Wikipedia:Reference desk/Archives/Science/2007 November 24

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November 24

Supercavitation

Hey I was reading a question on the science desk about the fastest way to move through water, and someone mentioned Supercavitation. I was wondering could Supercavitation be used on a plane or rocket to create a vacum throgh the air allowing it to move much faster?67.127.235.74 (talk) 00:03, 24 November 2007 (UTC)

No. The phenomenon of supercavitation depends on the fact that there is a phase boundary between liquid and gas. There can be no such phase boundary between gas and vacuum. Icek (talk) 01:48, 24 November 2007 (UTC)

IUDs and large penises

Are IUDs safe and reliable if the male has a large penis? —Preceding unsigned comment added by 189.15.179.115 (talk) 00:38, 24 November 2007 (UTC)

I would think (but IANAD) that they're just as safe and reliable as with small penises. IUDs, as their name implies, are placed inside the uterus, which is separated from the vagina by the narrow cervix. In normal women who aren't pregnant, the opening of the cervix (the external os) is tiny, maybe a few millimeters wide at most. Obviously the penis is much wider than that, so it never penetrates the cervix during intercourse. —Keenan Pepper 01:35, 24 November 2007 (UTC)

Well, not always entirely inside the uterus. The IUD article has a comment in the "Side effects and complications" section that appears to be the same information as in this ref of the article. Therein is a FAQ item:

10 Q: Should an IUD be removed if a woman's sexual partner complains about the IUD string?

A: Not necessarily. The couple may need reassurance and an explanation of what the string is. If this is not satisfactory, the end of the string can be tucked behind the cervix. If this too is not satisfactory, the string can be cut flush with the cervix. (This should be noted in the woman's record.) Such short strings will mean that the woman will not be able to check the strings and a provider will need narrow forceps to grasp the strings when removing the IUD. The woman should be given the choice of what she wants done, including whether the IUD should be removed.

Anyone know of a free diagram of a uterus with an IUD in it? Would be a good addition to our article. DMacks (talk) 19:35, 24 November 2007 (UTC)

Genetic possibilty of genius IQ

I am adopted and just discovered that my real father was a mathematical genius. I am not saying that I am a genius but I was in gifted/advanced classes for most of my life and read through books voraciously. Otherwise, I am a normal girl with normal wants/needs. However, I would sometimes freak out my friends for knowing more about a subject than necessary, etc. Is it possible that somehow that this could be genetic? --WonderFran (talk) 02:02, 24 November 2007 (UTC)

Yes, intellectual potential is partially determined by genetics. See genetics of intelligence and related articles. Dragons flight (talk) 02:18, 24 November 2007 (UTC)

One should be careful to read too much of a direct correlation between any given behavior and one's apparent genetic heirs. Genetics are complicated and the only direct relationships one have between genes and behavior are for extremely rare things (usually disorders)—the relationship between genetics and IQ is at this point known only in a purely statistical terms. To claim that your aptitude in school or love of learning is "only" the result of a genetic quirk both devalues your own effort but also the efforts of those around you—in reality, all things genetic require development to even become recognizable as traits, and we are not simply reflections of our genes. --24.147.86.187 (talk) 07:57, 24 November 2007 (UTC)

You might want to take note of the twin studies, which compared traits of identical and fraternal twins that were separated at birth. Identical twins showed more similarity than fraternal twins in a variety of areas, with IQ being at an intermediate level of genetic influence. So, IQ is a product of both genetics and environment. -- HiEv 16:45, 24 November 2007 (UTC)

I completely understand your point of view however, I grew up on the projects and anyone who has can contest the difficulty of achieving academically....not to dismiss the few who have.....--24.151.103.18 (talk) 08:05, 24 November 2007 (UTC)

Antisocial personality disorder

How many famous people have this disorder? —Preceding unsigned comment added by 76.64.130.224 (talk) 02:45, 24 November 2007 (UTC)

See antisocial personality disorder#prevalence. Fame is relative and somewhat subjective, so I don't see how it can be included in the calculation.--Shantavira|^{feed me} 08:25, 24 November 2007 (UTC)

BOOLEAN OPERATORS

boolean operators —Preceding unsigned comment added by 75.26.161.182 (talk) 03:38, 24 November 2007 (UTC)

Boolean operators. Someguy1221 (talk) 04:08, 24 November 2007 (UTC)

Pluto

Why didn't Voyager 2 pass Pluto? 124.176.190.64 (talk) 04:33, 24 November 2007 (UTC)

Presumably because the planets weren't lined up nicely enough for it. Someguy1221 (talk) 04:54, 24 November 2007 (UTC)

The orbit of Pluto is way out of alignment with the orbits of the planets, so although Voyager passed beyond the orbit of Pluto, it would have been far far away at the time.--Shantavira|^{feed me} 08:33, 24 November 2007 (UTC)

This answer confuses two issues. It's true that all the other planets orbit in something close to the same plane while Pluto's orbit is somewhat inclined, but it's not inclined so much that it would be unreachable. The problem is that, at the time of the Voyager probes, it was in a different part of its orbit.

The whole Voyager 2 mission was only possible because the four gas giant planets were in roughly the same direction from the Sun at that time, a rare occurrence. See Planetary Grand Tour. As it says in that article, it would have been possible to reach Pluto by directing the probe appropriately on leaving Saturn -- but then the probe would not have passed Uranus or Neptune.

