorb collision energy, the static velocity of the people in the car may kill them anyway....NINJAW —Preceding unsigned comment added by 124.120.200.29 (talk) 07:05, 10 August 2008 (UTC)

As above, Speed matters: the energy to be absorbed goes up withthe square of the collision speed. Another important point: you can in fact build a bumper that can absorb and release all that energy, but it will make the vehicle unacceptably large and unacceptably heavy, thus ruining the gas milage. -Arch dude (talk) 13:34, 10 August 2008 (UTC)

In part, because Ronald Reagan directed the massive weakening of the United States vehicle bumper standards when he became president, so now your car costs a few dollars less but can suffer several thousand dollars worth of damage when tapped by another car attempting to parallel park. See Talk:Volkswagen Golf for some background on this.

Atlant (talk) 19:37, 14 August 2008 (UTC)

Control group in statistics

In statistics, it is considered necessary to have a control group (placebo group). However, in some situations (heart surgery, smoking and cancer, etc) a control group is not possible. How can I deal statiscally with these cases? Mr.K. (talk) 18:50, 9 August 2008 (UTC)

You might try the Mathematics desk for this question. Also, maybe a bit more detail. What sort of data do you have in mind, and what sort of questions do you want to ask? --Allen (talk) 19:01, 9 August 2008 (UTC)

In medical research it is unusual for the control group to be untreated, since it would be unethical to not treat a group of people. Trials are usually done comparing two different treatments (one established and the other new). The other option is to test the new treatment on people that have exhausted all other treatment options, so in the absence of the trial, they wouldn't be treated anyway, so there is no ethical dilemma in not treating half the patients (at least until the treatment is shown to work). --Tango (talk) 19:24, 9 August 2008 (UTC)

The short answer is that you cannot deal with them statistically, so you must use other means. In more detail: when you do a proper, controlled trial, you choose half the subjects randomly, and put them in the treatment group, while the other half go in the control group and get a placebo. ONLY when you have assigned subjects to groups randomly can you say that the treatment was the cause of any difference between the groups. In ALL other cases, the experiment itself does not show causation, although you may infer this if you have strong reasons for doing so anyway.

For example, if you do a smoking trial, and somewhat unethically, you order half the subjects (chosen randomly) to smoke, and the other half not to, you can say that any increase in diseases (or beneficial effects) in the first group were caused by the smoking. If you take the two groups from among the populations of existing smokers and non-smokers, the results do not necessarily show the effects of smoking. There could, for example, be socioeconomic differences between the two groups causing different health outcomes. Even so, the second group can be used like a control group, and likely confounding variables such as income can be corrected for. When you assign the groups randomly, that is called a randomised trial, otherwise (I think) it is just called non-randomised.

In trying to deal with confounding variables, you can simply include them in your model, with regression analysis. For example, instead of being dismayed at the effect of income in the non-randomised smoking study, you can record it for each subject, and find the best equation that relates smoking, income, and life expectancy. This requires that the researcher anticipate these possibilities, and collect the data in the first place. The ideal is to collect data on several indicators, and randomise for the rest.

In any case, all inferences depend on the scientific method, which most importantly includes debate within the whole scientific community. In the case of medical research, one of the key aspects of debate is that conclusions about cause and effect must have a plausible mechanism. Thus, even if you did a randomised trial and your results showed (by pure fluke) that homeopathic medicine worked, scientists would remain skeptical. It's been emotional (talk) 18:27, 11 August 2008 (UTC)

With the current flood of medical data, control groups are being formed for research using existing data. It is easy to find large groups of people who are not treated, but have a rich medical history. For example, if I wanted to do research on a drug to control HbA1c levels, I can get a large group of people who have had their HbA1c labs performed over the last few years and had no treatment and compare them a group I'm now forming to take a new treatment. I can also include the control group in my study to show how they responded without treatment before the study and with treatment afterwards. At this time, medical data is a growing field of study. So, most researchers do not use existing data. In areas where there is a strong effort to collect medical data, researchers are using the data for, at a minimum, forming a hypothesis for study. -- kainaw™ 18:38, 11 August 2008 (UTC)

