Wikipedia:Reference desk/Archives/Science/2010 August 10

= August 10 =

What's the prospect of scientists creating enzymes that can treat prion diseases?
Based on what we know about prion diseases, what's the prospect that someday scientists will discover or engineer enzymes that can be used to safely break down amyloid plaques? --71.185.169.212 (talk) 01:50, 10 August 2010 (UTC)
 * I can't see any reason why it couldn't be done. They're just proteins and there are plenty of enzymes that break down proteins. I don't know enough about the subject to say how difficult it would, though. If scientists can find people with a genetic immunity that works by them producing a particular enzyme, then they might be able to track down the relevant gene, transplant it into a bacteria and then extract large amounts of the enzyme and administer it to patients, but no part of that process would be particularly easy. --Tango (talk) 03:10, 10 August 2010 (UTC)
 * Amyloid plaques consist of fragments of a larger protein, that build up outside of neurons. The key problems are those of drug delivery, as getting large molecules into the brain is extremely difficult, and selectivity: how to find a protein that digests amyloid plaques while leaving healthy proteins unaffected.  I don't know for sure, but the amyloid plaques may be as much a symptom as a cause of the disease -- the breakdown of their parent protein may be the actual cause.  Alzheimer's is also characterized by aggregation of an intracellular protein involved in maintaining the neuronal cytoskeleton, which would be even harder to target as it's intracellular. --Atemperman (talk) 05:25, 10 August 2010 (UTC)
 * Some care should be taken here with nomenclature. The prion protein PrP forms the amyloid plaques that appear in the transmissible spongiform encephalopathies ('mad cow disease' in cattle, or Creutzfeldt-Jakob disease in humans) &mdash; but not all amyloid plaques are formed from prions.  Indeed, when neurologists discuss amyloid plaques, the first thing that comes to mind is usually the amyloid beta peptide associated with Alzheimer's disease.  (It is in this sense of the term that Atemperman's response above answers the question.)  Amyloid beta does form amyloid plaques, but it is not believed to be a prion &mdash; amyloid just describes protein accumulations with a cross-beta structure and which exhibit certain histopathological features (like apple-green birefringence when stained with Congo red).  It's also worth bearing in mind that PrP is not the only prion protein (several others have been identified in yeast, though PrP is so far the only known mammalian protein which can form a prion).
 * I'm going to assume that you're interested in the breakdown of PrP amyloid for the remainder of my response here. Developing a suitable protease requires meeting three major challenges.  The first is that the protease even be able to digest PrP amyloid.  This is a steep challenge all by itself; one of the hallmarks of PrP amyloid (and indeed, of most amyloids) is their resistance to protease digestion.  PrP amyloid is resistant to detergent solubilization and even to exposure to proteinase K, which is able to digest normally-very-durable proteins like keratin (in hair).  Nevertheless, there has been some work in this area; this paper describes some very aggressive enzymes secreted by thermophilic bacteria that might be able to do the job.  (I'm not familiar with the literature in this area; that was just one of the first Google hits with the relevant keywords.)
 * The second challenge is one of specificity. Letting large amounts of an aggressive protease loose in human tissue is a recipe for disaster, unless it can be specifically targeted to PrP (ideally, just to misfolded PrP).  The third challenge is delivery &mdash; Atemperman's response above describes the problem.  TenOfAllTrades(talk) 18:51, 10 August 2010 (UTC)

Why did they change the iphone's antenna?
I don't recall any problems with the old one. What was the purpose of the change? 148.168.127.10 (talk) 13:24, 10 August 2010 (UTC)
 * They probably keep mum about this. But essentially, they wanted a new design (and may have needed more space or a different layout to realize the enhanced functionality). And from a technical point of view, reusing the structural elements of the frame as an antenna is exactly one of those very elegant ideas you fall in love with that unexpectedly turn around and bite you... --Stephan Schulz (talk) 13:37, 10 August 2010 (UTC)