The thing is that when you want to use the gravity of planet A to direct the probe onward to planet B, this fixes the course that your probe must take near planet A, and it probably won't be the same course you'd route it on if you were only interested in A. NASA was under such budget constraints at the time that they decided it was more important for the Voyagers to be well placed to visit Jupiter and Saturn than it was to pick up all three of the other planets. Only after Voyager I had succesfully visited Saturn, taking the pressure off Voyager II, was the latter probe placed on a course that would allow it to continue with the Grand Tour after Saturn. (Sorry, no cite, but that's what I remember reading.) Without a third probe, there was no way to reach Pluto as well.

--Anonymous, 12:40 UTC, November 24, 2007.

Pluto orbits the sun once in every 248 years (and at a very odd angle to that of all of the major planets). Voyager is heading away from the sun at about 35,000mph and Pluto is only 1500 miles across. The odds of it happening to be in the right place for the trajectory of the spacecraft to intercept it is quite remote unless the mission planners specifically designed things to come out that way. When they designed the route of the spacecraft they had specific things they wanted to survey - they must have had to make all sorts of compromises in order to manage the various gravity slingshot manouvers they did. Evidently they simply couldn't figure a way to get over to Pluto along the way. SteveBaker (talk) 21:56, 24 November 2007 (UTC)

See my answer above. They did figure out a way, but had other priorities. --Anon, 23:02 UTC, Nov. 24.

Path integrals and lightcones

As far as I know, in the the path integral formulation of quantum mechanics, one has to include faster-than-light paths. How would the predictions of quantum mechanics change if one would exclude faster-than=light paths? —Preceding unsigned comment added by 193.171.121.30 (talk) 09:11, 24 November 2007 (UTC)

I'm not sure if FTL paths are necessarily included when Quantum field theory is formulated in Minkowski space, but either way an exclusion of that sort would simply be a change in the geometry of one's space, which is suggested in the article to not have serious or significant consequences to the accuracy of the theory. SamuelRiv (talk) 17:24, 24 November 2007 (UTC)

Orbitofrontal cortex and ventromedial prefrontal cortex

Is the orbitofrontal cortex the same as the ventromedial prefrontal cortex? Or is it a part of the ventromedial cortex??? Lova Falk (talk) 10:33, 24 November 2007 (UTC)

I'm not sure, but they seem to correspond in location and function (see ventromedial prefrontal cortex): both process risk and judgement. SamuelRiv (talk) 13:13, 24 November 2007 (UTC)

Another question on the prefrontal cortex

In picture A one can see the vl-PFC (yellow) and the dl-PFC (blue). But what would be the name of the grey area between these two? Lova Falk (talk) 11:00, 24 November 2007 (UTC)

The mIPFC, medial inferior prefrontal cortex, i would imagine. It's been a while since neuroanatomy. SamuelRiv (talk) 12:50, 24 November 2007 (UTC)

But the medial prefrontal cortex (for some reason called MFd) is the reddish/brownish part of the picture. Would the medial inferior prefrontal cortex be in a complete different place? Lova Falk (talk) 14:13, 24 November 2007 (UTC)

Sorry, I meant medial lateral prefrontal cortex. I really don't know for sure though. Google suggests that this is a legitimate name for that region, but it doesn't look like it's used much in the literature. SamuelRiv (talk) 17:21, 24 November 2007 (UTC)

Symmetry of animals

Why are most of the biological organisms in animalia kingdom symmetric externally, though they are highly asymmetric internally? I was wondering why nature might have chosen the symmetric structure and what great benefits this symmetry brings to animals? This is not the case with plants, though their leaves and flowers also tend to be highly symmetric about at least one plane. I have never seen any asymmetric animal or plant leaf or flower, so to say. DSachan (talk) 19:28, 24 November 2007 (UTC)

I've always wondered about this, too. Here's an article I just found that might be helpful:[1]. 128.163.224.198 (talk) 20:15, 24 November 2007 (UTC)

Thanks for the link. But the article only discusses about what is responsible for the structuring of organs internally the way they are. It does not say anything about that skin deep super symmetry that exists everywhere and it also does not mention anything about why it might be so or the advantages and disadvantages of it. Though one thing I got to know from the article is that it is a pestering problem for scientists today and they are trying to speculate about the evolutionary benefits of skin deep symmetry and then asymmetry there onwards. Any other thoughts on the issue are welcome. DSachan (talk) 20:30, 24 November 2007 (UTC)

One explanation is that the macroscopic external world tends, on average, to be symmetric in the sense that there is no particular advantage to looking or turning left instead of right or vice versa. In fact, it is advantageous for many species to be able to see and turn equally well in either direction, since if they showed a preference for one direction over the other, other species (such as their predators) could evolve to take advantage of this. The easiest way to achieve this ambidexterity is to make the animal's body plan bilaterally symmetric; this also has the advantage of simplifying the ontogeny of the animal, since the development of both of its sides can be controlled by the same genes.

Another, related reason is that many methods of locomotion employed by animals, such as swimming, walking or flying, work best with pairs of symmetric limbs. A fish with bigger fins on one side than the other would tend to swim in circles, a human or any other land-dwelling animal with longer legs on one side than the other would have difficulty running straight ahead, and a bird with one wing bigger than the other could scarcely even take off.

