The Impossibility of Invisibility

[drzeus99]

Correct. Classic fictional tales of invisibility (a la "The Invisble Man") is physically
impossible to create.

However, letting an object appear as invisible to an observer is certainly technically
and physically possible, achievable, and almost certainly will be available in the future.

Bending light's wavelengths around an object would probably be the easist way to do
it.

If an object could be made to emit a strong enough electromagnetic field (or something similar), it could cause light (photons) to bend around the object, thus rendering it invisible to an observer.

Another method would be to create a material that has a type of display device (like
a tv screen made of fabric), and it always displays an image of what is directly behind
the wrapped object. (Obviously, there would have to be micro cameras imbedded in this fabric that filmed all areas in a 360 degree view so that this image can be projected on the material that is exacly on the opposite of the view(s))
This way, an observer would, in esssence, "see right through an object" by observing what visual sight is being blocked by the item (since it's being displayed on the display screen in the material that wraps the object).

The US Military has already been conducting tests on technology similar to this. This would allow the soldier of the future to wear a uniform with this display technology in it,
and he would appear "invisible" to a viewer he was appraoching. At worst, when this technology is perfected, it could make someone have an appearance similar to the "Predator" lifeform in the movie of the same name (and "AVP" too).

But as far as ever creating a true invisible man, nah...not gonna happen as far as we know the laws of physics today.

Cheers,
Dr Z

 

[Ron_Tomkins]

Correct. Classic fictional tales of invisibility (a la "The Invisble Man") is physically
impossible to create.

However, letting an object appear as invisible to an observer is certainly technically
and physically possible, achievable, and almost certainly will be available in the future.

Bending light's wavelengths around an object would probably be the easist way to do
it.

If an object could be made to emit a strong enough electromagnetic field (or something similar), it could cause light (photons) to bend around the object, thus rendering it invisible to an observer.

Another method would be to create a material that has a type of display device (like
a tv screen made of fabric), and it always displays an image of what is directly behind
the wrapped object. (Obviously, there would have to be micro cameras imbedded in this fabric that filmed all areas in a 360 degree view so that this image can be projected on the material that is exacly on the opposite of the view(s))
This way, an observer would, in esssence, "see right through an object" by observing what visual sight is being blocked by the item (since it's being displayed on the display screen in the material that wraps the object).

The US Military has already been conducting tests on technology similar to this. This would allow the soldier of the future to wear a uniform with this display technology in it,
and he would appear "invisible" to a viewer he was appraoching. At worst, when this technology is perfected, it could make someone have an appearance similar to the "Predator" lifeform in the movie of the same name (and "AVP" too).

But as far as ever creating a true invisible man, nah...not gonna happen as far as we know the laws of physics today.

Cheers,
Dr Z

Yes, that's exactly what I was talking about on my other post (which I think you missed). That's the "Invisible Suit" itself. And indeed, the person who mentioned this to me, also made the analogy to the "Predator" effect.

 

[TuftedPuffin]

There's another method as well, one that might even be simpler. I'm not sure if any actual scientists are investigating it, but it seems to crop up a lot in Sci-fi/Fantasy, the two biggest examples I can think of being witches in the Golden Compass and the Somebody Else's Business Field in Hitchhiker's guide: invisibility that works by making someone view you as part of the background. If someone doesn't perceive your presence as being a relevant detail, you are effectively invisible. While this might be quite complicated when dealing with humans, it could be much simpler with computer systems: hack in and remove your image.

 

[Dan O.]

Here is the way to think about the concept if you want practical invisibility: Start with a small object (say a sphere). Contained within this object is a set of projectors and cameras that cover a full 360º view field. The light given off by the projectors in any direction would exactly match the color and intensity of the light that would strike that object from exactly the opposite direction. This object is one pixel of your invisibility cloak. To build a cloak, each invisibility pixel would know its physical relation to the other pixels so one pixel would record the light that strikes it from a given direction and relay that information to the pixel that is inline with that rays original direction to be retransmitted along the same line.

Of course, such a system could never be perfect. There is a practical limit to the density of these pixels because the interconnect bandwidth goes up as the pixel size goes down. The resolution limit of the cameras and projectors would show up as a distortion against a high contrast background. There will always be a processing delay that will show up if the background is changing rapidly. The fake colors generated to fool the eye wouldn't fool a spectrum analyzer. And it would be difficult to generate enough light to fill the cloaking devices own shadow. But for some low demand applications it could work.

