Wikipedia:Reference desk/Archives/Science/2007 September 20

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September 20

Plant has its own division?

Apparently, there is one plant that has it's own division, does anyone know what this plant is? TheCoolestDude 13:35, 20 September 2007 (UTC)

Ginkgo biloba. Polypipe Wrangler 15:14, 20 September 2007 (UTC)

Our Ginkgo article agrees...although we should be careful to say that it's the only modern plant in that division, there must have been a number of others that are now extinct. It's likely that the modern Ginkgo is also extinct in the wilds - now only existing as a cultivated plant. SteveBaker 17:18, 20 September 2007 (UTC)

How can I jam a radio controlled car? (In the USA)

can I purchase a device that would jam my neighbors remote controlled car so he would use it elsewhere? his toy is causing problems for my dog as the sound aggravates her.

thank you

rob neal

irvine,ca —Preceding unsigned comment added by 216.154.252.184 (talk) 23:34, 19 September 2007 (UTC)

Someone into electronics correct me if I am wrong, but buying the same type of remote control car and using its remote should work because the two are on the same frequency. Personally I would talk to the neighbor about it first if possible, and if that fails try to move the dog elsewhere where the sound wouldn't bother her before going so far as to attempt to jam the remote controlled car. Another solution you could try if you can't get the neighbor to change his behavior would be to get a source of white noise for your dog. 71.226.56.79 00:36, 20 September 2007 (UTC)

Of course, it depends on the type of radio system. Some modern wireless systems are difficult to jam - especially with such a simple attack as a frequency tone jam. (For example, you can think of wireless mouse and keyboard - both are wireless systems but the communications protocol enables multiple access of some type or other. You should probably seek a non-technical solution to this, lest you escalate the conflict (or turn it into a systematic game of cat and mouse to see whose counter-counter-counter-measures are most effective). Nimur 02:42, 20 September 2007 (UTC)

On the other hand, if he can jam the radio secretly, then his neighbor may just think there's some type of interference and go elsewhere. For certain neighbors, even suggesting that they change their behavior may result in hostility, so that might not be the way to go.

I would think the first step is to find the proper frequency. Using some type of scanner when he uses his remote control might be the way to find the frequency (listen for some frequency where the sound changes when the car turns, for example). Then you would need something that jams that frequency. The car might use an adjustable frequency, though, in which case he might just go on to another. StuRat 03:11, 20 September 2007 (UTC)

The cheapest solution is to buy another radio controlled toy that operates on the same frequency. You'll of course need to find out the frequency it operates on...that would require some detective work. Most toys operate in the 27Mhz or 35MHz range. In the UK, I believe that only model airplanes are allowed to use the 35MHz band - in the US, it seems that both are used. However, within those two bands there are a dozen or more 'channels' - each at a slightly different frequency, you'd need to find the right channel. Some recent remote-controlled toys use infrared (like a TV remote) instead of radio - which will be almost impossible to 'jam' unless you are close to the toy at the time. SteveBaker 13:22, 20 September 2007 (UTC)

In the US, 27 MHz and 49 MHz are both common.

Atlant 16:18, 21 September 2007 (UTC)

You have overlooked the simplest route, beat him up and take it! That, or buy a faster one and run him off the road. Don't work harder, work smarter--Jmeden2000 15:29, 20 September 2007 (UTC)

Doggy issues aside, do bear in mind that generating "Malicious Interference" in the radio spectrum is a crime that the FCC doesn't appreciate much. Saturn 5 20:28, 20 September 2007 (UTC)

How about using that coil thing that uses sparks to generate a lot of radio noise? (Then you'd have the whole street mad at you). 80.2.197.120 20:43, 24 September 2007 (UTC)

Holographic Lightsabre

Can a holographic projector be used to create a safe lightsabre blade? --Ecyrblim 01:09, 20 September 2007 (UTC)ecyrblim

Sure. But can that kind of holographic projector be made in the first place? — Kieff | Talk 01:13, 20 September 2007 (UTC)

Holograms only work when you're looking at them. You can't "project" an image of a blade from the hilt unless the hilt, the blade, and your eye are all collinear, which only happens when you're looking at the blade end-on. -- BenRG 02:46, 20 September 2007 (UTC)

But what they could do is have you hold the hilt of the "light sabre", and see your reflection in a "mirror" which has the light blade added. This would make for a nice illusion at an amusement park. They could even give you a movie of you with the light sabre (for a suitable fee, of course). StuRat 03:03, 20 September 2007 (UTC)

How about just a regular "bright and focused" light and a room with a lot of dust in the air? Light projects out of hilt, reflects off dust in its path. DMacks 05:40, 20 September 2007 (UTC)

The problem with that is making the light stop abruptly three feet from your hand...there is no way to do that without some physical object being in the way. FYI: in the early StarWars movies they filmed the lightsabres by using a solid tube covered with a 'retro-reflective' material that was lit by a light next to the camera. Since retro-reflectors reflect light back towards the source, you ended up with a bright reflection from the lightsabres with very little light reflected off of anything else. In the later movies they used computer graphics (of course). Trust me, if there were any way at all to make a 'real' lightsabre, the toy makers (or at least the StarWars fanatics) would have done so LONG ago. SteveBaker 13:16, 20 September 2007 (UTC)

Most sources I've read state that the light sabers were done by rotoscoping.[1]. DMacks 13:37, 20 September 2007 (UTC)

• Our lightsaber article says: The lightsaber first appeared in the film Star Wars Episode IV: A New Hope (1977). At first, the effect was created by a handle with a motorized spinning reflector and pointing high-intensity light in their direction. The outcome was not satisfying, so in post-production the effect was augmented through rotoscoping. For episodes V and VI the use of reflective tape was abandoned and the effect was achieved by rotoscoping rods made of aluminum and later carbon fiber rods. For the prequel trilogy, the effect was created using computer animation. I have read that in the original Star Wars, right after Darth Vader kills Obi Wan Kenobi, his lightsaber is the original retro-reflective white, rather than red. The rotoscopers missed that scene. No doubt the bastardized re-releases have it colored in. --Sean 15:43, 20 September 2007 (UTC)

• Oh, look! This video shows the shiny spinning saber: [2]. --Sean 15:52, 20 September 2007 (UTC)

Regarding making the light "stop", sounds like a mean-free-path problem…will have to remember this one for a future pchem exam:) DMacks 03:35, 24 September 2007 (UTC)

Socks

Do they mainly were out from the inside or the outside? —Preceding unsigned comment added by 88.109.106.146 (talk) 03:47, 20 September 2007 (UTC)

They usually fail when my townails chop a hole through! So that would be inside. Graeme Bartlett 12:07, 20 September 2007 (UTC)

There's lots of variables to consider, including: How you wear them, e.g., only with shoes, or often just as socks with no shoes; the manufacture of the socks, e.g., whether they are lined, the quality of construction; personal hygiene, e.g., the length of toenails, how often and how the socks are washed. You'd need to define some of these before you could make any type of definite answer. --jjron 12:19, 20 September 2007 (UTC)

One hint, tube socks last longer because the heavy wear doesn't occur in just one spot, but rather in a ring, since you put them on in a different rotation each time. (They also avoid having the heel on the top of your foot when you get dressed in the dark.) StuRat 16:19, 20 September 2007 (UTC)

As Jjron says variables... My socks always wear out, sometimes very very fast on the heel. I believe this is because of the way I walk I somehow cause the heel section to wear out. Indeed it probably doesn't help that because I also wear out the heel section of my shoes this heel section then further damages the socks. I'm pretty sure that my socks therefore wear out from the outside in although possibly it's both but the outside wars out more (i.e. it actually meets in the middle). I've recently purchases some more expensive socks with more reenforced heels, waiting to see how these last Nil Einne 18:42, 22 September 2007 (UTC)

"...you'd get more mileage from a cheap pair of sneakers..." or, in this case, tube socks. Buy them by the pack. They are cheap, and last longer, and are more convenient. You can get them in black. StuRat 02:53, 23 September 2007 (UTC)

You could find out by wearing two identical socks one over the other and seeing which got the most wear. In winter I wear a cotton sock under a wool sock, and the wool sock seems to wear out more quickly. Perhaps this is due to the boot rubbing against the outer sock more, while the sock next to the foot does not move. The cotton is also much tougher though. 80.2.197.120 20:49, 24 September 2007 (UTC)

Engineering Projects

I have just begun with my first year of the four year engineering course in mechanical engineering... i was interested in making a research project... how do i start of?? I haven't a special interest in a particular field, i just follow a particular boom in the field.. as in nanotech,cad,automobile... how do i help myself out of this sitution developing an interest and the research part??? —Preceding unsigned comment added by 210.212.44.7 (talk) 06:32, 20 September 2007 (UTC)

You know, I was in a very similar situation once, and I eventually solved it by switching majors. Uh, but on a more helpful path, I went and found out what professors at my school were researching, and eventually found a project that just seemed really cool to me. I asked the guy, and he invited me on to the project. Someguy1221 06:43, 20 September 2007 (UTC)

So you have to decide on a research project in your first year? I don't think so. Wait until you need to decide. You'll have some ideas by then! —Preceding unsigned comment added by 88.109.4.191 (talk) 00:10, 21 September 2007 (UTC)

Aeroplane Speed -advantage when flying east?

