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In one possible future, WWIII progressed to the point where it was being fought off planet. It was something like a Warhammer 40k scenario, with the war going on for thousands of years. There was massive technological improvement, but both sides kept getting stalemated and it went nowhere. It continued like that with no end in sight.
A decision was made to try and retroactively prevent the war, or to just escape from it using Time Travel and keep looping back when they needed to if they couldn't stop it. They would try to live covertly as refugees, or build a new life in areas of time that were "unoccupied" by Humanity.
Some of the soldiers in the military back then (or in the future depending on your prespective) were heavily modified. Those whom weren't missing limbs from injuries underwent voluntary amputation of limbs and/or organs to replace them with weaponry and other enhancements.
These modifications were typically reserved for the more elite and experienced. This also led to slurs, with the unnmodified calling them "mechs" and "tinmen", even though the modified soldiers were the ones saving their hides most of the time.
Thousands of years of warfare had created innovations in technology that's lightyears ahead of anything in 2018. They figured out how to tap into unlimited free energy and how to funnel the energetic equivalent of a black hole through an extremely small footprint. Their nanotechnology could also completely rearrange matter down to the subatomic level.
A lot of them didn't lug around rifles back then. It creates an imbalance, and it's more efficient if their prosthetic arms could rearrange themselves into multi tools and weapons. There's less stuff to carry with them, and they can run, jump, and climb unimpeded. It also had the advantage that they couldn't really be disarmed in the unlikely event they were captured.
Prosthetic limbs can interface with the mind, or you can use voice commands, or even a touch panel. Whatever is most suitable for the operator at the time. You can reform it into specific saved presets, or you can mold it into something new. You could turn a finger into a key to open any lock, or change the thumb and forefinger into a pair of pliers if you needed to. You can even form it into devices with moving parts, such as a buzzsaw, and the nanotech can rearrange the edge into something harder than diamond.
It contains its own advanced A.I. that can provide navigation, guidance, and advise on situations, as well as a full adaptive computer hacking suite. There's also a user accessible memory storage for taking files and other information with you, along with full spectrum communication capability, which can adapt to all the things from the past such as Wi-Fi, cell phones, AM/FM radio, GPS, HAM, CB, 2G, 3G, 4G, 5G, etc, and can even adapt to not yet known communication channels from the future.
Some of the defensive capabilities include an energy barrier that can disintegrate, redirect, and otherwise render projectiles, explosives, and other weaponry harmless. The energy shield can expand in size and project itself out to a distance if needed, and can provide a type of cloaking ability that renders people and objects invisible. The barrier is able to be air permeable, or can be airtight to contain an oxygen supply, or protect against gas or other aerosolized agents.
That's just an idea of the tool and defense aspect of it. Since they're able to pack the equivalent of black hole energy in there, it makes for some extraordinarily powerful weapons. It's possible to form projectile weapons like railguns, or even conventional bullets if you wanted to, but they consume matter to form the projectiles and would require constant reloading to keep firing them.
The go-to long range weapon is usually a focused energy beam. When used, it's not fired at full power because the heat radiated from it could and would not only burn and destroy everything around it including the operator and the device itself, but the beam would continue on well past the intended target. To prevent this, the built in A.I. analyzes the target's composition and calculates the duration needed to penetrate or destroy without any overshot, while automatic tracking keeps the beam on target so it doesn't miss.
At close range, claws and other blade type weapons can be formed from the fingers and hands. Blades can be tuned to high frequency vibrations that oscillate the weapon which aids in ease of cutting power, and can cut through nearly anything with little resistance.
Lethal weapons are typically used as a last resort, or against enemy armor, vehicles, or drones. There's a full complement of non-lethal options as well. Some of the options include a sonic weapon like the LRAD, a bolo and net launcher, and a type of sticky foam that hardens and traps insurgents in place. The world's population in that timeframe was limited, so they tried to not kill people as much to prevent extinction.
There were some other non lethal means of incapacitating enemy soldiers. One was called "Brain Hijacking" which involved the injection of nanites into the body. This was used more for infiltrator type units. A finger could be formed into a syringe and stabbed anywhere on the body. The syringe could be made ultra fine to where you would barely feel it. The nanites would then travel to the brain and create an artificial bridge between the synapses.
It's possible to deploy these nanites with a simple handshake, or even a kiss, but injection into the blood stream is the fastest way to get them into the brain. Sometimes seconds count. In the case if infiltration, intimate contact would be used with a target instead of the syringe method. They could use the time in-between to gather information covertly while they took effect.
The legs aren't typically used as elaborately as the arms are, but they can reconfigure themselves like the arms if needed. They can reform into different presets that aid with climbing, and provide enhanced jumping and running ability. The feet can electromagnetically anchor themselves to metal, or configure into nanotech setules to stick to surfaces like a spider, as well as a built in anti-gravity option.
The arms and legs are also made to be able to plug into and interface with vehicles being piloted. Connecting the nervous system to the vehicle greatly enhances reaction time and also provides a greater degree of control. Being able to "feel" the jet you're flying provides additional incentive to pilot more carefully and avoid incoming fire. The artificial limbs also provide an advantage that allows pilots to withstand more G-Forces than normal since blood won't pool in the extremities.
All prosthetics come equipped with a self destruct that disintegrates them nearly instantaneously in the unlikely event they somehow get detached without authorization so they don't fall into enemy hands. There is also a fail safe that keeps them deactivated unless there's an emergency if there are any poisons or intoxicants in the user's system so they don't end up doing anything stupid in case they get blackout drunk.
