Wikipedia:Reference desk/Archives/Science/2009 November 16

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November 16

Centrifugal force

I'm doing a lab report on ficticious forces, and something doesn't seem to make sense. The design of the experiment is as follows: a stationary channel has a ball roll slowly through it, while the channel is overtop some paper on a rotating platform. Again, the ball is NOT rotating (just going in a straight line), but it's making contact with the paper that it rotating, and every 30 ms or so the ball makes a mark on the paper, so that's its trajectory is traced out. Now according to the lab manual (and in agreement with the results we obtained), the ball will slow down as it approaches the centre, and speed up while it's leaving, and apparently this is due to centrifugal forces.

Now to me, this doesn't make much sense. The ball itself shouldn't be experiencing any forces (ignoring friction), and even in the reference frame of the rotating paper, the ficticious force acting on it should be directed towards the centre, allowing it to continue in a circular path. After all, if the ball wasn't moving at all, it wouldn't start moving away from the centre, so I have trouble believing that there would be a ficticious force acting on it. But I wasn't actually there for the lab; is it possible that it was the ball that was rotating while the paper was stationary, and that my lab partner is just an idiot? —Preceding unsigned comment added by 24.200.1.37 (talk) 00:31, 16 November 2009

In the nonrotating lab frame, the velocity of the ball relative to the paper is the velocity of the ball relative to the lab (a constant) minus the velocity of the paper relative to the lab at the point where the ball is at that moment. The paper is moving more slowly near the center of the rotation, so it's reasonable to suppose that the difference of the velocities will be smaller nearer the center. That's why the marks on the paper were more closely spaced there.

In the real world (as real as academic physics gets, anyway), that would be enough. You don't have to analyze the problem in every frame, you can just pick one and do the analysis there. But since this is a lab, I suppose they want you to repeat the analysis in a rotating frame where the paper is at rest. That seems to be a lot more complicated in this case, so if the intent of the lab is to show you the usefulness of fictitious forces then it isn't doing a very good job. In the rotating frame, the ball experiences the centrifugal force (which acts outward from the center) and the Coriolis force (which acts at right angles to the ball's velocity). You're probably supposed to ignore the fact that the ball is constrained by the channel and supported by it against gravity; the important thing is that the ball moves as it would in empty space with no external forces (other than the fictitious forces). You can write down the equations of motion and try to solve them, but it's going to be ugly. I don't see any easy way to do it aside from solving the problem in the nonrotating frame and then changing the coordinates.

In the case of an object at rest in the nonrotating frame, the centrifugal force in the rotating frame points outward, but the Coriolis force points inward and is twice as large. So the net force is inward; that's the centripetal force that keeps the object moving in a circle. In this case, too, it's easier to see what's going on in the nonrotating frame. -- BenRG (talk) 01:11, 16 November 2009 (UTC)

If the ball is stationary, then it's radial velocity is zero. Since the coriolis force is proportional to the radial velocity, how can it cause a centripetal force? —Preceding unsigned comment added by 24.200.1.37 (talk) 08:21, 16 November 2009 (UTC)

An object following a circular path with constant angular velocity $\omega$ relative to a rotating reference frame which is itself rotating with constant angular velocity $\Omega$ (where axis of path coincides with axis of rotation of frame) requires a centripetal force of magnitude

$mr(\omega +\Omega )^{2}=mr\omega ^{2}+mr\Omega ^{2}+2mr\omega \Omega$

The first term is the centripetal force that is "expected" relative to the rotating reference frame; the second term is a "correction" due to centrifugal force; and the third term is a further "correction" due to Coriolis force. In BenRG's example the object is stationary relative to the non-rotating reference frame, so $\omega =-\Omega$, the Coriolis force term is negative, and the net required centripetal force (after the "corrections") is 0. Gandalf61 (talk) 11:59, 16 November 2009 (UTC)

Stone artifact need help with what it is...

I need help with a stone that my daughter found on Myrtle Beach S.C. It is very old and it is either indian or maybe even from the stone age. We have pictures and would appreciate any help with whom we could email for assistance. Thank you for your time —Preceding unsigned comment added by Donald Cohee (talk • contribs) 00:54, 16 November 2009 (UTC)

If you post the pictures, someone here might be able to help identify the stone. Otherwise, we don't have a directory of Wikipedians with expertise in identifying Native American artifacts. —Akrabbim^talk 01:23, 16 November 2009 (UTC)

Since you actually are in possession of this artifact, you might consider talking to a history or archaeology professor at the nearest university, who may be willing to spend some time looking at it. Nimur (talk) 03:07, 16 November 2009 (UTC)

It's worth noting that removal of antiquities can be regulated in some places. -- Scray (talk) 04:59, 16 November 2009 (UTC)

Also, Native American artifacts are "stone age", excepting those acquired through trade or from far into the "historic" period. Pfly (talk) 06:27, 17 November 2009 (UTC)

tuned circuit

I'm trying to create a tuned circuit for a science project to show how 60 Hz power line frequency intensity can be monitored and displayed on a computer based osciliscope. When I calculate the wavelength for 60Hz however I get an antenna length that is way beyond a length of wire that I can mount in the yard and tune. I know there must be harmonic that I can use with a reasonably shorter antenna and with a coil and capacitor circuit to cilate and perfectly match 60 Hz but I can not find any references or information as to how to calculate the length of the size of the coil and capacitor for a specific 60 HZ harmonic antenna length that will work. Does the Wikipedia have any references for the complete calculations? 71.100.7.189 (talk) 01:52, 16 November 2009 (UTC)

You will pick up 60 Hz whether you want to or not... I have a lot of experience setting out antennas of varying sizes and we always get 60Hz interference. Our AWESOME antennas pick up 60Hz and dozens of its harmonics; you can see its antenna size and download schematics for the antennas, as well as the preamplifier and data acquisition system. I think you can even purchase the hardware and equipment at-cost from Stanford for around $500. In any case, if your preamplifier and amplifier have a good low-frequency response (e.g. similar to a high-quality audio amplifier), you'll have no trouble tuning to the 60Hz at baseband ("zeroth harmonic") even with a small ~ 1 meter antenna. Nimur (talk) 03:15, 16 November 2009 (UTC)

I assume the horizontal lines are the 60HZ harmonics but what I need is a circuit specifically tuned to 60Hz or at least the equations necessary to create the antenna, coil and capacitor to resonate at 60Hz. 71.100.7.189 (talk) 03:47, 16 November 2009 (UTC)

The links I've provided show circuit schematics for a broad-band amplifier; if you want to tune to 60 Hz, take a look at tuned circuit and Q factor. The equations are trivially easy; the difficulty is finding components and getting a good amplifier at such low frequencies. That is why I suggest using a broad-band system (like the one I linked above) and digitally processing the result to analyze the 60Hz band. Nimur (talk) 05:03, 16 November 2009 (UTC)

So are Maxwell equations but their usefulness and comprehension exceeds the elegance of their simplicity. 71.100.7.189 (talk) 05:17, 16 November 2009 (UTC)