Please consider: a) the earth ratates on its axis at around 2000 kmph b) two airplanes takes off at same time and from same point in opposite direction (east & west) and fly at same speed. c) whether the one flying east will have any advantage over the one flying west because of earth's rotation ? Or whether both the planes will return the same start point at the same time ? If so, why? —Preceding unsigned comment added by 59.182.61.89 (talk) 08:45, 20 September 2007 (UTC)

The planes are flying at the same speed relative to the atmosphere, so what is important here is to understand how the atmosphere moves relative to the surface of the Earth. At low levels (and ignoring local weather conditions) the atmosphere is carried around at the same rate as the surface (we don't experience continuous 2000 kph winds at ground level, do we ?). At higher levels in the atmosphere things become more complicated - see our articles on atmospheric circulation and the jet streams. Gandalf61 09:11, 20 September 2007 (UTC)

My physics teacher explained it that they sort of kept the motion of the earth with them, so their absolute speed included the rotation of the earth - therefore, relative to the earth below, they went the same speed, and if you take the rotation of the earth into account, they also have that same motion in that same direction at that same speed as part of their overall speed, so it cancels out. Which is why when you go up in a hot air balloon, you don't end up traveling laterally - you stay "stationary" because you're really still moving at the same speed as on the ground - the speed of the Earth. Kuronue | Talk 00:00, 22 September 2007 (UTC)

• When I visited the US last year, the plane took significantly less time to return home (east) than it took to get there. If you consider transatlantic travel at about 11 km, there's definitely an advantage. If only JFK wasn't congested at the time, I might actually have reaped the benefits of the whole thing... - Mgm|^(talk) 09:41, 20 September 2007 (UTC)

This is most likely the effect of the Jet stream rather than the rotation of the earth. -- JSBillings 11:50, 20 September 2007 (UTC)

In Australia it's generally considered a cross-country west->east flight (say Perth-Melbourne or Perth-Sydney) will commonly take around half-an-hour less than the same east->west flight (average flight time is about 4hrs). However the reason for this is the prevailing winds rather than the Earth's rotation. The flight speed is commonly varied to allow for the winds, rather than having the planes arriving at unexpected times due to varying winds. --jjron 11:57, 20 September 2007 (UTC)

Any east/west bias in travel time is entirely due to the prevailing wind. It's reasonable to say that the velocity of the aircraft over the ground is it's velocity through the air plus the wind velocity. The rotation of the earth only has an effect to the degree that the coriolis effect affects the winds. Mostly we have to be concerned with large scale weather patterns at a particular altitude (eg the Jet Stream). SteveBaker 13:07, 20 September 2007 (UTC)

Be sure you are measuring true elapsed time for the flights, not comparing starting and ending times in local time zones. It's even possible the arrive before you leave, if following time zones (have we discovered time travel here ? :-) ). StuRat 16:28, 20 September 2007 (UTC)

Any advantage from being carried east by planetary rotation is exactly nullified because the target point is carried east at the same rate. —Tamfang 21:25, 20 September 2007 (UTC)

Indirectly (at least in North America and over the North Atlantic; I haven't flown elsewhere). The prevailing winds blow from west to east, due to the rotation of the earth; that gives flights from west to east a boost in speed over flights from east to west. Look at flight times listed on airline schedules.Gzuckier 13:50, 21 September 2007 (UTC)

differentiator

what type of the wave form pattern we get in case of of differentiator? —Preceding unsigned comment added by Patelshiv (talk • contribs) 12:54, 20 September 2007 (UTC)

The differential of the input signal? —Preceding unsigned comment added by 88.109.4.191 (talk) 15:34, 20 September 2007 (UTC)

The wave form out depends on the input naturally, if the input is rising the output is poisitive, if it is falling the output is negative. If you look at it in the Fourier domain, the output is multiplied by the frequency, so the higher the frequency the more it is amplified, and the lower frequencies are reduced in level. The opposite is the integrator. Graeme Bartlett 11:22, 21 September 2007 (UTC)

A sine wave will turn into a phase shifted sine wave, a triangular wave turns into a square wave. A square wave turns into non physical alternating infinite pulses. Graeme Bartlett 11:31, 21 September 2007 (UTC)

Chest clicking. Not medical advice?

I'm just wondering what interaction is going on here. When I stretch my arms and chest (like the stereotypical tired stretch), I sometimes click something in the front centre of my chest. I'm assuming it's similar to cracking joints. But I don't know of any joint in the front center of my chest that it could be! Any ideas? Please feel free to delete if you still feel this constitutes medical advice. Capuchin 13:03, 20 September 2007 (UTC)

I click sometimes when lying on one side, I understand one of my false ribs is detached at the end. DuncanHill 13:06, 20 September 2007 (UTC)

I'm not going to give you medical advice. This is information relevant to the phenomenon to which you refer;-) Have a look at this http://upload.wikimedia.org/wikipedia/commons/8/85/Human_skeleton_front.svg

and you will see that between the sternum and the ribs are connections of cartilage. These are not bendy articulations but they facilitate a little expansive movement to the ribcage and probably enough movement to produce a little bubble to cause the 'crack'. Thank God, really, because if these cartilage connections weren't there I've got a feeling everyone who underwent commpressive cardiac massage would need to get along to the orthopoedic department afterwards to get their ribs fixed. Richard Avery 14:21, 20 September 2007 (UTC)

I can feel the rib move. DuncanHill 14:58, 20 September 2007 (UTC)

You can dislocate ribs from those joints as well. I've done it twice. First time, I was hit by a bus. Second time, same rib, I woke up and somehow it just dislocated and hurt like hell. -- kainaw™ 15:04, 20 September 2007 (UTC)

Maybe you were the victim of a drug rape? Nil Einne 18:32, 22 September 2007 (UTC)

There's a joint between the xyphoid process and the rest of the sternum that can apparently "pop" in some individuals, for example when taking a deep breath (so says my doc). To my delight (and others' disgust) I can pop mine on command. --David Iberri (talk) 23:44, 20 September 2007 (UTC)

aquatic plants

what is the most favourable form of nitrogen for aquatic plants? —Preceding unsigned comment added by 202.141.78.125 (talk) 13:42, 20 September 2007 (UTC)

It is possible that Nitrification will provide some insights, although it doesn't mention aquatic plants in particular. It seems that the nitrogen-fixing bacteria are soil-dwellers, so there is probably an equivalent version for water-borne ecosystems (either in the form of aquatic bacteria or other nitrogen fixers). Nimur 15:22, 20 September 2007 (UTC)

I think the asker means either nitrate, ammonia as well as possibly nitrite and urea - I think it may depend on the plant which is most readily absorbed - but it is almost certainly one of the first two..87.102.87.157 15:15, 21 September 2007 (UTC)

Props go to any fan fan who answers this Q

Why aren't cooling fan blades shaped like an airfoil ? That is, why are they constant thickness instead of thicker near the leading edge ? I was asked this question and didn't know for sure, but this was my speculation:

• Airfoils are more expensive and more efficient. While this additional cost is justified for aircraft props it is not for electrical cooling fans, where it would be difficult to ever recoup the additional cost.

Was my speculation correct ? StuRat 16:14, 20 September 2007 (UTC)

The Sukhoi 29 is the probably most aerobatic aircraft in the world - as you can (just about) see, it has a symmetrical wing cross-section, showing clearly the myth of 'airfoil' shapes being responsible for generating lift SteveBaker 17:10, 20 September 2007 (UTC)

I don't think so. It's an alarmingly common myth that the cross-section of an airfoil is what causes an aircraft to gain lift...it has a very small effect but the major part of it is the angle of the wing to the airflow. The airfoil cross-section has more to do with controlling turbulance at low speeds - and getting the wing strong enough and yet still have a low drag coefficient. Consider a typical Ceiling fan. Those have flat, rectangular, plank-like blades - and they work very well. A desk fan, on the other hand has fat, more steeply angled blades. The choice of shape probably depends on the ratio of motor torque to blade diameter. A desk fan has to be compact - a ceiling fan doesn't. A desk fan is (typically) enclosed in a safety cage so the blades can spin much more quickly than a ceiling fan can. There are lots of little decisions that go into the design of such things. I would say that the blades of a desk fan more closely resemble a ship or submarine propeller - and the demand for small diameter with a high-torque motor probably drives both designs. SteveBaker 17:03, 20 September 2007 (UTC)

Thanks, but that doesn't seem to address why an airfoil shape is never used for fan blades. Are you saying that avoiding turbulence isn't important in fans ? I do think that turbulence occurs, since I note a small distance from the leading edge is clean, but after a half-inch or so, the fan blades become filthy. I take it that little dirt is deposited by laminar flow but turbulent flow "dashes dirt particles against the blades", causing them to stick. I would also reason that this turbulence would necessarily lower efficiency, since energy is being used to move air chaotically, not in the desired direction. StuRat 17:12, 20 September 2007 (UTC)