 * Apple claims, and several independent tests have supported, that the change improves reception in low-coverage areas -- provided that you don't hold the phone a certain way.  My personal guess is that Apple's internal testing just didn't uncover the flaw: they've got an AT&T tower on their campus, so local testing isn't bothered.  The phone that leaked in the field was enclosed in a case (which fixes the problem) -- so if that's typical, remote testing also wouldn't be bothered by the antenna placement issue.  But that part is just guesswork. &mdash; Lomn 13:41, 10 August 2010 (UTC)


 * Ironically, the new antenna was designed to improve the connection. My own personal experience has been that the iPhone 4 is indeed better than previous models as long as you don't hold it with the Death Grip.  A Quest For Knowledge (talk) 13:47, 10 August 2010 (UTC)


 * In addition to the suggestions above, relocating the antenna to the outside of the case frees up room inside. This allows more internal space for additional hardware or a larger battery.--Zerozal (talk) 14:08, 10 August 2010 (UTC)

watch what they do after September 30th. I bet by then they'll have sold all the ones produced with revision 1, and be selling the revision 2: what is that change? Perhaps nothing more than a resinous coating... 92.230.233.247 (talk) 14:45, 10 August 2010 (UTC)

is this text to speech or just a nerdy guy?
http://grail.cs.washington.edu/projects/videoenhancement/ I can't tell... —Preceding unsigned comment added by 92.230.233.247 (talk) 13:55, 10 August 2010 (UTC)
 * Sounds like a person to me, and his voice seems totally normal to me. Ariel. (talk) 16:50, 10 August 2010 (UTC)


 * Agreed. He enunciates a little funny, and doesn't modulate much, but it's probably because he's trying to be easy to understand (he's not speaking conversationally). He's no voice actor but he does fine by academic standards. --Mr.98 (talk) 00:03, 11 August 2010 (UTC)
 * Cool research. Definitely a real person.  A text-to-speech program wouldn't have gotten the rhythm of spoken language correct.  For a particularly striking example, consider the phrase around 4:47: "The resulting mat # is consistent with the occlusions in the scene."  The speaker puts a big pause at the "#" to let the listener focus in preparation for the coming big concept.  There's no punctuation there; it takes intelligence to decide to put it in.  Paul (Stansifer) 03:55, 11 August 2010 (UTC)
 * I think the speaker is using speed-up voice processing to limit the length of the video. Cuddlyable3 (talk) 10:13, 11 August 2010 (UTC)


 * yes! that must be it.  thank you. the actual audio does sound computationally produced (as opposed to spoken as we hear it), because it is, but is a real person, because it is. 92.230.232.58 (talk) 11:29, 11 August 2010 (UTC)


 * Eh, I don't buy it. Sounds like a regular voice to me. Perhaps not one that you yourself hear around often, but around universities and science departments, it is not uncommon. --Mr.98 (talk) 13:02, 11 August 2010 (UTC)

Frogs!
Is this a leopard frog or a pickerel frog? --Chemicalinterest (talk) 13:58, 10 August 2010 (UTC)
 * The striping would lead me to think it is a leopard frog as I don't think pickerel frogs have such stripes. Googlemeister (talk) 14:13, 10 August 2010 (UTC)
 * This picture of a pickerel frog (from Frogs in New Jersey) is what started my question. --Chemicalinterest (talk) 14:34, 10 August 2010 (UTC)
 * According to the pickerel frog article, a pickerel frog is a type of leopard frog. (It doesn't say so explicitly, but it has a sentence that reads "All other leopard frogs ...".) Looie496 (talk) 22:43, 10 August 2010 (UTC)

Cycle helmets - do they reduce vision and impair hearing?
Hi all, thanks for reading.

A the title asks really, do cycle helmets impair peripheral vision and hearing, leading to greater likelihood of being involved in an accident?

