Wikipedia:Reference desk/Archives/Science/2012 November 14

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November 14

Observation in quantum mechanics

Since wave function collapse requires someone to observe the system, does this mean that before humans (or other life), the Universe didn't exist? --168.7.231.3 (talk) 02:05, 14 November 2012 (UTC)

"Observation" is just the word we use for the event that occurs when a wavefunction collapses. It simply means that a certain kind of physical interaction has occurred. It does not require that any living thing be present. The confusion is understandable, but wavefunction collapse can happen whether or not humans are looking at the results. In fact, even physicists have a hard time defining what interactions will or will not constitute measurement. See measurement in quantum mechanics, wavefunction collapse and quantum decoherence. Dragons flight (talk) 03:26, 14 November 2012 (UTC)

Right. Just to be redundant, tyhis relies on the principle that there is no action at a distance, and that observation means somebody's bounced a photon off it or something similar. Which naturally means that the wavefunction is no longer what it was before you observed it. Gzuckier (talk) 01:44, 16 November 2012 (UTC)

Perfect number

I think I've found an error that make the information in the article contradicting. In the "even perfect numbers" section, it says: "As of June 2010, 47 Mersenne primes and therefore 47 even perfect numbers are known." Then later in the section it says: "It has not yet been proved that there are (or are not) others after the 41st." So I think it should have said "...other after the 47th."?174.20.101.190 (talk) 06:17, 14 November 2012 (UTC)

This is science reference desk, you should have asked the same on mathematics reference desk. Sunny Singh (DAV) (talk) 07:48, 14 November 2012 (UTC)

The explanation is quite simple. There are 47 known Mersenne primes, but only the first 41 are known to be consecutive - not all possibilities after the 41st Mersenne prime have been checked, so there may be as yet undiscovered Mersenne primes in the gaps between the 41st and the 47th on the known list. There is a 1-1 correspondence between Mersenne primes and even perfect numbers, so there are 47 known even perfect numbers but only the first 41 are known to be consecutive. This is what the perfect number article is trying to say - but I agree it is not very clear, and the wording could be improved. Gandalf61 (talk) 10:41, 14 November 2012 (UTC)

Thanks, Gandalf61. I've clarified it there. Duoduoduo (talk) 14:53, 14 November 2012 (UTC)

Heat energy vs thermal energy

Is correct to say that heat energy and thermal energy are different ? Sunny Singh (DAV) (talk) 07:36, 14 November 2012 (UTC)

There is a difference in a subtle way. Note that the term heat energy is bad english - heat is energy, so the term is like saying energy energy. The terms heat and thermal energy are often used interchangeably. However, more correctly in a scientific or engineering paper, heat is energy being interchanged from one media to another (e.g, the heat rejected by a gas to its surroundings when it is compressed), whereas thermal energy is the thermodynamic energy in a system. You may like to carefully read the WP articles Heat and Thermal Energy. Wickwack 58.169.249.183 (talk) 09:00, 14 November 2012 (UTC)

I was taught that heat was another term for waste energy. Plasmic Physics (talk) 11:46, 14 November 2012 (UTC)

In most engineering applications, heat is waste energy, i.e., unwanted or unusable, but in some cases it is the heat that is wanted. For example, the heat rejected in an internal combustion engine is waste, unless it is utilised in co-generation. The heat rejected by the burning fuel in a power station is not waste heat, but what's left over after boiling the feedwater and has to be lost in the cooling towers is waste. You may have mis-remembered or misunderstood your teacher. Wickwack 120.145.170.99 (talk) 15:00, 14 November 2012 (UTC)

No, I didn't mis-remember or misunderstand my lecturer, he used the second law of thermodynamics to explain it. Plasmic Physics (talk) 22:30, 14 November 2012 (UTC)

The 2nd law is essentially that in the conversion of heat into mechanical work, not all the heat can be so converted. How does that mean that that heat is waste energy? Not only is not all the heat "wasted", heat may be the desirable output as I said. Wickwack 120.145.143.165 (talk) 00:48, 15 November 2012 (UTC)

He didn't use "heat" in that sense. Plasmic Physics (talk) 05:04, 15 November 2012 (UTC)

It seems odd to read that heat is waste. Although most of the heat produced throughout the world become waste, yet it is a useful form of energy. It is true that we don't notice most of the heat produced, but it is not like that heat is waste. I don't think so because heat is used in steam engines to evaporate water, in ovens, in heating effect of many devices and many more. Talking about the second law of thermodynamics, it says all the heat cannot be converted into mechanical energy but some can. Sunny Singh (DAV) (talk) 09:16, 15 November 2012 (UTC)

It isn't that "heat is waste", which is a rather trite saying and like all trite saying has a tiny bit of truth packed inside too much approximation to have any real meaning. We should avoid triteness when describing things scientifically. Heat is waste in the sense that when two substances are in contact they exchange heat: the warm one heats up the cold one, until such time as the temperature between the two is equal. Here's the thing: The energy transferred from the hot body to the cold body cannot be utilized again to do work. That's because commensurate with the energy transfer is an increase in entropy, and you cannot recover that energy without "stealing" entropy from somewhere else in the universe, which would require you to heat up that part of the universe by more than you recover. That's the inescapable part of the second law of thermodynamics. Now, you can use heat to do work, as it is in the process of moving from the hot place to the cold place, but once the two places are in thermal equilibrium, you can't use that energy to do work again in an absolute sense: that energy is lost to the universe, i.e. there is a loss of free energy. So, in one sense, the trite statement "heat is waste" is correct, however it is not wholly correct because you can do work with heat; you just can't recover the energy after the work is done. --Jayron32 14:02, 15 November 2012 (UTC)