In aircraft, you need that thick part of the wing to contain the main structural spar (basically just a girder) - which supports the entire weight of the aircraft in the air and has to flex the other way and support its own weight against massive leverage when the plane is on the ground. This spar is at the center of gravity of the plane (in most designs anyway) - and it's wrapped in that teardrop shape merely to get the air to slip past it efficiently and to promote stability by placing the center-of-pressure as close as possible to the center-of-gravity. By virtue of smaller scale, fan blades can be simple thin plates that are mostly being flung outwards by centrifugal force and are not likely to break from the aerodynamic stresses alone. Hence no big fat girder running down the middle of the blade. So they can have a lower coefficient of drag than an aircraft wing that needs a fat bit in the middle to contain that chunky structural member. Turbulance isn't necessarily such a terrible thing in a fan blade whose main function is to stir up the air - so whilst it's probably undesirable, it's not a serious design issue. With ceiling fans, the cost of the things must be a lot less with flat wooden blades than with curved ones. I guess that with desk fans, which mostly have cast metal or plastic blades, you can have them be any shape you want - so they are more efficiently curved. Another issue with the design of fan blades is the range of speeds they have to operate at. It's possible that you are getting non-laminar air flow at the speed you happen to run your fan - but you'd get smoother airflow if it were running faster. The most likely thing is that aircraft are designed VERY carefully with wind tunnels and such - where ceiling and desk fans are designed as much for looks and cost as for efficiency. SteveBaker 17:33, 20 September 2007 (UTC)

(Come to think of it - if you look at one of those large modern windmills (which are just fans running backwards), the do have aerofoil shaped blades - presumably for the same kinds of structural considerations as aircraft wings). SteveBaker 17:35, 20 September 2007 (UTC)

Another consideration is that, given the RPMs of a desk fan on high, you probably don't want the blades generating lift, or your desk fan soon becomes a floor fan. Saturn 5 20:24, 20 September 2007 (UTC)

I believe "for every action there is an equal and opposite reaction" dictates that if you push air forward with a given force you also push the fan backwards with the same force, regardless of the means used to propel the air. I've definitely had fans fall over as soon as they are turned on. They need sturdy, weighted bases, or to be secured in some way, to prevent this. StuRat 02:01, 21 September 2007 (UTC)

Was my speculation correct ... partially, but I think it misses half the story:

• Airfoils act on the Bernoulli principal, which is a form of reaction - see Newton's third law.
• Curved blades, with no airfoil crossection, are usually designed to work with impulse - they change the direction of fluid flow without airfoil action - see Newton's second law.

Fans can benefit from both these concepts, depending on the design. Also, just guessing, a curved or angled blade made of stamped sheetmetal can probably be designed to act as an airfoil.

Cheap fans use simple shapes because they are cheap, even if an airfoil crossection could improve performance. In other cases, the fan might be an impulse design that doesn't benefit from an airfoil shape. I once saw a high performance fan - it had a stator and a rotor - that put 10 horse power in to the flow. It was less than twelve inches in diameter and only a couple of inches thick, and it could knock you over. It didn't have air foil shaped blades, just cheap stamped curved sheetmetal blades. --Duk 21:58, 20 September 2007 (UTC)

It really annoys me that just about every 'how things work' book says that Bernoulli's principle explains how airplanes fly - it really has almost nothing to do with it. This is one of the biggest pieces of misinformation in even quite serious publications. I offer as evidence the photo of the Su29 I posted above. It has a symmetrical wing cross-section and therefore has ZERO Bernoulli lift...yet it's one of the most nimble aerobatic aircraft in the world! Notice that something like a simple Cessna light aircraft that has a classic 'airfoil' cross section wing can fly inverted - with the Bernoulli effect pushing downwards on the wing. I've flown model planes with planks for wings (a rectangular cross-section). They fly just fine - again no Bernoulli lift whatever. Truly - this is a misconception that is perpetuated everywhere and it needs to be screamed and shouted from the rafters: THE BERNOULLI EFFECT IS NOT WHAT MAKES AIRPLANES FLY!!! SteveBaker 00:19, 21 September 2007 (UTC)

THIS SECTION IS NOT ABOUT WHAT MAKES PLANES FLY, IT'S ABOUT FAN BLADES!!!--Duk 00:51, 21 September 2007 (UTC)

With due deference to SteveBaker's volume, the Bernouilli Effect may not be the primary lift-generation mechanism, but it cannot be completely ruled out in terms of stability analysis and complex fluid flow modeling. The shape of the wing or control surface dramatically affects the airflow, whether in a lift-generating way or not. Nimur 17:57, 22 September 2007 (UTC)

Well, it has everything to do with the angle of attack of the wing. It's funny, just last week I performed an experiment in a wind tunnel using a NACA 0012 airfoil; its primary characteristic is that it is symmetric, yet it still produced lift. It still presents a typical $C_{L}$ v. $\alpha$ graph like the one shown on Lift coefficient, but the only difference is that it has a y-intercept of zero. It still presents stall, whether you are flying upright or upside down. What Bernoulli's effect does is that it generates a non-zero intercept for the graph. Titoxd^{(?!? - cool stuff)} 02:14, 21 September 2007 (UTC)

It seems to me that these discussions about "symmetrical cross sections" are all using the wrong frame of reference. The axis about which one should calculate whether aerodynamic symmetry exists is the air direction line that intercepts the center of gravity for the wing, is it not ? If the wing is symmetrical using this frame of reference, I would indeed expect there to be zero lift. StuRat 02:24, 21 September 2007 (UTC)

Not really. The calculations needed in fluid dynamics turn really ugly if you try to calculate them using a wind-based coordinate system. You always calculate them using a wing-based coordinate system, then do a simple transformation employing the angle of attack to bring them to the other coordinates. If the wing is completely parallel to the wind, there will indeed be zero lift; however, any deviation from that will cause air (or whatever the working fluid is) to have a longer displacement path on one surface, which causes higher speeds and lower pressures on that surface, generating lift. Titoxd^{(?!? - cool stuff)} 02:37, 21 September 2007 (UTC)

It sounds to me like you're saying the same thing as I did, just in a different way. Instead of starting with a wind-based coordinate system, they merely start with a wing-based coordinate system, then do a simple transformation, to get to a wind-based coordinate system. StuRat 04:14, 21 September 2007 (UTC)

Answering the original question: A flat blade at a good angle to the air provides more than enough wind without using much power. There is no need to increase the cost by planing down the blades into airfoils. The higher-quality blades are not airfoils. They are simply curved to better direct the wind in the preferred direction - a process that requires flexible blades, but is still cheap. As for the question about how much lift a simple air foil gives, consider common helicopters. The blades are airfoils for stability. They tilt to create lift. To touch briefly on two other subject mentioned: Stunt planes have wings curved on both sides to provide better stability for flying upside down. The wings (and the plane) feel the same to the pilot either way. For larger planes, such as passenger jets, the body of the plane provides a lot of lift. This goes back to TWA's first airplane purchase. They required the passenger jet to fly from Denver to Kansas City with one wing (I've also heard that it didn't have a tail, but I'm not sure about that). The wings are rather important though because most of the fuel is inside them. When it comes to jets though, keep in mind that they cannot fly at low speeds. As I was told when I started flying TWA's 727 and 747 simulators, jets don't glide when the engines stop. They drop like a rock. -- kainaw™ 02:52, 21 September 2007 (UTC)

Are you kidding us Kainaw? Just a few years back, some large commercial jet 'glided' for over 100 miles, if I remember correct. The plane ran out of fuel because of a metric/standard error. The tanks were filled with litres instead of gallons! I can`t remember the exact flight, but, the 'emergency' glide was to a small set of islands in the Indian or Pacific ocean. Sorry if I can`t remember more particulars, but this was BIG news, at the time. To conclude though, jets CERTAINLY can glide... with a glide-rate of around 16:1 to boot! 64.230.233.222 19:44, 21 September 2007 (UTC)Dave

No I am not kidding. The event you are referring to is Air Transat Flight 236. It "glided" at a fall rate of about 2,000 feet/minute. Note that the pilot knew about the fuel leak (not a metric conversion error) before he ran out of fuel. So, he descended and punched the remaining engine to get forward momentum. Then, he basically fell onto the runway. There is a huge difference between gliding along and falling at 2,000 feet/minute. But, you can define "gliding" however you like. -- kainaw™ 19:54, 21 September 2007 (UTC)

