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October 19

Global warming and agriculture

According to the article on climate change and agriculture, plant yields are expected to decrease as temperatures rise because each plant has an optimal reproductive and growth temperature which if exceeded leads to reduced growth. Why can't we genetically modify crops, whether through genetic engineering or more conventional methods, to have a higher optimal temperature? 74.15.138.165 (talk) 01:44, 19 October 2013 (UTC)

We don't even have to do that, just grow the proper crops for the new climate. For example, places which are no longer subject to winter frost become available for lots of new crops, like oranges. Also, in other cases, an additional harvest may be possible in a year for a given crop. Global climate change is not universally bad, it just requires that we adapt to the new environment, such as moving inland to deal with rising sea levels. StuRat (talk) 01:48, 19 October 2013 (UTC)

Wikipedia is not a source. The Middle Age and Roman warm periods were great for crops in Europe. Temperatures reached a maximum after the last ice age around 2000-3000BC. We are now in the cooling trend of a new ice age, called the neoglaciation. Recent trends have been local blips on that curve. See Canadian ecologist E. C. Pielou's, After the Ice Age, which is searchable at Amazon and well worth reading. μηδείς (talk) 01:59, 19 October 2013 (UTC)

Just to clarify: no credible scientist believes the world is actually cooling, just that it would be cooling if not for CO2 emissions. I assume that's what Medeis meant, although the natural Milankovitch cycles have been completely drowned out by anthropogenic warming in the past century. --Bowlhover (talk) 04:09, 19 October 2013 (UTC)

One century is 0.1% of the cycle of the ice ages, so how is that relevant? (Alternatively, how is the cycle of the ice ages relevant? At any rate, these two things have little bearing on each other.)  Card Zero  (talk) 18:43, 21 October 2013 (UTC)

The problem with climate change is not only the higher average temperatures but also that the changes in the local climate will happen fast and the local climate may well become inherently more variable. Higher evaporation rates will lead to more droughts, while the average amount of rainfall will also increase. This means that you'll have longer and more severe periods of droughts, interrupted by huge and sometimes catastrophic amounts of rainfall. Agriculture under such conditions will become more difficult, this may well lead to the collapse of our civilization. Count Iblis (talk) 02:12, 19 October 2013 (UTC)

You have moved the bar from global warming to climate change. Neat huh? μηδείς (talk) 02:35, 19 October 2013 (UTC)

No moving involved, only clarification. Global warming is the primary and most easily measurable effect, but no serious scientist ever thought that a general warming would have no or insignificant secondary effects on climate. --Stephan Schulz (talk) 18:51, 21 October 2013 (UTC)

This discussion has been closed. Please do not modify it.
The following discussion has been closed. Please do not modify it.
Why are you still allowed to post here? 68.0.144.214 (talk) 03:33, 19 October 2013 (UTC)

It depends on the crop. Field corn, for example, loves heat. But it also needs water at the right times, and a minimal growing season. The length of the growing season varies from north to south in the US. Northern varieties are bred to mature more quickly. If the North's summer becomes a lot longer (which does not really appear to be happening anytime soon), the corn with longer time to maturity could be grown farther north, and total yields would increase - provided the water and nutrients are also there. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:04, 19 October 2013 (UTC)

The funny thing is that after Trofim Lysenko spent half a century as the epitome of pseudo-science, some people have started actually doing some of those kind of experiments again. Turns out the Russian peasants were right -- crops inherit adaptation to stress conditions [1]. Wnt (talk) 18:13, 23 October 2013 (UTC)

Icebreaker

Three related questions: (1) Is it possible to make a hole in pack ice by firing a torpedo at it, as described in Ice Station Zebra? (2) Is it safely possible to use this method to free a ship trapped in ice? (3) Is it safely possible to free a ship trapped in pack ice by firing a (non-nuclear) cruise missile at the ice? 24.23.196.85 (talk) 02:35, 19 October 2013 (UTC)

The book synopsis for Under Ice: Waldo Lyon and the Development of the Arctic Submarine mentions "the Skate’s torpedo firing into ice", presumably USS Skate (SSN-578). As for (2) and (3), they sound rather dangerous. WAG: You'd probably have to fire very, very close to the ship to have any appreciable effect. Clarityfiend (talk) 04:19, 19 October 2013 (UTC)

As for using a cruise missile, an unguided torpedo is ever so many times cheaper than a Tomahawk missile at USD 1.5 million each. [2] It would be the world's most expensive hole in the ice. Also you'd have to get the thing to explode under the ice, which might not be as easy as it sounds - I'm not sure about that because they don't put the instruction manual on the internet. Alansplodge (talk) 08:35, 19 October 2013 (UTC)

Sometimes they do: [3] :-)Phoenixia1177 (talk) 08:45, 19 October 2013 (UTC)

Gosh! Alansplodge (talk) 19:14, 19 October 2013 (UTC)

(un-indent) Right, a torpedo is MUCH cheaper than a cruise missile -- but suppose there's a ship trapped in pack ice and being slowly crushed by the pressure, no icebreaker can get to it in time, and there's no submarine in the vicinity to fire a torpedo -- only a B-52 armed with cruise missiles (with conventional, non-nuclear warheads, of course). Is the trapped ship done for, in this scenario, or might it be freed without being sunk by the explosion in the process? 24.23.196.85 (talk) 19:32, 19 October 2013 (UTC)

With enough missiles you could free any ship I guess, not sure if one will do much good. A ship won't be crushed if it's trapped in a small isolated ice sheet, it's the pressure from the wind and sea currents on large ice sheets that can crush a ship, the same way it creates pressure ridges. An explosion on the ice will break up the ice in the vicinity and inertia will prevent the force being transmitted through the ice over long distances, but how many cruise missiles it would take and what the optimal distance (or optimal heigh of detonation) would be is anybody's guess...

Explosives were used in an attempt to free the Discovery, see Discovery Expedition: Second relief expedition. Ssscienccce (talk) 12:37, 21 October 2013 (UTC)

In other words, this method is doable, but inefficient and potentially dangerous. Thanks for the info! 24.23.196.85 (talk) 23:58, 21 October 2013 (UTC)

In any case you’ll have to aim very well!!

Iskánder Vigoa Pérez (talk) 04:42, 22 October 2013 (UTC)

Largest and most used Gastroenterology textbook?

What is the largest and most used (by universities) Gastroenterology textbook for students? Thanks. 79.182.146.170 (talk) 02:59, 19 October 2013 (UTC)

Asymmetric reciprocal effects

What are some of the most famous examples of asymmetric reciprocal effects?

What I mean is, you have two parameters, say volume and pressure, or capacitance and voltage -- I chose these randomly, these may not be good choices -- and when you increase one of the two, the other will respond and change, too, by a factor that relates to the increase in the first quantity. But when you reciprocate, i.e., increase the second quantity of the pair instead of the first, the first quantity will not change by the same factor but by much less or much greater a factor.

Apologies for my clumsy (and probably incorrect) use of technical terminology. — Preceding unsigned comment added by 89.204.130.121 (talk) 05:39, 19 October 2013 (UTC)

Anything with a non-linear relationship in it somewhere will be non-reciprocal, as will anything that is what is in electronics termed an active device. For example, all electrical networks consisting of nothing but passive components (ie resistors, capacitors, or inductors) will be reciprocal. A field effect transistor is an example of an active device, it can be set up so that a certain change in gate (input) voltage will produce a large change in drain (output) voltage, but any change in drain voltage forced by external circuitry will not change the input voltage. In radio, probably the most famous non-reciprocal (and active) device is the coherer, used in the early days to detect radio signals and convert them to audible clicks. Coherers compised a quantity of loose metal filings in an insulating (eg glass) tube. The incidence of radio wave energy caused the filings to clump together and drop in resistance, allwing current to flow in headphones. An electromagnet was typically empoyed to bang the side of it and loosen the filling again, ready for the next radio wave pulse. Obviously, no amount of banging or tapping on a coherer will produce radio frequency energy, so the device is clearly non-reciprocal.

Active devices, and non-reciprocal devices, can be made with magnetic materials, as the response of magnetic materials is inherently non-linear. Before the invention and refinement of transistors, magnetic amplifiers where widely used wherever it was desired to control a large current with a small input current - the regulation of factory processes, control systems for aircraft and missiles (use of magnetic amplifiers in missiles for flight path controls began with the very first - the German V2), control of elevators in multi-story buildings, and many more.

A carbon microphone, used in telephones for about 80+ years, is an example of a non-reciprocal active device. Sound pressure in the air causes change in electrical resistance and thus electric signal current. But no amount of electrical signal will produce a sound pressure. Carbon microphones are also an amplifier. Because the energy for the output current comes from a battery, the electrical power output was greater than the sound power input. That was important, as it overcame signal attenuation in the lines, and losses in the receiver, and allowed the deployment of telephone company networks before radio tubes and transistors were invented.

58.167.234.97 (talk) 06:03, 19 October 2013 (UTC)

That is very useful information to have, thank you very much. But unless I misunderstand you, the examples you give are of non-reciprocal relations only, whereas I am looking for reciprocal but not symmetric relations. (I increase parameter A from 30 to 40 and parameter B consequently increases from 100 to 101. But when I increase parameter B from 101 to 102, parameter A increases only from 40 to 41.) --89.204.130.121 (talk) 06:22, 19 October 2013 (UTC)

I gave you exactly what you asked for. My examples were:-

• Coherer: parameter A is field strength of radio frequency; paramter B is DC current. A affects B but B has no effect on A.
• Field effect transistor: Parameter A is gate voltage; parameter B is drain voltage. Again A affects B but B has no effect on A.
• Carbon microphone: parameter A is sound power; parameter B is electrical signal power.
• Another example is magnetostriction: In certain materials, a magnetic field causes the material to change in length and so apply pressure to something. But applying a pressure change does not produce a magnetic field.

Your use of the term "symmetrical" is what engineers term "reciprocal". "Reciprocal" to a mathematian means dividing into one, but to an engineer it means the translation of energy goes both ways between port A and port B. If changing A from 30 to 40 causes B to go from 100 to 101, AND if changing B form 100 to 101 (or changing it from 101 to 102) causes A to change from 30 to 40, then engineers would call it a reciprocal device. They would call it symmetrical if ALSO changing B from 100 to 99 would cause A to change from 30 to 20. That is, the change (on either side) had the same effect in both polarities. Almost all active devices are non-symmetric to some degree, as they are to a certain extent non-linear. (non linearity is of course the source of harmomic distortion in amplifiers) Electronic tubes follow a 3/2 power law. FETs are square law at low currents.

Perhaps you could give a set of numbers showing what you mean by reciprocal" AND "non-symmetric", as all my examples illustrate an, albiet more at a greater extreme, principle embodied in the numbers you used.

58.167.234.97 (talk) 06:51, 19 October 2013 (UTC)

Perhaps I should say "mutually influencing" instead of reciprocal? Other than that, I can't think of anything else that I could say to make my question clearer. (And I've given a set of numbers exemplifying what I mean in my previous post.) --89.204.130.121 (talk) 07:19, 19 October 2013 (UTC)

(I see that you've added some text to your latest reply in between my reading it and my posting of a response. Thanks but my question still remains. And in case the answer is, No that is impossible, nothing in nature works that way: Is that so not only for two mutually influencing parameters directly coupled to each other but also if they are just two mutually influencing parameters in a complex environment with many intervening links in one or more (non-linear?) cause-and-effect chain(s)? --89.204.130.121 (talk) 07:27, 19 October 2013 (UTC)

I'll have another go at making this clear for you. Here are the basic principles:-

• I'll use the term network. A network can be a network of electronic parts, mechnical parts, or whatever. In many fields, the term is system.
• Parts can be passive or active.
• Passive means that there is a one-to-one correspondence between a quantity and another quantity. A pipe is a passive part - the quantity of water leaving equals the quantity entering. Change one and teh other must change equallly - in sign and in magnitude. An electronic resistor is a passive part. The current in it is directly proportional to the voltage across it. Change either, and the other must also change in proportion, in sign, and in ratio.
• An active device is a device where one quantity controls another quantity. One example is a water tap - the position of the tap handle controls the water flow. Another example is a catalyst - the concentration of a catalyst (you can call this your "a") control the rate of reaction (your "B") of two other chemicals. Another example is a filed effect transistor - the gate voltage controls the flow of current in the drain.
• Practical parts can be almost ideal passives (eg electronic resistor), almost ideal actives (a transistor), or a mix of the two (eg a thermistor).
• A part can be linear, or non-linear.
• A linear part means that a change of x% in one quantity always occurs with the same x% change in another quantity - regardless of polarity, magnitude and offset from zero. An example is a resistor - a change in x volts will always cause the same y amps in current, regardless of whether it is a change from a to a plus x, or b to b plus x.
• A non-linear part means that the change in one quantity varies in magnitude, even when the change in the other quantity is always the same, when the offset from zero is changed. An example is a diode. As voltage is increased, the current increases ever more dramatically.
• A non-linear device is always non-symmetrical - that is a negative change has a different magnitude effect than an equal positive change. — Preceding unsigned comment added by 58.167.234.97 (talk) 11:07, 19 October 2013 (UTC)
• A network or system can comprise any number of linear, non-linear, passive, and active devices.
• A reciprocal network or system is where two quantities (your "A' and "B") each have an equal effect on each other.
• A network or system comprising only of pure linear passive parts is always reciprocal.
• A network or system containing at least one non-linear part or at least one active part will have a degree of non-reciprocity.

If this does not satisfy you, I suggest you give a set of numbers that show EACH term that you used. That is, give a set of numbers that show what you mean by non-recipocity, and another set of numbers that show what you mean by non-symmetricality. Then I can give examples to suit each. 58.167.234.97 (talk) 11:02, 19 October 2013 (UTC)

(outdent) You're starting to discuss a great deal of terminology now, which may be a good idea (to make sure we're all on the same page) but maybe not absolutely necessary. Here is your set of numbers:

Reciprocal:

Change in Quantity of A	Resultant Change in Quantity of B
30 -> 40	100 -> 101

Change in Quantity of B	Resultant Change in Quantity of A
100 -> 101	30 -> 40

Symmetrical:

Change in Quantity of A	Resultant Change in Quantity of B
30 -> 40	100 -> 101
40 -> 30	101 -> 100
Change in Quantity of B	Resultant Change in Quantity of A
100 -> 101	30 -> 40
101 -> 100	40 -> 30

But in what I am asking for, numbers might look like this:

Increase in one Quantity	Resultant Increase in other Quantity
QA 30 -> 40	QB 100 -> 101
QB 100 -> 101	QA 30 -> 31
Decrease in one Quantity	Resultant Decrease in other Quantity
QA 40 -> 30	QB 101 -> 100
QB 101 -> 100	QA 40 -> 39

I've called this "asymmetrical", for want of a better word. (Again, A and B are measurable physical parameters, such as volume, pressure, voltage, magnetic field strength, temperature, quantity of an element or compound, acidity, salinity, or whatever else one can imagine.)

Now if you tell me, that's preposterous, nature cannot work that way (or: simple logic tells us this is impossible), then o.k., my follow-up question is this:

Can there be a sufficiently complex system, in which A and B are but two of many distinct but mutually influencing factors, in which the interactions work in such a way that ultimately, changing A impacts B differently than changing B impacts A, in a manner as shown in my table above?