First, that is off the mark. Take a red hot body and a cold body, say a large fired up rod and a cup of cold water. Let's put these together. Energy flows from one to the other, and yes, that energy can still do work, since it will create steam that can be used in a heat engine. As for other misconceptions regarding the second law, I intend to knock it out of its ivory tower too. --Modocc (talk) 14:22, 15 November 2012 (UTC)

Um, why do you start a response with, "that is off the mark" and then say something that agrees with everything I say 100%. I am confused as to what parts of my explanation are incorrect. --Jayron32 14:33, 15 November 2012 (UTC)

To clarify, you said this, "The energy transferred from the hot body to the cold body cannot be utilized again to do work." Since you said "cannot be utilized again" I assumed you understood that the hot body already did work on the cold body, by heating it up, with the kinetic energy of the molecules being transferred. Its incorrect to conclude that this energy that was transferred cannot do any work again as you said though. In any case, entropy itself is elusive in the sense that it is inherently system dependent. In the 70's, entropy and Gibbs free energy were used to explain why diamonds, at the time, could be created only under extreme pressure and temperature, but in the 80's new catalysts circumvented the presumed entropy. -Modocc (talk) 15:01, 15 November 2012 (UTC)

Really? Once the rod and the water are the same temperature, how do you presume to use the thermal energy that moved from the rod into the water to do work again? Energy which is transfered from one form to another, or from one location to another, can be used to do work indefinitely except energy which is transferred as heat. You can only use that energy once; as it makes its trip from the hot stuff to the cold stuff you use that transfer to do work; but only that one time. Other energy transfers can be used indefinitely to do work along the way. That's the difference between the first law and second law of thermodynamics. --Jayron32 18:12, 15 November 2012 (UTC)

Let the temperature of the rod be 200celius. Let the rod cool to 199degrees and the water heat up to 199degrees. Assuming my cup has a lid on it, the water has turned to steam and is in equilibrium with the rod. Now, some of the heat of that rod has done work and you have said its not able to do work again, but it can. Open the valve on the lid, so the steam enters an engine. The heat energy of this steam gets transferred to the engine such that it is again does work. -Modocc (talk) 18:45, 15 November 2012 (UTC)

When you open the cup, in order for the steam to do work, it needs to be opened into a region of a different temperature/pressure than the inside of the cup. When you do that, you introduce a new heat transfer (from the hot inside to the cold outside). So you're dealing with different heat. You no longer have a closed system of just the rod and water, and if you take your system to be cup+rod+steam engine, the heat transfers have not been complete until all three are in thermal equilibrium. Once all three are, your system loses its ability to do work. My point still stands as 100% correct. --Jayron32 05:41, 16 November 2012 (UTC)

Once the steam leaves the cup, and I shut the valve, the rod should not rapidly lose much more heat if the rod and cup are insulated well. But I'm not sure what you mean by "dealing with different heat." I do mean a new heat transfer, but this basically just means that this heat gets utilized again. Also, your choice of closed/open systems are arbitrary. Its my understanding that work performed on any object is equivalent to its change in kinetic energy. Let the water initially begin with an ambient temperature and the water is heated by the rod, the kinetic energy of the water molecules are increased (and energy is absorbed too in order to break bonds with its heat of vaporization). Then, when this steam is used in the engine, the water will lose this extra energy, with some of it doing work on the engine's pistons, but the rest will not, with these amounts depending on the engine's efficiency. But all of that extra energy over and above the water's ambient heat content which the engine receives, but which it does not fully utilize, was provided initially by the rod's heat doing work on the water to raise its temperature above the ambient temperature. Thus, the water's initial ambient heat content plus the transferred heat energy that does work on the water prior to opening the valve equals the sum of the engine's waste heat energy and the energy of the work done on its pistons. A portion of this rod's heat energy does work on the water, and then as a part of the water's heat energy, it either gets wasted or performs work again on the piston. Your statement that "The energy transferred from the hot body to the cold body cannot be utilized again to do work." is wrong. -Modocc (talk) 06:42, 16 November 2012 (UTC)

Once you've redefined the system, it can. The difference is that you're working from a different defined system each time. You can keep passing heat energy from one place to another until the heat death of the universe and keep doing work at each transfer. My statement never claimed you couldn't do that. What you cannot do is use heat energy twice within a closed system. --Jayron32 18:53, 16 November 2012 (UTC)

Err, this assertion regarding what can or cannot happen within closed systems is wrong too, but I'm done with this topic for now. -Modocc (talk) 01:50, 17 November 2012 (UTC)

Seems to me this discussion has lost the plot with all this talk of triteness, reusability, and diamonds. Seems to me Jayron has merely established that heat can be waste (and in any thermodynamic process, some will be waste in a component process of a larger process), but heat isn't necessarily waste. Maybe that is a trite (= not novel) thing to say, but it is clearly different to saying heat IS waste (always), which is what PlasmicPhysics said. Plasmic said something wrong. Wickwack 120.145.0.141 (talk) 16:06, 15 November 2012 (UTC)