Kainaw, I certainly don`t want to be argumentative since 'falling' at 2000ft/min is surely quite a 'fall', but one must remember that while 'falling' at such a rate, that craft in question was also travelling forward at around 32,000ft/min. What I`m trying to get at is that given both numbers, the craft was far from "drop(ing)like a rock", as you put it. I`m quite certain with that forward speed, that aircraft surely was able to flare some, further decreasing its 'fall' rate. Looking up 'glide ratio' right here on Wiki gave some interesting values: Cessna 150 7:1, modern sailplane up to 60:1, hang "glider" around 12:1, paraglider around 7:1, airliners around 17:1, Space Shuttle 1;1! So, with those numbers in mind, wouldn`t you say THAT craft 'glided' pretty well? Thanks for finding those particulars that I missed,,,,I read the link. I found it very interesting. Thanks again. Dave64.230.233.222 21:00, 21 September 2007 (UTC)

The glide ratio is only half the need for gliding. The other half is the sink rate. Unfortunately, Wikipedia doesn't have an article on sink rates (yet). A good glide ratio and good sink rate make a glider. While a large passenger jet may be able to get a somewhat reasonable glide ratio, the sink rate stinks. So, the longer the jet is "gliding", the less forward momentum it has. The less forward momentum it has, the worse the glide ratio gets. The worse the glide ratio, the faster it falls. That is why the few cases where engines have completely gone out but the jet survived were cases where the pilots had warning and lowered the altitude while increasing speed. Lower altitude means less glide time to the ground. Higher speed means longer glide ratio. I'm sure someone will say that they can nose-dive and pull out to get forward momentum. Large jets are not constructed to handle that maneuver safely. (Although, I've seen a 747 do it in a simulator.) Then, there's the whole issue of the jet going into a crippled mode due to power failure. The little turbine that pops out gives only the basic necessities - not enough for a comfortable flight. All in all, if you set a rock in flight at the same height and speed as a jet, the rock will take a ballistic course. The jet will do a little better, but be much closer to the rock than a glider. -- kainaw™ 04:49, 22 September 2007 (UTC)

Thanks for the extra 'work' Kainaw...good job. A quick 'calculation' shows that the plane 'fell', just taking the vertical aspect of the fall into consideration, at an average speed of about 22-23 mph. I think an ordinairy, i.e. no 'special' aerodynamic qualities, rock would fall with a terminal velocity of, just a guess, 120-150 mph. The high-performance modern glider would 'fall' at about 5-6 mph, I think. Refering to your last sentence above, I believe that 22-23 mph, if correct, and if all other guesstimates are close to accurate, then the jet glided much closer to the rate of the glider than an ordinairy rock. Please correct me if I`m wrong. This will be my last post to this FAN question. Apologies to those 'forced' to listen to these side-tracks. Thanks all. Dave 64.230.233.222 09:07, 22 September 2007 (UTC)

Also check out Gimli Glider for another example of a Large Jet (in this case, a Boeing 767) gliding to a safe landing after running totally out of fuel Bunthorne 03:23, 24 September 2007 (UTC)

What gives airplanes lift

HEY, STEVE BAKER, WHAT GIVES AIRPLANES LIFT ?????????--Duk 00:58, 21 September 2007 (UTC)

Steve, aren't you kind of splitting hairs? The pressure difference between the upper surface of the wing and lower surface create lift. If the aggregate of vectors of lift and thrust are sufficient to overcome gravity then the plane will rise. The Bernoulli equation can be used to estimate lift from airflow over the wings if the geometry of the wings and the angle of attack are known. The longer streamlines generate lower pressure since the air must flow faster from the leading edge normal to airflow, specific location controlled by the angle of attack, to meet back up at the trailing edge. If this condition fails then you have flow separation and a resulting stall. Most standard airfoils have greater curvature on the "top" to increase the lift when flying right side up. If you fly with your symmetrical airfoils at an angle of attack of zero then you have identical streamlines both above and below, zero pressure differential and thus zero lift as predicted and calculated by the Bernoulli equation. Your inverted Cessna better have a proper angle of attack to make the streamlines on the undersurface of the wing longer while upside down or your "Bernoulli lift" will indeed be down rather than up. The reason the airfoil on your aerobatic plane, photo above, is symmetric is because it is designed to give equal "lift" either up or down as per the angle of attack selected by the pilot while flying aerobatically. Large subsonic widebody jets on the other hand will have asymmetric cross sections to maximize lift (greater curvature on top and flatter on bottom) when flying right side up. Indeed, they also have flaps to further extend the asymmetric curvature and maximize lift while landing ... as predicted and calculated using the "Bernoulli Effect". Lazyquasar 01:00, 21 September 2007 (UTC)

And to the Su29 wing design, they are shaped that way so as to provide equal lift regardless of which side is up, as LQ said above. Aerobatic planes tend to spend alot of their time upside down. The wing shape lets them get the same lift from the same alpha (angle-of-attack) in either orientation. Couple that with the massively overpowered engine which essentially "drags" the aircraft through the air regardless of lift, and you get the nimbleness that Mr. Baker described. Yes, Cessnas CAN fly upside down, but to maintain level flight as such requires a very aggressive alpha angle, and a steadier nerve from the pilot (if you've ever done this, you know how easy it is for the aircraft to "fall off" its lift...). I guess my point is that, while the wing angle is a significant element in maintaining lift, it is incorrect to say that Bernoulli Lift from the wing shape plays no part, or even an insignificant part. Saturn 5 14:36, 21 September 2007 (UTC)

Well as "steve baker" says - it's not the bernoulli effect - because the force downwards on the wind due to the curve on the leading top edge of an airfoil does not offset any force gained by having faster air above.

It's the angle of attack that gives lift. Consider "For every action there is an equal and opposite reaction" (newton).87.102.87.157 15:12, 21 September 2007 (UTC)

To be perfectly fair, it is a combination of Bernoulli and Newton that give lift. It is ignoring much physics to say that either Bernoulli or Newtonian lift play no part. Consider this: a "normal shaped" airfoil wing (what we refer to as a NACA 4415 shape, if you want to look it up), will still generate lift even when it is angled 3 degrees down relative to the airflow (3 degrees negative α ). At 4 degrees negative α, the Bernoulli lift is cancelled out by the downward Newtonian (so called Reactive) lift. Saturn 5 15:21, 21 September 2007 (UTC)

Does "bernoulli lift" violate the conservation of momentum principle? or not.87.102.87.157 15:41, 21 September 2007 (UTC)

For example this shape

   >>>>>          XXXXX
   >>>>>        XXXXXXXXXXXXXXXXX
air>>>>>      XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
   >>>>>    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
   >>>>>  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

In an horizontal airflow would it get lift, and what would be the forces on it?87.102.87.157 15:52, 21 September 2007 (UTC)

Actually, Bernoulli is a special case restatement of Newton, for a fluid. In your drawing, the air flowing over the top of the wing has further to travel due to the curvature, than the air flowing across the bottom. Therefore it must move faster than the air flowing over the bottom. Since Newton requires conservation of Energy, the faster moving air must be at a lower pressure than the bottom air. The pressure difference between top and bottom results in a net upward force on the bottom surface of the wing. All as a result of Conservation of Energy. Saturn 5 16:04, 21 September 2007 (UTC)

Rubbish - you seem to be assuming that two molecules starting at the front have to meet at the back at the same time. More worryingly what about the forces required to make the air move along the upper surface - total rubbish now fuck off87.102.87.157 17:05, 21 September 2007 (UTC)

Wow.... Sorry you feel that way, but I'm not making this stuff up. Read any physics book, its all in there. Oh and by the way, please also read WP:CIV. Saturn 5 17:11, 21 September 2007 (UTC)

I would like to make it clear that I'm not disputing that a wing with the cross-section shown in 87.102.87.157's post will generate lift. Yes, it will - as more or less every book on "How Airplanes Fly" will be sure to tell you. However, that's grossly misleading when you consider what keeps most airplanes in the air most of the time. This 'Bournoulli lift' effect is simply NOT a large part of the total lift in a normal aircraft and the wing cross-section has more to do with support structure and drag minimisation and stall-safety than it has to do with lift generation. The picture above misleadingly suggests that in normal flight, the flat bottom of the wing is parallel to the airflow. That's simply not true for almost every aircraft at almost every speed. For most planes, there is a well-defined "angle of attack" which is the angle that the flat bottom of the wing makes to the incoming airflow. You'd think that the air pressing against the bottom of the wing would immediately tip the plane's nose down until the wing sliced though the air nice and flat - like in the diagram...but airplanes are more cunningly designed than you perhaps think. Find a nice side-on photo of your favorite 'typical' airplane (try not to pick something weird) and look at the angle of the wing to the fuselage - and also at the angle of the tailplane to the fuselage. Notice how the wing is tilted upwards at the front compared to the tailplane. That's because in normal, level flight, the tailplane (being stuck WAY out the back of the plane, far from the center of gravity) exerts a downward force on the tail that pushes the front of the plane up. This results in the flat underside of the wing deflecting the air downwards just like it does on a typical slab-bladed ceiling fan. That's what's making the lift. There are exceptions...but as a general rule the whole business of the air taking a longer path over the top isn't what's generating the vast majority of the lift - the Su29 shows that elegantly by having a wing that's rounded on the bottom as well as on the top. If it's pointed directly into the airflow as per the diagram above, the air travels the exact same distance above and below - so no pressure differential - and no Bournoulli-effect lift...yet the plane flies perfectly well (in fact AMAZINGLY well in that case - it's one of the most manouverable aircraft in the world). If you are still not convinced, ask yourself why an aircraft points it's nose up at the sky on takeoff? If Bournoulli effect was getting it off the ground, it would keep the flat bottom of the wing parallel to the airflow and take off with the wings parallel to the runway. If we are dualing with references, read Angle of attack which says: The amount of lift generated by a wing is directly related to the angle of attack, with greater angles generating more lift. THAT is what makes (almost all) airplanes fly. There are some exceptions. SteveBaker 18:24, 21 September 2007 (UTC)

Please allow me to clarify a few things. First, before the other gentleman decided to cease having a civil discussion, my explanation of the wing diagram was deliberately simplified because dropping too much physics at one time, before some basic concepts are agreed to, tends to cause heads to explode. In fact, had we proceeded civilly, I would have gone into how, yes, the air molecules do NOT in fact arrive at the trailing edge at the same time. But anyway, you see how that went.

