I recently came across a public lecture "Dark matter decay?" by Sir Roger Penrose. In his lecture he states that the two equations $E=mc^2$ and $E=h\nu$ can be combined to form a formula for the frequency $$\nu=mc^2/h$$ He also states that "massive particles are clocks". I didn't quite understand this statement. I understand the derivation but why does this equation show that "massive particles are clocks". What is the concept behind this statement?
Suppose you annihilate an electron and an antielectron, producing two gamma rays. Your equation then gives the frequency of the gamma rays. This is a sort of universal time unit. If we get in contact with extraterrestrials through SETI, one way of correlating our time units would be this.
On the other hand, suppose we lived in a universe without massive particles, only massless ones such as gluons, photons, and gravitons. Then there would be no numbers or standards to start with in defining such an absolute scale of time. The technical terminology is that under these circumstances, the laws of physics are conformally invariant. This is a universe in which time becomes kind of a timey-wimey thing that lacks any absolute meaning.
Penrose had a failed theory based on this called CCC.