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.
@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:)