Maybe a dumb question. Is freq/amplitude/pm/etc. possible in free space without the signals being on a say pc board. In other words if two signals crossed in free space would you get a third signal that is modulated by the two frequencies. I swear its not homework. This question has bothered me for years, just couldn't think of a good way of testing it out.

Thanks, for any answer that makes fun of or helps me understand

 

In general, no. In true free space (vacuum), the answer may even be categorically no (I'm not willing to go out on a limb like that off the top of my head). In order to get modulation, you need some kind of a nonlinear medium or process. For EM signals intersecting in a material (gas or glass or something else), if the intensities are high enough it could drive the medium into nonlinear behavior and you would get modulation. There are devices, particularly in the laser world, that rely on this mode of operation.

 

From your answer, I suppose you need extremely high energies of EM waves for them to react in a vacumn. Searching this further, Gamma rays show this behavior.

Thank you for the reply.

 

Although radio waves can never mix in a vacuum regardless of how much power one has (this is the Superposition principle), the CAN mix in the ionosphere. This is the whole basis of ionospheric modification facilities such as HAARP, and the now defunct Hipas Observatory, where I worked for years. The non-linearity of the ionosphere was first recognized as the "Luxembourg Effect" and is a fascinating branch of plasma physics.

Eric

 

At the frequencies we normally think as radio vacuum is linear but at the much higher wavelengths and energies seen at the gamma ray scale vacuum becomes non-linear and EM "mixing' happens in bulk space.

http://en.wikipedia.org/wiki/Schwinger_limit
http://en.wikipedia.org/wiki/Nonlinear_optics

 

Good article - shed a lot of light on my understanding of pair-production.