 

[rsaavedra]

Well, here's some new development that might be aligned with possible invisibility cloaks:

http://www.wired.com/science/discoveries/magazine/15-11/st_alphageek

Her first trick was slowing a pulse of light to a crawl — 15 mph as it traveled through the BEC. Since then, Hau has completely frozen a pulse and then released it. And recently she shot a pulse into one BEC and stopped it — turning the BEC into a hologram, a sort of matter version of the pulse. Then she transferred that matter waveform into an entirely different BEC nearby — which emitted the original light pulse.

Cloaks built with thousands or millions of those "BECs", but somehow appropriately interconnected, so that light hitting a BEC on one side of the cloak is expelled identically and in the right direction from the right BEC on the other side of the cloak.

 

[Molinaro]

Here is the way to think about the concept if you want practical invisibility: Start with a small object (say a sphere). Contained within this object is a set of projectors and cameras that cover a full 360º view field. The light given off by the projectors in any direction would exactly match the color and intensity of the light that would strike that object from exactly the opposite direction. This object is one pixel of your invisibility cloak. To build a cloak, each invisibility pixel would know its physical relation to the other pixels so one pixel would record the light that strikes it from a given direction and relay that information to the pixel that is inline with that rays original direction to be retransmitted along the same line.

Of course, such a system could never be perfect. There is a practical limit to the density of these pixels because the interconnect bandwidth goes up as the pixel size goes down. The resolution limit of the cameras and projectors would show up as a distortion against a high contrast background. There will always be a processing delay that will show up if the background is changing rapidly. The fake colors generated to fool the eye wouldn't fool a spectrum analyzer. And it would be difficult to generate enough light to fill the cloaking devices own shadow. But for some low demand applications it could work.

But different people looking at an individual pixel from different angles would need to have a different colour projected. How does your pixel figure that out?

 

[Ladewig]

Here is the way to think about the concept if you want practical invisibility: Start with a small object (say a sphere). Contained within this object is a set of projectors and cameras that cover a full 360º view field. The light given off by the projectors in any direction would exactly match the color and intensity of the light that would strike that object from exactly the opposite direction. This object is one pixel of your invisibility cloak. To build a cloak, each invisibility pixel would know its physical relation to the other pixels so one pixel would record the light that strikes it from a given direction and relay that information to the pixel that is inline with that rays original direction to be retransmitted along the same line.

Of course, such a system could never be perfect. There is a practical limit to the density of these pixels because the interconnect bandwidth goes up as the pixel size goes down. The resolution limit of the cameras and projectors would show up as a distortion against a high contrast background. There will always be a processing delay that will show up if the background is changing rapidly. The fake colors generated to fool the eye wouldn't fool a spectrum analyzer. And it would be difficult to generate enough light to fill the cloaking devices own shadow. But for some low demand applications it could work.

An additional consideration would be depth of field. If one's field of view includes an object in the foreground and an object far way, then the human eye cannot hold both objects in focus. If the projecting field showed both objects in focus at the same time, then it would be obvious that one was looking at a created image.

 

[thomps1d]

A true double blind.

So would that mean that scientifically controlled tests would be triple-blind? Or would they be quadruple blind?

 

[rsaavedra]

But different people looking at an individual pixel from different angles would need to have a different colour projected. How does your pixel figure that out?

Will take the liberty to take a guess at replying to that, though it wasn't directed to me.

The notion of pixel is actually misleading. Dan O had mentioned start with a small object, a sphere. I would take that notion a little differently, thinking of an irregular object, not necessarily a sphere.

The object would be covered by some sort of invisibility "fabric", which has tiny "projectors" which wouldn't be tiny flat areas producing one unique color each. I think those projectors would be tiny domes or hemispheres, projecting in principle billions of colors, or at least capable of projecting many different colors in many different directions simultaneously.

If fact, each tiny dome has to project the whole scenery behind the cloaked object and appropriately in all possible directions, so that any observer looking at that tiny dome from any angle and location in space, will see the light that would have been received if there was nothing blocking the light coming from the direction of that tiny dome toward the eyes of that observer. So each dome has to project in a given direction the light that was received in that same direction by some tiny fish-eye camera placed on that invisibility cloak exactly on that same direction but on the other side of the covered object.

So alternated with those thousands or millions/billions of projectors there would need to be equally numerous tiny domed cameras, catching light from all possible angles on the hemispace in front of them. The only catch, all domed cameras have to send their information to all domed projectors in the invisibility cloak, since each possible camera-projector line defines a possible line of view for a potential observer outside the covered object intended to appear invisible. So this is one complicated fabric to build for sure.

An additional consideration would be depth of field. If one's field of view includes an object in the foreground and an object far way, then the human eye cannot hold both objects in focus. If the projecting field showed both objects in focus at the same time, then it would be obvious that one was looking at a created image.

