Reality Is Not What It Seems: The Journey to Quantum Gravity

by Carlo Rovelli

It’s a fine story but, on reflection, it leaves us perplexed – and this perplexity goes to the heart of the problem of time. How could Galileo know that his own individual pulse-beats all lasted for the same amount of time?fn41 Not many years after Galileo, doctors began to measure their patients’ pulses by using a watch – which is nothing, after all, but a pendulum. So we use the beats to assure ourselves that the pendulum is regular, and then the pendulum to ascertain the regularity of the pulse-beats. Is this not somewhat circular? What does it mean? It means that we, in reality, never measure time itself; we always measure the physical variables A, B, C … (oscillations, beats, and many other things) and compare one variable with another, that is to say, we measure the functions A(B), B(C), C(A), and so on. We can count how many beats for each oscillation; how many oscillations for every tick of my stopwatch; how many ticks of my stopwatch between intervals of the clock on the bell-tower …