Carlo Rovelli Quotes

Whatever the statistical state ρ is, there exists always a variable t_ρ, measured by the thermal clock, with respect to which the system is in equilibrium and physics is the same as in the conventional nonrelativistic statistical case!
This observation leads us to the following hypothesis.

I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role at all.

In his youth Albert Einstein spent a year loafing aim- lessly. You don’t get anywhere by not ‘wasting’ time – something, unfortunately, which the parents of teenagers tend frequently to forget.

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.

The thermal time hypothesis. In nature, there is no preferred physical time variable t. There are no equilibrium states ρ₀ preferred a priori. Rather, all variables are equivalent; we can find the system in an arbitrary state ρ; if the system is in a state ρ, then a preferred variable is singled out by the state of the system. This variable is what we call time.

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...

Children grow up and discover that the world is not as it seemed within the four walls of their homes. Humankind as a whole does the same.

This means that the time between [points on a line] is 24 hours here but... more if you're... higher, and less if you're less [high]. ...Now it's measurable to 30-40 cm [difference in] altitude in the lab.

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

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.

[I]n pre-relativistic mechanics, time is a special physical quantity, whose value is measured by physical clocks, that plays the role of the independent variable of physical evolution.

The difference between past and future... cause and effect... memory and hope... regret and intention... in the elementary laws that describe the mechanisms of the world, there is no such difference.

[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.

... 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.

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.