A purely theoretical question: if a wire carries an alternating current, it emits electromagnetical emissions in the form of radio photons, but, as far as I know, only radio waves can be produced this way; what would happen if the current frequency was higher and exceeded the radio spectrum to enter higher frequencies (IR, visible, UV, X and γ)? Would there still be photon emissions in their spectra, or would the energy emissions be limited to an alternative magnetic or electric field without photonic carrier? If so, what would be be the field carrier?
If you were able to create a wire that would carry an alternating current with a frequency of hundreds of Terahertz it would indeed emit visible light.
In fact it works so well that almost all electrical energy would be converted directly into light. It is thus very hard to keep a current of these frequencies in your wire. The only way to create such a current would be to create an alternating electric field of the same frequency. This can easily be done as it corresponds to shining light on the metal. for a very short time there will be an alternating current in the wire, however it will immediately be emitted again. Something which we observe as reflection. For some materials you might be able to form surface plasmons, which survive slightly longer.
If you go to even higher frequencies, the wavelength of the oscillations will reduce to the interatomic distance and the simple image of a wire as a conductor breaks down and you have to realize that the wire actually consists of electrons interacting with nuclei and with themselves. In the UV range you will start to kick out electrons out of the material.
If you go to even shorter wavelengths, the nuclei can no longer be seen as stable objects and you will have to take the behavior of the protons and neutrons into account.
The conduction electrons in a metal behave surprisingly like a free electron gas. Indeed this description is unsurprisingly known as the free electron model (and while we're talking about boring names the slightly more accurate version of this is known as the nearly free electron model).
Anyhow this gas has a natural oscillation frequency known as the plasma frequency, which is around the frequency of visible light for most metals. If you try to make the electrons oscillate faster than this they can't keep up so they won't oscillate. As you increase the frequency of your applied voltage you'll find that the EM emitted falls as you approach the plasma frequency and is effectively zero above it.
So applying frequencies in the UV, X-ray and gamma ranges will produce no EM emission from the metal.