<Radio did not exist on this planet a hundred years ago, and might become obsolete.

Oh?

Unless we discover something akin to "subspace frequencies" a la Star Trek - whatever they're supposed to be - all we have is the electromagnetic spectrum. Using some kind of modulation of either frequency or amplitude, or both, of a "carrier wave" to encode information is, equally, all we have. Fundamentally, television is radio; merely a particularly-defined encoding/decoding method thereof. Without too much of a stretch, one could also include old-fashioned telegraphy through copper wire, and optical fiber communication as well, within a very broad definition of "radio".

I wonder, though... even if we discover or invent something that will make radio obsolete for communicating, we will still have radio telescopes for the same purpose they serve now: to study objects in space at certain frequencies. We will, if we continue to listen closely, continue to be able to detect ETIs "younger" or "less advanced". One can easily postulate "more advanced" ETIs purposely retaining the same capacity and detecting our signals, even if radio communication is to them what heliographs are to us.

E'