Wikipedia:Reference desk/Archives/Science/2011 August 8

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August 8

Shortest division of time

Can time be infinity divided? --DeeperQA (talk) 00:25, 8 August 2011 (UTC)

Yes. --Jayron32 00:32, 8 August 2011 (UTC)

Any time interval less than the Planck time is theoretically unmeasurable. (In practice, we cannot get anywhere close to measuring intervals that small.) I am not sure whether it is really agreed upon whether there is a "real" quantum of time or not, or if it is "just" a measurement problem. If there is a quanta of time, the answer is "no". If there is not, the answer is "in some sort of metaphysical sense, maybe, but practically, no." --Mr.98 (talk) 00:50, 8 August 2011 (UTC)

As a measurement problem it sounds a lot like the function of y=arctan(x) where no matter how large x becomes, y will never attain a value of one with the practical problem being that of measurement where the difference between 1 and y is not measurable although y is still separate from one. --DeeperQA (talk) 01:15, 8 August 2011 (UTC)

No Jayron32. One of the assumptions of Zeno used in his dichotomy paradox is that between any two different points in time there is always another point. Without this assumption there are only a finite number of intervals between two points, hence there is no infinite sequence of movements, and the paradox is resolved. Cuddlyable3 (talk) 08:51, 8 August 2011 (UTC)

Well, the dichotomy paradox is resolved anyway (the infinite geometric series converges for r = 1/2), so that's no argument. Classical mechanics and standard quantum theory use the real numbers to represent time mathematically, hence time can be divided infinitely in the framework of these theories. The question is: are these theories useful models for reality for very short time intervals? Answer: no. We expect quantum theory to break down at around the Planck scale, i.e. for time intervals on the order of the Planck time, at which point we need a new theory (which reduces to quantum theory at longer time intervals) to describe reality at these scales. Physicists are working on finding such a theory but they are far from achieving that goal. Ideas often assume that time is in some way discretized with a unit on the order of the Planck time or shorter, but at the moment those are more or less speculations. So the answer to DeeperQA's question is: We don't know. --Wrongfilter (talk) 10:46, 8 August 2011 (UTC)

DeeperQA mentions the arctan function that is relevant when displacement s is a function of time t i.e. s = f( t ) . We may take the arctan of the change in displacement during an interval of time. Now suppose we keep reducing the time interval. At the limit where both the time interval and the corresponding displacement are both infinitesimal, the arctan value is defined as ds/dt . This is an operation in Differential calculus. Cuddlyable3 (talk) 11:33, 8 August 2011 (UTC)

DeeperQA brought up the arctan as an example of asymptotic behaviour if I'm not much mistaken. --Wrongfilter (talk) 12:56, 8 August 2011 (UTC)

Rather than one I meant π/2 but in any event even at x=infinity y still does not and cannot reach π/2, although for the purposes of the Calculus we treat it as though it does. --DeeperQA (talk) 13:42, 8 August 2011 (UTC)

You might also look over the time article. It is not a straightforward thing and there are a lot of very unusual ideas about it. Treating it as some sort of straightforward quantity is not a correct physical way to think about it. At some level if one wants to talk about the divisions of time, you have to start really getting specific about what you mean by it. Do you mean, can I determine the interval between two events to an arbitrary precision? (Which gets you into an even more specific question: how do you determine the interval between the two events? With other intervals... and so on.) It's a tricky thing to think about carefully, which was one of Einstein's big observations (there is not a "universal clock" out there in the world that is independent of our frame of reference). --Mr.98 (talk) 15:20, 8 August 2011 (UTC)

I'm not so sure... even though various elements with red or blue shifted spectral lines have the same separation from each other as I recall. --DeeperQA (talk) 16:00, 8 August 2011 (UTC)

It's called a chronit. It is the time two of the smallest possible side-by-side particles take to pass each other in opposite directions at the speed of light. I made that up one afternoon eating pizza and playing D&D after applying the practical science of Doctor Who to Zeno's paradoxes. μηδείς (talk) 01:32, 9 August 2011 (UTC)

How hot is my oven?

The directions for cooking a pizza are "bake at 400F for 22 minutes". I set the oven's thermostat at 350F and after 13 minutes the pizza is slightly burned. How hot is the oven? --Carnildo (talk) 01:41, 8 August 2011 (UTC)

Too hot. 64.134.153.212 (talk) 01:47, 8 August 2011 (UTC)

Indeed, you may want to invest in a cheap oven thermometer to test it out. Also, ovens don't heat uniformly, so you may have hot spots and cold spots. You can probably get an oven thermometer (a cheapo dial kind) at the grocery store for like $5.00. My guess is, though, that your thermostat is broken, and the oven isn't cycling off. --Jayron32 01:59, 8 August 2011 (UTC)

Agreed. I suggest a test. Put it on the lowest temperature setting for 10 minutes (set a timer !) and see if it still gets very hot. If so, the thermostat must be broken. If this is the problem, you shouldn't use it like this, since it could start a fire by going beyond the temperature it's designed for. And, of course, it will destroy pretty much anything you cook in it. (Note that if the oven doesn't get very hot, it doesn't necessarily mean the thermostat is OK. It might always say it's 300 degrees, for example.)StuRat (talk) 02:50, 8 August 2011 (UTC)

It depends on how funky is your chicken. --Trovatore (talk) 02:02, 8 August 2011 (UTC)

... or possibly on how green is your valley. --Trovatore (talk) 02:03, 8 August 2011 (UTC)