As for why animals don't tend to be more symmetric, this is also explainable by environmental factors: on Earth, gravity breaks up-down symmetry, leading to most animals having distinctive bottom and top sides, while the need to move rapidly tends to create a need for a specialized front and rear end. It's worth noting that quite a few marine species, such as starfish and jellyfish are, in fact, radially symmetric without a distictive front end — but few if any of these are species whose survival strategies would involve rapid movement. —Ilmari Karonen (talk) 20:26, 24 November 2007 (UTC)

Someday I'd like to run artificial life in a universe of higher dimension, and see what kinds of symmetry are favored in the critters there! —Tamfang (talk) 20:36, 24 November 2007 (UTC)

Ilmari Karonen, your logic of locomotion is working fine with the animals, but what about leaves and flowers? I see only ontogenical argument of yours working there. Will this be the only reason in leaves and flowers? and that means to say, nature is also fed up with having a large number of genes required to express the characteristics of biological organisms and it wants to get rid of as many as it can.

The other point is that there are some features even in us which I see having no advantages of them being symmetrically located. For example, the navel (belly button) is also symmetric about the sagittal plane in the middle of our belly. Does this point in our belly tend to be the point making the umbilical cord shortest in length in the early stages of our life? If this is not so, what else is the reason? DSachan (talk) 20:52, 24 November 2007 (UTC)

For plants, I suspect it may indeed be just a case of symmetric leaves being simpler to produce than asymmetric ones. A leaf essentially starts with a stem and then fans out — it's probably simplest to have it fan out equally in both directions. Also note that asymmetric leaves would tend to droop due to gravity pulling the heavier side down, which might be suboptimal for catching sunlight, at least when the sun is high in the sky. As for the navel, I'd guess its location along the body's certerline may be just an accident of evolution — although it's worth noting that mammalian embryos start out highly symmetrical (spherical, in fact) and then gradually develop various asymmetric features as they mature. Since the umbilical cord forms very early during embryogenesis, at the time when the embryo just begins to acquire a distinct head-tail axis, it makes sense for it to be aligned symmetrically; at that stage, everything in the embryo is still symmetric. —Ilmari Karonen (talk) 22:38, 24 November 2007 (UTC)

I read somewhere (I forget where - sorry) that one possible reason for symmetry is that it requires less DNA to code for it - and less DNA means less to go wrong and less 'stuff' to carry around in your cells. SteveBaker (talk) 21:42, 24 November 2007 (UTC)

Plus, it's 'easier' to evolve. Plant evolves leaf. Plant's descendants have simple mutation to carry out 'reading' of leaf code twice. Plant has two symmetrical leaves. Skittle (talk) 23:36, 25 November 2007 (UTC)

Another possibility is that the symmetry seen around us can be traced back to fundamental mathematical symmetries. The golden ratio and the Fibonacci series are good examples of this. It could also reflect the underlying symmetry of the materials from which we are made, and the biochemistry. Build things up in a symmetric way, and the result will be symmetric. Redundancy, as well (only need instructions for one half of the object). Evolutionary history as well - many of our bodily structures still reflect our evolutionary history. We've inherited the symmetry in those early forms of life. Carcharoth (talk) 22:05, 24 November 2007 (UTC)

Great answers, Ilmari Karonen. – b_jonas 12:11, 26 November 2007 (UTC)

Also, symmetry is a simple and often accurate way of detecting the health of an animal. Because of that, most species with bilateral symmetry see symmetry as a form of beauty. Take a look at the faces of attractive people and you will often find a higher than average amount of symmetry. Thus life has evolved to select for external symmetry. See Symmetry (physical attractiveness) for details. -- HiEv 12:41, 26 November 2007 (UTC)

Here's an asymetrical animal, although a result of development rather than original design. Mattopaedia (talk) 12:49, 27 November 2007 (UTC)

Lunar warfare

I've been thinking about what warfare would be like if it took place on the Moon in the near future. While science fiction is in love with laser weapons, it seems the worlds military are rather more conservative, in that they're very happy to go to war with weapons several decades old, but that they know work. My thoughts are below: my question is to ask y'all if I'm on the right track about the science:

• Conventional guns work pretty well. Bullets fly on shallower trajectories and (lacking air turbulence) don't need to be spin stabilised (so barrels are unrifiled). With a decent scope a sniper could be a threat at 10 miles away. Moondust and propellant residue must be cleaned from a gun's action with a can of compressed gas. For hand-held guns recoil is more of an issue that on Earth (because the firer has less weight with which to counteract it by leaning into the shot) so muzzle breaks are found on most guns. The big problem with guns is dumping excess heat. Single-shot and semi-auto guns have black radiative fins to try to dump heat, while automatic weapons must have cooling systems (which work by using the excess heat to warm dry ice, which sublimates and is then vented)

• Conventional unguided rockets work well. They don't need fins for stabilisation (again due to the lack of atmosphere), an have an effective range several times that of comparably sized terrestrial equivalents. Guided missiles must use high-performance motors (e.g sodium azide cells) to adjust their course midflight.

• a Lunar Positioning System (LPS) can be erected much like GPS. Reasonable advances in portable electronics and antennas mean that a system with fewer satellites (in wider orbits) will be sufficient.

• The weapons of artillery pieces and tanks work well (although the vehicles that propel them are obviously different). As with firearms, cooling is a major issue. When integrated with a LPS and an electronic battlefield system they can attack targets well over the horizon.

• No equivalent to close air support is possible. Tactical support of land forces is supplied by artillery or ground-to-ground rockets. Strategic bombing is achieved either by long range g2g missiles or missiles fired from orbital weapons platforms.

• With no cover, no weather, limited opportunities for camouflage, and the extreme ranges at which simple weapons are effective, everyone on the battlefield is very vulnerable. Humans, who are yet more vulnerable in pressurised suits and vehicles, are largely absent from the battlefield; most combatants are semi-autonomous robot vehicles.

