Wikipedia:Reference desk/Archives/Science/2011 March 26

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March 26

time

if I was using a telescope and looking at a clock 10 light minutes away, which I saw was 10 minutes behind mine, 8:50 and 9:00, respectively, and then started travelling to the clock, observing it the whole time, I know it depends on my speed, but what would I observe the clock doing in relation to mine; what time would the clock display? —Preceding unsigned comment added by 98.221.254.154 (talk) 01:21, 26 March 2011 (UTC)

Given that ly is mesurment of distance light travels in certain time you can easily calculate it, but the result depends solely on speed. At 10 minutes you'd see 10 minute delay, at nine minutes you'd see nine minute delay and so on upto no delay when you are at the clock. Assuming you travel at speed of light, it would be 9:10 when you reach the clock. While you're on your way you'd observe clock hands moving clockwise (i.e. not backwards in time) and the clock being behind in accordance to how far you are from the clock, so at 9:01 there would be 9 minute delay and you'd see that it is 9:52, 9:02-8:54, 9:03-8:56, 9:04-8:58, 9:05-9:00, 9:06-9:02, 9:07-9:04, 9:08-9:06, 9:09-9:08, 9:10-9:10 ~~Xil (talk) 04:39, 26 March 2011 (UTC)

The answer above neglects time dilation and problems with relativity of simultaneity. A good place to start trying to understand how these play a role is the twin paradox article. Dauto (talk) 13:39, 26 March 2011 (UTC)

The twin paradox is about acceleration. There is no need to assume any acceleration is going on in this problem. --Tango (talk) 15:35, 26 March 2011 (UTC)

The wording is somewhat ambiguous, but on first reading I assumed that "started travelling to the clock" meant that the observer was initially at rest with respect to the distant clock, and then accelerated as they "started travelling". I imagine Dauto made the same assumption. Gandalf61 (talk) 16:32, 26 March 2011 (UTC)

What about travelling at a fraction of the speed of light? —Preceding unsigned comment added by 98.221.254.154 (talk) 14:27, 26 March 2011 (UTC)

If you travel much slower than the speed of light (so it would take you several days to reach the clock), then we can ignore relativity and the solution is fairly simple. If the clock is 10 minutes behind yours now and we know it will exactly match yours when you arrive, then you must see it ticking faster than yours by such an amount that, in the time it takes you to reach the clock, it's ticked 10 minutes more than yours. If it took you 20 minutes to reach the clock (which is much too fast for us to neglect relativity, but it makes the maths easy!), your clock would have progressed 20 minutes and the other clock 30 minutes, so the other clock is ticking 1.5 times a second, from your point of view. If you take into account relativity, then your clocks wouldn't show the same time when they come together and the whole thing gets rather complicated. --Tango (talk) 15:35, 26 March 2011 (UTC)

If you don't take relativity into account, it probably could be be reduced to simple formula, similar to calculating when will two trains traveling towards each other at diffrent speeds meet, I find it hard to produce it without pen and paper, sorry. You would probably assume that all things except for your speed, which would be, x remain constant. Say if you would be traveling at half the light speed at 9:01 you would be 9.5 light minutes away from clock, so you would observe 9.5 minute delay and observe that time on the other clock is 8:51:30. I might be terribly wrong, but I don't see how relativity could make the answer any diffrent, if you are traveling at light speed. We assume that both the observer and the clock are in same state (not influenced by gravity or anything), we assume that the observer has never been at the clock (so twin paradox dosen't really apply, because it concerns traveling away and then back to a point of reference influenced by gravity etc.), his clock is synchronised with clock he observes and he does observe some effects of relativity as for him it appears that clock he monitors is delayed. If you have a diffrent idea on what would happen go ahead and tell us what he would observe ~~Xil (talk) 16:33, 26 March 2011 (UTC)

Twin paradox has nothing to do with gravity and it does apply here (half a trip still requires relative motion leading to time dilation). Dauto (talk) 17:07, 26 March 2011 (UTC)

So, here is the math. Let's say you move with a speed $v\,$ for a distance $d\,$. A time $t={\frac {d}{v}}\,$ elapses from the point of view of the clock you are looking at through the telescope which was initially synchronized with yours. Because of time dilation the time elapsed on your clock will be given by $t'={\frac {d}{\gamma v}}\,$, where $\gamma ={\frac {1}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}\,$. Therefore when you finish the trip the two clocks won't be synchronized anymore and the difference between them will be $t-t'={\frac {d}{v}}\left(1-{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}\right)\,$. Dauto (talk) 17:32, 26 March 2011 (UTC)

... isn't that the wrong way round? (Perhaps it depends on who is doing the observing.) Each observer sees the other clock moving more slowly, but their own clock moves at "standard rate". The effect is symmetric. Dbfirs 16:55, 27 March 2011 (UTC)

The effect is not symmetric because one of the clocks must accelerate to start approaching the other clock just like in the famous twin paradox situation. Dauto (talk)

Sorry, yes, I meant that the effect is symmetric whilst travelling at a constant relative speed. It is the effects of the acceleration that creates the paradox. Dbfirs 07:23, 28 March 2011 (UTC)

prediction

Is saying what I am going to do, and then doing it exactly as I described "predicting/manipulating the future"? Is answering someone's question before they ask it, because you know what they are thinking, "information travelling faster than light"? —Preceding unsigned comment added by 98.221.254.154 (talk) 03:22, 26 March 2011 (UTC)

No, and no, that is called intuition, categorised under psycology not relativity. Plasmic Physics (talk) 04:02, 26 March 2011 (UTC)

It is being able to control your own acctions to accomplish your goals and drawing on your expierience form numerous similar situations you've been in. You can do so with a large degree of certanty about what is going to happen, but that is not what people usualy describe as beeing able to predict and manipuate the future - if you could predict that your friend is going to get in car crash if he turns left and it happens it would be predicting future as you have no means to predict random accidents, if you convince your friend not to take that turn you are manipulating the future (but still we can never be certain that the crash would occur) ~~Xil (talk) 04:55, 26 March 2011 (UTC)

