One of a series of quick posts on the six sections of my book The Big Picture — Cosmos, Understanding, Essence, Complexity, Thinking, Caring.
Chapters in Part One, Cosmos:
- 1. The Fundamental Nature of Reality
- 2. Poetic Naturalism
- 3. The World Moves By Itself
- 4. What Determines What Will Happen Next?
- 5. Reasons Why
- 6. Our Universe
- 7. Time’s Arrow
- 8. Memories and Causes
It wasn’t easy to settle on how to organize all the material in The Big Picture. Ultimately I decided to start with the universe — a bit of cosmology, yes, but also some basic features of how the universe physically operates. Features that played a crucial role in the transition from an ancient view of the world, which still lingers on in our informal “manifest image” of how things work, to our modern scientific view.
One important step in that transition was the seemingly-innocuous realization that momentum is conserved. In Aristotle’s physics, to keep something moving you had to keep pushing it. That’s a very sensible thing to believe, since it’s absolutely true in our everyday experience. It took many centuries of thinking by very smart people (including Persian polymath Ibn Sina, right) to realize that things tend to move by themselves, and are only slowed down by external forces such as friction. That’s important for physics, of course, but there is a deeper implication for our picture of what kinds of things we need to invoke to explain the universe. As I put it in the book:
Aristotle’s argument for an unmoved mover rests on his idea that motions require causes. Once we know about conservation of momentum, that idea loses its steam. We can quibble over the details — I have no doubt Aristotle would have been able to come up with an ingenious way of accounting for objects on frictionless surfaces moving at constant velocity. What matters is that the new physics of Galileo and his friends implied an entirely new ontology, a deep shift in how we thought about the nature of reality. “Causes” didn’t have the central role that they once did. The universe doesn’t need a push; it can just keep going.
It’s hard to over-emphasize the importance of this shift. Of course, even today, we talk about causes and effects all the time. But if you open the contemporary equivalent of Aristotle’s Physics — a textbook on quantum field theory, for example — words like that are nowhere to be found. We sometimes talk about causes, but they’re no longer part of our best fundamental ontology.
What we’re seeing is a manifestation of the layered nature of our descriptions of reality. At the deepest level we currently know about, the basic notions are things like “spacetime,” “quantum fields,” “equations of motions,” and “interactions.” No causes, whether material, formal, efficient, or final. But there are levels on top of that, where the vocabulary changes. Indeed, it’s possible to recover pieces of Aristotle’s physics quantitatively, as limits of Newtonian mechanics in an appropriate regime, where dissipation and friction are central. (Coffee cups do come to a stop, after all.) In the same way, it’s possible to understand why it’s so useful to refer to causes and effects in our everyday experience, even if they’re not present in the underlying equations. There are many different useful stories we have to tell about reality to get along in the world.
The idea that “cause and effect” isn’t fundamental to the workings of the universe hasn’t spread as widely as it should have, despite the efforts of smart people such as Bertrand Russell. In this first section of the book I sketch how we moved from a picture of the universe animated by causes and reasons to one that obeys patterns, without the need for anything to cause it or sustain it. Of course the idea of causality is still crucial to our everyday lives, so I talk a bit about how cause-and-effect relations are emergent phenomena in a macroscopic world with a pronounced arrow of time.