After reviewing the few questions you have asked, I think the best advice for you is to review some basic physics; we have some good articles on electronics, electromagnetism, and atomic theory that are all very well-sourced. If you still have specific questions after you are up to speed on the fundamentals, or if you're having conceptual problems understanding parts of the articles, feel free to ask here. Nimur (talk) 06:52, 16 November 2009 (UTC)

You should not be trying to establish a pattern of thinking based on any of my questions since only two questions I have asked originated from the same member of our group. Also if you don't know the answer just say so. 71.100.7.189 (talk) 07:42, 16 November 2009 (UTC)

I would avoid constructing a passive resonator circuit for so low a frequency as 60Hz because of the inconveniently large LC product needed. It is easier to make it either as a bandpass Active filter or as a digital Goertzel algorithm. THe latter is used in DTMF detectors in telephony and might be readily implemented in your computer based oscilloscope. Nimur has given good advice above. BTW I have found that in many urban areas one need only bury a couple of earth rods a few meters apart to be able to pick up a few tens of millivolts of power line frequency between them. Cuddlyable3 (talk) 10:45, 16 November 2009 (UTC)

The phrasing of your initial question implies that you think it is necessary to have an LC circuit tuned to 60 Hz in order to "monitor and display" 60 Hz EMF. You also seem to believe that the antenna has to be resonant at 60 Hz to detect that low frequency. Responders have pointed out the error in these assumptions. You need not imply that the responders "Don't know the answer." You may just be asking the wrong question so far as your research goal is concerned. EMF is commonly measured by small instruments which allow display on PC's without some impossible 2500 km antenna. I have used them myself, and they are fairly small. I expect that some of the electronics hobbyist magazines have published "build-it-yourself" instructions for such a meter with 3 small coil antennas. A pickup coil producing 1 mV per 20 mG is available for under $5 online [1]. Edison (talk) 20:16, 16 November 2009 (UTC)

Here is a warning if the OP is considering measuring the electric field near high voltage power or overhead tram cables using some kind of homemade probing antenna. Don't. Cuddlyable3 (talk) 21:29, 16 November 2009 (UTC)

You will easily pick up lots of mains frequency (60Hz) signal just by connecting a short piece of wire to your oscilloscope input. To make the waveform look more sinusoidal, just use a passive RC low pass filter directly at the scope input and connect your wire to that. —Preceding unsigned comment added by 79.75.63.71 (talk) 00:30, 18 November 2009 (UTC)

Tsar Cannon with two ton cannonballs?

Sorry about the random inquiry, was just interested with reading a random article I found. Apparently the Tsar Cannon has cannonballs placed in front of it, weighing two tons each according to my interpretation. The cannon was also fired at least once, with gunpowder, according to analysis. Although the diameter of the cannonballs is larger than the barrel diameter of 35 inches, and they were made only for the purposes of decoration, if the cannonballs could fit, would a reasonable amount of gunpowder be sufficient enough to propel a two ton cannonball? And if so, to what distance? Thanks, ♠The Ace of Spades^♣♥_♦ 02:00, 16 November 2009 (UTC)

Not a cannonball, but two tons. Nimur (talk) 03:55, 16 November 2009 (UTC)

Well, this Tsar Cannon gun maybe isn't a suitable design; but a modern battleship from the 1980s was armed with a 66-foot long main battery capable of firing a 1200 kg projectile up to twenty miles. The explosive powder propellant is much more advanced than "gunpowder" (as it would have existed in the 16th century). Our article describes the propellant as "smokeless Powder Diphenylamine (SPD)" with a "titanium dioxide and wax compound known as 'Swedish Additive'"; and polyurethane jackets placed over the powder bags. When the USS New Jersey bombarded the coast of Lebanon, the Navy referred to the projectiles as "flying Volkswagens"[2] in an effort to demoralize the enemy[3]. Targeting for these 2-ton projectiles was grossly inaccurate (blamed on calibration errors in the explosive powder mixture). Shells from this barrage missed their intended targets by many miles, resulting in massive civilian casualties; this was cited as a motivation for the subsequent attack on the United States Marine barracks in October 1983. Nimur (talk) 03:43, 16 November 2009 (UTC)

Perhaps, seeing that the grapeshot that the Tsar Cannon fired weighed about a ton and three-quarters, according to the article. bibliomaniac15 04:15, 16 November 2009 (UTC)

20 miles? ha ha ha ha ha ha ha. Lucky if it went 20 yards. Here are my calculations, first the initial assumptions;

• explosive pressure of black powder = 67 ton/m² (might need adjusting - I got that from a fireworks article)
• energy content of black powder = 3 MJ/kg
• mass of projectile = 800 kg (from article - don't know where Bibliomaniac got ton and three quarters, I don't see it)
• calibre: 890 mm
• barrel = 5.34 metres

From the calibre I get a csa of 0.622 m² and from the pressure a force of 44.3 tons. Newtons second law arrives at an acceleration of 543 m/s². From $\scriptstyle v={\sqrt {2ax}}$ we get a muzzle velocity of a measly 76 m/s - you would probably do more damage head butting the enemy. Maximum elevation and ignore aerodynamics etc gets you a flight time of 7.77s and a distance just over 300 metres. The kinetic energy on leaving the barrel is 118 MJ which, if energy is transferred 100% efficiently, requires about 40kg of black powder.

For a 2 ton projectile it comes out at 118 metres and 16 kg of charge. Of course, many subtleties are neglected here, not least of which is the assumption that the charge continues to burn all the way up the barrel and is completely used by the time the projectile exits. That can be wrong both ways, the charge could burn up before we are done accelerating the projectile, or the charge might not burn fast enough leaving part of it uselessly burning after the projectile has exited. An ideal weapon has the barrel length "tuned" to the missile you are trying to launch and the propellant you are using to launch it, but this one most probably isn't. SpinningSpark 22:23, 16 November 2009 (UTC)

Actually, I've made an error in the amount of powder calculation, it comes out the same for both cases and is less than 1kg, but I don't believe for a minute that is the correct load for this gun in reality. SpinningSpark 22:48, 16 November 2009 (UTC)

The 20 mile range I quoted was never for the Tsar Cannon. It was for the main battery of an Iowa-class battleship; I only mentioned it to illustrate that large artillery does exist, but it is a very different and much more modern design than Tsar Cannon. As for "head-butting", an 800 kg object moving at 70 meters per second would have an impact equal to a large car going faster than highway speeds (150+ mph); I don't know why you would consider that weaker than headbutting. It's still no battleship, though. Nimur (talk) 01:01, 18 November 2009 (UTC)

You are assuming the enemy are too stupid to move out of the way. They will have plenty of time, it must take all day to load the thing, and even in the seven seconds flight time, they could still run a considerable distance. SpinningSpark 18:42, 18 November 2009 (UTC)

Thanks, all. ♠The Ace of Spades^♣♥_♦ 03:34, 17 November 2009 (UTC)

String theory and GUTs

Could there be string vacua that approximate the standard model without allowing proton decay? 76.67.73.245 (talk) 04:09, 16 November 2009 (UTC)

Nobody knows whether there are string vacua that look enough like the standard model to be consistent with experiment. Of course, the proton lifetime doesn't have to be infinite to be consistent with experiment, it just has to be larger than 10³⁴ years or whatever the latest lower bound is. If you're wondering whether string vacua with an infinite proton lifetime have been ruled out, then I don't know, but I imagine not. -- BenRG (talk) 23:08, 17 November 2009 (UTC)

Recreating extinct species

Okay, this is mostly just a small idea that I've been toying with. With current genetics research, I am fairly sure that at some point we will have the capability to clone and reintroduce extinct animals to the wild, but only some have enough intact DNA to do this. I was just trying to think of a list of the more important extinct species that could be brought back. For example, the passenger pigeon and the Carolina parakeet have enough DNA remaining via feathers to clone them and have genetic variation, but the dodo bird likely does not. I was wondering if anybody could add to the list of animals where enough material remains to clone them and retain genetic variability in the cloned population?

