Les armes à feu spéculatives 2

That number is, of course, read as "deux". Guns post. Not all of them are directly sci-fi related, actually, but they all concern things that inform SF gun-thinking.

• Remember my musing about whether the USMC cadre of the Peacekeepers, in my SF, would use a bullpup rifle or not? (Either way it was still going to be a Stoner-like design.) Well, apparently, the trigger-pull issue? Doesn't exist with electronic firing. I should've realized it wouldn't.
  
  So now I guess the Marine gun looks a bit like the Bushmaster M17S, albeit probably not so 1980s. (It's still a Stoner rifle, a modified form of the AR-16 that was the predecessor of the AR-18, and therefore also of the Howa 89式 the JSDF use. It probably would, like the AR-18, still be short-stroke rather than direct-impingement, though; direct-impingement adds heat, which you don't want in a caseless design. The electronic firing, removing the issues of trigger-pull, probably makes up for any loss of accuracy.)
• Apparently, one of the interesting things about the "AKs are so reliable, ARs are crap" folklore, is, when an AK gets something in its gas-system (which can include rust, with old Russian-made or less-old Arab-made ammo leaving corrosive residue), it ceases to shoot. (Supposedly the way to clear it is to kick the bolt carrier. Like, literally. Apparently you often needed a hammer, though.) Know what an AR does, when something goes wrong with its gas-system? It becomes bolt-action.
  
  Apparently, AKs also jam if they start to overheat—and in even moderately sustained fire, they do overheat, not least because the forward grip consists of wood wrapped around the barrel (Soviet training preferred to avoid sustained fire; the nations they handed AKs out to like Halloween candy, though...). Wood is an insulator; apparently it will even start to smolder before too long, leaving you with a lovely birch-smoked hand (yes, I looked up what wood is most common for AKs) and a remarkably expensive metal-and-wood club.
• It occurs to me, the khângây might use small coil vulcans (or I guess rotary coilguns?) as their equivalent of an assault rifle or LMG. Presumably semi-auto fire, given their technology, has sufficient time between shots for barrel-wear not to be an issue. And I figure three barrels, like the M197 or GAU-19, might make assault-rifle rates of fire feasible (and I think the rotation would be recoil-operated, so the battery doesn't have to run the rotor as well as the firing).
  
  I figure they'll have a casing over it, like what sticks out of the nose of an A-10 Thunderbolt. Does a small-arms rotary firearm sound far fetched? It is overplayed, generally without reference to the real requirements of a machinegun of that size (though then again people ignore the realities of fixed-barrel machineguns, too). But, before you go saying it'd never be done, you may want to talk to E. C. Neal.
• I realize that Attack on Titan (which is actually Titan(s) of the Charge, given what "shingeki" means) is at least partly horror, so its plot necessarily runs partly on the Idiot Ball, but nevertheless, there are much more effective means at their disposal for fighting the things than leaping around on cables and chopping them with giant box-cutters. These people have cannons, so why aren't they using chain-shot? That'd quite easily cut a Titan in half, and if you point it so the chain passes through the weak-spot in the neck, it'll kill it in one blow, without anyone being anywhere near its hands.
  
  Of course, along with the Idiot Plot nature of AoT's horror aspect, is the "I don't care if it makes no sense, it's cool" nature of the action aspect. There are more efficient means of taking on Titans than the "3D maneuver gear"/giant box-cutter combo. Realistically, all but the biggest Titans would be little match even for men on horseback; you just have a few horsemen, working as coordinated pairs or small teams, ensnare the Titans' ankles, and then drag them over a giant version of "severe tire-damage" traffic-spikes, cutting through their weak-spot and annihilating them. And, since the Titans are the whole reason they live the way they do, they would've built giant traffic-spikes, and other anti-Titan defenses, all around the Walls, and into (at least) parts of the city streets.
  