See explanation above, the "pixels" would not flat pixels, but domes, so light would be reproduced seemlessly to all possible angles within a half space from all domed "pixels" (that half space defined by the base of that dome.) So light would be projected from all sides of the object at the right intensities and colors, as if light had just passed through empty space, without that invisibly cloaked object blocking light sources. In that sense, depth of field should be preserved, the eyes would be tricked perfectly (in ideal technical conditions and ultimate resolution of those tiny domed cameras and projectors.)

 

[TX50]

Modern camouflage is pretty good at hiding people already. Camouflage
patterns are now computer generated (no longer designed by artists) and
designed to be effective at different ranges (older patterns only really work
close up). There are even quite effective IR camouflage schemes now that
help hide men (for a while) from thermal imaging sensors.

I always thought that the movie "Predator's" cloaking device thingy was total
overkill in the jungle. It is easy to hide in dense jungle even just wearing a
set of tropical combats and a bush hat.

If you were somehow able to make someone's tissues totally transparent,
then they couldn't eat anything. You'd see their lunch gooshing around in
their transparent stomach. Eeeeugh!

 

[Ladewig]

See explanation above, the "pixels" would not flat pixels, but domes, so light would be reproduced seemlessly to all possible angles within a half space from all domed "pixels" (that half space defined by the base of that dome.) So light would be projected from all sides of the object at the right intensities and colors, as if light had just passed through empty space, without that invisibly cloaked object blocking light sources. In that sense, depth of field should be preserved, the eyes would be tricked perfectly (in ideal technical conditions and ultimate resolution of those tiny domed cameras and projectors.)

I strongly disagree. As I look out my window I see a small bush 1 yard from where I am sitting and a large tree 60 yards away. If I focus on the small bush, the large tree becomes out of focus. If I focus on the tree, the small bush becomes out of focus. If I were to look out my window and see both objects in focus, then I would know that my window has been replaced with a photo or some image generating device. I have to adjust my cornea to change the focus and can feel what it is like to focus close to me or far away. Many cameras can also change their field of focus and depth of field, but the "invisibility device" could not know which object I am focusing on and therefore would not know whether to focus on the near object, the far object, or both objects (by greatly narrowing the aperture).

 

[fuelair]

No, I didn't. That's why I came to this forum. It is filled with skeptics, many of them with far more knowledge of scientific facts than I do. I came here to exchange information. I'm not claiming anything at all (If anything, I'm claiming that invisibility is impossible. I just want to know exactly why and how is it impossible)

Not meant as a putdown - I do not know you. Purely based on your statements, you cannot be here to exchange information (though you may choose not to see/accept that). You are here to get information.

An answer or so:A) molecular transparency one won't work well as you will be blind (parts of the eye need surfaces to do stuff involved with seeing - those surfaces must do more than let light just pass through.
B)military is looking for something LIKE full uniform of overlapped fiber optics going 180, point for point around the body so observer in front sees only what is behind you, obs in back sees what is in front of you, left side sees what is on right of you, etc.

Try : http://www.wired.com/wired/archive/11.08/pwr_invisible_pr.html

 

[rsaavedra]

I strongly disagree. As I look out my window I see a small bush 1 yard from where I am sitting and a large tree 60 yards away. If I focus on the small bush, the large tree becomes out of focus. If I focus on the tree, the small bush becomes out of focus. If I were to look out my window and see both objects in focus, then I would know that my window has been replaced with a photo or some image generating device. I have to adjust my cornea to change the focus and can feel what it is like to focus close to me or far away. Many cameras can also change their field of focus and depth of field, but the "invisibility device" could not know which object I am focusing on and therefore would not know whether to focus on the near object, the far object, or both objects (by greatly narrowing the aperture).

I think you missed the key point in my explanation, let me try to further explain it.

The fabric I described is *not* just like a flexible LCD panel, in which each pixel, at a given point in time, produces one and only one color in a given intensity, to all viewing points in front of it.

Remember, here each "pixel" is a sort of "dome" projector, and once again, it will be capable of simultaneously producing thousands if not millions/billions of light rays in different colors and intensities in different directions, depending on how it might be required.

If you moved in front of this object covered with such invisibility cloak full of these dome projectors, with every little move you make with respect to the object, you'll perceive different sets of rays and colors coming from each "dome", not the same colors you perceived in your former location.