1. Passenger Pigeon
2. Carolina parakeet
3. Elephant bird - long shot unless the eggs retain enough DNA.
4. Great auk
5. Thylacine
6. Quagga

Thanks, Falconus^{p t c} 05:11, 16 November 2009 (UTC)

[4] looks like an interesting read. It's currently cited in the Passenger Pigeon article. --Jayron32 05:19, 16 November 2009 (UTC)

Thanks, that was interesting. Also, apparently feathers don't actually contain DNA. Falconus^{p t c} 05:31, 16 November 2009 (UTC)

Bad idea. Eventually they'd figure out how to mate and then we'd have to listen to some flirty nonsense about chaos theory. In all seriousness though, the issue is going to be the "intact" part. DNA samples won't hold very well and the amount of reconstruction one would have to do would be enormous, to a point where you would likely have a very different animal (assuming viability). It's doable for things like the recreation of the 1918 flu since there are only 8 genes. ~ Amory (u • t • c) 13:58, 16 November 2009 (UTC)

The mammoth is often put forward as a likely candidate for the "Jurassic park" treatment. Large amounts of mammoth meat can be found deep-frozen in the north of Siberia - and it's thought that a modern elephant could provide the necessary egg & womb for turning DNA into living animal. Pidgeons & parakeets could be similarly dealt with if the DNA is in good enough shape - but finding a parent animal for an elephant bird, auk thylacine or quagga might be tough. SteveBaker (talk) 14:03, 16 November 2009 (UTC)

Steve, have a look at Quagga Project. ZUNAID●FOREVER 14:57, 16 November 2009 (UTC)

And Neanderthal genome project for a jockey?.Cuddlyable3 (talk) 21:21, 16 November 2009 (UTC)

Re-expanding a shrunk cotton shirt

Some questions regarding the hydrophilic properties of cellulose fibers such as cotton and wool:

• Why is it that materials made from cotton or wool shrink when immersed in hot water and allowed to dry? I mean, what's happening to the polymer? Does the water become permanently bound into it, resulting in permanent swelling of the fiber, thereby decreasing its length, or does the water evaporate and somehow leave behind a shrunk fiber?
• Why does this not happen with cold water?
• Is it possible to re-expand the fibers to the original length after shrinking in this fashion?

My inquiring mind was unable to find answers in the articles about cellulose, cellulose fiber, or hydrophilic. ~Amatulić (talk) 06:54, 16 November 2009 (UTC)

This is not a perfect fix but it helps if you hang a shrunk tee up wet. It would be nice to understand what is happening. My uninformed guess is that being wet allows the shirt fibers to become more disordered, increasing the entropic force of the fabric against stretching. -Craig Pemberton 08:42, 16 November 2009 (UTC)

The process of turning cotton fibers into threads and then weaving them into clothes involves pulling and twisting the fibers. As a result many of the fibers (and their constituent molecules) are distorted into an elongated state. They are weakly bound into that state by binding against other fibers and threads. Heat, water, and/or agitation (e.g. a hot wash) cause some the fibers to free themselves and relax into their natural, unstressed state. It's that act of relaxing that causes the clothing to shrink. Cotton and other organic fibers that have complex textures are more prone to stretching and shrinking than simple synthetic fibers. To a degree one can reduce / counteract shrinkage by manually stretching out the article with applied force but this will never be more than a partial solution since no about of pulling will really replicate the effects of making thread. Dragons flight (talk) 11:21, 16 November 2009 (UTC)

I guess, then, making a garment from pre-shrunk cotton thread won't prevent the garment itself from shrinking later, because the act of weaving would stress the threads. ~Amatulić (talk) 18:28, 16 November 2009 (UTC)

Flying Machine

Could this machine fly?

Are there theoretical limits on size? (I mean way beyond Spruce Moose size) TheFutureAwaits (talk) 07:26, 16 November 2009 (UTC)

The machine depicted appears to have no wing structures - even in a cutaway diagram one might expect to see spaces for their supports: is it actually meant to be an aircraft, or merely an airscrew-driven hydroplane?

If flight is postulated, I suspect it would be too heavy, especially given the old fashioned and bulky natures of many of its components. I don't think there are any theoretical limits on an aircraft's absolute size, but there must be practical limits determined by the aerodynamics of a given atmosphere and the strengths and weights of available materials. A major consideration would be the square-cube law, which dictates that as the linear size of an object increases, its surface area increases proportionallty to the square of the linear increase, but its volume (and hence weight) increases proportionally to the cube - this rapidly increases the wing loading of a conventional aircraft design beyond what will allow flight. 87.81.230.195 (talk) 08:49, 16 November 2009 (UTC)

You could try to demonstrate infeasibility by using a rough and ready calculation for the torque which would be applied to the propeller shaft, and see if any known material has the required strength. -Craig Pemberton 09:08, 16 November 2009 (UTC)

I don't see anything that indicates that this isn't just a multihull boat.-Craig Pemberton 09:11, 16 November 2009 (UTC)

The pontoons are labeled as being "retractable," which I would think would be more likely in the design for a plane than for a boat. My presumption is that it's intended to be a plane, with the wings just not visible in the cutaway. Red Act (talk) 09:22, 16 November 2009 (UTC)

Silly; any vehicle this extravagant is bound to be pimped out. -Craig Pemberton 22:47, 16 November 2009 (UTC)

Also, the rudder sticks up above the craft, not down below it like would make sense for a boat. Red Act (talk) 09:27, 16 November 2009 (UTC)

From the style of the sketch (that's all it is) especially looking at the cars, this was drawn around 1920-30. Anything can "fly" if it gets enough lift. So start by estimating what this monster weighs. Cuddlyable3 (talk) 10:21, 16 November 2009 (UTC)

From the drawing I would have thought that this is meant to be a blended wing body / flying wing aircraft. See alsoJunkers G.38. Boeing clearly thinks this to be an option for future aircraft and is designing one for 800 passengers which can use existing runways. --Cookatoo.ergo.ZooM (talk) 11:06, 16 November 2009 (UTC)

You only showed us one picture...there are more! [5] and [6]. Clearly it's not intended to be a boat. It's very clear that it wouldn't have stood a chance of flying...at least not with the internal design as shown. The artist clearly didn't understand anything about airplane design.