I suspect that you are doing something wrong, such as thawing a frozen pizza before cooking it. Looie496 (talk) 02:10, 8 August 2011 (UTC)

...or cooking it in a toaster oven. Looie496 (talk) 02:11, 8 August 2011 (UTC)

I do that all the time. It works great so long as you set the thermostat correctly. (I like to imagine that I'm using less energy this way, but I'm not sure. )APL (talk) 02:49, 8 August 2011 (UTC)

Some gas ovens generate move radiant heat (i.e. infrared radiation) when starting up than when operating at their target temperature. Essentially this is because they turn the burner up high when warming up and lower it when maintaining constant temperature. Most of the time this makes little practical difference but for some sensitive foods, such as breads and cakes, this can cause additional searing (e.g. surface burning) if you put the food in the oven without preheating. So, if you didn't preheat before putting the pizza in, you might try doing that next time and see if it makes any difference. Dragons flight (talk) 08:07, 8 August 2011 (UTC)

I do mine for about three minutes in a microwave. No burning. 92.28.240.88 (talk) 10:33, 8 August 2011 (UTC)

Don't have to worry about burning maybe, but you end up with a pizza flavored slab of rubber. Googlemeister (talk) 19:31, 8 August 2011 (UTC)

That happens when you microwave it for too long. Try halving the time. 92.24.191.250 (talk) 20:59, 8 August 2011 (UTC)

Responding to a number of points that have been raised: I'm cooking the pizza from frozen as directed, I'm preheating the oven, the oven is an electric oven, the heating element does cycle on and off, and what I'm wondering is the temperature it's cycling at. --Carnildo (talk) 00:48, 9 August 2011 (UTC)

If that's the case, then buy a stand alone oven thermometer, as I said above. They sell them pretty much anywhere kitchen supplies are sold; I have bought one at my local super market. The fact that it is cycling on and off doesn't mean that it is cycling on and off at the right temperature given what you have it set at. The thermostat may be working, but so badly calibrated that it is off by tens of degrees. --Jayron32 00:54, 9 August 2011 (UTC)

And, if everything is OK except that the temperature range is off, I suggest you use the oven thermometer to determine the true temperature at each setting, and either write that directly on the knob, or make a chart mapping the actual temperature to each temperature setting. This should save you lots of money over having the oven serviced or replaced. StuRat (talk) 03:39, 9 August 2011 (UTC)

You are probably at the bottom of a mine shaft, or perhaps an over-pressurized deep sea craft. Either that or you misread Cees for Effs. μηδείς (talk) 01:38, 9 August 2011 (UTC)

I would be quite impressed if a standard household oven could reach 350C (about 650F). Googlemeister (talk) 13:14, 9 August 2011 (UTC)

As for burnt pizza, if it's burnt at the outside only, and not in the middle, you need to cook it longer at a lower temperature. If, on the other hand, it's burnt evenly everywhere, then you just need to take it out sooner or lower the temp. StuRat (talk) 03:39, 9 August 2011 (UTC)

I've seen pizza burned in a short time as the OP described when an oven was accidentally set on "Broil" instead of "Bake." I agree that buying an oven thermometer makes sense, to see if the thermostat on the oven is broken. Edison (talk) 12:51, 9 August 2011 (UTC)

Ha, if you have trouble baking the supermarket pizzas, you don't want to try this:

"5) Cooking:

Using a wood or aluminum peel, and a little flour, the pizzaiolo transfers, the pizza using a rotary movement and a quick shake, on to the cooking surface of the oven without disturbing the prepared pizza. The cooking of the "Pizza Napoletana STG" must be done exclusively in a wood fire oven which has reached the cooking temperature of 485C, (905F), which is essential to cook the Pizza Napoletana.

The pizzaiolo should monitor the cooking of the pizza by lifting up its edge. Using a metal peel, the pizzaiolo rotates the pizza, changing the edge that is facing the fire, and taking care to always replace the pizza on the same spot on the cooking surface, to ensure that the pizza does not burn because it is exposed to different temperatures.

It is important that the pizza is cooked in uniform manner across its entire circumference.

At the conclusion of the cooking, the pizzaiolo removes the pizza from the oven with a metallic peel, and places it on a flat, dry work surface.

Cooking time should not surpass 60-90 seconds.

After the cooking, the pizza should have the following characteristics:

The tomato should have lost all excess water, and should be dense and consistent;

The mozzarella di Bufala DOP or the mozzarella STG should be melted on the surface of the pizza;

The basil, garlic and the oregano will develop an intense aroma, and will appear brown, but not burned.

The following temperature guidelines should be followed:

Cooking surface temperature: 800ºF about.

Oven dome temperature:800ºF about.

Cooking time: 60-90 seconds.

Temperature reached by the dough: 60-65C.

Temperature reached by the tomatoes: 75-80C.

Temperature reached by the oil: 75-85C.

Temperature reached from the mozzarella: 65-7C."

Count Iblis (talk) 00:07, 10 August 2011 (UTC)

Acetone

Is Acetone a type of VOC?

Yes. See volatile organic compound for more details. Dragons flight (talk) 06:45, 8 August 2011 (UTC)

Self combusting essential oils?