Neglecting obvious speculation about energy weapons (which I appreciate would be more effective in a vacuum) does this seem, erm, airtight? 86.131.206.94 (talk) 20:20, 24 November 2007 (UTC)

Instead of weather you get the long lunar night, which may be lit by earthlight, or only starlight on the backside of the moon. Things could very cold when not lit by the sun for weeks. Another factor is the extreme vulnerability of people to puncture wounds in their air containment. Something more like a shotgun may be able to cripple dozens of unprotected people. Graeme Bartlett (talk) 20:34, 24 November 2007 (UTC)

Don't forget that conventional guns usually need air to fire properly, as they propel by a rapidly-expanding gas. New propellants or rail guns are probably a better option. SamuelRiv (talk) 20:55, 24 November 2007 (UTC)

Any recoil vector not tangential to the surface would make the firer jump or fly up, which could be a big factor for artillery. A tank firing a sabot could put the penetrator into low orbit, so it would need to be careful about trajectory and try to figure out where not to be when it comes back around if it screws that up. The same goes for any weapon firing a projectile that goes faster than about 5,500 ft/s. Runaway missiles and stray shots would be raining down all over the moon for days or weeks after a battle, going as fast as they were when fired. Barrels would still be rifled, as the stabilization helps overcome perturbations caused by asymmetry in the gas blowby at the muzzle, which will envelope the projectile quite a ways downrange. Anything like aircraft would only be needed for emergency reconnaissance, as ballistic weapons could hit anything anywhere. Nukes could be used with impunity over the horizon, as all personnel would already be suited or indoors anyhow, and no blast would be felt. --Milkbreath (talk) 21:19, 24 November 2007 (UTC)

I don't believe that's true. Conventional guns turn solid propellant into gas (during its explosion) and that's what pushes the bullet out. Gas from the atmosphere just gets in the bullet's way. -- Finlay McWalter | Talk 21:02, 24 November 2007 (UTC)

I'm not sure that there would be little opportunity for camouflage. The lunar surface isn't particularly flat in most places, so (depending on time of day) there may be lots of shadows to play with and lots of structures to hide behind (erosion is very slow on the Moon...). As well, the surface colour and texture is fairly uniform compared to Earth (just a couple of different broad classes of rock, dusted over in many places with regolith) makes supplying camo uniforms easy (none of this mucking about with separate desert/jungle/winter/city uniforms). Of course, waste heat from warm bodies and equipment will be a dead giveaway on any sort of infrared imaging during the chilly lunar night.... TenOfAllTrades(talk) 21:21, 24 November 2007 (UTC)

Yeah - you don't need air to fire a gun. The oxidizer is mixed into the propellant.

• Conventional guns work pretty well. True.

For hand-held guns recoil is more of an issue that on Earth (because the firer has less weight with which to counteract it by leaning into the shot) so muzzle breaks are found on most guns. - Not true. Every action has an equal and opposite reaction. F=ma - the mass of the bullet (NOT WEIGHT) times it's accelleration produces a force that is absorbed by your body - so your accelleration (the 'recoil') is the mass of the bullet times the accelleration of the bullet divided by your body mass. Hence the recoil will be identical to what it was on earth. Actually rather less because you're wearing that huge chunky space-suit.

The big problem with guns is dumping excess heat. Single-shot and semi-auto guns have black radiative fins to try to dump heat, while automatic weapons must have cooling systems (which work by using the excess heat to warm dry ice, which sublimates and is then vented) - Maybe that's a problem...it's hard to know. I doubt dry ice solutions would be practical though.

• Conventional unguided rockets work well. They don't need fins for stabilisation (again due to the lack of atmosphere), an have an effective range several times that of comparably sized terrestrial equivalents. Guided missiles must use high-performance motors (e.g sodium azide cells) to adjust their course midflight. - The inability to use fins for stabilisation would be a major problem - but you shouldn't be thinking of long, thin missiles - they can be any old shape. Probably gyroscopic stabilisation will suffice.
• a Lunar Positioning System (LPS) can be erected much like GPS. Reasonable advances in portable electronics and antennas mean that a system with fewer satellites (in wider orbits) will be sufficient. - Probably.
• The weapons of artillery pieces and tanks work well (although the vehicles that propel them are obviously different). As with firearms, cooling is a major issue. When integrated with a LPS and an electronic battlefield system they can attack targets well over the horizon. - There is no problem (in principle) with making over-the-horizon weapons here on earth either. But you have to consider the problems of locating your target and that they can sense your incoming weaponry with sufficient time to get out of the way. Hence you need guided weapons...which in turn brings a whole other slew of problems.
• No equivalent to close air support is possible. Tactical support of land forces is supplied by artillery or ground-to-ground rockets. Strategic bombing is achieved either by long range g2g missiles or missiles fired from orbital weapons platforms. - See below.

I don't think you are thinking far enough 'outside the box'. Because there is no atmosphere, you can in principle orbit at very low altitudes - just above the tallest mountains would be just fine. Orbital speeds can be pretty amazingly high with no air resistance or aerodynamic forces - and a weapon in polar orbit can be arranged to cover the entire moon over enough orbits. Also, you don't need to burn fuel to stay up there. So injecting an enormous cloud of low-yield rocket/bomb/satellite things up there - with primitive guidance, a camera and a small rocket to nudge them out of orbit would produce a lethal combination. They could act as their own surveillance - when they spot a likely target, they call someone who is nice and safe a long way off who decides kill/no-kill - and the next available satellite nudges itself out of orbit and comes screaming in for the kill. The speeds would be amazingly high - you probably wouldn't even need explosives. You'd be able to nominate a target to hit from the ground and just have the next available unit drop out of orbit to take it out. With weapons like that, you'd obsolete almost all of the other things you've come up with. However, with both sides taking the same approach, there simply won't be any targets to hit.