Caffeine timing and sleep

If I want to fall asleep by 12:30 am, what time of day do I need to stop having caffeine? NeonMerlin 19:49, 9 August 2008 (UTC)

My experience says that caffeine sensitivity varies wildly between people both in terms of length and powerfulness of effect. I personally need about 6-8 hours for a large amount of caffeine, but I've known other people with other timing. Magog the Ogre (talk) 19:58, 9 August 2008 (UTC)

Concurred...it's very variable (see Caffeine and maybe Caffeine and healthCoffee and health for general info). I can down 4-6 cups and go to sleep within an hour, but I'm a heavy coffee drinker and am usually sleep-deprived. DMacks (talk) 20:35, 9 August 2008 (UTC)

(fixed link) DMacks (talk) 21:54, 9 August 2008 (UTC)

Well the half-life of caffeine is 9-11 hours, according to our article. It depends on how quickly your body absorbs the caffeine and how good your liver is at breaking down the stuff. —Cyclonenim^T@lk? 00:21, 10 August 2008 (UTC)

It definitely varies by person. I recommend experimenting on yourself. By trial and error, I found that for myself, if I stop drinking coffee before 3pm I usually won't have trouble falling asleep at 11pm. ike9898 (talk) 17:36, 11 August 2008 (UTC)

Space-time equation

Since space and time are relative, could scientists ever come up with an equation to turn time into space and vice-versa ? 69.157.227.243 (talk) 20:08, 9 August 2008 (UTC)

The long answer: almost anything is theoretically possible, given how little we know about the nature of the universe, and the possibilities that may exist.

The short answer: no: the two are completely different measures. Magog the Ogre (talk) 20:24, 9 August 2008 (UTC)

I don't know about "completely different measures." See Spacetime. --Allen (talk) 20:41, 9 August 2008 (UTC)

I'm not sure what you mean... an equation describes something, it doesn't do anything. Space and time are distinct (it comes down to a minus sign in the appropriate places), but are closely interlinked. You can come up with a new co-ordinate system that combines the usual space and time co-ordinates, but you'll always have one time-like and three space-like co-ordinates. Interestingly, within the event horizon of a black hole, time becomes space-like and the radial space direction becomes time-like, but that's not really time and space swapping round, it's just a technical thing. --Tango (talk) 21:33, 9 August 2008 (UTC)

Yes, I suppose I didn't quite read the question carefully. The answer is actually yes, they can be interchanged, but not in a practical way. By traveling at an extremely high rate of velocity (i.e., very close to the speed of light), one can essentially skip the time in between. This is time dilation. One could theoretically travel to the other side of the galaxy and back in a few minutes, and millions of years would have passed back at Earth. This would of course require astronomical amounts of energy.

This might not be the pretty solution you're looking for, but I don't know what else you might want: it's not like scientists could just remove a triangular piece of space. Magog the Ogre (talk) 02:50, 10 August 2008 (UTC)

That's not really interchanging space and time, it's just interchanging the rate at which you travel through each. --Tango (talk) 02:57, 10 August 2008 (UTC)

I thought the whole point of talking about "space-time" is that they are just properties of the same stuff, dimensions within the structure of the universe, and that our manner of perception creates the duality. I feel like your question is similar to asking if you could interchange between surface area and depth. Is my understanding of spacetime that far off? --Shaggorama (talk) 14:46, 10 August 2008 (UTC)

The whole point of Einstein about spacetime is that when you try to measure space, you are also making measurements of time, and when you try to measure time, you are also making measurements of space. This itself is not very surprising, except when you throw into this bit, relative mix the fact that there are a number of key invariants, things that aren't relative, like the speed of light. Then there are important consequences. It is a geometrical argument about the structure of measurement and its consequences. It's quite clever. But it's not "space and time are exactly the same thing and you can change one into another" or anything like that. It's that space and time are, quite logically and not mysteriously, inextricably linked to one another and there are practical consequences of that. --98.217.8.46 (talk) 15:21, 10 August 2008 (UTC)

Recent Horrible News!