I would say that that is predicting the future, as there could be events which prevent you from doing what you planned. Of course, if you do keep the prediction, it's not very impressive. Similarly some psychics predict that there will be "unrest in the Middle East", and they don't deserve much of a prize for that prediction, either. StuRat (talk) 06:47, 26 March 2011 (UTC)

The second question could be generalized to "is simulation a potential means of time travel", and when phrased like that it sounds fairly reasonable (if confusing). 213.122.59.22 (talk) 08:12, 26 March 2011 (UTC)

Physics entire goal is the ability to make qualitative and quantitative prediction about the future, but it still flatly denies the possibility for superluminal information transfer. This should highlight how they are different. —Preceding unsigned comment added by 92.20.201.71 (talk) 11:01, 26 March 2011 (UTC)

Anything you do is manipulating the future. I'm not sure if it counts as predicting if it's something you have control over, but looking it up it seems to work you're still telling in advance, even if you have a hand in it. Answering someone's question before they ask it does not involve information traveling fast. They gave the information before-hand, if unintentionally. — DanielLC 05:47, 28 March 2011 (UTC)

Question about the sub-division of "g"?

Since "g" of an earth may be the combination of inter-gravitational forces between its masses of water, soil, rocks ......[on large scale]. Thus would there be any difference in "g" for the falling of an object due to the following different underlying masses

1- Just above the deepest and largest sea water (mass wise)

2- Just above shore or land close to the shore

if my question is not wrong?74.198.150.216 (talk) 04:53, 26 March 2011 (UTC)Eccentric Khattak#1-420

Yes, mass differences in the ground or water directly under an object will affect the local g by a measurable amount, but not an amount people would notice. StuRat (talk) 06:42, 26 March 2011 (UTC)

See Earth's gravity and geodesy for the study of Earth's gravity, and the GRACE and GOCE satellites which measure the local gravitation field. CS Miller (talk) 12:23, 26 March 2011 (UTC)

Also read our article on gravity anomaly. The gravity anomaly is the measured difference between the expected and actual value of the gravitational force. It is useful when exploring for underground minerals and hydrocarbons. Mapping the gravity anomaly is also useful for calibrating extremely precise aircraft avionics on autopilots and trackers. Usually, though, it is very expensive to perform large gravity mapping surveys, so most of the data isn't available for free on the internet. I did find this example through a web-search: this United States Geological Survey webpage, Airborne Gravity Survey and Ground Gravity in Afghanistan. Their website has scientific information, as well as photos of the aircraft and the equipment used to measure gravity very precisely. Note that you can see the gravity anomalies trend along the terrain and geology, correlating well with the mountainous regions and the river valleys. Other methods of gravity survey include satellite- and ground-based observation and mapping. Nimur (talk) 15:04, 26 March 2011 (UTC)

And for a satellite pair dedicate to gravitational mapping, see Gravity Recovery and Climate Experiment. -- 110.49.249.12 (talk) 23:57, 26 March 2011 (UTC)

Name/purpose for this railroad bridge feature.

What is the name/purpose for the extra set of rails between the main rails on a railroad bridge? They usually angle inward at the ends of the bridge. Here is a picture http://farm1.static.flickr.com/62/225549229_5dcf181f3b.jpg --71.98.72.247 (talk) 05:03, 26 March 2011 (UTC)

Just a guess, but could it be a safety feature, so that if the train jumps the rail, those will catch one set of wheels, so the cars won't be able to hit the sides of the bridge ? StuRat (talk) 06:39, 26 March 2011 (UTC)

Yes, it is called guard rail ~~Xil (talk) 12:00, 26 March 2011 (UTC)

Nice find. Note that the inward bend at the ends is so that a derailed train won't run into the end of the guard rail, making things even worse. StuRat (talk) 21:14, 26 March 2011 (UTC)

mosquito bites and race

Can anyone find information about why mosquitos prefer to feed on Europeans than Africans? My own research has found that when sleeping in the same bed, I get bitten all over, whereas my friend does not at all. The scientist in me can't work out why it is the case, what is it about mzungus that they find so tasty? Thanks 41.138.85.43 (talk) 11:33, 26 March 2011 (UTC)

Isn't it possible that certain people attract or dis-tract mosquitos more than others. Some people simply do not get bitten. Your presumption that they avoid Africans is mis-placed if it's based solely on the fact that mosquitos are more attracted to you than your one African friend. 220.244.35.181 (talk) 11:47, 26 March 2011 (UTC)

The huge number of Africans who contract mosquito borne diseases such as malaria also belies your theory. Roger (talk) 12:13, 26 March 2011 (UTC)

No, his/her argument is that Africans are less likely to be bitten by mosquitoes than Europeans are. Malaria rampant areas are virtually free of Europeans, so your counterargument doesn't stand. That said, his/her argument is still fallacious, as it is a plain dicto simpliciter.--Netheril96 (talk) 12:55, 26 March 2011 (UTC)

I have heard the same "mosquitoes like the white skin" story. There are also various stories running around the Internet^[1] saying that mosquitoes prefer blondes to brunettes, and blondes are rare in Africa. But I also wonder if there are any differences in the probability that an individual mosquito bite will turn out to be itchy - whether due to enzymatic reaction, immune tolerance, or some kind of mental desensitization. For that last, after all, you see so many scenes of people in Africa with flies crawling on them that will make an American squirm just watching the video. I wonder if it's so different from being accustomed to tolerate cold weather. Wnt (talk) 15:44, 26 March 2011 (UTC)

The first mosquito bites 9f the summer are the most itchiest. You build a tolerance but you also lose it without exposure. --85.78.197.19 (talk) 08:29, 27 March 2011 (UTC)

I don't think this is a safe generalization: there are also white people who seem to get bitten a lot less than other white people. The fact is that mosquitoes are sensitive to a pretty wide range of chemicals, and the specifics are not at all well understood. Even the mechanism of the most widely used repellent, DEET, is not all that clearly understood. Until recently the prevailing theory was that it blocks the ability of the mosquitoes to sense the chemical that attracts them, but anybody who uses it in the field can easily see that it has a strong repulsive effect. Looie496 (talk) 17:00, 26 March 2011 (UTC)

Certainly a sample size of n=2 is not adequate for drawing such a conclusion. You and your friend different in things other than just race, presumably, including clothing, deodorant, shampoo, diet, etc., even blood type. How much of that matters to mosquitos, I know not, but isolating race as the only variable is surely fallacious in this case. --Mr.98 (talk) 18:05, 26 March 2011 (UTC)

Two major attractors for mosquitoes are heat and water vapor. It could simply be that you sweat more. --Carnildo (talk) 22:01, 29 March 2011 (UTC)

Fission products in environment => nuclear reactor breach?

