Wikipedia:Reference desk/Archives/Science/2013 January 8

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January 8

How strong magnetic field at a distance from a magnetic source?

If a magnetic field is 2.4 T and another object is 1 meter away. How strong field in Tesla will that object experience? Electron9 (talk) 00:54, 8 January 2013 (UTC)

This is not enough information to solve for the field strength at one meter distance. If you provided information about the source, we could estimate the strength at other points. For example, our article on dipoles provides a simple formula for calculating the magnitude of a dipole source. Many magnetic field sources are well-approximated as dipole sources. Nimur (talk) 01:19, 8 January 2013 (UTC)

1 meter away in what exactly? In vacuum or air? Or in another substance, like water, or separated by metaglas? Different media have different magnetic permeabilities. 72.128.82.131 (talk) 01:51, 8 January 2013 (UTC)

Or from what. You can't exactly have a point source of magnetism, see magnetic monopole. StuRat (talk) 02:00, 8 January 2013 (UTC)

Er... magnetic monopoles have not been proven not to exist, they are perfectly valid in hypothetical scenarios, and at any rate that is the essence of Nimur's comment... 72.128.82.131 (talk) 02:21, 8 January 2013 (UTC)

The source is a speaker sub 8" with other magnetic sensitive equipment at 0.2 - 0.3 meters distance. The media is ordinary air. The only obstacle may be tree panel ~2cm thick. Electron9 (talk) 08:48, 8 January 2013 (UTC)

You still have not provided sufficient information. Firstly, there are two main types of speaker magnetic circuit construction: 1) cylindrical magnet in centre and soft iron flux return circuit outside; 2) Soft iron pole piece centre and magnet in the form of a toroid on the outside. Type (2) is normally used if the magnet is a ferrite magnet. Because the flux density of ferrite magnets is less than for metal types, ferrite magnets are usually oversized so that the pole piece is saturated or close to saturation. This means a lot more flux "escapes" with type (2) magnet systems. Also, the amount of flux at the distance range you indicated will depend on the diameter of the magnet system - this can vary 2:1 or more for the same size speaker overall diameter. The amount of flux that "escapes" also depends on the voice coil gap - the wider the gap the more flux escapes. Hi fi speakers are often made with ferrofluid lubricant in the voice coil gap - this will reduce the amount of escaping flux. The construction and material used for the speaker frame/basket will also have an influence. Be aware that speakers are available in low field models - these have a surrounding magnetic shunt to contain flux that would otherwise escape. These speakers were developed for use in TV sets with cathode ray tube (CRT) displays - such displays are rather sensitive to magnetic fields, especially colour displays. A high efficiency high quality speaker made for car radio applications can have an external magnetic field very considerably stronger than that of a speaker made for a cheap TV set.

All this means that, unless you have access to comprehensive magnetic circuit data from the manufacturer, the amount of external flux will be impossible to estimate and you'd be better off to obtain a candidate speaker and test it. If you have no means of testing, use a compass. Position the speaker so that it's field is at right angles to the Earth's magnetic field at your location, as shown by the compass. Move the speaker towards the compass so that the compass needle deflects. You can then calculate approximately the field strength of the speaker flux as a fraction of the local Earth magnetic field strength from the angle of deflection. For instance, a 45 degree deflection would indicate identical strengths.

How sensitive is your sensitive equipment anyway? I have some quite standard 8 inch speakers that produce no deflection of a compass at a separation of 200 mm. Keit 58.170.141.154 (talk) 10:28, 8 January 2013 (UTC)

As data on the speaker is missing. Perhaps one can figure out how many times less the field is at a distance than at the magnetic source ? A compass also has some inertia (but I like the idea). Electron9 (talk) 13:14, 8 January 2013 (UTC)

No. You could, if the point of interest was remote from the magnet, determine the fall off at another point even further away, as you could then assume a far field from a simple round rod magnet (the proverbial spherical cow approach). But you said you are interested in a point as close as 200 mm from an 8 inch (200 mm) speaker. Even if you assume the frame to be non-magnetic, if the speaker has a type 2 magnetic circuit (as I described above), the diameter of the magnet toriod will be around 50 to 80 mm. This puts you in the near field, so a far field approximation cannot be used. And you would still have to make a field strength measurement anyway for the reasons I gave - you haven't identified the magnet structure (type 1 or 2 or some other), you haven't identified the magnet diameter, presence or not of magnetic shielding/shunting etc etc. You gave the magnet strength (2.4 Tesla) but if this is speaker manufacturer's data it will be the total flux in the voice coil gap. Almost all of this flux is contained within the soft iron return circuit and shield (if fitted for TV use). You don't know how much of it escapes outside the speaker structure, but it will be a very small portion.

To see what I am talking about using the terms "far field" and "near field" consider a rod magnet of some arbitary area x mm² having a total internal flux y. Close up at the poles, the flux density will be y/x. Close to the sides it will be less. At a point z metres far from it the flux density will be then (y/x).k where k is a reduction constant depending on distance and bearing relative to the magnetization axis. There is a standard formula for calculating k - Nimur gave you the link - but that need not concern us now. Now, take a second magnet made of the same material, but twice the area. Clearly, close up the flux density will be y/x i.e., same as for the first magnet. But at the far point the flux density will be 2(y/x).k, twice as strong as with the first magnet.

