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

Science desk
< June 18	<< May | June | Jul >>	June 20 >

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.

June 19

Coefficient of thermal expansion...

Those who are following this list may have spotted occasional references to my current pet project - refurbishing (well, throwing away and rebuilding from scratch) my home-made computer-driven milling machine (essentially, a robotic 2 horsepower router).

I'm thinking about the calibration of the machine. I'm using three precise stepper motors to rotate threaded steel rods that move the cutter up and down, left and right and in and out. The thread on the rod has a pitch of 16 turns per inch - and the stepper motor can move by just 1/200th of a revolution at a time. So, I have a resolution at around 1/3200'th of an inch! (Well, it's not really that good because of backlash etc...but it's not bad).

The problem is that the steel is going to expand and contract with temperature. So I'd need to calibrate the thing every time I turned it on. The drive system has limit switches - so I can move all the way down/left/in until the limit switches all trip - then move up/right/out until the other limit switches trip - and I'll know how many steps there are between the two limit switches.

But the limit switches are mounted onto the MDF (Medium-density fibreboard) frame - which (I'm hoping) doesn't expand and contract anywhere near as much as steel...but is that really true? Our article says: "Subject to significant shrinkage in low humidity environments."

So - to cut a long story short:

• What's the coefficient of thermal expansion of MDF?
• What's the coefficient of humidity-related expansion of MDF?
• Is it really less than steel?

SteveBaker (talk) 12:23, 19 June 2009 (UTC)

Steve (I assume)

Carbon steel is 10.8, other steels, depending on composition, range from 11.0 to 13.00 according to our own article on co-efficients. "The thermal conductivity of MDF varies slightly with thickness with the usual range being 0.05-0.08 kcal/mh°C (0.12 - 0.15 W/m°K). Like natural timber, MDF has a low thermal capacity. Within the normal range of temperature variation, MDF is dimensionally stable" according to this trade datasheet.

Humidity seems to be more of an issue, as 30% humidity at 20C can result in 5% moisture content, rising to 12% at 85% humidity. Moisture causes changes in length about 0.03 - 0.06% for every 1% change in moisture content, and by 0.3 - 0.5% in thickness. If your frame is 2 inches wide, this might come to 1/100th of an inch, which is substantial if you are working to 1/3200th accuracy. - KoolerStill (talk) 08:01, 19 June 2009 (UTC)

(Sorry - I forgot to sign...and for some reason sinebot took a vacation.) The longest distance over which expansion would occur is about a meter. The operating volume of the robot is 1m x 0.5m x 0.25m. So with a range moisture content from 0% to maybe 30%, I might see 30x0.06=2% !! Wow. That is a lot! I hadn't planned to paint the MDF - but now I might...just to control the uptake of water in it! Over (say) a 30 degC temperature range, the steel is going to expand by 30x11=330 parts per million..which is only a third of a millimeter over the longest dimension. So that gets me thinking...if MDF doesn't change size with temperature - and we know that steel doesn't change size with humidity - I wonder if I can have the robot somehow figure out which dimensions are changing. Because the distance between my two limit switches is determined by the moisture content of the MDF and the number of stepper motor steps it takes to move between them is determined by the temperature of the steel...tricky. SteveBaker (talk) 12:23, 19 June 2009 (UTC)

Ah! I think I have it figured out. If I change the design a bit and mount one pair of limit switches on an aluminium bar - then I can count the number of steps to span that pair of switches (which will be independent of humidity) and I should be able to use the difference in thermal expansion rates of aluminium and steel to calculate the temperature and correct for it. Aluminium expands about twice as much as steel - so I'd get maybe a third of a millimeter difference over a 30 degreeC temperature range - which is around 30 steps of my motor - so this would allow me to measure the temperature accurate to about a degree (is it a robotic milling machine or is it an exceedingly silly thermometer?!) Then I can count the number of steps it takes to get between the limit switches in the other axis (where the limit switches are on MDF) - correct for the expansion of the steel and then I'll know how much the MDF has changed size by and I can figure out how all of the dimensions of the machine have changed and correct for that too. That way, I don't have to care whether the MDF changes dimensions with temperature because I'm just measuring the size directly.

This is so much fun! (Well, if you're a serious geek it is...I imagine some people are yawning at this point.)

SteveBaker (talk) 13:02, 19 June 2009 (UTC)

Steve, your whole setup, including the workpiece and jigs, will expand and contract. If you intend to work with materials that all have the save coeffiecients of expansion as you machine, then you can simply ignore the problem. If not, then you need to consider the whole system, not just the machine itself. Perhaps you can invest in a single reference block of invar (or something) and calibrate against it each time. -71.0.188.190 (talk) 13:54, 19 June 2009 (UTC)

That's certainly true - and it's certainly something I've thought about. It's clear that nothing that's made of wood has precise or stable dimensions...and that's mostly what this machine is making. The previous incarnation did a wonderful job of doing wood carving. You could design a 3D pattern of leaves and flowers and stuff by processing a photograph using "shading-to-shape" techniques - then just hit the GO button and about an hour later - you'd have something that looked like someone had spent a month out of their lives carving it. The new version will be much better than that...I'm just exploring the envelope a little here...seeing if I can do yet better without it costing me much. SteveBaker (talk) 14:58, 19 June 2009 (UTC)