Can anybody explain from a scientific viewpoint how this phenomenon might occur. On the face of it I cannot see how these oils can just ignite at room temperature. Does the washing have any effect on the oils that might alter their nature and make them unstable in some way. Richard Avery (talk) 07:02, 8 August 2011 (UTC)

The article Spontaneous combustion specifically cites Linseed oil as having a high potential for spontaneous combustion. As to why something may spontaneously combust, the process is complex; concepts like Flash point, Fire point, and Autoignition temperature all come into play. --Jayron32 12:18, 8 August 2011 (UTC)

The linseed oil article explains that linseed oil has "drying properties", i.e. the unsaturated lipid components such as alpha-linolenic acid (which is also present according to that article in other drying oils such as perilla oil and walnut oil). That article explains how oxygen slips in and cures/cross-links the oils. This oxidation generates heat, and as explained in the spontaneous combustion article, the heat is produced in greater amounts on the exposed surfaces of rags, and builds up more when they are piled up in an open container. So essentially certain oils like linseed oil are already on "fire" in a chemical sense the moment they're exposed to the air - after that, it's just a matter of building up the heat until an open flame is produced. Wnt (talk) 13:29, 8 August 2011 (UTC)

An interesting consequence, which seems to be agreed in various web forums, is that rags oily with motor oil (which isn't supposed to form a varnish on your engine parts) are in fact no great threat. I think..... Wnt (talk) 13:42, 8 August 2011 (UTC)

Thanks both, very helpful. (damn, why didn't I think of that link). Richard Avery (talk) 06:27, 9 August 2011 (UTC)

How to interpret my test results

To clarify off the bat: I am not asking for medical advice. I've already been to my doctor. What I'm looking for is encyclopedic information to help me interpret my test results. My doctor did some blood test that reveals how my blood sugar level has been for the past 6 weeks. (I didn't know it was possible for blood to store information for 6 weeks, but apparently it is.) The result was a 6.6, which was called "too high". So my question is, 6.6 what? And how does my blood know what its sugar levels have been for the past 6 weeks? Pais (talk) 13:38, 8 August 2011 (UTC)

"6.6" will be your blood glucose level in mmol/L (sometimes shortened to mM).Noticing below, I should say maybe, I'm not au fait with long-term tests. But certainly that is a typical size for a blood sugar reading. Grandiose (me, talk, contribs) 13:41, 8 August 2011 (UTC)

I don't know if this is one of your tests, but the HbA1c test looks for glycated hemoglobin - hemoglobin chemically modified by exposure to sugar. The more sugar in the blood over a roughly 6-week interval, the more the hemoglobin has been altered. I notice that this article says there is a 6.5% threshold for diagnosing diabetes (partly) using this test.

Editorializing a moment, I think it is just appalling that lab results are given in such obscure formats, that don't clearly explain and identify the test or even the units involved so that the patient can understand them. The ideology is that they're for a doctor to see, but of course that only applies if you keep one doctor and you are entirely reliant on him for treatment options. Yet diseases like type II diabetes depend on lifestyle and respond well in early stages to nutritional and herbal options, so the patient deserves and needs primary responsibility for his health, and the respect due an adult. Wnt (talk) 13:52, 8 August 2011 (UTC)

The tests are indeed not for the patient to understand. They lack the competence to do so. They are for the doctor to interpret, based on many years of education and experience, and knowlege of the patient's clinical picture. A responsible adult will seek competent medical attention in case of a serious disease like diabetes, and not risk his health by self-treating themself with nutritional or herbal remedies, which are bound to do more damage than good. Anyone who plays around with something like diabetes is being childish and irresponsible. Dominus Vobisdu (talk) 14:32, 8 August 2011 (UTC)

So instead I should blindly trust someone who has a vested financial interest in keeping me sick? Pais (talk) 14:47, 8 August 2011 (UTC)

Exactly. Doctors would love to give you a prescription for high blood pressure, even though most people could improve it with proper diet and exercise. Heck your BP doesn't even need to be all that high before they try pushing pills onto you. Googlemeister (talk) 14:48, 8 August 2011 (UTC)

And yet, telling the patient to change his diet (with few details), and waiting to see what happens on the next test, are the first responses to such conditions. Certainly nutritional responses are appropriate in borderline diabetes - see [1] for example. Herbal medicines as simple as ordinary "cinnamon" (actually cassia, one of the 50 Fundamental Herbs of traditional Chinese medicine, which is sold as cinnamon in the U.S.) have observable benefits. (See PMID 19930003, PMID 19627193, PMID 21298562; see also [2])

I think it is perfectly possible for a patient to understand the HbA1c test, and just about any other lab test. I think Wikipedia helps greatly in this regard, though there are many other useful sites. Wnt (talk) 14:59, 8 August 2011 (UTC)

Patients can do whatever they please with themselves. Doctors need to get off their high horses. Patients have a right to know every bit of information in their files, even if they do not necessarily understand. --Nricardo (talk) 16:00, 8 August 2011 (UTC)

Agreed. Note, however, that's it not a binary choice between "doing what the doctor tells you" and "taking care of yourself". Doctors often initially minimize the importance of diet and exercise, and push meds instead. However, if you mention that you'd like to try improving your diet and exercise habits before taking meds, in marginal cases (or in addition to the meds, in more serious cases), most doctors will support such decisions, say by recommending a nutritionist. Diabetes is a case where lifestyle changes make a major differences, to the point where just taking meds without any attempt to lose weight (if you're obese) and reduce your intake of sugar and high GI foods, is seriously irresponsible. StuRat (talk) 17:04, 9 August 2011 (UTC)

If a doctor initially pushes meds rather than diet and exercise, he's probably just being realistic. In my own case, patient compliance with taking medicine is far higher than it is with dieting and exercising, and I suspect I'm not the only one. Pais (talk) 21:25, 9 August 2011 (UTC)