The object covered with this fabric full of those dome projectors will replicate in any direction (e.g. towards any viewpoint around it) all light rays hitting the object from exactly the other side of the object towards that exact point space where the viewing point is. This happens simultaneously to all possible viewpoints in space around the object no matter how close or far, and for all dome - viewpoint possible lines. That's why the covered object becomes apparently invisible after all, no matter where it is and where you are. Because of that, our stereoscopic vision would be perfectly tricked, and depth of field would be preserved seemlessly with each and every move of ourselves or of the "invisible" object in front of us.

Hope that attempt of an explanation clarified it.

As an extra verbose example:

Imagine the covered object is in front of a black wall. All domes in the front part of the cloaked object will project black (e.g. no light) to the front in most directions, so wherever you are in front of the object, you'll see the black of the wall behind. Now imagine someone turns on a flashlight to the right of the object, the flashlight is facing left, so the light goes perfectly horizontally in a line parallel to the black wall, and parallel to the floor. If you are still in front of the object, everything still looks black. You start moving to the left and everything remains appropiately black until some part of the object is between you and the flashlight's light, then you still will perfectly see the light coming from the flashlight as if there's no object in between, because the domes on this side of the object project towards where you are exactly the light coming from exactly the other side with respect to your point of view. Yet those same domes projecting a bunch of light towards you, at that very same time are not doing such light projection towards the front. Anyone in the front will still see the pitch black of the wall behind the object with respect to their position, even from those same domes that are sending powerful light in your direction reproducing the light of the spotlight to anyone in your location. That's the trick of these domes.



PS. In fact, since we are in the realms of sci-fi, let's stretch this invisibility a little further.

Imagine that the dome cameras and projectors work not just with light, but with the whole electromagnetic spectrum. If they are capable of doing so, the object will be invisibile no matter what you hit it with, other than actual matter. Hit it with a laser, you'll still get equally powerful laser on the other side. By the way, let's also assume that this invisibility cloak (full of millions of these tiny dome cameras and dome projectors) is also capable of producing absolutely no reflection of whatever hits it. So hit the object with a laser, the fabric will perfectly absorb all of that energy hitting it on that side (you won't detect any reflections from the receiving side of the object) and will project the laser identically from the projectors on the other of the object, in exactly the same direction. Hit it with X-rays or gamma rays, no problem. Hit it with infrared, radio waves, sound, infrasound... same thing. Absolutely perfect invisibility! Now wouldn't that be a nice little gadget, such an invisibility cloak. Even Harry Potter's would pale in comparison.

 

[Ladewig]

I think you missed the key point in my explanation, let me try to further explain it.

Your explanation has nothing to do with my objection. I will concede every point that you made. I still maintain that depth of field cannot be reproduced correctly with an invisibility device. Wikipedia article on depth of field.

ETA: I will agree that your device will work in limited settings such as a small room. It cannot fool a person if it is used outdoors or in a large room.

 

[rsaavedra]

Your explanation has nothing to do with my objection. I will concede every point that you made. I still maintain that depth of field cannot be reproduced correctly with an invisibility device. Wikipedia article on depth of field.

Of course it has everything to do with your objection. And whether the setting is small or large has nothing to do with how light and this invisibility works.

Once again, the invisibility I described would simultaneously preserve and reproduce the direction, intensity and color of *all* light rays hitting the object, no matter where they came from, and would reproduce those rays on the other side of the object, in their respective directions, as if there had been nothing there blocking that light. Hence, light would work exactly the same way whether coming from the sources directly at different distances in front of you, or whether the invisible object is somewhere between your eyes and those objects.

I'm a computer scientist and have worked with computer graphics quite a bit. Even though the technology to build this invisibility cloak doesn't exist, a computer program could perfectly model this fabric and could easily show that perfect invisibility would be achieved within the simulated rendered world up to whatever rendering resolution you'd want to achieve. (Of course, this is just a phrase, and it proves nothing; the invisibility provided by this simulated cloak would need to be shown in a computer simulation/rendering test, but I would confidently state that it can be done.)

Now instead of posting a Wikipedia link to something I understand well, please explain further your position, in sufficient technical detail; it is not clear at all why you maintain such position, that depth of field won't be preserved.

 

[Ladewig]

Of course it has everything to do with your objection. And whether the setting is small or large has nothing to do with how light and this invisibility works.

Once again, the invisibility I described would simultaneously preserve and reproduce the direction, intensity and color of *all* light rays hitting the object, no matter where they came from, and would reproduce those rays on the other side of the object, in their respective directions, as if there had been nothing there blocking that light. Hence, light would work exactly the same way whether coming from the sources directly at different distances in front of you, or whether the invisible object is somewhere in between.

Yes, I agree with everything you say. However that has nothing to do with my objection.