Why don't you measure the MDF fittings, buy a chunk of Hardened Steel, rework your fittings, and design or buy a small coolant pump (a tray underneath to recycle the used coolant, picture ;-) none goes down the drain) which will work quite well with only a fraction of the flow shown in that picture. You will have no end of improvement to your finish and be much kinder to your tools. Keep the workpiece cool rather than the machine (when it heats up too much it makes crap out of the piece anyway! How can you properly tighten your fittings to MDF without damaging it? You can't.) Use coolant, settle for a frictionless finish rather than all this messing about with burnable stuff! Coolant is dilutable, oils your stuff and recycles itself. It shouldn't cause any major overhead and is the standard method. ~ R.T.G 14:15, 19 June 2009 (UTC)

Why build a thermometer-machine? can't you put a simple real thermometer/temperature sensor into it? reading that will take less calculation in your already very_tight_on_memory controller chip than having to calculate temperature from relative expansion measurements. It would save having a set-up routine to do the measuring. How are the bars being fixed to allow for this amount of movement without stress or shearing? - KoolerStill (talk) 14:53, 19 June 2009 (UTC)

I don't think you understand what I'm doing. This is a hand-made machine I keep in my garage for making small (mostly wooden) things - with the new setup, I'm hoping to be able to mill copper circuit boards and drill holes for components - maybe mill a occasional aluminium part for a classic car restoration. I can't have pumps squirting stuff all over the place! If the workpiece gets too hot while I'm milling it - I'll just adjust the software to move the cutting head more slowly and shave off thinner layers - maybe pause for a while between tool passes to let everything cool off. Anyway - the machine has been doing great work for several years now - it's just that all of the cheap rollerskate bearings I used are getting worn and other parts are starting to fail and some of the wires have been flexed back and forth one time too many and broke...so I'm refurbishing it. And in the process using chunkier motors with more precision - new computer interfaces - that kind of thing. Since I have much more precision in the new machine than the old one - I thought I'd do a bit of 'due diligance' and check on the size of the thermal expansion error - and see if some simple steps (SIMPLE!) would reduce it. (Before, the errors due to thermal expansion would have been negligable compared to the backlash in the gear boxes, the lack of precision in the shaft-encoders, etc). My entire budget for building the machine is $200 ($120 of which went into the three big-assed stepper motors). I use the shaft encoders from an old USB mouse for figuring out how far the thing has rotated...that kind of thing. This is a "toy" compared to a professional CNC milling machine...but an amazingly useful toy...well worth a few weekends of work and $200. SteveBaker (talk) 14:52, 19 June 2009 (UTC)

Would like to say woodworking and PCB baords can't cause too much heat but can't say. Still be much better for the tools and the piece if you cool it when you are cutting metals. You could set up an open top gallon drum with a tube and a tap, chill in the fridge refill it occasionally, use a well aimed trickle, don't want to short the router either. The powerful gush in the CNC picture is probably the optimum but also the maximum. The coolant is reusable and dilute so isn't the most expensive in itself. Fair play but it seems you are agonising rocket science on what you could just make your own hardened steel fittings for. Good luck with it. ~ R.T.G 15:58, 19 June 2009 (UTC)

You should add it too Wikibooks and Wikiversity unless you are keeping the copies on it. Hardly a mean feat to put together and the motor is as powerful as your average 7-8 foot machine which lasts forever if you replace bits. I guess you are still working out bits for now or even selling the idea. ~ R.T.G 11:09, 20 June 2009 (UTC)

Without seeing SteveBaker's machine, I think using limit switches mounted on wood as a length standard is not a good idea. Not only is wood the least dimensionally stable material in the machine, the switches themselves presumably contain deformable springs. But if you dissect mice then you have light emitters and detectors with which to arrange optically sensed limits of a length standard. See this table of expansion coefficients. If you choose two readily available materials (steel and aluminium) and make an optical length standard based on each material, your software can calculate a temperature-independent length from the observed difference between the lengths of the materials. Cuddlyable3 (talk) 23:46, 19 June 2009 (UTC)

Err . . . you are talking about computer mice, aren't you, Cuddlyable3? I had a brief Girl Genius moment there. 87.81.230.195 (talk) 19:53, 20 June 2009 (UTC)

Why not measure the thermal and humidity related expansions directly with a Strain gauge or two? They are cheap and rather easy to use. Noodle snacks (talk) 12:05, 21 June 2009 (UTC)

remedy or something else?

On an episode of Emergency! before the opening credits, Roy was gargling warm salt water when Johnny and Chet came into the locker room. A minute later, Chet said his mother made him gargle with vinegar and soybean oil when he had a sore throat. Can those two fluids do the same thing salt water does when gargled?69.203.157.50 (talk) 04:25, 19 June 2009 (UTC)

Sore throat in common usage has a wider meaning than Acute pharyngitis. It can range from simple dry mucous tissue to something viral or bacterial among other things. Salt water helps with the hydrostatic pressure of the cell walls. Some bugs are sensitive to acids and the ph of many tissues in the human body is slightly acidic. This environment can be changed in favor of pathogens during an infection. Oil will coat the tissue and help against further drying. It may also keep a couple of the nasties from entering cells. So, depends on what you got if it has much of a positive effect, but at least it doesn't do harm. Unlike some OTC and prescription drugs that can seriously upset your system. (Patients who urge their doctors to prescribe antibiotics ignore the fact that those don't work on viruses.) 71.236.26.74 (talk) 07:42, 19 June 2009 (UTC)

Sustainability

Is it possible for humanity to live in a sustainable global society?