Incidentally, in Engineering Thermodynamics, D B Splading and E H Cole, Edward Arnold publ, a standard undergraduate text for many years, in Chaper 5 Heat, it gives a formal definition of heat: "the interaction between systems which occurs by virtue of their temperature difference when they communicate" (page 86 in 3rd Edition). It then goes on with 7 more pages about what heat is and what it is not. There is not a word about waste or any similar word like it. I have several other textbooks on thermodynamics and they all give the same definition, albiet in their own words, without any statement about "waste" until they come to actual examples, where for example, in a Carnot process some of the heat must be unusable (wasted). Wickwack 120.145.0.141 (talk) 16:06, 15 November 2012 (UTC)

All too true, and the above usage of "waste" is simply being used synonymously for the concept of entropy. So to define entropy is to define waste. In my neighborhood, we do a great deal of recycling though, so as to reduce the amount of waste that gets wasted. We've been binging on fossil fuels for some time now, but its time to sober up and do a far better job of recycling energy than we have. -Modocc (talk) 16:31, 15 November 2012 (UTC)

I also said the same thing heat is not waste. According to me, the overall discussion goes in favor of Modocc and Wickwack. Sunny Singh (DAV) (talk) 12:08, 16 November 2012 (UTC)

Calling heat "waste" is kind of silly when you think that the Sun, the furnace in your house etc. are all providing "waste". Best to call heat heat and leave the rest to semanticists. As I understand it, given that it appears the universe never ceases to expand and thus to cool, the heat should never become unrecoverable to work at any time in the future; there is no heat death of the universe, though the article seems a bit unsatisfying on the point. Wnt (talk) 20:59, 17 November 2012 (UTC)

The fundamental definition of heat as given in modern textbooks is energy transfer from one body to another that is not due to macroscopic work. This definition does not involve thermodynamic concepts, because you need to have this definiton to build up the thermodynamic concepts. Macroscopic work is defined as energy transfer due to a change in the external parameters of a system. What one chooses as the external parameters (e.g. the volume) is entirely arbitrary in priciple, so this makes the separation of energy transfer into a work part and a heat part entirely subjective. If you choose to describe the system exactly in terms of all its fundamental degrees of freedom, then all of the energy transfers will be work, and the entropy will be the so-called fine grained entropy which is always equal to zero (which expresses the fact that there is no loss of information at the fundamental level). Count Iblis (talk) 19:17, 16 November 2012 (UTC)

New Sub-Question:-
Leave this long discussion as it doesn't answers my question. Talk about the question. I read the article heat and thermal energy for another time as suggested above, but I have some confusions. I'll thankful if someone happily resolves my confusions.

1. When two bodies having different temperature are brought in contact with each other, both bodies attain thermal equilibrium after sometime. Here during energy transfer, heat is transferred between bodies or thermal energy i.e., which one of the latter two is being transfered. Sunny Singh (DAV) (talk) 05:45, 18 November 2012 (UTC)
2. Section 'Distinction of thermal energy and heat' of the article thermal energy mentions "Statistically, thermal energy is always exchanged between systems, even when the temperatures on both sides is the same, i.e. the systems are in thermal equilibrium. However, at equilibrium, the net exchange of thermal energy is zero, and therefore there is no heat". In the first sentence it says thermal energy can be exchanged when the systems are at thermal equilibrium and in the second sentence it says thermal energy cannot be exchanged. How is it possible ? Sunny Singh (DAV) (talk) 06:21, 18 November 2012 (UTC)

Answer to #1. Your english is a bit cryptic, so I hope I'm answering what I think you must be trying to ask. The correct term for what gets transfered is Heat. The reason why heat is the correct scientific term is bound up in the theory of thermodynamics. However, as I said in my first post, you will often find the terms heat and thermal energy used interchangeably in the literature. Thermal energy can mean other things, but whether an author means heat or something else can be determined by the context. The heat (which is a form of energy) gets transfered by one or more of the following ways, depending on just wht the bodies in contact consist of: Conduction (thru inter-molecular forces and collisions, and Radiation (electromagnetic radiation eg infra-red). If one of the bodies is a gas, then heat can be transfered within it by convection as well, sometimes referred to as mass transfer.

Answer to #2. What the author is trying to say is this: There is a two-way flow of heat between bodies when both are at a finite temperature - for bodies A and B there is a flow of heat from A to B (which would decrease the temperature of A ind increase it for B), and also a flow from body B to A (thus increasing A temperature and decreasing B). If the two bodies are at the same temperature, the two heat flows are equal, and since they are in opposite directions their effects in altering the temperatures cancel. You are perhaps surprised that there are two flows, and not just a flow from the hotter to the colder. To see why there are two flows, consider two bodies separated by a gap. Because each body is at some temperature (ie not at absolute zero), it must be emitting radiation, which is a function only of its temperature and its own properties (area, emissivity), which propagates across the gap as electromagnetic energy. After a propagation delay, the radiation hits body B, which must absorb some of it, as a function of its transparency. Meanwhile, Body B must be also radiating, since radiation is a function only of its own temperature and properties. If the two bodies are touching, then heat can flow by conduction (and perhaps convection) - that really doesn't change the fact that as both bodies are at some non-zero temperature, both must be emitters.

I am sorry that the discussion went away from just answering your original question, and I'm partly responsible of causing that when I corrected the erroneous assignment of heat as "waste" by PlasmicPhysics. Why he brought in the term "waste", which is not generally used in thermodynamic textbooks, is something only Plasmic can know.