Second, if you notice, I have not said that the lift generated by deflection of airflow off an angled wing does not play a significant part in airfoil lift. In fact, at cruise (30,000 feet, 500kts or so), I'd guesstimate that a good 70-75% of total lift is produced that way. At takeoff, that goes even higher. I'm not arguing that point!!! My comments have been to simply point out that completely discounting the Bernoulli effect is not correct. Bernoulli lift is important because the Reaction lift (equal-opposite reaction from the deflected air molecules), while more powerful than the Bernoulli, is rather unstable by itself, and very sensitive to the slightest change in airflow such as one would find in the turbulent atmosphere. The way one's hand reacts when held out the car window on the highway gives a good example of this.

I'm not duelling with references... the only reference I've cited was CIV, for obvious reasons. :-) Saturn 5 18:41, 21 September 2007 (UTC)

Yes - so we don't actually disagee. In cruise you say that at least 70% of lift comes from the angle of attack - and the rest from the airfoil effect - and I'd probably agree with that (for at least most 'normal' aircraft - we'd have to argue about the Sukhoi and the Stealth fighter for example). Up until a few months ago I used to design flight simulators - mostly for military aircaft - and I could probably quote you some exact numbers - but one never knows what's classified - so I won't. Of course in the case of the Su29, the figure is 100% and I'm pretty sure we could find some kind of a weird-assed plane for which the number was much smaller (although I'm finding it hard to think of one right now). My problem is not denying the existance of this airfoil effect - it's that without fail almost every single book on the subject (especially for kids) says that the airfoil effect is "What Makes Airplanes Fly" - which is to completely miss the point! The measure of misinformation is a crisis level! Go to your local library and take a look at books on the subject and I guarantee that 95% or more get it wrong. In discussion of this subject one should FIRST talk about the angle of attack effect - discussing how that angle is maintained by the tailplane and how the elevators work by pushing up and down on that long lever arm (which is why aircraft have such long tails) to adjust the angle of attack. Someplace way back in Appendix Q of the most erudite books on aeronautics, it should mention the airfoil effect. Issues of stability and flight through turbulant are are VASTLY more complicated - effects such as lift from the fuselage, dihedral/anhedral, tailplane lift...it's all insanely complicated - and it would be oversimplifying to say that this is why we need Bournoulli lift. However, it is true to say that in aircraft where stability is NOT desirable (fighters, Su29's, etc), a symmetrical wing is desirable...so there is clearly some contribution to stability from having a flat-bottomed airfoil. But no matter what - can we PLEASE fix the textbooks?! SteveBaker 19:02, 21 September 2007 (UTC)

(Ok, 8 indents is plenty I think):-) Yes, my 70-75% was based on the 4415 sort of shape, and more specifically the 747-400. Something like 1000 lbf/sq-ft from Reactive lift, 240-ish for Bernoulli. At least my textbook got it right. :) Saturn 5 19:16, 21 September 2007 (UTC)

Then of course we have the F16 which exhibits 'black magic' vortex lift...no Bernoulli and kinda perpetually in stall...weird. Works great just so long as your computer keeps running! SteveBaker 19:59, 21 September 2007 (UTC)

I believe many of the pre-Wright-brothers aircraft were designed around the assumption of pure Bernoulli lift -- which is why they tended not to fly very well. --Carnildo 22:33, 21 September 2007 (UTC)

Yep. One thing that can be said without excessively simplifying the subject is that an unsymmetrical airfoil will have an initial lift at $\alpha =0$. However, that lift is much smaller than lift caused by angle of attack. Then, of course, you have to worry about flow separation and all those nice things... Titoxd^{(?!? - cool stuff)} 22:50, 21 September 2007 (UTC)

Ooooohhhh! I forgot to tell the skeptics to try my poor-mans wind tunnel demonstration! The next time you are riding shotgun in someone's car - take along a paperback book - preferably an nice slim 200 pager - not a 1000 page Stephen King monster! Open the window when the car is going nice and fast and hold the book out into the airflow with the spine pointing into the wind - hold it up high enough to get out of the turbulance caused by the door mirror. You now have a 'wing' with a rectangular cross-section. If you tilt the spine of the book up - you'll easily be able to feel the lift caused by the "angle of attack" effect. OK - so now, push on the pages on the top of the book to make an 'airfoil shape with a flat bottom and a slight bulge on the top. Hold the book so that the flat, bottom surface is parallel to the airflow and see how much 'Bournoulli effect' lift you get. When I tried this, it was hard to tell that there was any lift at all - but if you curve the top edge just right, you can kinda-sorta feel it. If this demonstration doesn't convince you, you're a lost cause! SteveBaker 19:02, 21 September 2007 (UTC)

Well, yes, ramming air against a geometric plane can provide lift, but the high drag that creates makes for rather inefficient use of fuel, does it not ? I believe that's why the airfoil is used, to provide lift without excessive drag. StuRat 18:07, 22 September 2007 (UTC)

• For those who are wondering what on Earth is the NACA xxxx, there's the NACA airfoil article. Titoxd^{(?!? - cool stuff)} 22:50, 21 September 2007 (UTC)

After getting back into flying, my current instructor first recommended, then insisted, that I get the book Stick and Rudder by Wolfgang Langewiesche. My first thought was "why bother with such an old book" (vintage 1944). Hoo boy, was I wrong! It goes deeply into the science of flight. To make a long story short, he agrees that the Bernoulli explanation leaves a lot to be desired. In effect, the airplane goes up because the wing causes a whole bunch of air to go down (remember Newton?). If you're the least interested in the science of aviation, I recommend, nay, insist, that you get hold of his book Bunthorne 03:46, 24 September 2007 (UTC)

Moran's Introduction to Aerodynamics is also quite good for understanding the actual fluid mechanics involved in aerodynamics. It's relatively cheap for a textbook too. Titoxd^{(?!? - cool stuff)} 19:41, 24 September 2007 (UTC)

Dominant and recessive

Why human genes are dominant and recessive? Is it because of their chemical structure or something else? And is it possible to interchange their properties? --85.132.14.38 17:02, 20 September 2007 (UTC)

This question is answered in the Dominance relationship article. To put it simply, it has to do with the fact that we carry two copies of each gene (for the most part), one chromosome from your father and one from your mother. -- JSBillings 17:47, 20 September 2007 (UTC)

Come now, anyone who asks about gene dominance already knows about diploidy. To put it still rather simply: if one allele produces a functioning enzyme and the other doesn't, usually the presence of the former is enough to have full effect; presence dominates absence like light dominates darkness. —Tamfang 21:21, 20 September 2007 (UTC)

Just to clarify, many genes don't have simple dominant and recessive relationships. Even for example the defect in haemoglobin which causes sickle cell anaemia. While the actual disease is only a problem in recessives, carriers of the gene (people with one copy of the 'normal' gene) have a high resistance to malaria (which is believe to be one of the key reasons why the disease occurs in relatively high frequency in some African populations) Nil Einne 18:45, 22 September 2007 (UTC)

It is important to remember the genes themselves do not create the phenotype, it is the proteins (and RNA) they produce which ultimately causes the differences in organisms we call phenotpyes. Dominance and recessiveness are often (but not always) correlated with how the respective alleles' protein products function (or not). There are many ways that dominance or recessiveness can be "created" through mutation so I will give some protein related examples. 128.196.149.20 00:56, 21 September 2007 (UTC) (This is User:Sifaka who can't sign on right now)

• Reccesiveness can be caused by a loss of function mutation. Suppose you have two alleles A (normal) and B (mutated) that make some protein. Allele B is mutated so that its protein can not function properly. If allele A produces enough normal protein to make up for allele B's defective protein resulting in a normal phenotype, Allele A would be considered dominant to B. If allele A alone can't produce enough protein to make up for the deficiency so an intermediate phenotype results, then A and B might be incompletely dominant.