And if my follow-up question is nonsensical because there is something wrong with its internal logic or understanding of nature, please point to the error in my thinking. 89.204.130.121 (talk) 15:11, 19 October 2013 (UTC)

Ok, that makes it clear what you are asking. And I should have picked up what you were getting at earlier. Your third case is what an electrical engineer would call a two-port device with dissimilar forward and reverse gain. "Gain" is the ratio of an output quantity to an input quantity. Devices with dissimilar forward and reverse gain are an important class of non-reciprocal devices. So important that most engineers would just say its non-reciprocal. Many devices and systems in electrical/electronics, mechanics, chemistry, etc have this property. The word assymmetrical is not a bad word for it, but in electrical/electronics, assymetry means something else as previously described. Such devices are not at all remarkable. The examples I gave earlier, carbon microphones, transistors, radio tubes/valves, magnetostrictive devices, chemical catalysts, worm-gear mechanical drives with regard to torque, etc etc, all have dissimilar forward and reverse gain. In carbon microphones, for example, the forward/reverse gain ratio is infinite. In worm gear drives, by altering the gear pitch, you can have pretty much any forward/reverse torque ratio you like. In transistors, the ratio is very high (>1000:1 in low power audio transistors) but you can increase or decrease it by adding circuitry around the transistor. In fact, for any device with any arbitary dissimilar forward/reverse gain ratio, you can in theory add feedback to get any desired ratio. For example, if you feed some of the output back to the input at the right level and in opposite sense/polarity, you can completely cancel out the reverse gain, producing an infinite gain ratio. This is called neutralisation. 58.167.234.97 (talk) 16:00, 19 October 2013 (UTC)

Thank you, this is helpful! 89.204.138.111 (talk) 20:06, 19 October 2013 (UTC)

But just for curiosity's sake: The examples you've given -- water tap, transistor, microphone, worm gear -- are all machines. Can you also name a couple examples of natural systems meeting this definition? 89.204.137.240 (talk) 01:03, 20 October 2013 (UTC)

Don't forget catalysts, which I mentioned. Catalysts can be natural as well as man-made. Biological systems use complex catalyst systems to extract energy from food and use it. There's probably a vast multitude of biological systems that display dissimilar forward/reverse gain. DNA: In the simplest cases, the quantity of an enzymes etc is proportional to the number of genes for that enzyme (other factors also affect the rate of enzyme production), however the quantity of any enzyme does not affect the rate of DNA production. Drugs: Narcotics work because they are closely similar to molecules naturally made in the brain. If you take a narcotic, the brain over time automatically cuts down its own production - that is one of the mechanisms of addiction, because if you stop, you're in trouble until the brain ramps up production again, driving you to take more of the drug. So there narcotic you take can be your Quantity A, the brain's natural production can be regarded as your quantity B. The gain or coupling in each direction is obviously not the same. Various climatic parameters are coupled in more or less complex ways with dissimilar gains. For example an increase in temperature increases average wind strength, and an increase in wind strength affects temperature, but depending on distance from coasts etc, the temperature may increase or decrease. 60.230.213.251 (talk) 01:26, 20 October 2013 (UTC)

That's very interesting. Speaking of climatic parameters, we know from paleoclimatic records that in Earth history, an increase in temperatures generally was followed by an increase in carbon dioxide, but not the other way around or not as often. Could the temp v. CO2 relationship be an example of dissimilar forward/reverse gain? You mentioned feedbacks. How well are all the intervening feedbacks between CO2 change and temp change that may damp so-called climate sensitivity understood? Can the principle of dissimilar forward/reverse gain help us better understand the lull in global warming since 1998? 89.204.137.240 (talk) 02:04, 20 October 2013 (UTC)

I don't want to take this any further as I am not a climate scientist. I am an electrical engineer. While engineers have a lot of good mathermatical tools that helps their understanding of non-electrical subjects, there is a danger in straying from one's qualified field. However, my impression is that climate science is not well understood, as the complexity of the calculations demand a super-computer. And even then, too many simplifications have to be made in order to make the problem manageable. I suggest you ask your question on climate modelling again, without all the preample that is in this question. Then any climate scientists in Wikiland may answer you. When questions and their answers get too long, it tends to inhibit others from reading it all. 60.230.213.251 (talk) 06:28, 20 October 2013 (UTC)

i ask you again:is there any picture or graphic of the mixed flow compressor from the Pratt and Whitney pw610f?

is there any picture of this compressorß cause i surfed the internet and found the patent but there is no picture about this compressor and google gives e nothing aswell and surfing the internet in general helped nothing. so i give you the question again is there any picture of this compressor?Saludacymbals (talk) 15:02, 19 October 2013 (UTC)

You can get patents from various online sources. The patent should have a diagram, but on some online sources you only get teh overview text. So, if you tell us the patent number, the patent titel and assingnee, and which site you got that from, we'll have a chance to find another online patent source that has the diagram(s). 58.167.234.97 (talk) 15:40, 19 October 2013 (UTC)

Assingee is PRATT & WHITNEY CANADA CORP. the patent number is EP 1322865 B1 the title is Mixed flow and centrifugal compressor for gas turbine engine but there is only the cuttet diagram and no picture of the compressor section and google and internet dont shows more Saludacymbals (talk) 20:57, 19 October 2013 (UTC)

Haven't found one... Ssscienccce (talk) 15:05, 21 October 2013 (UTC)

Special dietary treatments for Autism

Is there published research on the effectiveness of special diets, such as the Specific Carbohydrate Diet, in controlling autism/ADHD?

This is a field of medicine where research is scant. Many parents treat or control their child's autism with special diets they believe to work, or drugs like ritalin the consequences of which haven't even been confirmed - for example whether it changes the patient's blood pressure. Authoritative publications like Proceedings of the Royal Society B, The Lancet and so on simply haven't covered in in the same way as, say, lung cancer or Alzheimer's Disease. I'm looking for sources like professional journals and books. Thanks 92.28.225.238 (talk) 15:47, 19 October 2013 (UTC)

There is quite a bit of published research -- not all of it high-quality though. See https://www.aacp.com/pdf%2F2104%2F2104ACP_Review2.pdf for a systematic review that discusses diet along with other treatments. Looie496 (talk) 16:05, 19 October 2013 (UTC)

Note that, although both autism and ADHD can lead to social interaction problems, the two conditions are quite different. Also with high functioning autism, it can be strongly argued that it is wrong to try to correct it. A degree of autism allows high achievement in subjects requiring technical detail and is thus of benefit to society. Many autistics like themselves just as they are, thank you very much. Whether or not ADHD is a real disease is controversial. The incidence of ADHD diagnosis is high in cultures where parents and teachers are taught/counselled/compelled not to use corporal punishment, whereas in previous generations the offender would have got a good smack or the cane, and that would quickly end the problematic behaviour. This is not to suggest that their cannot be a disease or gene expression problem leading to ADHD, but it is very likley that many diagnosed with ADHD do not have anything wrong with them other than just needing discipline. 58.167.234.97 (talk) 16:15, 19 October 2013 (UTC)

Autism and ADHD are comorbid conditions - meaning that if you have one of them, the odds of you also having the other is significantly higher than for the general public.

There are ways to alleviate the symptoms of ADHD (eg with drugs like ritalin) but there is nothing that helps with Autism - other than teaching sufferers social skills and such - which helps them to live with the symptoms without in any way diminishing them.

We don't entirely understand why Autism happens - and it's really only a description of a collection of symptoms - not all of which apply to all sufferers, so there could be many underlying conditions that produce the symptom group that we call "Autism". Worse still, it's a spectrum condition - varying from people who are almost normal to people who are profoundly cut off from the rest of the world.

I have Aspergers' Syndrome - which is now 'officially' called "High functioning Autism". There are some common symptoms that I don't have (eg extreme sensitivity to certain sounds and smells) - but I do have the social interaction issues and the beneficial symptoms such as improved mental focus and the inability to empathically understand how other people are feeling. For all of that, I'm one of the lucky ones who doesn't want to be cured.

Having met people with many more of the symptoms - it's really hard to believe that there is just one underlying condition - which means that it's highly unlikely that there can be just one cure. For that reason, I am super-suspicious of "cures" that are just anecdotally supplied.

I think that many parents of the less severely afflicted kids are being quite cruel to their children because they don't understand what's really going on. I have seen parents who tried to "cure" the social interaction problems by creating a meet-up group where all of their children could meet up and play together. For someone with Asperger's, it's hard to imagine anything more horrible than doing this!

It's also worth mentioning that the idea that innoculations are the cause of Autism is busted, busted, busted. For chrissakes don't avoid innoculations for your kids in order to "prevent" them getting Autism...it won't work and it's a dangerous thing to do.

SteveBaker (talk) 20:36, 19 October 2013 (UTC)

Autisma is a spectrum disorder. No two autistic persons are alike in their symptoms. For some children, parentally or teacher driven social interaction may be cruel or it may not be. A couple of things:-

• Generally, high functioning autistics (Aspergers) gain good, even very good, social interaction later in life. That's because they have enough built-in social function to learn from their experience. The best time to learn is when you are young. So an autistic child may be uncomfortable in social groups, but the experience will give him a better adult life. Sometimes you have to be cruel to be kind. Like birds, who when hatched don't know how to fly and must learn from watching parents, all people have to learn how to do social interaction. It's just that for normal people, learning it is easy, but for autistics it is hard. High functioning autistics learn social skills and can end up as good at as anybody - it just takes them longer to do it. The earlier they start the better, just as playing piano is difficult - but start at age 5 (ie as soon as the arms and hands are big enough), practice every day and you can eventually be very good.
• Like other brain aberations leading to poor social skills, many autistics may have poor social skills and lack the ability to bond with friends. But that doesn't mean they don't crave friendship and human interaction. Some couldn't care less, but some do care. Often, childhood social interaction doesn't result in the autistic child gaining lots of friends, but the frienship bonds they do form can after a while be very strong and last a lifetime.

60.230.213.251 (talk) 01:42, 20 October 2013 (UTC)

Thank you. 92.28.225.238 (talk) 08:54, 20 October 2013 (UTC)

There are a handful of reports about "purine autism" (autism with hyperuricosuria), a pretty small subset of people who have specific issues with purine metabolism, and even a claim that the drug allopurinol can address it [4]. I think potentially a number of traditional Chinese gout remedies that work by a similar principle of inhibiting xanthine oxidase, such as cassia/cinnamon, would have a related beneficial effect, though not as potent, but I don't think any real parent would want to fool around with an experiment on top of an experiment where their kids are concerned, so I don't think there's any data on that! But though there's a lot of research about metabolic variations in autism, I don't feel like much of it has carried through, and of course simply diagnosing and subcategorizing the condition into several subtypes is a step that, though needed, isn't even being done yet on a regular basis. Wnt (talk) 16:05, 20 October 2013 (UTC)

To follow up, it looks like there is some Chinese traditional medicine basis for dealing with autism, though it is hard to tell the difference between truly traditional treatment for the "Syndrome of five delays" and modern ideas to repurpose existing medicines for other conditions [5]. Still, various sources of unknown veracity such as [6] lead me to think that there is an acknowledged relationship between autism and "kidney qi" and "liver qi" which seem otherwise involved in metabolic disturbances such as gout, and herbs recommended for it do indeed include cinnamon, among many many others. As hard as it is to pick through information like this, I suspect that any nutritional help for autism is more likely to come from a careful, skeptical, systematic examination of the existing body of semi-empirical observations than from some sudden inspiration based on the genetic or neurological data. Wnt (talk) 16:39, 20 October 2013 (UTC)

There are indication that vitamin D is involved in autism. The evidence is not conclusive and sometimes contradictory, but this may be because wat is considered to be a normal vitamin D level may still be way too low. If the hypothesis that normal calcidiol levels should be around 120 nmol/l or higher is correct, then the autism epidemic in the West may well be the making of the medical community itself. The medical community had issued RDAs for vitamin D that were too low by today's standards, and today's standards are not compatible with what is observed in indigenous people in Africa. Particularly during pregnacy there is a big problem.

Until very recently the official guidelines were that expecting mothers should not use any vitamin D (due to the now debunked theory that this could cause damage to the unborn child), so their calcidiol levels would be very low. But measurements of the calcidiol levels of pregnant Maasai and Hadzabe women that were performed last year show that they have very high calcidiol levels, some had levels higher than 250 nmol/l (until recently it was thought that 250 nmol/l is the highest level that is obtainable by natural means alone). Also, huge spikes in the calcitriol hormone levels were observed during pregnacy, extremely high levels that according to common wisdom should be dangerously high levels, are apparantly part of the normal pregnancy processes. Count Iblis (talk) 20:02, 20 October 2013 (UTC)

I'm underwhelmed by the study that article is based on, because it shows that people with autism have lower levels of vitamin D -- but that could be because those people simply spend more time indoors, for example. Wnt (talk) 01:26, 21 October 2013 (UTC)

Here are a few Cochrane Reviews on the subject:

• Gluten and casein-free diets for autism spectrum disorder. It concluded, "Research has shown of high rates of use of complementary and alternative therapies (CAM) for children with autism including gluten and/or casein exclusion diets. Current evidence for efficacy of these diets is poor. Large scale, good quality randomised controlled trials are needed."
• Vitamin B6 and magnesium in combination for children with autism spectrum disorder. It concluded: "Due to the small number of studies, the methodological quality of studies, and small sample sizes, no recommendation can be advanced regarding the use of B6-Mg as a treatment for autism."
• Omega-3 fatty acids for autism spectrum disorders (ASD). It concluded: To date there is no high quality evidence that omega-3 fatty acids supplementation is effective for improving core and associated symptoms of ASD. Given the paucity of rigorous studies in this area, there is a need for large well-conducted randomised controlled trials that examine both high and low functioning individuals with ASD, and that have longer follow-up periods.

You can find other autism related related Cochrane reviews here (not all about special diets/supplements). And I want to re-emphasize the message hinted by User:Looie496's above: unless you (ie the OP) are medically trained, or have justifiable confidence in your ability to properly survey and interpret the results, you should really be looking at only high quality reviews and ignoring individual medical reports on the subject. Abecedare (talk) 02:01, 21 October 2013 (UTC)

Quite right. But, again, correlation does not mean cause. It may be that kids with a degree of autism do better because their parents take an interest, and try lots of things - including forcing contact with other kids. And the kids will be aware of their parents' interest, and believe they can interact socially - the placebo effect. Kids whose parents take the easy road and give the child what he wants, i.e., stay in his room and persue his hobby interests, are never going to learn social skills. Heck, even completely normal children will not learn social skills if they don't have contact with other kids. 60.230.213.251 (talk) 05:31, 21 October 2013 (UTC)

Potential Energy and Kinetic Energy

Assuming two massive bodies collided with each other due to gravitational potential energy. In such case, it is expected that collision will result in heat loss. How do we (approximately) calculate this increase in temperature at the surfaces of collision? Can we say that: Heat rise = Kinetic energy difference = potential energy difference? --Almuhammedi (talk) 16:04, 19 October 2013 (UTC)

It's virtually impossible to calculate the temperature right at the surface of collision, because so many factors affect it. In fact it really isn't even meaningful, since the collision surface is obliterated at the instant of collision. Looie496 (talk) 16:08, 19 October 2013 (UTC)

Indeed. In the simple case of two uniform homogenous bodies where nothing gets torn apart and there is no elasticity, the deformation of the surface must take a finite time, and that tiem can be estimated. In that time some of the heat will flow into the bodies within and away from the deformed surface, so there will be a temperature profile. Following the collision, heat will continue to flow within the bodies until the temperature is uniform thoughout. You can easily calculate the final uniform temperature, by relating the kinetic energy to the specific heat. This is a not uncommon question in asked of students in physics courses. By knowing the thermal conductivity of the bodies, you can with a lot more math, calculate the peak surface temperature rise. In reality though, your two massive bodies probably won't be uniform homogenous, will exhibit some elasticity, the elasticity will be non-linear (ie not obey Hook's Law) and some of the material will be sheared/split/broken off. Then a proper cacluation is near impossible. 58.167.234.97 (talk) 16:37, 19 October 2013 (UTC)

You can calculate the total thermal energy added to the colliding bodies - that's easy. But figuring out how much of the heat energy ended up in each body...and precisely where within each body is virtually impossible. It depends on the shapes, the initial temperatures, the material it's made of, the ambient air temperature...a huge number of unknowns.

