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sub-millimetre wavelengths, terrahertz waves
They're like microwaves but with even smaller wavelengths. So, instead of having frequencies in the gigahertz to tens of gigahertz, and wavelengths in the so-many centimetres range, they have frequencies in the terahertz range and wavelengths in the region of a millimetre. "Between microwaves and infra-red" is one way to describe it.
In order of frequency, radio is at the lower end, and in increasing frequency then there's microwaves, T-rays, infra-red, light, ultra-violet, and X-rays, in that order. In the past it's been difficult to generate T-Rays by any efficient practical method, but now there is news, as a breakthrough has been made.
Here are a few helpful links about this:
www.technologyreview.com/Nanotech/19910/?nlid=766 - new developments at Argonne Illinois
www.tgdaily.com/content/view/34999/118/ - Josephson Junctions
www.lbl.gov/Science-Articles/Archive/ALS-t-rays.html - using a large synchrotron
www.space.com/businesstechnology/technology/t-ray_camera_020613.html - includes pictures taken with a T-Ray camera
Note that T-rays are not X-rays, except in a colloquial sense, as in "to have X-ray eyes". T-rays are the opposite side of light in terms of frequency.
T-rays can not see through the body or through metal, but they can see through wood, plastic, etc.
I have seen websites that claim that the use of T-rays will put an end to some of the nonsense with airport security. However, my opinion is that airport security has very little to do with prevention of terrorism and has much more to do with being an excuse for authoritarian government to annoy people and get away with it. The idea you have got to take your shoes off to go through airport security has always been absurd. See antiterrorist nonsense
T-rays have medical uses, as well as communication. Also, there is considerable interest in T-ray astronomy. It's a natural sequel to radio astronomy and should allow stuff unseen in the universe to be revealed. Searching for submillimetre astronomy finds some interesting things which have become possible by improved technology.