Wikipedia:Reference desk/Archives/Science/2009 December 19

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 19

Urban gulls on a cold day...

This is just something I was musing about to myself as I was riding the bus this afternoon and watching the gulls hunkering down on the rooftops and chimneypots, all fluffed up and looking rather cold.

On a cold day, are urban gulls more likely to perch or sit on chimneypots that have gas fires (I don't imagine that a gull would sit on top of one that was belching smoke from a coal/wood fire for long) burning down below in order to take advantage of the heat from the flue gases? It would seem reasonable common sense to suggest that they do this - but does anyone know if there's ever been any research done in this area? --Kurt Shaped Box (talk) 00:05, 19 December 2009 (UTC)

Not gulls, I know, but I have got Chimney Perching for Warmth in Starlings and also Chimney Perching Behaviour in Birds. You will need access to JSTOR or to pay to see the full articles. SpinningSpark 03:04, 19 December 2009 (UTC)

Original research - but: Part of my daily commute is along a gigantic 8 lane freeway with a bunch of electrical cables strung over it. This time of year, we get massive numbers of birds migrating through the area. I've noticed that in the mornings, when the northbound lanes are full of traffic and the southbound are deserted, the birds are all sitting on the wires over the northbound traffic - and in the evenings when the traffic is mostly heading south, they are all sitting over the other lane. Since you'd imagine they'd want to avoid the noise and pollution rising from the traffic - yet they clearly actually SEEK the lanes with the traffic in them - I can only imagine that they are choosing to perch there in order to take advantage of the heat rising from all of those car engines. SteveBaker (talk) 12:32, 19 December 2009 (UTC)

Thanks very much for the answers, guys. Hey Steve, do you think it's possible that the birds could just be keeping an eye out for fresh roadkill or edibles (e.g. crisp packets with crumbs in the bottom, half-eaten service station crap) being thrown from car windows? This would depend on what kinds of birds you have where you are, of course. I can imagine gulls and corvids doing this... --Kurt Shaped Box (talk) 23:01, 20 December 2009 (UTC)

consuming lab products

Say I'm synthesising a product known to be edible. What workup precautions would I have to use, or what things could I measure to ensure that it is reasonably safe to consume in minute quantities? Like I might imagine there might be trace heavy metals, but in so far as I am tasting the product (I intend to use a thiol as part of my starting materials!) I basically want to avoid anything that would make acutely sick. Will recrystallisation do the trick? How do companies do it? John Riemann Soong (talk) 02:58, 19 December 2009 (UTC)

I'm not so sure this sounds like a good idea. In any way. Just a gut feeling. --Jayron32 03:16, 19 December 2009 (UTC)

Jayron is right. Eating in the chem/bio lab is a bad idea. Eating something you synthesized in the chem/bio lab is a really, really bad idea. It is not safe in any quantities. --Dr Dima (talk) 08:34, 19 December 2009 (UTC)

Presumably some food additives are created in labs, so surely there would be a safe way...? Vimescarrot (talk) 11:44, 19 December 2009 (UTC)

Short answer: Don't do it.

Longer answer: I don't know what your situation is, but whether you're in high-school, university, or working in a commercial or government lab, you'll notice fairly prominently located signage telling you not to eat or drink in (or of) the lab. Unless you're in a food lab, you've got no idea what the equipment or reagents you're using have been used for, or used with, and it's quite likely that you could inadvertantly consume something seriously bad for you. So, again, don't do it. --Mattopaedia ^{Have a yarn} 12:00, 19 December 2009 (UTC)

There are really three serious problems:

• There could be trace amounts of some really seriously poisonous stuff on your glassware or other utensils.
• Your ingredients will not be 100% pure - and while contaminants in (for example) a bag of sugar that you buy in the supermarket will have been tested to ensure that they aren't life-threatening, the contaminants in your stock reagents could be almost anything!
• Your procedures will be unlikely to produce a pure result - leaving all sorts of reaction byproducts lying around in your final product. Are you 100% sure that all of those are safe?

