Three Wild Concepts of Time
Different efforts to unify quantum mechanics and relativity have led to vastly different concepts of the meaning of time. Here are three fun ones to think about...
1. Time Is a Dimension on Par with Space (the Block Universe Meets Quantum Mechanics)
In the early 1900s, Albert Einstein and Hermann Minkowski gave us relativity theory and spacetime. Time became a dimension on par with space. A ball can move forward and backward in time, like it can move there and back in space. We also received the relativity of simultaneity. Observers moving at different speeds find different sets of events to be simultaneous. For two events that I find to exist separated in time, there can be a moving observer who finds the same events to exist simultaneously. If existing is a transitive property, then from an outside perspective all events in time can be considered to coexist.
This is the block universe: every event in the lifetime of the universe is where it was / is / will be in space and when it was/is/will be in time. The block universe perspective depends on the laws of nature being deterministic and time-reversible. Does quantum mechanics ruin the block universe?
According to quantum mechanics, an electron in an atom is like a cloud around the nucleus. We can only know the probability of finding the electron, upon measurement, within any defined volume of space in the cloud. And the measurement process is apparently irreversible. Bad news for the block universe. But, before measurement, the wave function (i.e. the cloud) describing the electron evolves deterministically and reversibly over time, as governed by Schrödinger's equation. One way to save the block universe is to declare the wave function itself to be what is real. The physicist Sean Carroll explains this in his 2010 book about time, From Eternity to Here.
When you measure the electron's location, its wave function—which is the electron in a superposition of possible locations with particular likelihoods—does not collapse irreversibly. Instead, your wave function (yes, you are a wave function, too) becomes entangled with the electron's wave function, forming a combined wave function. And the superposition is even bigger than this: the electron, you, and everything else are part of one wave function of the universe. The "you" reading this now is merely one possible outcome of you within this grand wave function and its many worlds.
If we live in a block universe, why does time appear to pass in the direction it does? Answer: the second law of thermodynamics and the low entropy big bang.
The energy in the universe tends to change from useful to useless. Entropy is a measure of how useless energy is. Making memories generates useless energy (heat). The low entropy condition at one end of time (the big bang), the tendency for entropy to increase, and our creation of memories in the direction of increasing entropy all establish what we perceive to be the direction of time.
2. Time Emerges from Change
Aristotle believed that time depends on change. His belief holds up in the quantum gravity equation developed by John Wheeler and Bryce DeWitt in 1967. The physicist Carlo Rovelli is working on a modern version: loop quantum gravity. He describes an amazing aspect of the theory in his 2018 book, The Order of Time: there is no time variable in the fundamental equations. Instead, the theory's equations describe "how things happen in the world in relation to each other." Change is fundamental, and from change, time emerges.
Change that follows a pattern allows us to keep time. This idea is familiar to us. The Earth orbits the Sun; we define a year. The Earth rotates about its axis; we define a day. We divide a day into hours, an hour into minutes, a minute into seconds.
3. Time Is Fundamental
The physicist Lee Smolin comes to a different conclusion when contemplating the laws of nature and the cosmos. Time does not emerge from something more fundamental. Smolin believes, as he expresses in his 2013 book, Time Reborn, that "Time will turn out to be the only aspect of our everyday experience that is fundamental." The conclusion does not follow from a theory the way time is a dimension on par with space follows from relativity and time emerges from change follows from loop quantum gravity. Time does not follow from anything. Time is bedrock.
If time is the only thing that is fundamental, then the laws of nature are not fundamental. They evolve in time. Smolin's theory for how laws evolve is called cosmological natural selection. The parameters of the Standard Model of physics - the masses of particles and the strengths of forces - can change a little each time a new universe is born. The hypothesis is that new universes are born inside black holes. A black hole forms when a massive star collapses due to gravity. The collapsing star explodes to form a new universe (which we cannot see from outside the black hole's event horizon). Universes with parameters that result in the creation of copious black holes outproduce other universes.
Is this not wild?