I've been ot the CTC site ( http://www.ctc.org.uk/ ), and looked up some reports on helmet safety through google scholar, but they all seem to focus on injury patterns for helmet wearers and non wearers involved in accidents. I'm trying to find out if wearing a helmet actually does increase the likelihood of being involved in an accident.

Some facts/figures/reports would be great to back this up if you guys could.

This isn't homework, I'm having a robust conversation with a number of people on some forums, and its something I've been curious about for a while, but could never find the research to back-up my assertion that helmets do impair sensory perception, and in doing so, increase the likelihood of being involved in an accident.

Cheers all, Darigan (talk) 14:01, 10 August 2010 (UTC)


 * These guys seem to manage OK. They're just the best in the world. [[Image:TourDeFrance 2005 07 09.jpg|thumb|The [[peloton]] of the Tour de France]]


 * Sorry, I know that's not an answer, but yours does seem to be a question based on rebellion and an unwillingness to change. HiLo48 (talk) 14:25, 10 August 2010 (UTC)


 * Look at this helmeted fellow. He would have to strain to see the helmet, and it's hard to believe it has any significant effect on his hearing. This is a bit like arguing that seatbelts are bad because they might trap you in a burning car.  Perhaps true in a tiny minority of cases, but statistically marginal compared to the times they prevent serious injury. --Sean 14:28, 10 August 2010 (UTC)


 * I bicycle a lot. Helmets are uncomfortable when it is 38C (100F) outside, but they do not seem to impair my hearing because they do not normally cover the ears. Normally they do not cover any of the sight range either (at least for my Schwinn helmet). --Chemicalinterest (talk) 14:32, 10 August 2010 (UTC)

There would be many factors to control for in any study. For example, maybe cyclist who where helmets are simply more careful (in general) than cyclists that don't, and so would be expected to have a lower rate of accidents. You would want to start by asking yourself how many cycle accidents are caused by ignorance of peripheral vision or hearing on the part of the cyclist (whether wearing a helmet or not). As a personal opinion, I don't buy the peripheral vision argument: a cyclist should always look over the shoulder to avoid the blind spot, and the blind spot is not affected in the slightest by wearing a helmet or not (it's way to the front of any helmet design). Physchim62 (talk) 14:42, 10 August 2010 (UTC)


 * Cheers all
 * @HiLo48 - You may have a point, and LOL at the Tour pic.


 * @Physchim62 - That was the main issue with the studies I did come across - the variables, particularly the ones that you mention, as well as issues of variable cycling competency, and the localities covered in various studies (obviously, road conditions, but also, localities with higher number of cyclists might record lower a lower than expected ratio of accidents because motorists the frequent that area are more familiar with sharing the road with cyclists etc).


 * I accept that my boldly proclaimed opinions on the subject elsewhere may have been entirely factually incorrect. Darn.


 * Thanks all, Darigan (talk) 15:01, 10 August 2010 (UTC)


 * On the hearing front, I'd suggest that cyclists have more to be concerned about with the increasing numbers of virtually silent electric cars, than with helmets. HiLo48 (talk) 21:37, 10 August 2010 (UTC)


 * HEY LOOK WE HAVE AN ARTICLE Bicycle helmet WHICH COVERS MUCH OF THIS. Although it doesn't seem to mention the way that drivers will assume you are more experienced if you wear a helmet, despite that being almost exactly the opposite of the pattern, and hence will give you less space and you'll have more serious accidents. 82.24.248.137 (talk) 21:47, 10 August 2010 (UTC)