Wickwack 121.215.50.212 (talk) 10:15, 18 November 2012 (UTC)

Two Last Sub-Questions:-

1. Suppose I have two iron block of 1kg. I, somehow, increased the volume (mass remaining the same) of first block and increased the mass (volume remaining the same) of the second block. Assuming all blocks have the same temperature, which one has more thermal energy ?
2. Suppose I have a solid block of iron of 1kg and gaseous nitrogen in a large container, nitrogen has also the same mass of 1kg. Which one - iron block or gaseous nitrogen- has more thermal energy ? Sunny Singh (DAV) (talk) 03:59, 19 November 2012 (UTC)

It would be better to ask new questions as new questions, not as sub-questions to an old question. This is because (a) you are more likely to get answers from various contributors, and (b) given the length of discusion on this one, probably nobody wants to contribute any more, even if they are still looking.

Answer #1: This question cannot sensibly be answered, as you cannot in any real way take a block of iron and alter its mass or its volume without changing both in proportion. By increasing the temperature, you can increase the volume a tiny bit without changing the mass, but the tiny change possible is not relavent to thermodynamics, and you excluded a change in temperature anyway.

Answer #2: Each element and each chemical compound has a specific heat that can in theory be deduced from its molecular and electron orbit structure. Specific heat is the amount of heat that a unit mass of a substance absorbs when its' temperature is increased by 1 unit (1 kelvin, 1 degree farenheit, etc). Particularly for gasses, specific heat varies with temperature. From standard tables, iron at 25 C (ie solid iron) has a specific heat of 450 J / kg.K; nitrogen has a specific heat at constant volume of 742 J / kg.K. Nitrogen, weight for weight, holds the greater amount of energy.

When talking about heat contained within a mass, it is necessary to know what this heat could be transfered to. The above answer is relavent if any heat flow in or out of the 1 kg iron or nitrogen is with respect to something else at close to 25 C. If the nitrogen was coolled so that it liquifies, for example, it would give up additional heat according to its latent heat of vaporisation.

Wickwack 58.170.164.182 (talk) 11:09, 19 November 2012 (UTC)

I was not talking about heat, I was talking about "thermal energy"; this is too late, so, close this discussion, I'll ask the same later. Thanks to those who contributed to this question and special thanks to Wickwack. Sunny Singh (DAV) (talk) 13:51, 19 November 2012 (UTC)

Vitamin D

Vitamin D is already present in cow's milk or it is added to milk by humans. Sunny Singh (DAV) (talk) 07:52, 14 November 2012 (UTC)

Most of the vitamin D in milk is artificially added. I can't find a source online that says exactly how much vitamin D is naturally present. Someguy1221 (talk) 08:20, 14 November 2012 (UTC)

Yes, I read the same thing in the last line of second paragraph of the article vitamin D. I am also confused how much vitamin D is naturally present in cow's milk. What about mother's milk (referring human), it contains vitamin D or not. Sunny Singh (DAV) (talk) 08:40, 14 November 2012 (UTC)

I found the same thing as with cows milk. It contains some vitamin D, but not enough to meet even a baby's daily requirements, but I didn't find a source that gave the exact amount. Someguy1221 (talk) 09:42, 14 November 2012 (UTC)

This site says that milk in the UK is not routinely fortified with Vitamin D. I also found this: "Whole cow’s milk was found to contain 38 i.u. vitamin D/I. Whole human milk contained 15 i.u. " from "The total content of vitamin D in cow's milk and human milk", Leerbeck (1980) - Journal of Human Nutrition. (There is a link on Google, but I've had a lot of trouble trying to get it to download and I'm not sure about putting the reference in here.) --TammyMoet (talk) 10:37, 14 November 2012 (UTC)

According to recent research, mother's milk contains enough vitamin D for the baby (more than 400 IU per liter) provided the mother's calcidiol level is above 120 nmol/l, which requires a daily intake for the mother of at least 4000 IU/day [1]. This is one of the arguments for higher calcidiol levels (120 nmol/l or higher instead of 50 nmol/l) despite there not being strong evidence for better health outcomes for adults, or that evidence being disputed. Obviously if babies would routinely need vitamin D pills because they would otherwise not even get the 400 IU/day for which we know their health is adversely affected, then that also implies that adults need more than 4000 IU/day. If we are right about this not being necessary for cancer or heart disease, then for other reasons we don't know anything about yet.Count Iblis (talk) 16:56, 14 November 2012 (UTC)

The use of ultraviolet light to enrich milk with vitamin D was discovered by Harry Steenbock. It's not so much that the vitamin D is added (there's is not some separate container of vitamin D that is mixed into the milk), rather the milk is vitamin D enriched/fortified, as there are pre-vitamin D compounds in milk that are transformed by the ultraviolet light into active vitamin D, in the same fashion sunshine (which contains ultraviolet light) can convert pre-vitamin D compounds to vitamin D in the skin. -- 205.175.124.30 (talk) 22:46, 14 November 2012 (UTC)

USDA's National Agricultural Library has all sorts of interesting information. This and this compare the nutrient content of 3.25% whole milk with and without added Vitamin D. Zoonoses (talk) 07:07, 15 November 2012 (UTC)

Forest fire vs candle flame

Candle flame goes off by the effect of wind but forest fire increases with increasing velocity of wind. Why ? Sunny Singh (DAV) (talk) 09:03, 14 November 2012 (UTC)