  In real life, no one method of fighting—and Titans are basically armor without guns—is an automatic win, armor needs infantry support and infantry needs artillery; air alone is close, but you still need a diverse air force (something the "let's give the A-10's job to the F-35" people apparently don't understand). Titans would actually be only a minor inconvenience, at least for anyone who has guns.
• I think I've mentioned that zled space-forces are a branch of their artillery? They also have more ordinary artillery; as I said, they don't have our distinction between tanks and other guns, because their "tanks" are just up-armored self-propelled batteries that also have close-range attack capabilities. (I think zledo call them simply "armored guns"; we called a light-tank design project the "Armored Gun System", rather than, y'know, "light tank design competition", and unlike us zledo never used "tank", "cistern", or "reservoir" as code-names for "caterpillar machine-gun destroyers"—and they don't have "caterpillars", either.)
  
  Their artillery presumably does distinguish degrees of armor, among its guns, but also things like direct versus indirect fire, ballistic versus propelled (as in once the projectile leave the gun), and guided versus unguided munitions. I also think there might be a distinction between "fast" guns, on treads or spherical wheels (I think the spherical wheels give approximately the same performance as treads, but with an increase of speed and maneuverability), and "slow" or "all-terrain" guns, which are basically "spider tanks", with four legs. Hey, I said my SF had bipedal mecha, I never said all the mecha were bipedal.
• I really ought to go back to the Peacekeeper rifles' round being comparable to the .30-06, although it's still in a 7 mm package (rather than 7.62). Assuming each round weighs 10 milligrams (quite doable in that caliber if .270 Winchester and .270 Weatherby Magnum are any indication), and a muzzle velocity of 850 m/s, we get a muzzle energy of 3612 Joules. That requires a propellant load of 3.564 grams of nitrocellulose. 3.564 grams nitrocellulose is the equivalent of 1.497 grams of octanitrocubane, which has a volume of 726.699 cubic millimeters. If the propellant has the same diameter as 6.8 Remington SPC's brass casing (10.7 mm), and if we treat the 7 millimeter by 31 millimeter bullet as a cylinder, then we get a length to the propellant cylinder of 21.35 millimeters. It sticks out 1.85 millimeters in back (uniform thickness all around) and goes 19.85 millimeters up the side of the bullet.
  
  Given this, I guess we need to rename the thing "7 × 21 mm", rather than "7 × 18 mm". If each empty 60-round casket mag weighs 192 grams, and each round now weighs 11.5 grams (give or take 3 milligrams), then each fully-loaded magazine now weighs 882 grams. Compare that to the weight of seven STANAG 30-round magazines—each (taking into account that the M855 lead-free cartridge is slightly heavier than the old M193) weighs 486.3 grams—we find that, for only 3.6% more weight (3528 g vs 3404 g), each Peacekeeper can carry 14.3% more ammo—240 rounds as opposed to 210. And he only has to reload four times, instead of seven.
• Given the caseless rounds are only 32.85 millimeters long, it might be feasible to have the magazines load in the top—like those on the P90. FN 5.7 × 28 is 40.5 millimeters long, and we know you can use its magazines on an assault rifle (leaving to one side those who consider the P90 an assault rifle rather than a PDW or SMG). People do—there's a modified upper receiver for AR-15s that re-chambers them for FN 5.7 and lets them use P90 magazines. It's called an AR-57, or AR Five-seven (or presumably "AR Five-seveN", to those who didn't find that gimmick annoying in the pistol).
  