As a flying machine - it's pretty impractical. Using a steerable rear propeller and more props out on the "wings" for turning the thing is a really clunky idea (especially as there is no indication of how those things are powered) - and the de-icing trick results in the need for a double skin which would be heavy. There is a pathetically small fuel supply - yet the fact that there are "quarters" even for the crew and vast amounts of water storage suggests anticipated flight durations of more than just a few hours. There is essentially zero internal structure that would give the thing strength - all we see are partition walls separating rooms...no structural girders except in areas where the artists couldn't think of any useful purpose for the interior space. The weight of things like fuel and engines - plus water tanks and heavy cargo are not well-distributed through the structure - it looks like it would tear right down the middle where the super-weak elevator shaft splits the two heavy sections! The center-of-gravity of an airfoil needs to be at the same place as the center-of-pressure - and that's at the thickest part of the cross-section - this doesn't look like it would have enough weight up-front...but then with those gigantic diesel engines...who know?

There are certainly a lot of oddities to the design...the elevator motors are gigantic! Any idiot could see that if they had moved some of the cargo storage forwards into the bottom of the craft and put the reception areas and offices on upper floors - they'd have needed far fewer stairwells and elevator shafts and perhaps stood a chance of getting the center of gravity in the right place.

This is a classic example of something an engineer would never come up with. Some artist sketched a design for an interesting-looking exterior in 2 minutes flat and then spent the next couple of days carefully filling every cubic inch with something that a cruise liner might need - without giving any consideration whatever for structural strength, power-to-weight ratio, center of gravity, etc.

Bottom line: No - it wouldn't fly.

SteveBaker (talk) 11:18, 16 November 2009 (UTC)

It would make a great submarine. Cuddlyable3 (talk) 11:57, 16 November 2009 (UTC)

Especially if there was a large body of water right at the end of the runway! :-) SteveBaker (talk) 13:58, 16 November 2009 (UTC)

Look how much it's banking in that cover shot! I hope nobody is in those bathtubs! APL (talk) 16:21, 16 November 2009 (UTC)

It's an ocean liner with wings. It is designed just like an ocean liner and would probably weight as much (thousands of tonnes). There is no way to get something that heavy to fly by conventional means. It's not that it was designed by a non-engineer, it was designed before the realities of flight were well understood. It looks like it was designed by a nautical engineer (or someone with experience of ship design) thinking flight would be the same kind of thing. --Tango (talk) 20:05, 16 November 2009 (UTC)

Now I've seen the full magazine item at the page linked to below I see it doesn't have wings, per se. It is supposed to be one big wing. This seems like an obvious idea until you realise that greater weight requires greater wingspan, which means that you need to be able to increase the wingspan without increase the weight much, which means having light wings. That necessitates a design like a regular aircraft with a central thick body and thin wings (although the wings don't have to stick out the sides, they could go forward and back - there have been some interesting ideas along those lines). --Tango (talk) 21:10, 16 November 2009 (UTC)

The design is from 1932 or 1934 (the page says both), and is estimated by its designer to be 10,000 tons. Dragons flight (talk) 20:31, 16 November 2009 (UTC)

It's from '34. The article refers to an earlier design from '32. --Tango (talk) 20:59, 16 November 2009 (UTC)

BTW, we have an article on the guy that designed this thing: Hugo Gernsback. --Tango (talk) 21:17, 16 November 2009 (UTC)

Another couple of problems with the design:

1) Transonic propellers: At 80 feet in diameter, and at the speeds those propellers would need to rotate to keep this monstrosity aloft, the tips would go supersonic, while near the shaft they would remain subsonic. The forces on such propellers would rip them apart.

2) Unstable flying wing design: A flying wing is inherently unstable, requiring continuous control surface adjustments to keep it flying straight. This made flying wings a bad idea in the 1930's. They are more feasible today, since we can use a computerized fly-by-wire system to make those adjustments many times a second. However, if the computer fails, you're in trouble. StuRat (talk) 23:20, 17 November 2009 (UTC)

not exactly dreaming?

I think dreaming refers to REM sleep, a stage that occurs after you've been asleep for a while. There's another dreamlike experience when initially falling asleep. It's possible to forget where you are and what you're doing, and confuse imagined activities with reality, while drifting off. Is there a name for that, if it's not technically the same thing as a dream? 69.228.171.150 (talk) 07:29, 16 November 2009 (UTC)

The article Hypnagogia may give you some leads, both in itself and through the links in its "See Also" section, such as Hypnopompic. 87.81.230.195 (talk) 08:34, 16 November 2009 (UTC)

Thanks, that article is great. 69.228.171.150 (talk) 21:20, 16 November 2009 (UTC)

mathematcal model of PMSM/AC servo motor

hi everybody my project is to control the position of AC servo motor by FPGA by sliding mode control so i need the mathematical model i am try to derive the model but i m facing a lot of problem i m very new to the electric machines i know the basics of the servo motor but it is three phase motor not like Dc motor which is qite simple and its modeling also so any body on wiki kindly help me i want state space modellike[A][B][C][D] matrix so that i can simulate it in MATLAB/SIMULINK and then implementation of sliding mode control technique on AC servo motor i want help about the sliding mode algoritham thanks in advance kindly reply

regard abhay

anembryonic gestation

does anembryonic gestation negatively affect further pregnancy?202.83.57.17 (talk) 09:33, 16 November 2009 (UTC)remy

Not an answer, I'm just adding a link to the Anembryonic gestation article, for others who might be interested. --NorwegianBlue ^talk 15:20, 16 November 2009 (UTC)

Perhaps you meant subsequent -- apparently, spontaneous abortion is thought to include the subset of anembryonic gestation. DRosenbach ^{(Talk | Contribs)} 17:18, 16 November 2009 (UTC)

Special Relativity

In special relativity, if you measure a particle of having an average lifetime of say n seconds as measured in the rest reference frame, and you know its speed, do you just use distance = speed x time to find the average distance it travels in the rest reference frame? —Preceding unsigned comment added by 94.193.67.204 (talk) 10:24, 16 November 2009 (UTC)

If by "rest reference frame" you mean the reference frame of the laboratory, then the average distance a particle travels as measured in that frame, is calculated as Average_speed x Average_rest_life_time x Gamma, where Gamma is 1/sqrt( 1-v^2/c^2 ). The average "time" that was measured of the "still" particles gets dilated by a factor gamma when the particles are moving. DVdm (talk) 10:48, 16 November 2009 (UTC)

The average lifetime is measured as the particle is moving, in the laboratory reference frame, so to find the average rest life time would you divide the average lifetime by gamma? —Preceding unsigned comment added by 94.193.67.204 (talk) 11:26, 16 November 2009 (UTC)

Yes, if the average lifetime is measured on moving particles. Average_rest_life_time x Gamma = Average_moving_life_time. See Time dilation#Time dilation due to relative velocity.