If you had a wind in proportion to the size of the flame, you could blow it out. For a forest fire, you'd need a hurricane, at least. StuRat (talk) 09:33, 14 November 2012 (UTC)

Also a forest fire is much, much hotter than a candle flame. Even if you could "blow out" the actual flames of a forest fire, the timber would still be burning. You can't really blow out any fire other than a tiny one such as a candle flame.--Shantavira|^{feed me} 09:45, 14 November 2012 (UTC)

I disagree. You'd just need a long and powerful enough wind to allow for the heat to dissipate. This might be several hours at several thousands of miles per hour, though. StuRat (talk) 09:49, 14 November 2012 (UTC)

I think Shantavira was talking about the real world. Caesar's Daddy (talk) 15:52, 14 November 2012 (UTC)

Is it correct to say that- wind in case of forest fire bring flame from burning tree to neighboring tree and in this way fire increases rapidly, but this doesn't happen in the case of candle flame. Will same thing happen if we stick some candles in a row and blow wind ? Sunny Singh (DAV) (talk) 11:09, 14 November 2012 (UTC)

Yes, wind helps a forest fire to spread, but as Shantavira says, pre-existing heat is a major factor. It's worth considering a blacksmith's forge, where moving air from a bellows or similar is used to increase the intensity and heat of a fire.

• Oil well fires are typically put out using the "Wind" from a large high-explosive blast: the shockwave from a high-explosive charge "blows out" the flame in exactly the same manner as you blow out the candle: Bigger flames require bigger winds to blow them out. --Jayron32 15:40, 14 November 2012 (UTC)

See also the fire whirl article. Count Iblis (talk) 17:27, 14 November 2012 (UTC)

• The analogy is flawed because one single candle has nothing near the geometry of a forest. Pack a shoebox with upright birthday candles, light one, wait a few seconds for it to warm up, and then blow on it with a speed scaled to a gale force as if the candles were 60 foot tall trees. Get back to us after you extinguish the charred box full of burnt wicks and molten wax. μηδείς (talk) 05:36, 16 November 2012 (UTC)

... and even a single candle in a light draught burns much more rapidly, often spilling wax in the process. Dbfirs 07:21, 16 November 2012 (UTC)

Parrot Drone or Quadrotor

Is there any project that try to develop Parrot Drone or Quadrotor that can lift a person or two persons? What's the obstacles to built it? Is it motor problem or battery problem? Is Parrot drone can fly very stable? What if another two motor added horizontally, can it fly faster? Will these two motors cause disruption in flight stability? Thanks... roscoe_x (talk) 09:10, 14 November 2012 (UTC)

What's a parrot drone ? I don't think a quadrotor is particularly safe. If any one of the four rotors fail, I assume it then crashes. StuRat (talk) 09:36, 14 November 2012 (UTC)

Parrot AR.Drone apparently. No flying machine weighing more than a few ounces can get off the ground it runs from a battery.--Shantavira|^{feed me} 09:52, 14 November 2012 (UTC)

Not so. The Sikorsky Firefly is a manned electric helicopter. For that matter, a bit more searching reveals the E-volo VC1 (see picture in the article), a manned electric quadcopter-style rotorcraft. That said, there are major design tradeoffs that make battery-powered helicopters completely impractical given current or near-term future tech, just as the quadrotor concept isn't really solving a problem in the field. — Lomn 15:45, 14 November 2012 (UTC)

I suspect that mostly it's a function of "no need" once you scale up to manned aircraft. Helicopters and gyrocopters are well-developed technologies that don't need the additional complexity that a four-rotor design introduces. For your final question, there's the compound helicopter, which adds horizontal thrust for additional speed -- the Sikorsky X2 is a recent example that reached 250 kts in level flight. There's also the V-22 Osprey, a tiltrotor. Both types of aircraft have considerably more engineering difficulties than traditional helicopters. — Lomn 14:43, 14 November 2012 (UTC)

I found an idea for a two-person quadcopter named The Skyflyer and a suicidal Chinese man. I think less is more when it comes to personal helicopters. Trio The Punch (talk) 19:36, 14 November 2012 (UTC)

It seems odd that those two quadrotor designs put the pilot above the rotors. This design is less stable and obscures the view of the ground, which is critical for landing. StuRat (talk) 19:55, 14 November 2012 (UTC)

Its probably much safer to sit on top of them in case of a crash landing and if the rotors are above you you'll need extra big and strong (=heavy) landinggear. Trio The Punch (talk) 22:20, 14 November 2012 (UTC)

Interesting answers, thank you. Especially the news about the farmer who built his own flying machine. roscoe_x (talk) 00:58, 15 November 2012 (UTC)

Some of those farmers are pretty innovative - see Richard Pearse. Zoonoses (talk) 07:10, 15 November 2012 (UTC)

This thing is pretty cool, but unmanned. Trio The Punch (talk) 15:08, 15 November 2012 (UTC)

chemical / molecular cross-section

What is the formula for the chemical reactivity cross-section of a molecule? The article cross-section only gives info about light absorption cross-sections and neutron cross sections. 128.143.1.142 (talk) 12:12, 14 November 2012 (UTC)

Reaction rates depend on the details of the chemicals involved as well as their physical states and abundance. In general, there isn't a single cross-section that would be useful for all molecules or all possible reactions, though you can use tables of things like standard electrode potential to determine what chemical reactions are likely to proceed. If you want more detail, you probably need to ask about specific molecules and reactions. Dragons flight (talk) 18:24, 14 November 2012 (UTC)

this is what is the compuound of Splenda, sugar and cloro form this, can we understand if it is a natural usefull compound for the human body?