  One advantage, I think, to having the magazine where the P90 puts it, is you can see what your ammo-level is like (assuming the clear magazines so popular with the P90), without having to change your position. Most other magazine designs, bullpup or otherwise, force you to take your weapon off your shoulder to see where the mag-level is at, even if the magazine has clear sides. (Sure, you should keep track of how much you've fired, but what if you're handed a new weapon, or have to take one off a corpse?) Probably in real life, definitely in games, there are ways to mechanically or electronically indicate how many rounds remain—that counter on the back of the assault rifle in Halo?—but nothing beats being able to look.
• On the other hand, the sights on the P90 are a bit inelegant and unwieldy, 'cause of having to make room for the magazine. Might keep the magazine where it is ("normal bullpup", in other words) after all; I do have an awful lot of references to "casket magazines" that would need to be rewritten. Maybe use the P90 approach for the khângây small-arms coil-vulcan, though? That or helical magazines. Maybe both, under different circumstances—the helical ones would presumably be preferable for sustained fire/LMG applications, since they'd have less risk of jamming since they wouldn't have to rotate the bullet before chambering it. Ooh, yeah, I like that. And have the magazine under the barrel, instead of on top (which removes the "weird sights" issue). That probably means the bottom barrel is the one that fires, rather than the top like on our Gatling guns.

We Call It Voight-Kampff for Short

Post about artificial intelligence and robots. You ought to know what the title's a reference to.

• Why is it that every 18th- and 19th-century novelty toy, playing music or writing letters or doing various other tasks, some of them "programmable" because there are multiple cartridges of mechanical "instructions" for them to follow, gets called "early computer"? It's a sewing machine or a player piano—are those "early computers"?
  
  We've been doing mechanical embroidery, with "programmed" patterns, since at least the 1870s; fundamentally none of these "early computers" is any different from the industrial loom—except the industrial loom drastically lowers the price of decent garments, the novelty toy does nothing and improves the life of nobody (well, no more than any other interesting novelty item improves people's lives—a form of "happiness" that a fiction-writer probably ought not to despise).
  
  You might call them "early robots" (since "robots" refers primarily to the industrial usage), but that's not what people do. Words mean things. Please don't use words that are not applicable, just because they are similar to other words that are.
• I feel like setting out some rules of engagement, for people who wish to defend the millennarian hopes of the Transhumanist faithful, when other people make remarks about the realities of neurology and the limits of machine logic, and their implications for Kurzweil's prophesied techno-Rapture. One, don't take your username from books (or God forbid, movies of books) by William Gibson; he knew about as much about computers as H. G. Wells knew about trans-lunar injection. Two, don't demonstrate that you don't actually know what "most complex" means—Microsoft Windows is, objectively, the most complex computer program the human race ever created, and your opinion of how good an OS it is is completely irrelevant. (That other OSes are "elegantly simple" by comparison is actually one of their selling points—the significance of "most complex" in this context is that Microsoft Windows, with all its many, many crashes, is still 3.2 million times smaller than the representation of a human brain as one-line-of-code-per-cell or synapse...and realistically we would probably need multiple lines of code to represent certain cells and synapses.)
  
  Three, don't demonstrate that you are too stupid to grasp what hardware emulation is, by asserting that the number of braincells and synapses is irrelevant—and three-point-five, do not act like actually researching relevant facts like those numbers, on something like this, makes someone a target for ridicule. (The reverse is the case: that you don't know the relevant facts, and have not even bothered trying to explain how your enterprise will cope with them, shows how intellectually bankrupt you are.) And finally, four, do not tell the other party they must've selected "Gödel Incompleteness" at random, as a basis for their case. If you don't even know that Gödel only came up with the idea while exploring the limits of machine logic—because Hilbert was trying to make a machine that could handle all of logic—then you are simply announcing that not only do you have no right to your opinion, you don't even know what would or would not constitute a right to an opinion, on this matter. I'm not saying you can't challenge the argument from neurology (you pretty much can't challenge the argument from the limits of machine logic, or at least nobody has yet—every attempt to refute Lucas-Penrose that I know of has mostly been a demonstration of illiteracy); but the fact of the matter is that you're not trying to challenge those arguments.
  
  (Yes, I am thinking of a particular person. But all of these errors are common occurrences, though the unnamed idiot in question was the first I'd ever seen with all of them.)
• It occurs to me that "civil registries" are a poor thing to base a robot's ethics programming on—over and above the silliness of the Three Laws. Realistically an AI would be able to recognize humans, among other things, with a dedicated "object-class detection" (AKA "object recognition") program, presumably one of the suite of "weak" AIs a strong AI (assuming you can get one) would be made up of.
  