The inertia of the compass needle is of no importance whatseover, as you only need a stable deflection off normal.

Keit 124.182.27.187 (talk) 14:22, 8 January 2013 (UTC)

Syrian airforce

What kind of laser guided bombs do the Syrian airforce jets have?--Jonharley667 (talk) 03:16, 8 January 2013 (UTC)

Wasn't this just asked ? StuRat (talk) 03:26, 8 January 2013 (UTC)

No, apparently missiles and bombs are different so I want to know about their bombs--Jonharley667 (talk) 09:00, 8 January 2013 (UTC).

IC4-trains

how many IC4 trains are in operation at the DSB?--89.249.2.53 (talk) 10:19, 8 January 2013 (UTC)

Thirty seven...I think...the history of these things is complicated, confusing and very ugly! According to the last two sentences of our IC4 article: "In November 2011 two IC4 trains each failed to stop at stop signals. This caused the authorities to ban the IC4 from running until the problems had been investigated. The investigations are currently ongoing." ...and... "On 2 July 2012, the DSB announced that Trafikstyrelsen (Transportation Authority) has approved the Danish railways to be able to put into service the park 37 IC4 who had been withdrawn from service in November 2011."

So if they'd all been withdrawn from service in 2011 - and 37 put back into service last July - then there are (presumably) 37 in service today....although I suppose it's possible that more have entered service in the past 6 months...or that although DSB was approved to put 37 back into service, they might not have done that for whatever reason. Those possibilities aren't mentioned in our article or the sources it references - so the best answer I can find is "thirty seven". SteveBaker (talk) 13:22, 8 January 2013 (UTC)

Electrons

1. Why doesn't the electron fall into the nucleus?
2. Which type of motion do electrons possess in an atom - they move randomly or they move in orbits like planets ? Parimal Kumar Singh (talk) 13:58, 8 January 2013 (UTC)

Well, the answer is a bit complicated. Let's shoot for something simple and let the guru's here complicate the answer to the point that neither of us will be able to understand it!

Electrons are often thought of as tiny little balls that go around the nucleus of an atom like planets around a star - that's a nice mental image - but it's not really true. The "truth" is a bit different from that. At the level of atoms, the world isn't like the familiar world of human-scaled things. Particles are waves and waves are particles - mass and energy are the same thing. That's true at the scale of humans too - but it's very hard to tell. But at the scale of an electron, those weird things come to dominate any explanation. Nothing has a definite location. So an electron is more like a fuzzy cloud of "probability" - that's to say that the exact location of the particle isn't a definite thing - all we can ever know is that it has some probability of being at some point - and that probability is higher here than it is there. When you consider that, the questions you pose stop making any real sense.

SteveBaker (talk) 14:21, 8 January 2013 (UTC)

You might find this short history of electron models helpful in conceptualizing how the idea of the electron changed over time. --Mr.98 (talk) 17:18, 8 January 2013 (UTC)

Adapting my answer from this old thread: electrons do fall into the nucleus. The electron of a hydrogen atom doesn't orbit the proton, it's simply superimposed on it. They don't collapse to a point because the uncertainty principle forbids it. A decrease in position uncertainty beyond a certain point implies an increase in momentum uncertainty which leads to an increase in position uncertainty. Instead they settle into an equilibrium state where these two effects balance out. The electron ends up with a much larger uncertainty of position because its much smaller mass means that a small uncertainty of momentum counts for much more in terms of velocity. That's why the atom looks like a small nucleus of positive charge surrounded by a big cloud of negative charge.

When there are more electrons, the exclusion principle means that they can't all sit in the same place as the first one, so they end up in higher-energy states. Some of these are just more energetic versions of being superimposed on the proton (2s, 3s, etc.) while others (p, d, f, etc.) have angular momentum, making them more like orbits. The motion (if you can call it that) is nonetheless quite chaotic because of the uncertainty principle; the electron ends up spread out over the entire orbit. But electrons far from the nucleus in a Rydberg atom can have orbits that look almost like planetary orbits for a short time. Electron orbitals and planetary orbits are two extremes of the same thing. -- BenRG (talk) 17:42, 8 January 2013 (UTC)

Perhaps this wil help. --Guy Macon (talk) 18:22, 8 January 2013 (UTC)

Related question: From the formula of the 1s orbital, it looks that the electron is more likely to be within the nucleus than in any other shell of equal volume (i.e. with positive inner radius). Is that right?

Heisenberg to teacher: "When I did my homework I got bored and measured its momentum. And now I don't know where it is..." 217.251.167.131 (talk) 09:57, 9 January 2013 (UTC)

Not sure about it, but I came to the same conclusion. The notion that the electron is never at r = 0 comes from the fact that the probability to be at any given point is zero. However, being within the nucleus doesn't imply that, it implies that r is very small but can be nonzero. Thus, electrons can enter and exit the nucleus without interacting.