Or are there physical limiations on our global impact in that we will always need to consume more resources than we can replace? TheFutureAwaits (talk) 09:55, 19 June 2009 (UTC)

It's certainly possible - but almost certainly not at anything like the present population levels. If there were (say) ten million people in the world - we would have negligable impact - but at seven billion, I think it's quite clear that it's unsustainable - somewhere between those two numbers is the limit. SteveBaker (talk) 11:54, 19 June 2009 (UTC)

I disagree. It isn't at all clear whether or not the current population is sustainable. For example, if economies were all free markets with good protection of property rights, it is possible that we could support more people using fewer resources and with less environmental degradation. Wikiant (talk) 12:07, 19 June 2009 (UTC)

I'm sorry - but that's incredibly naive.

We can manage less environmental degradation...yes - for sure...but ZERO degradation? That would mean zero use of fossil fuels, zero increase in atmospheric carbon (in fact, a significant REDUCTION in the present levels) - zero mining of metals (100% recycling of every scrap of metal we use...that's insanely difficult!) - zero over-fishing of the oceans. Abandonment of non-recyclable plastics and other chemicals based on fossil oil.

Consider just one case: It's not high on the list of environmental panics - but did you know that the world only has 60 years worth of copper reserves left? Yep - we're going to run out of copper during the lives of some of our younger readers. Even if we go into recycling overdrive - there are no recycling processes that reclaim even 50% of any given resource. So we have 120 years of copper supplies - at most - assuming everyone treats this stuff like gold. Can we do better? Are we geared up to recycling every single scrap of copper we ever use? It's very difficult to do that. Sure - when a building is demolished, we can go in there and salvage all of the copper pipes and electrical wiring - and when a car is recycled, we can pull all of the copper wiring out of the alternator and starter motor. We could stop using copper in coins. But those tiny little copper contacts inside the battery compartment of your kid's toys? Will those get fully, 100% recycled? No - I don't think so. Also, many of the uses of copper require us to mix it with something else - eg to make Brass. You can't get the copper out of brass in order to recycle it without huge expenditures of energy (which - you'll recall - we are also trying to save!)...so gradually, no matter what we do, we cannot sustain the use of copper. We're probably going to run out of copper in industrial quantities within 100 years...the price will rise until it's the price of platinum or something - and we'll have to resort to mining less and less viable mines - resulting in us tearing up yet more of the landscape and consuming yet more energy. Meanwhile - what do we use for conducting electricity? Aluminium works - but it has lots of problems with corrosion in the air and it's not as flexible as copper - so no more bendy wires between your toaster and the wall socket! Silver makes great conductors - but it's kinda rare too.

And that's just one of a whole bunch of things we're running out of. Helium supplies are dwindling - all sorts of the more exotic metals used in modern electronics come from single-source mines that are unique to a particular place on the planet and have predicted lifetimes measured in decades - not centuries.

We are very, VERY far from living sustainably!

SteveBaker (talk) 13:25, 19 June 2009 (UTC)

Steve makes some great points and I'm really curious about the implications of them. I just don't see how even in theory we can live sustainability given our resource requirements and the difficulty in reusing and recycling these resources. Which also means unless we all move off-world (which is really, really impractical so can we not get into that?) it seems like we're doomed. Maybe to an agrarian culture? Maybe to not exist at all. Am I wrong? TheFutureAwaits (talk) 13:34, 19 June 2009 (UTC)

How is "moving off world" sustainable? All you have done is generalize all resources that we consume into one "planet resource", which you have casually discarded when it becomes easier to go somewhere else. Nimur (talk) 14:56, 19 June 2009 (UTC)

Before we get too much into the carrying capacity discussion I should clarify I'm wondering if it's possible for people to live a 100% sustainable lifestyle and then apply this on a broad scale. In my experience even the greatest green technologies always produce waste. On the other hand when we lived without technology we apparently had major ecological impacts (the death of native species in the Americas and Australia upon our early arrivals comes to mind) so I don't think abandoning technology would offer much of a solution.

