Here, an idea to be wrestled with: what is time, and why do we experience it? 

Ms. Tippett: Right. So, you said somewhere — you wrote, “The passage of time is internal to the world, is born in the world itself.” So, here’s one very intriguing thing you say again as a physicist. To the question of what explains it for us, time seems to pass or to flow. And you say you believe “this is connected to the connection between time and heat,” that the difference between past and future exists only when there is heat. That is such a baffling and fascinating idea. I mean, can you explain that a little bit?

Dr. Rovelli: Oh, yes. Oh, this is something that curiously has not been said enough. And the known physicists don’t know it, but it’s not something new, and it’s something well established. In fact, since not the last century, but the previous century, every time we give a description of the world, of phenomena where there’s no heat, we cannot distinguish the past from the future. Every time there’s something that distinguishes the past from the future, there’s heat......

So, take a movie or something, and you run it backward. Imagine you take a movie of the Moon going around the Earth. You run it backward, and you see a Moon going around the Earth the other way. It’s completely consistent with the laws of physics, and there’s no heat there. But if you throw a pen on the table, and it stops, and you take a movie of that, if you run the movie backward, you see something totally absurd, a pen that starts moving from nothing. And in fact, when the pen stops, it heats the table because there’s friction, and there’s heat.

So only when there is some heat around, the phenomena are different in one direction of time from the other. So the direction of time is deeply connected to the existence of heat. That doesn’t explain the direction of time, but is the first step toward understanding it. The direction of time has to do with the presence of heat.

But, look, I don’t think that I, as a person, exist without the rest. I am my friends, my love, my enemies, my — everything that I interact with.

“To take away the breath.” Bella da togliere il fiato.

Carlo Rovelli is the author of Brief Lessons on Physics and Reality Is Not What It Seems: The Journey to Quantum Gravity. 
This excerpt is from an interview produced for the radio broadcast, "On Being", All Reality is Interaction, by Krista Tippet.  Available here: http://www.onbeing.org/programs/carlo-rovelli-all-reality-is-interaction/