That Heisenberg line is brilliant (but quite unrelated to the question). I only knew Schroedinger: "My cat ate my homework, and now the poor thing looks like half-dead." - ¡Ouch! (^{hurt me} / _{more pain}) 07:32, 10 January 2013 (UTC)

Sub Questions
It is clear that electrons do not move in orbits, but still I have some confusions. Do electrons move around nucleus or they remain at the same site ? If a electron is not at rest, the orbital in which it lie should also move with this electron. Does this happen ? Can a electron change its orbital ? Parimal Kumar Singh (talk) 13:26, 9 January 2013 (UTC)

As I understand it, dozens of illustrations have taught us that electrons are little black spheres, and neutrons and protons are not so small spheres, red and white in color. We need to lose that image when thinking about "real" electrons.

"Real" electrons are not at one point of the orbit, but within a certain volume, at best. Due to uncertainty (Heisenberg was not that off-topic after all), one cannot determine both velocity and location of an electron. Pinning the location down to a volume smaller than the apparent radius of the orbital would allow for velocities sufficient to escape the atom; that measuring method would give us a good location but ionize the atom in the process.

The orbital can "move" (it can snap towards other atoms and blend with their orbital), but it doesn't move with the electron. The point of the orbital is to relate each possible location to the probability that the electron will be nearby. s-prbitals are spherical, but p-orbitals are not.

Electrons change orbitals when they receive or lose the right amount of energy. That's the very principle of emission. absorption, and spectral analysis: different atoms mean different energy levels, thus different frequencies (E = h f, h being Planck's constant). - ¡Ouch! (^{hurt me} / _{more pain}) 07:52, 11 January 2013 (UTC)

That sounds like someone has been recycling the balls from the stick and ball models of molecules. I've not heard of a code like that for elementary particles but it possibly could be useful in diagrams. Dmcq (talk) 22:40, 12 January 2013 (UTC)

Is Anthropogenic Global Warming falsifiable?

I had an epistemological question which has to do with the fact we probably can't reverse the continuing increase of greenhouse pollutants which are posited to be the inextricable causes of global warming, thus losing the chance to conduct an experiment to determine whether reducing our greenhouse emissions will lead to a cooler climate. If anything were possible, we would create a perfect clone of earth and make everyone live 100% perfectly "green" while this earth continues the status quo, thus a replicable experiment that can be repeated dozens of times with identical outcomes that would be unassailable.

Background from here it says "NOAA: 2012 was warmest year ever for US, second most 'extreme'" & it is reasonable to say that both "global warming" is going up & so are the "greenhouse pollutants" going up also. However, correlation does not always imply causality.

I used to be a non-believer of AGW due to this page and also because of the climategate scandal, but now that I'm beyond the politics & the election is over, I can finally ask my question here on the reference desk without people bashing mitt romney or republicans (knock on wood!)

So here is my honest, legitimate question: do the 95% of scientists who are in concensus that AGW is real & caused by humans and/or meaningfully exacerbated by humans even though the earth might be warming as we speak if humans were all dead--do those same zealous scientists have weather predictions they definitively stand by? (such as 6 outta the next 10 years will be hotter than any of the last 100 years unless we cap greenhouse gases)

I'm agnostic now about AGW and do not feel like I can learn anything more from wikipedia about AGW after reading the articles without asking for some human assistance from those at the ref desk knowledgeable about the epistemological basis for how do we know what 95% of scientists believe. This is also a chance to share knowledge with someone who is ready to listen to someone on the pro-AGW camp of Wikipedians. Thanks in advance for any answers or book recommendations or online resources that are provided. Bests, the Tomato expert1 (talk) 19:03, 8 January 2013 (UTC)

The short answer is yes. However, many of the most publicised predictions are ones which, if they come true, will mean that we are already too far gone. I would be interested in shorter term predictions such as you suggest. Personally I am fairly firmly convinced of AGW, but that's mostly on the basis of retrospective studies. I am therefore interested in the short- to medium-range predictive power of the theory.

However, as a former student of chaos theory, I'm bound to observe that the weather is an intrinsically difficult subject, and so a prediction being met would not absolutely guarantee the theory, nor would a prediction being missed completely falsify it. As using less power is cheaper than using more, and we are running out of fossil fuels anyway, I find it pragmatic to act as though the theory were watertight; I don't think I can reasonably lose out that way. AlexTiefling (talk) 19:13, 8 January 2013 (UTC)

PS: Delingpole's critique is not based in science, but in a frankly paranoid conspiracy-theory view of the scientific community which cannot reasonably be taken seriously. This response [1] to his blog is instructive. AlexTiefling (talk) 19:18, 8 January 2013 (UTC)

You can check the resources listed at User:Wavelength/About Earth's environment/Climate change.

—Wavelength (talk) 20:10, 8 January 2013 (UTC)

Firstly, I think that our use of vocabulary here is crucial. The greenhouse gas emissions problem is not about "weather" - it is about "climate" - the difference is crucial. The overall climate of the planet changes fairly slowly and somewhat predictably. Weather is chaotic (in the mathematical sense) - so no prediction of weather change is either proof or counter-proof of anything very much. The word "warming" in the term "global warming" is distracting because the weather in some places in the world will not experience a temperature increase - and might even feel a decrease (eg if the north atlantic conveyor current reverses) - and people can only directly experience "weather" - you need a ton of complicated science and statistics to measure "climate". So it would be better to stick with "global climate change". Furthermore, climate change is only one result of the prediction of the effect of greenhouse gas increase - others include sea level rise, loss of arctic and antarctic ice, the diminution of glaciers and so forth, increased chaos in the day-to-day weather of most of the world. Obviously these effects are all inextricably linked.