When the soldiers came back, their modifications remained intact, but because the limbs and other modifications are extremely dangerous. they had to be deactivated since it's way too dangerous to be walking around outside a warzone in normal society with them. This process is referred to as "demilitarization", just as you would do with a firearm.
It provides a huge advantage over people in the past, and the temptation would be too great for some of the soldiers to not just blow open a vault in a bank and make off with all the money, or raise too much suspicion by unrealistically overachieving in sports. Not to mention the sudden Earth shattering consequences to society if people found out about them. Technology stocks would likely plummet since they'd feel like they had been wasting all their time on stone age tools.
Many of them who actually lived through war long enough also suffered from severe cases of PTSD which would make them a liability, so it required memory wipes. In that future, WWIII had begin before adequate treatment for PTSD victims had been found, and once the war was going their scientists were focusing their time elsewhere since most soldiers didn't live that long anyway.
There was also the factor of Time's Corrective Mechanism. When dealing with something like lotteries, it's easy to just change numbers to prevent someone from winning, but when you have some very powerful and dangerous tanks with incredible destructive force walking around, the results can be unpredictable, so they thought it would be best to just not mess with it.
When not in combat mode, they look and feel just like regular arms and legs. These aren't just cybernetics, but more like ultra realistic bionics. The difference between conventional cybernetics and bionics is that cybernetics can be elegant, but are typically more on the crude side. Bionics are built to mimic structures and processes that exist in nature.
They sweat and they sunburn and can bleed too. They rearrange themselves to increase in size if you work out or gain weight. They have a perfect nerve interface that provides indistinguishable tactile function to the real thing. They're extremely expensive, especially during a wartime budget. That's why they were typically reserved only for upgrading or rebuilding the elite soldiers.
Part of the reason for doing it this way was for the quality of life factor. Not everyone wants some big gaudy looking robot arm. Some of the replacements were due to involuntary injury and it was ideal for the victim to live as normal a life as possible. In some cases, people were able to continue as if nothing ever happened. Especially if shock therapy was involved and the memory of the incident had been erased. They wouldn't even know the difference between the body part that was gone and the new one.
They can be removed, but certain conditions must be met first. If an attempt were made to remove them before the conditions are met, the prosthetic remains in deactivation mode, even after removal. For example, if someone like a government or group in the past found out about them and wanted the technology for themselves and just tried to cut an arm off, it would continue to appear as if it were a normal flesh and blood arm and they wouldn't learn anything from it. It would rot and eventually disintegrate like any other flesh.
The security they had in mind to unlock them had to be complex situations that wouldn't be encountered in everyday life, so someone can't just bump into a button or say a password or something and the limb comes off while walking down the street.
It involves biometric security, and only select individuals have clearance for it. A pass phrase is involved, but the specific passphrase can't just be spoken, it has to also invoke a biological response when used. For example, that response would have to have an effect on the vital signs of the person, like heart rate and respiration, even for a split second, along with a change in hormone levels which is all monitored by the prosthetic's built in A.I.
Then, certain frequencies of sound must be present in the background in a specific order while this is happening. Finally, there are "buttons" on the prosthetic that must be pressed. They're not obvious like indentations or ones that stick out, it's more like grabbing the limb in a certain area. The A.I. also senses the fingerprints. Finally, the prosthetic releases itself from the body.
It's surreal to actually watch it happen. The flesh begins to separate smoothly on its own in a pattern. It's almost in the same fashion an elevator door opens and closes. You can see the layers of skin, fat, and muscle, but then you begin to see the mechanical parts underneath as they move apart. There are what looks like colored lights that blink in patterns, while some look solid, but are actually just rapid blinking. These are the nerve interfaces transferring information, kind of like fiber optics, but far more advanced.
The user can actually retain full control over the removed limb wirelessly. Not only can it move, but you can still have sensation in it. It's unknown how far the range is. I saw some rudimentary testing done on this outside of laboratory conditions. A guy had his arm removed and they had him blindfolded in a sealed room with earmuffs while they poked his fingers and he was able to tell them which ones they were touching.
Then they basically took his arm on a drive a couple hours away while he stayed behind. He was able to still move it around and grip the steering wheel. I'm sure they could have kept driving, but due to time constraints and the security risk involving this type of technology, they didn't want to screw around too much.
To summarize, in order to release the prosthetic, you would have to put all the conditions together at the same time for it to work. So for example, it could be something like taking a specific person who has the biometric clearance, then they speak a passphrase that invokes a biological response.
It could be something as simple as "I love you", but there would have to actually be a connection with the person, otherwise it won't affect the vital signs and release dopamine and oxytocin. This would prevent the enemy from removing it, since you'd basically hate their guts. While doing this, the audio frequencies could be something like a certain song playing. Then they would just need to grab the limb in a specific way and it should release from the body.
You combine all these together and you have some serious security that would probably be greater than a one in a billion chance of ever accidentally occurring while walking in public. Unfortunately, due to memory alterations, many are unaware of how to remove them, or even that they have the prosthetics, but I'm sure the information exists out there somewhere.
This technology was later adapted for use with MIB for time travel enforcement. Due to the sensitive nature of the matter, it made things a lot easier for protection of operatives, escorting important people, and to ensure the ability to counteract any threat to the Timeline. This is also where the old term "arms" came from when referring to weapons. It was both literal and code.