Yes, but some comply completely, and probably just about everybody makes some effort, if not entirely successful, so they should all be told that diet and exercise are critical to the control of diabetes. To not do so is equivalent to not telling someone with lung cancer or emphysema that they should stop smoking. StuRat (talk) 04:02, 10 August 2011 (UTC)

Note that this likely varies from country to country. From my admittedly limited experience in NZ doctors push diet and/or exercise first where suitable and appropriate, generally with the knowledge many patients won't be compliant but that it's better to at least try, and then move on to drugs when it becomes clear the diet/exercise route isn't working (or perhaps after giving the diet/exercise spiel). And AFAIK this is the general recommendation of most medical associations. I know there are some doctors who argue statins in particular would benefit a large number of people above a certain age regardless of diet/exercise but that's IMHO a somewhat different matter. Of course if it's someone's regular GP, the may have some experience with their patient and some expectation of what will work and what will be desired. Nil Einne (talk) 13:59, 11 August 2011 (UTC)

Whenever I undergo a medical test, from blood work to X-rays, I always ask for a copy of the report. Everyone should, I think. Imagine Reason (talk) 02:36, 10 August 2011 (UTC)

chock2

in every home there is a tube light along with chock. my doubt is what is the back emf at the time when the supply to the chock is off since at the time of switch on the supply for chock we can calculate the back emf with the inductance value,resistance value,voltage and also we know time constant but iam asking AT THE TIME OF SUPPLY IS OFF SINCE AT THE TIME OF SUPPLY IS OFF, COLLAPSING MAGNETIC FIELD IS VERY VERY FAST AND ITS TIME CONSTANT IS VERY VERY LOWER THEN THE TIME CONSTANT AT THE TIME OF SUPPLY IS GIVEN TO THE CHOCK. IAM ASKING BACK EMF AT THE TIME WHEN THE SUPPLY TO THE CHOCK IS OFF DUE TO THE VERY HIGH RATE OF CHANGE OF COLLAPSING MAGNETIC FIELDvsnkumar (talk) 14:04, 8 August 2011 (UTC)

Is this a repeat of the question #CHOCK, above? (For reference, I believe that chock as it is used here is probably a mild mistranslation of choke.) TenOfAllTrades(talk) 14:10, 8 August 2011 (UTC)

Disconnected from the tube, the ballast will produce a back-emf of several thousand volts (depending on leakage), but the strike voltage is typically 600 to 1000v, or lower for shorter tubes, so this high voltage should not normally be generated. Dbfirs 16:49, 8 August 2011 (UTC)

who started the "information processing theory" ?

thanks

According to this reference cited in the article, it looks like George Armitage Miller and his paper "The Magical Number Seven, Plus or Minus Two" is considered the seminal work in the field. That doesn't mean that there weren't earlier psycologists that worked in the area, or that Miller invented the concept out of whole cloth (that never happens in science), but his work is important enough and early enough to be given prominence in the field. --Jayron32 16:13, 8 August 2011 (UTC)

need some recommendations for digital cameras that are good at recording insects (specifically fruit flies)

It so happens that I can make suggestions on how to spend my lab's budget.

We have a ton of cheap 480x640 web cameras (plus some 1024x768) and while they focus quite well (they take beautiful black and white photos of macroscopic surfaces even with a shallow depth of field) and I'm trying to get better image quality than some of the photos I've taken so far:

Fruit flies 1 (1280x1024+crop)

Fruit flies 2 (1024x768)

I'm trying to create an automated tool for fly tracking and fly counting. However, I also need really sharp images with preferably a large sensor and decent depth of field at that magnification. (Shallow enough that that most objects immediately above the bottom are out of focus, but not too shallow that only one part of the fly is in focus at any one time.) We also want cameras for multiple vials, so cameras under $100 each would be good. These photos were taken at differing resolutions from this camera. Are there any cameras that deliver a live feed that can do better at a good budget? I would also love to use my dslr, or some other point and shoots, but I don't know how to have them deliver a constant feed that can be captured by ImageJ, TWAIN, etc. elle _{vécut heureuse} ^{à jamais} (be free) 18:40, 8 August 2011 (UTC)

Just a thought, but, since it seems like magnification of the image is going to be the main expenses, perhaps instead of getting cameras with expensive lenses, you could just get a magnifying lens that sits on the vial, then take the pictures with less expensive cameras. The magnifying lenses would also work for just viewing the fruit flies. Or perhaps you're already doing this ? StuRat (talk) 03:28, 9 August 2011 (UTC)

Oh I already have magnification equipment -- the problem is the field of view becomes too narrow -- I need to record the entir bottom of the vial. elle _{vécut heureuse} ^{à jamais} (be free) 23:51, 9 August 2011 (UTC)

Do you really need to reinvent the wheel here? I'd think there must be somebody working on sleep in fruit flies, or fruitless, or something, who has had to come up with decent optics on a budget. Wnt (talk) 05:41, 9 August 2011 (UTC)

We have a user (User:KimvdLinde) who specializes in the study of fruit flies. Maybe you can ask her? -- Obsidi♠n Soul 05:46, 9 August 2011 (UTC)

If the vials are transparent and contain flies of a uniform size, a single measurement of the overall light transmission of the vial from a diffuse light source gives a value proportional to the number of flies. Cuddlyable3 (talk) 15:34, 9 August 2011 (UTC)