In labs that are set up to produce drugs, food and food-additives from chemical feedstocks, all of those things are carefully monitored and the final product may go through a bunch of purification steps and final purity checks before they end up inside a human body. Since you're very unlikely to be able to do that - we have to STRONGLY advise you not to do this. SteveBaker (talk) 12:26, 19 December 2009 (UTC)

Could I feed it to lab mice to see if there are any acute effects? John Riemann Soong (talk) 15:52, 19 December 2009 (UTC)

You can't just test things on animals for the hell of it. There are animal testing regulations, and I doubt frivolously testing chemicals on them to see if you can taste them would pass muster with an Institutional Animal Care and Use Committee. Fences&Windows 16:12, 19 December 2009 (UTC)

You can absolutely not test anything on animals before you got an official permission to do so, and before you have your own test animals to work with. There are no "no man's mice" in the lab. Every mouse is a subject in someone's experiment, and that person went through the whole process of getting the experiment funded, getting it approved by the ethics committee, having the mice purchased / bred / cared for / treated / trained and so on. If you test something on someone's mice, you are interfering with someone's experiment. That is an extremely immoral thing to do. The repercussions are likely to be severe, as well. --Dr Dima (talk) 17:58, 19 December 2009 (UTC)

Well I was actually thinking of using my own mice... John Riemann Soong (talk) 18:12, 19 December 2009 (UTC)

I think the RSPCA (or similar organisation in your country) might be interested in that... --TammyMoet (talk) 19:56, 19 December 2009 (UTC)

Er. No. That's a very very bad idea. Labs can do things to animals that private citizens are prohibited from doing to pets or wild animals. Under Virginia law, "Animal cruelty occurs when a person overrides, overdrives, overloads, tortures, ill-treats, abandons, willfully inflicts inhumane injury or pain not connected with bona fide scientific or medical experimentation, or cruelly or unnecessarily beats, maims, mutilates, or kills any animal. It also occurs when a person deprives any animal of necessary food, drink, shelter or emergency veterinary treatment. These activities constitute a Class 6 felony if the current violation or any previous violation of this subsection or subsection A resulted in the death of an animal or the euthanasia of the animal."[1] Fences&Windows 21:40, 20 December 2009 (UTC)

In a sense, cooking is a form of chemistry. (In particular, you might want to take a look at molecular gastronomy.) It's just chemistry done with food-grade reagents, using methods that are know not to (usually) produce toxic byproducts. The advice given by SteveBaker and others above is generally sound: don't do your cooking with items and materials from the chemistry lab, for the same reasons you wouldn't do it with things found in the toilet or in the garage. You just don't what's in there, and it's not meant to be eaten.

That said, if you want to produce edible results with chemistry, the basic rule is to treat it as cooking, not as lab work. So start with food-grade ingredients and only use dishes and utensils designated for cooking. If you want to cook your food in a graduated Pyrex flask, you're welcome to do so — but use a flask that's only been used for cooking edible things, not one that someone else might've used earlier to brew god knows what mix of toxic chemicals.

I actually remember a class project in high school chemistry where we did produce an edible end product: ammonium chloride. It was in fact done in the chemistry lab, but we used a specific set of reagents and laboratory ware set aside for that particular project, not the haphazardly washed ones that we'd normally use. I should also note that ammonium chloride, though indeed used as a flavoring, isn't really something one could or should consume in large quantities; the amount we produced was barely more than a pinch, just enough to taste and observe that, yes, it does taste like salty liquorice. —Ilmari Karonen (talk) 17:41, 23 December 2009 (UTC)

Ether

Why did they name the group of chemicals after the classical element? --75.15.162.235 (talk) 04:22, 19 December 2009 (UTC)

Not sure, but often, the classical element is spelled aether while the organic functional group is spelled ether. That is, they may not be related. Or they may. Just sayin'... --Jayron32 05:28, 19 December 2009 (UTC)

Diethyl ether explains that the name ether was coined (for the class) in 1730 by August Siegmund Frobenius. It is uncited, and does not explain why that name was chosen. --Jayron32 05:30, 19 December 2009 (UTC)

The name was used for the substance first in Latin as aether, which was derived from Greek word aither, from aithein which means to burn: (from Collins Dictionary). This root also gave rise to ethyl- ethane, ethereal Ethernet. presumably the name was chosen because ether could burn easily. Graeme Bartlett (talk) 11:43, 19 December 2009 (UTC)

Etymonline says "for its lightness and lack of color". It also gives 1757 as the date it was named. 213.122.6.175 (talk) 12:00, 19 December 2009 (UTC)

Quantum Field Theory

What are some current or future applications of QFT? 74.14.108.212 (talk) 06:34, 19 December 2009 (UTC)

Homework? SpinningSpark 12:30, 19 December 2009 (UTC)