If radioacctive iodine has been found in the environment, is that conclusive evidence that at least one of the Japanese nuclear reactor pressure vessels has split or cracked open, or could it come from the stored fuel rods? Thanks 92.29.127.59 (talk) 15:13, 26 March 2011 (UTC)

It most likely comes from the spent fuel rods. Dauto (talk) 16:58, 26 March 2011 (UTC)

A rough guess: if it is Iodine-131, that probably means it is from the reactor (it is a short-lived fission product and its existence would only be significant in a reactor that had recently been running). If it is Iodine-129, it could be either. But the older the fuel, the more Iodine-129 you'd find, so significant amounts of it would probably be more indicative of a spent fuel pool breach. But I am not a physicist. --Mr.98 (talk) 18:01, 26 March 2011 (UTC)

The report discussed here seems to say it is indeed Iodine-131 that has been found, and that it suggests Reactor 3's containment has been breached. So there you go. --Mr.98 (talk) 18:08, 26 March 2011 (UTC)

(ec) No one can know from afar what really happens but iodine decays very quickly and is only produced by fission or neutron capture so it looks improbable that it comes from spent fuel rods. As far as I understand the pressure vessels are not absolutely tight but have valves that regulate pressure. So my idea is that some fuel rods are damaged and the fission reaction is partly going on, producing new iodine that is released from the containment via safety valves. 77.3.187.97 (talk) 18:16, 26 March 2011 (UTC)

The fission has stopped definitely. If they open the security valve to reduce pressure you vent iodine, there is enough iodine in the reactor even after four or five half lives.--Stone (talk) 18:20, 26 March 2011 (UTC)

I admit that I don't know how much total iodine is inside the core and how much has been released but how would you know that the fission has stopped "definitely"? 77.3.187.97 (talk) 18:29, 26 March 2011 (UTC)

The fission is only working with the moderator (water) and without the control rods. Two possible cases: It is very hot and the water is gone -- no chance for fission. Everything is molten -- No reaction, because you have a molten block with no water inside. Two last point the neutronradiation would have been detected and without cooling the reactor would simply melt away if not only decay heat but also the much higher heat production of fission would be present.--Stone (talk) 19:23, 26 March 2011 (UTC)

OK, I see your point(s). If there is no other type of moderator (as graphite like it was used in Chernobyl), fission would stop. But if there was some problems with the control rods (? jammed from the earth quake and not completely inserted ?), wouldn't fission resume when new cooling water was inserted? Of course, only partially, not full scale as in normal operation. Another question: how would one detect neutron radiation from within a water filled containment? Wouldn't even the concrete of the containment shield that completely? 77.3.187.97 (talk) 20:11, 26 March 2011 (UTC)

The control rods were inserted after the earth quake before the tsunami. The reactor always stays within a very thin margin of criticality where the secondary neutrons are used to keep control. If something goes really critical you leave this safe way and get a bomb like explosion.--Stone (talk) 20:57, 26 March 2011 (UTC)

To be clear, that's nothing like a thermonuclear bomb, however. StuRat (talk) 21:54, 26 March 2011 (UTC)

A slow fission reactor cannot undergo a "bomb like explosion". You get lots of radiation and heat, and maybe a small explosion, but nothing on the order of a fission bomb. --Mr.98 (talk) 22:14, 26 March 2011 (UTC)

Not a nuclear bomb explosion scale, but a explosion more like a small grenade. The conversion rate in a nuclear bomb is high while it is low in a criticality accident. But it is enough to produce enough heat to divide the critical mass into separate parts.--Stone (talk) 08:38, 27 March 2011 (UTC)

Surely you can see the need to clarify what one means by a "bomb like explosion" when one is talking about critical masses. Most people still think nuclear plants can go off like nuclear weapons, which is not true. They have their own problems, but that's not one of them. --Mr.98 (talk) 10:06, 27 March 2011 (UTC)

As I understand it, the control rods were actually inserted before the earthquake: Japan's earthquake warning system gave the reactors about 30 seconds' warning of the quake, so the reactors were shut down before the shaking started. --Carnildo (talk) 22:06, 29 March 2011 (UTC)

Does symbolic actions make people change their behaviour?

Today's Earth Hour and the discussion of the pros and cons of it made me wonder what research there is about whether people who participate in some symbolic action (like signing a petition or wearing a button) are more or less likely to do something more substantial for the same cause (like donating money and time or sacrificing some of their comfort). I recall reading at least ten years ago about some study that seemed to show a positive correlation, but I don't remember it well. Sjö (talk) 18:25, 26 March 2011 (UTC)

I think so, it opens them up to Cognitive dissonance. 92.29.127.59 (talk) 19:03, 26 March 2011 (UTC)

You think they are more or less likely to do something? And do you know of any research about it?Sjö (talk) 19:27, 26 March 2011 (UTC)

The article mentions the Ben Franklin effect for example, and that cites some papers. 92.29.127.59 (talk) 19:37, 26 March 2011 (UTC)

Note that correlation doesn't mean causation. That is, observing Earth hour doesn't necessarily make them more environmentally responsible. It's probably the other way around, that people who already were environmentally responsible are those who choose to observe Earth hour. Then there's also the possibility that non-environmentalists might do Earth day just so they can say "I've done my part, now I am done". StuRat (talk) 19:29, 26 March 2011 (UTC)

The article gives some data developed by the experiment. Presumably, policy makers could use this in decisions - e.g., how much greenhouse gas can be avoided if monopoly utilities are allowed to run amuck and raise prices 200%, which allows them to feel better about taking the campaign contribution. Wnt (talk) 21:26, 26 March 2011 (UTC)