Not really. Some flies may be more transparent than others, there could be food media in the tubes of variable transparency, and the vials may get dirty with time, as from fruit fly excrement. Water condensation on the inside may also tend to scatter light. StuRat (talk) 18:21, 9 August 2011 (UTC)

Hmmm, I don't suppose you can do flow cytometry of fruit flies? (Imaging the eye fluorescence, perhaps) Can you do "hydrodynamic focusing" of air mixed with flies? Wnt (talk) 20:42, 9 August 2011 (UTC)

It's funny! My friend at MIT suggested that too. However, that is not possible, because I am not doing inventory management. At the moment we are investigating clock genes and the dopamine receptor pathway; the flies are basically dosed with cocaine and we have a small ten-minute window to record their "high" -- the amount of flies lying incapacitated at the bottom versus the amount of flies able to walk (and therefore not present at the bottom) is one way to assay different genotypes' sensitivities to cocaine. Fly counting is only the most fundamental and basic assay that we plan to do (we eventually want to track their behavioural patterns). elle _{vécut heureuse} ^{à jamais} (be free) 23:50, 9 August 2011 (UTC)

Okay, I have been looking into this problem. There are two options.

1. CTRAX. Ctrax is software that does tracking of flies in populations, is largely developped and is free. The downside when I tried to use it is that it requires uncompressed AVI, which nowadays is a rare thing except with some more expensive camera's. But that was some months ago, maye they have gotten to the point where you can use different types.
2. Do it yourself. To the contrary of your idea, you do not need extreme detailed focus, what you need is sufficient difference between background and flies. Once you have those, you can determine where the flies are and where they move too by subtracting two subsequent images. If the background is nicely constant, the pixels with the large change are the ones where the flies moved. By estimating if the change was from background to fly or visa versa, you can determine direction of the flies. If you do not need to track individual flies, but only populations, that would be rather easy to program (treat a single frame as a 3D-matrix with length pixels by width pixels by color channel and subtract that from the matrix of the frame before). I have done somethings with a cheap webcam of less than 25 dollars.

Okay, this are justa few things straight out of my head. The main question is, what data do you want to collect exactly from what set-up? -- Kim van der Linde ^{at venus} 00:49, 10 August 2011 (UTC)

AS for lighting, I used a strong light and a white background, that gives a really nice contrast. I played around with background light, which can work well as well. -- Kim van der Linde ^{at venus} 00:52, 10 August 2011 (UTC)

Wow thanks! So far, I have been using a dark room and red light (although I suspect red glass reflects off glass more -- glare is an issue), while covering top 7/8 of the vials with construction paper -- that way flies not on the bottom are both out of focus and darkened. I am willing to experiment with other setups though. The problem is that with the images here is that automated counting algorithms do not work well! Fly wings can get counted as separate flies, and it becomes more difficult if there's slight overlap. I am working on a morphological editing technique (one that would try to factor legs into the counting, as part of "this is a fly body and not a fly wing" clue). Does Ctrax take fly shape into account? I have 30 flies to a vial, on average, although I can propose a protocol that reduce this to 15 or 20 for my lab. The fly tracking thing as an algorithm sounds promising. At the same time I also would like to automate our lab's most simple bottom counting method, too, which counts the flies on the bottom at any one moment. We currently count in about one-minute intervals, so perhaps I could somehow devise a method in which I take ten images in rapid succession and use motion to sort out any ambiguity. elle _{vécut heureuse} ^{à jamais} (be free) 01:24, 10 August 2011 (UTC)

Okay, I think we are getting to the point that I suggest you contact me directly as we are going into the details, and I think it might be wise not to publish all details about your experiments publicly before publishing. Contact me at my e-mail. If you prefer to keep discussing here, let me know. It sounds like your research has a lot of similarities with what we just published about circadian modulation of alcohol sensitivity. -- Kim van der Linde ^{at venus} 16:47, 10 August 2011 (UTC)

Treeline and Kangerlassuaq

What is the minimum temperature necessarily for a tree to grow. I thought it was 52 F. Does the average high or daytime average has to be 52 F? Kangerlassuaq's west coast of Greenland daytime summer have average high of over 60 F about 2/3 in July, it's daytime average cannot be under 52 F, if west coast of Greenland's summer is warm enough how come it can't grow trees. Do Kangerlassuaq, Greenland's climate ever qualify as tundra, it should it be subarctic Dfc-it's summer is quite warm. Is Narsasuaq, Greenland's southwest Dfc subarctic or is it tundra. Our article said some land may occasionally top to 86F, which land does it.--69.229.5.134 (talk) 19:53, 8 August 2011 (UTC)

It's not a simple case of some magical temperature below which no tree will grow, and above which trees will flourish. There are complex environmental reasons why an area will be wooded, and temperature is but one of them. The article Tree line may have some more information for you to read in this regard. --Jayron32 20:05, 8 August 2011 (UTC)

((edit conflict) but in agreement with Jayron) It should be noted that it's not merely temperature that inhibits further tree growth. Altitude, kind of soil, wind velocity, even the species of trees, etc. can factor in. A tundra is also not defined by surface temperatures alone but by the presence of permafrost in the ground and the length of the seasons. But yes, southwest Greenland is subarctic. Anyway, as mentioned in the article on Tree line, Greenland simply does not have seed sources. It is after all, an island, and was completely covered with ice before. It's a bit more difficult for new forests to take root there after the retreat of the ice cover. The few (birch) forests that survived the Ice Age are genetically isolated.

There are, however, afforestation experiments in Kangerlassuaq. And they're thriving quite well from the sound of the articles I've come across.