The bigger question is why there are targets out there anyway? You won't have huge civilian cities - and with no 'nuclear winter' concerns, why not just nuke the mines, factories and launch facilities out of existance? (In fact - forget the mines and factories - just take out their launch facilities and their mines and factories are irrelevent. I can't see the need for humans and tanks and stuff to be there at all.

SteveBaker (talk) 21:30, 24 November 2007 (UTC)

• Hold on there. It's true that without air resistance an orbit at very low altitude is possible, but it won't necessarily be stable. The Moon's gravitational field has irregularities and the Earth causes sizable perturbations. Your orbiting weapons would need fuel for stationkeeping, although I don't know how soon. Also, from a position close to the ground, they could only strike targets along a narrow path on each orbit... so you'd need an awful lot of them to be able to cover a reasonable amount of the Moon's surface. --Anon, 23:17 UTC, November 24, 2007, Earth.

That's true - but it's likely to be a pretty tiny amount. You only have to account for gravitational variation - a very gentle adjustment is all that's likely to be required. The craft need some kind of motor to nudge themselves out of orbit anyway - so I doubt that's a huge deal - a hydrazine thruster would probably suffice. As for the number you'd need - that would depend on how urgently you need to hit your target. A polar orbit would take half of a lunar month (14 days) to cover the entire surface - but the orbital speed would be something like a kilometer per second - taking about two hours to complete an orbit - with the orbits being about 80km apart at the equator - it doesn't take much of an orbital 'nudge' at 1km/sec to deflect your orbit by 80km. So if you had 14 of these satellites you could hit any target within a day of deciding to do so. If you had 150 of them, then you have to plan your attacks a couple of hours ahead of time. With 3000 of them you can hit any target within 5 minutes - which is pretty good by military planning times. I envisage these things as being cheap - highly networked (so you can't take out a command unit or disrupt their communications because they can pass messages over very short range radio from one satellite to the next) - and with each one carrying a camera (which they'd need for station-keeping and targetting) - you have constant surveillance of the entire moon. You'd launch them from orbit and since they're cheap you'd have a heck of a lot of them. With the kinetic energy from travelling at 1km/sec, something about the size of a football is all you'd need to vaporize your target. I had in mind satellites like the size and complexity of a cellphone - with the same communications ability and a similar kind of camera - plus a few ounces of hydrazine fuel and gyroscopic stabilisation. Cost in bulk could be thousands of dollars each - so a few million dollars (about the cost of one tank and about a lot less than the cost to get one infantryman to the moon) would allow you to bring down huge destruction on anything on the surface of the moon within a few minutes. SteveBaker (talk) 17:33, 25 November 2007 (UTC)

(edit conflict) Okay, my bad, according to MythBusters_(season_4)#Guns_Fired_Underwater, oxygen is not needed with modern guns. The ignition of gunpowder and primer does not require oxygen. I guess enough gas is produced to accelerate the bullets effectively: Smokeless powder converts almost everything to gas, but gunpowder only gets about 40% yield. There also should not be any leakage of gas behind the bullet, though maybe some is there as the cartridge is loaded... if this is the case, you'll want to manufacture some new guns and bullets that are optimized to this change in environment. Heat buildup is a big problem, as that's normally dealt with by air cooling (yes, in the end almost everything here is air-cooled). Any type of radiative cooling would kill any hope of camouflage, but I don't see any other solution besides dumping excess heat into some type of electromagnetic radiation. SamuelRiv (talk) 21:45, 24 November 2007 (UTC)

You could drive a big spike into the lunar soil and use that as a heat sink. If you find somewhere in permenant shadow, it'll be pretty amazingly cold. If you have cheap access to water (unlikely), you could build a cooling jacket that oozed water to the surface. It would boil off in the vacuum taking substantial amounts of heat with it. But I very much doubt that conventional projectile weapons are the way to go. Infantry on the ground would be horribly vulnerable - even a blast of low speed buckshot would penetrate a spacesuit. Firing a bazillion small ballbearings on a ballistic trajectory (with no air resistance to slow them down) would take out large formations of infantrymen very easily indeed. Ergo there won't be large formations of infantrymen. The lack of air resistance (and hence the absence of terminal velocities) means that if you are killable by high speed metal - you're dead. So you have to be moving fast or buried underground or very expendable. With no reason not to use nuclear weapons (no civilians, no wind-born fallout, no nuclear winter issues - and if your technology is good enough to get you to the moon in the first place, then it's good enough to build nukes with), being buried a little way underground won't help you - so you'd have to be in a gigantic bunker - very tough to construct in a lunar environment. Moving fast is possible - but moving fast in a nice, predictable, straight line is death - so you need to be accellerating unpredictably. For that you need something with no humans inside. Since you can use insanely low altitude orbits to stay airborn without using fuel, you probably want very manouverable, very low altitude satellites. Being very numerous is another way to be safe - so (as I said before) a vast number of very low altitude orbiting bombs can take out anything that moves on the surface.

But still - why fight on the moon to start with? If there are any people there at all then clobbering their supply lines from Earth is the simplest solution. Those supply craft will be big, sluggish and predictable. Fire a cloud of ball bearings at a few hundred mph in the direction of a resupply craft and they are without food and water within a month. Do it two or three times in a row and they are dead.