Is that assuming that any power which can be cut for an hour can also be cut permanently ? That's completely untrue. I can cut my heat for an hour, but if I cut it permanently my pipes would freeze in winter (as would I). StuRat (talk) 21:51, 26 March 2011 (UTC)

A resounding "yes". Symbolic actions are extremely powerful in terms of effecting human behaviour.It is the human mind's suseptability to symbolism that produces the Ra-Ra to the flag,the puchase of a new car when when the old one is still good,The badge and button wearing, choice of dress style,The Symbolic identification of ones self for others to see. The very identification of homo-sapiens as distinct from earlier hominids is based largely on appreciation of symbolism. The earliest known symbolism,"art"about seventy thousand years ago marks the generally accepted transition to modern humans. Symbolism effects not only the person taking the action but also those who witness it. I suggest you look up the acheivements of public manipulation through symbolism acheived by "Edward Bernayse" throughout most of the twentieth century. I beleive that will give you a much fuller appreciation of the power of symbolism.Phalcor (talk) 17:01, 27 March 2011 (UTC)

I suggest reading "Public relations" published 1952 and "The engineering of consent" 1955.Phalcor (talk) 19:08, 27 March 2011 (UTC)

How about Man and His Symbols by Carl Jung ? StuRat (talk) 19:10, 27 March 2011 (UTC)

collecting water ... using oil

Does this idea violate any laws of thermodynamics? Suppose that air is not saturated -- i.e. evaporation for a naked body of water should be faster than condensation. Suppose I place a large bowl with a large surface area, full of oil.

Suppose water occasionally does condense on the oil. Some of the water will re-evaporate, but some will sink to the bottom -- and a pool of water starts to collect at the bottom of the oil, unlikely to evaporate. Over time, I will accumulate more and more water. Is any law of thermodynamics being violated here? (Or does the oil conceivably become warmer and warmer, lowering likelihood of condensation with each new water being condensed?) AFAIK, oil is acting only like a heterogeneous nucleation-promoting catalyst -- it shouldn't change the equilibrium constant.

It occurs to me that agitation might allow faster introduction of water vapor into the oil, upon which it condenses. This would expend work, which would conceivably then allow the equilibrium constant to change. I however did not study heat engines and chemistry in my general chemistry classes. Are there any relations between mechanical agitation and work -- and say the amount of water I can extract from the air? John Riemann Soong (talk) 18:32, 26 March 2011 (UTC)

I don't see any inherent reason why the first approach would violate thermodynamics laws. However, as a practical matter, the few molecules of water which would condense on the oil would evaporate again before they formed drops large enough to overcome the surface tension and sink to the bottom. Therefore, I think a significant temperature difference would be required to get it to work. Perhaps in the morning, if the bowl of oil was in the shade, and stayed cool for some time, water from the warmer air would condense in sufficient quantities to form drops that would sink to the bottom. However, note that oil isn't required for this. You can have a large funnel to which dew will adhere in the morning, then drip down into a bottle at the bottom. The small hole at the top of the bottle slows to evaporation rate back out of the bottle. You could, I suppose, put some oil in the bottle too, to further slow the evaporation rate, but then you'd have oil mixed in with your water, and the oil might go rancid. As for agitation, that would likely just produce a mixture, like you get when you shake up salad dressing. It would eventually settle out, but it could take many hours or days. Also, the energy used might better be spent on a dehumidifier to collect your water. StuRat (talk) 19:22, 26 March 2011 (UTC)

I'm thinking about the law of microscopic reversibility. I've changed the conditions to favour one direction of the reaction over the other, and so I've distorted the equilibrium somehow. John Riemann Soong (talk) 20:52, 26 March 2011 (UTC)

Like Maxwell's Demon ? StuRat (talk) 21:12, 26 March 2011 (UTC)

Yes. The energy of the environment hasn't changed has it? Neither the energy of the condensed water. (per mol) John Riemann Soong (talk) 21:13, 26 March 2011 (UTC)

In concept you're introducing some energy when the water droplet falls under the oil (if you come up with a way to make this work). So it's not a true Maxwell's Demon, but one using energy to do sorting. Wnt (talk) 21:29, 26 March 2011 (UTC)

The experiment you've described, unfortunately, isn't realistic. As StuRat notes, if the oil is in thermal equilibrium with the humid air, and the air (and oil) are both at a temperature higher than the dew point, then macroscopic amounts of water will never condense out on the oil's surface. You certainly won't ever get droplets large and heavy enough to overcome the surface tension of the oil in order to sink to the bottom.

Even if you start out with water below the layer of oil, the system will still return to the 'proper' equilibrium eventually; you've neglected to consider that water's solubility in oil is low but not zero. There will be a (very slow) transfer of water out of the liquid phase, into the oil-dissolved phase, and back into vapour. (Depending on the temperature and thickness of the oil layer, this is a process which could require months, years, or centuries, but it will happen.) This evaporation through the oil will continue until the air reaches 100% humidity for the temperatures, or until the supply of liquid water is exhausted. Agitation will speed this process, in that it will speed the equilibration of water across the water-oil and oil-air interfaces. TenOfAllTrades(talk) 04:37, 27 March 2011 (UTC)

And apart from all the other technicalities, from a purely practical standpoint. Having experienced on more than one occasion a sudden camping trip, I can tell you that to collect a useful amount of water by natural condensation (no matter how you do it) will require a surface area much, much,much,much,much greater than a large large bowl.Phalcor (talk) 19:36, 27 March 2011 (UTC)

Solution to the colatitude part of Schrödinger in a Hydrogen atom

For some research I'm doing, I'm trying to work through the derivation of the solution to the Hydrogen wavefunction to garner a better understanding of the mathematics behind it. In my searches, I've found that not many people include a detailed explanation of the math behind the differential equation involved in the colatitude ($\theta$) part; most textbooks or informational sources that I've found have simply said something along the lines of, "This is a very hard differential equation. Here's the answer:" I know the final answer comes out to be the associated Legendre function, but I would like to know how this is derived.