By the way, please sign your messages with 4 tildes (~~~~)? Thanks. DVdm (talk) 11:38, 16 November 2009 (UTC)

Hang on. I think there is some confusion here. You only need to use gamma if you are converting from one frame to another. So, if you are measuring the average life time and the distance travelled in the same frame then you can just do distance=speed*time. If the average life time is measured with the particle at rest (which is the normal value quoted) and the distance is measured in the laboratory frame, then you need to convert the life time at rest to a life time in the lab frame by multiplying it by gamma. You can multiply that by the speed to get distance. --Tango (talk) 19:40, 16 November 2009 (UTC)

Yes. To avoid that confusion I introduced the names Average_rest_life_time and Average_moving_life_time. DVdm (talk) 20:55, 16 November 2009 (UTC)

Recycling

In the United States, is it generally a problem or not to include paper envelopes with plasticine address "windows" with mixed paper recycling? Thank you --71.111.194.50 (talk) 13:10, 16 November 2009 (UTC)

No it isn't. Recycling plants have techniques for removing small amounts of contaminants like this. I can't believe I just answered a refdesk question with a link to Yahoo Answers... —Akrabbim^talk 15:46, 16 November 2009 (UTC)

Bahahahah... you're a regular though, so we have to let that slide. :-) ♠The Ace of Spades^♣♥_♦ 03:32, 17 November 2009 (UTC)

Many such envelopes have a plastic lining that makes them fairly unrecycleable. If you wet your fingers and rub the envelope, the paper will rub off and you'll see a very thin plastic sheet. Ones like that should just be thrown away. --Sean 17:17, 16 November 2009 (UTC)

I don't think you meant plasticine... --TammyMoet (talk) 19:46, 16 November 2009 (UTC)

I have seen many that simply have a hole cut in the paper, and replaced with a clear plastic piece. —Akrabbim^talk 19:59, 16 November 2009 (UTC)

Yes indeed - but that's simply clear plastic, not plasticine, which if you read the linked article, you will see is based on modelling clay! --TammyMoet (talk) 10:23, 17 November 2009 (UTC)

The OP's "plasticine" was clearly a slip for "glassine". I don't think glassine is used much in window envelopes these days, having been superseded by clear plastics as noted above, but it used to be. Deor (talk) 13:02, 17 November 2009 (UTC)

Just to check if my notions are still correct, recycling still is a worthless waste of resources to persue an unrealistic hippie dream right?Bastard Soap (talk) 11:34, 18 November 2009 (UTC)

Perhaps for some materials. We are pretty good at recycling metals though. Googlemeister (talk) 14:13, 18 November 2009 (UTC)

Do arrows have "lift"?

My question concerns the following claim made in this version of the "Quarrel" article:

"Bolts and arrows have different flight characteristics. Bolts fall at the same rate, independent of the speed of flight. Arrows, in contrast, depend on gaining lift in flight, and hence have their centre of balance at a point about 9% of their length forward of their center. This is called 'point planing'."

No reference is (at present) cited in-line for this information. What I'm wondering is: does that make any sense?
On the "Quarrel" talk page I made the comment:

"To have lift, you need an airfoil, but the fletching article makes it clear that the fletchings are there only to "stabilize the arrow in flight" (ie., they are not wings). There is no mention of "lift" in the arrow article."

But then I started wondering about eg. paper airplanes, hang gliders, etc. – the "wings" of which do not (necessarily) seem to have an airfoil shape, either. Some of the "lift" in those cases apparently derives from "angle of attack" issues, but could that really be what's going on (in a consistent, reliable way) with arrows, too? Wikiscient 17:20, 16 November 2009 (UTC)

Air foil shape is useful for getting lift while minimizing drag. Ever see pieces of plywood flying around during a hurricane? Those are not airfoil shape and certainly have lift. Googlemeister (talk) 17:29, 16 November 2009 (UTC)

Well, that refutes my "to have lift, you need an airfoil" remark, but I had already done that myself by bringing paper airplanes etc. into it.

I'm still trying to understand better how "lift" works with arrows, if, in fact, there is any (which, presumably, would be attributable somehow to the fletchings, which neither bolts nor bullets have). Wikiscient 17:40, 16 November 2009 (UTC)

Some (but not all) crossbow bolts do have fletchings. See Crossbow#Projectiles, and this picture. Red Act (talk) 18:15, 16 November 2009 (UTC)

Ok, thanks for the clarification. I guess that just makes the statement in the crossbow bolt article which prompted my question all the more interesting or "incorrect," I'm still not sure which! :S Wikiscient 18:31, 16 November 2009 (UTC)

Wouldn't a crossbow bolt also have that same sort of lift if it was fired at the appropriate angle of attack? APL (talk) 17:32, 16 November 2009 (UTC)

I'm not completely certain of this, but I think the answer to that would be: yes, but not much! Wikiscient 17:43, 16 November 2009 (UTC)

That doesn't make sense to me - shouldn't the centre of balance be further back than the centre of the arrow in order for it to have positive angle of attack and thus have lift? That said, I think the key difference between arrows and bolts is that bolts go faster. That means they don't really need any lift in order to maintain level flight, there just isn't time for them to fall far before hitting their target. --Tango (talk) 18:31, 16 November 2009 (UTC)

Well, again, my question is about this quote (from the crossbow bolt article):

"Bolts fall at the same rate, independent of the speed of flight. Arrows, in contrast, depend on gaining lift in flight..."

There is no reason an arrow couldn't be launched with a higher velocity than a bolt, so I don't think that is the "key difference" in this case.

The "key difference" does seem to have something to do with the location along the length of the projectile of the center-of-gravity. And I am wondering the same thing: placing that forward of the center-of-length would, intuitively, seem likely to create a downwards angle-of-attack force! On the other hand, if the fletchings were providing lift the way an airfoil does (ie. even in level flight), you'd want to have your center-of-gravity forward of center-of-length to compensate for that "upwards-from-the-rear" force.

So, anyway, I'm still trying to understand what the quote in question is trying to say... Wikiscient 19:00, 16 November 2009 (UTC)

See, I'm just confusing myself with this, *sigh*...Wikiscient 19:04, 16 November 2009 (UTC)

Arrows are fired by a bow, bolts are fired by a crossbow. The ways those two devices work mean that bolts are fired faster. That is (part of) why different projectiles are used. As I understand it, the fletches make the arrow spin, providing stability by the gyroscopic effect. Since the arrow is spinning, there is no way the fletches could provide lift (any such force would rotate with the arrow and could average itself to zero). --Tango (talk) 19:31, 16 November 2009 (UTC)

I should clarify - the fletches probably are aerofoil shaped in order to get optimal torque. Each fletch will provide a force in a different direction and, since they are equally spaced, they will all cancel out so there is no resultant force, just a torque (rotational force). --Tango (talk) 19:33, 16 November 2009 (UTC)

No, both of these claims are wrong. Fletches are, modulo production errors, straight and symmetrical. They do not stabilize the arrow by rotation, but simply act as aerodynamic fins. If the arrows velocity is not parallel with its heading, the increased drag of the fletches acting on a long lever will right it again. --Stephan Schulz (talk) 22:39, 16 November 2009 (UTC)

Agreed. So, hmmm... Wikiscient 22:50, 16 November 2009 (UTC)

Assuming that an arrow/bolt behaves in a similar manner to a javelin, the center of gravity is placed relative to the center of pressure to achieve optimal lift. By sport rule, javelins have the center of gravity pushed forward to make them plummet and stick into the ground as opposed to gliding along and skipping along the ground when they land. -- kainaw™ 18:53, 16 November 2009 (UTC)