Equally obviously, the climate change prediction of greenhouse gas emission increase is falsifiable. If we went for 100 years with the average temperature of the atmosphere decreasing steadily while greenhouse gas emissions continued to increase - then the theory would be proven false. This most certainly isn't a matter of falsifiability. It's a matter of whether the theory is true or false. (Being shown to be true or false is not related to the issue of falsifiability. An unfalsifiable theory is one such as the existence of God. We can't prove it false - there is simply no conceivable way to do that. We also can't prove whether we live in a computer simulation of the universe - that's also unfalsifiable.)

So we are beyond falsifiability. It's climate change due to greenhouse gas emissions is falsifiable...for 100% sure.

The important question for unbelievers is now "Is it false?" - which is not at all the same thing as "Is it falsifiable?"

The question of whether predictions have been made that came true is one solid way to show that some theory is likely to be true. So one prediction is that average temperatures around the world would increase...and they have. Another is that polar ice would retreat - and it has. Another is that migratory animal species would live at latitudes increasingly far from the equator - and they are indeed moving that way. Yet another is that while the climate would slowly warm, the actual weather would become more extreme, and that too is clearly true. So there are a TON of predictions that are playing out - and they are fairly consistent in showing the truth of the theory.

But it's extremely hard to prove any scientific theory beyond all doubt. After all, there is always the possibility that some extremely powerful extraterrestrial being is dead set on messing with our heads by screwing with our experimental equipment. You can't absolutely, 100% prove or disprove anything. That's really why this rather vague-sounding word "theory" is used in the sciences when "fact" would have a nicer ring to it. In the end, we have to say "With all that we've looked at - is the balance of evidence that we should take extreme emergency measures to combat the release of greenhouse gasses?" - and to that level of satisfaction, there is no longer any reasonable doubt that global climate change is real and that emissions of CO2 and methane that we have made are by far the biggest culprit.

Sure, we might be wrong (I'd give it a one in a thousand chance) - but the consequences of cutting emissions drastically when we didn't need to are so vastly less than the consequences of inaction if we're correct, that if there were even a small chance that we're correct, then there really shouldn't be any doubt as to the smartest course of action for all humans everywhere.

Oh - and your figure of 95% of scientists believing in climate change is out of date. In 2011, the percentage of scientist believers was over 97%. More importantly, the few percent who did not express belief were not necessarily saying that they actually disbelieve - merely that they aren't convinced yet. The number of scientists who actively disbelieve is down below the threshold at which it can be reasonably measured.

SteveBaker (talk) 20:29, 8 January 2013 (UTC)

...Just to add, a number of scientists quibble over minutia of details and definitions. So, they may be "skeptical" about certain statements; or they may "disbelieve" certain conclusions; but it would be inappropriate to say that they do not believe in climate-change or human effects on climate. My take is this: "anthropogenic climate change" is not a hypothesis: it is not a statement or prediction that can be tested; so it's invalid to call it "true" or "false;" or even to say one "agrees" or "disagrees" with it. You might as well ask if a scientist "agrees" with impressionist painting, or whether "impressionism" is falsifiable. This phrase, like "anthropogenic global warming," encompasses an entire field of study, a roughly-agreed-upon body of known work, and a whole bunch of opinions about those works; the name of the field is not, in itself, an assertion or prediction that can be tested. A better question should be used to query a scientist's position, or their confidence in the evidence. For example, "to what extent do you believe CO2 concentration determines the total warming?" The response to that type of question would be a scientist's evidence-based opinion; or a statement of fact; or a hypothesis that could be proven true or false. Nimur (talk) 20:50, 8 January 2013 (UTC)

Also "believing in climate change" needs to be defined. The number of scientists that believe that it isn't changing is smaller than the number of scientists that believe that humans have little do with it, which is smaller than the number who believe that climate is changing and humans are largely the cause but don't believe that it is stoppable, which is smaller than the number who believe that climate is changing and humans are largely the cause but don't believe that it is stoppable by the US and Europe if China, India, Africa and/or South America don't cooperate.

We have some nicely written articles on this at Scientific opinion on climate change, List of scientists opposing the mainstream scientific assessment of global warming, and Surveys of scientists' views on climate change. --Guy Macon (talk) 20:49, 8 January 2013 (UTC)

A couple points:

1) Most scientists prefer to call it "global climate change" rather than "global warming", as a few spots may actually get colder (like Northern Europe, if the thermohaline circulation fails).

2) Random variations swamp out long-term effects in the short run, making it quite difficult to predict the climate for the next few years. Only the long-term trends can show changes regardless of these random events. StuRat (talk) 21:05, 8 January 2013 (UTC)

Climate scientists definitely do have quantitative climate predictions, and you can help with the prediction yourself. See this distributed computing project, which also includes links to some of the papers they've published.