Recently I've gone through an article of NASA where it was stated that the Earth will be annihilated in 2012. Is it so truely? Is there no way to escape it? Anyone to answer is thankfully welcome.117.201.96.57 (talk) 20:04, 9 August 2008 (UTC)

No, it's completely false, despite claims by some (e.g., An Inconvenient Truth) - where did you read this, and did you truly find it plausible? Magog the Ogre (talk) 20:18, 9 August 2008 (UTC)

An Inconvenient Truth does not claim the Earth will be annihilated in 2012. I'm sure you were joking, but someone not familiar with the movie might not realize that. --Allen (talk) 20:22, 9 August 2008 (UTC)

Yeah, I was kidding (sorry, too much Glenn Beck on CNN anymore). Magog the Ogre (talk) 20:25, 9 August 2008 (UTC)

Actually, I've read these news on the Magazine 'Science Reporter'. Thank you all, for answering. I am relieved from a tension now.117.201.96.57 (talk) 20:27, 9 August 2008 (UTC)

The Reference Desks have already had two discussions on this matter; see [4] and [5]. --Bowlhover (talk) 20:32, 9 August 2008 (UTC)

An article by NASA? That sounds unlikely... was it actually from NASA or was it from some random person claiming that it was NASA that had originally said it? If the latter, they are simply lying. It's just a load of crackpots talking nonsense. --Tango (talk) 21:35, 9 August 2008 (UTC)

I'm with tango. I'm challenging you to produce this article, as I'm sure we'd all love to read it. --Shaggorama (talk) 14:40, 10 August 2008 (UTC)

None of the feature article descriptions of Science Reporter that I looked at (January 2008 - August 2008) had any mention of the alleged 2012 apocalypse. Which issue was it in? Confusing Manifestation(Say hi!) 23:31, 10 August 2008 (UTC)

Comet of Sodom

Look at this : they found the bits and pieces of an asteroid that destroyed the Sodomites in 3123 BC. [6] Explain this one ! San Francisco watch out! 69.157.227.243 (talk) 20:36, 9 August 2008 (UTC)

What do you want explained? An asteroid hit an ancient city, people at the time didn't understand what had happened and assumed it was the wrath of god and, after a few rounds of Chinese whispers, it got written down in a book that eventually became part of the Old Testament. Doesn't seem at all odd to me. There is no reason to believe it actually had anything to do with the residents of the city being particularly sinful. --Tango (talk) 21:39, 9 August 2008 (UTC)

I wonder if "March 31, 2008" explains this one? I think we have more to fear from people who don't use all the facts at hand and look for excuses to justify their biases than from comets:) But even today, otherwise-intelligent people forget (or willfully ignore if it suits their agenda) that correlation does not imply causation. Stock market went up the other day just after I drank coffee, but I'm not gonna sink my life savings on Wall Street and head to Starbucks to make myself rich. DMacks (talk) 22:38, 9 August 2008 (UTC)

From the article: "Dr Hempsall said that at least 20 ancient myths record devastation of the type and on the scale of the asteroid’s impact, including the Old Testament tale...." Emphasis mine and mine alone. -- MacAddct  1984 ^{(talk • contribs)} 22:40, 9 August 2008 (UTC)

Ummmm, there is no mention of any "bits and pieces" being found. The article is about working from half of a 4-inch clay copy of a 2.4 thousand year old clay tablet to pinpoint the hour, 5000 years ago, of an observation that lasted no more than 10 seconds.

Meteorites have impacted Earth many times. How about this and this and this and this or any of these? It happens because of gravity and chance. Saintrain (talk) 23:17, 9 August 2008 (UTC)

Asteroids of the size mentioned in the article, however, only strike Earth once every 80 000 years. If an asteroid "more than half a mile across" really did wipe out Sodom, the incident would be the only major impact event in human history that we know about.

