Carlo Rovelli Quotes

[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.

[I]n a fundamental description of nature we must "forget time"... this can be done in the classical and in the quantum theory.

When two friends meet... after one has lived in the mountains and the other at sea level, the watches... will show different times. ...Neither is truer than the other. ...Times are legion: a different one for every point in space. ...there is a vast multitude of them. ...Every clock has its proper time. ...Einstein has shown how to calculate the difference... The world is not like a platoon advancing at the pace of a single commander. It is a network of events affecting each other. ...Physics does not describe how things evolve "in time" but how things evolve in their own times, and how "times" evolve relative to each other.
Time has lost... its unity.

But it isn’t absence that causes sorrow. It is affection and love. Without affection, without love, such absences would cause us no pain. For this reason, even the pain caused by absence is, in the end, something good and even beautiful, because it feeds on that which gives meaning to life.

What is then the physical content of a solution of Einstein's 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.

... theoretical physics is less clean than the way it's usually solved. You can always change parameters and save yourself. ... it's very rare that theories are ruled out by just an experiment or a group of experiments. Theories usually come with flexibility. Theoreticians can add flexibility. And so new experiments — you can just patch up your theory.

In the elementary equations of the world, the arrow of time appears only where there is heat. The link between time and heat is... fundamental: every time a difference is manifested between the past and the future, heat is involved.

I am aware that ...[this] answer ...is only one among many possibilities. Other authors have argued that the notion of time is irreducible... Until our theoretical and experimental investigations tell us otherwise... what is important is to put the alternatives clearly on the table...

Don't trust your teachers.

The way evolution is treated in general relativity, is... subtle... Change is not described as evolution of physical variables as a function of a preferred independent observable time variable. Instead, it is described in terms of a functional relation among equal footing variables... as... (T₁,T₂)... In general relativity, there isn’t a preferred and observable quantity that plays the role of independent parameter of the evolution... General relativity describes the relative evolution of observable quantities, not the evolution of quantities as functions of a preferred one. ...[w]ith general relativity we have understood that the Newtonian "big clock" ticking away the "true universal time" is not there.

I think that physics is about escaping the prison of the received thoughts and searching for novel ways of thinking the world, about trying to clear a bit the misty lake of insubstantial dreams, which reflect reality like the lake reflects the mountains.

Intuitively (and imprecisely) speaking, time "flows", we can never "go back in time", we remember the past but not the future, and so on. Where do all these very peculiar features of the time variable come from? ...[T]hese features... emerge at the thermodynamical level. ...[T]hese are all features that emerge when we give an approximate statistical description of a system with a large number of .

While non-relativistic time is the observable quantity measured (or approximated) by physical clocks, in general relativity clocks measure s along their worldline, not t. The relativistic... t is a freely chosen label with no direct physical interpretation. ...The physical content of a solution of Einstein's equations is not in its dependence on t, but ...in what remains once the dependence on t (and x) has been factored away.

What makes the difference is the mass. ...[T]he earth is a big mass and it slows down time. If you go to a bigger mass, like Jupiter, it's stronger. If you go near a big star it's stronger. If you're near a black hole... it's even more strong, so strong that if you go very near... time essentially stops down. It goes very, very, very slowly.