Why such eagerness to assume that the question is homework? I thought the policy here is to assume good faith. QFT is an advanced and very technical subject. A question about the applications of QFT is not the same as one asking about the everyday applications of electricity. As a non-physicist with a general interest in science, I too would like to know learn something about the applications of QFT, if someone can explain it meaningfully in simple terms. I've taken a quick look at the Wikipedia article on quantum field theory, which is very technical, and I'm not sure if the OP's question is answered there. If you want to be helpful, how about a few pointers to good intro materials on the subject understandable by non-experts? --173.49.9.184 (talk) 19:52, 19 December 2009 (UTC)

Why? Because it's written in a style typical of a school homework question, and asked in a manner typical of someone coming to the refdesk in the hopes that someone can do their homework for them. Maybe it's not homework, but basing conclusions on adherence to repeated patterns isn't particularly bad faith. If it's a curiosity question, for example, people generally go on to explain why they're curious (in the hopes that someone will understand their reasons and thus offer up a better answer). You did exactly that, by expanding and explaining why the obvious sources you linked to were no good to you. KageTora - (影虎) (Talk?) 20:35, 19 December 2009 (UTC) (originally by Vimescarrot (talk but he was too shy to post this))

For the OP: you may want to check this out. Not sure if it's what you are looking for, but anyway. --KageTora - (影虎) (Talk?) 21:18, 19 December 2009 (UTC)

Solar System and Orbits

Why does it seem like most objects in the solar system orbit the Sun along the same plane? While I realize that some have tilted orbits, traditional representations of the solar system always seem to put all the planets and asteroid belt within the same plane. Why isn't there any objects that orbit perpendicular to the orbit of the Earth? 24.125.65.172 (talk) 07:46, 19 December 2009 (UTC)

The solar system formed from a rotating disk of gas. See Formation and evolution of the Solar System and Protoplanetary disk. In addition to the plane of the ecliptic we do have the Oort cloud which is spherical. 88.112.56.9 (talk) 09:39, 19 December 2009 (UTC)

There are some objects that are inclined well off the invariable plane such as 4 Vesta at 35° and 1566 Icarus at 23°, but as you point out, the majority of the mass of the solar sytem is orbiting in one plane. The collapsing nebula which formed the solar system started off with objects moving in many random directions, but if all the angular motions (L) are added together there will be a net L in some definite direction. This ultimately becomes the preferred direction of rotation. As the cloud collapses L must be conserved according to the laws of motion, but a smaller orbit means higher velocities. Objects not orbiting in the preferred L direction are gradually eliminated by collisions or expulsion from the solar system altogether. The Oort cloud does not show this property because it represents "left-over" material that did not take part in the original collapse because it is too far out. SpinningSpark 11:31, 19 December 2009 (UTC)

There is also an inherent stability to flat disks. It's the same mechanism that keeps Saturns rings flat. If an object strays off at some angle ("upwards" say) - the gravity fields of all of the other objects will sum to produce a net downwards force pulling it back into line. However, the gravity fields of those distant planets are pretty small at those distances - so recently captured objects like Pluto may take a very long time to get gently nudged into that same plane. SteveBaker (talk) 12:16, 19 December 2009 (UTC

The majority of the mass of the solar system (excluding the Sun) is orbiting in precisely one orbit - that of Jupiter. Jupiter makes up about 66% of the Solar System, if the Sun is excluded. All the planets together make up over 99% of the Solar System by mass. Asteroids, comets, Kuiper Belt objects and Oort cloud objects are trivial contributions in terms of mass. --Tango (talk) 18:51, 19 December 2009 (UTC)

Effects of moving from hot water to cold water on vision

Today I went to some hot springs. I stayed in a hot pool, one that was painful to get in to and one that most only stayed in for a couple minutes. After that I moved to a very cold pool. It was relaxing at first, but then I lost feeling in most of my extremities. I got out of the pool, and then everything in my field of vision kept spinning upwards (not at all what I normally see when I'm light-headed or faint.) I was fine a few minutes later, but I'm curious as to why my vision was affected like that.Sjmcfarland (talk) 12:19, 19 December 2009 (UTC)

This question appears to be a request for medical advice. It is against our guidelines to provide medical advice. You might like to clarify your question. Thank you. Responses containing prescriptive information or medical advice should be removed and an explanatory note posted on the discussion page. If you feel a response has been removed in error, please discuss it before restoring it.

