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EDIT: Question timeframe changed from early XXI to mid-XXI century. Lets say 2040 decade-ish.


You are the engineer in charge for the construction of the latest <secret agent name>'s stealth suit.

It should be the best personal cloaking device mid-XXI century tech could deliver. But also must:

  1. Be lightweight (maximum 60lbs)
  2. Dampen sounds or outright eliminate them
  3. Able to eliminate IR (heat) emissions for at least 15 minutes in a 60F/15C temperature and above
  4. little to no extra electromagnetic signature (beyond the normal human body's)
  5. be no more than a light hindrance on the agent's movements (so no full-body mirrors on both sides, please) - including a basic aikido roll - can be done with a small backpack.

for the cloaking device and other devices, estimate your weight. ALso, explain some of the "science" behind your choices.

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  • $\begingroup$ Based on the technologies available to the early 21'st century (as in right now), it's not possible. Why? If it were possible, the government would have done it already :P The main issues with this are power source, weight, and frailness of the suit. However, if you change the question to allow for some future tech, it would be possible to explain. $\endgroup$ – Aify Apr 22 '15 at 3:41
  • $\begingroup$ @Aify ok. lets place the question in near-future, like 2040 $\endgroup$ – Mindwin Apr 22 '15 at 4:40
  • $\begingroup$ @Mindwin, could you clarify what you mean by "dampen sounds or outright eliminate them"? I can think of at least three ways that could be interpreted. Do you mean that moving the suit creates very little sound or foot falls make no sound? $\endgroup$ – Green Jul 5 '15 at 13:57
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My first thought was metamaterial cloaking. Interestingly, metamaterial cloaking is mentioned in the Cloaking Device link in the question.

If you've done a highschool science course, you should be vaguely familiar with the wave-properties of light. This is what what allows a prism to split a white light into a rainbow.

Prisms have their limitations, so higher-levels of science often refer to things called Diffraction Gratings when teaching optics and quantum mechanics. They're cheap, reliable, and can be manufactured with very specific properties. The property most commonly sort after is the slit size - the size of the gaps in the grating. Different sized slits will bend different colours of light to different degrees. The angle the light hits the grating is also key, but that's a simple matter of just shining your light at the correct angle in the first place. :P

There is a downside to all of this; your grating's properties are fixed. One grating may be useful in one experiment, but useless in another. This makes diffraction gratings a pretty poor candidate for a high-tech cloaking device.

Goals:

  1. We want to be in control of spacing between gratings,
  2. and we want to control the angle light hits the grating.

Lets turn the first problem on its head: instead of moving the gratings closer or further apart, let's make them bigger or smaller. This is good, but we still have the problem that Mr. Secret Agent is only invisible when people are looking at him from very specific angles!

We solve the second problem by being able to point these in the direction of our choosing. And this is made more difficult by the fact that suits change shape. They bend and deflect as you move about in them. Not only do we have to align the gratings correctly, we have to re-align them every time we do something!

Why don't we create a grating out of hair-like structures? Maybe ones that you can control the angle of? If we turn to the science books, we see that nature has something that almost fits this bill!

Eukaryotic Cilia!

Mr. Secret Agent's cloak is going to have a thin layer of hair on it. I lie! These hairs are actually thin sheets, say made from graphene, rolled into tubes. Picture a sheet of paper rolled up into a tube. How tightly you roll the tube will determine how big or small it is. At the base of the tube is a Kinetosome - what's effectively the world's smallest biological electric motor. Use this to wind and un-wind the roll. This is your spacing problem solved. More Kinetosomes, artificial muscles, or maybe piezo-electric actuators, point the hairs in the directions of your choosing.

Which leaves one final problem: choosing the correct direction at any point in time.

I'm afraid there's no amazing futuristic science involved in this step. It's just mathematics. Very complicated mathematics, but ones that we understand today. You may need to integrate a small supercomputer into your suit, but given that Moore's Law still holds, and most major semiconductor manufacturers are working towards miniaturising their stuff, it's no stretch of the imagination to have this available (at great expense) to a government-funded R&D engineering team.

Just a side note, these hairs, no matter what they're made of, are most likely not going to like fire or being in contact with hot things. A gust of wind is going to be an annoyance too.

Other features

Heat emissions. Solve this by coating the inner lining of the suit with Peltier Coolers. It'll get nice and toasty for Mr. Secret Agent, but he comes from a tropical environment and handle a little heat just fine. The limit to the system is going to be how hot Mr Secret Agent is, and how much power is available to pump the heat back as it dissipates. If the operator is climbing about, hanging from the ceiling, stealing things, and being a general nuisance, the suit can operate for about fifteen minutes.

If the operator sits perfectly still, and remains calm, it's perfectly reasonable to think the suit could operate for a few hours. Maybe even an entire night if the wearer lies and down and has a sleep.

Nothing here really weighs that much. The cloak, in theory, could operate equally well on non-visible spectrums of light... though probably not at the same time as visible light. There's very little in the way of moving parts, so the noisiest thing is going to be Mr. Secret Agent's mobile phone if he forgets to put it on silent.

