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Recently I was reading about a technology that uses radio waves to stimulate neurons to fire. The radio waves have the advantage of being able to pass through the skull (hence being non-invasive) but they are very coarse. Would it be possible to target individual neurons with radio waves (for example by having a bunch of low-powered radio waves converge on a focal point only nanometers large without diffusing to surrounding tissue)?

http://sfari.org/news-and-opinion/toolbox/2012/radio-waves-turn-on-gene-expression

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My original answer is that it is not possible in a laboratory setting, the wavelengths are enormous,

The wavelength is the distance from one peak of the wave's electric field to the next, and is inversely proportional to the frequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters (983,571,056 ft) which is the wavelength of a 1 hertz radio signal. A 1 megahertz radio signal has a wavelength of 299.8 meters (984 ft).

In order to focus a wave one should be able to interact with the beam, refract it, in a coherent manner, create a lens. The size of the lens has to be commensurate to the wavelength, nanometers are not in the ball park. Look at the size of the antennas used for radio communications .

Nevertheless, all the above is for conventional materials and lens geometries.

Searching on the internet I found in MIT news :

Researchers at MIT have now fabricated a three-dimensional, lightweight metamaterial lens that focuses radio waves with extreme precision. The concave lens exhibits a property called negative refraction, bending electromagnetic waves — in this case, radio waves — in exactly the opposite sense from which a normal concave lens would work.

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To test the lens, the researchers placed the device between two radio antennae and measured the energy transmitted through it. Ehrenberg found that most of the energy was able to travel through the lens, with very little lost within the metamaterial — a significant improvement in energy efficiency when compared with past negative-refraction designs. The team also found that radio waves converged in front of the lens at a very specific point, creating a tight, focused beam.

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The device, which weighs less than a pound, may be used to focus radio waves precisely on molecules to create high-resolution images — images that are currently produced using bulky, heavy and expensive lenses. Ehrenberg says that such a lightweight device could also be mounted on satellites to image stars and other celestial bodies in space, “where you don’t want to bring up a hefty lens.”

So in principle the answer is yes from now on.

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    $\begingroup$ Thank you for this fine answer. However, I recently read this: physics.stackexchange.com/questions/1263/… I thought about it really hard (by imagining waves as ripples which is what they really are) and realized that the focal point can be arbitrarily small, much smaller than the wavelength. And at that exact point, the amplitude will vary with the correct frequency. Would you agree? That leaves me wondering exactly what problem the MIT "metamaterial" was supposed to solve, if we could already achieve high-resolution focusing with phase arrays. $\endgroup$ – pete Dec 29 '14 at 9:29
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    $\begingroup$ @pete - Materials with negative indices of refraction are very interesting and useful. The metamaterial mentioned in anna's post is apparently reducing the energy lost when a radio wave passes through the material (compared to previous efforts). The other advantage is that these materials need not be enormous in size/area to focus radio waves, compared with more traditional methods. I recall listening to a lecture while in grad school about these negative refraction materials and I was amazed by what they could do, though unfortunately my memory of the specifics is hazy. $\endgroup$ – honeste_vivere Jan 1 '15 at 15:19
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something like the microwave generator in the microwave oven can shoot out energy waves of different spectrum. and its is relatively focused like the laser beam. human brain wave of thoughts present in brain as small area patterns. mind control/manipulation and detection hence achieved.

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