1,6-dicloro-1,6-dideossi-β-D-frutto-furanosil 4-cloro-4-deossi-α-D-galattopiranoside o C12H19Cl3O8. — Preceding unsigned comment added by 195.110.143.127 (talk) 16:26, 14 November 2012 (UTC)

It looks like you've given the IUPAC name for sucralose. Splenda is the brand name of a sucralose-based sweetener that also contains dextrose and maltodextrin. Do the sucralose and Splenda articles answer your question? They both have referenced sections on health and safety. 209.131.76.183 (talk) 18:49, 14 November 2012 (UTC)

Artificial 2-toned lobsters

Is it currently possible to manipulate a lobster so that it looks like this one? Are we able to control the pigmentation selection process at the first cell division of the embryo (of certain species)? Is this technically possible in all multicellular beings (if you invest lots of time and money for research)? Are 2-toned humans theoretically possible? Trio The Punch (talk) 16:27, 14 November 2012 (UTC)

It's called heterochromia which in this case is caused by mosaicism, an early mutation in one cell after fertilization; but can also be caused by Chimerism, the merger of two embryos. There are people with such a condition, but due to differences in the splitting of cells in mammals and arthropods a symmetrical half-and-half mixture would be unlikely. See these images. Yes, a mad scientist could make one, but almost certainly not as striking as the lobster. μηδείς (talk) 17:04, 14 November 2012 (UTC)

As a rule, the kind of straight-down-the-middle color boundary so beautifully illustrated here is more common in arthropods. See gynandromorph for other such examples. (A Google Image search will be rewarding also) The split doesn't have to honor this midline boundary, however, even in arthropods. In mammals, the contribution of different cells in chimeras is pretty close to completely unpredictable. Wnt (talk) 18:29, 14 November 2012 (UTC)

Thanks a lot guys, very interesting links! Trio The Punch (talk) 19:14, 14 November 2012 (UTC)

Cthulu, before he escaped the lab

Note again that the lobster in this case is a mosaic, not a chimera. Humans can be both mosaics due to a mutation in an early embryonic cell or chimeras of fused non-identical twins. Interspecies chimeras have been created, such as this horrific house mouse/deer mouse chimera. Note the monstrous lack of symmetry in the eyes due to the developmental gene expression. Do that with a Human and a Chimp and you'll get something out of H. P. Lovecraft. μηδείς (talk) 20:33, 14 November 2012 (UTC)

I hope our mad scientist creates a Cthulhu-like creature. Trio The Punch (talk) 22:12, 14 November 2012 (UTC)

Yah, I've been daydreaming the human-chimp thing since the eighties... we could use an interpreter or two for interspecies communications. :) Wnt (talk) 22:44, 14 November 2012 (UTC)

Nyeah, bad idea. Reminds me of Asimov's The Ugly Little Boy. -- OBSIDIAN†SOUL 10:13, 15 November 2012 (UTC)

I don't know why would someone ever want to have 2 skin colored. It looked pretty abnormal and doesn't look good at all to me. Plus it is not ethical to do that anyway. You can't just test that on people.174.20.41.202 (talk) 21:58, 15 November 2012 (UTC)

Agreed, see wrongful birth and wrongful life. The picture of the chimerical mouse I linked to should scare the pants off any ethical experimenter. A human-chimp hybrid would be symmetrical and even-colored--but it's still not worth the risk. μηδείς (talk) 22:23, 15 November 2012 (UTC)

Named particles?

The Oh-My-God particle was a specific particle which was named for its surprisingly high speed. This is the first example I'd ever heard of a specific particle having a name. (Not a class of particles, but this particular proton.) Are there other examples? Staecker (talk) 16:35, 14 November 2012 (UTC)

Not a particle, but of the same idea: See Wow! signal. --Jayron32 17:09, 14 November 2012 (UTC)

This is strictly a case of notability. Anyone can arbitrarily name anything they want. For example, I name all of my protons, and some of my better mesons, but I'm not famous enough for any of my pets to have their own encyclopedia articles. Nimur (talk) 17:11, 14 November 2012 (UTC)

Yes I know. I'm asking if any other individual particles have achieved this level of notability. Staecker (talk) 00:15, 15 November 2012 (UTC)

gravity = side effect of vacuum?

Could it be that the displacement of vacuum by massive objects causes the density of the vacuum closest to the object to be denser than the vacuum any distace farther away from the object, thus causing the denser vacuum to "suck" smaller objects to that area, which just happens to be the surface of the object?165.212.189.187 (talk) 20:45, 14 November 2012 (UTC)

Gravity is the effect of mass causing curved space-time. (relax.... its non intuitive and complicated) Spacetime — Preceding unsigned comment added by Ap-uk (talk • contribs) 21:25, 14 November 2012 (UTC)

Thanks, (and no offense) but that sound like regurgitated double-talk165.212.189.187 (talk) 21:31, 14 November 2012 (UTC)