Now instead of posting a Wikipedia link to something I understand well, please explain further your position; it is not clear at all why you maintain it.

Look at something at least 20 feet away from you. Now hold up a finger 7 inches from your face while still looking at the far object. You will notice that your finger appears out of focus. If you focus on your finger, the far object appears out of focus. The human eye is built in such a way that a very near object and a very far object cannot both be in focus at the same time. The human eye is versatile enough that it can easily and relatively quickly change its focus from near to far objects. Well-made cameras can also focus on a near object only, a far object only, or (unlike humans) both objects at the same time. If your invisibility device places both a very near object and a very far object in focus at the same time, a human could recognize the image as not being lifelike because the human eye can never produce that effect. If your invisibility device focuses on the near object only, then the human will recognize it as a imaging device because the human will not be able to view the far object in focus. If your invisibility device focuses on the far object only, then the human will recognize it as a imaging device because the human will not be able to view the near object in focus.

I posted the wiki link because it has a large number of images that illustrate the effect. Those photos show the effect much more efficiently than I can describe the effect.

Is there anyone else in this thread that can describe my position more clearly?

 

[rsaavedra]

If your invisibility device focuses on the near object only,

There's your problem, absolutely wrong understanding of how the devices we are fantasizing about work.

The device doesn't focus on anything in particular. For the n-th time, each dome camera just preserves the direction, intensity and color of all light rays hitting it, regardless of where they were coming from. The little dome "cameras" don't have to focus at any particular distance. They just detect light from all directions, and appropiately (and instantly by the way) share that information with the right dome projectors somewhere else on the fabric.

Is there anyone else in this thread that can describe my position more clearly?

My guess is no there isn't, unless they have the same misunderstanding of how things are working here.

 

[rsaavedra]

They just detect light from all directions, and appropiately (and instantly by the way) share that information with the right dome projectors somewhere else on the fabric.

A minor clarification to my last post.

In fact, absolutely all domed cameras share information they gather with absolutely *all* domed projectors in the fabric. This is because the direction connecting any camera with any projector could be the direction of a ray of light hitting the object towards a possible viewpoint exactly on the other side of it in that same direction. I explained this in an earlier post, but my last post suggested the information sharing happened between each camera and just subsets of the projectors; not the case. It would need to happen from all cameras with absolutely all projectors. What does happen is that each camera doesn't send all info they get to all projectors, just exactly the info associated to that virtual direction connecting them in space.

That implies something we haven't mentioned: each nano-device on that fabric has absolutely perfect spatial location knowledge of all other devices on the fabric over the 3-dimensional space. This is required for invisibility to work regardless of the shape of the covered object, or when that shape changes over time.

 

[drzeus99]

I strongly disagree. As I look out my window I see a small bush 1 yard from where I am sitting and a large tree 60 yards away. If I focus on the small bush, the large tree becomes out of focus. If I focus on the tree, the small bush becomes out of focus. .

Here's 2 ways to look at it.

Watch a movie (like I'm watching now).
I can see a person up-close on the screen, a tree 20yrds away, and mountains that look 50 yards away. If I look at the up close person, are the views behind it blurred? Nope, not when I look at them. It's an infinity shot.

The way this works (not perfected yet) is to make the image appear as if you were looking through a window. The window shows behind it what is outside. The objects don't need manipulating if you can't see them clearly, your vision needs to focus.

With this, it would be like looking through a window. You would focus your sight on what you choose. It wouldn't take a set snapshot, but rather keep a continuous view of everything behind it, like presenting a clear window to the viewer from either side.

If a projector can give you a liefelike look, as if the image were mearly a window to what was behind it, then the viewer would focus to view the image they wanted. The image doesn't change, just the viewer's focus.


Cheers,
DrZ

 

[Dan O.]

The focus effect is nothing more than the divergence of the light rays that strike either side of the lens in your eye. The rays from a close object are diverging faster than the rays from a far object. If the object is at infinity, the rays are parallel. To a limited extent, an invisibility cloak can reproduce this effect and therefore produce a proper 3D image. For this to happen, each camera/projector "pixel" would have to be significantly smaller than the lens in your eye or about the size of the pixels on the screen of your monitor and each of these "pixels" would be projecting the full 180º image as would be seen from that point. The scale of the image generating element required within these "pixels" is much smaller than the wavelength of light so this cannot be built using current technologies.

On a larger scale of making more distance objects (say 100 meters away) invisible where everything already appears to be close to or at infinity, the focusing issue is not a problem. The diffraction limit of the human eye for visible light is about 5cm at that distance so we wouldn't need to make the "pixels" any smaller than that and could probably get by with much larger.