Optimal lift for an arrow or bolt would, presumably, be the amount of life required to keep the flight level (so you don't have to compensate for trajectory when aiming). That, I would think, would require the centre of gravity to be just behind the geometric centre. (Of course, arrows are sometimes fired up so they travel a greater distance and hit the enemy from above - that would have a different optimal lift, although to get maximum range I would thing it would still want the centre of gravity behind the geometric centre.) --Tango (talk) 19:31, 16 November 2009 (UTC)

Google finds many sources for the "point planing" claim and 9% figure but I leave it to others to decide if any meet WP:RS. Cuddlyable3 (talk) 20:57, 16 November 2009 (UTC)

Almost all those hits look like Wikipedia mirrors to me. --Tango (talk) 21:41, 16 November 2009 (UTC)

Has anyone found a good way to exclude those pesky mirrors from search results? A lot of them don't even say where they got it from so a simple -wikipedia doesn't work. —Akrabbim^talk 22:32, 16 November 2009 (UTC)

If you only want to exclude mirrors of a particular article, just choose a sentence from the article that doesn't sound likely to appear in other articles by change and do -"that sentence". --Tango (talk) 22:35, 16 November 2009 (UTC)

However, "-" terms don't always work in Google if the page uses frames. I think it applies the search criteria separately to each frame but returns the whole page. Unless it's changed since I asked about it and they answered. --Anonymous, 23:52 UTC, November 16, 2009.

Yes, there is a lift force that acts on arrows. There is also lift on crossbow bolts (especially those bolts that have fletching), but the lift is not as significant there.

The primary purpose of the fletching on an arrow is to provide stability. The stability is the strongest in the case of a helical fletch, which imparts a strong rotation around the roll axis. However, a helical fletch results in a slower arrow, and can cause problems with the fletching clearance on the arrow rest. For that reason, a straight fletch is often used, or an offset fletch, which is intermediate between a helical and a straight fletch.

In addition to stability, the fletching also provides lift. Fletching is not shaped like an airfoil, as a glance at the pictures in the fletching article will show, but that's not necessary for lift to occur.

To visualize the forces involved, it's convenient to first think about a horizontal arrow that you just drop, and then consider how that picture changes due to forward motion of the arrow. That won't work well quantitatively, because Bernoulli's principle makes life complicated, but it will at least help to picture some of the physics involved qualitatively.

When you just drop a horizontal arrow, the air resistance on the arrow causes an upward force, especially on the fletching, that wouldn't exist if the only force involved were gravity. That upward force is considered to be drag, which is defined as the component of the aerodynamic force acting opposite to the direction of the arrow's motion, which in this case is downward. Now suppose the arrow also has a horizontal component to its velocity. The upward aerodynamic force still exists, but now that upward force is mainly considered to be lift, since that's defined as being the component of the aerodynamic force that's perpendicular to the arrow's direction, which is now mainly in the horizontal direction. The fletching doesn't have an airfoil shape, so with a straight fletch and an arrow moving purely horizontally (implying a zero angle of attack), there would be no lift. But during the second half of the arrow's trajectory, at least, the arrow is also falling, and the air's resistance to that vertical motion counts as lift.

The quarrel article gives a poor explanation of why an arrow's center of gravity is placed where it is. The reason is that it's a tradeoff between lift and stability.

First, consider how the arrow's center of gravity affects lift. Suppose most of the arrow's mass was right near the tip. Now if you hold the arrow horizontally and let go, the tip of the arrow will drop faster than the nock end, because the upward force on the fletching is in this case much more effective at holding up the nock end of the arrow than the tip. Now consider the same arrow, with the tip still sagging below the nock end, but now with also a horizontal motion to the arrow. Due to the attack angle of the fletching, there isn't going to be as much lift on the arrow, or the lift might even be negative. So the arrow's going to take a nosedive. In contrast, if the arrow's center of mass was back far enough that it stayed horizontal when it was dropped, the loss of lift would be avoided. So for the sake of lift, it's good to keep the center of mass back away from the tip.

Now, consider how the arrow's center of gravity affects stability. Suppose most of the arrow's mass was in the nock, behind the fletching. In this extreme situation, the inertia of the nock tends to keep the nock going at a constant speed, while the drag on the fletching works to make the fletching part of the arrow lag behind. So the forces on the arrow favor the arrow turning around, in obviously a completely unstable situation. As the center of mass is moved forward, the arrow's stability improves, with the maximum stability occurring when the center of mass is at the tip.

Trading off keeping the center of mass away from the front for the sake of lift, and keeping the center of mass away from the back for the sake of stability, leads to a (somewhat empirically determined) optimal compromise between the two.

For a good discussion of the technical details of arrows, go to here, and click the "Archery Technical" link. The "Fletching Arrows" and "Arrow Rotation" articles are particularly helpful toward this question. Red Act (talk) 01:59, 17 November 2009 (UTC)

Thank you for a very clear explication of the issue! Wikiscient (talk) 21:10, 17 November 2009 (UTC)

Dreaming

Why does it seam so realistic, as to be virtual reality?Accdude92 (talk to me!) (sign) 20:28, 16 November 2009 (UTC)

No one can tell you that it's all nonsense without waking you up, so what else can you believe? Cuddlyable3 (talk) 21:01, 16 November 2009 (UTC)

If it seemed unreal then you could pass off whatever messages your subconscious is giving you as being unimportant. Vranak (talk) 22:48, 16 November 2009 (UTC)

This is a Reference Desk, so let's at least attempt to supply some references, people. My only reference is the obvious Dream article which seems to have a lot of problems and does not address your excellent question. It has many, many links to other interesting articles. Comet Tuttle (talk) 23:07, 16 November 2009 (UTC)

Freud, subconscious, ego. I thought this stuff was common knowledge! Vranak (talk) 01:58, 17 November 2009 (UTC)

I have to point out that Virtual reality is not nearly as realistic as dreams. Not yet, anyway. APL (talk) 23:41, 16 November 2009 (UTC)

Don't forget that it's not your eyes that see during the day, it's your brain. If a dream is just a series of images created by your brain, well then that's no different that what happens when you look at something normally. While awake, you know and can tell the difference between what you are seeing and your imagination (the latter is probably hampered by the former) but you don't have that luxury while asleep. ~ Amory (u • t • c) 23:53, 16 November 2009 (UTC)

That's a good point. It ties into the ideas of some contemporary neurologists (Damasio, Ramachandran), who say that memories of sensory experience seem to activate the same brain centers as actual physical sensations. But memories usually seem less real because we have a constant stream of more vivid sensory information coming in to contradict them. If new sensations stop coming in (sensory deprivation, phantom limbs, possibly other neurological conditions like schizophrenia) we can't make that distinction anymore, and we experience convincing hallucinations. Neat stuff. Indeterminate (talk) 07:33, 17 November 2009 (UTC)

microwave cooker question

What would happen to a person who is standing in front of your standard home microwave for 2 minutes if it was running with the door open? I presume most of the microwaves are focused towards the center of the device and you would only encounter a few stray bits that were reflected? Googlemeister (talk) 21:16, 16 November 2009 (UTC)

The Wikipedia article on Microwaves leads me to believe you might get burns and the radiation could possible cause cataracts, though it seems no long term effects (other than tissue damage) would be suffered. Obviously nothing good can come of this, and since it is non-ionizing radiation, there's no way to get super powers 206.131.39.6 (talk) 21:27, 16 November 2009 (UTC)

Someone with metal or electrical implants might fare worse. Before microwaves were properly shielded they posed a threat to people with pacemakers. APL (talk) 21:30, 16 November 2009 (UTC)

Two summers ago I was living in a place with a shared kitchen and I used the microwave a number of times before I realized that there was a hole two-inches in diameter in the front door. I never used it again, and did not develop anything afaict. ~ Amory (u • t • c) 23:49, 16 November 2009 (UTC)

How could an infinite universe expand?