What you can say, easily and immediately is that the total heat energy created is equal to the sum of the kinetic energies of the colliding bodies - which (if this is a collision caused entirely by one object falling under gravity onto the other) is equal to the initial gravitational energy. SteveBaker (talk) 20:18, 19 October 2013 (UTC)

No, not correct. In purely elastic deformation, all of the kinetic energy is returned to the two bodies. Two bodies colliding dead on will rebound, heading apart again, without any conversion to heat. If two elastic bodies collide but with a glancing blow and not dead on, part of the knietic energy will end up as the bodies spinning while they fly apart again. For conversion to heat, plastic deformation is required, as Ssscience pointed out below. This generally means that calculating the amount of heat is not trivial, and temperature rise most decidely non-trivial, because plastic deformation, specific heat, and thermal conductivity are all non-linear in real bodies. Not at all easy. 60.230.213.251 (talk) 02:19, 20 October 2013 (UTC)

Without plastic deformation, the decrease in kinetic+potential energy will equal the increase in "heat energy". When plastic deformation has occured, part of the energy may have gone into breaking of inter- or intramolecular bonds, changes in surface energy, stored as internal stress... Ssscienccce (talk) 00:30, 20 October 2013 (UTC)

What you can do here is compute the relative speed just before impact. If this is much more than the speed of sound of the materials the bodies are made of, then you can use the same methods that are used to compute the effect of impacts in the Solar System. A lot is known about this from theory, simulations and experiments, there are ready to use formulas that are derived from these results using scaling relations. Count Iblis (talk) 14:53, 20 October 2013 (UTC)

Recrystalization vs. acid-base extraction

In an experiment in which benzoic acid is purified using two methods, recrystalization (heating and cooling) and acid-base extraction (which involved mixing benzoic acid with ethy acetate and changing its pH in a separatory funnel), which should one should result in a higher percent mass recovery (i.e. recovered mass similar to initial mass)? Also, I have no idea how, but my recovered mass from the extraction method was twice the initial mass. This doesn't seem right. --FutureTrillionaire (talk) 16:18, 19 October 2013 (UTC)

Extraction should give a MUCH higher percent recovery -- recrystallization relies on the decrease in solubility with temperature, so a good portion of your product will go into the waste with the solvent (been there, done that!) As for why your recovered mass is twice the initial mass -- the most likely explanation is that your extracted product still has A LOT of solvent in it (or maybe it somehow reacted with your solvent because of a pH excursion, though I think this is unlikely). 24.23.196.85 (talk) 19:25, 19 October 2013 (UTC)

Cool. Thanks man!--FutureTrillionaire (talk) 19:38, 19 October 2013 (UTC)

With a solubility of 68.0 g/l at 95°C and 2.1 g/l at 10°C (1.7 at 0°; 2.9 at 20°), recrystalisation could be quite efficient, depending on the amount of water used... Ssscienccce (talk) 21:10, 19 October 2013 (UTC)

Health effects of low exposure to pesticides

Is there any evidence of harmful health effects from consuming fruits and vegetables containing a small amount of residual pesticides? The article on health effects of pesticides wasn't clear about this. 74.15.138.165 (talk) 17:20, 19 October 2013 (UTC)

Short answer: yes, see paraquat for example. Nuanced answer: one reason the article is not clear is that the answer is not entirely clear -- among other factors it depends on what pesticide is involved and what amount. There is some reasonably solid evidence that exposure to certain pesticides can increase the likelihood of Parkinson's disease, for example (see our article). Looie496 (talk) 17:40, 19 October 2013 (UTC)

Well, that depends on how small is your small amount -- if you're talking about 1 part per trillion, then no pesticide can possibly have an effect at such a low concentration, whereas if you're talking about, say, 100 parts per million, then long-term harmful effects are very much possible, depending on the pesticide in question. 24.23.196.85 (talk) 19:51, 19 October 2013 (UTC)

What do you mean by exposure? Does that mean exposure at the typical concentrations that you would find on a vegetable in a supermarket? 74.15.138.165 (talk) 20:12, 19 October 2013 (UTC)

Birth defect rates were higher (in U.S., 1996 to 2002) when conceived in the period april to july, same period in which concentration of pesticides in surface water (and possibly tap water) is highest. Increase (in birth defects) was 3%. Doesn't mean causation of course... Ssscienccce (talk) 22:48, 19 October 2013 (UTC)

• There is sometimes a paradoxical measurable benefit from a small exposure to certain toxins. I am annoyed I cannot think of the term for this. Perhaps it will occur to someone else. μηδείς (talk) 00:44, 20 October 2013 (UTC)

Hormesis 89.204.137.240 (talk) 02:52, 20 October 2013 (UTC)

Or Desensitization (medicine), immunization and even homeopathy, 175.38.168.70 (talk) 03:02, 20 October 2013 (UTC)

October 20

Measuring tension in a belt

I have a long, thin drive belt running between two pulleys that are 1.2 m apart. I'm trying to estimate the tension in the belt - but it's hard to do directly. What I did was to buy a machine that fishermen use to weigh their catches - a little gizmo with a hook on one end and a digital display on the other. If I hook the machine over the belt midway between the two pulleys and pull on it until the belt deflects by 5cm - it reads 2170 grams.

Is this enough information to calculate the approximate tension in the belt? What would be the equation?

SteveBaker (talk) 04:38, 20 October 2013 (UTC)

I don't think you can calculate the tension in the belt when you aren't pulling it sideways - depending on the elasticity, it could in theory be completely slack. AndyTheGrump (talk) 04:49, 20 October 2013 (UTC)

Oh - yeah, I appreciate that. What's the tension when it's deflected? In this case, I doubt the tension is much different because drive belts are fairly inelastic and the deflection I'm applying is tiny compared to the overall belt length of around 2.4 meters. SteveBaker (talk) 04:59, 20 October 2013 (UTC)

(Hmmm - if it helps, I can measure the force to deflect the belt by two or more different amounts.) SteveBaker (talk) 05:00, 20 October 2013 (UTC)

(ec) You can approximate the tension by assuming the belt is a string (i.e., assuming its tensile forces are symmetric and isotropic). Then convert the measurement from your spring scale - you want to convert from grams (as indicated) to force (in newtons, or pounds) by multiplying mass times g (9.81 m/s). (Actually, the spring-scale measures weight: force on the spring, not mass, so it ought not display in grams!)

Then, you can draw out the geometry of your deflection: a force of (2170 · 9.81 N) gives a 0.05m deflection, let's say at a perfect right-angle to the belt, which is 1.2 m long. Draw the triangles, and solve for the tension along the hypotenuse - just straightforward trigonometry! The forces on an ideal string, in x- and y-, are exactly proportional to the deflections in x- and y- so you can use "similar triangles", or compute the tangent, or use whatever other favorite trigonometry formula you like.

The catch is, belts are not perfectly isotropic and symmetric; it is not "string-like; tension out of the plane is not perfectly additive with tension in the plane of the material. Even worse, your measurement is highly sensitive to error because of the tiny deflection you can realistically create. A tiny measurement error will be multiplied by an immense scale factor. (If you look online for belt tension gauges, as used in automobile maintenance, those devices work around this problem by controlling the geometry - they hold two points of the belt stationary while a second armature performs the deflection. No matter how long the belt is, the measured vertical deflection is always about the same size as the measured horizontal span, which keeps your calculations in a less pathological part of the tangent function; or put more simply, you want to avoid measuring a deflection using a long and skinny triangle!) My number-crunching, using Steve's data, estimates about 100 pounds tension in the belt: not impossible, but higher than I'd expect.

Among our many contributors, one of us has an ASME handbook; we can probably dig out an appropriate "engineering fudge factor" to adjust the simple equations, if your belt is made of a standard material, size, and shape... Nimur (talk) 05:01, 20 October 2013 (UTC)

Ah - so I understand the sensitivity issue. What I need to do to get more precision then is to perhaps add two fixed pulleys - maybe 10 cm apart and pull on the belt midway between the two to get the 5cm deflection. The problem with that is that the belt would need a lot of force to deflect it by so much over such a short distance. I'll play around with it...maybe I can figure out something better.

Many thanks! Having more eyes on the problem always helps. SteveBaker (talk) 16:59, 20 October 2013 (UTC)

I think you should lay out which alterations are possible and which ones aren't - also whether you need to know the tension moment to moment or if you are just measuring it once to position the pulleys. For example, if you have the option to anchor one of the pulleys by a material with Hooke's Law elasticity such as a spring, you can calibrate how far the pulley moves with varying forces you apply yourself, and then measure the position of the pulley moment to moment to know the tension. If you have the option to modify the belt itself, you could have a region of the belt with a known elasticity, with a small insert that is inelastic anchored to one end of the region, and have it report how far the other end is displaced. Wnt (talk) 15:56, 20 October 2013 (UTC)

I can't change the geometry or material of the belt - both pulleys are fixed (and moving them would alter the tension - which kinda defeats the object of measuring it). SteveBaker (talk) 16:59, 20 October 2013 (UTC)

Can you do the following?

• make two marks on the belt and measure precisely the distance between them
• take the belt off the pulleys and put clamps on the belt at the two marks
• apply measured tension to the clamps to stretch the part of the belt between the clamps, to the extent that the two marks are the same distance apart as when the belt was on the pulleys
• use the applied tension as an estimate of the tension of the belt when it's on the pulleys

--173.49.18.190 (talk) 08:34, 21 October 2013 (UTC)

Could you remove the belt, get another 2 pulleys, fix one, attach a newtonmeter to the other, and move it to the correct position (ensuring it is held by only the newtonmeter and the belt? The distances won't be 100% accurate, but it's probably closer than you'll get by the other techniques. If you set up some kind of fixed sand 120cm+x (where x is the length of the newtonmeter, plus a fudge to account for the extension while in use) from the fixed pulley, then you don't even need to hold it. MChesterMC (talk) 08:46, 21 October 2013 (UTC)

If the belt is sagging measurably by it's own weight, and you know the weight per unit of length of the belt, you could calculate the tension without applying springs or other devices by using the formulas for a catenary. But that's unlikely, drive belts usually have high tension compared to weight... Measuring the same way you did for a few other deflections (2, 3 and 4 cm, best not larger, don't want to stretch it) and comparing it could give you some idea. You have to measure the deflection in respect to a straight line between the pulleys, not from the belts "resting position" (because of sagging). To give approximate value, use 120*M/D with M= measured grams and D is deflection in cm. Not completely exact, but for small deflections it doesn't matter in comparison with the measuring accuracy. Ssscienccce (talk) 13:36, 21 October 2013 (UTC)

Is reading in your DNA?

Is the skill or tendency to be good/easily learn to read in human's DNA? Throwaway62 (talk) 04:47, 20 October 2013 (UTC)

The capacity for language is practically universal in humans. It's the exceptions and the struggles which get the attention, such as dyslexia. That suggests that language capability is hard-wired, as sure as the ability to walk upright. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:17, 20 October 2013 (UTC)

It's certainly both, as being exposed to reading early on also affects one's ability and willingness to read. StuRat (talk) 13:16, 20 October 2013 (UTC)

You can say it in a sense, because of course there are genetic learning disabilities that can prevent people from learning the skill. However, despite some interesting phenomena such as dyslexia which involve a more visual-level confusion that people can circumvent with training, I'm not aware of any genetic syndrome that specifically prevents reading while allowing other activities to work normally. This should not be surprising since reading was developed too recently for evolution to affect specifically to any great degree. Wnt (talk) 15:46, 20 October 2013 (UTC)

That last sentence doesn't make much sense to me. Reading is based on symbol recognition, and that ability goes way back in human evolution. In fact, it may even predate humans, as other animals seem able to recognize some symbols, too. So, there should be genes for that, which will affect the ability to read. StuRat (talk) 02:17, 21 October 2013 (UTC)

Well, if it comes to recognizing whether a symbol is a "b" or a "d", dyslexia affects that, and that may be very vaguely comparable to an animal trying to decide if an insect is an edible or inedible species. But recognizing that three letters together in a certain sequence, without spaces, but with spaces around them, have a particular meaning... it's hard to picture an equivalent from other animals. (There's a whole article on reading (process) which I should refer to rather than philosophize about from scratch, I suppose) Wnt (talk) 05:14, 21 October 2013 (UTC)

Though on consideration, it's possible I'm pursuing a No true Scotsman fallacy here... Wnt (talk) 17:12, 21 October 2013 (UTC)

Yes, I think very similar logic is used to determine that "that object is a chair" as is used to determine "that object is the word 'chair' ". It's all just various forms of pattern recognition. Heck, in some languages the word chair might actually be a picture of a chair. StuRat (talk) 17:23, 21 October 2013 (UTC)

But then there's grammar, and also the tendency of, say, Egyptian hieroglyphs to pull away from meaning exactly the thing represented in the picture (how exact that can ever be anyway is a side-track) into the domain of metaphor and sounds. You can say that symbols for sounds are combined into words and the words recognised as patterns representing things, but some of those things are very context-dependent abstract concepts or helper words, some of the words may be new but still understood, and it's all cognitive in fancy ways that amount to something more than pattern recognition ... arguably.  Card Zero  (talk) 18:59, 21 October 2013 (UTC)

This 2009 paper on the genetics of reading disability by by Tracey Petryshen and David Pauls says: "although there may be some disagreement regarding the genetic contribution to specific components of [reading disability], all studies demonstrate that [reading disability] is influenced by genetic factors. However, the specific underlying genetic mechanisms are not known. " Gandalf61 (talk) 09:21, 21 October 2013 (UTC)

Just a note that while the ability to read may be coded in our DNA, the skill is not. Skill in reading falls squarely on the nurture side of nature versus nurture and is not heritable. q.v. Epigenetics.--Auric talk 22:58, 21 October 2013 (UTC)

Dropping a Slinky

I was watching an episode of QI yesterday and on it Stephen Fry claimed that we don't know why the bottom of a dangling Slinky remains stationary until the top compresses down on to it at which point the whole thing falls to the ground. You can see what I'm referring to here. The man in that video though seems to suggest that we do know why this happens. Granted, it's simplified for laymen. Oversimplified you might say. I'm curious though and want a bit more explanation. I'm not looking for math formulas and things though because that would be over my head. But could someone explain it a bit more in depth? I have an idea in my head but I don't really know how to best express it. Something about the tension in the up vector, mid-drop, being greater than the force of gravity.

Note: This is not a request for medical or legal advice. Please do not hat this question. Thanks, Dismas|^(talk) 05:50, 20 October 2013 (UTC)

Isn't this the same as Steve's question above, about belt tension? Or is that just the delusion of generality perpetuated by the addled brain of a physicist...? Yes, tension in the real world is more complicated than the idealizations we teach in elementary statics!

The answer is quite obvious, and the video you linked above explains it very well. The tension at the bottom of the spring exactly counters the force gravity. At time t=0, when the top of the spring is released, the tension at the bottom has not changed. So, there is no net force on that part of the spring material; and it remains at rest.

Why hasn't the tension changed yet? Information about the dropping of the top of the spring should travel fast - at the speed of sound. In spring steel, that's something crazy fast, like eight kilometers per second! Even at 300 frames per second, that would take less than a frame of video to reach the bottom of the spring.

But slinkies are crazy contraptions! Tension between the coils is a totally separate entity from tension along the coils, and if we wanted to, we could describe the slinky as a "meta-material" with multiple speeds of sound! When energy propagates as a compression wave among the coils, (rather than as a compression wave along the interior of the metal material), the speed of propagation is quite slow.

And finally, since we have to satisfy action and reaction, consider the top of the slinky. At time t=0, the exact same rules apply. The tension has not changed and is still pulling that part of the spring downward; this is added to the force of gravity; and the contact force where the man was holding the spring has suddenly vanished. There is a net force: gravity plus downward tension. The top of the spring falls downward, faster than a free-falling object.

If you had enough time - say, you dropped the slinky off a tall building - that compression wave might reach the bottom of the slinky a long time before the slinky hit the ground, and the bottom would move. The wave might reflect, and the slinky would expand and contract as it fell; and then you'd get a wobbling slinky. Chances are pretty good that the entire slinky would start tumbling, unless you were absolutely perfect in the way you dropped it, and air flow was perfect, and so on.

If you study advanced (i.e., continuum) mechanics, your teachers will force you to solve you will have the opportunity to solve the equations of motion as a continuum - rather than treating the spring as a point particle, like you would in engineering statics (or "high school physics"). You will have to express the tension, and in fact the mass, as a function of spatial coordinate, and then construct an equation of motion that satisfies all the constraints; forces must be resolved, and you may also require that the spring remains intact (as a single, solid but perhaps non-rigidly connected object). Usually, this requires learning how to live and work in Lagrangian coordinates. Once you learn that technique, most of the rest of physics - even the weird stuff like relativistic motion and atomic physics - all seems pretty mundane! Nimur (talk) 06:09, 20 October 2013 (UTC)

Even though the speed of the actual movement of the object might be slow wouldn't tension in a substance like that cause effects that may propagate at about the speed of light.175.38.168.70 (talk) 08:41, 20 October 2013 (UTC)

(edit conflict) Further to Nimur's comments above, a simple way of looking at the situation is that the centre of gravity of the slinky falls as expected, but at the same time the slinky contracts because it no longer has a stretching force. Thus the top falls at twice the "expected" speed (because it has both gravity and internal tension acting on it) and the bottom doesn't fall at all until the contraction ceases. Dbfirs 06:19, 20 October 2013 (UTC)

Thank you both. I think I understand better now. And I don't read the Science Desk on a regular basis, so I wasn't aware of Steve's question above. Dismas|^(talk) 06:59, 20 October 2013 (UTC)

Going back to Nimur's answer above, in the HD video here at about one minute in, the slinky is dropped from a tall building and eventually - at about 1.30 - the top of the coil appears to overtake the base, which is still stationary. Richerman (talk) 10:02, 20 October 2013 (UTC)

The mathematical explanation can be found here . Ssscienccce (talk) 13:53, 21 October 2013 (UTC)

It really isn't counterintuitive. With a slinky you can see the upward force, i.e. the tension that is implied by the spring being stretched out. Since the area right above the bottom has the stretched shape, you know full well the bottom can't drop because it's held up by that. What's harder to accept viscerally is that the center of gravity really has to follow Newtonian physics. I mean, why does the very top of the slinky have to start moving at the exact rate needed so that the mass it has will move at the speed it does so as to make up for all the mass in the center and the bottom? I suppose I see that its speed will be faster from the same force the smaller its mass, and the force will be greater the larger the mass it supports, so it has to work out, unless it's hindered by air resistance or the speed of light or something in which case I suppose the Newtonian model doesn't apply any more, but it's still an unexpected place to see math poking its nose into. :) Wnt (talk) 18:49, 21 October 2013 (UTC)

Power use in simple remote control

Help! Recently my girlfriend went to a battery shop and got them to change the battery in her remote garage door opener. The man who did it told her that the battery would last much longer if she only used the remote when she was really close to the door, rather than out on the street. It sounded like the usual rubbish "technicians" come up with to me, but then a recent question on the Ref. desk about radio communication got me thinking that maybe I was too quick to judge! I assumed that simple remotes like this just broadcast a coded signal at a certain strength. What is the real answer? 122.108.189.192 (talk) 07:19, 20 October 2013 (UTC)

I think your first instinct is right, and that's BS. One exception is if she starts trying to use it when out of range and ends up pressing the button more than once. StuRat (talk) 13:14, 20 October 2013 (UTC)

StuRat is probably correct, but there are hundreds of types, makes, and models of door openers; the electronics inside them are usually subcontracted recursively to various small electronics companies; so it's nearly impossible for us - or the retail technician, for that matter - to answer with great certainty. Even if we had a lot of time, money, and skilled scientists and engineers, reverse-engineering garage door opener radios is not as easy as it sounds!