It's much easier to regurgitate sound-bytes describing our most non-intuitive physical explanations than to understand them! In any case, whether we understand it or not, gravity is a phenomenon that we observe in the universe. It is one of the most fundamental interactions we know, and it appears to contribute to the dynamics of almost everything we know about (with only a very few exceptions). We can describe the effects of gravity in many different ways. For many purposes, it is useful to describe gravity as a force, whose strength depends on a quantity we call mass. This is the "classical" gravity that physicists attribute to Isaac Newton. Einstein and more recent physicists used a different mathematical formulation to avoid the trouble with inexplicable action-at-a-distance, and to resolve some of the more subtle effects of gravity, like the way it interacts with electromagnetic waves. But, whichever formulation of gravity you use to model your observations, very few physicists ever talk about the "density" of a vacuum. And I have never heard any reputable physicist suggest that vacuums "cause" gravity as a side-effect; I can't really even figure out what that would mean. "Vacuum" is just a term we use to describe a region of empty space. It sounds as if you're trying to stretch an analogy of gravity to some type of buoyant force, and you're hypothesizing some type of gravitational phlogiston fluid - but if you carefully study all the implications of that hypothesis, I think you'll find that theory does not correspond to what we observe in nature. You are not alone; many great scientists throughout history have tried to use a hydraulic analogy to describe as-yet unknown effects of heat, electricity, and chemistry; but in each case, these hypotheses have been superseded by more parsimonious explanations. Nimur (talk) 22:26, 14 November 2012 (UTC)

Dammit, Jim, I'm a lawyer not a physicist, but 165.212.189.187 it seems to me that you've got the cause and effect wrong. Vacuum density is a measure of how much matter — gas and other particles — are within a space. Those things are denser near an object because of gravity, or to say it backwards, if it were not for gravity the density would not be higher near the object. Someone who's a real physicist will probably tell me I'm wrong, but that's my first impression. Regards, TransporterMan (TALK) 21:30, 14 November 2012 (UTC)

No offence taken , it's to do with perspectives. Mass, space and time are interlinked but our general default brain setting is one of 17th century mechanics. The proof of Einsteins curvature of space time explained 1) Mercury's weird orbit 2) Displacement of starlight around the sun. Both are shown here : Tests_of_general_relativity ... To show that time is not absolute a 1971 experiment is good fun :check out the "Hafele–Keating experiment" Ap-uk (talk) 23:00, 14 November 2012 (UTC)

• One theory of gravity is that the vacuum exerts a positive pressure, and that the proximity of two bodies blocking that pressure from each other causes them to move together. I am not sure what that theory is called, so I can't get you references, but perhaps someone else here does. μηδείς (talk) 23:59, 14 November 2012 (UTC)
  • You may be referring to Le Sage's theory of gravitation. Richard Feynman says it doesn't work, and that's good enough for me. Someguy1221 (talk) 04:37, 15 November 2012 (UTC)
• Three things. First, gravity exists and can be measured in the absence of vacuum or pressure forces, or even in the presence of opposing forces - a heavy weight dropped in the ocean will sink, even though the water pressure surrounding it in every direction is roughly equal, and that the water pressure increases with depth rather than decreasing. If low pressure (or vacuum) caused gravity's attraction, the weight would more likely float up into the sky. Second, attraction due to vacuum cannot explain how a small, very dense object like a black hole or neutron star can exert a much stronger gravitational effect on larger but less dense objects. This observation also affects the positive pressure theory Medeis mentioned. Third, because pressure in a given area tends to like staying equal, vacuum 'sucks' on an object in space roughly equally in all directions. With equal force pulling the object in every direction, the net force effect is zero, not 'sucked toward the nearest other object' as the question suggested. So in short, no, vacuum cannot explain the observable properties of gravity. – NULL ‹talk›
  ‹edits› 04:32, 15 November 2012 (UTC)

Arent atoms' volume 99% empty space?165.212.189.187 (talk) 16:35, 15 November 2012 (UTC)

@ Someguy and Null. Le Sage's theory of little particles is not the same one I was thinking of, but rather a Casimir effect caused by vacuum energy. The geometry is the same, but in the second theory it is a force, not particles. This is mention in this section of the Le Sage article, and no criticism or refutation is given. Second, the vacuum energy is someowhat of a misnomer. It's supposed to exist every, including in relative vacuums, not just in or because of vacuums. Perhaps the OP means vacuum in a different way from vacuum energy, he'll have to speak for himself. μηδείς (talk) 17:54, 15 November 2012 (UTC)

• Just to be sure ... how does Le Sage's notion of ultra-mundane particles differ from the notion of virtual particles filling all space, e.g. a Dirac sea? If gravity is an exchange of virtual gravitons, is this the same as the blocking of exchange of certain virtual antigravitons which would be Le Sage's particles? Wnt (talk) 17:11, 16 November 2012 (UTC)

A question on economics.

Here in the UK a lot of economics statistics and also general work statistics (ONS) are used by the media who report it as fact.

Many years ago I got a degree in science and struggle to see how accurate these statistics actually are especially with economics. The general trend is to make it all look too difficult to the average person so they don't question things and also there is a lack of a glossary and explanation as to what has been left out or included.

1) Is economics a pseudoscience?

2) Is there some kind of bias in the statistics to make things look slightly less intimidating to the public? I notice for example that with regard to wealth stats the 69% of people who die in debt are left off the table, the wealth stats are then tabulated to take the super rich out (in fact you are in the top percentile with a net worth of only £423,000).