As far as I understand it the possibility of an infinite universe has not yet been removed from the drawing board but how would this play into the big bang and inflation, can infinity originate from a point and can infinity expand?Bastard Soap (talk) 22:22, 16 November 2009 (UTC)

If the universe is infinite then it as always been infinite. At the big bang it wouldn't have been a point of infinite density, it would have been an infinite volume of infinite density. However, the observable universe (all the space light has had time to reach us from) is definitely finite, so that would have been a point. People often refer to the observable universe as simply "the universe" (since it contains everything that can have any effect on us, meaning we can just ignore anything outside it), which is the cause of this confusion. --Tango (talk) 22:26, 16 November 2009 (UTC)

Think about all the real numbers in [0,1]. They are uncountably infinite. If you take a new set, 2[0,1], it is "larger", yet it is still infinite. You can do the same thing with an unbounded set such as [0,$\infty$). Cardinality theory might help you. -Craig Pemberton 22:54, 16 November 2009 (UTC)

The OP wasn't asking about different sized infinities, but rather how something of zero size can being infinite, which is a good question. Also, what makes you think the OP understands interval notation and terminology like "unbounded set"? While they might well do, it is best not to assume certain prior knowledge without good reason. --Tango (talk) 23:30, 16 November 2009 (UTC)

As BenRG puts it, universal expansion is just "stuff moving away from other stuff". Think about it that way and it shouldn't be so confusing. --Trovatore (talk) 23:03, 16 November 2009 (UTC)

I don't know if that's the quote I want to be remembered for. But, yes, when astronomers say the universe is expanding what they mean is that the galaxies are moving apart, and they can do that no matter how many of them there are—even if it's an infinite number. -- BenRG (talk) 11:46, 18 November 2009 (UTC)

An infinite volume of infinite density? That stuff is still on the drawing board? Doesn't that involve infinite energy?92.251.33.207 (talk) 19:30, 17 November 2009 (UTC)

Does it bother you that an infinite universe might have an infinite energy? Why? Dauto (talk) 19:52, 17 November 2009 (UTC)

Wouldn't infinite energy rule out a final heat death? Googlemeister (talk) 21:36, 17 November 2009 (UTC)

We need Stephen Hawking for this question. Is Stephen Hawking in the house? Bus stop (talk) 21:45, 17 November 2009 (UTC)

Why do you keep saying that? -- BenRG (talk) 11:46, 18 November 2009 (UTC)

Dauto is it being picky now to expect the universe to have finite energy? Universes these days, going to hell in a handbasket I tell ya!! Must be that rock music and violent video games! Can you have the positive and negative forces equalling themselves if mass is infinite? Googlemeister seems to me that if the universe can expand from a state in which every god damn piece of it is undergoing fusion to the current state, it must also be able to expand to ultimate heat death state, at least my common sense suggests so, don't know if the universe is laughing maniacally at my puny notions though.193.188.46.64 (talk) 11:31, 18 November 2009 (UTC)

Generally speaking, because physics is local (speed-of-light-limited), it doesn't matter how much stuff there is out there. What happens in one part of the universe only depends on what happened earlier in that part. If you assume the universe is (approximately) homogeneous, then what happens in one part also happens (approximately) in other parts, for parallel reasons. So there's no intrinsic problem in introducing a large or even infinite number of galaxies that evolve largely independently from one another, but in similar ways. -- BenRG (talk) 11:46, 18 November 2009 (UTC)

How does this fit in the free lunch theory of the universe? Is infinity equal to infinity?Bastard Soap (talk) 08:40, 19 November 2009 (UTC)

Why isn't more money spent on SETI?

Why isn't more money spent on SETI, or a similar search for extraterrestrial signals? Although there is no guarantee that there are other intelligent civilisations, or that they would broadcast information that we could use in a short timeframe (rather than us having to make contact and then wait decades, or hundreds of years) - it would seem that the potentially massive technological benefit if we happened upon a broadcast of an advanced civilisation would warrant a bit more of a gamble. Billions, likely trillions, are spent every year on various kinds of scientific research, whereas it seems that SETI has a budget of around $500,000, and finding this stream of information could bypass all of that other spending. On a similar subject, is there an organised effort to transmit our knowledge out into space? I guess that it is likely that any civilisation receiving it would likely not be much less advanced than us, so it may be that our knowledge would not be useful in a pure technological sense, but would be in an athropological sense. I guess the above reasoning for spending money on SETI could also be applied to the development of AI, but it seems that workable that advances our knowledge AI is many, many years away, whereas that signal from the clever aliens could be available right now. QuickSnow (talk) 23:14, 16 November 2009 (UTC)

On the former point, it all comes down to the likelihood of success (small), the enthusiasm for donating money to it (modest), and the political difficulties of dumping public money into a program that has absolutely no realistic prospects for public rewards (difficult), and the fact that trying to communicate with extraterrestrials is rightly considered political poison (crank stuff). All of this, of course, avoids the fact that most people think there are more pressing uses for money (and there are a lot of scientific competitors out there in the "long-shots that could actually pay off" category). Add to it a squeamishness about the fact that meeting extraterrestrials does not, on the face of it, sound like the kind of thing humans would be very good at (how often has the encountering of one advanced civilization with a less advanced one gone well for the former?). On the latter point, see Arecibo message. --Mr.98 (talk) 23:29, 16 November 2009 (UTC)

Transmitting our knowledge into space might not be a good idea. If we ever contact and successfully communicate with aliens we're going to want something to trade with. APL (talk) 23:39, 16 November 2009 (UTC)

What if we're the advanced civilization and find a less-advanced one? The cost of sharing all of our technology could destroy our economy... Seriously though, SETI and similar programs don't get a lot of play because it's hard for people to see the benefit. In the long run, yeah there might be something but compared to that, living on Mars is right around the corner. People would rather see money go towards other things, and it is usually hard to justify spending billions on SETI instead of AIDS or other medical research that will have a much larger benefit much sooner, or any of a thousand other projects. ~ Amory (u • t • c) 23:46, 16 November 2009 (UTC)

I would expect any civilisation we made contact with to be more advanced than we are - we have had the ability to send and receive messages between stars for only a few decades (and even now we can only barely do it). Chances are slim that we would make contact with another civilisation during those few decades where they are less advanced than we are now.