Most garage door remotes are simple unidirectional broadcast transmitters, so they expend the same transmitter power whether the signal works or does not work. However, there are some garage door openers with more advanced radios - probably not as sophisticated as the mobile telephone radio protocols we discussed last week. But it's plausible that your remote might "retry" broadcasting the command until it receives an acknowledge response from the base station. That would mean that it would waste a lot of power retrying (until it succeeds, or times out), if the reception is poor or the distance to the base station is too far.

Why does this matter? Because power savings of a few microwatts might extend the life of the battery by several years!

And, there are other reasons to care about garage door openers. Some years ago, I learned a lot more than I ever expected to know about garage door radio protocols, because (as you can read in this 2008 JIEDDO summary report), door opener remotes were a very popular and very sinister IED trigger in Iraq and Afghanistan. They're cheap, they're easy to buy, and the radios inside them are all totally different. Retailed door opener products just get slapped together by contract manufacturing designers with any radio circuit that happens to be available that week! That made detecting and jamming the bombs almost impossible for American electronics warfare soldiers. "JIEDDO has spent more than $2.3 billion to develop, procure, field, and sustain electronic jamming technology and techniques to thwart simple, cheap triggering devices such as two-way radios or garage door openers." As it turns out, when the lives are on the line, it is actually very hard to know everything there is to know about a garage door opener radio protocol. Nimur (talk) 05:48, 21 October 2013 (UTC)

There is nothing in the JIEDDO report that indicates some remote garage door systems send back a confirmation signal from the receiver. Doing so would double the cost and increase the current drawn form the remote battery, even if the first ping worked. And it is totally unnecessary: If your garage door doesn't open, simply hit the button again, a bit closer. As in recent years all such devices operate in the high UHF band, the range is consistent from one day to the next. The owner will soon learn how close he/she needs to be. 60.230.213.251 (talk) 06:45, 21 October 2013 (UTC)

You're right; the report I linked only establishes that a multitude of garage door radio exist; it does not describe them in detail; but that was an overview report written for politicians. Technical details would bore the politicians, so those sorts of boring details are kept in confidential reports that aren't available on the internet at large. Nimur (talk) 15:51, 21 October 2013 (UTC)

I have a Chamberlain garage door opener which uses rolling codes for security (so somebody can't just scan the code you use then come back later and replay it to open your garage door). Logically, they must send a confirmation signal, or how else would you keep the rolling codes in synch between the sending and receiving units ? Of course, if the confirmation code isn't received, then sending the code again and again automatically from the sending unit would be unwise, in case the button is pressed when you are nowhere near the garage. StuRat (talk) 16:26, 21 October 2013 (UTC)

Assuming you have a KeeLoq encoder, see Rolling code and this datasheet linked from it, particularly pages 16 and 17. The transmitter just increments its code by one every time you press the button - the receiver keeps track of the last code sent successfully. If the received code is within 16 of the last code, it's accepted. If it's more than 16 but less than 32k away from the last code, you need to resynch the transmitter and receiver by pressing the button twice, so that the receiver gets two sequential codes. If it's more than 32k away, the transmitter is locked out and you'll have to reset it. But no information is transmitted from the receiver to the transmitter. Tevildo (talk) 19:21, 21 October 2013 (UTC)

That's too bad. That method means it will accept something like 32 codes, making it much easier to hack, than if it worked the way I hoped. StuRat (talk) 20:59, 21 October 2013 (UTC)

Adaptive signal strength is useful in cel phones because of the length of the transmission, but implementing it in a car door opener seems unlikely, the two-way transmission would use additional power for the required receiver, besides there are legal limits on the signal strength of these devices so they're limited to short distances anyway. Ssscienccce (talk) 14:12, 21 October 2013 (UTC)

(OP) Thankyou all very much, an interesting comment from Nimur particularly, Scary stuff!! 122.108.189.192 (talk) 06:44, 22 October 2013 (UTC)

Can airplane runways be sloped?

I think you can save energy and time by landing while going up the slope, and take off while going down the slope. What is the problem with this?--朝鲜的轮子 (talk) 07:40, 20 October 2013 (UTC)

A Google search on "sloping runway" gives lots of hits, and some dramatic images. HiLo48 (talk) 08:09, 20 October 2013 (UTC)

Yes. Here are some tips on taking off and landing at sloped fields. Dismas|^(talk) 08:14, 20 October 2013 (UTC)

While it's possible, it's avoided where possible, and when not possible may result in quite some limitations. Civilian air traffic is highly safety-conscious (which I as a semi-frequent traveller very much endorse ;-). Part of that safety is achieved by regulation, training, and routine - to the degree that not only all air traffic controllers and pilots speak English, but that they are even expected to use a particular set of words and phrases defined by the ICAO. Having a non-standard runway layout decreases the value of that training and routine, and hence increases the risk of operating such an airport. Similarly, you have to define specific procedures for just that runway, and you would have to do a careful safety analysis that cannot draw on past experience from the large number of normal airports. Finally, most runways can be operated in two directions. Runway 21 is not the 21st runway of an airport, it is its single runway when operating in the direction that is 210 degrees off north. The very same strip of concrete (or asphalt, or dirt ;-) is known as runway 153 when when it's operated in the other direction. When there is significant wind, runways are always operated in a way that aircraft take off and land into the wind. Strong crosswind is a hazard that can even lead to an airport being closed - this is why many airports have runways in two different directions. --Stephan Schulz (talk) 08:35, 20 October 2013 (UTC)

Correction -- the other end of Runway 21 is Runway 3, not Runway 15. 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

Right! Thanks for the correction! --Stephan Schulz (talk) 11:57, 21 October 2013 (UTC)

See also Courchevel Airport, one of the most dangerous airports in the world. Rather you than me.--Shantavira|^{feed me} 08:37, 20 October 2013 (UTC)

Thanks for the explanation on runway numbering. I've wondered about that when I've flown, but never enough to look it up once I got back home. The configuration of Chicago Midway International Airport presumably takes advantage of different wind directions, and obviously they can't use both parts of the "cross" at the same time - and it's clear that a slope would be of no use at all. In fact, I would think airplanes are designed with the assumption that they're taking off from a flat surface. I've seen smaller airports where some taxiing runways have a slight slope due to the natural topography, but that's not a problem. As regards Courcheval, I infer from the article that it's not too bad IF you're flying a small plane and IF it's a clear day. And it's not especially comforting that it's only the seventh most dangerous airport in the world. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:59, 20 October 2013 (UTC)

You should have seen the sloping (and short, and rough, and high-elevation) runway at Matekane, Lesotho! 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

Also note that over the length of a runway, building up even a modest slope would require moving massive amounts of Earth, especially if you don't want the ground to drop away right at the edges of the runway. So, the expense is yet another reason not to do this, in the case where the ground is naturally flat. StuRat (talk) 13:12, 20 October 2013 (UTC)

However, in a VERY few cases, a runway may be deliberately built on a slope, in order to allow heavily overloaded aircraft to take off downhill -- an example would be the airstrip at Scholkovo (east of Moscow, near the town of Zhukovsky) from which Chkalov took off on his flight across the Arctic Ocean. 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

It would also mean that almost every single airport would need to double the number of runways if they were sloped. As an example if the airport has a 13/31 runway and the top of the slope is at the 13 end, then another parallel runway with the top of the slope at 31 is also going to be required or aircraft may be landing downhill and departing uphill. CambridgeBayWeather (talk) 06:51, 21 October 2013 (UTC)

The NAZI buzz bombs used sort of a sloped "runway". Since they didn't need to land, that eliminated the doubling of runways issue. I believe other unmanned aircraft use the same trick. Some are caught in nets, while others land on level ground. StuRat (talk) 16:51, 21 October 2013 (UTC)

A more recent example and also rather specialized is the Flight deck#Ski-jump ramp. Alansplodge (talk) 17:24, 21 October 2013 (UTC)

It's not obvious to me that you would save anything overall by taking off downhill anyway. You might reach take-off speed sooner, but at take-off you would be pointing in a less advantageous direction in terms of climbing to altitude, and it might take just as much fuel to correct that as was saved on the runway. 86.176.215.115 (talk) 19:49, 21 October 2013 (UTC)

Redirection would be a simple matter of making the end be like a ski jump hill, right? (but less extreme). I wonder what the G-force would be. Sagittarian Milky Way (talk) 11:03, 23 October 2013 (UTC)

That depends on the plane's speed and the angle of the ski jump; for a navalized Mig-29 taking off from the carrier Admiral Kuznetsov, it's on the order of 2.5-3 G. 24.23.196.85 (talk) 00:11, 25 October 2013 (UTC)

Galvanised nails as plant water sensors

I'm a serial plant killer. I would like to make an arduino-based project to protect my herbs from drying out. How long would galvanised nails function as water sensors? Would they need to be recalibrated as they age? I have six pots, all on the same window sill. — Preceding unsigned comment added by 2.97.26.56 (talk) 13:03, 20 October 2013 (UTC)

I assume the problem is you forget to water them ? If so, you might want to get some of those inverted bulbs which slowly deliver water. I haven't used them myself, though, as I don't have that issue. Also, are you using any type of plant food/fertilizer ? StuRat (talk) 13:07, 20 October 2013 (UTC)

I've not used any fertiliser but these herbs didn't really last long enough to use it. I'm not sure how well those globes will fit but I'm going to buy some and find out. Thanks for the suggestion! I may still decide to make an electronic system, just for fun, but it can now go on the backburner along with everything else! --2.97.26.56 (talk) 13:23, 20 October 2013 (UTC)

You're welcome. Also, you might want to use some simple reminder method. For example, if you use a paper calendar, you could write a W in the corner of each day, then cross it out after you water the plants that day. Or, if you make coffee each morning, maybe you could put a "water plants" note inside the coffee can. StuRat (talk) 13:33, 20 October 2013 (UTC)

Responding, we should note that arduino is a sort of open source single board microprocessor; at least the original premise is presumably to measure how dry the plants are rather than to assume it. However, there may be a problem there, if I'm correct in assuming the galvanized nails would be intended as electrodes, because pure water is not a very good conductor, but ions make it conductive. So fertilizer, accumulated salinity, etc. would throw off the measurement. You'd need some sort of actual hygrometer. It seems like it could be as simple as a human hair ...... but I suddenly realize that I have absolutely no conception of what the "relative humidity of dirt" is, how to measure or even define it, even though it's something that we take as a matter of common sense every day! Wnt (talk) 15:39, 20 October 2013 (UTC)

• Our relevant articles are available water capacity, Hydrology#Soil_moisture, Soil#Soil_water, and Permanent_wilting_point. We could probably use a whole article on soil water content, if anyone is interested in putting a stub together out of the links above :) SemanticMantis (talk) 17:54, 20 October 2013 (UTC)

One could also use some sort of scales for this: you add water, system notes the maximum weight and plays the fog horn once the plant+pot is 50 grams (or whatever) lighter. 88.148.249.186 (talk) 17:27, 20 October 2013 (UTC)

And then you go to make a big meal and cut a lot of herbs, only to get a fog horn sounding right next to you. :-) Katie R (talk) 19:02, 21 October 2013 (UTC)

Err. Let us brake things down to their components parts. (1) Galvanised nails: Are these truly galvanised nails or nails that have been quickly and cheaply zinc plated and sold as being ' galvanised'? Because the thickness of zinc will affect their working life. (2) Although zinc is important for plant growth... too much is toxic. One does not leave this type of hydrometer in a pot all day and every day – unless one is a serial plant killer.--Aspro (talk) 22:15, 20 October 2013 (UTC)

There are very cheap and very simple Arduino soil moisture sensors available on Amazon - they're basically nothing but traces on the PCB. I suspect that over time they will corrode and lose effectiveness, much like a simple nail-based solution would. Katie R (talk) 14:57, 21 October 2013 (UTC)

Googling arduino soil moisture gives many discussions and several types of hardware. Seems like a common "almost anything above entry-level" model is to alter the metal to avoid corrosion. DMacks (talk) 15:12, 21 October 2013 (UTC)

A plant moisture meter I've seen in a store consists of two different metals (such as aluminum and copper with an insulator between them, at the end of a probe which is stuck in the soil. At the top of the probe is a simple meter movement with a pointer and dial. You could insert a galvanized nail or strip of zinc at some small distance from a strip of copper or a copper nail and periodically measure the voltage between them, which should relate in some way to the soil moisture, though it would probably not be a linear relationship. If you constantly drew current from this electrochemical cell, it would consume the zinc electrode or the zinc coating on the iron nail. Iron and copper would have a different voltage output from zinc and copper. Some judgement would be needed as to what output voltage represented ideal moisture, too dry, or too wet. For a nail and a piece of copper wire, there would be an appreciable fraction of a volt output, but the available current would be extremely low, so design your measurement circuit accordingly. Edison (talk) 04:16, 23 October 2013 (UTC)

Effects of high speeds in outer space

Since there are no g-force and air in the outer space, what are possible effects of, say, putting an arm (of course, in a space suite and in a isolated compartment) out of a window of a spacecraft travelling at cosmic speeds (say, 500,000 kmh and over)? Or just traveling externally fastened to a spacecraft, going with those speeds (excluding possible injuries from meteors)? --93.174.25.12 (talk) 20:12, 20 October 2013 (UTC)

Space is a near vacuum, not a true vacuum. So, you would be hit by some objects. Individual atoms or molecules presumably won't each put a hole in the suit, but will wear away at it, like dripping water wears a hole in a bathtub. I'm not sure how long this would take, though. Also, this would cause heating of the suit. And there are also micrometeoroids that would put a hole in the suit. StuRat (talk) 20:19, 20 October 2013 (UTC)

(ec)There are g-forces in outer space, of course. But apart from that: If you are in a spacecraft in free fall, there is no problem with sticking your arm out, or flying alongside, unless you come to really high speeds (when even the very tenuous gas and dust in interstellar space will start to become significant). --Stephan Schulz (talk) 20:24, 20 October 2013 (UTC)

At constant speed in an absolute vacuum and all other things being equal, none I can think of. This is interestingly related (Space travel using constant acceleration) not sure about the case here. Ok I suppose going out side the craft tethered would be like absailing at whatever g the ship is pulling. — Preceding unsigned comment added by 122.111.243.217 (talk) 04:32, 21 October 2013 (UTC)

@StuRat, "like dripping water wears a hole in a bathtub", what? where? when? Never in my life ... Caesar's Daddy (talk) 06:58, 21 October 2013 (UTC)

It would take a lot of drips, and might be more likely to happen to an old-fashioned ceramic or iron tub than something made of plastic as they are now. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:13, 21 October 2013 (UTC)

Not sure if plastic tubs are any better at resisting this. It might just be that they aren't old enough yet to show this effect. It takes decades. StuRat (talk) 16:18, 21 October 2013 (UTC)

Good point. Also, plastic doesn't rust. But a steady drip-drip-drip over many years ought to have some effect eventually. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:04, 21 October 2013 (UTC)

Yep, I'm old, and old-fashioned. I've seen that on old bathtubs. HiLo48 (talk) 09:39, 21 October 2013 (UTC)

Yes, so have I, though it's often combined with corrosion because of the constant damp. For pure erosion, the effect is more clearly seen in old stone sinks. For a more dramatic effect in nature, see Panhole (called swirlholes in the UK). Dbfirs 10:01, 21 October 2013 (UTC)

At that sort of speed, the atoms and ions will be like cosmic ray radiation and penetrate deep into that arm, causing radiation damage. Graeme Bartlett (talk) 10:49, 21 October 2013 (UTC)

500,000 km/h isn't that fast. Solar wind peaks at close to 3,000,000 km/h - prolonged exposure to peak solar wind outside the protection of Van Allen radiation belt will increase a person's lifetime cancer risk. And our solar system rotates around the galactic center at 700,000 km/h, while our galaxy is on a 500,000 km/h head on collision course with the Andromeda galaxy. Measured against the cosmic microwave background our galaxy zips along at 2,000,000 km/h[7]. It's all relative. 88.112.41.6 (talk) 15:17, 21 October 2013 (UTC)

Did the Nazis performed unethical psychological experiments too?