So is it possible through the application of technology to live a 100% sustainable life and to apply that on a massive scale? Or are there simply limits to the degree we can minimize our impact? TheFutureAwaits (talk) 13:28, 19 June 2009 (UTC)

We could certainly revert to stone-age existances - but there is no way for seven billion people to live that way! We'd all need stone tools - will the supply of flinty rocks be enough? Are there enough natural caves for us to live in? Would the much lower density of wild animals be able to support such a population without dense farming with fertilizers and irrigation? No! Not by a million miles! For us to revert to a stone-age lifestyle - we'd need a stone-age population density - which means that we would have to wipe out 99.9% of humanity to get down to about 5 to 10 million people (that was the estimated world population in 10,000 BC when farming and such like started). If we did that - then we could live very close to sustainably in the lap of luxury with all of modern technology. Copper would run out in 60,000 years instead of 60 years. Global warming would cease to be a problem - we could all drive gas-guzzling Hummer's and the planet would hardly notice. The problem isn't our technology - it's our sheer numbers. SteveBaker (talk) 13:37, 19 June 2009 (UTC)

I disagree, it seems like population reduction (say everyone had one child for the next several generations) would only delay the inevitable. Sure copper might last 60,000 years instead of 60 but we're just moving the problem further downfield. It seems like eventually, no matter what we do, we're going to hit that wall. That's why I'd like some evidence that we won't; that there is some way to live sustainably at least until the sun runs out of hydrogen. TheFutureAwaits (talk) 13:48, 19 June 2009 (UTC)

Certainly there is no way to consume anything whatever and last forever. Even that stone-age existance with just 1 million people would result in them running out of stone to make tools - eventually. The rate of new rock formation just isn't high enough. We'd have to revert to being great apes with zero technology. That probably requires a genetic change - we'd have to de-evolve back to a more primitive form. But there is a massive difference between having 60 years worth of copper reserves and 60,000 years worth. In 60,000 years we could produce the technology to mine the asteroids - spread onto other planets - build robotic mining machines to dismantle entire planets in search of copper (or whatever). With the resources of the entire solar system and 60,000 years of technological advancement, I have no doubt whatever that we could survive until the extinction of the sun without reverting to pre-stone-age techniques. But there is no way we're going to find a way to mine the asteroids for copper in the next 60 years.

A chinese-style one-child-per-family approach would drop the population by a factor of two in every generation. If we could enforce that world-wide for 10 generations then switch to a two-child-per-family approach, we'd survive this. But that's not such a simple thing to do. Each new generation of workers - has to support two retired people each in addition to themselves...that's tough! But if we could do it - we might be OK. If we take copper as our benchmark (we could choose oil or atmospheric carbon instead) - then in the 60 years it takes the copper to run out - we may have halved our population - which means that we extended the life of the copper reserves by what? 15 years (assuming linear reduction in consumption)...but we can't halve the population again in the next 15 years without killing a lot of people! So the copper is gone...there is nothing we can do to stop that. For oil, I think we could reduce our population faster than we're running out - and for global warming, we could certainly avert disaster by halving our population over the next 60 years.

But applying such draconian measures is tough! China did it only because they were a totalitarian state with relatively compliant people. Could you imagine doing that in the USA?!! It's pretty inconcievable. Religious limitations on birth control would prevent large swaths of the roman catholic world from following that reduction rate. If some countries decided to make the supreme sacrifice and others didn't, you'd find that the sharply decreasing population of the "good guys" would result in them being overrun by the numerical superiority of the "bad guys". Governments would realise this - and we'd never get agreement to do this on a global scale.

Sadly, we're probably doomed.

SteveBaker (talk) 14:08, 19 June 2009 (UTC)

I strongly disagree that asteroid mining in the next 60 years is infeasible. 60 years is a long time, technologically speaking. Think where we were 60 years ago. We were still 8 years away from putting something into orbit. Now trips to LEO are strictly routine and there are serious plans to long-term occupation of the moon (as opposed to everyone assuming there would be such plans sometime next week but nobody actually planning it which has been going on for decades). Manned missions to Mars are intended for sometime in the next 30 years (although 40 might be more likely). Once you can get to Mars, getting to the asteroid belt isn't much harder (near-Earth asteroids are even easier and wholly robotic mining operations might be a better option as well). If the price of things like copper increases as your numbers suggest it will, asteroid mining will easily become economically viable, which means private business will get involved. Private businesses generally move much faster than national space programs since they don't have to contend with massive budget cuts. I wouldn't be at all surprised to see asteroid mining within the next 60 years (although I know better than to make definite predictions that far into the future where technology is concerned). --Tango (talk) 22:53, 20 June 2009 (UTC)

What is sustainability, anyway? How long does something have to be sustainable? After all, our energy comes exclusively from stellar fusion, so we really only need to be as sustainable as the sun.

All life is an energy-intensive process. I make the vague-and-handwavey claim that "higher forms of life are less efficient Carnot engines." In the most basic life-like things on Earth (say, a virus), the energy consumption is near zero until it is time to reproduce. The next steps up on the chain, the autotrophic prokaryotes, are pretty darn efficient - they absorb solar or geothermal energy, and use it to drive a simple chemical reaction and produce complex chemicals. The next few steps upward on the chain of "complex life" move into the realm of multicellular organisms, and the first heterotrophs emerge. These are much less energy-efficient organisms, in that they can't even generate their own energy, but must prey on some other thing. So, solar energy gets concentrated by algae, and then multicellular pondscum comes along and eats on the solar-fed algae. Pond scum, in my naive definition, is thus a "higher form of life," because it has a more complex energy pathway, and uses the excess energy to produce more complicated molecules (like enzymes for digesting the cellulose that they're eating).