The question of falsifiability is not as simple as asking whether you can do a controlled experiment. Obviously it's not possible to create another Earth to see whether carbon dioxide is the cause of warming, just like it's not possible to create another solar system to see whether Mars' orbit is due to gravity. All we can say is that we have a model of gravity that worked in the past, there's no reason to suspect it wouldn't work for Mars, and it correctly predicted where Mars would be for millenia, so it's reasonable to trust the model's predictions of the future. Climate is vastly more complicated and harder to predict than gravity, not to mention we have much less data to work with, but the essence of climate modelling is to find a physically reasonable model that accurately explains the past climate and apply it to predict the climate 20 years from now. In 20 years, the model could easily be proven wrong--if the global climate was predicted to increase by 1 degree but instead decreased by 4, a change that's easily measurable with modern instruments, the model is inaccurate. --140.180.245.22 (talk) 21:45, 8 January 2013 (UTC)

To elaborate on Steve's points above: The crucial bit is the difference between weather and climate. Weather is a chaotic process, and looking sufficiently far into the future, weather (say the weather in 3 weeks time, or even whether next summer is going to be a hot one) is essentially random. No person, scientist or not, can predict the future. Climate on the other hand is aggregated weather. Climate is the likelyhood of having a particular type of weather. Mathematically, climate is the probability distribution of weather. The crux is that even if the individual events are random/unpredictable, the probability distribution itself is not random and we can build fairly robust models for it. Robust enough that these can be falsified, given enough data. A single data point (like the US average summer temperature in 2012, or a cold spell in Russia) is not going to tell you anything, but given 20 or so years of observations, you can make quite firm statements such as the likelyhood of seeing this pattern without climate change would have been unreasonably small. Much depends how you define unreasonable, but given enough data you can falsify (or proof) at any threshold level. So yes, climate change certainly is falsifiable. A more interesting question is whether it is falsifiable or provable with the data we are able to gather before it is too late to take action. I fear the answer is probably not. As to whether scientist can make weather predictions that they can definitely standby: No, certainly not. Probably never. Weather is random, chaotic, too complex for predictions. But that's not the fault of the scientists and certainly not a fault with global warming. It is just the nature of the beast. Predictions about climate are entirely different. 86.185.161.4 (talk) 00:05, 9 January 2013 (UTC)

Note also that different parts of the AGW theory make quite specific and testable predictions. We can directly observe the changes in the infrared spectrum of earth caused by atmospheric CO2 using satellites like IRIS. We can measure the Suess effect to confirm the fossil sources of CO2. And so on. --Stephan Schulz (talk) 13:54, 9 January 2013 (UTC)

• The way to think about the relationship between climate and weather is what many other sciences call principles of mass action. In chemistry, for example, the position and specific motion of individual molecules are not predictable, but that doesn't mean we can't describe the properties of a substance. The properties of a steel canister filled with oxygen are extremely well understood and consistent, though the specific motion of a specific molecule of oxygen is not. It's the exact same way with weather and climate: in climatology a single year's weather is the "molecule" while climate is the "tank of oxygen". --Jayron32 14:01, 9 January 2013 (UTC)

I think a better question for someone to ask is how much of their money should be spent on various options considering their personal values. If a doctor says you have cancer and there's a 90% chance of you living more than 5 years if you have some operation do you have the operation? I'm sure Delingpole could find lots of surgeons who have botched operations or papers casting doubt on various techniques, does that mean you ignore the doctor or does it mean you take a second opinion? If 9 out of ten doctors say go for it what then? Does a general say "I don't know exactly where the enemy is, therefore I won't do anything"? They try and work out the best thing to do, for a general that might even mean tossing a coin sometimes! Too much of this looking behind by people who are not qualified in the minutiae strikes me rather like this [2]. Dmcq (talk) 15:12, 9 January 2013 (UTC)

The original question is kind of large and vague for scientific purposes (no offense). But the CO2 greenhouse gas "global warming" model offers numerous points where it can be falsified; like any chain of cause/effect, disproving a single link would disprove the entire hypothesis.

Examine the theoretical model: we're burning a huge amount of carbon that hasn't been in the atmosphere for hundreds of millions of years, burning carbon produces CO2, the additional CO2 will make the CO2 levels in the atmosphere rise, CO2 absorbs near IR energy at the frequencies the earth radiates, the energy absorbed will end up dissipated in the atmosphere, making it not just warmer, but more energetic in general. That overall model could easily be falsified, if any part of that chain were proved false. Setting aside any partisanship here as much as humanly possible, I'd say every step in that chain is pretty well proved past the likelihood of disproof at this point. So, the question of "disproof" becomes one of either providing an additional, equally well established effect that will negate or interfere with this process, and/or (preferably and) demonstrating convincingly that this is not happening in reality. In the absence of either or both, all of the other hypothetical causes of warming offered as "disproof" are irrelevant to the question; just as any discussions of how your house warms up every summer, or the possibility that your house is getting warmer because the furnace is on, become irrelevant to the question of whether your house is getting warmer because it's on fire, once you know that there are actual flames eating your draperies. And the models suggested for effects that interfere with the CO2 greenhouse gas model described above, such as the various iterations of the "higher temps will lead to more clouds which will reduce solar energy absorbance", are so far from being even demonstrated, let alone proved, at this point that they may be dismissed as handwaving; and the kind of evidence cited to demonstrate that in reality the model isn't functioning, such as "it hasn't warmed in the last 2/3/4/8/whatever years" don't meet any standard of evidence either, given that literally dozens of similarly sized flat periods can be demonstrated in the process of the unequivocally rising temperatures of the last century. Thus, yes, the theory is quite falsifiable, but nothing so far has come anywhere near falsifying it. Gzuckier (talk) 04:49, 10 January 2013 (UTC)

