@Susan. Noise-cancelling headphones would never work. I can hear the little thing's bark when he's inside, I'm inside, and all windows in between are closed. The headphone are good for background noise, not these intermittent shrills of a bark.
I finally figured a few weeks ago how I could be such a fan of Star Trek and never take to Harry Potter, even for my kids' sake. It just didn't grab me. I realized that Star Trek's technology was something that sparked my imagination and it was something we could strive toward: space travel, tricorders, alien species, teleporting, communicators and on and on.
With Harry Potter, every situation or fight was resolved with a wave of a wand. How do you aspire toward that, or get inspired?
That said, the moral conundrums and ambiguities addressed in Star Trek had me fascinated too: who didn't want to be a starship captain with alien women fawning all over you, or say "fire phasors" to avoid being assimilated:) Harry Potter's broom and his 'something-leviosomo' didn't quite cut it:)
Having been a fan of both Star Trek and Harry Potter, I leanred from Star Trek how to analyze the events for what made sense and what didn't. There are plenty of instances on both that don't make sense or are never explained. One place where the HP story breaks down is right at the very end when Ron suddenly speaks Parsel Tongue. It comes out of nowhere and IMO is a cheap way out.
You can apply the same analysis to the Wizard of Oz books as well as the Wicked series.
I really need to read/see The Hunger Games Series.
Active radar cancellation, I like that! It makes sense and could have some very practical applications for weight reduction and accommodation of diverse surface material types.
And one of course cannot help but wonder: Assuming federal and state laws permit the required local radio emissions at special frequencies, anyone care to predict how long it will be before the first and most obvious consumer cloaking device of all hits the shelves: a vehicular radar-gun canceller?
The final line of this article makes no practical scientific sense "Theoretically, the approach should also work at visible wavelengths"
This is a signal cancellation system, so stop and think what that means in terms of visible light.
For mono-static radar, the source transmits a radar pulse at the target. This active target responds with a signal that cancels out the signal from the transmitter, but ONLY in the direction of the transmitter. This makes the target look "black", i.e. it doesn't seem to reflect the radar pulse signal. In the context of an aircraft against the background of an empty sky then this would indeed make it look invisible, but only becuase in radar terms it would be "black" against a "black" backround.
With visible light, we don't beam light from our eyes and observe the reflection, we look at an object illuminated with diffuse illumination from multiple sources. It is therefore virtually impossible to "noise cancel" that, even in just the direction of a single observer. Even if you could achieve that, the object would appear black, and would NOT be transparent, which is most people's definition of the word invisible. Also, becuase the observer isn't transmitting the illuminating light, the object to be made invisible wouldn't know the direction of the observer. In order to do signal cancellation, it is necessary to know the direction of the observer. Also, for signal cancellation you need to align the phase of the light so it cancels at the receiver. At visible frequencies, this would require knowing the distance to your eyes to sub-micron accuracy.
I agree with this statement for what can be done with the level of sophistication currently in this paper and that the whole "cancellation/distortion" of the source pulse is about as much as what is shown here (which has a lot of very interesting applications, so it's not to be trivialized).
In principal though, this "concept" can work. For instance, a simple analogy would be that if you had a thin opaque wall that you wanted to cloak. It is possible to put detector/emitter combo on the surface of the wall and directly emit whatever is detected on the other side and it will even be practically invisible on the either side (I think there was a demo of this where someone was wearing a set of camera's on his back and had a white screen that was in front of him that had a projected image of what was behind him on there... some university in Tokyo?). Phase matching is not an issue for practical applications as the eye is not a single event detector and is a continuous amplitude integrating sensor. If you were in a room lighted by a scanning coherent RGB lasers, or watching some display that utilized phase matching to create the image, that's a different matter, but in normal diffuse light situations, you don't need to match the phase, but would probably need it to calculate what would need to be projected on the other side.
However, in practice, I think this is beyond the processing capability to do on anything beyond a very thin sheet and for looking at things from relatively far away. The number of antenna's and the processing that would be necessary to do to translate input from one antenna to the output of the others real time in a more complex 3D object is huge and impractical as anything beyond a thin sheet would not be a direct translation and for visible spectrum, you would need emitter/detectors at the resolution limit of your closest target observer (which could be a spacing of every 500um of less if you wanted to cloak at something at arms length (calculate how many of these you would need to cover even a computer mouse...).
With that said, for psuedo stationary objects that needed portable cloaking from detection far away where a moderate time lag between what is projected and what is detected on the other side doesn't matter, I think it is possible. Think of setting up a military field station tent and wanting to reduce it's detection from binoculars... a "dome" like this would be ideal to make it very hard to detect from 500 yards away.
The idea propsed in the article is phase cancellation, (not a "camera on one side, +display on the other" type of invisibility.), so that is the only concept I am commenting on. I grant you that other solutions may indeed be viable.
I was specifically criticising the impossibility of the solution they proposed when it was glibly extended to visible wavelengths and a non-emitting observer.
To say "Phase matching is not an issue for practical applications as the eye is not a single event detector and is a continuous amplitude integrating sensor.", is not a correct statement in this case, as to phase cancel out light you need phase coherent cancellation and the non-phase sensitivity of the eye is irrevlevant to the physics of this.
Their idea is to have "destructive interference" of the signal at the receiver, and this requires phase coherency. This approach is easy for mono-static radar, and impractical for light.
You're actually correct in everything you said, and I was incomplete in what I wrote as the message was getting too long.
For an "invisibility cloak" you actually have to do both things, what you mentioned and what I mentioned. I took the leap of how to apply this concept to make an invisibility cloak, which is to detect what's coming in on one side, send a cancellation pulse backwards, AND send a propagating pulse forward as though your object didn't exist.
So, as you said, what's described in the article has all the issues you correctly identified. However, a logical extension of the same concept (calculate and project what is behind the object) gets around the whole "black on black" issue for the visible range.
Hope this clears things up a bit an sorry that my reply was a bit incomplete. I would consider this extension a part of the "concept" of utilizing detector/emitter combo to make something invisible, but definitely see that what's described in the article is not at this level of complexity.
For the backwards cancellation, the phase matching is absolutely necessary, but for the forward propagation, it's not unless you're trying to fool an actual sensor which can detect phases instead of the human eye, so once again, you are in the strictest sense correct.
@Nicholas.Lee I told Dr. Eleftheriades about your objection to a visible light version of his cloak--that cancelling visible light would just create a black target, rather than transparency as most people interpret "invisibility," and this is what he replied:
No I do not agree with this: What is cancelled out is not the total field surrounding the object to hide. Only the scattered field is cancelled. Hence you will not see anything black around the object. You will just see what light was there when the object was absent.
What is true though is that for light, the cloak cannot respond instantly. Short but finite time is needed (size_of_object/speed of light) for the information to flow behind the object. Hence momentarily the object will show up before it disappears. However, this is true for any electromagnetic cloaks, including the metamaterial based ones (where the situation is worse since they are thicker and light will need even more time to travel around it).
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 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.
Instead of using metamaterials, Eleftheriades's approach surrounds the object to be cloaked with tiny antennas tuned to the frequency band in which the cloaking is to occur, such as radar.
Now, that's interesting. Colin, you've been writing about this "invisibility cloak" topic for many years. It's such a tantalizing idea. We are all hoping that this will come to life within our life time!
I am just imagining a scenario when both the attacking planes and the target warships are cloaked. This could be a virtual war where no war-photographers will be able to show it live on their TV channels!
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?
@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.
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.
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.
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.
"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?