I have been fascinated by astrophysics for a while. When I was younger, I used to read Stephen Hawking’s books as soon as they hit the stores. There is something fascinating in imagining the structure of the universe, defying common sense perceptions—time and space are relative, for instance. There is something incredible as well in the fact that us, limited just slightly intelligent apes, are leaving aside many of the tools given to us by evolution in order to improve our knowledge. And in this setting, Carlo Rovelli, an Italian theoretical physicist, has written a wonderful book that goes from the evolution of the way we think about the universe to his particular stance on how to reconcile relativity and quantum physics.
The theory of relativity, developed by Albert Einstein, shows, among other things, that mass curves space-time. If I were to travel close to a black hole, and then come back, I would find that time on Earth has happened much faster than for me. When Matthew McConaughey and Anne Hathaway come back from the first planet, which is near Gargantua—a black hole—the other passenger is much older than when they descended.
Quantum physics is much harder to explain. As Richard Feynman put it: “I think I can safely say that nobody understands quantum mechanics.” Particles are no longer thought of as small balls in space; they are now defined as a set of probabilities and possible (discrete) positions—each position with an attached amount of energy. While intuitively hard, quantum mechanics has been so successful that it is the leading (by far) theory on particle physics.
In most cases, these two theories—the most important developments in the field in the XXth century—do not overlap. Inside an atom, particles barely have any mass to “curve” space-time. And massive objects like planets and stars, where relativity matters, are too big to “suffer” the quantum effects. Yet there are situations in which both theories should be important, such as the case of microscopic black holes or the big bang. Situations where mass is compressed in very small spaces. The problem, however, is that both theories, as they stand, are incompatible. And looking for ways to reconcile them has been a huge endeavour for decades now in the field.
The main working unifying theory has been string theory—the idea that particles can be thought of as strings, and that what makes them different from one another how they vibrate. Any attempt on my part to go into more detail is surely to get many things wrong, so I will not make it. Suffice to say that Rovelli—and others—have put forward an alternative theory, called quantum loop gravity. And more or less the second half of the book is devoted to it.
What is quantum loop gravity? Well, I am going to give my understanding of it. In quantum mechanics it is understood now that the forces that operate in it—electromagnetism, for instance—are the result of the interaction of fields—electromagnetic fields in this case. According to quantum loop theory, we should understand space (and time) in the same way: they are fields and the way they interact at subatomic level is what generates gravity. Sounds simple (and convincing) enough to me, but, you know, Dunning-Kruger effect.
But there are some reasons to support this theory versus string theory. First, many theoretical physicists have been working on string theory for decades without a huge success. (A remark here: I am not making any claim about the value of their contribution—with hindsight is very easy to say what works and what does not.) Not only that, but the experiments in the LHC at CERN have failed to generate (so far) some particles theorised from some versions of string theory that require super-symmetry. And quantum loop gravity does not treat gravity as something different: it brings the current state of quantum physics—quantum field theory—into space.
What does that mean? Well, among other things, space is not continuous but discreet. The same way a particle can only take some states (quanta), space can only take some forms. This happens in a very small scale, of course; this scale is called Planck’s distance. We cannot divide this distance in half; that’s it, that is a building block of the universe. This is also true for time. Planck’s time, which is just the time it takes a photon at speed light to cover Planck’s distance, is the building block of time. It cannot be further divided. Space and time as we know and experience them do not exist at that level.
This is extremely fascinating. Carlo Rovelli does a good job explaining the intuition of these theories, but it is slightly technical in some parts. I appreciate that, but readers less versed in physics might not. But it is without a doubt the best book on the topic I have read in ages. Rovelli has another book, more basic, titled Seven brief lessons in physics, that might be a better starting point for many. A special mention should be given to the way he links these ideas to pre-Socratic Greek thinkers. The first half of the book, less technical and focused on describing the evolution of the thinking about the universe, is simply outstanding.