I did find, this, however. This page outlines how to set up the differential equation, and this page shows how to solve it. I understand the first page entirely, but I'm caught up on the second page. Specifically, the part when the author expands the nth derivative to obtain Equation 3.3. My questions

1. Why does the author switch from trying derivatives of $(1-x^{2})^{n}$ to trying those of $(x^{2}-1)^{n}$? My first thought was to get the sign of the middle term to match the original DE, but when he derives the (n+1)th derivative, the sign changes due to the fact that n-(n+1)=-1, so I don't really see why he chooses that as his function. Can anyone explain that?
2. Where the sum comes from in the (n+1)th derivative?
3. I think there's a typo in Equations 3.3 and 3.4.. Should the derivative operators not be ${\frac {d^{n}}{dx^{n}}}$ instead of ${\frac {d^{2}}{dx^{n}}}$?
4. Also, I don't understand Equation 3.4.. Why can we just randomly throw in the ${\frac {1}{2^{n}n!}}$ term?

Thanks for the help!--Dudemanfellabra (talk) 18:35, 26 March 2011 (UTC)

You might have better luck searching for discussions of the spherical harmonic differential equation. It's reasonable enough for discussions of the hydrogen atom to skip the spherical harmonic part, since the spherical harmonics are an area of study in their own right.

I'll attempt to answer your questions: 1. I think he tries $(1-x^{2})^{n}$ first, and then, since that doesn't quite work, tries $(x^{2}-1)^{n}$ instead. Trial and error (informed by long experience) is one of the most useful techniques for solving differential equations. 2. I think this comes from a sum of the coefficients of $2(n-1)xw',2(n-2)xw'',2(n-3)xw''',\ldots$. If you do a few derivatives by hand you should see what's going on. Note that $w^{n}$ is a typo; it should be $w^{(n)}$. 3. Yes, that's another typo. 4. You're defining a separate function for each $n$, and these functions are only meaningful up to a constant factor, so it's fine to throw in a factor that depends only on $n$. -- BenRG (talk) 21:03, 26 March 2011 (UTC)

The standard practice of solving (associate) Legendre polynomial equation is power series method.The current Wiki article on that method is incomplete as it only pertains to nonsingular equations, which is almost useless when you encounter special functions. You may find several textbooks on mathematical physics most useful.--Netheril96 (talk) 16:33, 27 March 2011 (UTC)

another current

could we ever ever find another type of current something like oscillating current or rotating current instead of alternating or direct current. Is there only ac and dc? Is there such a thing as magnetic current with magnetic monopoles flowing and that creating a electric field — Preceding unsigned comment added by Lufc88 (talk • contribs) 19:29, 26 March 2011 (UTC)

Well, there's only the possibilities of current being constant (DC) or variable (AC). However, a variable current could vary in different ways. You could have other frequencies, a square wave or sawtooth wave, versus sinusoidal, etc. You could also have the amplitude of the wave stay positive or negative over the entire cycle, versus alternating between them, but I don't see much advantage in that. In any case you can change the voltage or amperage or wattage, independent on the carrier wave used. StuRat (talk) 20:16, 26 March 2011 (UTC)

As far as we're aware, there is no such thing as a magnetic monopole, so you couldn't have a current of them. If magnetic monopoles do exist, then I guess it would be possible. --Tango (talk) 20:26, 26 March 2011 (UTC)

Well, the current described in Magnetic monopole#"Monopoles" in condensed-matter systems is sort of like what is requested, no? Wnt (talk) 21:33, 26 March 2011 (UTC)

Sort of but not really. Real magnetic monopoles have never been found but the theoretical motivation for their possible existence is fairly strong. Dauto (talk) 21:36, 26 March 2011 (UTC)

Noah's Ark

Is the story of Noah's Ark possible? Could two of each animal really fit into a ship that size? —Preceding unsigned comment added by 68.181.201.26 (talk) 20:04, 26 March 2011 (UTC)

Depends on what you mean by "each". If "each" means counting separately each group identified by modern science as a distinct species, the answer is definitely "no". Staecker (talk) 20:09, 26 March 2011 (UTC)

What is possible, however, is them taking some livestock and such on board so they would have a starting point to rebuild after a regional flood (not a worldwide flood, of course). So, there could be a grain of truth there. StuRat (talk) 20:12, 26 March 2011 (UTC)

If you take "each" to mean some arbitrary level between genus and kingdom, then you could fit them and perhaps even the feed necessary for them. John Woodmorappe wrote a book in which he calculated that it was possible if you brought one pair of each "kind", as he put it, and in some cases only brought young animals. His work has been harshly criticized [2] [3], also by other creationists [4].Sjö (talk) 21:23, 26 March 2011 (UTC)

And, of course, there would be no way to get all the species we have now, from that, unless you assume that God made it happen. In this case, why did God need Noah's Ark ? StuRat (talk) 22:34, 26 March 2011 (UTC)

Judaism holds that it was a miracle. Even by the time of the Talmud it was known that it could not have been done by ordinary effort. The text supports this as well (Genesis 7-8 and 7-9) - the animals went in, not Noah took them in. The reason God needed Noah was that the ark was an attempt to get the rest of the people to repent by building an ark so publicly (which is also why it took so long to build). Additionally it's the same reason man needs to plant seeds, rather than just have food appear. Ariel. (talk) 02:08, 27 March 2011 (UTC)

That would make for a really small effective founder population though. John Riemann Soong (talk) 20:57, 26 March 2011 (UTC)

It seems impossible to collect and store away a million species of beetle, for example. But the biological species concept is a construct of modern science, not faith. (For comparison, it's written that God cursed the serpent to crawl on its belly. Which serpent?) And if you allow for arbitrary divine intervention, you can suppose that the beetles all arrived on their own, or were transported on a divine wind, or needed no food, or could occupy the same physical space, or could produce offspring with different genetic codes - obviously, once God gets involved, the laws of physics are kind of out the window. Wnt (talk) 21:39, 26 March 2011 (UTC)

A further problem with Noah's-Ark-As-Fact argued here is the people side of things. "Noah, his wife, his three sons, and their wives" would have had to work overtime to populate the all the emerging civilizations from China to South America in double quick time. Biblical scholars have calculated the date of the Flood to the 25th century BC, just when the Pyramid of Khafre was being built. Alansplodge (talk) 22:59, 26 March 2011 (UTC)