Think Monster Study, Stanford Prison Experiment without cutting it short, Learned Helplessness with humans, and so on. OsmanRF34 (talk) 21:01, 20 October 2013 (UTC)

I don't see anything listed in the Nazi human experimentation article. If anyone finds reliable info, please add it (with cites, obviously). DMacks (talk) 21:57, 20 October 2013 (UTC)

I don't believe so. Would they believe such psychology applied to them so what was the point for the war effort? They were mostly interested in how to treat various sort of trauma, transplants and diseases and viewed the prisoners as lab rats without even today's concerns. Dmcq (talk) 09:12, 21 October 2013 (UTC)

October 21

Metric

The Fisher information metric is understood to describe infinitesimal changes in the Kullback-Leibler divergence. However its quantum analog, the Bures Metric, is understood to describe infinitesimal changes in the Bures distance, which is defined with reference to the Fidelity, and not the Quantum Relative Entropy (the quantum analog of the KL divergence). What is the reason for this breakdown in analogy?

Has a metric describing infinitesimal changes in the QRE been studied? Does its behaviour differ from the Bures Metric? — Preceding unsigned comment added by 144.82.191.250 (talk) 10:51, 21 October 2013 (UTC)

Just linking some of the terms above for reference: Fisher information metric, Kullback-Leibler divergence, Bures metric, Bures distance, Quantum relative entropy. -- The Anome (talk) 11:21, 21 October 2013 (UTC)

Also Fidelity of quantum states. Red Act (talk) 17:18, 21 October 2013 (UTC)

Maybe the Mathematics reference desk is a better place for this ... Ssscienccce (talk) 14:27, 21 October 2013 (UTC)

This is unarguably within the remit of physics. — Preceding unsigned comment added by 86.134.240.24 (talk) 19:45, 21 October 2013 (UTC)

Regarding the absence of a very well known info on SEXUAL DESIRE on wikipedia

This question is related to sexual desire of males. In the article SEXUAL DESIRE i could not find a very well known fact. It is generally seen that in sexually active active males, the time gap between the ejaculations affects their sexual desire. For example, if a man masturbates or ejaculates regularly(say once a day or once every two days or twice a day) and suddenly stops it, his sexual desire(or what we call libido) increases..It is also observed that the orgasm after a period of abstinence are also intense and getting and erection in a short term abstinence period is quicker..Why does this happen..? And why does wikipedia not mention i under the factors affecting sexual desire ..? I know that males are made to release their sperms once in a while. But why could i not find a reason or mention of it on wikipedia. .. Please give a scientifically proven reason for your answer....many thanks — Preceding unsigned comment added by Ed beerman (talk • contribs) 11:46, 21 October 2013 (UTC)

If it's that well-known, you should have no trouble finding valid sources on it, and could improve the article yourself. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:42, 21 October 2013 (UTC)

You are generalizing with no links to any studies. Your statements amount to speculation and can be discarded.217.158.236.14 (talk) 13:24, 21 October 2013 (UTC)

It is noted (with cite) in Libido#Physical_factors. Libido is one of the concepts linked and discussed in Sexual desire, but I don't understand the distinction between those two concepts...seems like there is a lot of overlap between their articles. DMacks (talk) 15:07, 21 October 2013 (UTC)

• The most relevant article is probably refractory period (sex). Looie496 (talk) 16:05, 21 October 2013 (UTC)

yes,i have read about Refractory period(sexual)..But this article only deala with the time period after ejaculaion during which it is impossible for a person to get an erection..i am talking about a different thing(although we can relate it to refractory period)...i am asking about the reason that greater the gap between two ejaculations(at least in short run) , higher is the desire to ejaculate ...why is sit so..? — Preceding unsigned comment added by 117.207.94.35 (talk) 17:27, 21 October 2013 (UTC)

At a guess, I'd imagine that it has at least something to do with the changes in pressure in the ejaculatory system caused by the build-up and release of fluid, but that would be pure unfounded speculation on my part; there are so many mechanisms that could be related to it -- hormonal, nervous (voluntary and involuntary), muscular, vascular, psychological, learned behaviours, etc. etc. that it wouldn't be suprising if it was still an open question. Have you tried to take a look at the medical literature on the topic, e.g. via Medline? -- The Anome (talk) 00:42, 22 October 2013 (UTC)

Are you sure this is actually true? I usually masturbate (or have sex) around 4 times a day, if I'm busy and not able to, and am aware of this, then what you're discussing holds. On the other hand, sometimes I don't notice, in which case it doesn't make a difference at all. In other words, at least for myself, it seems that if I want sex/masturbation and can't have it, then I want it more- and if I don't really want it and don't get it, then I still don't want it. What you seem to be describing is wanting + not getting = really wanting; but that holds for hunger, thirst, the need for a cigarette, or any other desire. I think the key is where you say "and suddenly stops it", rather than doesn't feel like it- if you avoid satiating anything, again, you're going to want it more.Phoenixia1177 (talk) 04:11, 22 October 2013 (UTC)

"Oh, no, not again. The creature stirs..." (Momentarily contrasting the desperate intellectual deprivation of the 80s with our fortunate forum, though I think Harry would have missed the sound of a human voice.) Wnt (talk) 05:20, 22 October 2013 (UTC)

Four times a day sounds like above average frequency for male orgasm. I have heard "Age 20: tri-weekly. Age 40: try weekly. Age 60: Try weakly." So no scientist has published reliable sources looking into the temporal link between getting off and horniness? Amazing. Think of all the studies that could be done with experimentally varying the hydrostatic pressure in the seminal vesicles and seeing how it affects horniness, or just measuring the horniness as a function of the interval from last orgasm, or administering hormones and seeing their effects, whether on rats, monkees or human volunteers. Edison (talk) 04:02, 23 October 2013 (UTC)

It is above average frequency, I suppose, I've always had a strange fascination with sex. I think, though, as far as the question goes, the focus is too narrow- the same relations hold for most wants. If you miss a meal, you might eat more at your next, or at least think more about it. The main element seems to be having an unsatisfied desire, not anything specifically sexual.Phoenixia1177 (talk) 05:06, 23 October 2013 (UTC)

'You regularly have it(like you 4times a day) and you suddenly stop it, then you want it more(if you are aware and not having it), this is what i want to say..does the urge to masturbate not really increase if you dont have sex for a,say a week(when you regularly have it)..? are the orgasms after a peiod of short term abstinence not more intense than the usual..? Also it is so common to say that we get a wet dream to release the fluids when the tank is full..Sexual desire is also like hunder or any other biological need.. Now i arrive at two simple questions: When will the sexual desire(or stimuli) be more strong--When you are masturbating fouth time in a day..? Or when you are masturbating after four days Should "time gap between ejaculations" be a factor affecting sexual desire on wikipedia..? If no, Please tell why..many thanks 12:16, 22 October 2013 (UTC)~~ — Preceding unsigned comment added by Ed beerman (talk • contribs)

I would think that the plumbing, hydrostatics and hydrodynamics contribute very little. The main stimuli are psychological and pheromonal. Dbfirs 12:56, 22 October 2013 (UTC)

I can only speak from personal experience to answer, so bear that in mind. I notice that the more I want sex, the more intense the experience is, and the more I think about it- and the worse it is if I can't have it (I work long hours some days). Some weeks, I get wrapped up in other things of interest and sex is far from my mind (as are things like regularly eating and sleeping), in those cases, I don't really miss it, and, when it returns, it doesn't come back in a "flood", but slowly increases back to normal (hunger and the interest in sleep do the same). On the other hand, sometimes I'm very fixated on sex, in those cases, if I can't have it, it's all I want to think about- in that case, I masturbate more, think about it more, etc. As for when the sexual experience is the most intense: during those times when I am masturbating more frequently, the experience is stronger- since I'm not doing this back to back, but a few hours apart, each experience is equally intense (there is variance, but it is a factor of time of day and schedule more than where the event is in sequence). As I said, I think the major factors in intensity and want is desire and availability- if I really really want sex and get stuck at work for a 20 hour shift, it's very amazing when I finally get home- on the other hand, if I'm at a low period of interest and work a 20 hour shift, when I come home, sex is less of a "yippee!" and more of a "I'm sleepy, but my wife likes sex, so okay".Phoenixia1177 (talk) 05:06, 23 October 2013 (UTC)

Opportunities for Exploring Comets and Minor Planets

• List of missions to minor planets
• List of missions to comets

Where can I get information about proposed projects and/or theoretical opportunities of unmanned space explorations?

Is there a list of future "windows" for the exploration of solar system objects?

Launch windows are determined partly by the available propulsion power and the location of your launch facility. If you have an ion rocket and a very light-weight spacecraft and your rocket base is on the equator, you probably have larger windows for visiting every solar system body. Otherwise, you may have smaller or no windows at all.

I just want to see a list of the "ON SALE" (least expensive) days for visiting comets and minor planets. -- Toytoy (talk) 15:14, 21 October 2013 (UTC)

I think you will need to narrow you criteria; there are over 1 million asteroids and comets known. Rmhermen (talk) 15:43, 21 October 2013 (UTC)

There must be some useful rules for solar system tours.

If you want to travel beyond Mars, you may need to use Mars as a slingshot. Then you have to look-up the nearest window for Mars at first.

I think there must be many rules and restrictions to narrow down available possible destinations. -- Toytoy (talk) 07:41, 22 October 2013 (UTC)

See Exploration of Mars#Launch windows. Duoduoduo (talk) 17:43, 22 October 2013 (UTC)

Also Exploration of Venus#Under study, BepiColombo, Solar Probe Plus#Trajectory and mission, Venus In-Situ Explorer, Venus Entry Probe, Venera-D, Europa Jupiter System Mission, and Titan Saturn System Mission. Duoduoduo (talk) 17:54, 22 October 2013 (UTC)

I heared a presentation on which asteroids you can visit and bring a sample back. Although there are a lot of asteroids out there the number was small like 5 or 10. First you have to get ride of all with high inclination. The ones which are far out are off limits because of the too high reentry speed. Third is to eliminate all fast spinners. At the end you elimated all but 10 and you end up at the point that OSIRIS-REx and Hayabusa 2 had too look for the same conidates.--Stone (talk) 18:21, 22 October 2013 (UTC)

The problem is always you can go for venus mars and earth flybys to get to speed and with that you have a lot of oportunities. --Stone (talk) 18:21, 22 October 2013 (UTC)

Soda pop can exploding when frozen

Why do soda pop cans explode when put in the freezer too long? I've put 20 fl oz plastic bottles of cola in the freezer, but they don't explode. --209.203.125.162 (talk) 17:51, 21 October 2013 (UTC)

Aren't cans usually full to the top, with no room for the ice to expand, while bottles usually have air in the neck? μηδείς (talk) 17:56, 21 October 2013 (UTC)

Plastic bottles seem like they can stretch a bit from internal pressure, whereas metal can certainly deform but not as readily to increase volume? DMacks (talk) 19:00, 21 October 2013 (UTC)

Yes that's it. Soft plastics can undergo a great deal of plastic deformation (no surprise there), while metal isn't as flexible. StuRat (talk) 20:55, 21 October 2013 (UTC)

However, as I understand, the recent incidents of "dry ice bombs" in LAX were plastic bottles stuffed with dry ice and then capped tightly. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:59, 21 October 2013 (UTC)

That's because it's easy to put dry ice in a plastic bottle and seal it. If you fill it completely with dry ice, then it will have to pop. For it not to pop would require it to be able to contain a gas at a pressure of about 1000 bar. Count Iblis (talk) 00:26, 22 October 2013 (UTC)

Which is essentially what makes them work. If you tried the same with a can (somehow, not sure how you'd seal it), you'd probably get a smaller pop, since it would rupture at a much lower pressure. MChesterMC (talk) 08:50, 22 October 2013 (UTC)

So if the cola inside is frozen, why does it explode all over the freezer? Why doesn't the can just burst open, leaving a frozen cola cylinder ? --209.203.125.162 (talk) 01:17, 22 October 2013 (UTC)

Not all of the cola is frozen. Enough freezes for it to break the seal, through which the remaining liquid escapes. 81.147.166.89 (talk) 01:31, 22 October 2013 (UTC)

The freezing process involves the unmixing of the water and the cola ingredients, you get pure ice from which the rest of the cola is expelled. The freezing temperature depends on the concentration of the ingredients, so, when a little ice forms the cola ingredients have been expelled from the ice, the concentration will have increases in the remainder. So the remainder will only freeze at lower temperatures. This means that at some given temperature, the freezing process will stop before everything is frozen. But of course, if the can cannot hold a large pressure it will break open. Most of the contents will be a liquid which then explodes out of the can as it was held under pressure. Count Iblis (talk) 01:35, 22 October 2013 (UTC)

There are a number of phenomena going on here. The big ones are the liquid freezing from the outside in, forming a rigid container of ice around an ever-decreasing volume of liquid, and freeze distillation forcing dissolved carbon dioxide out of the liquid as it freezes, forming a bubble of high-pressure gas. In a relatively inelastic can, at some point the pressure reaches the point where the ice shatters, the can ruptures, and the remaining liquid and gas spray all over the place. In an elastic bottle, the bottle will stretch as the ice breaks, letting the formerly-dissolved gas join the bubble at the top of the bottle. --Carnildo (talk) 01:45, 23 October 2013 (UTC)

The can bursts because water expands when it transforms from liquid to solid, unlike many other fluids. I wanted to create a smooth hemispherical dome on the bottom of a Diet Coke can so it would work well as part of a Van de Graaf generator using "found" materials .I had observed that a cola can has a recess at the bottom and that when it freezes, the recessed bottom as well as the top expand out and become convex. I used Diet rather than Regular Coke because the sugar lowers the freezing point. Anyway, after a while the can at about zero F had bulged out at the bottom into a dome with a few rings and the top was deformed outward as well. It was placed bottom up for better observation. I decided to give it another 5 minutes to try and smooth out the dome a bit more with increased interior pressure. When I came back, the poptop had failed and the Coke slurry inside had come out like a rocket, and the can had clearly bounced around in the freezer. The actual instant of failure and resulting rocket action would make a cool Youtube or Mythbusters episode (perhaps motivated by checking the "myth" that Diet Coke freezes and bursts before regular Coke in a deepfreeze or very cold outdoor environment). Edison (talk) 03:46, 23 October 2013 (UTC)

Meat from unstressed animals

The claim that meat from unstressed animals tastes better is regularly made, but is there any scientific evidence that it is actually true? All the sources that I can find merely assert it without presenting any evidence, or use words such as "people say that...". 86.176.215.115 (talk) 19:42, 21 October 2013 (UTC)

Science has certainly backed this up. Lack of references may come from generations of our forefathers experience that take this as a given – so our forefathers didn’t require any authority to tell them what they already knew - so that's the origin of "people say that...".

• This 'patent' give some of the science bit as background (with refs) to explain the patent:[8]
• This link explains why it is important in to the meat industry:[9]

--Aspro (talk) 21:13, 21 October 2013 (UTC)

The type of food an animal eats supposedly can make a significant difference in taste (I've been told that deer that feed on corn taste a lot better than deer that have to resort to leaves and twigs), so it's not unreasonable to expect that the animal's general well-being could also affect the taste. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:01, 21 October 2013 (UTC)

That's an interesting point which is worthy of a separate discussion. In short: You know how Coca-Cola did blind consumer market research and tried to introduce New Coke only to find it was then rejected – as not the real thing. I think is a bit like corn feed deer. Some people used to eating domesticated animals I can well imagine, find that the taste of 'farmed deer' more familiar. I myself, prefer what I am more familiar with - i.e., Scottish wild deer. My tasted bud are now getting a bit jaded (alas) by the passage of time but younger members of the family refer to farmed deer, as a bit like funny tasting lamb! In other words, lamb has been slaughtered before it has developed the richer taste of mutton. Farmed deer are slaughtered before they have developed a rich flavor and so like lamb, non have the taste that my hungry mouths around the dinning table associate with meat from a deer hunted on a scottish estate where they feast on a natural Highland diet. But that has nothing to do with the stress that the animal is put through before slaughter. PS. The food the animal eats does has a very big effect on taste - there is no supposing about it.--Aspro (talk) 18:18, 22 October 2013 (UTC)

You may check out this article.