Doubling the carbon dioxide only has a small effect in absolute terms because most of the effect already happens - the climate skeptics are right about that - but what they then fail to take into account is that temperatures are not relative to zero degrees Centigrade, they are relative to absolute zero which is -273°C. The changes we're talking about like 2-5°C are quite small compared to that. Everything being small in absolute terms and the overall complexity makes it difficult to calculate the effects accurately but there is no real doubt about the general outcome. When a thermostat is turned up you need to show something like that a door has been opened and that it is cold outside to avoid the conclusion that the house will warm up. Dmcq (talk) 14:35, 10 January 2013 (UTC)

How Ears and Eyes Extract Frequency?

I know that we can extract frequency information from raw signals using methods such as FFT, but how do our organs do it without any apparent computation? 75.228.142.113 (talk) 19:50, 8 January 2013 (UTC)

For the ear, the shape of the cochlea selectively allows certain frequencies to resonate at different spatial locations; in other words, the shape of the cochlea selectively converts temporal-frequency of acoustic vibration into spatial frequencies. This is expressed in great detail in the (not-freely available) book Hearing: Its Psychology and Physiology, the reference for which I got from the excellent (and totally-free, available-online) Physical Audio Signal Processing (in the section on perception of advanced sounds).

For the eye: most studies indicate that the extraction of spatial frequency is performed in the brain, not the eyeball. This naturally becomes pretty complicated in a hurry; but you can start by reading this website, the "Cornea Lab" in the Vision Science Program at Berkeley; here's a good starter paper, on spatial frequency selectivity of human vision; it cites dozens of books and papers for background reading. The extraction of frequency of light - in other words, color, is performed inside the specialized cells of the human retina: what we commonly call the cone cell. These cells contain photochemical that makes them selectively receptive to photons of specific color (frequency of light).

Finally, you might be surprised at how straightforward it is to build a discrete cosine transform to extract spatial frequencies out of a focused optical system. This can be expressed mathematically through the framework of Fourier optics. A fast fourier transform isn't necessary, because such machinery works in the analog domain and does not depend on sequential digital calculations. You can extract spatial frequencies simply by intelligently combining signal levels from various combinations of the input (i.e. the retina cells) when stimulated by a focused image. This is the exact analog representation of computing the inner product of the image against each basis-function in the transform domain. Nimur (talk) 20:15, 8 January 2013 (UTC)

For the eye, we have to be careful to distinguish the frequency of the light from spatial frequencies and even temporal frequencies (eg strobe lights).

• The light frequency isn't measured at all well - at least not compared to the ear. We have three kinds of color sensors that detect light as amount of energy within a range of frequencies with a roughly gaussian fall-off. The center frequencies of the three kinds of cell are at the colors Red, Green and Blue. We perceive pure yellow light (eg from a sodium lamp) because both the red and green detectors are producing a mild response and the blue is not. However, to use an acoustic analogy, a "chord" of red and green light looks exactly the same as yellow light - we literally can't tell the difference. It's as if a chord of the C and E keys on a piano sounded exactly the same as the D key by itself! Another set of cells detects the overall brightness of the light and is more sensitive to motion.
• The spatial frequency appears to be an ugly mixture of pre-processing in the retina and post-processing in the brain. There isn't enough bandwidth down the optic nerve for each cell to report back to the brain individually - so we know that there is a considerable amount of pre-processing going on in the retina. We know that edge-extraction happens in the retina - and that's a component of frequency analysis. Everything is very much complicated by the fact that our eyeballs continually vibrate in their sockets - this scans the scene with the retina to eliminate "pixellation" such as you get with a TV camera. If a camera with the same number of pixels as the retina is used to take a picture, you can see the individual pixels - but the eyes don't do that because of this vibration. That increases our spatial frequency perception beyond the resolution of the retina. When your eyes get tired (or you get drunk!) this vibration ceases and your vision goes blurry - that blurry vision is the "natural" resolution of the eye. All of this complication greatly enhances the difficulty of answering this kind of question!
• Temporal frequency is another strange thing. We don't see single "frames" of video like a TV or a film projector - instead, we're getting signals from the retina like "An edge, sloped at 45 degrees is moving left to right at such-and-such speed"...but it's not a discrete time-sampled signal - which is why we don't see strobing and temporal aliassing in normal daylight.

Interpreting the way the eye works as an analogy of a digital camera is a very bad idea! The entire system from the mechanics, the optics and the image-processing "circuitry" works totally differently to any man-made camera. SteveBaker (talk) 21:12, 8 January 2013 (UTC)

"The center frequencies of the three kinds of cell are at the colors Red, Green and Blue" is false, as I and others have pointed out to you repeatedly in previous threads. As shown here, two of the cone types are maximally sensitive in the yellow-green-cyan range, and the third peaks in the violet.