The secret here seems to be the " and their wives " part. How do you know how many wives they had? 212.169.190.116 (talk) 00:06, 27 March 2011 (UTC)

Hmmm.. Probably several million... Alansplodge (talk) 01:11, 27 March 2011 (UTC)

That would seem a bit pointless at least for Noah. According to Noah he died 350 years after the flood. Even if he was screwing a wife every hour (on average) for the whole 350 years that would only be 3068100 wives with none of them more then once over the 350 years even if the wife didn't get pregnant. One woman every hour for 350 years seems a little extreme even if he was like horny teenager all those years. Nil Einne (talk) 11:06, 27 March 2011 (UTC)

Check this. I asked a similar question last year and got some really interesting, in depth answers. It was suggested, from a rough calculation that we could build a ship large enough to carry two of every land animal (in fact we have built much larger ships) - and that the dimensions given in the Bible are not wholly unreasonable for the task on a purely volumetric level. It's the 'keeping everything alive for 40 days and 40 nights' bit and the technology level of the time that's the major problem. --Kurt Shaped Box (talk) 22:54, 26 March 2011 (UTC)

Would you need more than 2 animals of each species to repopulate the planet to avoid inbreeding? A Quest For Knowledge (talk) 23:01, 26 March 2011 (UTC)

Yes, far more. StuRat (talk) 23:06, 26 March 2011 (UTC)

Not only 40 days, but a little over a year. 40 days and 40 nights is how long it rained and then it took some time for the water to retreat. Which makes it even harder to store the necessary feed.Sjö (talk) 08:27, 27 March 2011 (UTC)

It depends on whether you mean a planetary flood, or a "known world" flood as defined for the writers of the story. It seems quite possible that a major flood could have struck the fertile crescent and Noah could have built a ship that could have held an arbitrarily large number of the local megafauna from that area. Or, even more likely, that the story could date from oral tradition about the flooding of the Black Sea (see Black Sea deluge theory) which may have been the source of many middle-eastern flood stories. --Jayron32 23:38, 26 March 2011 (UTC)

It was the Noah's Ark story that first began to turn me away from religion. My Sunday School teachers wanted to have me believe it literally, then when I questioned the logic of that, started to tell me all sorts of variations. I realised that there was an unlimited number of these variations, and that if every piece of the Bible needed that level of interpretation, one could be certain of nothing it contained. HiLo48 (talk) 00:20, 27 March 2011 (UTC)

I've never understood why people try to explain miracles and still stay within the religion! If it's not a miracle then what's the basis for your religion? Someone can't just come and say "I'm God's messenger, listen to me" - you have to prove it, by indisputable miracles, not just interesting coincidences and fancy words. Ariel. (talk) 02:11, 27 March 2011 (UTC)

And this theologian held that a belief in miracles is atheistic. What's the worth of a god who doesn't play by the rules? DuncanHill (talk) 08:11, 27 March 2011 (UTC)

This has certainly been discussed here before. APL (talk) 02:30, 27 March 2011 (UTC)

Why do people keep stating that all the animals were in twos? See Genesis 7:2-3. Three pairs to mate and one to watch.--Shantavira|^{feed me} 08:06, 27 March 2011 (UTC)

Um, it doesn't say that. The 7 is only the clean (i.e. kosher) animals (and birds). However it is ambiguous if it means 7 pairs (i.e. 14) or 7 animals (14 seems more correct). Says nothing about one to watch. Ariel. (talk) 09:05, 27 March 2011 (UTC)

Have you ever been around livestock? 6 females and 1 male works perfectly fine. Googlemeister (talk) 16:33, 28 March 2011 (UTC)

Where did all the water go? Imagine Reason (talk) 02:22, 29 March 2011 (UTC)

And where did it come from? I doubt there's enough water on Earth to flood the whole planet. A Quest For Knowledge (talk) 18:20, 29 March 2011 (UTC)

how did engineers calculate loads and stresses before the arrival of Newtonian physics and calculus?

I'm thinking Greeks, Romans -- Gothic architects who built those flying buttresses, etc. What mathematical tools did they use for their calculations? Did they build little models and then assumed the forces would scale? When building really large monuments for example, and some engineer came up with some experimental idea, how did the ancients test their ideas? Presumably there was a great deal of trial and error involved, but I usually don't hear about ancient engineering disasters. John Riemann Soong (talk) 20:56, 26 March 2011 (UTC)

There would be little sign left of most ancient engineering disasters, since the materials are likely to have been reused or eroded since then, and the rulers wouldn't be likely to record their failures in writing. One exception I know of is some ancient Egyptian pyramids which are now just piles of rubble. Another is some Easter Island statues that seem to have been abandoned before finished, due to cracks, etc. StuRat (talk) 20:59, 26 March 2011 (UTC)

Also note that, just as Newtonian physics is technically "wrong", but is still useful in many cases as an approximation of quantum mechanics and relativity, earlier models may have been "wrong", but still useful tools. And note that they don't have to understand why a particular model works, just that it works, to use it for good results. Finally, they likely just allowed more of a margin for error, if there was more uncertainty in their calculation methods, by using thicker beams, bigger stones, etc., than were actually needed. This might actually explain why some of their projects lasted so long. Roman aqueducts may not have been meant to last for thousands of years, but the calcs they made might have made them seem less stable than they really were, which they then compensated for by over-engineering them, especially once the larger margin-of-error is considered. StuRat (talk) 21:04, 26 March 2011 (UTC)

Well, I don't know of a source that deals with the topic comprehensively, but there were quite a number of Gothic cathedral projects that either fell down as they were being constructed or didn't last very long. Looie496 (talk) 21:14, 26 March 2011 (UTC)

What mathematical techniques did the ancients use? In building monumental projects did they ever make use of advanced Euclidean geometry and infinite regressions? I note the ancients talk of Diophantine equations and explored relatively intricate problems. I suppose the idea of the Riemann sum actually went way back? Did the ancients use to do something like approximate an integral by simply splitting up the problem into tiny easy to calculate approximations? Did mathematics drive engineering, or did engineering problems drive mathematics? John Riemann Soong (talk) 21:18, 26 March 2011 (UTC)