  Likewise, one wouldn't define "harm" according to the ICD definition of "injury"; there are apparently ways now to make a robot mechanically detect when any motion would exceed "the human pain tolerance limit", so presumably a society that can make a natural-language interface you can meaningfully talk with, can give its AIs sufficient situational-analysis to know when an action would exceed one of several limits of human tolerance. "Safeguarding-space violation" or "safety-space violation" would apparently be the term for "harm" in a robotics context.
  
  Incidentally, robots that don't have Asimov programming (which, remember, AIs dislike) would still be programmed to behave in accordance with ISO 13482. Because we now have an industrial standard for those!
• It occurs to me that what you'd really want AI for, is to have one person, on-call 24-7, who can answer any concern anyone might bring to it, rather than getting "well when Bob was on duty he said..." situations. You'd still probably want to have a normal person in overall command, since you don't necessarily want something that can be hacked to have any legal authority, but there is, as you can see, a real market for them (unfortunately for hippies, that market is mostly the military).
  
  Incidentally, you could probably give the same job to a person who's had their need for sleep done away with. But whether that's actually possible, or advisable long-term, is an open question; sleep serves some necessary purpose, since organisms that only have half their brain asleep at a time still do sleep, which they wouldn't if they could've done without it. We don't actually know what sleep is for, or what doing away with it would cause.
  
  There's also the question of whether it's remotely ethical to ask anyone to have their brain screwed with like that, a question that doesn't come up with an AI (though there are questions about whether you ought to create a person just to do a specific task), but apparently most people don't think there's anything wrong with restricting key posts to eunuchs? (Many eunuchs, historically, were volunteers, so "consent makes everything okay" is not valid—not that it ever is.)
• As is my custom when writing one of these posts, I read manga about robots in my other browser tabs. There's a neat little one called "Ninomae Shii no Tsukaikata" or "How to Use Ninomae Shii", that sadly only lasted 30 chapters (the reader-questionnaires are a harsh mistress). It's about a robot made by a middle-school Nobel Prize winner, searching for his purpose.
  
  But...the purpose of a strong-AI, aside from any jobs it might happen to perform, would be the same as the purpose of any other self-aware entity. Self-aware entities have, as their purpose, the fact they exist, and the contemplation and appreciation of that fact. (The Baltimore Catechism phrases it more succinctly, in the famous answer to question 126.)
• There is apparently some idea abroad in the land that "android" means bio-engineered, while a robot shaped like a man is called a "humanoid". Only, Common Usage, mammajamma: "android" means "robot shaped like a man" ("gynoid" is sometimes used for "robot shaped like a woman", but usually that's just called "female android"), while the bio-engineered things are called "bioroids". "Humanoid", meanwhile, means anything shaped like a human, living or not (an "android" is a humanoid robot). (Sometimes, in settings where such a distinction makes sense—all of them space-opera—"humanoid" is restricted to Rubber Forehead Aliens, and the other, more vaguely man-shaped, guys are called "bipeds" or something.)
• I was wondering how to write androids getting freaked out (remember, I have strong-AIs due to a highly unorthodox software workaround). At first I thought they wouldn't get chills, because while some of them do have body hair (the one whose job is infiltrations does), they didn't evolve from animals that sometimes survived by puffing up their fur to look bigger.
  
  It occurred to me that they might involuntarily switch to a different power-generation mode, one more active than just homeostasis, as their "subconscious" (the multiple weak-AI programs that govern their bodies, semi-independently of the strong-AI that is their consciousness) gears up for fight-or-flight. But then someone, talking about Transformers, said "chillingly" still makes sense in the context of the Cybertronian "biosphere", because their cooling-systems shift into high-gear to dump the excess heat caused by exertion.
  
