Read: Reality Is Not What It Seems
If you’re curious about the most current ideas of space and time from the perspective of physics, I’d recommend this book or its audiobook counterpart
It is only in interactions that nature draws the world.
Carlo Rovelli, Italian physicist and author, starts the journey (Part 1) with the concept of reality being ‘granular’ following ideas of 450 BCE philosophers in Miletus, Greece, and tails out (Part 4) with current (international) efforts to define reality combining relativity, quantum, and information theory.
It is written in poetic and accessible prose with a well outlined narrative. The heroes in this part of the story (in order of appearance) are Anaximander, Democritus, Leucippus, Plato, Socrates, Aristotle, Zeno, Lucretius, Pythagoras, Ptolemy, moving on to the comparatively contemporary Copernicus, Kepler, Galileo, Dante, Newton, Faraday, and Maxwell.
Part 2 is a great chapter for those seeking to understand the key principles of quantum field theory — granularity, indeterminacy, and relationality. The key figures in this part of the story are Einstein, Hilbert, Bohr, Heisenberg, Dirac, and Feynman.
The world of quantum mechanics is not a world of objects: it is a world of events.
Granularity refers to the idea that reality is granular. For example, the manifestation of particles from their corresponding quantum fields: photons from the electromagnetic field, electrons from the electric field, and so on. Indeterminacy refers to the idea that we cannot predict things with certainty and that the future does not unequivocally depend on the past (in time) and is ultimately statistical (the whole idea of probability in quantum mechanics). And relationality states that all events of nature are always interactions and occur in relation to each other.
Perhaps because we must not confuse what we know about a system with the absolute state of the same system. What we know is something concerning the relation between the system and ourselves. Knowledge is intrinsically relational.
Part 3 is the juice of the book: quantum space and relational time. Enter Leavitt (finally, one female!), Lemaître, Bronštejn, Wheeler, DeWitt, Hawking, Bianchi, and Boltzmann. This part of the book presented many key learnings on what really is space per quantum loop theory:
The gravitational field is not diffused through space; the gravitational field is that space itself…and the quanta of the gravitational field are quanta of space: the granular constituents of space.
That is to say, the same way that light co-emerges when matter/energy interact with the electromagnetic field, space co-emerges when matter/energy interact with the gravitational field.
As we abandon the idea of space as an inert container, similarly, we must abandon the idea of time as an inert flow along which reality unfurls.
Einstein showed us that time does not pass in the same way everywhere in the world through his theory of general relativity: the closer you get to the Earth, where gravity is more intense, time passes more slowly. But the equation that combines, mathematically, the ideas of quantum mechanics and general relativity to describe a quantum field (the Wheeler-DeWitt equation) does not feature time as a variable. That is the equation used to compute the evolution of something in time does not consider time as a variable but considers interactions between events.
The quanta of gravity do not evolve in time. Time just counts their interactions.
To explain further, the absence of the time variable from the fundamental equation does not imply that things don’t change, but that change is everywhere, and on a micro scale ‘time’ has no preferred direction. This concept is analogous to jazz musicians who keep time relative to each other.
The passing of time is intrinsic to the world, it is born of the world itself, out of the relations between quantum events which are the world and which themselves generate their own time.
In part 4, to bring the concept of time home, Rovelli gives the analogy of time to temperature and even relates time to temperature. For those unfamiliar with the definition of temperature, it is not a physical quantity itself but it is a measure of the average velocity of particles in a system. Our common day perception of hot and cold has more to do with relationality. Derek explains it brilliantly in this video:
So if temperature (or our notion of heat) is a measure of the average velocity of particles in a system, then time is the measure of the average relationality of a system—keeping in count which events are reversible and which are irreversible. That is irreversible processes eventually govern the arrow of time giving it its forward direction.
If you think about it, all phenomena where we detect the passage of time are co-involved with temperature. Time is not a fundamental constituent of the world, but it appears because the world is immense, and we are small systems within the world, interacting only with macroscopic variables that average among innumerable small, microscopic variables. We, in our everyday lives, never see a single elementary particle, or a single quantum of space. We see stones, mountains, the faces of our friends — and each of these things we see is formed by myriads of elementary components. We are always correlated with averages. Averages behave like averages: they disperse heat and, intrinsically, generate time.
Bravo Rovelli! The final bit before the conclusion goes further into the statistics of averages and connects to the information theory. Rovelli has a lot to offer in the book by way of example and explanation and I leave you with a picture from the preface on the nature of reality:
Quantum fields draw space, time, matter and light, exchanging information between one event and another. Reality is a network of granular events; the dynamic which connects them is probabilistic; between one event and another, space, time, matter and energy melt in a cloud of probability.