As we continue to move up to higher organisms, we see that the total amount of energy for each organism is increasing; and the total efficiency of the food chain is decreasing. When we reach macroscopic organisms like frogs and monkeys, these guys are consuming just enormous quantities of energy compared to the single-celled organisms. So, you need to have billions of algaes so that the pondscum zooplankton can eat them, and millions of zooplankton to feed thousands of fish, and thousands of fish to feed one or two monkeys or frogs ("sustainably"). The root energy supply is still solar, but the ecosystem is getting more towering and the food chain is very sophisticated by this point.

After several million years, some creature decides that this is actually an inefficient way to extract energy from the environment, and begins to develop technology to circumvent the foodchain. They domesticate sheep and cows, and learn how to farm, to speed up the process of concentrating solar energy into usable food. They also develop fire, and waterwheels, and nuclear fission, and petroleum refining. This is a new step up on the hierarchy of "advanced forms of life" - they are able to extract a larger quantity of energy by collecting solar energy that has been stored somewhere for the last millions of years by trillions of algaes. Amazing! Biology has been circumvented with engineering! Now, more energy is extracted, but as always, it is a less-efficient way of using each individual joule of solar energy. This is sort of necessary, though, because thermodynamics teaches us that by physical law, every energy transfer process has some waste. (Maybe somebody who is familiar with thermodynamics can voice their interpretation of the Second Law?)

By extrapolation (making stuff up) it would seem that the most advanced forms of life will continue to consume more and more energy. SteveBaker's robot brains will start to take shape - a server room full of ten thousand blade servers will consume far more energy (in Joules) than the human that it is simulating. But, it will be able to use electricity, instead of trying to digest rice and cow; and it will be able to speed its simulation up or down or whatever. But it's using more energy, which is the only nonrenewable resource.

So, I propose Nader's Law Nimur's Law?: an optimal form of life is the one which matches its energy consumption to the rate of maximum return before the heat-death of the universe. In other words, the pitiful little algaes are extremely energy efficient but their form of life is so primitive and limited that they won't accomplish much. At best, they can hope to make lots of sugar and... sweeten the ocean or something. That's their maximal impact. We humans are successfully changing our atmosphere, launching manmade objects into deep space, building cities and art and pornography, setting fire to cities and art and pornography, raising sheep, developing advanced languages and computers, walking on the moon... so sure, we're not as energy-efficient as the algae, but we do a lot more cool stuff. We've managed to concentrate our resources into a form that is useful for these sorts of tasks - at the expense of some of those resources. Maybe our robot computer replacements will be even better at that sort of stuff with less waste (but I doubt it - they'll find needs for larger energy budgets, with the accompanying extra resource waste). It's entirely possible that some form of life higher or lower than us will actually be more effective, over the cosmological time-scales of our universe, to use energy more efficiently than us to accomplish the same things - but there is a ticking clock - if we don't use the energy up, entropy will dissipate it for us anyway. Nimur (talk) 15:22, 19 June 2009 (UTC)

When human load exceeds supply, something like a Malthusian catastrophe could result. Remember that we'd run out of natural resources if everyone in the world had an ecological footprint as large as that of say, an average American, in fact we'd need about 5 Earths' worth of resources. ~AH1^(TCU) 01:08, 22 June 2009 (UTC)

On Hawaii

When did the first settlers arrive in Hawaii? I'm not talking about americans, mind you, but the original people of hawaii?Drew Smith What I've done 10:46, 19 June 2009 (UTC)

According to ancient Hawaii, it was some time between the third and eighth centuries. Algebraist 10:50, 19 June 2009 (UTC)

Big Bang

Is there a formal name for the big bang theory? or is that all there is? 84.97.254.29 (talk) 10:55, 19 June 2009 (UTC)

It's formally called the Big Bang theory. An earlier name is l'hypothèse de l'atome primitif, if you want an alternative for some reason. Algebraist 10:59, 19 June 2009 (UTC)

Yeah - it's kinda sad. The guy who named it (Fred Hoyle) didn't believe in the theory and was being incredibly sarcastic about it in a radio interview - he pretty much thought up the name on the spot as a way to mock the people who believed in it! So the name of the theory that explains how the entire universe started doesn't get a fancy, grandiose name...but a joke. <sigh> SteveBaker (talk) 13:29, 19 June 2009 (UTC)

Hoyle did coin the name, but I don't think he meant it to be insulting, it was just meant to contrast with "steady state". The articles Fred Hoyle and Big Bang have a couple of sources backing this up. It's better than the earlier names "primeval atom" and "cosmic egg", which are just ridiculous. The problem with all of those names is that they emphasize the wrong thing, the singularity where the model breaks down instead of the well-behaved region where it makes predictions. "Big bang cosmology" is not about a big bang, it's about the evolution of an expanding universe. You might as well call general relativity "the theory of black holes". Of course, "general relativity" is a stupid name also. So is "quantum mechanics". Oh well. -- BenRG (talk) 16:27, 19 June 2009 (UTC)

Why are "general relativity" and "quantum mechanics" stupid names? A Quest For Knowledge (talk) 16:31, 19 June 2009 (UTC)

General relativity isn't a great name because it only makes sense in the historical context of it being a generalisation of special relativity. And, of course, special relativity only makes sense in the context of it being a special case of general relativity. It is something of a circular definition. The only part which has any meaning in itself is "relativity", which offers no suggestion of how it is different to Galilean relativity which had been around for several hundred years before Einstein came on the scene. "(General/Special) invariant speed of light theory" would be more descriptive, albeit it a little less catchy. I don't have any real problem with "quantum mechanics", though, although I would rather name theories by their hypotheses, not their uses. The theory has nothing to do with mechanics, that's just what it is used for. "Quantum energy theory" might be better. --Tango (talk) 16:41, 19 June 2009 (UTC)