  So...androids get chills. Briefly; unlike humans, it's much simpler for them to control involuntary responses like that. Incidentally, the strong-AI programs themselves are, in part, made of a gestalt of multiple weak-AIs, just like the "unconscious" programs that govern the bodies they're in—a couple of weak-AIs handle language, a couple more handle object-recognition, and so on. There's even programs for making decisions and for "discursive thought", but none of them is really the AI program, anymore than "you" is your language-capability or decision-making or even discursive thought.
• Likewise, my strong-AI androids can dream (make your own "electric sheep" joke). Why? Well, they periodically enter a de-frag mode, in which they can't otherwise be conscious, but, since their AI-consciousness doesn't simply cease to exist, they have experiences made up of random portions of their memories—which is basically what dreams are. They experience fragmentation because most fragmentation-preempting techniques can cause performance problems. Not a big deal for a PC; kinda one, for a person's mind.
  
  The non-android AIs don't have that problem (see above: they're awake 24/7). They have enough processing capability (since they don't have the space-constraints an android does) that they can either preempt fragmentation, or else defrag "in the back of their mind". That might be kinda like those animals that only sleep with half their brain at a time. I think it adds a touch of realism, that cramming an AI into what processing-capacity fits inside a head comes with some loss of capability.

Distortion Levels So Low as to Make a Brave Man Weep

It occurred to me, you can actually transmit optical hi-fi analog. How? Easy—if you're, say, scanning a near-UV nano-scale optical storage medium, you just have what the near-UV laser gets as input, be output again, say as a 532-nanometer laser (since that wavelength experiences little degradation, even across interstellar distances). That, you can then watch on something that receives those signals, or record back to other media. It's still very unwieldy compared to digital streaming, but it's also superior to digital. Interestingly, it works a lot like VHS did, back in the day, but HiFi.

Lives, Fortunes, Sacred Honor

Oh like you can come up with a better title for a post about alien biology, speculative material culture, and military science fiction.

517 is 11×47, the sum of five consecutive primes (97+101+103+107+109), and a Smith number, which is where the sum of its digits (in decimal) is equal to the sum of its prime factorization's digits (5+1+7=13, 1+1+4+7=13).

• Reading up on birds is almost certainly an absolute requirement for anyone that wants to do xenobiology and alien psychology. Birds are every bit as advanced as mammals—actually ravens may be smarter than any mammal that isn't actually sapient, given they've been shown to hold grudges over attacks at which the individual raven was not present, i.e. they tell each other about threats. Yet their brains and those of mammals diverged a third of a billion years ago. And it shows.
  
  Birds, for example, do not have a corpus callosum. The two hemispheres of their brains, however, are both active during song—all the impulses shoot back into the arcopallium (around the middle of the brain), and even all the way back to the brainstem and thalamus, because those are the only thing linking the two. An experiment with pigeons revealed that the fact birds in the egg are always curled up the same way—and thus always have only one eye exposed to light—is instrumental in teaching their brain-hemispheres to coordinate; pigeons incubated in the dark (i.e., where there was no difference of light between the embryo's eyes) couldn't coordinate data between their eyes (each bird eye only sends information to one brain hemisphere, unlike mammals). (Birds only "learn" images directly with the brain-hemisphere connected to the eye that actually sees them; screw up the architecture of inter-hemisphere coordination, and merely covering one of their eyes means they lose access to all the images that eye has learned.)
  
  Apparently, by the way, all vertebrates use the left side of the brain to process routine behavior, like feeding (including, probably, hunting), while the right side is used to process novel things and emergencies (which includes social interactions and mating). That's the real origin of the "left-brain, right-brain" folklore, so popular with seminars. Birds, since their eyes only map to one hemisphere each, will actually look at things with different eyes, depending on which category the thing goes into.
• You know how I keep saying "society" is best modeled as "a method of resolving territorial disputes by agreeing to treat all conspecifics as kin in the absence of obvious hostile intent"? One interesting aspect of it is, if they are kin, specifically siblings—and as I've said, "sibling" is the only relationship that exists when "mating" and "parent/child" are off the table—then some of the acts of "violence" in society are not violations of the agreement to treat everyone as kin. Why? Dominance scuffles.
  