• Imagine if the gene encodes for a red pigment in petals and the mutation in B causes a noncolored protein causing white petals in double recessive individuals. A normal phenotype is red. If allele A can make up for B completely, the flowers are still red. If A doesn't make enough protein though, the flower might be pink.

• Dominance of a mutant allele can be caused if the mutant protein fails to be regulated properly. Imagine a gene with two alleles C (normal) and D (mutated). Allele D has a mutation so that the protein produced has a defect in a regulatory region which controls its activity so that it can't be "turned off". The Allele D is dominant over Allele C because only one copy of allele D is needed to make mutant protein which causes the mutant phenotype.

• Imagine the gene encodes for a red pigment in a flower whose normal phenotype is red on the edges and white in the center. There is a regulatory protein produced in the cells in the center of the flower where it is normally white which tells the red protein not to work here. The mutant protein created by allele D doesn't respond to that signal creating the mutant phenotype of an entirely red flower.

• A gene has a part that encodes for the protein, and also regions to regulate its transcription. A mutation need not be in the parts encoding for a protein (thereby creating a mutant protein), a mutation to a key part of the regulatory region may drastically change the the gene's expression, causing a mutant phenotype.

FTL and the size of the universe

Begging your pardons if this is the wrong place but I couldn't find the information or answers anywhere else on Wikipedia.

I need a logic and reason check on this statement, please:

"If faster-than-light travel is possible, then the universe is not infinite."

Or, if you prefer:

"If the universe is infinite, then faster-than-light travel is impossible."

The logic is as follows:

If the universe is infinite, then there has been infinite opportunity for an advanced intelligence to develop FTL travel and visit us. As we see no evidence for extra-terrestrial intelligence, then at least one of the following must be true:

1. The universe is finite.

2. FTL travel is impossible.

"An infinite universe and faster-than-light travel are mutually exclusive." (corrected stupid typo in this statement AJKGordon 18:28, 20 September 2007 (UTC))

Therefore if experiments conclude that FTL travel is possible, then the conclusion that the universe is finite is automatically deduced. And vice-versa, of course. —Preceding unsigned comment added by Ajkgordon (talk • contribs) 17:54, 20 September 2007 (UTC)

Anyone care to correct or confirm? AJKGordon 17:46, 20 September 2007 (UTC)

There's a clear and fundamental flaw in your logic. An infinite universe (presumably you mean in size) does not necessitate that everything which might happen has happened. — Lomn 18:04, 20 September 2007 (UTC)

Yes, there are indeed quite a few problems with that assumption. First of all, it assumes that advanced intelligences CAN exist and are not somehow eliminated by the laws of physics. Secondly, it assumes that an advanced intelligence would WANT to search the universe for other life - maybe not. Thirdly, it assumes that this has not already happened (maybe they visit suitable planets, seed them with unicellular life and stand back and wait until WE find THEM!). It assumes that FTL travel means infinitely fast travel...maybe you can go faster than light but only twice as fast. Also how long does it take an infinite number of intelligent species, flying at infinite speeds to search an infinite number of stars in order to find an infinite number of less intelligent species like ours? The answer is something like infinity divided by infinity - which can be any number you choose...including zero or infinity. So it's perfectly possible for there to be infinite everything but for there still to be only a one in a trillion chance of us ever being reached and communicated with. There are plenty of holes in your argument that make it perfectly possible to have an infinite universe with infinitely fast travel and no alien visitations whatever. A similar argument to yours (without the need for FTL travel) would say that there are an infinite number of black holes in the universe so how come we haven't been swallowed by one of them? Well, there might be an infinite number of them - but they are spread over infinite distances - so the actual density of them can be whatever you could imagine. So, sadly, your argument is without merit. For the record, we believe that the universe is finite and FTL travel is impossible. However, because space can expand (as a result of the big bang) faster than light (this is NOT the same thing as travelling faster than light) - there are parts of the universe that are receding faster than we could fly towards them - so we can never reach them...which means nothing from those regions can reach us. In that sense, it doesn't matter whether the universe is infinite or not - in either case, we cannot possibly ever reach all of it. SteveBaker 18:05, 20 September 2007 (UTC)

Thanks both. Just want to make it clear it's not my logic! Kids sometimes ask difficult questions! I will reflect on your responses. Thanks again. AJKGordon 18:17, 20 September 2007 (UTC)

Ah - In that case, the short-form, kid-safe answer is: The trouble is that infinity is a slippery concept. If the universe is infinite then it also contains an infinite number of cats - so by your logic, there must be one sitting on my lap right now - and there isn't (you'll have to take my word for that!). That's because those infinite cats are spread over an infinite volume - so the distance between them is infinity divided by infinity which could be any number at all. We know the universe isn't infinite - in fact, about 13.7 billion years ago it was a mere 'dot', then we had the big bang and it's been expanding ever since - but it's not expanding infinitely quickly or we wouldn't be able to see any distant galaxies - so it must still be finite in size. Hubble's law says how fast it's expanding and that gives us an idea of the size - which is most likely about 78 billion light-years. SteveBaker 19:24, 20 September 2007 (UTC)

Hmmm... predicting his response...

A finite but infinitely old universe and FTL travel are mutually exclusive." AJKGordon 18:26, 20 September 2007 (UTC)

Hmmm - I misunderstood that comment initially. No - even if the universe was finite but infinitely old, the aliens might not be here. A finite (spatially) universe only contains a finite number of civilisations at any given point in time - although over the entire past history there must have been an infinite number of them. Because a civilisation doesn't necessarily last forever (wars, etc) - each civilisation might last long enough to look everywhere in the finite universe using their super-fast spaceships - but we weren't there at the time, we were still in our 'primordial ooze' stage and now (millions of years later) maybe the aliens are all dead. Now we are here and they aren't - so they might never find us because we never overlapped in time. But still, it's an academic point because..... SteveBaker 19:51, 20 September 2007 (UTC)

A finite (in size) but infinite (in age) universe is already a mutually exclusive proposition...forget FTL. We KNOW the universe is expanding (even a very basic telescope+spectrometer can show the red-shift in more distant galaxies) - and we know that if it ever stops expanding, gravitation will force it to shrink again and then nothing can stop it from ending in a 'big crunch'. If the universe was INFINITELY old then either it crunched already (it didn't or we wouldn't be here) - or it has already expanded to infinite size. So infinite in size and infinite in age are possible - and finite in both size and age are possible. But besides, we can already look back at the universe as it was soon after the big bang (Cosmic microwave background, etc) - we know for sure it's not infinitely old - hence we know it's finite in size - and we're pretty much 100% certain that FTL travel is impossible. So, sadly, we have finite everything and boring old spaceships that are forever going to be too crappy to let us see much of it. Gene Roddenberry would roll in his grave. SteveBaker 18:48, 20 September 2007 (UTC)

Infinite in size but finite in age is also possible. In fact it's what follows from the simplest models having negative or zero asymptotic curvature.

Of course the observable universe is finite. --Trovatore 18:55, 20 September 2007 (UTC)

Human history is only 10000 years old or so (much less if you limit it to people capable of productively processing the concepts of space and aliens). Even if space was chock full of species with warp drives, maybe they just haven't been interested in making their presence known to a boring backwater civilization like ours. 76.231.189.193 19:52, 20 September 2007 (UTC)

OK thanks, Steve and others. Not sure I get your logic about finite age = finite size and I think that "[virtually] 100% certain that FTL travel is impossible" is over-cooking it a little but that's enough to keep the offspring entertained. Thanks again. But I may be back! AJKGordon 20:18, 20 September 2007 (UTC)

Oh - that's an easy one. The universe is 13.7 billion years old. Back then, the universe was a dot - zero sized. Then we have the big bang and it starts growing. It doesn't grow infinitely fast - the growth rate is finite and approximated by the Hubble's law. So if it started off at zero size and grew at a finite rate for a finite amount of time - then the universe has to be finite in size...and we estimate it's 79 billion light years across right now. Hence finite age means finite size (assuming finite growth rates - which is what we see when we look out there and watch distant galaxies rushing away from us). If there was at some time a brief moment of infinite expansion before things settled down to where they are now then we'd be in deep trouble because the finite amount of matter that was in the universe shortly after the big bang would be spread out over infinite space - and things would look kinda empty around here! So we can be reasonable sure the universe is finite size and finite age. But that's not 100% for sure. Anything further away than 13.7 light years is too far away to see because it would take longer than the life of the universe for the light to get to us...so there is a good fraction of the universe that we'll never be able to examine...maybe it's a lot different from our bit and maybe those parts ARE moving infinitely fast...it doesn't seem likely though. FTL travel is fairly certain to be impossible...sadly...the experimental results testing Einsteins theory of relativity have absolutely all shown it to be 100% correct - and it most certainly says that faster-than-light travel is out of the question. It's not looking likely. SteveBaker 00:01, 21 September 2007 (UTC)

The part of the finite-universe reasoning that you can't check is the "the universe was a dot" part. What we actually get by running the equations backwards to the big bang is that the universe had infinite density, not that it had zero volume. Certainly, if its mass is finite, then it would follow that it had zero volume, but that's begging the question.