 * I would caution readers that the source linked above (a BBC story) doesn't quite jump to the same conclusions that the poster does in his text. The story only describes a study which noted that vehicles tend to pass closer (8.5 cm or about 3.3 inches; roughly 10% less total space) to cyclists wearing helmets than they do to cyclists without.  The authors of the study speculate that drivers may assume a greater level of cycling competence, experience, and/or predictability on the part of the helmeted cyclists, and therefore those drivers may pass the cyclists more closely.  The article does not indicate that helmeted cyclists are at greater overall risk, nor does it describe any research into this effect.
 * Incidentally, our article on bicycle helmets seems to be written in a very non-standard essay-like format, and probably should be reviewed. TenOfAllTrades(talk) 22:15, 10 August 2010 (UTC)


 * I just tried on my helmet, which I think is a pretty ordinary one, and noted that I can't see any part of it whatsoever while I am wearing it, even with my peripheral vision. Nor does it cover my ears. Looie496 (talk) 22:48, 10 August 2010 (UTC)
 * Some helmets come with sun visors, which normally can be snapped off. I think it's a good idea to snap them off &mdash; they do make it a bit hard to see forward when you're bent over for a descent, unless you hold your head in a tiring and unnatural position.  The downside is that you lose the sun protection, but there's sunscreen and UV-protecting glasses for that. --Trovatore (talk) 22:51, 10 August 2010 (UTC)


 * There's some information about research into the benefits and drawbacks of helmets here. The evidence taken as a whole is certainly not clearly in favour of helmet-wearing and may be against. As a regular commuter cyclist I do not own and never wear one. AndrewWTaylor (talk) 18:07, 11 August 2010 (UTC)


 * I don't wear one when commuting either, but a person who rides at high speed or down steep hills without wearing a helmet is a suicidal maniac, in my humble opinion. If there is a significant chance of a Close Encounter of the Automotive Kind, I would also very much want to be wearing a helmet. Looie496 (talk) 00:24, 12 August 2010 (UTC)
 * In a collision with a car a helmet would be no use at all; they are designed to protect against low-velocity impact. AndrewWTaylor (talk) 20:23, 12 August 2010 (UTC)
 * If the car hits you in the head at a high relative speed, maybe the helmet doesn't help. I'm having trouble visualizing how that would happen, though.  The more relevant scenario is:  Car knocks you down, head hits the blacktop. --Trovatore (talk) 20:27, 12 August 2010 (UTC)


