I'm trying to determine the feasibility of some advantages and disadvantages of literal light infantry. This type of infantry is basically the same as the rest but weighs much less.

To truly determine the advantages and disadvantages of light infantry I want to eliminate what specific light materials and such would do for them, so I'm assuming that this infantry is exactly the same as a normal human but lighter. Its bones have the same strength, resilience, elasticity etc, it's fat has the same energy content, it's brain is just as capable, it's muscles just as strong etc. Nothing is different but one thing: Their weight. Currently, I'm exaggerating their weight difference in saying that the lightweight human is 10% of the weight of the "real" human. As an example, an 80 kilo human would have a lightweight version of 8 kilo's. An old-timey 40 kilo human would have a lightweight version of 4 kilo's etc. The individual weight differences do not matter for this check, just the general advantages and disadvantages of being significantly lighter.

Advantages as far as I can determine:

• higher jumps
• faster acceleration to their maximum speed
• slower terminal velocity
• can fall from higher before a fall will wound or kill
• better climbers
• can carry more weight before it becomes a problem -->techniqually not true. They can carry the same amount of weight, but the reduced amount of bodyweight means more weight of equipment can be carried.
• requires much less equipment to hover or fly.

Disadvantages as far as I can determine:

• slower falls --> when jumping or trying to run, it takes longer to reach the ground again, meaning you can run slower and take longer to recover from a jump.
• When in a car crash (or explosion) you are more likely to die --> your organs are easier to accelerate/decelerate causing them to receive more damage. This might be a moot point as there's also less force behind your own organs, IE they require less force to be stopped and thus might experience fewer problems from a crash.
• Similar to a car crash, being in a fight will mean getting hit is more lethal to you (might be just as moot a point).
• In hand-to-hand combat, you are much more ineffective as any blunt-force attempt has much less weight and therefore energy behind it (any other technique still works assuming you don't need your weight). Can be countered by wearing more heavy gear to compensate.
• high winds have much more effect on the light human
• much more problems handling recoil. --> the era does not matter for this question, just that any type of recoil (or wind for that matter!) will have more effect on a light human than a heavier one.

Clarification on the time period:

This question does not tell you what time period these lightweight humans are in as it is about the general differences between a light and normal human, and not the individual specific differences between say a sword-wielding lightweight vs normal human.

The comments aren't exactly clear why the time period matters. There's two interpretations that I could identify as to why it might be important. The first is that the type of weapon matters. But does it really matter? with melee combat in medieval times the lightweight user has certain disadvantages. With melee combat in current and future times the lightweight user still has the same disadvantages. It also doesn't matter if the lightweight user is using a bow, a musket, an SMG or a handheld tank canon. In all cases the lightweight user is at a disadvantage over a normal weight user (bows have a small amount of forwards recoil rather than backwards, but this is comparatively a similar amount of recoil per kilogram as that of some modern rifles have on normal weight humans!). The answer does not change based on the weapon or time the weapon is used.

The second way it could be interpreted is what type of conflict was the most common and how efficient the lightweight humans would be in such a conflict. While an interesting question in itself that I might ask later, the question I ask isn't about the efficiency of lightweight humans wielding a particular weapon set or engaging in a particular kind of conflict. It's just pure and simple about the general advantages and disadvantages a lightweight human would have over a heavyweight human. It is a reality check with some advantages and disadvantages I thought would apply, this can be answered with a yes/no question and some additional information as to why you think it is correct or not.

I hope that clears up some of the confusion about my question.

Lightweight humans (whose sole difference is weight, with strength and resilience unaffected) are basically a straight upgrade from normal humans. That improved strength ratio (muscle capacity vs body weight) means they can move faster and carry more stuff.

Your question identifies positives and negatives pretty well, but I would make a few corrections

• Fall speed is largely unaffected. Recall Galileo's experiments that objects fall at the same rate regardless of weight. Fall speed only changes significantly when the object is so light that aerodynamic forces [ie. air resistance] are similar to the gravitational force acting on an object. A lightweight human doesn't meet that criterion unless they are wearing a wingsuit or are EXTREMELY low density, like a balloon or aerogel.

• I think hits will be LESS lethal, as their bodies are easily accelerated away with an impact, rather than their bones/organs being forced to elastically resist an incoming blow while the body's momentum changes to match the blow. Imagine trying to pop an airborne balloon with a hammer.

Other thoughts:

• There will be a range of inconveniences for the lightweight humans interacting with objects designed for heavy humans. Heavy doors would have to be braced against to open them (hell, they might even lack the friction against the ground needed to open a big door), a backpack full of stuff would overbalance them (totally changed gait for carrying objects perhaps?), Heavy weapon martial art forms would be unusable as the centre of rotation for a human holding the weapon is vastly different.... Basically, I'm saying the lightweights will move and interact with the world in unusual ways, they are kind of like humans permanently living on the moon.

• We might expect the lightweight humans to end up weaker than normal humans, just because they don't experience the continual weight training of lugging a fat body around. Training may be necessary.

• I would expect lightweight humans to develop solo spear-based combat styles, taking advantage of their superior speed to evade and make lightning, piercing strikes to the opponent from range.