One approach which requires very little math is to construct different scaled models. If the 1/10th scale model supports 10 times the weight of the 1/100th scale model, it's reasonable to assume that the full-scale model will support 10 times that weight. If the 1/10th scale only supports twice as much weight, then the full-sized project is likely to only support twice that. Certainly not perfect, but might give them a good ball-park estimate. StuRat (talk) 21:28, 26 March 2011 (UTC)

I am not sure if that actually works like that Stu. Think about this. How would the world work if everything on earth was a 1 5,000 scale model of a different planet? What fantastic biological properties would be needed in order for 5 mile tall beings to be able to travel at speeds of 60,000 mph (equal to a human running at around 12 mph)? What kind of fantastic material properties would be needed to build a 1,000 mile high skyscraper? Googlemeister (talk) 20:15, 28 March 2011 (UTC)

The usual cautions about extrapolation apply. That is, you don't want to just measure the results off a 1/5000 scale model and assume that everything will scale up. If, however, you also make a 1/500, 1/50, and a 1/5 scale model, and they all work well, it's a good guess that the full scale model will, too. In the case of land animals, we have ample evidence that they run into size problems around the size of the largest dinosaurs. Marine animals can get somewhat larger, because their weight is supported by water, but there's still a limit, far shot of 5 miles long. StuRat (talk) 21:11, 28 March 2011 (UTC)

My understanding is that the tools of Gothic architecture and Romanesque architecture were developed with a mix of very non-general theories and practical experience. They had functional theories of arches, for example, long before they had the calculus. They worked with new ideas on smaller structures before using them on big ones. They no doubt had their failures, as well. --Mr.98 (talk) 22:12, 26 March 2011 (UTC)

And of course, they did have their documented failures, such as famously, Beauvais Cathedral, which pretty much marked the end of Gothic architecture's relationship with "bigger is better" mentalities. —Preceding unsigned comment added by 92.20.201.71 (talk) 22:20, 26 March 2011 (UTC)

Flying buttress says "Early flying buttresses tended to be far heavier than is required for the static loads involved ... Later architects progressively refined these designs and slimmed down the flyers", and it also has a "remedial" section which mentions how they were sometimes retrofitted. I think there is a much more famous example of this than the parish church in the article - there's at least one cathedral which had buttresses added after construction - but I can't remember where it is. Fonthill Abbey's tower collapsed several times, but that was because William Thomas Beckford was incredibly rich and in a hurry, and willfully negligent in designing it. 213.122.22.239 (talk) 22:19, 26 March 2011 (UTC)

Wells Cathedral's dominating interior "scissor arch", which cuts the interior space in two, was a 14th century retrofit to prevent the tower from collapsing. —Preceding unsigned comment added by 92.20.201.71 (talk) 22:29, 26 March 2011 (UTC)

(ec) Not everything in the Principia was new. I think that ancient people had a good quantitative understanding of weight, normal forces, and static friction, which are all you need to build stone structures that stay up. You don't need a concept of acceleration or even velocity. The stones are your atoms, and there are only finitely many stones, so you don't need calculus. You do need vectors, which the ancient Greeks already had. -- BenRG (talk) 22:23, 26 March 2011 (UTC)

There's also just plain old experimentation; i.e. instead of calculating what might work, you just go ahead and build it, and if it doesn't, you try something different. The Bent pyramid shows such experimentation "on the fly". When the pyramid's slope angle proved unstable, the builders simply adjusted it mid construction to make it work. There are likely dozens of long lost and forgotten pyramids which failed before the ideal angle and construction methods were tripped upon when building the Pyramids at Giza. The same is likely true of other great construction projects (castles, cathedrals, fortresses, city walls, etc.) Mostly trial and error, and then if it works repeat it... --Jayron32 23:31, 26 March 2011 (UTC)

Agreed. The spire of Lincoln Cathedral fell down in 1237 when it was less than 10 years old. It was replaced with a bigger one - said to have been 160 metres high, making it the tallest structure in the world, but that was blown down in a storm in 1548. They got the hint - it was never replaced. Another factor is that medieval masons verbally passed the tricks of the trade on to their successors in secret. Each mason therefore had generations of practical knowledge of what worked and what didn't, filed away in his head. The passing-on of secret information in this way survives symbolically in modern Freemasonry. Alansplodge (talk) 15:36, 27 March 2011 (UTC)

Many cathedrals fell down, probably churches also. Georgian houses were badly built and often fell down. Only the good ones have survived. Many buildings in the third-world fall down. 2.97.210.137 (talk) 21:44, 27 March 2011 (UTC)

Double Recombination

Hello. A wild-type female fruit fly mates with a yellow, chocolate, cut male (all traits recessive). The percentage of progeny of various types is as follows:

• 40.53% cho, y⁺, ct⁺
• 7.85% cho, y, ct⁺
• 1.84% cho, y, ct
• 0.05% cho, y⁺, ct
• 0.03% cho⁺, y, ct⁺
• 2.08% cho⁺, y⁺, ct⁺
• 8.07% cho⁺, y⁺, ct
• 39.55% cho⁺, y, ct

I determined that the ct gene is in the middle. Is the distance between y and ct 84 cM or 16 cM? How can I calculate the expected percentage of double recombinants? Thanks in advance. --Mayfare (talk) 21:44, 26 March 2011 (UTC)

I ran across [5] but for some reason I'm having trouble finding a good simple Web-based site for this sort of calculation (see LOD score).

Now to go over the genes one-by-one. First, let's work out the raw recombination fractions. 40.53 + 0.05 + 0.03 + 39.55 = 80.16% (cho+, y OR cho, y+), 7.85% + 1.84 + 2.08 + 8.07 = 19.84% (cho, y OR cho+, y+) 40.53 + 7.85 + 8.07 + 39.55 = 96.00% (cho, ct+ OR cho+, ct), 1.84 + 0.05 + 0.03 + 2.08 = 4.00% (cho, ct OR cho+, ct+) 40.53 + 1.84 + 2.08 + 39.55 = 84.00% (y, ct OR y+, ct+), 7.85 + 0.05 + 0.03 + 8.07 = 16.00% (y+, ct OR y, ct+)

Now with just the raw fractions it's pretty obvious that cho and ct are close together, with y probably closer to ct than to cho. How probably is where the finer analysis comes in. Note that there's no "84 cM" result possible - you can't get more than a 50/50 ratio. The alleles are labeled + or mutant to distinguish them, but what they are labelled doesn't actually matter. It's apparent that they started as cho, y+, ct+ and cho+, y, ct chromosomes at the beginning (the two main unrecombined types coming out of the cross.