Sorry. SteveBaker (talk) 13:55, 19 December 2009 (UTC)

The ref desk is allowed to answer medical questions. Medicine is after all an important scientific discipline. What we are not allowed to offer are diagnosis. Just because the questioner has framed his inquiry with a first person anecdote does not make it a request for medical advice. If the effect to which he refers is known, then I'm sure the OP would like to know all the medical information that the public domain has to offer. —Preceding unsigned comment added by 92.23.58.199 (talk • contribs)

The questioner describes symptoms in the first person and asks for a diagnosis. According to Wikipedia:Reference desk/Guidelines/Medical advice this does make it a medical advice question. Gandalf61 (talk) 16:29, 19 December 2009 (UTC)

The OP is describing an effect and asks for an explanation. It is possible that this can be completely explained without offering a diagnosis. (eg if I posted, "I pressed my hand to my chest and felt a beating, what is this", that would nto be a request for medical advice, and the answer, "that is your heart" would not be a diagnosis). —Preceding unsigned comment added by 92.23.58.199 (talk) 17:11, 19 December 2009 (UTC)

This discussion should not be conducted on the RefDesk proper. That's what the Talk page is for (and you'll see I've linked to the appropriate section, which you've just created). -- Scray (talk) 17:26, 19 December 2009 (UTC)

Hmmm... I did describe the events in the first person because they happened to me, but I'm not interested in a diagnosis at all. I was simply interested in the phenomenon. I experience Orthostatic hypotension quite often as I have low blood pressure, but never has it been anything like what I experienced then. I was simply curious what could possibly cause one's range of vision to spin laterally (which is what I experienced) instead of horizontally (which is what one normally feels if dizzy.) My apologies for the ambiguity. Sjmcfarland (talk) 17:31, 19 December 2009 (UTC)

You experienced a symptom and you're asking for a diagnosis - and we're simply not allowed to do that. SteveBaker (talk) 17:57, 19 December 2009 (UTC)

Elephants

Why are elephants afraid of mice? All I found in the archives was : this --Drogonov 13:10, 19 December 2009 (UTC)

Snopes.com has this to say. Gabbe (talk) 13:40, 19 December 2009 (UTC)

For once - I'm not sure Snopes is right. Their page was last updated in 2007 and offers none of their usual incontrovertable evidence. Their references are mostly newspaper articles and "wacky but true" types of sources - which (as we all know) are really about as effective at spreading myths as quelling them! Since then, the Mythbusters did (vividly!) demonstrate that wild elephants are indeed amazingly wary/cautious around mice - although "afraid" might be an overstatement. There is definitely something behind this old story. However, it's really impossible to get inside the mind of another animal (or another human for that matter) - so understanding the "why" of it is almost impossible. SteveBaker (talk) 13:47, 19 December 2009 (UTC)

Epiblast and primitive endoderm (Developmental biology)

When do these two tissues arise in embryo development? Maybe at late blastula? --82.57.143.146 (talk) 15:22, 19 December 2009 (UTC)

The epiblast article is quite short, but the table at the bottom charts out what happens during different weeks of (human) embryogenesis. The blastula stage is earlier and quite distinct from the blastocyst stage. Basically, to answer your question, after formation of the inner cell mass the cells organize into two layers called the epiblast and hypoblast. You could also have a look at human embryogenesis and specifically human embryogenesis#Inner cell mass differentiation. --- Medical geneticist (talk) 19:33, 19 December 2009 (UTC)

• Ok, thanks; so the split is a little bit later than I thought. --87.18.126.231 (talk) 10:43, 20 December 2009 (UTC)

Downforce

I'm studying downforce for my investigation in advanced higher physics and i required an equation to calculate downforce. I found an equation on this page : http://en.wikipedia.org/wiki/Downforce The equation near the top of the page is the one i have a question about. I cannot see how this equation would work if the angle of attack of the way was zero degrees. The equation would be rendered useless if the angle was at zero. If i'm missing something obvious, could someone kindly please explain this to me? —Preceding unsigned comment added by Michaeljohnston92 (talk • contribs) 15:29, 19 December 2009 (UTC)

Is there some reason for thinking that the downforce will not be zero at zero angle, as predicted by the equation? The real aerodynamics are considerably more complex than is suggested by that equation, and it should be treated as a first approximation. Dbfirs 17:24, 19 December 2009 (UTC)

That equation is only a very rough approximation. A wing with a symmetrical cross-section certainly doesn't generate any downforce when the angle of attack is zero. (Think about it: If everything is symmetrical - how would the wing "know" which way to generate the force!?!) But that is not true for an asymmetric wing. Downforce is just the same thing as lift - but directed downwards. The same equations that determine what makes an airplane fly - or a ships' propeller push it through the water are applicable to calculating the downforce from a spoiler on a car...except in the case of the car, the force is directed downwards - in an airplane it's upwards and on the ship, it's forwards. The equations in Lift_(force)#Methods_to_determine_lift_on_an_airfoil are more credible than the one in the downforce article - but none of them are going to be perfect because subtle differences in the shape of the airfoil cross-section matter a lot - and (especially in the case of a car) the direction and turbulance of the airflow as it arrives at the airfoil matter a lot. So if you're designing a spoiler for a car - the equations should be regarded as a first approximation step and to know for sure whether your design is as effective as you need, you either need to do scale experiments with models in a wind-tunnel or use mathematical models in a virtual wind tunnel on a computer - or you need to try a range of different designs on the actual car. SteveBaker (talk) 17:53, 19 December 2009 (UTC)

