While non-relativistic time is the observable quantity measured (or ap — Carlo Rovelli

"[I]t is the statistical state that determines which variable is physical time, and not any a priori hypothetical "flow" that drives the system to a preferred statistical state. When we say that a certain variable is “the time”, we are not making a statement concerning the fundamental mechanical structure of reality. Rather, we are making a statement about the statistical distribution we use to describe the... properties of the system... The "thermal time hypothesis" is the idea that what we call "time" is the thermal time of the statistical state in which the world happens to be, when described in terms of the macroscopic parameters we have chosen."

"The new coherent picture is not yet available. With all their immense empirical success, G(eneral)R(elativity) and Q(uantum)M(echanics) have left us with an understanding of the physical world which is unclear and badly fragmented. At the foundations of physics there is today confusion and incoherence."

"What is then the physical content of a solution of Einsteins equations..? ...[C]onsider what is actually measured in general relativistic experiments. ...Consider a clock at rest on ...Earth and a clock on a satellite in orbit around the Earth. ...Call T1 and T2 the readings... Each measures the along its own worldline, in the Earth gravitational field. ...the theory predicts the value of T2 that will be associated to each value of T1. Or vice versa."

"Quite remarkably, this was understood before having good clocks... It is one of great intuitions by Einstein. In 1915 he completed the theory of relativity... At the time clocks were not good enough... it was a theory, a good speculation... Today it is not a theory... Its a fact that we measure in the labaratory. ...Its so clear and definite that technology needs this to be taken into account."

"[T]ime is much more elastic than our intuition of this rigid passage... The time interval between one point and another... depends on where you are."

"The notion of time familiar to us may... be reconstructed in special physical situations, or within an approximation, as... the "surface of a liquid" disappears ...[at] the atomic level, or "temperature" ...makes sense only in certain physical situations and when there are enough degrees of freedom."