An impactor usually creates a crater twenty times its own diameter, so a half-mile-wide asteorid would create a 10-mile-wide crater, one that should have been discovered long ago. It's also interesting that the article claims the asteroid's impact released energy "equivalent to 1000 tons of TNT exploding", which would imply it was much less energetic than the Tunguska event. A realistic figure would be 50 000 million tons. --Bowlhover (talk) 06:25, 10 August 2008 (UTC)

OK, yeah, glad to see I'm not the only one casting a skeptical eye on this (I've seen other junk articles from timesonline, btw). A piece of pottery talking about a meteorite from 2300 years beforehand, before there was writing, while there were no accurate measurements around? I suppose it is possible (the Semites had a sick amazing oral tradition), but it seems highly unlikely - along the lines of Atlantis. Magog the Ogre (talk) 02:57, 10 August 2008 (UTC)

As Macaddict and Magog tO, there may be the impeccably si-ick oral traditions that leave our media in the shade with feats of memory making silicon valley look primitively mechanistic but when it comes to the causal connections, there's magical thinking to consider. Julia Rossi (talk) 10:01, 10 August 2008 (UTC)

I can't find my mistake

I want to find an equation for the final velocity of an object that is dropped from a height h (not assuming g remains constant), but the answer I keep getting is wrong.

${\vec {F}}=m{\vec {a}}$

${\frac {GM_{e}m}{{\vec {r}}^{2}}}=m{\vec {a}},$ G is less than 0.

$d{\vec {v}}={\frac {GM_{e}}{{\vec {r}}^{2}}}dt,dt={\frac {d{\vec {r}}}{\vec {v}}}$

$\int _{{\vec {v}}_{0}}^{\vec {v}}{\vec {v}}\,d{\vec {v}}=\int _{-(r_{e}+h)}^{-r_{e}}{\frac {GM_{e}}{{\vec {r}}^{2}}}\,d{\vec {r}}$

${\frac {{\vec {v}}^{2}-{\vec {v_{0}}}^{2}}{2}}={\frac {GM_{e}}{r_{e}}}-{\frac {GM_{e}}{r_{e}+h}}$

${\vec {v}}={\sqrt {2GM_{e}({\frac {1}{r_{e}}}-{\frac {1}{r_{e}+h}})+{\vec {v_{0}}}^{2}}},$ where G=gravitational constant, re=radius of the earth, Me=mass of the earth, and h=height the object was dropped at.

Everything seems right, but the final answer's imaginary: somewhere I've made a mistake with a negative, but I don't know where. Please help, and thanks.

Edit: I'm considering down (i.e. towards the surface of the earth) as positive. —Preceding unsigned comment added by 76.68.246.7 (talk) 23:25, 9 August 2008 (UTC)

Why have you said that G is less than zero? It isn't. G is positive, and you need a minus sign in the second equation. Those cancel out, though, so won't affect the final answer. Next, why are the limits on your integral -r_e and -(r_e+h)? Where did those minus signs come from? The radius of the Earth is a positive number. I think making those limits positive will fix your problem. --Tango (talk) 00:09, 10 August 2008 (UTC)

Sometimes G is considered negative and the minus sign in the second equation is ignored, it doesn't make a difference, people have different preferences. As for the limits in the intergral, they're negative because I'm considering the downwards direction as positive. In other words, the initial position of the object is -(r_e+h) because the object would need to travel a positive distance (r_e+h) to reach the earth's centre. Notice that change in displacement is a positive number (h), but if I treated the limits as positive, the change in displacement would have been negative.

To better illustrate this, consider the object as undergoing a uniform acceleration (down is positive). In this case, v=2aΔs, v=2a(r_f-r₀), and because r_f is 0, this equation wouldn't make sense if r₀ was positive.

You can't both take G as negative and down as positive, that's self-contradictory. Gravity is directed towards the center of the Earth and hence the force of gravity must have the same sign as "down". Dragons flight (talk) 01:47, 10 August 2008 (UTC)

Exactly. Gravity acts to reduce distance, which is why there's a minus sign in Newton's universal law of gravity (or, equivalently, G is taken as negative, although I've never seen that). If you measure distance backwards, you'll have to get rid of that minus sign. --Tango (talk) 02:13, 10 August 2008 (UTC)

Well, if down is taken as positive, then r would be negative, and so G (or Newton's universal law of gravity) would still have to be negative, right?

Edit: Of course, r^2 is still positive, my bad. Thanks. —Preceding unsigned comment added by 76.68.246.7 (talk) 03:10, 10 August 2008 (UTC)

I think it's best just to keep G as positive always and add in minus signs where you need them. Changing the sign of a physical constant to suit your co-ordinate system just seems confusing to me... --Tango (talk) 03:30, 10 August 2008 (UTC)