So-called Downforce should enable a car to be driven upside-down through a tunnel. Cuddlyable3 (talk) 00:25, 23 December 2009 (UTC)

General relativity

"Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime"

How or why does matter curve space-time? —Preceding unsigned comment added by 59.93.11.222 (talk) 15:50, 19 December 2009 (UTC)

To get to the other side. - Nunh-huh 01:29, 20 December 2009 (UTC)

Thats not really what science concerns it self with. Its just that assuming that the object 'spacetime' exists, and that it is curved by mass produces a gravitational model which is extremely accurate, and until we have a better model, we cannot dismiss it. —Preceding unsigned comment added by 92.23.58.199 (talk) 17:08, 19 December 2009 (UTC)

I disagree the idea that there are things that science does not concern itself with. There are certainly things we don't yet know the answers to - and this is one of them. If we could figure out a way to answer this question - then scientists would be only too happy to get concerned about it. But right now, all we know is that this is how things are. The concept of 'mass' and how this interacts with 'space' is as yet, not well understood. It's thought that if the LHC can find the Higgs boson then we may gain some understanding of how 'mass' works - and who knows what that may tell us? SteveBaker (talk) 17:37, 19 December 2009 (UTC)

Any theory which explains how spacetime acts in the way it does is equally valid as long as it still posits that it acts in the way we know it to. A question to which any answer is equally valid, is for all intents an purposes in itself invalid. Spacetime is completely described by GR, any questions that can be asked about it that GR cannot answer are essentially invalid.

All true to an extent, but having an explanation for something often allows us to improve and extend our predictions. This is why theories are preferred to empirical laws. Kepler's laws of planetary motion are great for working out where a planet will be in the night sky, but useless for pretty much anything else. Newton's universal theory of gravity allows us to predict all kinds of things since it explains the reason for Kepler's laws and that reason applies to far more than just planetary orbits. In this case, an explanation for how mass distorts spacetime could, for example, all us to extend GR to very small scales and we would have a theory of quantum gravity just by simple derivation from that explanation. --Tango (talk) 01:06, 20 December 2009 (UTC)

Einstein's approach was axiomatic—find some basic principle and see if adhering to it clarifies nature's behavior for you. For general relativity, this works pretty well—and is something of an improvement over Newton's "mass sends out a force called gravity." But neither really tell you why or how. As SteveBaker points out, this is a perfectly valid question for science, but we don't have a great explanation at this point. --Mr.98 (talk) 18:30, 19 December 2009 (UTC)

You can always ask why to any answer, luckily science knows not to play infantile games. Physics mathematically describes objects behaviour, the rest is philosopy and stamp collecting.

All science used to be philosophy until about 1725. "Why" questions created science. 213.122.60.32 (talk) 01:17, 20 December 2009 (UTC)

I don't think all science has been philosophy since antiquity. We have had mathematical science for a long time. Geometrical optics for example is well over a thousand years old. On your second point, it may be "why" questions that make science, but it does not always offer answers, more it offers predictions. To me asking why do masses effect spacetime is as good as asking why is c a constant, or why is energy conserved; it just is, because our best models recquire it, and all the evidence suggests it. —Preceding unsigned comment added by 129.67.116.96 (talk) 23:19, 20 December 2009 (UTC)

In a way, Einstein's theory provides a mathematical description of observed phenomena. By asking the "how or why" question, you are asking for an explanation. But what exactly is the nature of a scientific explanation? I submit that a scientific explanation reduces the phenomenon being explained to more familiar, fundamental phenomena. In other words, it shows how a seemingly unfamiliar phenomenon is merely the manifestation of more familiar, fundamental ones. However, this process of reduction cannot be continued indefinitely. At some point, you have to stop with behavioral descriptions that are accepted as fundamental, and stop demanding that those fundamental behaviors be explained in terms or even more fundamental ones—there may not be such explanations to be found. That mass distorts spacetime might be such a fundamental phenomenon. --173.49.9.184 (talk) 05:37, 20 December 2009 (UTC)