If economics is a pseudoscience why are no top scientists complaining or is it safer (as regards future financial science funding) to just pick easy targets like religious people ;)

--Ap-uk (talk) 21:19, 14 November 2012 (UTC)

1) Economics is a social science. 2) It would really depend on what you are looking at, who compiled the wealth stats you are looking at? They might have reasons to present data in a particular way other then "making it less intimidating".. And of course, nothing stops an economists biasing data if they have an agenda to push. Vespine (talk) 21:38, 14 November 2012 (UTC)

Your answer 1) is open for discussion. I wouldn't call it a science at all, social or not. Unless you are testing and improving your theories, do not call it a science. Science operate on the principle of falsifiability. The most amazing thing about economists is that they don't even care if their theory is wrong or not. It's an ideology pretending to be a science.~On the other hand, they indeed do use, although poorly, scientific tools like statistics, but that's not enough. OsmanRF34 (talk) 23:42, 14 November 2012 (UTC)

This is one of the sillier generalizations I have seen on here in awhile. There are indeed many branches of economics that involves compiling huge datasets and testing theories against them. There are branches of economics that intersect heavily with behavioral psychology and involve running clinical studies. It is a very broad field in terms of methodology. I have never met an economist who didn't care of their theories weren't correct. Saying "economics is an ideology pretending to be a science" is just reflective of your ignorance or prejudice and nothing more. --Mr.98 (talk) 15:44, 17 November 2012 (UTC)

I got the wealth stats from the ONS (Office of National Statistics) however it was not straight forward to find out the answers that I wanted and I had to glean it from several documents even then it was not 100% clear. The ONS did become separate from the government recently however I still see traces of "non-controversial" bias. — Preceding unsigned comment added by Ap-uk (talk • contribs) 21:55, 14 November 2012 (UTC)

This reminds me of the control systems lecturer we had when I was an electrical engineering student. He loved to poke fun at fields that were not like engineering, which is 100% based on fundamentals. He used to say that when an Engineering lecturer writes this year's version of the exam, he changes the questions. When an economics lecturer rewrites the exam for the year, he keeps the questions and changes the answers!

More seriously, economics when compared to things like engineering is a bit like psychology. Both economics and psychology DO have fundamentals, but the subjects are not fully understood and day-to-day issues have to be solved by postulation. Examples of fundamentals are: In psychology - all learning is either clasical conditioning (http://en.wikipedia.org/wiki/Classical_conditioning) or operant conditioning (http://en.wikipedia.org/wiki/Operant_conditioning); in economics, in a closed economy, the total value of money is equal to the total value of goods, services, and property. But can a psychologist predict what sort of responses will be triggered by my post, or can an economist predict how the price of widgets will react to Hurricane Sandy? Well, yes they can, but not by a process solely involving calculation and logical deduction starting with fundamentals. Engineering is quite different. In engineering, all solutions are derived from calculation and logical deduction starting with fundamentals like ohm's law. Then you get climate change science, which is not even as solid as psychology or economics. Wickwack 120.145.143.165 (talk) 01:13, 15 November 2012 (UTC)

• The answer is sort of complicated. When economics is quantitative, it is a science. But because economics is so closely tied to politics, there are aspects of it that are not scientific, as for example the work of Karl Marx and the Austrian School. Looie496 (talk) 03:31, 15 November 2012 (UTC)

The category of pseudoscience may not be as useful as you have been led to think. Classifying something as pseudoscience begs the question whether there is a generally applicable criterion by which to do so, the famous demarcation problem. Certainly no-one would argue that the economy is something beyond scientific study, and certainly most economists are quite earnestly trying to understand, quantify, and describe it. On the other hand, there are many scientists, including economists, who have made scathing critiques of the state of mainstream economics, for example that in economics, the theoretical side and the empirical side exist in fatal separation from one another (in a very interesting paper by an economist, but naturally I forget her name – I think she was from the University of Chicago). This, if true, would make economics bad science but not pseudoscience.

As to official government figures relating to the economy, you do well to take them with a good pinch of salt, since massaging the figures is usually the easiest way to make a problem go away. Inflation can be conveniently suppressed by adjusting the consumer price index, unemployment be kept down by defining people out of the ranks of the unemployed, etc. (I have no idea if and how much this is done in the UK, but would be surprised if it isn’t.) The “official” net worth of an individual person may not reflect their actual wealth with any degree of accuracy, since much of what the very rich own will often be nominally owned by some corporation or otherwise be distanced from them. But if you know how to read the figures correctly, they do provide useful information about real life. If the media report them uncritically, it’s the media’s fault, not the statisticians’.--Rallette (talk) 07:23, 15 November 2012 (UTC)

There are websites who specialise in checking out statistics, especially those quoted by politicians to back up their claims. check out Full Fact and Fact Check. (UK urls given as OP is UK) --TammyMoet (talk) 09:54, 15 November 2012 (UTC)

If there isn't enough information for you on how the official stats are collected, crunched and presented, there are always contact details of ONS staff who can answer your questions Some reasons why the methods might not be what you would regard as optimal include the need to ensure comparability with other countries, comparability over time, cost of collecting data and data protection. Journalists will never take any notice of the caveats that come with the data. Academic researchers should, but they also slip into professional conventions. The national productivity statistics are one of the most striking examples of the phenomenon you identify - they measure something, and they show that one country consistently "does worse" than another, but what they measure doesn't appear to be productivity. Itsmejudith (talk) 02:40, 17 November 2012 (UTC)