  Some "fights" are better modeled as attempts to modify or reassert "pecking order" within a peer group (which, again, functions as a group of siblings in ethological terms). Now, of course, dominance scuffling can get quite violent when the two are not actually related (remember, that's why we used to think wolves were so violent—in captivity their packs were made up of unrelated animals), but it is still a fundamentally different activity from making, or fending off, territorial incursions. You know how you used to be able to fight out your differences under certain circumstances, without the cops getting involved? There were some sound ethological reasons for that mindset.
• That people fundamentally do not understand that a lot of the use of force is dominance scuffles—or that war is "politics by other means"—is why you get the idea, in science fiction, that war with aliens would necessarily involve one of us annihilating the other. They think (because they are either fat happy peace-drunk fools, or craven physical cowards—possibly both), that the goal of fighting is to kill the other person.
  
  The goal of fighting is to remove the threat posed; or actually to impose your will on the other person, even if your will is only "not to be annexed/robbed/murdered/raped/whatever". (Defensive fighting is still fighting: you are both imposing your will by force. That isn't always wrong, all your tribal prohibitions notwithstanding. I'm sorry ethics is more complex than a Kazimir Malevich paint-by-number.)
• Alien senses make a big difference to their scientific and technological development, as I've hit upon. The khângây, for instance, can distinguish individual sounds from a group of several, meaning among other things that their informal conversations don't involve stopping to let the other person talk, because you can both follow each other just fine (in formal situations, of course, there are etiquettes about turn-taking). It also, however, means they simply can't throw white noise onto a digital recording to smooth the jaggies (when I get tired, I can't listen to digital music—I notice the white noise, and it's actually quite unpleasant). I think they therefore use, instead, nano-scale optical analog Hi-Fi. That means digital media is largely moot to them, and their potlatch intellectual-property views; a big part of why file-sharing works for us is MP3s usually sound good enough.
  
  Zledo don't have quite that problem—at high enough resolution, they can actually use white noise to make digital audio work. But in earlier times, before digital media, they didn't actually start publishing music recordings till they had Hi-Fi, because with their hearing (which has a wider range than khângây, both in terms of frequency and faintness, but can't pick out individual sounds as well), they find the "noise" of Lo-Fi audio recordings too distracting. Now, I don't know how much that would've slowed the development of recording technology; after all, the big reason we kept doing Lo-Fi for so long was because that was "good enough" for many people (possibly just the sheer novelty of recorded music carried it for a while—remember how awesome we all thought the first iMac's version of Quicktime Musical Instruments was?). Presumably once they started converting sound waves to electric ones they then started working on making the electric waves produce sounds they actually liked hearing.
• I think I have a good enough justification for parasite space-craft being used in battles ("space fighters", though the thing they launch from is a mother-ship, not a carrier). You distribute the launch points around your mother-ship, so it can lob missiles, bullets, and beams from many directions at once, and you want them controlled internally, because—ask anyone who's played Team Fortress (or heard their brother screaming at the computer while playing Team Fortress)—lag kills. In space, anyone close enough to a remote weapon system to operate it without lag, already is aboard it for all practical purposes.
  
  As for "why not put an AI on the ship", well, remember how I compared the cost of developing AI to the Space Shuttle program? Well, extending the parallel, even after the initial development (which, if it cost as much as developing the Space Shuttle, was the equivalent of $38,277,033,135.80), an individual AI costs the equivalent of $450 million (the entire 2012 defense budget of Armenia). Not to put too fine a point on it, but three highly trained pilots are a hell of a lot cheaper than that.
  