 * It's not going to make any difference at a reasonable speed, if a cager hits you there are other things to worry about. When downhill riding a full face and goggles is prudent and reasonable protection.
 * On the other hand I do wear mine while commuting. I had one incident a while ago where I sideswiped a wall at about 15 mph and given the damage to the helmet I'm glad I had it on, I did break my shoulder though.  Also in London it's useful, it's all pretty slow speed but most cages don't leave room and it offers some protection from glancing blows.
 * Swings and roundabouts, an ex was knocked off by a van and if she'd had a helmet on may well have lost an eye...
 * The point about visibility or hearing restriction is pretty specious though, if one is riding defensively one should be looking round pretty frequently anyway.
 * ALR (talk) 20:36, 12 August 2010 (UTC)
 * Not going to make a difference because there are other things to worry about? That doesn't make any sense.  Sure, the car could run over your chest and put a piece of rib through your heart, and in that case it won't make a difference whether your skull is broken.  But in lots of other scenarios it might. --Trovatore (talk) 22:03, 12 August 2010 (UTC)
 * If I come off at 20-25mph because a car has hit me from behind I'm more likely to break my neck than get any value from a helmet. Similarly if a car clips me on the way past I'm going to fall into the vehicle rather than away from it, again it's more likely to break my back.  Alternatively if I end up on the road in the wake of a car that's hit me, then the one immediately behind will hit me.  That's on roads where the cars are doing between 50 and 70 mph.
 * At any kind of reasonable speed in traffic a helmet isn't going to make a significant amount of difference.
 * If I compare that to inside London, where the best I'm doing is about 15mph the cars generally aren't passing me so the threat is quite different, so a helmet can help.
 * Context and threat awareness are pretty significant. I'd say for about 60% of my commute I don't get any value from my helmet.
 * ALR (talk) 22:18, 12 August 2010 (UTC)
 * I think you're simply incorrect. --Trovatore (talk) 22:19, 12 August 2010 (UTC)
 * Ever commuted in Britain?
 * ALR (talk) 22:24, 12 August 2010 (UTC)
 * Do either of you have any studies to reference, or are we working on anecdote and speculation alone here? Algebraist 22:28, 12 August 2010 (UTC)
 * The reference above highlights that the evidence is inconclusive, some of the other discussion in CTC material highlights that the value is context dependent. Speed of cyclist and surrounding cages, both relative and absolute, varies the impact of the helmet, with little effect at high speeds.
 * The majority of work done in the UK focuses on urban accidents, so slow speed and the deaths are usually chest, pelvis or femur related.
 * I would add that yes, I am informing the interpretation by experience. In 35ish years of regular cycling; commuting, road racing and mountain biking, I don't recall anyone in a high speed hit by motor vehicle incident that hasn't had significant chest, back and arm injuries.
 * ALR (talk) 22:56, 12 August 2010 (UTC)
 * So first, I have to apologize that I hadn't really read your remarks completely before commenting. I do agree that if you get rear-ended by a car doing 70, you're quite likely dead regardless of helmet.  But if you get sideswiped by a car doing 70, in my opinion you have a lot better chance with the helmet on.  Yes, I agree you're likely to have significant injuries, but that's not the same thing as dead.
 * As for "the reference above", to be honest that looked like an advocacy site to me. --Trovatore (talk) 21:09, 13 August 2010 (UTC)
 * Well, here's from a recent Cochrane Review, : BACKGROUND: ... Head injury is by far the greatest risk posed to bicyclists, comprising one-third of emergency department visits, two-thirds of hospital admissions, and three-fourths of deaths. ... MAIN RESULTS: ... Helmets provide a 63%-88% reduction in the risk of head, brain and severe brain injury for all ages of bicyclists. ... REVIEWER'S CONCLUSIONS: Helmets reduce bicycle-related head and facial injuries for bicyclists of all ages involved in all types of crashes including those involving motor vehicles.Looie496 (talk) 22:47, 12 August 2010 (UTC)
 * Original research - when I used to commute by bike into London, a car overtook me, pulled in and braked sharply. I was coasting downhill at the time, 10-15mph. My front wheel hit his rear bumper, I head-butted his hatchback resulting in a hemispherical dent in the car but no damage to head or helmet. Conclusion - I'm glad I was wearing one! Alansplodge (talk) 17:06, 13 August 2010 (UTC)

paint stirrers
what are those free paint stirrers made from? are they real wood? —Preceding unsigned comment added by Tomjohnson357 (talk • contribs) 14:06, 10 August 2010 (UTC)


 * No, they are just leftovers from cutting real wood. 92.230.233.247 (talk) 14:07, 10 August 2010 (UTC)
 * Aren't "leftovers from cuttings [of] real wood" real wood? Regards, --— Cyclonenim | Chat 17:06, 10 August 2010 (UTC)
 * No the real wood has been sold as boards, two by fours, what have you. Like sawdust, these stirrers are just what falls to the ground during the cutting.  92.230.233.247 (talk) 17:59, 10 August 2010 (UTC)
 * I get where you're coming from, but those cuttings come from the original 'real' wood. So aren't the stirrers, by composition, exactly the same? Perhaps with a few impurities. Maybe I'm just not understanding you properly, I'm pretty slow today. Regards, --— Cyclonenim | Chat 18:08, 10 August 2010 (UTC)
 * But then by using that logic, particle board (basically a sawdust-glue mixture pressed flat) could be called real wood. Googlemeister (talk) 18:43, 10 August 2010 (UTC)
 * I actually have to agree with Cyclonenim here. If the stirrers are made from a solid piece of scrap wood, they're still 'real' wood by any meaningful usage of the term. Particle board isn't comparable because that's a composite of particles of wood (sometimes not even that but other fibre) held together by a binder. In a similar way, someone may say a meat patty of some sort (burger, meatball, nugget etc) isn't 'real' meat but small pieces of meat which may be scrap and used for a stir fry or whatever is still 'real' meat. Note we have an article on Engineered wood, I'm pretty sure scrap wood of the form described by 92 doesn't fit in to it. In fact, very often people may use scrap wood from a building site or whatever to make things, no one is going to say that isn't 'real' wood. Nil Einne (talk) 19:14, 10 August 2010 (UTC)