I also think you're assuming too much about Einstein's work applying in regimes in which it's never been tested (why don't you like possessive apostrophes, BTW?) but we already know we disagree on that one. --Trovatore 00:29, 21 September 2007 (UTC)

• Fermi paradox. --JWSchmidt 22:36, 20 September 2007 (UTC)

Couple of things I understood differently, Steve. 13.7 billion light years is simply how far light has travelled since the Big Bang, not how far away the furthest objects in the observable universe are away from Earth - that works out as 47 billion years. The 79 (or 78) billion light year size is simply the lower limit of the size of the universe, i.e. the furthest that can be seen with no repeating patterns - repeating patterns suggesting that the universe might be curved and finite within observable limits. (I think I got that right). I was also under the impression the question of whether the universe was infinite or not was still pretty open. (No pun intended). AJKGordon 00:54, 21 September 2007 (UTC)

What about the Fermi paradox, JW? AJKGordon 00:57, 21 September 2007 (UTC)

I pointed to Fermi paradox because this section started with, "....opportunity for an advanced intelligence to .... visit us. As we see no evidence for extra-terrestrial intelligence, then.....", which is the starting point for the "Fermi paradox". People have been thinking about this for a long time and some of the resulting chains of thought are outlined at Fermi paradox. --JWSchmidt 03:06, 21 September 2007 (UTC)

The trouble with the Fermi paradox is that it's not really paradoxical. There are some rough calculations as to the number of possible alien civilisations out there. The numbers have vast error bars on them - so it's possible that the answer is 1 (vis, Us) - and suddenly there is no paradox. Also, (as previously discussed) we shouldn't be surprised that we aren't visited by aliens because of the limitations of the speed of light - and the lack of radio transmissions is unsurprising if their radio technology is on a par with ours because our strongest transmitters would be undetectable by our most sensitive receivers at 4 light years distance (the distance to the nearest star system). The universe could easily have a human-level civilisation on every single planet of every single star and there would still be no possible way for us to figure this out. SteveBaker 04:07, 21 September 2007 (UTC)

Technological civilizations might have developed on other planets of our galaxy hundreds of millions of years ago, providing what strikes some people as plenty of time for evidence of their existence to have reached Earth. Some people have speculated about panspermia and other possible forms of "contact" that might not be easy for us to detect. --JWSchmidt 06:25, 21 September 2007 (UTC)

They may have developed, and they may have died out, like many civilizations have done on this planet. Our own Western civilization has seemed precariously teetering on its own destruction for the last century, and we've only made it to our moon a handful of times. (I also want to just add, that even if we had FTL travel, it would have to be pretty significantly FTL to make interstellar travel something people really wanted to spend the time doing. Even if you were traveling at c it would still take you a long-ass time to get anywhere beyond our solar system. A four-year trip to the next nearest star is still a bit too long for casual travel, even if those on the ship didn't experience any time as passing—you wouldn't want to go on a long trip and come home to find your kids grown up, family dead, etc.) --24.147.86.187 13:09, 21 September 2007 (UTC)

It's impossible to say what FTL travel might do to human perception since all of the math and physics say it can't happen. But the effects of time and distance dilation have a massive effect on this stuff. If we could travel very close to the speed of light, distances between stars would contract and we could move between them in less elapsed time. You don't need FTL travel to zip around the universe like on StarTrek - 99.9999999% of 'c' is plenty. However, this has the side-effect of speeding up time around us so whilst the occupants of a fast ship would live long enough to travel between stars - the remainder of their civilisation would die of old age waiting to hear the results of their travels. At exactly the speed of light, the universe shrinks to an infinitesimally thin two-dimensional object - which is too small to contain your spacecraft - while time speeds up to an infinite degree so the universe ends (big crunch or heat-death...either way) before you can press the "OFF" button on your hyperdrive...never a good thing for crew morale! Beyond the speed of light - we are "in mathematical la-la-land" (a new phrase that I'm becoming fond of) in that lengths, time and masses all become complex numbers (like the square root of -1) - which is simply not possible in our universe...so speculation as to what FTL travel might or might not do is pointless - it can't happen so we can't say what would happen if it did. Things would get plenty freaky at 99.99999% of 'c'. SteveBaker 15:14, 21 September 2007 (UTC)

Science fiction writers have come up with various imaginative ways of side-stepping the Fermi paradox and explaining why our galaxy is not overflowing with alien civilsations. In the Culture novels of Iain Banks, sufficiently advanced civilisations become bored with the material universe and join the Sublimed. In Vernor Vinge's novels FTL travel (and other advances such as sentient computers) is only possible if you are more than a certain distance from the galactic centre - which the Solar System, unfortunately, is not. Gandalf61 13:33, 21 September 2007 (UTC)

I don't think they NEED to do that. The Fermi paradox relies on the Drake equation for it's prediction of the number of civilisations within reasonable distance of us. According to our article, the Drake equation states that:

$N=R^{*}\times f_{p}\times n_{e}\times f_{l}\times f_{i}\times f_{c}\times L$

where:

N is the number of civilizations in our galaxy with which we might hope to be able to communicate;

and

R* is the average rate of star formation in our galaxy

f_p is the fraction of those stars that have planets

n_e is the average number of planets that can potentially support life per star that has planets

f_l is the fraction of the above that actually go on to develop life at some point

f_i is the fraction of the above that actually go on to develop intelligent life

f_c is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L is the length of time such civilizations release detectable signals into space.

Some of these numbers we have good estimates for. We know R*, we have enough evidence about f_p to guess, but so far, the value if n_e is unknown. We've found maybe a hundred stars with planets (and it starts to look like f_p is maybe 0.1 or so) - but so far, none of them have planets remotely like earth. So n_e could be very close to zero. It could be 1.0 divided by the number of stars in the universe - which would mean we are utterly alone here. As for L and all of the f_whatever terms - we have literally no clue about those. We can guess - but the answer could be wildly wrong. So where is this paradox? It's not paradoxical for n_e to be a very tiny number - we don't know enough about planetary formation to say whether Earth is an amazing flook of nature - a unique result of a spectacularly unlikely series of coincidences. So it's not a paradox - it's a result of some of those numbers being rather smaller than we estimated. Science fiction writers don't need to come up with elaborate reasons why the universe is or is not full of aliens - it's simply the result of some assumptions about some unknown numbers in an equation. If you want a Star Wars universe where every tiny little rock (including asteroids) is teeming with life then crank n_e to around 5.0 and f_l to 1.0 and put f_i to maybe 0.5 or so. If you want StarTrek - where most of the universe has been colonised by just a few intelligent species but where most planets are colonisable - then set n_e to a bit less than 1.0 but f_l to a very small number. If you want Asimov's Foundation series where humans are everywhere but no alien life of any kind exists - then push f_l to a smaller number still so that everywhere is livable but all life comes from earth colonisation and set f_i to 1.0 so the one place that has life has intelligent life. By playing around with the unknown numbers in the equations you can get any science fiction setting you like. The Fermi paradox is nonsense until/unless we somehow prove that the f_whatever terms should somehow be big numbers - and yet we still find no life. But how would scientists show that the probability of a civilisation developing technology is a big number when we only have one example (ourselves) to use as evidence? Nah - forget the Fermi paradox. SteveBaker 15:35, 21 September 2007 (UTC)

Steve, I think you might be making the same mistake that many many people have made in the past - claiming that something is impossible when we don't really understand it. Sure, travelling conventionally in accordance with Einstein's famous equations, it seems fairly bullet-proof that FTL is impossible. But we also know, through those very same equations, lots of stuff about folding space and so on that could, theoretically, allow some of these hard and fast rules to be bypassed. Even though we have no idea how to generate exotic matter and other fanciful notions for, say, worm-holes, or indeed if it's even possible, that's "just engineering". Science and technology are two very different things. And the latter doesn't necessarily need to understand the former! AJKGordon 18:35, 21 September 2007 (UTC)

The trouble comes when you add infinite variables to a question... infinite sets can act and interact in funny ways. Probably a bit too complicated for young children, but these discussions of infinite sets and events can be good brain candy (depending on your sweet tooth). --SB_Johnny | ^PA! 19:57, 21 September 2007 (UTC)

Thanks Johnny. 84.98.245.216 20:51, 21 September 2007 (UTC)

Finding Average Acceleration from a Position-Time Graph

So there's this physics question which I can't seem to get. There's a bus, it's got a varying speed along its 3.5 hour trip. From t = 0 to t= 1 hour it position changes from 0 to approx. 25 (the graph is small). Then on leg B of it's journey, it goes from t=1 hour t~2.2 hours. Its position changes fromm x = 25 to x = 33. Then, on leg C of the journey, from t = 2.2 to t =3.5, the bus's position changes from x = 33 to x = 28. Now, I have to find the average acceleration. The answer I know, -8.3km/h². However, the process I don't. I first began by calculating the velocity's (using the slope) of A, B, and C.