See the following:

• Sarah Lyall (October 28, 2007). "Warming Revives Flora and Fauna in Greenland". The New York Times.
• Kenneth Høegh (October 28, 2007). "Extended article - "Green forests in Greenland"". POST Greenland.

And the academic paper (if you have access to it, I don't though):

• Ødum, S (1979). "Actual and potential tree line in the North Atlantic region, especially in Greenland and the Faroes". Holarctic Ecology. 2: 222–227. doi:10.1111/j.1600-0587.1979.tb01293.x.

Disclaimer: not an expert by any means. Cheers.-- Obsidi♠n Soul 20:33, 8 August 2011 (UTC)

What vehicle make-&-model has had NO accident fatalities in its entire history?

NASA Crawler-transporter

I wonder what vehicle make/model has had no fatalities in any of its accidents in the entire history of it since the first vehicle of that model was produced. It's so that if I get this vehicle, I'll have less to worry about. Thanks, --68.95.72.154 (talk) 20:51, 8 August 2011 (UTC)

I would speculate that, excepting odd models with very limited runs (say, numbering in the few hundreds or less) or truly unique, one off cars, that there are no wide-production commercial automobiles that have zero fatalities. It is literally impossible to design a car that would be impossible to die in, even extremely rare models where no one has coincidentally died in likely has nothing at all to do with the safety features, but rather just has to do with the fact that so few are on the rule^{Did you mean "road?"}, no one has had the chance to die. There are car models which are safer than others, for various reasons, but no car is "deathproof" and expecting one to be is entirely unrealistic. --Jayron32 20:59, 8 August 2011 (UTC)

I'd think that you could make a car that's almost death-proof, if money was no object. I'd start with an armored personnel carrier, then put in speed limit controls, an alcohol breath test on the driver before it would start, software to refuse to start without seat belts fastened, or with brake faults detected, or with improper tire inflation pressure, etc. Yes, this car would be a total pain, but it would be rather difficult to have a fatal accident in one. StuRat (talk) 03:23, 9 August 2011 (UTC)

Lunar rover. Wanna buy it? Only one owner. Course, you do have to pay for shipping. Clarityfiend (talk) 21:41, 8 August 2011 (UTC)

As far as I know, the NASA Crawler-transporter has had no accident fatalities (maximum speed 2 mph unloaded). Looie496 (talk) 22:30, 8 August 2011 (UTC)

It's important to identify the part of a car that causes most accidents. It is the nut behind the wheel. The OP would be safer buying a car model that has had many fatal accidents, which increases the likelihood that it has used up all the fatal accidents that it will ever have. Cuddlyable3 (talk) 23:03, 8 August 2011 (UTC)

I certainly hope that last sentence is a joke, because the logic is totally faulty. StuRat (talk) 03:14, 9 August 2011 (UTC)

That's a corollary to one I heard a long time ago (obviously)... that when you board a plane, you should carry a bomb with you, because the probability of TWO bombs being on the same plane is virtually zero. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:15, 9 August 2011 (UTC)

That sounds convincing but I smell a rat somewhere. Cuddlyable3 (talk) 15:26, 9 August 2011 (UTC)

<nerdiness>If that was a serious remark, then it is because if the probability of a bomb is P(B), then the probability of two bombs, P(B₂|B₁)×P(B₁) is [P(B)]², assuming the events are independent/not centrally organised. However, by bringing a bomb on yourself you have guaranteed that there will be a B₁, so the probability of another bomb is just P(B₂|B₁)=P(B), same as it would be had you not brought a bomb (Baye's theorem). The only difference now is that the probability Q of a bomb going off accidentally on your plane, previously only P(B)×P(Q), is now just P(Q), i.e. greater by a factor of the reciprocal of the original probability that there would be a bomb on the plane (or always there). 50.19.15.136 (talk) 16:28, 9 August 2011 (UTC)</nerdiness>

Modern bombs are reliable but hard to conceal i.e. the probability D of discovery P(D)>P(Q). If a bomb is discovered while boarding, a plane is evacuated at once. When Baseball Bugs brings his bomb onboard, the probability P(Q) of it going off accidentally is more than offset by the doubled probability of a bomb being discovered and consequent saving of the passengers from the other bomb which then has a low probability of not exploding P(Q') in the empty plane. To maximise safety, all passengers should bring bombs. Cuddlyable3 (talk) 00:03, 10 August 2011 (UTC)

The Ford Model U. If you want speed, on the other hand, try the Lamborghini Estoque, the first 4-doored Lambo car. I can confidently assure you that those two models have never been in an accident that involved a fatality of any occupant. (Good luck obtaining them though!) --70.179.163.168 (talk) 03:55, 9 August 2011 (UTC)

That's probably the answer, there are probably a number of concept cars that have never had accidents. That wouldn't give me piece of mind, though. Many of them are probably horrible death-traps. APL (talk) 08:34, 9 August 2011 (UTC)

How many MPH is the bumper rated for on our MRAPs and M1A2 Abramses?