I agree that there is a hierarchy of explanations, and at any given time one or more of them must necessarily be the most fundamental we have. And it is not at all an infirmity for a theory to be the currently-most-fundamental in its domain. But it does not follow that "you have to stop" trying for even more fundamental explanations. It is not given that we will find any, but the moment we stop trying, science is dead. –Henning Makholm (talk) 05:56, 20 December 2009 (UTC)

Wormholes and Time Travel

Could a wormhole theoretical provide time travel to a point in the past before the wormholes creation? I know it is theoretically possible to make a wormhole to another point and then slow the passage of time for one of the mouths so that one is in the present and the other in the future (for example: If Gracey and Bob create a wormhole and Gracey stays on earth with her end while Bob takes his into space and flies around at extremely high speeds for say a day, time will slow down for Bob (due to general reletivity). If Bob were to then land on earth after a day of fling around, he would seem to be in the future because time slowed down for him but not the rest of the universe. Lets say as soon as Bob was done with his trip and landed safely on earth he sent Gracey a signal. If gracey on earth waited until she recieved Bob's signal, then hopped through the wormhole she would find her self in the future). But my question is is there a way to open a wormhole to the past, not the future? --74.67.89.61 (talk) 23:56, 19 December 2009 (UTC)Connor Goham —Preceding unsigned comment added by 74.67.89.61 (talk) 20:41, 19 December 2009 (UTC)

Not within GR. I'm not sure if it has been proven absolutely impossible, but I don't believe any method has been found for travelling back to before a time machine was created that is consistent with GR, wormholes are no exception. --Tango (talk) 21:15, 19 December 2009 (UTC)

From a geometry of space-time perspective there is no theoretical reason why a new wormhole formed today (from the perspective of a person at one end) couldn't happen to have an exit in what that person perceived as the past. However, since we have no idea how to construct a wormhole, or even if it is ever theoretically possible to construct a stable wormhole, I'd say the real answer is that no one knows. It's entirely possible that no actually realizable physical process will ever create a useful wormhole and that they are simply a mathematical curiosity of how we have come to describe space-time. Dragons flight (talk) 21:22, 19 December 2009 (UTC)

Backing up yet one more step - we don't actually know that Wormholes exist - or even can exist. So this is an all round "NO!". Of course future science may change that - but we can only talk about what we know. SteveBaker (talk) 01:01, 20 December 2009 (UTC)

The OP clearly means us to assume wormholes can exist (which is essentially equivalent to assuming there is matter with negative mass, I believe) - the question is nonsense otherwise. Therefore, I don't think our lack of knowledge about the existence or not of wormholes has any bearing on the answer to this question. --Tango (talk) 01:09, 20 December 2009 (UTC)

In that case, we're in science fiction territory. I think the answer must be, agreeing with Dragons flight, that there is no hard GR reason why a past-pointing wormhole absolutely cannot exist. Even if wormholes were created connecting present-to-present (corresponding events at the two ends have spacelike separation, seen from outside), taking one end of the wormhole on a long relativistic journey (see Twin paradox) would transform it into a past-pointing one (with timelike separation). So if Connor wants to write a story that features a past-pointing wormhole, that should be within his poetic license.

On the other hand, some physicists argue that if a wormhole ever ends up an a past-pointing configuration, it would be possible for photons or other particles to come out of the "early" end with just the right velocity to eventually end up hitting the "late" end and becoming themselves from an earlier round. And (if I understand this correctly) quantum fluctuations would necessarily create an unbounded number of such self-causing particles, until their collective gravity makes the wormhole collapse, with rather ill-understood but potentially spectacular effects. Note that this is a speculative theory, not a universally accepted argument.

So, based on current knowledge, it would also be within Connor's poetic license to write a story in which past-pointing wormholes are impossible, if that suits his plot needs better. In such a world, if you try to make a past-pointing wormhole by the twin-paradox trick, it will (for example) explode once the two ends enter each other's light cones. The complexities of preventing this from happening through a cascade of wormholes each of which is present-to-present (essentially Einstein's telegraph-into-the-past configuration) would be considerable, which can either be tacitly handwaved away, or used as plot fuel to justify a tight bureaucracy that regulates wormhole kinematics around the galaxy... –Henning Makholm (talk) 06:38, 20 December 2009 (UTC)

Junk food

What's the evolutionary advantage to thinking that junk food (which is very unhealthy) tastes good, but healthy food, which is much better, tastes bad? --70.247.248.43 (talk) 23:41, 19 December 2009 (UTC)