So then I got answers such as V(a)= (25km-0km)/1hr = 25km/hr, V(b) = (33km-25km)/(2.2hr-1hr) = 6.6km/hr, and V(c) = (28km-33k)/(3.5h-2.2h) = -3.84km/hr.

Now I proceed to find the average acceleration of each segment, by diving the change in velocity of each segment by the change of time. a) (25km/hr-0km/hr)/(1hr) = 25km/hr². b) (6.6km/h²-25km/²)/(2.2hr - 1hr) = -15.33km/h². c) (-3.84km/h-6.6km/h)(3.5hr-2.2hr) = -8.03km/h².

That adds up to a grand total of of + 1.64km/h²....which is well far from the answer. If any could help me, I'd really appreciate it. Thanks.

207.161.45.29 22:48, 20 September 2007 (UTC)

It seems odd to ask for average acceleration, rather than average velocity, in a problem like this. That said, look at the first equation in velocity, which says how to calculate average velocity; acceleration is to velocity as velocity is to position, so you should be able to use a very similar equation for your problem. (Hint: you'll need some but not all of the numbers you've given here, and some but not all of the ones you don't need to use are wrong.) --Tardis 23:32, 20 September 2007 (UTC)

I do use that same equation, in the second part where I wrote avg. a = (v2-v1)/(t2-t1). I'm not really sure what you mean by the last sentence.. I need to use some of the numbers I came up with, but some of those are wrong? —Preceding unsigned comment added by 207.161.45.29 (talk) 23:42, 20 September 2007 (UTC)

No...this is wrong, wrong, wrong. You've approximated the position curve by three straight lines. A straight line in position means a constant velocity which means zero acceleration. But at the precise point where the two straight lines meet, the velocity changes instantaneously. That's an infinite accelleration for zero amount of time - which throws the math off into la-la-land. SteveBaker 23:48, 20 September 2007 (UTC)

I know the acceleration is zero from Point A to point B, from Point B to C and zero from C to D, but as a whole the acceleration changes because the velocity changes. How would I work this out? —Preceding unsigned comment added by 207.161.45.29 (talk) 23:58, 20 September 2007 (UTC)

No - you don't understand. You have approximated the position-versus-time curve as a straight line from A to B - right? So you can calculate the average velocity over that time from the slope of that line. But if you assume the velocity is constant - then what is the acceleration? Acceleration is the rate of change of velocity over time. Well, if the velocity is constant then there is no change - so the acceleration is zero. OK - so from A to B there is ZERO acceleration. If B-C is also approximated by a straight line (albeit one with different slope from A-B) then the same argument applies - a straight line for position-versus-time means constant velocity and zero accelleration. The velocity changed at point B. So let's plot a graph of velocity versus time. It's a horizontal line from A to B - then the graph goes up vertically and you get another straight line from B to C. Acceleration is the slope of the velocity/time graph. So you had zero acceleration most of the time - but at precisely time B you had...infinite accelleration. So for most of the journey, there was zero accelleration - but at time B there was infinite acceleration. SteveBaker 00:09, 21 September 2007 (UTC)

Okay that does makes sense...but then why in the world does my textbook (wiley 7th edition) give an answer of -8.3km/h²? I've tried nearly every way to get that number pop up in my calculator but it won't. Is the textbook wrong then?

Well, let's assume that your approximation of the three sections of the graph as straight lines is wrong. I think the average accelleration should be the velocity at the very end of the graph minus the velocity at the start of the graph - divided by the time. The rate of change of velocity over time. I don't care what happens to the position and velocity between those two points...right? So measure the slope of the curve at time A (that's the initial velocity), measure the slope at time C (that's the final velocity), subtract one from the other and divide by the total time. SteveBaker 00:25, 21 September 2007 (UTC)

Ahh, and there it is. Thanks for devoting so much time and effort. Thanks again! 207.161.45.29 00:34, 21 September 2007 (UTC)

Please do point out to your instructor that the illustration is crap, in that infinite acceleration is needed between the flat lines, which, of course, is impossible. StuRat 02:17, 21 September 2007 (UTC)

Indeed - yes. I'm a little dissatisfied with the definition of an 'average' - in a situation where you have zero acceleration throughout the period being sampled - with the exception of an infinite acceleration for a zero amount of time...I'm not sure that the mathematical concept of an 'average' can actually be calculated. However...it looks like we answered this one to at least some degree of satisfaction. SteveBaker 14:44, 21 September 2007 (UTC)

The notion of average is actually one of the motivations for introducing such things as the Dirac delta function. If you work at a goldsmithy and get paid in the form of a continuous trickle of gold from the casting pot, we can talk about your wage as a literal instantaneous derivative of value in the gold that's yours, and that derivative is constant (at least over the work day). If your company decides to instead give you weekly paychecks via direct deposit, and the average rate of payment is the same, then the paychecks (which are instantaneous adjustments to your wealth) must each have the value of the week's worth of trickle. The checks can be seen as the limiting process of giving you a larger portion of the "flow" for a shorter period of time: you could have all the output for, say, 3 minutes a week instead of having the trickle. Then the plot of your wealth-rate over time looks like a series of top hats corresponding to those 3 minutes each week. The paychecks are just the limit as the paying-time tends to 0 while the paying-rate grows without bound in such a fashion that their product (integrated over the longest time interval involved, like a week) is constant. On longer time scales (like calculating your yearly salary) the average is all that matters, and the divergent instantaneous values are unimportant.

In the OP's problem, the discontinuous changes in velocity merely correspond to changes that are fast enough that we don't worry about them. A bus could easily change speeds from 25 to 6.6 or from 6.6 to -3.84 kph in a very short time compared to an hour or more of travel at each speed, so (in terms of its overall position) the details of how it changed speeds are unimportant (except to the passengers, who do not want to experience the conceptual infinite acceleration). The average acceleration is quite well-defined, so long as we are sure that we don't cut the graph off in the middle of a change: if the bus came to a stop at the 3.5 hour mark, our value for the average acceleration changes drastically if we include that change or not (or include half of it!) in the analysis. The reason for the trouble is that we are told to evaluate the average acceleration on a precise time interval, so if there's a change at one end or the other there's no way for it to be so fast that we don't have to worry about its precise duration and shape. --Tardis 19:33, 21 September 2007 (UTC)

There is something else confusing about "average acceleration", how to deal with negative acceleration (deceleration). For example, if you accelerate at 1 g to the half-way point of your journey to the Moon, then turn around and decelerate at 1 g for the second half of the journey, does that mean your average acceleration is 1 g or 0 g ? There are different ways of looking at it. I suppose we can call the 1 g answer "the average of the absolute values of accelerations" and the 0 g answer "the average acceleration", to avoid this confusion. In this case, the 1 g answer is definitely more useful, when considering the effects on humans. (Note, this isn't how actual Moon trips have worked, but would provide the most comfortable, quick journey, if energy was not a constraint.) StuRat 16:08, 22 September 2007 (UTC)

Heat Absorbtion

Please go in depth on the following question. Do different colors absorb heat diffently? If so, why? —Preceding unsigned comment added by 208.61.108.71 (talk) 23:06, 20 September 2007 (UTC)

Yes. A coloured object looks coloured because it's absorbing some frequencies of light and reflecting others. The light energy it absorbs turns into heat (in almost all materials). So a black object (which absorbs all frequencies) gets hotter than a white object (which reflects most of it). However, it's not quite a simple as that because lots of the sun's energy comes in the form of Ultraviolet and InfraRed light - and two objects might appear to be the same colour but in fact absorb different amounts of UV and IR making one get hotter than the other. Beyond that, it's a complicated matter to actually predict the total energy gain because when an object gets hotter, it sheds energy by emitting IR radiation too. So some objects emit energy better than others - which greatly confuses matters. SteveBaker 23:45, 20 September 2007 (UTC)

To clarify on Steve Baker's answer, a black object absorbs all visible light frequencies, but not necessarily those outside our vision range. User:Sifaka who is currently IP 128.196.149.20 01:27, 21 September 2007 (UTC)

People never seem to mention that darker objects emit energy easier, too. Because of this, when objects are cooling down, darker object will cool faster and end up being colder than light objects. — Daniel 01:46, 21 September 2007 (UTC)

Are you sure about that ? Why would it's color in the visible frequency be related to it's ability to radiate heat (in the infrared frequency) ? StuRat 02:05, 21 September 2007 (UTC)

I think he assumes that darker objects in the visible spectrum are also typically darker in other wavelengths. --Spoon! 02:38, 21 September 2007 (UTC)

Well, if they were then they'd cool off slower. No - what's going on here is that EMISSIVITY is not necessarily correlated with REFLECTIVITY. The former is associate with shedding heat - the latter in absorbing it. Whilst it is commonly the case that visible colour and IR reflectivity and emissivity are correlated - it's not necessarily true. SteveBaker 03:54, 21 September 2007 (UTC)

Kirchhoff's law of thermal radiation --Spoon! 04:21, 21 September 2007 (UTC)

And Black-body radiation. --jjron 09:01, 21 September 2007 (UTC)