Whereas ordinary civilian vehicles typically have 5-MPH bumpers, how many MPH of speed can an MRAP and an M1A2 Abrams withstand a collision before their bumpers start to dent? --68.95.72.154 (talk) 20:51, 8 August 2011 (UTC)

Regarding the M1A2, tanks don't have bumpers, which even a cursory glance at a picture would confirm to you. Regarding the MRAP, that information would have required someone to have actually tested the dent resistance of that bumper on that vehicle, and given the use of that vehicle, it seems quite unlikely that anyone would have bothered. Which is not to say that tanks and mine-disposal vehicles aren't tested rigorously for their ability to withstand damage; they certainly are, but standards which apply to civilian road vehicles, like the dent-resistance of bumpers, makes absolutely no sense in the context of tanks and other similar military vehicles. --Jayron32 21:02, 8 August 2011 (UTC)

They might have tested the MRAP bumper. For example, if it's necessary to push another vehicle out of the way, they'd no doubt prefer the bumper not to be torn off in the process, and getting it hooked on the other vehicle's bumper could be fatal in a combat situation. StuRat (talk) 03:10, 9 August 2011 (UTC)

True, but that's not exactly "dent" testing, which is pretty cosmetic and not likely the first thing that worries the builders of mine-resistant vehicles. --Jayron32 03:24, 9 August 2011 (UTC)

Just want to point out that MRAP refers to an entire category of vehicles. Y'all are discussing it as if there is only one type of vehicle called an "MRAP". Roger (talk) 09:01, 9 August 2011 (UTC)

Satellite dish ears

Inspired by this question, what would a sapient race's hearing be like if they had ears shaped like satellite dishes? Subliminable (talk) 21:27, 8 August 2011 (UTC)

they wouldn't be able to hear much at all! In fact if their ear was made out of very sound-reflective material they may be totally unable to hear! A satellite dish is designed to focus all incoming transmissions onto the receiver, which is suspended at the focal point of the dish. As a result a hole in the dish itself, for an ear, would not be able to receive much all. HominidMachinae (talk) 22:12, 8 August 2011 (UTC)

See Parabolic microphone. There is no reason why an alien race could not evolve little Cassegrain reflectors to help focus the sound. Their hearing would be very directional and ineffective at low frequencies. Their sapience is irrelevant. However if we assume a human-like form, they would save neck strain by have spherical rather than parabolic ear-reflectors because that allows steering by the sub-reflector instead of by turning the whole head. The same principle is used to steer the Arecibo radio telescope. Cuddlyable3 (talk) 22:52, 8 August 2011 (UTC)

Assuming that they need a tympanic membrane like our ears have, it would be difficult to have a structure like a parabolic microphone, with a receiver at the focal point of the dish. If they were a silicon-based lifeform though they could evolve exactly like a parabolic mic. Electret ears would be pretty cool. HominidMachinae (talk) 23:02, 8 August 2011 (UTC)

chernobyl nuclear plant

Is the Chernobyl nuclear plant site still radioactive? if it is how long would the radiation still last?

According to Chernobyl Nuclear Power Plant, "It is likely that with no further decontamination work the gamma ray dosage at the site will return to background levels in about 300 years. However, as most of the alpha emitters are longer lived, the soil and many surfaces in and around the plant are likely to be contaminated with transuranic metals such as plutonium and americium, which have much longer half-lives." Clarityfiend (talk) 22:13, 8 August 2011 (UTC)

it's very radioactive. the "corium" melted material in the core is composed of all the metal from the reactor and contains heavy radioactive elements with very long half-lives, like on the order of billions of years. Chernobyl will probably be radioactive long after humanity has come and gone, in fact the uranium in the core itself will have more than trace radioactivity when the sun expands and consumes the earth. HominidMachinae (talk) 22:18, 8 August 2011 (UTC)

You need to keep in mind, though, that the nuclides with the longest half-lives are also the least radioactive. That's by necessity; a nucleus radiates only when it decays, and half-life is inversely proportional to decays per nucleus per unit time. Granted, other things affect how dangerous the radioactivity is (for example tritium is considerably less dangerous than you might think, given its short half-life, because the decay mode is a low-energy beta particle).

The half-life of U-238 is about 4 billion years, but precisely because of that, it is only slightly radioactive. --Trovatore (talk) 22:47, 8 August 2011 (UTC)

That is true, but as I understand it non-radioactive material was melted into the corium as well. Which could be acting as a neutron moderator. Plus there is way more than a critical mass lumped together into a non-favorable geometry. HominidMachinae (talk) 23:09, 8 August 2011 (UTC)

I don't think anyone thinks there active criticality at the Chernobyl site anymore. At least, I've never heard that. It seems really unlikely that it would be the case all these years later (any criticality would have long since melted itself down further at this point). It's not an easy thing to make a reactor go critical; it won't just happen incidentally.

The issue with long-lived isotopes is not that they will zap you quick, but that many of them can get deposited in the human body. On the outside of a human body,w a weak alpha emitter is not a problem (it can't penetrate the skin). On the inside of a human body (in your bone marrow, for example) they can do a lot of cell damage (e.g. leukemia). Certain long-lived isotopes are easily taken up into the body, which is bad. Others get flushed through pretty quickly and so are not as much of a problem (though they do increase your cumulative radiation exposure by a little bit, which is not good if repeated for long periods). --Mr.98 (talk) 01:44, 9 August 2011 (UTC)

My main concern was actually to respond to the inference HominidMachinae appeared to be making, that "it's very radioactive" because it "contains radioactive elements with very long half-lives". A lot of people seem to be under that impression, and it's important now and then to point out that things actually go in exactly the opposite direction — other things being equal, longer half-life means less radioactive. --Trovatore (talk) 03:01, 9 August 2011 (UTC)

I think it's important to emphasize that both long and short half lives pose different types of radiological risks. --Mr.98 (talk) 20:27, 9 August 2011 (UTC)