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    $\begingroup$ Peltier coolers are stunning inefficient, around 10%. This means that they generate around ten times as much heat as they draw from the cold side. And remember, a person usually emits around 100 W of thermal power. I don't think Mr. Agent would last a minute, much less 15. $\endgroup$ – 2012rcampion Apr 22 '15 at 13:37
  • $\begingroup$ @2012rcampion is absolutely right. Peltiers will only worsen the issue by producing additional heat. $\endgroup$ – Ghanima Jun 2 '15 at 22:21
  • $\begingroup$ I love the idea of cillia to form diffraction gratings! But I don't see how that would enable invisibility. $\endgroup$ – JDługosz Jul 5 '15 at 9:15
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How about an active image display? The suit surface is covered with light emitting microscopic dots.

There are also sensor eyes covering the surface, so the scene on the back side can be read even if he's right up against it.

The scene reproduced on the front side only works for a specific viewpoint. So, track the viewer's eyes and create the scene to match what he should be seeing.

Now two eyes, or worse yet multiple people and cameras... different images are projected in different directions. The suit surface is covered with tiny bumps, and the lights are highly directional and arranged over the bump.

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This answer assumes considerable advances in materials science, power storage, sensor miniaturization, and computation power in the next 25 years to make this stealth suit possible.

Lightweight (maximum 60lbs)

For batteries, let's assume that Lithium-Sulphur batteries become commercially available and small enough for use by a human. Current power density of LiS batteries are 0.5KW•H/kg (though this could improve significantly in 25 years). That may not sound like much but if the Li-ion batteries in a Tesla Model S were swapped for LiS, the Model S would triple its range to about 900 miles on one charge.

LiS offers 1.8Mjoules/kg equivalent to 4285.7 calories or .5kg of fat. Were a human electrically driven (rather than chemically driven), a two kg LiS battery would be enough to power a very active human for almost three days.

Dampen or eliminate sounds

Simple soft-soled shoes should do the trick for walking/running sounds. I can't think of a way to dampen the sounds of running across a metal surface by a design feature in the suit. Running fore-foot strike might get the required quietness.

We also need to account for the suit rubbing against itself while moving. Rigid textures on the surface of the suit would need to be eliminated as much a possible. Velvet like materials or flexible TPU coatings would go a long way here.

Able to eliminate IR (heat) emissions for at least 15 minutes in a 60F/15C temperature and above

As stated in other answers, passive cloaking solutions such as metamaterials are tricky for several reasons. Instead of heating or cooling the suit to match ambient temperatures, we instead used a technique similar to noise canceling headphones to make the suit appear to be ambient temperature but really isn't. (Quick review of noise canceling technology: A microphone on the outside of the headphones measures the amplitude and frequency of incoming soundwaves, then a microprocessor calculates an inverse soundwave that when added to the incoming soundwave cancels it out to zero. The effect is that in a really noisy environment like an airplane at 30K feet, the listener doesn't hear a thing.)

Things admittedly get a bit hand-wavy at this point because the computation requirements alone are staggering. The suit knows the incoming IR radiation from all directions using micron sized directional temperature sensors. This omnidirectional heat map is fed into the primary IR cloaking processor which accounts for the position and orientation of the suit plus the skin temp of the wearer. It then instructs micron-sized directional IR interference generators spread across the suit to emit IR energy of the correct frequency and amplitude to make the IR emissions of the wearer appear to be that of the background.

A similar technique for IR can be used for visible light cloaking too.

Given the power availability of our LiS batteries, 15 minutes of stealth mode shouldn't be any trouble.

Little to no extra electromagnetic signature (beyond the normal human body's)

I'm assuming the question is referring to radio and microwave emissions and not black-body radiation. Properly shielded, all of the computational equipment and battery packs shouldn't give off any additional EM signals. If optical CPUs are used then shielding is even easier and considerably faster then current copper/silicon based CPUs.

Unrestricted movement

If the suit is formfitting and doesn't have any inhibitions to flexibility or range of motion, the wearer should be able to do everything they would normally be able to do while wearing a plain lycra bodysuit. With the addition of a thin strength enhancement layer to the suit, agility should be greatly augmented though without increased load bearing capacity built into the suit the wearer will still be limited by skeleton strength (no jumping off 10 story buildings and landing safely).

Suit Construction

The suit will need the following layers to achieve its mission.

  • Power distribution layer
  • Insulation/Environmental control layer
  • Movement augmentation layer
  • Ballistics Protection layer
  • Camouflage layer
  • Sensor layer

Admittedly, several of these layers can be combined or assist each other and can be combined.

Weaknesses The camouflage technique used by this suit can't account for strong heat or light sources. It may be able to match the frequency of the light source but it can't match intensities. So an intruder wearing a suit like this would have to make sure to not walk in front of a strong light or heat source. Perhaps the wearer would have genetically modified eyes to detect IR radiation in addition to the normal visible light.

Because of the processing time involved in calculating the IR/visible output of the camo system, a security system could measure the lag between when a light was turned on and when it appeared to turn on to a sensor some distance away. Processing the heat map can never be as fast as the flight of an IR ray from a heat source.

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