However, regardless of who is we make contact with, we won't get any meaningful communications for a long time. The nearest civilisation might, at best, be 20 light years away, that means a 40 year round trip for any message. The first few messages will probably just be establishing a language to communicate in (it is possible the first message sent will be able to teach a usable language to the other civilisation, but I would expect a bit of trial and error to be required - as we discussed on the Language desk a few days ago, it is very difficult to learn a language (even one designed for the purpose) without 2-way communication. So it would likely be centuries before we could exchange technology. Getting funding for such long term projects is very difficult. Public funding is difficult to get for anything that won't happen before the next election. You'll only get commercial funding if you can turn a profit within a few years, certainly within the lifetime of the investor. That leaves charitable donations and you'll get limited amounts that way.

All that said, there is actually quite a lot being spent on SETI. Consider the recently launched Kepler Mission. That wasn't cheap. While it isn't solely devoted to SETI, discovering terrestrial planets would make it much easier to know where to look for ETIs (or where to send messages to them on spec).

--Tango (talk) 23:55, 16 November 2009 (UTC)

I was imagining a one way communication in the sense that a civilisation mght decide to transmit some, or all (perhaps with the exception of information it thought could be dangerous to itself or the receiver), of its technological informaton along wth the means to decode/translate it, as a constant transmission on a loop. There's no proof that such a signal exists, but it would seem very likely that advanced civilisations do exist out there and perhaps it is likely that they would choose to send such a signal (purely for altruistic purposes, no different to one country sharing its technology with another, simply for the benefit of the receiving country). I can understand that spending billions on this might not be politically sound, but $500,000 a year seems very low (I take on board your point about the Kepler mission though), as there are a lot of people who would spend close to that on a new car. When you take into account that tapping into such a signal would possbly replace decades (or perhaps even hundreads or thousands of years of research)it would seem that somebody somewhere would take a slightly bigger gamble (maybe not with public funds, but with private ones perhaps). QuickSnow (talk) 00:21, 17 November 2009 (UTC)

Finding a way to transmit a message containing instructions to build advanced technology without any pre-existing common language would be very difficult. Anyway, the receiving civilisation would need to technology to receive the message. Assuming a similar technological progression to ours, that puts them at no more than a few decades behind us, we wouldn't have much to teach them, probably nothing. Another issue is that people don't like to devote time and money to something that they won't know worked - we would need a reply to know we had achieved anything, so that's still going to take more than a lifetime for anything but the nearest stars. Also, working out what technology wouldn't be dangerous would be hard - even the simplest technologies can be harmful in the wrong hands. --Tango (talk) 00:59, 17 November 2009 (UTC)

I'm not sure why you think it is "likely". As the Drake equation does a good job of showing, there are a lot of probabilistic factors involved in whether there are even civilizations out there that could communicate with us during the tiny (tiny!) period of geologic history that we have had the ability to listen for them. Taking those and then saying, "so what are the odds that said advanced civilization has decided to broadcast their information to us with a strong-enough signal to be detected and on a continuous loop over the course of at least hundreds of years?"... to me, it doesn't look very likely. The biggest problem here is time itself—look at the history of just our own planet, and how long our civilization has been able to do this kind of thing. Now extrapolate a little bit and say that any civilization that can really grok radio-wave transmission is going to also, within a century or so, grok nuclear weapons and genetic engineering. Now extrapolate the likelihood of any given civilization to survive in a world where there are creatures that grok nuclear weapons and genetic engineering—how many centuries of a "window" does that give you before your civilization can no longer send or receive signals? Much less if we postulate them having anything like politics. Obviously all of this is intangible—we don't know how likely our situation is, much less theirs—but it does seem to imply that there is a much better chance that there is no one out there sending/listening at the same time we can send/listen. --Mr.98 (talk) 00:59, 17 November 2009 (UTC)

I hear this "alien civilisations will probably destroy themselves" argument a lot. The evidence doesn't support that. 100% of species known to have achieved civilisation are still going strong. --Tango (talk) 01:49, 17 November 2009 (UTC)

Human societies have regularly collapsed, e.g. Amazonian civilisations, Ur, Easter Island. So the premise is that if human societies can collapse so easily, so could alien civilisations. See our articles on Societal collapse, Risks to civilization, humans and planet Earth, Existential risk and Extinction event. Fences&Windows 03:35, 17 November 2009 (UTC)

I was careful to speak in terms of species. Human societies and civilisations have certainly collapsed, but they have always been replaced by others. --Tango (talk) 04:03, 17 November 2009 (UTC)

Assuming an alien civilization's lust for bandwidth is similar to ours, they compress their data, which is virtually indistinguishable from background noise. A Quest For Knowledge (talk) 03:50, 17 November 2009 (UTC)

Except it is concentrated in a specific band. However, that only refers to us overhearing their internal communications. It is more likely that we could detect signals intentionally sent into space (we are moving towards wired communication for long range communications so it is likely that other civilisations will do the same, so they would only send out significant amounts of radio pollution for a few decades, we're unlikely to be looking for them during that slot). They would hopefully start with sequences of primes or something else that would stand out from noise very clearly. --Tango (talk) 04:03, 17 November 2009 (UTC)

"It is more likely that we could detect signals intentionally sent into space". Really? Given that a) Homo Sapiens is the only known species who have intentionally attempted such a communication, and b) they've only attempted it once, what is the likelihood that another species would so such a thing? A Quest For Knowledge (talk) 04:28, 17 November 2009 (UTC)

Perhaps it is unlikely that another civilisation will send such a message, but it is even less likely that we'll detect an unintentional message. With the amount of noise we're sending out at the moment and the strength of our radio telescopes we could only detect ourselves if we were at best one light year away, so we could only detect the noise from other civilisations if there is much more of it than we emit and it looks like we aren't going to emit much more than we are now (since we're moving towards wired communications for long range things - ie. the internet vs. regular TV and radio). --Tango (talk) 04:48, 17 November 2009 (UTC)

And that signal power would need to be squared to double the distance right? Googlemeister (talk) 17:18, 17 November 2009 (UTC)

Not quite. Power needs to be multiplied by x² to multiply the distance by x, so you have to quadruple power to double distance. I expect that is what you meant. --Tango (talk) 18:02, 17 November 2009 (UTC)

You all forgot to mention another explanation. It is not scientific (potential benefit and probabilities) or economical (cost effeciency) but political, there is conspiracy in high level government circles to conceal the information that the ETI has been already found and possibly contacted. (Igny (talk) 04:59, 17 November 2009 (UTC))

We didn't all forget - we're all part of the conspiracy. -- Scray (talk) 06:07, 17 November 2009 (UTC)

You were part of it. I'm sure the black helicopters will be arriving soon. --Tango (talk) 08:19, 17 November 2009 (UTC)

Thanks for the warning. I was beamed out by my "friends" just in time. -- Scray (talk) 00:29, 18 November 2009 (UTC)

Somebody forgot a small tag close. Or maybe it was... abducted? Nimur (talk) 01:02, 18 November 2009 (UTC)