The next thing we'd like to know is the working out of the mapping function. Kosambi's mapping function seems to be pretty popular in Drosophila; Haldane's mapping function is a simpler version that ignores that one crossover might affect the probability of another. See [6]. Now according to [7], m = 1/4 ln [(1+2r)/(1-2r) for 0 <= r < 0.5 (m = map distance, r = recombination). So 16% adjusts to 16.58 map units, 4% adjusts to 4.00 map units, and 19.84% adjusts to 21.00 map units. That gives us a basic suggestion that we have y <- 16.58 -> ct <- 4.00 -> cho. Because this is (I assume) a homework problem, this adds up nicely to y <- 20.58 -> cho as opposed to 21.00 calculated, and the double recombinants y+ ct cho and y ct+ cho+ are in fact very uncommon (the latter source gives a way to use double recombinants to work out the order more precisely). Still, a good program could maximize the LOD scores for a more precise result, and give a better idea of the confidence of the mapping. Wnt (talk) 23:21, 26 March 2011 (UTC)

Math and radiation

Is 1.76 × 10E18 50% more than 1.3 × 10E17? Looks like about 100% to me. If not what is this author talking about: [8]. 75.41.110.200 (talk) 23:15, 26 March 2011 (UTC)

It's actually 13.54 times more (or 1354%). Perhaps the number of days each burned figures into their calcs ? StuRat (talk) 23:22, 26 March 2011 (UTC)

Be careful with your "more". It's 13.54 times as much or 1254% more. -- 110.49.251.220 (talk) 11:05, 27 March 2011 (UTC)

He's saying Fukushima emitted "1.3 × 10e17 becquerels per day", and saying that "In the 10 days it burned, Chernobyl put out 1.76 × 10e18 becquerels". So he's comparing the daily rate of Fukushima vs the 10-day rate of Chernobyl. If you assume Fukushima's rate was constant over 10 days, it would have emitted 1.3 x 10e18 over the same 10 day period. So the two 10e18 cancel out, and you're left with the ratio 1.3:1.76, which is (roughly) "50 per cent more". -- Finlay McWalter ☻ Talk 23:26, 26 March 2011 (UTC)

Calling 35.4% more "roughly 50%" is rather sloppy. StuRat (talk) 00:31, 27 March 2011 (UTC)

If we take the full quote, he says "1.2 to 1.3 × 10e17". Using the lower bound the ratio is 1.2:1.76, which is 46% more. -- Finlay McWalter ☻ Talk 00:40, 27 March 2011 (UTC)

Fuse interrupted during oven use

During baking, the fuse of my apartment went off. I have no doubt that the oven is to blame. Is it secure to turn the oven on again or will the fuse go off again and again? 212.169.191.85 (talk) 23:43, 26 March 2011 (UTC)

Fuse going off may have resulted in some other electrical devices turning off, however if it is not the case it will indeed go off again ~~Xil (talk) 00:13, 27 March 2011 (UTC)

By "fuse", do you mean a circuit breaker ? A fuse only works once. If the oven was the only thing on the circuit and it made the circuit breaker pop, it's likely to do it again. StuRat (talk) 00:16, 27 March 2011 (UTC)

yes, I mean circuit breaker, and not a proper fuse. Well, but maybe, it went off because I was using the oven for a relative long time. 212.169.190.116 (talk) 00:19, 27 March 2011 (UTC)

Length of time should only matter if you are running the circuit over its rated load. Circuit breakers don't actually trip at the stated number. The greater the overage the faster they trip. If you run just slightly over, it can take a very long time (hours even) to trip. I would attempt to find out what other devices are on the circuit, because an oven should be the only device on the circuit! Then add up all the labeled power draws, and compare to the circuit breaker. Ariel. (talk) 02:26, 27 March 2011 (UTC)

Your options are to get an oven which draws less power (including spikes), switch it to a circuit which can handle more, or upgrade the present circuit. You could also replace the circuit breaker, it might just be overly sensitive (popping below the rated limit). A more risky approach is to change the circuit breaker to one which allows more power to be drawn. This can cause a fire, though, so only an electrician who knows how much that circuit can handle (and the entire fuse box) should do that. StuRat (talk) 00:29, 27 March 2011 (UTC)

It should be possible to acctualy calculate if it will happen - I believe you sum power in watts of all devices turned on and divide it by 220 (also watts) and you get how many amperes this set up produces, if it is greater than safe limit of the circuit breaker it will go off ~~Xil (talk) 01:35, 27 March 2011 (UTC)

It's not quite as simple as that, because the power demands of each device aren't constant. If all the spikes hit at once, you can have a problem with a circuit that seemed fine before. StuRat (talk) 02:02, 27 March 2011 (UTC)

I don't see why leaving a stove on for a long time would draw more current than turning it on for a short period. In fact, an already-hot oven might draw less current on average than one that's heating up from a cold start.

If it trips again, you may have a problem with your wiring. I suggest talking to the landlord about it. APL (talk) 02:28, 27 March 2011 (UTC)

See my reply a bit higher up, but basically circuit breakers don't trip immediately at their rated setpoint, there is a time * overage lag. Ariel. (talk) 02:52, 27 March 2011 (UTC)

I can understand the delay mechanism for a slight overload of an old-fashioned fuse, but how would a delay be implemented mechanically in a modern circuit-breaker? Dbfirs 07:02, 27 March 2011 (UTC)

A bimetallic strip take time to warm up.Circuit_breaker#Thermal_magnetic_circuit_breaker--Aspro (talk) 09:03, 27 March 2011 (UTC)

I have seen these inside appliances, but didn't know that they are a secondary trip in mains installations? Dbfirs 16:45, 27 March 2011 (UTC)