A sufficiently long half-life means essentially no radiological risk. Take bismuth-209 as an extreme case. I'm fairly skeptical of claims of radiological injury from, say, depleted uranium, though I recognize that they're possible in principle. --Trovatore (talk) 22:01, 9 August 2011 (UTC)

I don't think anyone's disputing that. But there are plenty of isotopes that are "long" (from a human standpoint) and "dangerous". Pu-239 has a 24,000 year half-life, but it's still nothing you'd want embedded in your lungs. --Mr.98 (talk) 00:20, 12 August 2011 (UTC)

Put it this way: people who are in affected areas are still having their life expectancies reduced by considerable amount. (I was once told removing them for a period of a month would extend their life by a year, but I don't know if this is true.) Grandiose (me, talk, contribs) 20:29, 9 August 2011 (UTC)

It's not. μηδείς (talk) 17:10, 10 August 2011 (UTC)

Water and Electricity

Water and electricity are not a very good mixture. Why is that? We often see fire fighters spraying water onto burning buildings. Surely there will be a mixture of water and electricity. Why does that not cause problems? — Fly by Night (talk) 22:38, 8 August 2011 (UTC)

Pure water is a very shitty conductor of electricity. However, if there are any ions in the water (a mixture known as an electrolyte), the ions are very GOOD conductors of electricity. Finding water without ions is basically very difficult to do (water is called the "universal solvent" for a reason, and in many conditions it can be quite easy to dissolve a trivial amount of electrolytes in the water and make it a decent enough conductor). Generally, if a fire has progressed enough to need the firemen, its probably damaged the house's wiring to the point where electricity is not as much of an issue as you may think; however firefighters can and do get get electrocuted when trying to put out a fire, see [3] for about 1000 examples. Its part of the inherent danger of being a firefighter, and why they have to have balls the size of melons (or ovaries the size of coconuts, for the ladies) in order to do that job. --Jayron32 22:47, 8 August 2011 (UTC)

for the misspelled sentence above. Cuddlyable3 (talk) 22:57, 8 August 2011 (UTC)

Yes, I probably missed an apostrophe. Now cut that shit out. It is getting obnoxious. --Jayron32 23:35, 8 August 2011 (UTC)

Jayron32: Thank you very much for your answer. It made me wonder: a car battery uses distilled water, which I assume is the "pure water" that you talked about. If pure water is a bad conductor, then why use it in a battery? (I'm not trying to trip you up, I'm asking because I don't know.) On a personal note: just ignore people that try to annoy you. Your reply was fine, and I understood it perfectly. You helped me to answer my question, and that is the point of any reference desk. Let attacks be like water off a duck's back. — Fly by Night (talk) 01:45, 9 August 2011 (UTC)

The electrolyte in a lead-acid battery (the old-style car battery that you would top up with distilled water) is actually a solution of sulfuric acid in water. (As the battery discharges, the acid is partially consumed, forming lead sulfate on the battery plates.) The sulfuric acid solution is quite a bit more conductive than distilled water would be. But why, you might ask, do we not need to replenish the sulfuric acid? First, the formation of lead sulfate is (essentially) completely reversible on charging the battery. Second, sulfuric acid has a very low vapor pressure; it's not going to evaporate fast enough to matter, even over the entire lifetime of the battery. The water component of the electrolyte, on the other hand, escapes in two ways. Water can evaporate, and small amounts can escape from the battery in this way given sufficient time. Second (and more important) the process of charging the battery breaks down small amounts of water into hydrogen and oxygen through electrolysis. These gases are lost, reducing the amount of water in the battery over time. TenOfAllTrades(talk) 02:39, 9 August 2011 (UTC)

To make it a little clearer what TenOfAllTrades is talking about, you used to add distilled water (essentially pure water) to old car batteries because you don't want to foul up the existing electrolyte system by introducing impurities. They didn't contain distilled water; far from it, as he notes they contained highly corrosive sulfuric acid. However, the voltage that a battery delivers will be dependent on the concentration of the electrolyte, so if there is too much, or to little, water it will throw off the voltage which in some applications needs to be carefully calibrated (dry cells, like AA batteries, avoid this by being in the solid state; solid state chemistry is much less finicky in regards to concentration for reasons I won't go into now). You used to need to "top off" old car batteries to fix this problem; but the electrolyte is most definitely there. See the Nernst equation for the deep nitty-gritty with regards to how voltage in wet cells is highly dependent on concentration. But to reiterate, you needed to top off old car batteries with distilled water not because they used distilled water, but because the carefully calibrated electrolyte inside the battery could be contaminated if you added non-distilled water. See Automotive_battery#Fluid_level for more on this; most modern car batteries have a sealed case and no longer need to be "topped up" in this manner. --Jayron32 03:19, 9 August 2011 (UTC)

Thanks to both TenOfAllTrades and Jayron32 for your answers; you've really helped me to fill in some gaps. One little thing: less of the past tense when referring to car batteries. Up until recently I had a 1989 E-class. It was as old as the hills but it looked cool as hell! (Sadly it's gone to car heaven now.) — Fly by Night (talk)

Please, I drive old cars, and I drive them until they croak. The car I owned immediately before my current one was a 1983 Toyota Tercel; I drove it from 1998-2002. Even in that car I used a sealed-case battery, however. My current car is a 1999 Ford Escort; I plan on giving that one to my 5 year old son when he's old enough to drive. --Jayron32 05:40, 9 August 2011 (UTC)

We learn a lot about people here in Wikipedia. Cuddlyable3 (talk) 13:55, 9 August 2011 (UTC)