Once upon a time, things like salt, fats, and sugars were scarce but necessary in our diets. So it's an evolutionary advantage to crave foods containing those, to induce us to go out of our way to obtain and eat them. But now that we've got an industrialized processed-food industry, it's to their profit advantage to indulge our cravings. —Steve Summit (talk) 23:49, 19 December 2009 (UTC)

Healthy food doesn't taste bad. Bad tasting healthy food tastes bad. There is plenty of good tasting healthy food out there if you look for it. Therefore, I don't think it is an evolutionary thing, it's a social thing - for some reason people have got it into their heads that they don't like healthy food and then food manufacturers make bad tasting healthy food because otherwise people won't believe it is healthy, thus reinforcing people's misconceptions. --Tango (talk) 23:50, 19 December 2009 (UTC)

I think it's more because people want straight-across replacements for unhealthy foods. So they wind up with pale imitations. (Like how there are plenty of tasty vegetarian foods, but direct meat-replacements like veggie-bacon are always terrible compared to the genuine article.)

There is none. Junk-food exists as a function of your diet - all food is 'good', it is just that differing foods are required in differing quantities. Your Diet is what is important, not an individual item of food (ignoring allergy etc.). ny156uk (talk) 23:51, 19 December 2009 (UTC)

(after Edit/Conflict)

Partial Answer : Things like sugar and salt are vital for our well-being. So it's no surprise that we crave them. However, now we have the technology to concentrate them like never before. We evolved in a time when sweet food items took some effort to find, we have not adapted to our current environment where you can just go guy a pound of sugar for a few bucks.

(It doesn't help that we mostly don't exercise as much as our 'wild' ancestors.) APL (talk) 23:57, 19 December 2009 (UTC)

Sugar is not required by humans, and salt is only required in very much lower amounts than the typical western diet gives. I'm not sure if salt is required at all. Sweet tasting things have more energy, hence a preferance for then evolved. 89.242.211.123 (talk) 22:05, 20 December 2009 (UTC)

Yes, salt is required. See salt. It really, truly is required. Absolutely required. Animal life needs salt to live. Earth animals die if they have no salt. Sodium chloride is necessary to animal life on Earth. Too much is toxic, but so is not enough. This easily researched fact is one of many found in this handy online encyclopedia I have just discovered, not too far from this very page. Oh, the wonders of technology. 86.176.191.243 (talk) 23:51, 20 December 2009 (UTC)

Note that the UK recommended amount of 4g of salt per day is given as an achievable target - the actual optimum amount is less. I would think that the low level of salt required is going to be found naturally in food, without any needing to be added. 89.242.211.123 (talk) 00:05, 21 December 2009 (UTC)

Yes, but that's a world away from it not being required at all. It will be found naturally in some food, particularly food in marine areas. If you exclusively eat things like lettuce, potatoes, tomatoes, with no added salt, you will have a salt deficiency. The UK reference nutrient intake (RNI) is 4g of salt a day, but our article seems to have misunderstood the source given. The achievable population target is the 6g of salt a day set as a target maximum for adults, which is the amount they advertise as a target. The 4g is the actual recommended amount, based on an average adult.

The Scientific Advisory Committee is looking out for public health on a population scale. As they say (you really should look up the source this article uses, it's very readable)

"There is no agreed definition or clinical criteria to identify

salt sensitivity. The greatest benefits are likely to be

achieved by taking a population approach to reducing salt

intakes, rather than through individual targeted advice."

This means, the target of 6g of salt a day is aimed at reducing the intake of the whole population so that the few individuals who are sensitive to high levels of salt in their diets do not develop problems which then have to be treated. This is exactly the sort of thing the Committee is supposed to be doing, but does not mean more than 6g of salt a day is harmful to most people. And the 4g a day is not a population target: it is the Reference Nutrient Intake, a very different thing. I would edit the article, but it's protected. 86.176.191.243 (talk) 02:32, 21 December 2009 (UTC)

We have not evolved in contact with junk food. Feasibly some early humans may have had high-fat diets available, but they also seem likely to have been forced by circumstances into lots of exercise. Even if they were somehow able to get very fat, this is only unhealthy in the sense that it lowers modern life expectancy, killing you, say, of heart disease at age 50. If our ancestors, fat or not, tended to die before that age anyway of other causes (other diseases, weather, being gored by something) then that type of unhealthiness would be evolutionarily irrelevant. 213.122.60.32 (talk) 00:13, 20 December 2009 (UTC)