• http://www.journalofanimalscience.org/content/83/2/440.full

If a pig has been scared for too long before slaughter, you may see its meat whitened unnaturally and lose its meat juice. -- Toytoy (talk) 08:09, 22 October 2013 (UTC)

Cline variation

What is meant when it is said that human variation is clinal not racial? — Preceding unsigned comment added by 70.49.46.102 (talk) 19:54, 21 October 2013 (UTC)

It basically means that the prevalence of specific human genes and phenotypes generally varies gradually instead of suddenly across geographic regions, making it difficult and arbitrary to try to meaningfully divide humans into discrete races biologically, if you consider humanity as a whole. See Cline (biology). Red Act (talk) 20:10, 21 October 2013 (UTC)

So essentially it is hard to draw the line where for example black race or white race or asian race starts? But it is easy to do that, we all know what an Asian, or African or white person looks like? — Preceding unsigned comment added by 70.49.46.102 (talk) 20:15, 21 October 2013 (UTC)

No we don't. There is so much regional variation those terms are almost meaningless sometimes. That of course doesn't mean there aren't' genetic differences between different populations, but they don't necessarily map to the traditional 'human races'. Fgf10 (talk) 20:49, 21 October 2013 (UTC)

A couple points:

1) It isn't always easy, with people of mixed ancestry.

2) The genes that describe things we think of as race (skin color, hair color and texture, etc.) are only a small number of the total genes. So, you could have Asian genes that control your appearance, and yet have non-Asian genes as well, which don't contribute to your appearance. StuRat (talk) 20:52, 21 October 2013 (UTC)

It can sometimes work reasonably well as a model to divide people into races, if you only consider the people who live in one geographic area, due to historical immigration patterns from separated locations which have obscured the clinal nature of the genetic and phenotypic variation among humans as a whole. For example, a lot of people arrived in North America from Europe, mainly Northern Europe and Western Europe, during the European colonization of the Americas, and a lot of people arrived in North America from Sub-Saharan Africa during the Atlantic slave trade. People in northwestern Europe and people in Sub-Saharan Africa tend to look rather different, so if you're only considering people who now live in North America, it works moderately well to categorize at least some of the population in North America into "white" vs. "black" races. However, if you take into consideration everybody in the world as a whole, what people typically look like varies gradually if you consider a sequence of populations extending from Sub-Saharan Africa up through northwestern Europe. For example, people from Egypt (in Northern Africa) don't typically look like they came from England or France, but they don't look like they came from Senegal, either; instead, they look very roughly somewhere in between (although the variation is more than one-dimensional). But if you're considering only people who are currently in North America, you aren't taking into account too many people with largely Egyptian ancestry, because there wasn't a large wave of immigration to North America from Egypt in North America's history. Similarly, the concept of "Asian" as a race also works somewhat in North America, because historically a lot of people arrived in North America from East Asia and Southeast Asia, but not as many people arrived in North America from Central Asia or Eastern Europe, so the clinal nature of genetic and phenotypic variations along the populations in between East/Southeast Asia and northwestern Europe is obscured. Red Act (talk) 22:18, 21 October 2013 (UTC)

This is even further obscured by our limited understanding of history. If a large wave of immigration to North America from South America (or vice versa) occurred before recorded history, we would have no knowledge of the event. --Auric talk 22:38, 21 October 2013 (UTC)

Archeologists could figure it out, say if the pottery style from one South American tribe suddenly turned up in North America. StuRat (talk) 17:55, 22 October 2013 (UTC)

Thank you very much, that clears the confusion for me! 70.49.46.102 (talk) 01:11, 22 October 2013 (UTC)

Resolved

StuRat (talk) 02:05, 23 October 2013 (UTC)

Why are screw propeler more efficient than paddle wheels?

Couldn't well-designed paddle wheels (not just a flat board) be equally efficient? OsmanRF34 (talk) 22:00, 21 October 2013 (UTC)

Intuitively paddle-wheels seem less efficient because they waste energy lifting water and submerging air. Also, only a portion of the water they do move goes in the proper direction. StuRat (talk) 22:28, 21 October 2013 (UTC)

Is a screw propeller actually more efficient? They are superior for many purposes because paddle wheels don't work well if the water is not flat, but I would like to see a reference before accepting that there is an advantage in efficiency. Looie496 (talk) 00:09, 22 October 2013 (UTC)

The advantage of screw propellors is that the direction of flow imparted to the water is parallel to the screw rotation axis. That allows propellors to be made with blades of shallow pitch at the tips, gradually increasing to a more agressive pitch as you move toward the centre. In this way the velocity imparted to the water is the same throughout most of the blade length, even though the tangental velocity of the blade is a lot less near the centre than it is at the tips. Also, the blade tips can be rounded, so that in the last 5 to 10% or so of the blade, the water velocity is gradually brought down, avoiding a large shear, which would waste energy in turbulence.

Paddle wheels have the direction of water flow at right angles to the direction of rotation. That means neither variable pitch nor blade tip tapering can be used. As StuRat said, paddle wheels move some water in the wrong direction, they also move some of it at the wrong velocity, and the large shear at the bottom of the paddles causes much useless turbulence.

The problems of paddle wheels could be overcome to a limitted extent by using co-axial nested paddlewheels of huge diameter, with the inner wheels turning faster, and the very outside wheel turning very slowly. Or by using some sort of linkage to keep the paddle blades vertical. But its never going to be as good as a propellor, with anything like a practical robust assembly. Why would you bother, when a simple propellor does the job. 60.230.213.251 (talk) 00:43, 22 October 2013 (UTC)

There are various tracked amphibious vehicles which propel themselves using their tracks ("some sort of linkage to keep the paddle blades vertical"). The DARPA Captive Air Amphibious Transporter looks fun.  Card Zero  (talk) 01:04, 22 October 2013 (UTC)

besides every movement in a propel contribute to the ship displacement, in the wheel you have to waste energy raising up the paddles over the water surface, plus the wheel weight more and require more space, etc.

Planes have to be very very efficient, so they has “propel like” devices (helices turbofans etc)

Iskánder Vigoa Pérez (talk) 02:13, 22 October 2013 (UTC)

Actually, the raising of paddles is not a problem. It takes energy, stored as potential energy in the paddles, but it is balanced out by other paddles moving down into the water, which give back their potential energy. Wheel weight also have no effect on efficiency. A high wheel weight just means that the wheel acts as a flywheel, and a flywheel running at constant speed does not consume energy. In any case, propellors have considerable weight too. 60.230.213.251 (talk) 06:17, 22 October 2013 (UTC)

See also here. Count Iblis (talk) 02:39, 22 October 2013 (UTC)

Hmmm... what if the paddles were made up of some sort of "molecular origami" that was held by some thin, strong, rod, but otherwise had substantial freedom to furl and unfurl in an arbitrary shape by computer command throughout the stroke? If you had the ability to force a succession of, say, cupped parallel planes through the water, might it work better than the screw shape? Wnt (talk) 05:15, 22 October 2013 (UTC)

Well, as I said above, a paddle wheel cannot be more efficient than a propellor, and any real design will be less efficient because with a paddle wheel the water flow is at right angles to the axis of rotation, whereas for a propellor the water flow is parallel to the axis of rotation. That allows profilling the blade width and pitch along the length of the blade, so over most of the blade, the motion imparted to the water is consistent in the desired direction. Yet shear at the tips is minimised by tapering the blades, reducing turbulence. Have a look at the photo of a modern propellor in the propellor wikipedia article. See how the blade pitch increases as you move to the centre. See how the blade width is taperred of to nothing at the blades tips. You can't do that with a paddle wheel - it has no geometical meaning.

Incidentally, while a paddle wheel can be improved by arranging the paddle blades to remain vertical, that still leaves us with excessive turbulence due to shear at the bottom of the blades. And increased shear at the sides. Again, why go for a complex mechanism when a simple propellor does the best possible job? Also, cupped shaped blades do not alter any of this.

60.230.213.251 (talk) 06:30, 22 October 2013 (UTC)

I don't think that one can say that either is more efficient, without fixing some parameters. One could fix the impeller area and total thrust, with the Reynolds number being very large. Under these conditions, the impeller that accelerates water over the largest area will generally be most efficient (least total kinetic energy imparted to the fluid per unit time for a given thrust). Since a paddle wheel only effectively accelerates the fluid over the area of a blade, but a propeller does so over the area that it sweeps out, the latter's advantage is kinda obvious. At a low Reynolds number (imagine treacle), viscous drag dominates and adds a significant energy loss mechanism, when this argument does not apply. — Quondum 03:14, 23 October 2013 (UTC)

Really? The value of computing the Reynolds Number is that it tells you whther or not the flow is going to be turbulent (high Reynolds) or laminar (low Renolds), just from the speed of the fluid past surfaces. You don't want turbulence, it is just a waste of energy. So, paddlewheels or propellors, you want to operate at a Reynolds Number below the transition value. Below that value things are pretty much linear with speed. 124.178.152.227 (talk) 03:31, 23 October 2013 (UTC)

October 22

Penicillin-producing fungi in the human body

According to its article, Trichophyton interdigitale is a species of fungus that's often responsible for causing fungal infections in humans and other species, and it's also one of the species known to produce penicillin. If wounded (e.g. punctures or cuts), are areas infected with T. interdigitale less likely to develop bacterial infections than areas not infected by the fungus? Or does the fungus produce such small amounts that it doesn't have a substantial effect? Nyttend (talk) 04:57, 22 October 2013 (UTC)

Well, the effect is apparently more just that the bacteria become resistant [10] but that does suggest there is some real antibacterial activity at some point. Wnt (talk) 05:25, 22 October 2013 (UTC)

House dust mite - Helping me with this could very well improve my life

Soon i am gonna move to a new place so it's kinda in late, but, is there any actually affective anti-House dust mite (Anti-HMT) Spray that i could buy from the net and spray upon my bed or in the room? something that'l really do the trick? thanks from all heart. Ben-Natan (talk) 09:10, 22 October 2013 (UTC)

I am not aware of any spray that will kill dust mites AND be harmless to humans. However, there are other things you can do:-

• Give the house a really thorough vacuuming with a high-suck vacuum cleaner like the 2kW Volta. Not one those useless litle round robot things. Deep pile carpets and matresses, especially matresses with a top wool layer per American practice, are nice to sleep on but they are dust mites' best friends.
• Then, get a commercial cleaning service in to wash all carpets.
• Wash the bedroom curtains. — Preceding unsigned comment added by 60.230.213.251 (talk) 09:40, 22 October 2013 (UTC)
• Use fine-weave cotton sheets on beds. You can get special fine weave sheets expressly for this purpose. They work, because the mites hide in the mattress during the day, then come up though the sheets to bite you when they sense your body warmth. With fine-weave sheets they have to walk the long way round.
• Before you move in, get a referral to an alergy specialist and have him test you for dust mite response. If you have no response, most likely with normal good cleaning stanards you won't have a problem. If you do have a response, you will need to be extra scrupolous in cleaning until the dust mite poulation falls.
• If you are moving into a furnished home, give the matresses away to the needy, and buy your own new. This is always a good idea anyway. All sorts of body oozes collect in matresses, even when they look clean.
• Wash the bedsheets each time you do so (should be once each week) with a small amount of eucalyptus oil added with the detergent. The smell's not at all overpowering, most people think its nice, but dust mites hate it. — Preceding unsigned comment added by 60.230.213.251 (talk) 09:43, 22 October 2013 (UTC)
• If you remain concerned, say by itching, have yourself tested again 3 months after you move in.

Lastly, the main problem with dust mites is that they make you itch. So do lots of other things - gardening in short sleaves without gloves, old age causing hormone levels to fluctuate (especially with females, but men too), taking certain presciption medications, alcoholism, and taking illicit drugs.

60.230.213.251 (talk) 09:37, 22 October 2013 (UTC)

Some of the information given above is bordering on medical advice which we are not supposed to give. You will find information from the professionals on how to deal with dust mites here and here. You will find lots more information if you google "dust mite allergy" Richerman (talk) 13:54, 22 October 2013 (UTC)

• I didn't think that dust mites caused itching - according to [11] there are other mites that do that. A search readily turns up resources like [12] that discuss both physical and medical approaches. Wnt (talk) 14:00, 22 October 2013 (UTC)

Which echoes my concern about some of the above being medical advice. I think the uncited advice from the anonymous editor above should be removed. Richerman (talk) 14:11, 22 October 2013 (UTC)

I want to make sure we are talking about dust mites and not bedbugs. Dust mites are microscopic and universal, and the best we can do is to reduce their numbers and the accumulations of their feces, by careful, frequent cleaning. Bedbugs, on the other hand, are large enough to see, and come out to bite people at night and suck blood, and need to be addressed by professionals, as they can cause medical problems. StuRat (talk)

Can dust mites be killed using radiation? It seems to me that irradiating your mattress with intense gamma rays would get rid of them. Count Iblis (talk) 15:48, 22 October 2013 (UTC)

Right -- just whip out your handy-dandy portable Tevatron and you're ready to go. Or would you prefer a tiny nuclear explosion? Or covering the bed with plutonium, maybe? Looie496 (talk) 16:06, 22 October 2013 (UTC)

The basic concept of sterilization by irradiation is sound (but men, don't try standing in front of the microwave as a "poor man's vasectomy"). However, home irradiation isn't practical, as it requires handling dangerous radioactive materials. I suppose somebody could offer a service where you bring your items in and they irradiate them, but just putting them in a chamber devoid of oxygen might be a lot safer. StuRat (talk) 16:15, 22 October 2013 (UTC)

You might say that radiation is used to kill bedbugs ... infrared radiation, as they are heated up. There's a whole little industry of tenting houses and heating them to 130 degrees F, and lesser expedients will apparently work: [13] Wnt (talk) 16:39, 22 October 2013 (UTC)

Ha, beat me to it. Yeah, insects are prone to dehydration. One way to ensure you're not carrying bugs from your old place to your new oine, for example, is just leave the truck wit all your junk in it parked in the sun for a day or more. So maybe if you just cranked up the heat in the new place as high as possible for as long as possible. This is all conjecture, of course, and note that these guys will probably breed back to the original level pretty quickly. 206.213.209.31 (talk) 17:19, 22 October 2013 (UTC)

Your thermostat goes up to 130°F ? Is that a Vulcan brand thermostat ? :-) StuRat (talk) 17:52, 22 October 2013 (UTC)

Damp clothes smell

After getting wet in the rain, I notice a musky smell on my clothes or body, that smells a little like clothes that have been forgotten in the washing machine. What causes this? Is it related to the same process for clothes left damp in the washing machine? --129.215.47.59 (talk) 10:05, 22 October 2013 (UTC)

See mildew. --Jayron32 11:04, 22 October 2013 (UTC)

Agreed. Clothes must remain wet for some time for mildew to grow. However, when they dry off or are washed with detergent alone, this doesn't kill the mildew, it simply remains dormant until the next time the clothes are left damp. I find using bleach in the laundry to be the most effective way to kill the mildew. StuRat (talk) 16:19, 22 October 2013 (UTC)

anaphilaxis and andrenaline

Why when someone needs to inject Epipen for elergy he needs to do it in her knee? why the other place of the body are not for that? If it needs the mussels there are a lot of them in the body, so what is the special in this place?176.13.29.148 (talk) 13:24, 22 October 2013 (UTC)

The muscles of the thigh (note—not the 'knee') are a large, easy-to-hit target, and one that is easy for most people to reach on their own body. The prinicipal concern with epinephrine autoinjectors like the EpiPen is that one might accidentally hit a vein, thereby allowing a single, concentrated dose of epinephrine (adrenaline) to be delivered straight to the heart. The outer thigh is an ideal injection site, as it contains no large blood vessels. From the manufacturer's web site [14]:

...The outer thigh is the safest site for IM injection as there is minimal risk of injection into major blood vessels or nerves in this area. Intravascular injection (which is possible on the front of the thigh) could lead to acute cardiovascular compromise. Injection into a major nerve (which could occur on the posterior thigh or buttock) can cause significant damage; also, injection into the buttock may not be effective for a severe allergic reaction. Accidental injection into the hand, particularly the digits, can cause serious injury and possibly gangrene....