• Galileo's experiments were about the objects of the same material (or thought experiments about a free fall). Light infantry's bodies would have much smaller density. – Alexander Jan 30 '19 at 19:32
• Density matters to falling speed only insofar as it affects the ratio of gravity to air resistance. I think my phrasing is more precise and gets to the physics behind the problem. If it were just about density, we would expect a wooden brick to fall at the same rate as a sheet of paper. – Mark_Anderson Jan 30 '19 at 19:48
• I will clarify the answer a little further though to highlight that an extremely lightweight human would have an appreciably different fall rate – Mark_Anderson Jan 30 '19 at 19:51
• Newton's second law, F=ma, proves that the impact would be lighter. F being the force, m the mass expressed in kilos and a the gravity constant 9.82m/s^2 (on Earth). Aerodynamics and air resistance are negligible for the relatively short distances humans can fall without taking damage. – Amarth Jan 30 '19 at 20:11
• Lighter humanoids would have a lower terminal velocity, which might matter if they are being used as paratroopers, but I agree with @Amarth that this effect will be rather negligible for falls that are survivable unassisted. – Nuclear Wang Jan 30 '19 at 21:15

One huge advantage is much easier logistics. Lighter people will burn less energy while doing anything than heavier people will. So they will need less food and maybe less water. They'll also be easier for vehicles to transport, especially in the case of aircraft. A big consideration for transport aircraft is load balancing. Lighter people will make that less of a problem. And for fighter or strike aircraft, every kilogram of dead weight you can save is one more kilogram of ordnance or fuel you can carry, or one less kilogram slowing you down. This kind of thing holds true even for pre-modern societies. Lighter people are easier for horses to carry, and they also are easier to pull in a chariot.

Light humans would also probably float better (if they are the same size than a normal human, they are less "dense") and could be able to swim with more burden than a normal man.
I think a lighter human would work very well in amphibious operations (crossing rivers, landing on beaches...): being able to swim carrying a heavier equipment, they could easily establish well-equipped bridgeheads. Similarly, they would behave well also in difficult and soft terrains (mud, ice, high mountain in general, crossing rivers on improvised bridges), since they would stick less and the terrain would be less likely to collapse under their weight

• It depends on why the terrain is difficult. They may have MORE difficulty in slick terrain because there will be even less friction between them and the ground. – Ryan_L Jan 30 '19 at 21:34
• @Ryan_L: good point. In case of a terrain where they would have more difficulty to move, they could carry some extra burden (having anyway an advantage over heavier soldiers, who would need to walk with less equipment)... – McTroopers Jan 30 '19 at 21:41
• @Ryan_L: I'm not sure that slick terrain would be any different for them than for anyone else. Sure, they'd have less force available, but also less force required by exactly the same proportion, so the effects cancel. If they were trying to move something, though, that didn't have its full weight on them, then it'd be a problem. – AaronD Jan 30 '19 at 22:36
• @AaronD is spot-on here. Lighter individuals have a lower normal force from the ground and a smaller maximum friction force before slipping (F = umg), but they also require require less force to accelerate (F=ma). Mass cancels, so the only thing that affects movement is the coefficient of friction and not the individual's weight. Pushing an external object that didn't add to the normal force would be an issue though, as mentioned. – Nuclear Wang Feb 4 '19 at 19:04

Word play aside, Your light infantry is probably not going to be a really good infantry at all. They could be useful at so many other things.

What you are proposing is that the light humans are the roughly the same size and shape of normal humans, just without the mass. To get to your proposed mass, you need some seriously radical changes to the basic materials in human anatomy. Think about it, your skelton alone is 15% of your overall mass. That by itself is half again more than your proposed light people. A bit can be done with air sacs in the bones, like birds, but probably not enough. So magic magic or radical science would be necessary to get a similar strength from an alternative material. Next you have muscle and other tissue That's a lot of volume to fill if you want to cover a normal sized human skeleton. Last but not least you have blood. it's made of water (sort of) and masses in a similar way. 5.5 liters of water masses 5.5 kg. So you end up in a place where the only way possible to get a 10% of the mass human is to use Magic or Extreme science fiction. But this only gives you a base of things you have to account for.

Now what can you do with them. Regular infantry is right out unless you have them carrying some sort of energy weapon with no recoil. That puts you in the sci fi category. In the era of muscle powered weaponry they simply will not have the mass to be effective. To strike hard with muscle power, you need momentum. Momentum needs mass, and these guys just don't have any. One standard human with a mace would wade right through a bunch of these guys in a world with normal physics. In a modern military, you need a variety of projectile weapons. These weapons have a lot of mass. When it comes to recoil, you do not get a pass on the laws of physics.

ONe thing you might consider for these ultra lightweight humans might be light Calvary. A role that means you have to move a lot of people and stuff really fast. Ranged weapons on a moving platform, whether that be horse or helicopter. Less mass means you can either move faster or carry more stuff. A light horseman can go farther, faster than the same horse with a standard human. Same goes for a helicopter or other conveyance. an 8kg pilot means you can carry 72 more kg of ammo.

Also, you could even look at all sorts of flight or glider possibilities for recon and intelligence gathering.

• I'll worry about their uses. Wall climbing, tree climbing/jumping, crossing gaps and rivers, airdrops (heavy equipment dropped seperately, allows them to function as normal weight humans but drop with less equipment) etc are things I'm working on. The reason I didnt name any specific materials is to get an idea of what advantages simply being light has, as opposed to naming different lightweight materials and having their disadantages&advantages affect the outcome. The 10% weight was chosen to exaggerate the differences instead of discussing a 90 kilo human vs a 70 kilo human. – Demigan Jan 31 '19 at 7:37
• Well with that in mind if you look to the real world, lighter generally means faster. It is easier to move and change direction with less mass and the same engine (whether that be a race car or muscle power). The downside is that it takes less motive energy to change a light soldiers direction, so recoil becomes an issue the lighter you go. Agility advantages go up at a similar rate to momentum related problems. As to armoring, keep in mind that armor has to be matched to the weapons placed against it. Armor for a light soldier is going to have the same mass as for the standard soldier. – Paul TIKI Jan 31 '19 at 16:41