SteveBaker (talk) 22:16, 24 November 2007 (UTC)

a cardiac procedure called mase

What is a cardiac procedure called mase? —Preceding unsigned comment added by 67.177.212.215 (talk) 18:50, 24 November 2007 (UTC)

There's nothing in the book of medical abbreviations on my desk. Could it be something done with a maser? —Tamfang (talk) 20:32, 24 November 2007 (UTC)

MACE appears to stand for "major adverse cardiac events", which includes infarctions and other (dunno what) bad heart happenings. [2]. But that's not a procedure. -- Finlay McWalter | Talk 20:34, 24 November 2007 (UTC)

See Maze procedure. --Arcadian (talk) 23:09, 24 November 2007 (UTC)

Artificial gravity

If a spaceship, which is not near any object large enough to cause significant gravity, would generate its own gravity by rotating around its own axis, what would happen? If someone standing on the edge would climb up all the way to the centre, and then keep on climbing further, would they start falling down, but actually not back where they started, but in the same direction that they had been climbing? And where is "down" exactly when it is towards an entire area, not towards a single point? Is it towards the point on the surface that is nearest to your current location? Does that mean that if someone were to jump up on a rotating spaceship, they would land on a different spot than they originally jumped from? JIP | Talk 20:38, 24 November 2007 (UTC)

If you can, watch the movie 2001: A Space Odyssey (film) - it does a great job of showing what spin-gravity would be like. But let's imagine a vast spacecraft that's a cylinder spinning around it's long axis. You'd build the 'decks' of the craft as concentric cylinders inside the craft. The decks closest to the outside of the craft would have the strongest gravity - those closest to the central axis would have less gravity - and at the very center of the craft, you'd be in zero g. If there were windows on the curved outside surface of the cylinder, they would be in the floor of the strongest-gravity deck. If you imagine a ladder running "up" from one of the outer decks, up through the center of the ship and through to the opposite side, then someone starting out to climb the ladder up from the outermost deck would feel strong gravity at first, then slightly less and less still until reaching the center of the spaceship where there is no gravity. If you continued 'climbing' the ladder through the zero g region, you'd start to feel like you were hanging upside down and the direction that was "up" is now "down"...and as you climbed further "down" the ladder, you'd find the gravity getting stronger and stronger until you reached the outermost deck - 180 degrees around the other side of the ship from where you started.

If the spacecraft was small enough with no internal decks - then yes, as you say - you could jump upwards gently and land back where you started from - but a really big jump would take you sailing up into less and less gravity - floating gently across the middle of the ship - then (alarmingly) plummeting head-down towards the opposite side of the ship with increasing accelleration until you whacked your head on the floor on the opposite side.

However, everything I've said has missed one important thing. Coriolis forces. Since you are moving sideways at a fairly large speed on the outer surface of the ship, as you go upwards, you'd find yourself not going in a straight line because the space craft is spinning beneath you. One of the problems with producing a spin-stabilised craft "for real" is that these coriolis forces might make it's occupants have all sorts of puke-making feelings from being pushed sideways everytime they stood up too quickly - or that different gravity between their heads and their feet would cause severe disorientation. Hence spin-stabilised craft have to be big enough to where coriolis forces are sufficiently negligable to not cause problems.

SteveBaker (talk) 20:58, 24 November 2007 (UTC)

(edit conflict) The acceleration you feel due to gravity on Earth is a pretty constant 9.8m/s/s. In a spinning spaceship, acceleration looks like $a=r\omega ^{2}$, where r is the radius and ω is the angular velocity, which is constant throughout. So as you go farther from the axis, as you noted, gravity feels stronger. So you could conceivably jump and get stuck in the middle of the ship or fall towards the other side, but the effect would be gradual, not sudden. Now if you jump and fall back to the same side, you may see yourself drift a bit due to the z-terms of the Coriolis effect. There shouldn't be any force from the air as it will reach an equilibrium position inside the ship--the acceleration is always outwards and constant at each point. SamuelRiv (talk) 21:09, 24 November 2007 (UTC)

Consider also what an outside observer sees. You jump off the rim – that is, you push yourself inward by a kick against the wall, as in a swimming pool – and proceed in a straight line. Your vector is the lateral velocity of the rim at that moment plus whatever impulse your legs can impart; so to reach the axis you need to jump at an angle to negate the rotation. You "land" when you collide again with the rim; where you land depends in part on how far the rim has turned during your jump, and thus on the speed of your jump. —Tamfang (talk) 22:13, 24 November 2007 (UTC)

It's only the friction between your feet and the deck - plus any air resistance from the air inside the ship that keeps you going around in a circle as you stand "still" on the deck. In true high-school physics style, let's neglect the air resistance. Let's suppose you can jump high enough to sail across the ship and land on the other side. Let's suppose you try to jump straight upwards through the exact center of the ship. The moment you leave the floor, there are no more forces acting on you. (Remember, this isn't "real" gravity - this is just a spinning cylinder in zero g). With no forces acting on it, your body travels in a straight line - at a constant velocity. So your velocity vector is the sum of a vector that is acting tangential to the floor at the moment your feet left the ground (friction) with a inward radial vector due to the force your legs applied in the jump. Let's call the lateral vector 'F' (friction) and the vertical vector 'J' (jump). From your perspective, you would have been trying to jump "stright up" towards the center of the spacecraft - but find that you miss that point by some amount that depends on the size of the spacecraft and the amount of spin it has (this is the Coriolis effect). In ADDITION, the spacecaft is rotating so that the point you were aiming for on the opposite wall has moved by some amount by the time you get there.