So, the risk of Bad Things is higher with other injection sites. TenOfAllTrades(talk) 13:40, 22 October 2013 (UTC)

Looking for articles or references on epidemics due to lack of immunity

Hi all, when European colonists met indigenous peoples in the New World, Australia, and other places, my understanding is that many of the locals there were killed by smallpox and other diseases, diseases to which which the Europeans had an acquired immunity. The story then goes that contact between populations is inherently dangerous, because of this potential for epidemics. Yet as far as I am aware, the reverse did not happen, that is, the colonists did not in turn die of diseases contracted from the natives. Why did one side get sick and not the other? Or is the premise false? IBE (talk) 16:23, 22 October 2013 (UTC)

There is extensive discussion of this in Diamond's book Guns, Germs, and Steel. DES ^(talk) 16:28, 22 October 2013 (UTC)

Thankyou - exactly what I was after. I shall take note of the reference, and put it on the to-do list, but I won't be able to do this until well after the thread is exhausted. Can you or someone give me a very quick (three or four point) summary? I am primarily interested to know if the premise is true, and whether it is due to smallpox being a particularly contagious and virulent disease that just happened to be prevalent around at the same time as the contact between civilisations occurred. IBE (talk) 16:40, 22 October 2013 (UTC)

The article linked to summarizes his arguments fairly well. As to your question, it is generally agreed that there were a number of very widespread epidemics in North and South America after contact by Europeans, and that these were particularly severe because the local populace had little or no immunity to the imported microbes. Going the other way, it has at least been alleged that Syphilis migrated from the Americas back to Europe in a similar way, and that its initial outbreaks were very severe because of a lack of immunity. As to WHY this effect was so much larger in the Americas than in Europe, I don't think there is a general consensus. Diamond asserts that the previous history of Europeans had involved more encounters with initially separate groups, and a much wider East/west span (for all of Eurasia), and a larger variety of domestic animals (often a source of disease) all leading to a more robust set of immunities among the European population. Not all scholars accept his views. DES ^(talk) 16:49, 22 October 2013 (UTC)

I wouldn't think you'd need to look farther than the relative population sizes of the New and Old Worlds. With more people, there would be more diseases to evolve, and immunity to develop, in the Old World. The differences in population would, in turn, be due to the larger land area in the Old World, and the fact that it has been populated far longer. StuRat (talk) 17:46, 22 October 2013 (UTC)

According to this article indigenous diseases may have actually started to affect the indigenous population more at the time of colonization. Count Iblis (talk) 20:03, 22 October 2013 (UTC)

is space what we really see?

Is the outer space really what we know, like we see them in photos and movies? Or is it to human eyes? Will cat or chimpanzees have a different look at it? Is there anything invisible (to us at least) out there? -anand, chennai. — Preceding unsigned comment added by 122.164.37.214 (talk) 16:48, 22 October 2013 (UTC)

There's dark matter and black holes, although black holes become visible as they swallow matter. Also, all forces are invisible, including gravity, electromagnetism, and dark energy.

And the universe would look only slightly different to eyes that see ultraviolet or infrared. StuRat (talk) 17:36, 22 October 2013 (UTC)

Actually, no. For example galaxies look very different depending on the wavelength. Eyes can see only a tiny part of the electromagnetic spectrum. One can play with this e.g. with chromoscope. Various kinds of objects can be very faint at the visible wavelengths and very bright in X-rays or far-IR.

Photos, e.g. those colourful ones NASA proudly displays, also are generally different than what we could see with naked eye. The observations are (almost) always designed to solve some scientific problem... for example, a galaxy could be observed at three different wavelengths: 1) narrowband H-alpha 656 nm to probe star formation 2) some broadband observations at optical, like V-band 500 nm 3) infrared K-band at 2.2 micron. One then combines them into a RGB single image, which looks pretty but is very different than one would see with naked eye. The big telescopes do not even have eyepieces--at least normally. Telescope opening ceremonies and celebrations are a different matter, at least the first 8-m VLT had an eyepiece then. 88.148.249.186 (talk) 18:23, 22 October 2013 (UTC)

I'd call that slight difference. If there were galaxies which only existed in the IR or UV spectrum, I'd call that a major difference. StuRat (talk) 22:21, 22 October 2013 (UTC)

Cats see things a bit differently than humans do because (most) humans have trichromatic vision, whereas cats have dichromatic vision. Chimpanzees, however, have trichromatic vision similar to humans. Red Act (talk) 18:32, 22 October 2013 (UTC)

You might be interested in the qualia article. Sean.hoyland - talk 18:33, 22 October 2013 (UTC)

The qualia problem has bee solved. See Stephen E. Palmer's work. μηδείς (talk) 02:58, 23 October 2013 (UTC)

Thanks for the name because I don't think I've read anything of his, so I will. I didn't really mention qualia as "a problem", more as a response to the notion that "outer space" or anything is "really" like how animals or machines perceive or represent it. Sean.hoyland - talk 07:51, 23 October 2013 (UTC)

Palmer's research is in vision science and visual perception. I don't see how that than can "solve" the much wider philosophical questions about what qualia are, or whether they have any objective existence. Qualia (if they exist) are a component of all conscious experience, not just visual phenomena. Gandalf61 (talk) 09:09, 23 October 2013 (UTC)

Judging from this, Palmer apparently wouldn't disagree with that view. Sean.hoyland - talk 10:04, 23 October 2013 (UTC)

Feeding habits of the subterranean invertebrata

So... my yard is a thin coating of topsoil over the naturally occurring red clay, varying from dense to essentially rockhard. I've been attempting to improve drainage in one corner, maybe 30 foot square (10 meters square), which involved pulling up the sparse lawn there last fall and doing some digging. The point being that this area has been essentially vegetation free for about a year, baked pretty dry.

During the digging I notice two things. Firstly, down in the clay, maybe up to a foot deep which is as far as I dig, where there is no trace of any organic matter and it's pretty dense clay, I find quite a few earthworms of large size. Not monster movie size, but up there on the upper end of the normal earthworm distribution size. I don't find these guys in the more soil-like layer anywhere, although there are smaller earthworms there in reasonable number. I can see their tunnels horizontally in the clay, without any evidence of their going up into the soil layer. So, what do these big guys eat? You can't eat clay for a living, can you, even if you're an earthworm?

Secondly, back in the soil layer, there are a lot of chafer beetle grubs, the usual kind which infest lawns and eat the grass roots. Like a few per square foot, fat and happy. Mind you, this is bare soil, no turf, the occasional weed is all, and the grubs don't seem to associate with the weeds. There are a lot of maple tree roots in the soil, and they appear to be more numerous where there are roots, but I couldn't swear to it. They're not where the roots branch out into terminal little rootlets, just near the big solid inch thick roots. However, the rest of the yard where the grass is growing into a decently dense lawn, is essentially grub free. ??? Maybe the parents find it harder to lay eggs where the grass is thicker, but my question is, what are these guys in the bare area eating? They don't eat tree roots, AFAIK. I suppose they could eat whatever organic matter is in the soil, but again, I thought grubs didn't do that, they were consumers of grass roots. They're thriving quite well in the absence of any living plants other than the tree roots. ????206.213.251.31 (talk) 17:38, 22 October 2013 (UTC)

I'm not an earthworm expert, but from your description it seems likely that the large worms are in fact living in the topsoil and you're only noticing their tunnels in the clay because they can't break through and instead are worming along the boundary, perhaps looking for deeper crevices. In the normal topsoil their tunnels would probably collapse more readily and not be as noticeable. My "yard" runs from mixed to very clayey and it seems the ability of the clay to hold shape makes the worm trails there more long lasting and noticeable. Matt Deres (talk) 19:51, 22 October 2013 (UTC)

Clayey's not a word! You just made that up! μηδείς (talk) 02:56, 23 October 2013 (UTC)

To be honest, I was surprised when the Firefox spellcheck didn't redline it. Huh, it's redlining both "spellcheck" and "redline", but not "redlining" (or clayey). What a world. Matt Deres (talk) 15:44, 23 October 2013 (UTC)

Cleaning from background radiation

By which means a human body can be completely cleansed from background radiation, including radon exposure? And what actions minimize their effect (perhaps water while taking a shower)? --93.174.25.12 (talk) 19:29, 22 October 2013 (UTC)

I'm quite confused. Background radiation is the stuff that is, like, everywhere, the radiation that exists as a baseline in all places. It cannot be avoided. See Background radiation which states that it is "ubiquitous". Excess radon exposure isn't really "background", if, for example, you have radon buildup in confined basement areas. If you wanted to minimize your exposure to background radiation, you could chose certain areas of earth to live on (the background radiation is ubiquitous, but not uniform). However, you cannot make yourself live your entire life exposed to literally no ionizing radiation ever. --Jayron32 19:40, 22 October 2013 (UTC)

Right - there is radiation coming from radon gas in the air, there is radiation from the food you eat and the water you drink, there is radiation from the rocks in the ground and from space in the form of cosmic rays. You could possibly reduce that somewhat by living in a lead-lined box in Antarctica *where atmospheric radon is at a minimum), eating food grown in another lead-lined box. That won't reduce the amount of radiation to zero - because you'd need an infinite thickness of lead to guarantee that.

But it's all a bit silly - we've evolved to live with a certain amount of background radiation - sure, there is a tiny increased cancer risk from background radiation - but eliminating it would come at such a high personal cost as to almost certainly make your statistical chances of dying early larger rather than smaller. There are better ways to spend money to live longer.

That said, if you live in an area with excessively high radon density, it does make sense to install good ventilation systems into your home and place of work.

SteveBaker (talk) 20:47, 22 October 2013 (UTC)

Is lead itself very slightly radioactive?  Card Zero  (talk) 21:39, 22 October 2013 (UTC)

Theoretically. But the observed isotopes of lead in nature do not show detectable levels of radioactivity. Someguy1221 (talk) 21:49, 22 October 2013 (UTC)

Not just theoretically but actually. We used to spend a fortune on special low radioactive lead for use in the lab. I think it came only from a few very deep mines but this is the only ref I can quickly fine to explain it. [15] Presumable the lead from these very deep mines formed so long ago that the radiation had reduced to very low levels. So yes, even lead give a slight radioactive signature -what ever its age and origin. Yet, to put it into context: my mom's apple pies are very much more radio-active – but too yummy to resist.--Aspro (talk) 22:07, 22 October 2013 (UTC)

...so they stay warm forever ? Yum. As for the lead, I don't think the lead from deep underground was formed earlier, if you are talking about the lead atoms themselves. However, there might be impurities in the lead ore (other elements) which have short enough half-lives that they significantly decay over millions of years. StuRat (talk) 22:17, 22 October 2013 (UTC)

What Stu said is right. The radiation is not coming from lead, but from contaminants. I hadn't thought of that wrinkle. Someguy1221 (talk) 22:25, 22 October 2013 (UTC)

@ StuRat. No, I am not talking about the formation of the lead atoms themselves but the age that they had crystallised and laid as Native metal in that deposit. Petrologist use the age of the overburden to help to attain and double confirm the latest age of the underburden. Thus, native lead from the deep mines exhibit lower alpha emitters – (or so the lead refiners that charged us an arm and a leg for just a few hundred weight told us). P.S. Mom's apple pies often got cold (when she was presumable able to keep them hidden from us long enough for the radiation (thermal?) to die down and they were just as good when cold. In which case I personally preferred a few cloves where as her hot pie benefited from a little aromatic cinnamon. Maybe a chief with a nuff (enough) culinary degrees to get a job as a cooks thermometer would like to comeback on this – as I am still experimenting to reproduce that elusive je ne sais quoi'' @Someguy1221 Now lets get to the 'term' lead. The post above said and I quote “living in a lead-lined box in Antarctica”. One assumes in this context... the common understanding of lead. forth definition. I.e., The stuff we come across every day. No mention, nor suggestion, intonation, et cetera, et cetera, of a practically unobtainable pure lead-lined box . Lets us now wave a 'red flag at a bull' . Tell me where I was wrong in my last post? --Aspro (talk) 14:35, 23 October 2013 (UTC)

A few weeks back I asked about ways to grow radiation-free food. Unfortunately, chemical elements like potassium, which we need for our survival, naturally include a significant amount of radioactive isotopes. The conclusion of the discussion was that it might be possible to produce foods using centrifuges and such to reduce the portion of radioactive isotopes, but it would be very expensive, and would only reduce the radiation somewhat. I do agree that, if it was possible to live a life free of ionizing radiation, then we should. Unfortunately, it doesn't seem to be possible. StuRat (talk)

First only use isotopical pure substances best the pure elements. Go and get carbon-12, .... only the non radiaktive ones. Buy the biosphere 2 disasamle it and bring it to a very deep mine building and clean it properly. Fill it with the isotopes you need and start introducing microbes to process the whole stuff to more complex chemical compounds. Algae would be best to start with. After some time you should be able to live in the biosphere 2. All the things you eat should come from inside the biosphere 2 and all what you want to get ride off should be dumped outside.After several years you should be fairly clean. To be 100% save wait one or two generations. A nealy non radioactive human will be at hand.--Stone (talk) 22:31, 22 October 2013 (UTC)

...and you'd better re-build you biosphere and all equipment using low-background steel. This could get very expensive :) Gandalf61 (talk) 08:28, 23 October 2013 (UTC)

Smog contest: Great Smog of 1952 in London versus today in Harbin, China

Did any scientists do air monitoring in the Great Smog of 1952 in London to see the level of particulates and pollutants?Has any reputable source at least estimated the actual pollution levels, beyond " blinding and deadly pea-soup?" I wonder how it compared to the present smog in industrial areas of China? Edison (talk) 22:49, 22 October 2013 (UTC)

Prob'ly not -- at that time, smog in London and other British cities was considered commonplace and not something to be too concerned about. It was only in retrospect, after examining the epidemiological data, that the actual harm caused by the Great Smog to the life and health of Londoners became apparent -- which led to clean-air legislation (which wasn't in place before the Great Smog). 24.23.196.85 (talk) 01:33, 23 October 2013 (UTC)

The CNN report on it says visibility was 100, and yesterday down to 20 - 30 meters. That's much more than during the Great Smog, according to our article visibility could be down to a metre or so... So, assuming similar composition, London was 20 - 30 times worse. They're both grey smog, not photochemical. Ssscienccce (talk) 08:43, 23 October 2013 (UTC)

My dad tells me that the worst day of the great smog was so thick that you could not see your feet while walking - but at as he travelled in to London by rail the visibility outside was only 50 feet or so because of fog. It surprises me that anyone would even try to travel in this, but evidently many people did by feeling kerbs and looking for lamp posts (road traffic had completely stopped). This makes a comparison of pollution levels difficult, it could well be that the thick natural fog meant that visibility was a lot less for the same pollution level. -- Q Chris (talk) 08:58, 23 October 2013 (UTC)

What is this medical process called?

Be prepared for some extremely unscientific and vague descriptions here.

Step 1. Person has a disease.

Step 2. Person survives the disease, either through doctor intervention or just surviving it.

Step 3. The Doctors use something from his body to cure others in the future.

What am I describing? I feel I've heard of this before, but Google can't find anything of this description. 68.111.166.27 (talk) 23:58, 22 October 2013 (UTC)

Sounds like antibody therapy, or the related intravenous immunoglobulin therapy. The basic idea is that you take the antibodies that one person has developed against a disease-causing agent and use them to attack that same agent in other patients' bodies. In modern medical science, these therapeutic antibodies are not harvested from human survivors, but from genetically engineered lab animals. The sequence for the antibody is, however, often derived from a human survivor's immune system. Someguy1221 (talk) 00:32, 23 October 2013 (UTC)

Thank you! 68.111.166.27 (talk) 05:26, 23 October 2013 (UTC)

October 23

Killed so quickly pain is not registered

Is is really possible to kill someone so quickly their pain receptors will not signal? I assume lasers would do as such; but what about something more conventional?— Preceding unsigned comment added by CensoredScribe (talk • contribs) 01:16, 23 October 2013 (UTC)

Yes, destroy their nerves faster than the rate of pain. Plasmic Physics (talk) 01:27, 23 October 2013 (UTC)

I don't know if you'd consider nuclear weapons to be "more conventional", but they can do the trick very well -- someone caught in the actual fireball would be vaporized before even the ion channels can open in the pain receptors, let alone the signal traveling to the brain! 24.23.196.85 (talk) 01:38, 23 October 2013 (UTC)

I have added a link to ion channel to your post. 220 of ^Borg 05:22, 23 October 2013 (UTC)

I’ve thought about this in the past and I suspect if a shipping container fell on your head without you “seeing it coming”, you’d be dead before you knew it, or felt any pain.. I think it takes about a second for your pain receptors to really kick in to full swing in your brain. Watch people who break a limb playing a sport, like soccer, it’s not uncommon that they don’t even realize it for a short while. Vespine (talk) 02:59, 23 October 2013 (UTC)

Isn't that just adrenaline? Sagittarian Milky Way (talk) 12:25, 23 October 2013 (UTC)

(edit conflict)The sensation of pain is 'created' in the brain in response to impulses from nociceptors. The brain itself though feels no pain, if I recall my anatomy & physiology classes, though the meninges covering it do.[16]

Therefore any action that destroys the brains' function quickly enough should cause no pain sensation.

