"The antennas then send out a signal that cancels out the reflected signal -- effectively cloaking the object"...if the reflected signals are cancelled, the object won't be visible, but it would leave a dark spot in the shape of the object isn't it?...how would the background appear?
@DrQuine, I agree with your comment! I could think of at least one way in addition to the known absorbsion method emplyed by current stealth technology (for radar anyway). The noise canceling method is both workable in theory and possible in practice but I would think that it would require significant horsepower to properly create the needed canceling signal. On the other hand, I had wondered why planes did not employ some sort of video masking through cameras and body sized LCD screens. If you can't see the plane with radar/visually then it is a lot harder to track it. Then again, heat seeking / IR sensors would still work..but would the ground crews know in time to employ them, that is the big question.
I should have pointed this out. Unless your optical source is a laser beam, an added complication in optical cancellation will be the incoherent nature of most artificial and natural light sources. Pretty hard to create a reflection that is 180 degrees out of phase with an incoherent source of light. Irt would take a whole lot of differtent cancellation beams at every frequency, in other words.
A simple optical analog of this radar cancelllation would be optical cancellation, in a blacked out environment, of a flashlight beam shined on the object. If one electronically transmits light that is 180 degrees out of phase with the natural reflections of the flashlight beam, then the object will appear just as black as the rest of the environment. That's the simplest case, and in principle, seems like it should work.
The difference people are assuming between optical and radar is only that a radar receiver is capable of focusing only on reflections from its own transmitteer beam, filtereing out any ambient radar frequency energy there may be around. So in fact, everything else always looks "black" to radar.
So the only disconnect here is in the initial assumptions. An optical cloaking device that works, say, to hide your car, at night, from the headlights of another car, should be feasible in principle.
On the other hand, making an abject appear transparent, by repeating what is "behind it" to the other side, is an entirely different mechanism, not in the least similar to this radar cloaking technique.
This technique seems like a similar, but alternate, application of MIMO. Instead of trying to capture reflections and use the to improve data bandwidth, this technique tries to cancel you reflections. Kind of like beamforming used in MIMO to enhance signal strength, this cancelt the signal out.
I think that the basic issue here is the use of the term "cancelling" utilized in the article.
Is it more accurately described as you're taking input and then transmitting it to the other side? Even if you were to be doing this, you would also need to do some level of cancelling to handle the reflections from the object.
So you still need to do the two functions of cancelling the object's reflections and retransmitting it to the other side.
I think the cancelling portion can be done pretty well and pretty easily, but the transmitting portion is going to depend on a lot more things (every antenna's input will have to be considered for every other antenna's output) and is quite a bit harder. Theoretically, it's a very similar function so I can agree that it's theoretically possible, but practicality of doing it is much more difficult. This is similar to the point that Nicholas was making of the difference in making an object "black" versus making it "invisible." Both can be done with the same device theoretically, but the difficulty level is drastically more difficult for the latter.
A result that would be more convincing in terms of invisibility is not to say that something disappears from radar (which can be done by various other means like just absorbing the frequencies that you're being scanned at), but to say that when you fire a radar pulse at a suitably large object that would normally block it, a detector on the other side can still detect the pulse as though the object is not there. If that's what was actually done, it doesn't really convey that in the article.
The other issue is that this is a classic scaling problem where to scale to the visible region, you would need to increase the number of "antenna's" (at least a n^2 relationship as it's surface area dependent) and operate at higher frequency (another order of n), plus take into account input from all the other nodes when calculating what to transmit (another n^2 or maybe even worse relationship). Scaling from radar to visible would seem quite a challenge computationally just as much as having suitable "antenna's".
This is a very interesting area of research for many industrial and military applications, but the specific question of whether this approach is really something that is extendable to visible light cloaking in the Harry Potter sense is a bit of a stretch still for me.
I've followed these topics a bit as I was doing metamaterial research for grad school, but as yet, I'm not convinced that there is any real solution that has any chance of succeeding in the visible spectrum. Fun stuff though and good luck in the research.
I think perhaps my comment has been misunderstood, and /or possibly the article fails to convey the process with sufficient technical detail.
I never suggested that the total field would or could be cancelled. Nor that anything "around" the object would be black. That would be impossible. I merely suggested that the object itself would "appear" to be non-reflective (i.e. radar-black), rather than radar-transparent.
In order to be truly "transparent" an object must receive signals from one side of itself and pass them through (or around) itself and (re)transmit them out of the other side. A "transparent" object, by definition, is also non-reflective.
My understanding from the article was that a signal from the radar source would be detected by the equipment, and a phased-array signal transmitted in anti-phase would be sent back towards the radar source, thereby cancelling any apparent (scattered) radar reflection from the equiment. By creating an RF "null" at the radar source, the equipment would appear to have not reflected the original radar pulse and would therefore appear "radio black", like the empty sky behind it. However, an object which does not reflect a signal looks "black", it is does not look "transparent".
If this basic understanding of the article is wrong then my subsequent analysis is naturally also invalid.
Either you are cancelling an incoming waveform with an signal that phase cancels it in only in one direction (where the "null" occurs), or you are not doing cancellation, you are being additive. Can we be clear on which it is?
Professor Eleftheriades's sytem is both innovative and militarily useful, but I fail to see how even theoretically it can make an object "transparent", rather than simply "non-reflective". Being radar "non-reflective" is still incredibly useful for an aircraft or military vehicle.
Perhaps he could elaborate with a clearer explanation on how transparency is achieved (at either radar or visible wavelengths).
I have a hunch that these cloaks will have the flaw that they work in one wavelength and fail in another. An airplane made of radar absorbing material will be invisible to radar but block the sky behind it to a camera. An object with light pipes or a camera and display system may be visually "transparent" but will pose a significant target to a radar. These systems may also become vulnerable when viewed from multiple angles.
I still find it amusing that after years of thinking that invisibility cloaks were impossible, as soon as I heard two years ago that they'd been made, I could think of 2 ways (both of which are now being implemented) of doing it without reading the article. How many other things are there that we can't do only because we're convinced it isn't possible?