Special Relativity. Lifetime.

[Alex K]

Not really. There is in principle no upper limit to frequency like there is for velocity, except that frequencies with periods shorter than Planck time are maybe impossible) but this limit does not derive from special relativity.

The physical concept frequency is related to more closely is actually energy. By virtue of the Schrödinger equation the two are basically the same.

The term frequency, seems to me, applicable to very different types of phenomenon. The frequency of a neuron is similar to the pulsation of a heart; they are individual entities with a limit to how fast pulsations can be produced. But the frequency of light seems different, it appears continuous or connected; it makes more sense to talk about "how many" as opposed to "how fast" when it comes to these waves. (I'm assuming an increase in the frequency of light corresponds to more waves fitting into the same space, not the same number traveling faster through that space).

So I think the lack of an upper limit, or its relatedness to energy, depends on what frequency is in reference to.

Hm, I don't know. The concept of frequency is really quite fundamental in physics and basically relates any physical process and the frequencies which occur in them, to energy states.
A frequency in the sense physics uses it refers to harmonic sine waves. Any other periodic change can in this sense be viewed as a combination of various harmonic oscillations with different frequencies on top of each other - google Fourier decomposition. The lowest of these frequencies determines the time it takes to complete a cycle, what one would colloquially call the frequency.