I had a professor who said that the name "special relativity" was ironic, because the key point of the theory was that the speed of light *wasn't* relative, but was the same for everyone everywhere. If I remember correctly, Einstein didn"t like "relativity" either, and prefered the term "invariance theory". (Doesn't seem to be mentioned on Wikipedia - Google ("invariance theory" einstein) for references.) -- 75.42.235.29 (talk) 03:26, 20 June 2009 (UTC)

Yes, Einstein later in his life thought it ought to be called invariance theory (because he got sick of people missing the point that the invariances are more novel than the relativity), and on GR he later (following Vladimir Fock) wanted just to call it a "theory of gravitation" or something like that, more straightforward and less emphasis on the relativity. Quantum mechanics was so named to differentiate it from (old) quantum theory. --98.217.14.211 (talk) 18:42, 20 June 2009 (UTC)

Most physicists would probably say "big bang cosmology" rather than "the big bang theory". The latter sounds like a layperson's term (and now like a TV series). You could also call it "Friedmann cosmology" or "Friedmann-Robertson-Walker cosmology". That calls attention to the expansion instead of the singularity. The currently favored version of big bang cosmology is called the "Lambda-CDM concordance model". It's the simplest version of Friedmann cosmology that's consistent with the currently available data. You could also call it "Lambda-CDM cosmology". Lambda-CDM might give way in a few years to another model with a different name which would still be under the umbrella of big bang/Friedmann cosmology. None of these are very good names, but they're a bit more formal sounding. -- BenRG (talk) 16:27, 19 June 2009 (UTC)

Nudibranch (or something)

Hey guys, could some marine enthusiast help me identify this nudibranch subspecies? The picture is from Crete. If the relevant article is in need of a pic, I'll gladly post this one. Thanx! P.S. At least I think it's a nudibranch! PervyPirate (talk) 11:11, 19 June 2009 (UTC)

It's surely a sea slug of some slimy sort, but whether it's a nudibranch or a Cephalaspidea, or a Aglajidae, or a Sacoglossa, or even a Onchidiidae is not clear to me. This website http://www.nudipixel.net/ might help. If they have a forum you could ask there too. (I know the name sounds sketchy but it is a legit site on nudibranchs. It's SFW as long as your boss isn't a gastropod.) Sifaka ^talk 19:28, 20 June 2009 (UTC)

It's probably a polycladida flatworm, maybe yungia aurantiaca, which looks pretty similar,[1] but there's plenty to choose from. Mikenorton (talk) 20:35, 20 June 2009 (UTC)

To Sifaka: What a wonderful site this is! I think I'm gonna start making T-shirts with all those nudibranchs! Unfortunately, the genus I'm looking for is not there... Oh, and my boss kinda behaves like a gastropod, throwing ink at people and looking annoyed if somebody bothers him (Yes, we are both scuba instructors!)
To Mikenorton: YES!! That's exactly the one I've been looking for... I wish we had a relevant article for me to add that picture to.. I guess I'll have to create it myself then.. "Orange flatworm" it is!

Thanx a lot to both of you! PervyPirate (talk) 01:03, 21 June 2009 (UTC)

ADD and stimulants

Hi there. I notice most treatment for ADD/ADHD are amphetamine stimulants like Adderol and Ritalin. How are the actions of those drugs different than say giving someone with ADD 20 cups of coffee? How is one stimulant different than another? --70.167.58.6 (talk) 13:34, 19 June 2009 (UTC)

It has a few side-effects! I think that 20 cups of coffee contains something close to the lethal dose of caffeine for a human. It's LD50 is 150mg/kg of body weight...and a cup of coffee has about 150mg of the stuff - so if you drink one cup of coffee per kilogram of your body weight in a period of a few hours - you have a 50/50 chance of dropping dead. So a 20kg kid and 20 cups of coffee will likely die. A 50kg kid would do better - but it's hardly healthy! One assumes that these other stimulants have much better controlled side-effects. SteveBaker (talk) 13:48, 19 June 2009 (UTC)

I was using hyperbole. ;) I didn't really mean for someone to drink 20 cups. Aren't amphetamines highly addictive? (Valley of the Dolls, Judy Garland, and all that?) --70.167.58.6 (talk) 14:42, 19 June 2009 (UTC)

Really, Steve? How long do you think it would take for you to drink 70 cups of coffee? Quite a bit longer than the ~5 hour half-life of caffeine, I would expect. – ClockworkSoul 16:47, 19 June 2009 (UTC)

In laymen's terms, the neurobiology of ADHD can be simplified in a similar way that depression is simplified. With depression, low serotonin levels cause depressive symptoms, and most antidepressants (TCAs and SSRIs) increase these levels. For ADHD, low dopamine levels cause irritability, lack of focus and lack of pleasure (from tasks such as reading), and most ADHD treatments (amphetamine, methylphenidate) increase these dopamine levels. In reality neurobiology is far more complicated than that (and the "monoamine hypothesis" - which states that low serotonin causes depression - is pretty much obsolete now). --Mark PEA (talk) 18:39, 19 June 2009 (UTC)