I think living to 50 was fairly common for hunter-gatherers (which are the only group of humans worth considering for evolutionary purposes, really - very little has happened since the beginnings of farming. There's lactose tolerance and alcohol tolerance, but that's about it.). Population density wasn't high enough for disease to spread much and there were no domestic animals to catch diseases off (diseases that have been in humans for a long time evolve to be fairly minor, otherwise their host dies before it can spread much), they were perfectly capable of building shelters appropriate to their environment (if they couldn't, the people that went there either died or went back to the nice place they had started off in) and they knew better than to stand in front of a charging boar. Life expectancy at birth was far lower than it is now due to infant mortality, but if you reached adulthood you could easily reach what we now called middle-age. Once we started farming, disease became a bigger issue and life expectancy dropped, but you could still reach a decent age if you survived to adulthood. --Tango (talk) 00:26, 20 December 2009 (UTC)

Yep, all true. I just wanted to wheel out the idea of an effect being evolutionarily irrelevant, and trundle it around a bit, because it's one of my favourites. Oh, wait, it can still be valid if we did all or nearly all our mating before age 50. No real reason to suppose that either, and the lack of hamburgers in the stone age is probably the real reason, but, you know. It was a thought. 213.122.60.32 (talk) 00:49, 20 December 2009 (UTC)

Good point. There is plenty of reason to suppose we used to mostly mate before age 50 - even now it is very unusual to have children older (especially for women). The ability to care for children and grandchildren does drive evolution a little for traits that are only relevant after child-rearing age, but it is less significant. --Tango (talk) 00:59, 20 December 2009 (UTC)

I think this question puts the cart before the horse. We don't like junk food just as a result of the fact that it is bad for you. Junk food is very carefully designed to be as "tasty" as possible at the same time as being quick, cheap and easy to mass produce with very high consistency. "healthy" was not a factor when junk food was first designed, so it was not included in the requirements, the result is unhealthy food. That has started to change, people are becoming much more health conscious these days and as a result fast food companies have had to start considering that, for example you can now buy a salad at McDonalds. Vespine (talk) 21:45, 20 December 2009 (UTC)

Absolute Zero and Special Particles

Is absolute zero really impossible? According to special reltivity everything travels at the speed of light through time and space combined, the faster you travel through space the slower time passes for you, but if you were to add up your speed through space and through time it would be equal to the speed of light. It requires massleses particles to travel at the speed of light through space (they don't travel through time) trying to accelerate a particle with mass to the speed of light is a futile attempt. Wouldn't the same be true for absolute zero, that a particle at absolute zero would travel at the speed of light through time but not move at all through space, and that only certain particles can exist at absolute zero the same way only certain particles can travel at the speed of light. Any attempt to decelerate a particle such as a proton to absolute zero would also be futile. This explains why we have not been able to decelerate known particles to absolute zero, because only certain particles can exist at absolute zero (perhaps their key characteristic would be that they have infinite mass (the opposite of particles that travel at the speed of light through space which have no mass)). This might also explain why we haven't detected any particles that exsist at absolute zero, they have no momentum through the spacial dimensions so they wouldn't react with any experiments in spacial dimensions. So is absolute zero really an impossible state? Maybe only specific particles can exist in such a state (Like how only massless particles can travel at the speed of light through space) and because they have no spacial energy, this would provide a natural explaination to why we have never detected such particles with current experiments. --74.67.89.61 (talk) 23:54, 19 December 2009 (UTC)Connor Goham —Preceding unsigned comment added by 74.67.89.61 (talk) 23:45, 19 December 2009 (UTC)

It's special relativity, not quantum mechanics, that talks about the norm of 4-velocity always being the speed of light. I don't know of any reason to believe there is an analogy between the speed of light and absolute zero. --Tango (talk) 23:56, 19 December 2009 (UTC)

I think Connor may have been confused by simplified presentations of the kinematic theory of heat. Since "absolute zero" then means that each particle has a velocity of zero, he thinks that the trouble of achieving it is that particles do not want to be at rest. And that is somewhat true (due to uncertainty), but it is not the fault of the specific kind of particle. First, relativity teaches us that "at rest" is not an absolute concept, so it makes no sense for any particle to refuse to rest. Second, looking at a single particle is not enough. The motion that is relevant for heat, is the motion relative to the macroscopic velocity of the matter, so the problem is not getting one particle to v=0; it is getting an entire ensemble of particles to have exactly the same velocity (not necessarily zero).

I believe the uncertainty principle gets into play to prevent absolute zero from being achieved, but I'm not good enough at thermodynamics to make any absolute pronouncements about it. –Henning Makholm (talk) 07:09, 20 December 2009 (UTC)