• For example, a gunshot in an appropriate spot (or large enough caliber) that causes brain death should work. There are though examples of people with grave head wounds involving large portions of the brain that survive with surprisingly little disability.[17], 43% brain lost

• Vespines unexpected "shipping container" on the head, & being on top of a nuke when it explodes as our IP editor 24.23... suggests, indeed anything that near 'instantly' totally destroys the entire head & brain would seem to be good candidates too. Hand grenades or other explosive held to the head, or a rocket propelled grenade head-shot too perhaps.

This seems to be a rather hard thing to prove (volunteers would seem hard to come by), but I wonder if any tests on animals have been conducted? 220 of ^Borg 05:22, 23 October 2013 (UTC)

Yes. It is of utmost importance to well run abbatoirs. For hooved animals at any rate, three methods have been shown to work well, provided the operator does what he is supposed to do: a) a "bolt" of the right diameter impact driven into the head to just the right depth and at the right spot. The animal may well suffer though if the operator does not do it at the right point. b) an large electric current into the brain. The device should contain automatic circuitry so that the animal gets instant lights out or nothing at all. c) carbon dioxide asphixiation (used on pigs in some places). It takes up to 10 to 15 minutes or more to kill, but the animal is rendered painlesslessly unconscious long brefore death. The downside is operators may not wait long enough, and pigs recover consciousness with brain damage during the butchering process. That's not good. See the Temple Grandin lectures on YouTube. 124.178.152.227 (talk) 08:14, 23 October 2013 (UTC)

I have my doubts about that last. CO2 poisoning (not "asphyxiation" but really poisoning; CO2 in high enough concentrations is lethal even if there's plenty of oxygen) sounds fairly nasty to me. Our hypercapnia article says "symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death". (They seem to have left out headache — is that a myth?) --Trovatore (talk) 09:01, 23 October 2013 (UTC)

I suggest you read up on the ways of dispatching pigs. The effect of carbon dioxide is more complex that the article suggest. But the article does include headaches. Small amounts of CO2 just make you breath faster, without any ill effects. For that reason, gas fire suppressant systems, such as Inergen, which work by driving oxygen out of the building by displacemnt with intert gasses, also include CO2 to stimulate deaper breathing so humans can better utilise what oxygen partical pressure remains. At higher concentrations than are used in fire suppression, it can produce the symptoms described in the article. I have experienced a full Inergen dump in a computer room and felt no discomfort whatsover. At really high concentrations, CO2 asphyxiates as the blood looses its capacity to carry oxygen, which the lungs can't get enough off anyway, due to displacement - unconsciousness follows quickly with little or no distress. Note however what I said before - if the animal is not held in a CO2 chamber long enough, the oxygen starvation of the brain WLL produce nasty symptoms if and when it wakes up. The article also does not make that clear. The table of effects in the article only covers low CO2 concentrations experienced for long durations as might be encountered in human activity and you want the person to live. Pigs are dispatched with high concentrations as you want them to die as quick as possible. 124.178.152.227 (talk) 10:24, 23 October 2013 (UTC)

I should note that the experience of breathing carbon dioxide is readily available to anyone who has just finished consuming a bottle of soda pop, by inhaling from it. What's surprising is that it seems painless in this form, or when breathing from above a container of dry ice, but I've found it can definitely give a soda-pop-up-the-nose discomfort if it is humid (cloudy looking) enough. (I was curious what I was putting the mice through...) Wnt (talk) 15:48, 23 October 2013 (UTC)

@ IP124.178 I was thinking more of having sensors or electrodes attached to the animal (likely in their brain) to detect if they feel any pain at the instant they are despatched, if that is possible. The CO² thing would seem to fall outside the original question posed by CensoredScribe. I think it's implied that a fully conscious lifeform is suddenly killed without warning. -Δ-220 of ^Borg 09:39, 23 October 2013 (UTC)

I don't think that would be allowed, unless they use that kind of medicine that works by simply slowing down nerve signal speeds to bearable levels (instead of blocking brain consciousness or getting you high (opiates)), and then figuring out the full no-drug speed. Or for full scientificness, crush the animal's foot and then destroy it's brain before the signal could get there. And time it's nerve speed. Myelinated signals travel at up to 0.1 to 0.15 km/s, so over 340 mph should be unfeelable, even if it started at your toes. Sagittarian Milky Way (talk) 12:25, 23 October 2013 (UTC)

The Guillotine was introduced for this very purpose but it's still a matter of debate whether it is painless or not see: [18]. Of course, knowing that you are going to be executed is obviously a 'painful' experience in itself. Richerman (talk) 12:51, 23 October 2013 (UTC)

Supposedly, decapitation was the preferred method for condemned royals, over the other option, hanging, as decapitation was assumed to be as quick and painless as it could get. Common criminals had no say in the matter - hanging was it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:15, 23 October 2013 (UTC)

Cryogenic neutrons

How low must the temperature become, before free neutrons are electrostatically bound? (Are we talking nanoKelvin, picoKelvin, etc.?) At standard pressure, would a neutron gas convert to a solid or liquid below this temperature? What sort of packing would the solid assume, HPC? Plasmic Physics (talk) 01:21, 23 October 2013 (UTC)

There are no bound states at zero temperature and standard pressure. Compare to Helium where you also don't have a solid state because the zero point motion would provide enough energy for the Helium atoms to escape. Count Iblis (talk) 01:28, 23 October 2013 (UTC)

Then what is the minimum pressure required for bound state. Plasmic Physics (talk) 01:39, 23 October 2013 (UTC)

You'll probably have to be careful defining "bound state". Liquids exist because of inter-particle attractive forces, which could lead to a "bound state": where particles remain in close proximity, even if it is en masse rather than individually. Liquid helium should be regarded as a bound state under this definition, especially as this state occurs at zero pressure and temperature. The context can then be transposed to neutrons (which, for all we know, could exhibit a miniscule van der Waal's force due to the neutron's internal structure (or other slight attractive force). The best approach might be to try to determine the phase diagram, or pressure–volume diagram of a neutron gas at zero temperature. Chances are such information would not be easily determined, though. And as Plasmic Physics subtly suggests, and overall pressure may lead to T–P–V points where two phases are in equilibrium, where one of these phases could be considered to be a bound state – maybe even for neutrons. — Quondum 02:55, 23 October 2013 (UTC)

Pin tumbler lock

Europrofile Cylinder set with 2 double cylinders and one single cylinder (left) and the screw to secure the cylinder

Today's main page picture is from pin tumbler lock. I understand that picture and the mechanism well. However I have never understood how locks where you can enter the key from either side of the door work - indeed you can fit two keys at once, although the second key often won't turn regardless.

How does the two sided mechanism work especially given that the key isn't symmetrical. -- SGBailey (talk) 08:20, 23 October 2013 (UTC)

It's a double cylinder, simply 2 identical tumbler mechanisms, one on each side. See pic, left a single, right a double one. Ssscienccce (talk) 08:52, 23 October 2013 (UTC)

So if you wanted to, then you could make a double lock with kay A for one side and key B for the other side? I had visions of complex levers actuating the pins. -- SGBailey (talk) 11:21, 23 October 2013 (UTC)

Escape velocity

Hi,

In this week's "what if? - xkcd" Randall says the following: "However, the weird thing about escape velocity is that it doesn't matter which direction you're going.". I'm inclined to believe him as he's rarely wrong. But how is this? Surely you are more likely to escape from a planet if you are moving directly away from the centre than if you are moving tangent from the surface? Can anyone explain this to me? Thanks! 80.254.147.164 (talk) 10:01, 23 October 2013 (UTC)

Ditto, have you ever thought about moving towards the planet, and expecting to escape from it? Good luck. The escape velocity must have a vector which is greater then tangential. If the escape velocity is equal to the tangential, then you would have a simulated orbit. Plasmic Physics (talk) 10:21, 23 October 2013 (UTC)

If the planet has no atmosphere and is not rotating and the escaping object is moving ballistically and its trajectory does not intersect the planet's surface, then its escape velocity would be independent of direction, and is more correctly an "escape speed" - it is simply the speed at which the projectile's kinetic energy plus its (negative) gravitational potential energy is zero. The xkcd article is discussing a hypothetical small non-rotating asteroid, and it says "If you go faster than the escape speed, as long as you don't actually go toward the planet, you'll escape" - which is correct in that context. In practice, planets have atmospheres and rotate and rockets are not ballistic projectiles, so confusion arises when people try to apply the "escape velocity" concept to rocket launches. It makes more sense when applied to orbital mechanics in space. Gandalf61 (talk) 10:31, 23 October 2013 (UTC)

Even if you throw the body towards the planet, but make a tunnel in the planet for the body to go through, it will escape from the other end of the tunnel - WikiCheng | Talk 12:01, 23 October 2013 (UTC)

The part about the chaotic orbits of elongated objects is interesting, but I think the article fails when it says you'd have to worry about going into a tumble if you run too fast. Provided you were otherwise accustomed to the weird gravity, staying vertical rather than having periods of fast spinning, etc., your "near orbit" run would not be in a chaotic domain, I think. Wnt (talk) 15:55, 23 October 2013 (UTC)

A counterfactual example might be helpful here. Suppose it was harder to scape if you started your orbit horizontally. Then you would have to reach a point of maximum altitude and start falling back. at that point you would have to move slower than an object following a circular orbit at that altitude - That must be the case because your orbit would have a smaller radius of curvature even though you were under the same gravitational acceleration. But an object in circular orbit has a speed smaller than the scape speed contradicting our original assumption that your speed was equal to the scape speed. Dauto (talk) 16:03, 23 October 2013 (UTC)

How do mirrors work ?

I've been reflecting on a problem in my head, the details of how an optical mirror reflects light. I'm using a particle model to try to understand. In that model, I believe the basic idea is that photons bounce randomly off a rough surface, and you get white, or they bounce in parallel off a smooth surface, and you get a mirror reflection. However, photons are subatomic particles, and surely all surfaces must be rough at that scale. That seems like bouncing tennis balls off a pile of boulders and expecting them not to go in a random directions.

So, do I need to completely abandon the idea of a photon as a particle here ? Even using a wave model, it's hard to see how a field of boulders doesn't scatter the wave packet randomly. Or does some quantum weirdness apply where the photon behaves as if it was a much larger particle, like in one variation of the double-slit experiment, where each electron seems able to go through both slits at the same time. StuRat (talk) 14:04, 23 October 2013 (UTC)

Actually, metallic surfaces are rather smooth even at that level due to the sort of metallic bonding (i.e. "sea of electrons") that minimizes the distinguishability between atoms. It's why metals make good mirrors and reflect so well. --Jayron32 14:40, 23 October 2013 (UTC)

That's not right. Seas of electrons, atoms, and metallic bonds are on a far small scale and have nothing to do with it, as StuRat has come to realise. Metals make good mirrors because they are ductile and easy to polish to a low surface roughness. Glass is not a metal. Everyone knows that in a domestic mirror, a greater amount of light is reflected off the silvering on the back surface, but you get quite a bit reflected of the front air/glass surface too. In vacuum tube colour TV cameras, unsilvered sheets of glass were used to split the light from the lens into 4 paths. It worked because each glass sheet reflected some light as a mirror, and let the rest straight through to the next piece of glass. Coloured film was used to block cyan from the red pickup tube, a magneta blocking film for the green pickup tube, etc. The fourth tube got unfiltered light and provided the luminance signal. As the eye cannot see detail in colour the luminence tube was the best possible, the other three were smaller cheaper versions. If you polish ceramic to a surface roughness less than 30 namometres, it will work nicely as amirror too. I've worked with ceramic pole insulators that you can see your face in. Not all ceramics can be polished that good though - it depends on what's in the mix before firing. We've all seen good reflections in water when there is no wind. Still water makes a good mirror becasue it is a liquid and thus self levelling.124.178.152.227 (talk) 14:54, 23 October 2013 (UTC)

This is where you need to look at it on the basis that light is a electromagnetic wave. Sometimes you need to consider it as photons, sometimes you need to consider it as waves. The wavelength of visible light is in the range of 400 to 700 nanometres, or 0.0004 to 0.0007 mm. To function as an optically smooth mirror, the surface variation needs to be sensibly small compared to the wavelength or scattering and destructive interference will occur. If the surface roughness is sensibly small compared to the wavelength, there is little time of arrival diffrence between waves and they will reinforce, only in the direction of propagation. Now what is a typical surface roughness of a reasonably good mirror? It's in the order of 30 nanometers or less. Check the wiki article on mirrors. Quite ordinary float glasses have a surface roughness of around 50 nanometers, which can be signifantly improved by polishing. 124.178.152.227 (talk) 14:42, 23 October 2013 (UTC)

Here you have to take into account the spread of the wavefunction in the perpendicular direction. Suppose you have a particle that moves in the z-direction. Then initially it will be within some area in the xy-plane, so it starts out as a beam with some cross section. By the uncertainty relation, the smaller that initial beam cross section is the larger the uncertainty in the momentum in the perpendicular direction will be, therefore the beam will diverge faster.

When light reflects off a mirror, you have to consider the area of the mirror that is illuminated. If e.g. the light from the source is collimated to a narrow beam then only a small area of the mirror will reflect the light. Each photon starting out from the light source that makes it into the beam will be spread out in the perpendicular direction when it arrives at the mirror over that area. Suppose the mirror is perfectly smooth. Then the reason why the photon would reflect off the mirror in the epxected way is due to intererence of all the possibilities that you have within that reflecting area. Perpendicular to the incident beam, the phase is constant. This means that at the reflecting area oin the mirror you have phase shifts. In the outgoing beam, the phase is again constant accross the beam, and that requires that the angle of the reflection is equal to that of the incident beam but opposite w.r.t. the perpendicular direction.

The narrower the incident beam on the perfect mirror is, the less well defined will the reflecting beam's direction be. You always get an inteference pattern where the central peak correpsond to the "correct direction" but the spread in that peak becomes larger then smaller the beam width is. Then what happens for incoherent light sources is that the interference pattern gets washed out (each photon will have a different interfence pattern), so you won't see inteference fringes, you will see that the reflecting beam diverges. Instead of a small incident beam, you can also use a tiny mirror here. If the mirror is not perfect, you will have the effect of introducing additional phase shifts accross the reflecting area which has the effect of distorting the reflecting beam.

The classical picture of the moving and reflecting particle is only obtained when the beam width is much larger than the wavelength, the so-called geometric optics limit. For perticles like electrons, you have such small wavelengths that you can pretend that the wavelength is zero. But for light you will easily see the effects of it havng a finite wavvelength, and then you get the paradoxical result that classical behavior only arises in the limit that the beam width is infinite, which is actually not classical at all. This is due to the uncertainty realation, if you want to have a well defined beam moving in a particular direction, the uncertainty in the momentum in the perpendicular direction has to be small, but that implies that it must have a finite width.

Example. Take camera with a perfect lens of 5 cm diameter. Photons of 500 nm from a certain direction will have an angular spread of the order of 500 nm/5 cm = 10^(-5) radians. This means that in a picture of an object at 1 km distance you can't resolve detals smaller than 1 cm. So, evewn the most expensive lens with that aperture on a gigapixel camera won't give you perfectly sharp pictures. Count Iblis (talk) 15:35, 23 October 2013 (UTC)

One thing that can throw you off is thinking in terms of "bouncing" at the atomic/photon level like billiard balls. Rather photons are absorbed and re-emitted by the electrons in the mirror's metal surface. The re-emitted photons interfere to give a coherent image. It gets into quantum electrodynamics I'm afraid. There is always QED: The Strange Theory of Light and Matter which talks a lot about mirrors. 88.112.41.6 (talk) 15:58, 23 October 2013 (UTC)

Ammonia to remove mildew from clothes?

Having established above that my clothes and towels are riddled with mildew, I'd like to kill off what I can. A gamma irradiator would be perfect for this, but the one I have access too is way too small and we're not allowed to clean our clothes with it :P Can I use ammonia indiscriminately or would I need to check each items material composition and avoid certain or all colours? I'd like to avoid damaging my clothes and towels. I don't really have the opportunity to hang my clothes outside (I live in an apartment block; I'm guessing that hung outside would UV-treat them, but I guess that also fades the colours). I don't mind using chemicals so long as they don't damage the fabric or my health. --129.215.47.59 (talk) 14:05, 23 October 2013 (UTC)

Have you tried either bleach or bleach alternative? Ammonia may damage the fabric. --Jayron32 14:36, 23 October 2013 (UTC)

Before doing either (bleach can damage fabric as well), I'd give it a run through the washer again with hot water and a good detergent to see what comes of it. But yeah, bleach would be the next step; not ammonia. Matt Deres (talk) 15:55, 23 October 2013 (UTC)