Well, I think those statements are somewhat controversial, but the key point is that all stimulants are not created equal. The stimulants that work for ADHD are psychomotor stimulants, which affect the brain's dopamine system. Caffeine operates by a completely different mechanism. (Nicotine, on the other hand, might have some effect, since it partly acts as a psychomotor stimulant.) Looie496 (talk) 00:04, 20 June 2009 (UTC)

Stimulants used to treat ADHD raise the extracellular concentrations of the neurotransmitters dopamine and norepinephrine which causes an increase in neurotransmission. The therapeutic benefits are due to noradrenergic effects at the locus coeruleus and the prefrontal cortex and dopaminergic effects at the nucleus accumbens. Hit up the refs on the Main ADHD page in the management section to find some decent scientific papers on the subject. I suppose you can compare that to the pharmacological effects of caffeine. People with ADHD are more prone to using psychoactive substances which include caffeine. As for how stimulants are different there is info at ADHD management, (the article could use a little cleanup for clarity...) Sifaka ^talk 19:11, 20 June 2009 (UTC)

kineto-baric?? what is this?

Hi

Here are a couple of links to a phenomena called kineto baric effects. Could someone tell me what exactly this is, found a Nasa page on it and mention on a book so Its not something a internet crackpot just made up. Can anyone please tell me exactly what this is??

Kinetobaric effects, a new phenomenon Peschka, W. http://adsabs.harvard.edu/abs/1974DFVLR......603P

http://books.google.co.uk/books?id=zENRkvFO0SEC&pg=PA226&lpg=PA226&dq=kineto+baric&source=bl&ots=qxeQVo7ZB_&sig=SN0aR7fGi9hy1lvD4Puozpjr9bc&hl=en&ei=OJc7SsWhMOeMjAfoxpAg&sa=X&oi=book_result&ct=result&resnum=4

Thanks in advance

Rob —Preceding unsigned comment added by 79.67.246.16 (talk) 13:56, 19 June 2009 (UTC)

It looks like junk-science to me. Your first link doesn't suggest that NASA endorses it - only that it's in their database of science abstracts. This work dates back to the 1970's - if they had truly found a new kind of force - you'd bet that mainstream scientists would have been all over it by now. So your evidence boils down to a 35 year old paper in an obscure German publication - and a reference in a book that's full of crackpot theories. A google search on the term turns up lots of crackpot/free-energy web sites. It's junk. SteveBaker (talk) 14:40, 19 June 2009 (UTC)

I second the vote for junk science, with the caveat that it's equally likely that the abstract has been mangled by poor translation. If you can get your hands on the original, you may be able to get a better translation. Nimur (talk) 15:44, 19 June 2009 (UTC)

The original article is not necessarily junk science—it might just be wrong science, or incomplete science, or premature science. --98.217.14.211 (talk) 18:35, 20 June 2009 (UTC)

Ok thanks guys, just thought I would ask.

Rob —Preceding unsigned comment added by 79.67.246.16 (talk) 18:24, 19 June 2009 (UTC)

I couldn't access one of your links. I had a look around for the German text by W. Peschka [2] His report looks like any other "our experiment has shown this result and we don't have any clue as to why" kind of scratching our heads report. They were re-creating some setup originally done by R. Zinsser during WWII. The effect is described as generating "mechanical energy from an an anisotropic gravitational field" Peschka has a reasonable track record in research at the German Aerospace Center. There doesn't seem to have been much further study since 1974. A followup study in 2004 could not re-create the effect and assumes that the previous results were caused by thermal expansion or air movement [3] However Peschka's report seem to have been adopted by some fans of Fringe science. 68.208.122.33 (talk) 19:04, 20 June 2009 (UTC)

Black rope has musty mildew smell. How to clean?

How can one remove a musty mildewy smell from a colored rope? My first instinct is to dip it in bleach, but that would destroy the color. Are there any color safe ways to remove mildew? --Navstar (talk) 14:35, 19 June 2009 (UTC)

Vinegar would work as well as bleach, and smell better too. Use white vinegar diluted no more than 1:3 with warm water, and move the rope around vigorously to make sure it gets in between the twists. Rinse in plain water. Then dry it in the tumble dryer, as it is residual dampness that causes the growth of mold or mildew.- KoolerStill (talk) 15:48, 19 June 2009 (UTC)

Many things you could do to remove mildew from a rope could reduce its strength, as could the mildew itself. Unless it is just a decoration, how about starting with fresh rope and taking better care to store it is conditions not conducive to mildew? [4] gives some hints on cleaning and caring for rope, and says that bleach weakens it as does battery acid. No mention of vinegar. Here is a 1956 Popular Mechanics article on rope care [5]. Here is a book on rigging with a section on rope care: keep it clean and dry.[6] The sources suggest thay poly rope is not likely to mildew. Is your rope natural fiber? Edison (talk) 18:43, 19 June 2009 (UTC)