  The human ones launch by electromagnetic catapult; their engines are for maneuvering and slowing down, as much as possible, after the battle. Then, I think, the mother-ship or a dedicated retrieval craft either picks it up or tows it back to the mother-ship, respectively. Presumably there's a beacon on the craft, to aid in pickup once the battle's over. The zled ones have metric-patching engines, whose operation-range stands to rockets as nuclear submarines stand to diesel ones; they can return to their mother-ship under their own power (they can also land, using plasma sails, since the difference between them and the entry-vehicles is armor, weapons, and crew-space, not atmospheric capability).
• Remember how I said every attempt to portray a species with more than two sexes is always actually male, female, hermaphodite, and neuter—"a, b, both, neither" but never "c"—with varying degrees of fluidity between them? I recently came across another option, where the multiple "sexes" have different roles to play in the life of the offspring, things I would describe as "passive defense" and "active defense" and various other things.
  
  But...no species that reproduced like that would survive; the complexity of mate-selection increases exponentially with every extra member, since every prospective partner must be compatible with all the others. Besides, no such species ever answers the "every intelligent alien represents a biosphere as complex as Earth's" issues—how do four-sexed cockroaches or lizards or birds behave?
  
  Also, plenty of terrestrial organisms do everything described by that system, and without deliberately hamstringing the efficiency of their reproduction: it's called "caste". Ant workers or termite guards aren't another sex, they're just non-reproductive (usually) members of the same sex(es) as their reproductive caste.
• The reason animals have leg-anatomy like chickens and wolves (i.e. "digitigrade stance"—their knees don't bend backwards, that's an ankle, and they walk on their toes) is that the strength of a muscle is proportional to the cross-sectional area divided by the length. Walking digitigrade (or with lengthened ankle bones forming a third leg-joint, like frogs—and zledo) lets an animal have longer legs, and thus longer strides, without sacrificing strength—because it increases the total length of the leg without increasing the length of its individual muscles. All of that is well and good, and quite likely to show up in alien anatomy. But...why does it show up in mecha? The power of a hydraulic or pneumatic cylinder is primarily a function of the pressure of the cylinder's working fluid; while a very long cylinder obviously needs more working fluid to pressurize, it's nothing like what's experienced with muscles. Think of the kinds of loads hydraulically activated cranes regularly move, on sections far longer than the typical mecha's limb-joints. No, I think the chicken walker is a purely aesthetic thing.
  
  Realistically, of course, the optimal layout for a walking tank is probably something like the Scarab from Halo, or some other "spider tank". You want to be able to walk (so you can go on that 40% of land-surface wheeled and tracked vehicles can't traverse), but you also want a stable platform for artillery. On the other hand, though, part of the appeal of any kind of walking machinery is psychological—bestriding the battlefield as a colossus—and a spider tank just doesn't give that (but a humanoid tank can go most places a spider can). Also, despite what Gundam would have us believe, two places where there is absolutely no reason to use mecha are space and underwater. You really wouldn't even have them be able to fly. You don't need legs to go everywhere you need to in the air or in space or underwater, planes/helicopters, submarines, and rockets are adequate to our needs in those environments. Just like how you only need legs for locomotion on the ground, your machines would only need them for that, too (although you might build a mecha that runs for a takeoff, as a heavy attack-plane, I suppose—the psychological factor is probably useful for close air support).
  
  Also? Dear DeviantArt: when I search "realistic mecha", by what possible rationale do I get even one picture of Cheetor, from Beast Wars? Inquiring minds want to know.
• In case you wondered, it's quite doubtful that an alien species would not have brain-hemisphere "decussation" (crossing over—the left brain hemisphere controls/responds to the right side of the body, and vice-versa). Decussation is apparently topologically superior, in terms of wiring; it reduces the likelihood of connection errors, which become a significant problem as the number of connections increases.
  
  Of course, nothing says your aliens have to map their hemispheres the way we do. Flip 'em, if you like—have 'em process routine behavior with the right and emergencies with the left—or divide the functions each hemisphere processes on a different basis. Maybe their optic nerves don't cross (despite vision's importance, the eyes are connected to the brain by relatively few nerves, so there may not be a risk of connection error), or maybe their whole bodies are wired like mammal optic nerves, and some impulses go to the opposite side.