 * I call on 92.230's claim that paint stirrers are scrap wood.  Looking at sites like this, it's clear that they're sawed and shaped in bulk, which does not scream "scrap wood" to me.  Comet Tuttle (talk) 18:44, 10 August 2010 (UTC)


 * Have to agree with CT here. See also   these Chinese sellers. Don't exactly scream 'scrap wood' to  me either Nil Einne (talk) 19:30, 10 August 2010 (UTC)


 * Paint stirrers are solid wood, made by splitting a billet of pine or other soft wood parallel to the grain, since sawing them would waste about half the wood. High quality billets of wood are used, not pieces full of knots, since those would break. The wood needs to be of high quality, but the pieces could be short lengths, I suppose. I have never seen or heard of a paint stirrer made of any sort of composition of sawdust and glue. As for "free," their retail value in the US would be about 10 cents each, if you wanted to buy a bunch (like for a craft project, or for labels for garden plants, or whatever) and weren't buying any paint. Edison (talk) 14:57, 11 August 2010 (UTC)

Transforming a principal stress tensor to maximum shear stress tensor?
The author re-posted the question to the Mathematics Reference Desk, which was the more appropriate Desk. See HERE. Dolphin ( t ) 22:50, 10 August 2010 (UTC)

Chemistry/Software question
This is kinda science related and also software-related, but I reckon the people who are best to answer it are at this desk rather than the computing one. Are there any open source/freeware programs that do the same job as the FULL ChemSketch? Personally I can't afford to buy the program and I don't want to resort to illegality in getting it. Regards, --— Cyclonenim | Chat 17:09, 10 August 2010 (UTC)
 * List of software for molecular mechanics modeling has a list; you might want to make a list of specific features you need (many on that list are computational molecular dynamics tools, with "sketching" or 3D image rendering as just a side-benefit). The article molecule editor has a less-well-organized list, but may be more what you're looking for.  List of molecular graphics systems, too.  I think we might need to consolidate (or just organize) these articles.  Nimur (talk) 18:24, 10 August 2010 (UTC)

What is a tensor?
I read the article on it, but I'm still not sure what it even is. In layman's terms? 148.168.127.10 (talk) 18:32, 10 August 2010 (UTC)


 * I struggled with these at university and only got them the third time round. If you understand vectors, then that's a start as they are 1st order tensors (I'm going back a few decades here so others will I'm sure correct me if I'm wrong). Some physical things such as stress and strain need 9 'components' to fully describe them and for elasticity, which relates stress to strain, you need 81 components and that's a 4th order tensor. As the tensor article says, the stress tensor (2nd order) describes the relationship between two vectors (1st order tensors). I doubt that helps, as it's very difficult to describe this in laymen's terms, but good luck anyway. Mikenorton (talk)