The claim that microsaccades increase the eye's effective resolution doesn't seem to be backed up by the article. The eye, unlike cameras, has a fovea, and we can see any small part of a scene in great detail by pointing the eyes at it, but those are large eye motions, not tiny vibrations. Everything outside the fovea is very blurry. The smallest microsaccades (according to the article) move about 2 arc minutes, which is several times the separation of photoreceptors at the center of the fovea (about 0.5 arc minutes).

"When your eyes get tired (or you get drunk!) this vibration ceases and your vision goes blurry" is wrong. The actual cause of the blurred vision (and seeing two of everything) is a failure of accommodation. -- BenRG (talk) 23:18, 8 January 2013 (UTC)

"The light frequency isn't measured at all well." True indeed. I was lucky enough to attend one of Edwin Land's lectures on his retinex theory, and he demonstrated that photographing a scene with red and green filters, then projecting those two B&W slides with two narrow wavelength yellow lamps whose wavelengths were 10 nm apart, resulted in perception of the full spectrum of colors, although at that degree of closeness the colors were definitely on the washed out side. But, perception of blue and red when the only light sources were both yellow? Definitely vision does not measure frequency of light the same way the ear does sound. Gzuckier (talk) 05:07, 10 January 2013 (UTC)

You may find chroma subsampling interesting, as well. Not only is the eye interpolating information about what color it sees; it also is very imprecise in where it sees each color. This is one reason that JPEG or MPEG compression, or even analog signals bearing ATSC television data, are so efficient from an engineering standpoint: because the human vision system is very easily fooled by very deviant reconstructions! The easiest way I can visualize this is to consider watercolor painting. If you look analytically at a sample painting and observe where the colored portions are, you see that they aren't where they should be! Cézanne can't even keep his brush inside the lines! But if you step back, the painting looks strikingly "realistic," even though the colors are in completely the wrong place - and who ever saw a purple human! Alas, it's easier to be an artist than an engineer... those guys can get away with anything! Nimur (talk) 04:30, 11 January 2013 (UTC)

liquid nitrogen condensing liquid oxygen

From Liquid nitrogen: "Vessels containing liquid nitrogen can condense oxygen from air." Is there an upper limit as to the percentage of liquid oxygen that gets condensed?Dncsky (talk) 22:23, 8 January 2013 (UTC)

No, as long as the temperature remains below the condensation point of oxygen, there should be no cesation. Plasmic Physics (talk) 22:33, 8 January 2013 (UTC)

Here's the article's source: [3]. I don't see an upper limit. If all the liquid nitrogen has evaporated, this may leave a small quantity of near 100% liquid oxygen (with maybe some dry ice and water ice mixed in). StuRat (talk) 22:37, 8 January 2013 (UTC)

That was actually my follow-on question, you mind reader you. Can someone else just confirm this: if I leave a bottle of liquid nitrogen in the open, it'll become a bottle of liquid oxygen (with impurities) at one point.Dncsky (talk) 22:56, 8 January 2013 (UTC)

No, if you leave a bottle of liquid nitrogen in the open, it'll eventually become a bottle of air. --Jayron32 23:21, 8 January 2013 (UTC)

I missed "at one point" in that sentence. I've added it now. I'm basically asking for a confirmation on what StuRat said. Dncsky (talk) 23:53, 8 January 2013 (UTC)

I'm not even sure that's the case, except in carefully controlled conditions, where the ambient temperature is below the boiling point of oxygen but above boiling point of nitrogen (between the window of 77 K - 90 K). At room temperature, trace amounts of liquid oxygen are probably forming within the liquid nitrogen, but these probably boil off along with the nitrogen, to the point where it never makes up a significant portion of the liquid. --Jayron32 01:30, 9 January 2013 (UTC)

Doesn't that contradict the article though? It's saying "increasingly enriched in oxygen", but you're saying it's just trace amounts. Maybe I'm misunderstanding what the article meant.Dncsky (talk) 02:30, 9 January 2013 (UTC)

(edit conflict)You'd need some special geometry of your container to get a noticeable amount. Boiling (i.e., evaporating) LN2 has N2 gas streaming off of it, which makes it hard for much room-air (source of O2) to reach the LN2 and condense in. I've left LN2 dewars and flasks open until it's all gone, and I usually have little if any water-ice in the bottom even in humid labs, but I do get water condensing on the still-cold glass surface after the LN2 has evaporated. DMacks (talk) 01:31, 9 January 2013 (UTC)

Thanks, that answers it. Dncsky (talk) 02:34, 9 January 2013 (UTC)

Actually, what DMacks reports is true only under limited (but very common) circumstances. Often, the evaporating LN2 pushes all O2 away from the surface, which can fool you into thinking it always works that way. If you have a container with a wide mouth and a cross-breeze you will definitely end up with all the LN2 boiled away and a smaller amount of pure LO2 in the container. Also, if you put LN2 in a regular (non-dewar) metal container, LO2 will condense on the outside, as seen here:

http://www.youtube.com/watch?v=EkK6xhORZnQ

Also,

http://www.chem.purdue.edu/chemsafety/Chem/ln2.htm

says this:

"Liquid nitrogen will condense oxygen from the air. This is most alarmingly demonstrated if a person leaves his/her vacuum pump's coldfinger in a Dewar of liquid nitrogen overnight. In the morning the coldfinger will contain LIQUID OXYGEN up to the level of the nitrogen in the Dewar." --Guy Macon (talk) 11:28, 9 January 2013 (UTC)

Um, 'coldfinger'? Assuming this isn't a stand-offish Bond villain, what is it? AndyTheGrump (talk) 01:00, 10 January 2013 (UTC)

Cold finger. DMacks (talk) 01:02, 10 January 2013 (UTC)

Thank you very much, Guy! After reading the LN article I just thought to myself: "How cool is it that when I leave substance A out in the open it'll turn into substance B! And B isn't even chemically derived from A in any fashion.". StuRat confirmed my suspicion and how you found the source to back it up. Thanks again. Dncsky (talk) 18:55, 9 January 2013 (UTC)

Rainbow colored clouds?

I was just on my way home from work today and in the south-western part of the sky, there was a cloud that appeared to be in the colors of the rainbow - yellow, red and faint blue in that order from right to left. The Sun was to the right of the cloud. I've seen plenty of rainbows before, but they accompany rain and span across the sky. There was no rain (that I am aware of) and it was localized in a very small area of the sky - just a portion of some clouds. I observed it for a good 10-15 minutes at least.
I tried to take pictures of it with my cell phone, but I was driving and it was almost impossible to keep the Sun out of the frame. I can try to upload a picture to Commons if any of them came out alright.
So, here's my question: Can anyone tell me what I just saw? I can't recall ever seeing anything like this before.
In case it helps, this was in the Chicagoland area and it was about 4pm local time. A Quest For Knowledge (talk) 22:54, 8 January 2013 (UTC)

Sunset was at 4:37 PM in Chicago today [4], so the Sun was low in the western sky. I suspect there was some distant rain, which the sunlight shone through, causing it to be diffracted, before it struck the clouds. Looking at a weather map, there does seem to be rain coming in from the West, so perhaps that was the first little sprinkle. StuRat (talk) 23:58, 8 January 2013 (UTC)

A rainbow is water droplets glistening in the sun. The physics determine that the colour they appear to you depend on the angle formed by the sun, the water droplet and the observer, creating the illusion of the rainbow. It does not have to be falling water, but can be mist, fog, or even clouds given the right conditions. The rainbow article has pictures of quite a few different examples. 86.185.161.4 (talk) 00:19, 9 January 2013 (UTC)

Or it could have been a circumzenithal arc (which is basically a rainbow but reflecting off ice crystals in the clouds rather than water droplets). What is the temperature in Chicago at the moment? 86.185.161.4 (talk) 00:33, 9 January 2013 (UTC)

38°F, as of 5:51PM. StuRat (talk) 00:40, 9 January 2013 (UTC)

Likely to be substantially colder at cloud elevation. I was thinking of CZA, too, given that the sun was low in the sky and the mercury was low as well. -- Scray (talk) 01:01, 9 January 2013 (UTC)

Could it have been a nacreous cloud? 24.23.196.85 (talk) 01:48, 9 January 2013 (UTC)

My copy is buried somewhere as a result of moving, but what you saw is almost certainly described somewhere in The Nature of Light and Color in the Open Air: Minnaert covers just about every atmospheric optical effect in it. --Carnildo (talk) 04:03, 9 January 2013 (UTC)

Hmmm...well, there was no discernable arc to the rainbow and it was a very, very small portion of the sky. Anyway, I Googled (well, Binged) for images this morning (I have no idea why I didn't think of that last night) and here's a few that are kind of what I saw.[5][6][7] A Quest For Knowledge (talk) 14:31, 9 January 2013 (UTC)

This article, What's with the rainbow clouds?, seems to describe what I saw, but it says that in the US, "that pretty much relegates any sightings to roughly around 6 weeks either side of the summer solstice." It's nowhere near 6 weeks of the summer solstice. A Quest For Knowledge (talk) 15:06, 9 January 2013 (UTC)

This page has some similar examples, and says that they are types of Halo (optical phenomenon). Alansplodge (talk) 15:09, 9 January 2013 (UTC)

I think this sounds like a sun dog but there are many other kinds of atmospheric optical phenomenon (that article actually gives a useful list to go through). Wnt (talk) 17:24, 9 January 2013 (UTC)

OK, here's one of the pictures I took yesterday. You can barely see the colors of the rainbow and it looks a lot brighter than it did yesterday. In fact, it kind of looks like a comet or a UFO. The Sun is on the far right, and the 'rainbow colored cloud' is towards the center (slightly offset to the right).

• 

A Quest For Knowledge (talk) 18:55, 9 January 2013 (UTC)

Well, I definitely see clouds, so that supports the idea of sunlight passing through water vapor (or ice crystals). StuRat (talk) 20:45, 9 January 2013 (UTC)

That looks like a sun dog to me. Wnt (talk) 00:11, 10 January 2013 (UTC)

@Wnt: I'll through that list you mentioned previously as soon as I get a chance, but as far as it being a sun dog, it is at the same height as the Sun and the red part of the rainbow was facing the Sun. But there was no halo effect. A Quest For Knowledge (talk) 13:00, 10 January 2013 (UTC)