Suppose your spacecraft is spinning clockwise from the perspective of our outside observer. If you aim your jump at a point exactly 180 degrees away from your start point then the coriolis effect (the addition of the small 'F' vector to your 'J' vector) forces you to land at a point a little anticlockwise of where you aimed - BUT while you were in the air, the rotation of the craft moved your aim point clockwise as you were in motion so you land even further anticlockwise of your aim point than you expect.

This means that the slower you jump, the bigger 'F' is compared to 'J' - the bigger the coriolis effect - and the further anticlockwise of your aim point you land.

But we just said that from the moment your feet leave the floor, there is no more gravity (there never was any gravity - it just felt like there was) - so how does this feel so natural? When you are standing still, the 'F' vector is pointing slightly outside the craft (it's a tangent to the curved deck), your inertia wants to put you outside the spaceship - but the deck is forcing you inwards all the time. That force feels just like gravity...well, almost.

What's going to be weird for the occupants is small vertical jumps. In a small vertical jump the coriolis effect and the rotation of the craft are more or less equal - so the point where you land is almost exactly where you jumped from - just like if you jump straight up here on earth.

The 'F' vector is tangential to the floor when you jump and the 'J' vector is tiny by comparison - so the combined vector points to a place on the deck only a little clockwise from you. More or less exactly where the deck plate you were standing on rotated to while you were in the air. However, if that distance is a significant fraction of the circumpherence of the craft - you'll be landing with your body at an angle to the deck. If the craft rotateds at 10 degrees per second and you spend a second in the air during your jump - then when you land, your body is leaning 10 degrees to the local "vertical" and you'll probably fall over when you hit the deck. So only small jumps feel natural - just like gravity providing the ship is huge and rotating slowly. If it's smaller and still trying to produce one g of similated gravity - then these peculiar coriolis-related matters will become very disturbing. Walking and bending down, picking things up and tossing them to your fellow astronauts will all feel ever so slightly 'off' from what you are used to on earth. If the space ship is too small, it's likely that nausia and other problems would be common.

SteveBaker (talk) 17:16, 25 November 2007 (UTC)

Short answer: In a spaceship rotating on an axis about its center of mass, the axis of rotation is "up" and the directions away from the axis are "down". The amount of "gravity" (actually centrifugal force) depends on how close you are to the axis of rotation and the revolutions per. minute of the spaceship. The further from the axis and/or the faster the rotation, the more "gravity" you feel. For more see Artificial gravity#Rotation. -- HiEv 13:16, 26 November 2007 (UTC)

That answer may be short - but it's not exactly true...well...no...not exactly.

(This is SUCH a fun thing to think about! I do hope people are still reading!)

Here is a thought experiment for you (again, neglecting air resistance). Suppose you started off with the spacecraft not spinning - so you are in zero g - you lift your feet off the floor - so you are floating six inches away from the curved deck. Now someone starts the spacecraft spinning (maybe using some thrusters mounted on opposite sides of the cylinder)...do you feel gravity and fall down? Nope! No gravity! You carry on floating there with the deck spinning just inches beneath your feet! So what happened to the gravity (which the previous respondant claim acts depending only on your distance from the central axis)??? The thing is that from the point of view of someone who is standing on the deck, whizzing around with it, everything appears to them as if the room is still, there is gravity - except that you are "orbiting" the deck at high speed! Now, if we add some air - which will gradually start to spin with the spacecraft due to friction/viscosity - it will gradually start to (from your perspective) speed you up to start to match the speed of the spacecraft's spin...or (from the perspective of your crewmate, standing on the deck) the air resistance will start to slow you down. This is in every respect like an inside-out planet...so (from his perspective) as you slow down, you fall out of orbit towards the deck. From your perspective, the wind applies a force that moves you off IN A STRAIGHT LINE - which means that you gradually get closer to the spinning deck until you hit it, get accellerated around into a circle and start to feel like there is gravity. Like an inside-out planet, gravity points outwards from the center instead of inwards. Objects can orbit "above" the surface if they have enough speed relative to the deck - so if you run around the deck at a speed equal to it's rotational speed and give yourself a little upwards 'push' you'll find yourself in one of those peculiar 'inside' orbits again! And (to extend the metaphor) if you can launch yourself upwards fast enough, you reach escape velocity (although the deck on the opposite side gets in the way of true escape). Coriolis forces also apply - just like on a normal planet - except they are reversed. Everything that happens on a real planet can happen here - but inside-out! SteveBaker (talk) 21:45, 26 November 2007 (UTC)

I was trying to be brief, and my assumption was that you were rotating with the spaceship. The original question did involve standing or climbing within the ship, not floating freely. Yes, there are some differences between this kind of "gravity" and the kind of gravity you'd feel on a planet, but in general what I said is reasonably accurate as far as how you'd determine "up" and "down". Some more information on the perception of gravity in a rotating space station can be found here. Also, a 900 meter radius station would need to revolve at 1 rpm, or 679 km/hr (398 mph), to maintain 1 G (source), so we probably won't have to worry about people jumping into "synchronous orbit" within the outer rim of such space stations. -- HiEv 22:24, 26 November 2007 (UTC)

