you presented your guesses as if they were well known facts and didn't need explaining nor warrant discussing
Because I was lazy and you guys weren't really reading any posts anyway, there wasn't enough of a bonus compared to time taken in explaining all that, and long posts go by unread. But since you asked, it would be worth int now
why do you say that nanotech will make AP useless? What exactly do you assume about this tech?
Nanotech would allow us to make extra strong materials, which would undoubtedly be used for armor. The concept of Armor Piercing is to get past armor. It is extremely difficult to go through something with a tougher material - you can try firing off a piece of chalk into plywood for an example.
You'd need an AP-bullet with a nanotech-strengthened tip to get past that. Cost-wise, that would be like putting a microchip on every bullet. You can do it, but it's going to be expensive. It will be done in a specialized way, but with infantry being much smaller than tanks, AT guns and RPGs will be much less effective.
Also going with how armors work, there's shock absorption and there's damper. Plate mail was developed by people with no modern scientific knowledge and it was of taking several shots from primitive bullets because of the way it deflected them. It was outdated mainly because of speed and because guns were cheaper (though cavalry stuck around for a while). A plate mail with modern shock dampening would be able to handle much more impact.
Why would a nanotech-based armour be weak at the back?
Because all armor is weak at the back. I think the thought is that you can't heavily armor everywhere, so best to put it where it'd get hit the most. And the way that shock will be distributed around would mean that it's more effective to focus the shock dampening mechanics at the front not at the back.
I reaally don't want to explain shock absorption in armor and can't seem to find an article on it, but the simplest way of saying it is that it's going to combine typical dampening and distribute it by deflection. I'd say that futuristic ones would work a bit like water - you hit something and that energy is displaced somewhere else in the form of ripples. That way, it's focused throughout the armor, say with 4000N of force, the part directly hit would take 2000N of force and distribute it to the rest of the armor.
This would result in some more flexible part being able to "ripple" more, but the drawback is that the part will be softer. Better to have that at the back. This will still be armored, of course, but only about half as much.
Why wouldn't explosion work against it? Why would a cannon work, if neither an explosion nor AP round does(what is it shooting then)
AP can work as well as cannons, but due to the shock dampening system, attacking over a large area would cause more damage than a more focused one.
What makes power armor more awesome than plate mail, Muz?
Speed and strength. Power armor could probably be designed to run pretty damn fast, faster than an unarmored human, but it certainly won't be as slow as plate mail. It beats tanks because it allows the user extra mobility.
Rods-from-god makes far less sense, because of things like air movement (it takes a supercomputer to calculate the weather as it is, and it's not even that accurate), air resistance, and most of all - terminal velocity. It doesn't become much more powerful than a standard missile
Actually, RFG accuracy isn't much of a problem at all; most missiles these days are equipped with things that help fine tune accuracy. But a giant block from the sky is going to have to deal with the rotation of the earth.. any initial velocity will be slowed down significantly by wind and air pressure, making it off track. Calculating the air pressure is not easy.
My experience in weapons also tells me that ethics will get in the way. Nobody ever complained about the invention of swords, chain mail, RPGs, or assault rifles. But there's always a huge controversy when someone invents a long range weapon that wins against better ones. Like crossbows, gunpowder, precision munitions, and nukes. You won't have much resistance to the development of power armor, but developing a WMD like RFG will have similar resistance as nuke/gunpowder development. Heh, arguably ethics never stopped anything, but it's the reason the world still doesn't have a defense system against nukes.