 * Not the OP How would that make them not vectors? Can't vectors have an arbitrary number of components? 82.24.248.137 (talk) 21:35, 10 August 2010 (UTC)
 * Well, they are vectors in some sufficiently abstract sense. But that doesn't really say much &mdash; it just says you can add them and multiply them by scalars, and the normal obvious things happen.
 * The general question "what is a tensor?" is probably not going to have a really satisfying answer right now. You could say that it's a linear transformation from some power of a vector space to some power of a vector space, but I don't think that really gives the sort of intuition you're asking about.
 * Instead, I'd recommend studying some particular simple tensors, preferably of rank 2. The stress tensor is a good one &mdash; think of it as a rule that takes a little flat piece inside the object (which you can think of as a vector, with magnitude equal to the area of the piece and direction perpendicular to the piece), and returns a vector representing the force through that piece.  Figure out why that's a linear transformation (hardly obvious!) and you've got a start.
 * Another one to examine is the moment of inertia, which takes a vector representing the object's angular velocity and returns one giving its angular momentum. --Trovatore (talk) 21:42, 10 August 2010 (UTC)


 * A tensor, in the simplest possible terms, is a multidimensional matrix attached to a point in a multidimensional space, with the numerical entries in the matrix changing in specified ways when the coordinate system for the multidimensional space is altered. Looie496 (talk) 22:33, 10 August 2010 (UTC)
 * Those might be the simplest terms in some sense, but they're not the most illuminating ones. Better to emphasize coordinate-free formulations. --Trovatore (talk) 22:38, 10 August 2010 (UTC)


 * The tensors that show up in physics are usually either symmetric or antisymmetric. I don't know if "layman's terms" includes calculus, but both of these special classes of tensors are probably best thought of in calculus terms.


 * A symmetric rank-n tensor is what you get when you generalize nth derivatives to functions of more than one variable. The first derivative of a real-valued function of real variables is a vector (rank-1 tensor), called the gradient, whose components are (∂f/∂x, ∂f/∂y, ...). The second derivative is a matrix (rank-2 tensor) called the Hessian. It's a symmetric matrix because the order in which you take partial derivatives doesn't matter. In the case of a function of one variable, you get a one-component vector and a 1×1 matrix whose sole component is the usual first or second derivative. The most useful thing to know about symmetric real matrices is the spectral theorem, which says that you can always make the matrix diagonal by some orthonormal change of basis variables. Thus you can think of a symmetric rank-2 tensor as a set of perpendicular coordinate axes (the diagonalizing basis) with a number (the corresponding element on the diagonal) attached to each axis. These axes are sometimes called "principal axes". In terms of the original scalar function, what this means is that every smooth surface can be approximated locally to second order by an ellipsoid (just as it can be approximated locally to first order by a plane).


 * An antisymmetric rank-n tensor is a differential form, which is the thing that you write after an n-dimensional integral sign. For example, f(x,y) dx dy is a differential 2-form. In matrix form it would look like $$\biggl( \begin{smallmatrix} 0 & f(x,y) & 0 \\ -f(x,y) & 0 & 0 \\ 0 & 0 & 0 \end{smallmatrix} \biggr)$$, assuming three dimensions with x and y the first two. It's antisymmetric because dx dy represents a little parallelogram in space (the "area element"); if you swap dx and dy the area element flips over, changing its sign, and if you make dx and dy the same vector it collapses to zero size. dx dy is more properly written $$dx \wedge dy$$, where $$\wedge$$ is the wedge product, which is this case is the same as the 3D cross product. The number of independent components of an antisymmetric tensor is given by entries of Pascal's triangle. In three dimensions antisymmetric tensors of rank 0, 1, 2, 3 have 1, 3, 3, 1 components respectively, and they are scalars, vectors, pseudovectors, and pseudoscalars respectively. In four dimensions the tensors of ranks 0...4 have 1, 4, 6, 4, 1 components. The first two are scalar and vector and the last two are pseudovector and pseudoscalar. The middle one, in 3+1 spacetime dimensions, can be decomposed into a 3D vector and a 3D pseudovector. This is how the electromagnetic field tensor decomposes into electric and magnetic field vectors. There's more that could be said about antisymmetric tensors, but I'll leave it at that. -- BenRG (talk) 06:00, 11 August 2010 (UTC)