Some OR on mildewed poly rope: The fibers themselves are not likely to mildew, but that doesn't mean that stuff can't grow in between the twists. There are a number of products on the market that claim to be safe on fabric. (e.g.[7]) If it's just the smell you want to get rid of, go over to the nearest dive shop. They sell little packets to use on dive gear. I'd second opinions voiced above on not using the rope for anything critical and replacing it sounds like a great idea. 68.208.122.33 (talk) 19:28, 20 June 2009 (UTC)

Fishing in Ontario lake

Was fishing today. Saw clearly a pink, almost red fish swimming around. Fish was big - I would say more then 2kg (umm, relatively big). Looked like oversized aquarium fish(only because of the color). Put fish like bait in front of her - but then realized that predator could not wear such color. Asked two fisherman - both come up with idea that it could be carp fish. Why pinkish red? One said that it could be ill(was not looking like ill one at all) and another one joked that it is too much uranium around... Is there a spice that could look like that? Or it is totally weird fish? I'm really a newbie in fishing(caught my very first fish today, Largemouth bass:)) DeadlyPenguin (talk) 17:42, 19 June 2009 (UTC)

Carp, Goldfish, and Koi are all very closely related species. It's very possible that there can be colored variants in the wild, even with never-domesticated fish. (There was a recent story about a yellow lobster caught from the wild, where there are also blue and white variants.) By the way, carp are omnivores/scavengers, so they would have no problem eating dead fish. -- 128.104.112.114 (talk) 20:38, 19 June 2009 (UTC)

Definitly a carp. As IP said before, carp, goldfish, and koi are all closely related, with the common ancestor being carp. Obviously carp ancestors had to have the light colored pigmentation gene otherwise we would not have such vibrantly colored goldfish and koi today. Selective breeding has beautified, and mangled goldfish completely.Drew Smith What I've done 14:36, 20 June 2009 (UTC)

Might be Chinook salmon if carp doesn't fit your bill. [8] —Preceding unsigned comment added by 68.208.122.33 (talk) 19:37, 20 June 2009 (UTC)

Question on J.J. Sakurai's notation

I've just read through the first chapter of Sakurai's Modern Quantum Mechanics, and I've begun working through the problems. The second problem has totally baffled me with notation. Throughout the chapter the dot, $\cdot$ , is used for explicit multiplication, and the boldface dot, which I'm not sure how to produce with this LaTeX, is used for the inner product. In problem 2 (page 60) Sakurai states

Suppose a $2\times 2$ matrix $X$ … is written as
$X=a_{0}+\mathbf {\sigma \cdot a}$
where $a_{0}$ and $a_{1,2,3}$ are numbers.

where the sigma, dot, and 'a', are all boldface, and he asks

a. How are $a_{0}$ and $a_{k}$ ($k=1,2,3$) related to $\mathrm {tr} (X)$ and $\mathrm {tr} (\sigma _{k}\,X)$?
b. Obtain $a_{0}$ and $a_{k}$ in terms of the matrix elements $X_{ij}$.

Since $X$ is $2\times 2$, clearly $a_{0}=a_{0}I_{2}=a_{0}{\begin{pmatrix}1&0\\0&1\end{pmatrix}}$, right? He uses this implied notation in the chapter. But what is $\mathbf {\sigma \cdot a}$? Clearly $\mathbf {a}$ is a 3-component vector. It seems like $\mathbf {\sigma }$ is also. But, if they're both vectors, and the dot is the inner product, then

$X=(a_{0}+\mathbf {\sigma \cdot a} )I_{2}$

and the questions make no sense…to me, anyway.

One further piece of evidence is that in Problem 3 Sakurai refers to the "determinant of a $2\times 2$ matrix $\mathbf {\sigma \cdot a}$" as well as $\mathbf {\sigma \cdot {\hat {n}}}$ "when $\mathbf {\hat {n}}$ is in the positive $z$-direction".

So, $\mathbf {\sigma \cdot a}$ can be a $2\times 2$ matrix! But how? Even if the boldface dot were an outer product, that would make it a $3\times 3$ matrix, which, in some sense, is even worse of a description!

Does anyone see or know what I'm missing? Thanks! — gogobera (talk) 18:54, 19 June 2009 (UTC)

Does he use the notation σ elsewhere? It might refer to the Pauli matrices which are a triple of 2x2 matrices, so it makes sense to dot them with a vector and get a 2x2 matrix. Algebraist 19:02, 19 June 2009 (UTC)

hmm. He hasn't defined them, yet. On p. 23 he writes "…when we discuss the Pauli two-component formalism in Chapter 3". On the other hand, the book expects some introductory quantum experience. I'll go with your suggestion though, for now, and see what comes out of it. Thanks! — gogobera (talk) 19:09, 19 June 2009 (UTC)

Oh yeah, and I know this is a math question, but it being in a Quantum text, I thought someone here might've had experience with it. I'll cross-post, I guess, but only if necessary. — gogobera (talk) 19:05, 19 June 2009 (UTC)

Is there a list of notation at either the beginning or the end of the book? --Tango (talk) 21:06, 19 June 2009 (UTC)

The $\sigma \,$ are the Pauli matrices, as Algebraist sugested. That's why I don't like Sakurai too much. He doesn't explain things well enough. Dauto (talk) 05:11, 21 June 2009 (UTC)