Invisibility versus transparency. What is possible with plausible advanced technology?

Several theoretically possible ideas come to mind:

• Counter projection. A powerful computer and camera system 3d scans the surroundings live and tracks where every person within range is currently looking. Additionally, the cloaked person is wearing an ultra-black bodysuit so that they'd appear as a dark blotch to peoples vision. Then, a complex laser projection system beams the appropriate background image directly into the retinas/camera sensors of any viewers. Since the bodysuit is completely (>99.999%) absorbing, the laser projection system can work even in low-light settings. Today, the bodysuit would be build able and the 3d scanning/tracking system too although you'd need to mainline a supercomputer. The projection system would be impossible though. Likely some sort of phased-array visible light system would be needed which does not yet exist.

• Cloaking field. This is pretty sci-fi and requires the ability to manipulate gravity and generate anti-gravity (negative) fields. It's been conclusively proven that gravity has many wave-like properties, and I wouldn't be surprised if humanity eventually figured out how to build a gravitational emitter of some kind. If you do have gravity manipulating technology, you can lense light around a subject, similar to how a black hole lenses light, but in reverse.

• Mass mind control. Not strictly "invisibility" but if you have scanners (radar, mri, whatever) which can achieve sub-neuron resolution at range, you can potentially couple this with some sort of remote, radio-based, electrostimulation setup. Then, you fire off neurons in people brains from a distance to make them selectively ignore you. This would however require monstrous computational capabilities along with a complete understanding of the human brain which we don't have yet. Alternatively, you could also do this with nanites which are dispersed in the air, breathed in, and then attach themselves to convenient neurons.

• Metamaterial shenanigans. Just like you can make a micro/nanostructure which traps light in a fractal-shaped pattern producing a very dark black, you can probably make a metamaterial that reflects light along a specific path. I imagine that you'd be able to coat a cylinder with such a coating and set it up to appear transparent when viewed from the sides. Unfortunately, the surface would be very fragile and it would only work from horizontally distributed viewing angles. Also, you wouldn't be able to make this into a suit because the ridgid cylinder shale would be critical to it's functioning. This technology is probably the closest to reality, and could probably be made within the next 10-20 years if there were a practical reason for it.

It depends if you whether stick to with a definition of 'invisibility' where light either passes through the object (transparency) or is bent around the object .

In terms of your question to date there is no way for organic matter to be made transparent (or indeed most other forms of condensed/cold matter either).

In terms of deflecting light around a target object there are, as other posters have noted several interesting technologies being developed. Since they seem to involve some form of shaped mea-material being placed between an object and the observer you could quite probably have your aliens travel is small vehicles that were 'invisible' even if they weren't.

There is a third option you could look at which basically involves intimating chameleons. As I recall there was a defense project many years ago looking at this option for armored fighting vehicles (don't know if its still progressing).

Basically the idea would be to develop a material layer containing lots of miniaturized photo-receptors and light emitting diodes or similar made from flexible electronics. Then you add a small compact power supply (it wouldn't need to be much, maybe it could ven be powered by the wearer's movements) plus a specialized chip to control it all

The receptors note the ambient light hitting the surface from 360 degrees around the target - and transmit that info to emitters on the exact opposite side of the target. So if red light hits the target from say 30 degrees the color & intensity is transmitted to emitters at 210 degrees.

If you had someone wearing a 'smart suit' with similar capability that person could say walk along street which had a brick wall on one side of the road and a park on the other.

Someone walking down the 'park side' would look across the road at the alien and see only the brick wall pattern reproduced on that side of the suit facing them. Someone walking past the alien on the other side (a short distance from the wearer) would glance to the side and only see the 'greenery' of the park.

The trick would be getting all the circuitry to work consistently, at high resolution in real time. Sudden movements, or changes in light might may distort the images being transmitted along enough for a blurry object to be seen.

Yes, Canadian camouflage company HyperStealth use conventional but little-known optics to make objects effectively invisible.

They made a flair splash with this and the technology clearly has a lot of applications, many of which are military. There are some good videos of their tech in action online.

This may not be what you're looking for:

Here's a video showing it in works.

What is happening here is that a fresnel lens is catching ambient light. Place the object you wish to hide far enough back from the lens, and the ambient light has been bent "around" the object, effectively concealing it (very effectively in some cases).

You can see, however, that the lens is blurring details. Against a low-detail background, it's really not a problem. But it stands out very clearly against (for example) flower-patterned wallpaper.

There's are three problems with this as a solution : (1) it is not "true" invisibility, (2) the lens needs to be looked at straight-on (or nearly straight-on); which can be resolved by adding more walls, and (3) the lens needs to be able to catch a lot of ambient light; as you add more walls for a more rounded effect, you start to have those additional lenses stealing light from one another (like shade from trees).

But those are problems that might be possible to work-around, if problems (1) and (2) don't kill the idea.

Metamaterial Electromagnetic Cloaking Device

Metamaterial cloaking is a mostly-theoretical concept which is to create a meta-material that can conduct light around it and thus be rendered 'truly invisible', or at least, undetectable by sight within certain frequencies of light. As the object is not hit by light in any way, this can be considered to be realistically close to 'true invisibility'. There are some downsides, namely, it can only be done on small scale and it can only be done within the microwave spectrum for now, that is, it's only invisible to microwaves. That, and there's the fact that if we do get a working prototype within the full visible spectrum range, then it can still be seen using methods outside the visible light spectrum, i.e. infrared, sonar, etc.

No

True invisibility isn't something achievable, at least with current science and current theoretical science. Like you say, invisibility is often confused. In the movie "tomorrow never dies", they have an invisible car that records with tiny camera's the background and projects it on the other side. Problem is perspective, elegantly explained in Gost Protocol: https://youtu.be/ydIPKkjBlMw

To be truly invisible, you need light to bend around you and then follow the same path, or you need transparency without refraction, neither of which exists.

Furthermore, you have a lot of other problems. If you are invisible, you can't receive light input and are blind. At least on the frequenties you bend or are transparent for, but that is tetriary to your question. Still I wanted to mention it.