<- This very nice Introduction to Tensors for Students of Physics and Engineering from NASA might help.  Sean.hoyland  - talk 09:03, 11 August 2010 (UTC)
 * My impression from reading this, trying to understand, specifically this part:

All scalars are not tensors, although all tensors of rank 0 are scalars (see below). All vectors are not tensors, although all tensors of rank 1 are vectors (see below). ... was that an n-dimensional tensor is just a subset of n-dimensional matrices (the subset where a change of coordinates doesn't affect how the system works). I'm still working to get this myself. - Craig Pemberton 08:12, 14 August 2010 (UTC)

Higher order tensors simply seem like "vectors of vectors" to me. Have you taken multivariable calculus yet? John Riemann Soong (talk) 18:28, 11 August 2010 (UTC)

satellites.
Have we (humans) placed any satellites that are in retrograde orbit around anything besides earth? Googlemeister (talk) 19:04, 10 August 2010 (UTC)


 * Our article Artificial satellites in retrograde orbit suggests that the answer is probably no, but it's a rather poor article. --M @ r ē ino 19:16, 10 August 2010 (UTC)
 * Yeah I looked at that article and thought the same thing, especially since it would require a massive energy expenditure around any extraterrestrial body I can think of except perhaps Venus. Googlemeister (talk) 19:18, 10 August 2010 (UTC)
 * No, I don't think so. For Earth, where you get the rotation speed for free (or have to compensate for it), the difference is major. But for any other body, the rotation of the body is irrelevant - you're dropping in from outside the system, anyways, and, as long as the rotation is not relativistic, the other body simply behaves as a point mass. You can essentially chose any orbit you like. I would suspect that probes often chose a polar orbit, since that allows them to map the whole body over time. --Stephan Schulz (talk) 21:25, 10 August 2010 (UTC)
 * Plus, craft that we put in orbit about other bodies are either doing scientific/photographic missions or dropping off landers - and the lower the relative speed between surface and orbiter, the sharper the photos can be and the lower the re-entry speed of any lander. Using a retrograde orbit would pretty much guarantee worse results. SteveBaker (talk) 02:48, 11 August 2010 (UTC)

Most intelligent bird
Can you tell me which bird species is generally considered to be the most intelligent in the world? --95.148.105.77 (talk) 21:01, 10 August 2010 (UTC)
 * Corvidae are generally considered the most intelligent bird species family, for the reasons (and sourced ones at that) presenting in the introduction to the article. You may also enjoy reading bird intelligence. Regards, --— Cyclonenim | Chat 21:09, 10 August 2010 (UTC)
 * Corrected, it's a family of birds rather than an exact species. It'd be hard to pinpoint between the species within the family for they are specialised at different things. Regards, --— Cyclonenim | Chat 21:11, 10 August 2010 (UTC)
 * Grey Parrots are recognized for their cognitive language skills, though ravens are very good at solving problems and can count to 6 or 7. Googlemeister (talk) 21:25, 10 August 2010 (UTC)
 * I would like our questioner to tell us what he or she thinks intelligence means, especially in birds. We have enough trouble agreeing on what it means for humans. When we try to apply the concept to totally different creatures it becomes very difficult to agree on what it means. HiLo48 (talk) 21:32, 10 August 2010 (UTC)
 * The smartest corvid would make for a dumb human. But at the same time, the smartest human would make for a dumb corvid.  HiLo48 is right.  Perhaps all you can say is that, amongst the various kinds of birds, the corvid's intelligence is most similar to human intelligence.91.104.151.200 (talk) 00:12, 11 August 2010 (UTC)
 * I remember a story from a few years ago in which a bird was able to form a hook out of a piece of wire then use it to retrieve some food at the bottom of a container, a behavior previously unobserved. I don't know what the species was, though.  Hemoroid Agastordoff (talk) 17:00, 11 August 2010 (UTC)
 * New Caledonian Crow, perhaps? Ghmyrtle (talk) 17:03, 11 August 2010 (UTC)