The trouble with a 900m radius ship is the mechanical strength required. Building something toroidal or cylindrical that's that big that's got to hold together under 1g is like building a 5.6 kilometer single-span bridge! The longest single-span we've ever built on earth is just under 2km - can you imagine the engineering effort to build one of those in orbit?! It's also an awfully large ship! So, yeah- we're not really going to be jumping around and such...fun though it might seem. We might want to start with something a little more reasonable. A more likely thing is to have a spacecraft where the main power plant (or something like that) is on one end of a long cable and the crew quarters are like an elevator cab hanging on the other. The two masses rotate about a common center. The cable has to be strong enough to support the entire weight of the craft under 1g of accelleration - but we have cables with very large breaking strains - so that's do-able (technically, it only has to be strong enough to bear twice the weight of the crew compartment). Of course you don't necessarily need a full 1g to maintain crew health and comfort. It's argued that if they are going to Mars (the most likely destination for our first spin-gravity ship) then they might as well get acclimated to Mars gravity on the trip - so you can spin three times more slowly or use a shorter cable and get the desired effect. The nice thing about a system like this is that you can keep the coriolis forces under control by the relatively simple process of making the cable longer and spinning the ship more slowly. SteveBaker (talk) 23:57, 26 November 2007 (UTC)

It's better (for humans) than no gravity at all, but to maintain 1g you have to stay at a specific radius and not move with or against the motion of the spinning, though with a very big structure the variations can be reduced. A lot of the issues where thought through reasonably well on Babylon 5, including the idea of falling from the centre to the deck, where you could go from weightless with a descent at arbitrarily low speeds, to going to 1g and landing on a moving surface the equivalent of being ejected from a car moving at high speed. Peter Grey (talk) 08:16, 27 November 2007 (UTC)

Which spacecraft was it where the science part had filing cabinets all around arranged in a cylindrical fashion where the scientists/astronauts noticed they could do their jogging kind of 2001 space odyssey style just by running around the (not very large) cylinder? Keria (talk) 12:18, 27 November 2007 (UTC)

That would be SkyLab - they used a left-over second-stage Saturn V rocket and kitted it out as a temporary space station. The interior was ENORMOUS even by ISS standards. SteveBaker (talk) 17:10, 27 November 2007 (UTC)

It's not a your mum joke, promise.

What is the relationship between gravitational and inertial mass? Why are they the same? Do they have the same cause? Please answer ungriftingly! Samuel P. Lemminghornsworth —Preceding unsigned comment added by 217.43.117.20 (talk) 23:07, 24 November 2007 (UTC)

Until someone who understands this turns up, you could do worse than read Mass#Inertial and gravitational mass. Algebraist 01:53, 25 November 2007 (UTC)

Inertial mass is the mass related to F=ma, one of Newton's laws. Gravitational mass is the mass related to F_g=mg or F_g=Gm₁m₂/r². Their equivalence was well known to Newton, but not understood by him. Einstein solved the problem by showing (in General relativity) that acceleration and gravitation are essentially the same thing. Someguy1221 (talk) 02:48, 25 November 2007 (UTC)

Actually, he didn't "show" they are the same thing, but rather assumed it. General relativity doesn't provide any compelling explanation for why it must be true (other than that GR works). We certainly have examples of forces (e.g. electromagnetism) where the ability to create force is determined by something (charge) that is different from the resistance to acceleration (inertial mass). There is no fundemental reason why one couldn't construct a GR-like theory in which something other than intertial mass appeared in relavant spots of the stress-energy tensor that determined the shape of space time. Dragons flight (talk) 03:14, 25 November 2007 (UTC)

General relativity works from the premise that accelleration and gravitation are indistinguishable (that's the thing that started Einstein off on his quest to establish it) - one consequence of which is that inertial mass and gravitational mass MUST be the same. If they were not, general relativity wouldn't work because you'd be able to tell whether you were out in deep space with a rocket motor pushing you along with a 1g accelleration or sitting still on the launch pad here on earth. The problem here is to ask whether general relativity is true BECAUSE of some amazing coincidence between the two interpretations of mass - or whether the two meanings of mass are the same BECAUSE general relativity is true. Asking WHY a fundamental law is true is an unanswerable question. It's more philosophy than science! SteveBaker (talk) 10:10, 25 November 2007 (UTC)

That's only true to the extent you assume there isn't an answer. Or put another way, it means you assume there isn't anything more fundemental than GR. Equivalence is still basically an assumption of GR, and my physicists' intuition wants there to be some deeper physical principle to explain why there must be a force whose magnitude is proportional to inertial mass. Dragons flight (talk) 12:36, 26 November 2007 (UTC)

Oh - I agree, you might well be correct. But it's also possible that there is a deeper reason why GR is right that's unrelated to the original principle that lead Einstein toward it. If that were the case then the reason that inertial and gravitational mass is the same thing is BECAUSE accelleration and gravity are the same thing and we are making an artificial distinction because as mere humans who can't directly see the curvature of space/time it seems that accelleration and gravity are different things. We don't know which of those viewpoints is "correct" - what is a cause and what is an effect? SteveBaker (talk) 21:23, 26 November 2007 (UTC)

What is the best way to self-study string theory?

Are there free online text books, etc? -- Taku (talk) 23:20, 24 November 2007 (UTC)

It really would depend on your level of knowledge to begin with. Are you a serious student of theoretical physics? Or are you an interested amateur? If the latter, you can't go too wrong by reading the popularized books first. --24.147.86.187 (talk) 23:42, 24 November 2007 (UTC)

My background is mathematics, so I'm basically interested in how math is used in physics. (For example, I vaguely know the use of functional analysis in quantum physics.) Since popular books tend to have almost no math, I was wondering if there is a textbook or something comparable on string theory (because academic papers are way beyond my knowledge.) -- Taku (talk) 08:58, 25 November 2007 (UTC)

A First Course in String Theory might be at the level you're looking for. I've only leafed through it, but it's aimed at undergraduates and fairly heavy on mathematics. -- BenRG (talk) 15:59, 26 November 2007 (UTC)