Twenty years ago, Stephen Hawking tried to explain time by understanding th
e Big Bang. Now, Sean Carroll says we need to be more ambitious. One of the leading theoretical physicists of his generation, Carroll delivers a dazzling and paradigm-shifting theory of time’s arrow that embraces subjects from entropy to quantum mechanics to time travel to information theory and the meaning of life.
From Eternity to Here is no less than the next step toward understanding how we came to exist, and a fantastically approachable read that will appeal to a broad audience of armchair physicists, and anyone who ponders the nature of our world.
The arrow of time
The most mysterious thing about time is that it has a direction: the past is different from the future. That’s the arrow of time. The reason why time has a direction is because the universe is full of irreversible processes—things that happen in one direction of time, but never the other. You can turn an egg into an omelette, as the classic example goes, but you can’t turn an omelette into an egg.
There are more ways to be disorderly than to be orderly, so an orderly arrangement will naturally tend toward increasing disorder: Entropy.
So if everything in the universe evolves toward increasing disorder, it must have started out in an exquisitely ordered arrangement. The arrow of time is the reason why time seems to flow around us or, why we seem to move through time. It’s why we remember the past, but not the future. It’s why we evolve and metabolize and eventually die. It’s why we believe in cause and effect, and is crucial to our notions of free will. And it’s all because of the Big Bang.
The mystery of the arrow of time comes down to this: why were conditions in the early universe set up in a very particular way, in a configuration of low entropy that enabled all of the interesting and irreversible process to come? Unfortunately, nobody knows the right answer but we have the tools to tackle the question in a serious way.
The past is present memory:
1. Time labels moments in the Universe
Imagine a movie in which every single transition between two frames was a cut to a completely different scene. When shown through a projector, it would be incomprehensible—on the screen it would look like random static. Presumably there is some French avant-garde film that has already used this technique. The real universe is not an avant-garde film.
If you want to meet someone, it’s not sufficient just to specify a time, you also need to specify a place. Physicists say that space is three-dimensional. Three numbers that are the coordinates of that location in space. What we call the universe is just the set of all events—every point in space at every moment of time. So we need four numbers—three coordinates of space, and one of time—to uniquely pick out an event. That’s why we say that the universe is four-dimensional: every point in space at every moment of time, as a single entity called spacetime.
Time serves to slice up the four-dimensional universe into copies of space at each moment of time—the whole universe at 10am in Jan 20, 2010 and so on. There are an infinite number of such slices, together making up the universe.
2. Time measures the duration elapsed between events
Time not only labels and orders different moments, it also measures the distance between them.
The primary criterion is that a good clock should be consistent. There are some objects in the universe that are consistently periodic—they do the same thing over and over again.
Think of planets in the Solar System. The Earth orbits around the Sun, returning to the same position relative to the distant stars once every year. But Mars, as it turns out, returns to the same position once every 1.88 years. And Venus moves around the Sun 1.63 times every time Earth orbits just once.
The Earth spins on its axis, and it’s going to do so 365.25 times every time the Earth moves around the Sun. The tiny crystal in a quartz watch vibrates 2,831,155,200 times every time the Earth spins on its axis. The reason why quartz watches are reliable is that quartz crytal has extremely regular vibrations.
As human beings we feel the passage of time. That’s because there are periodic processes occurring within our own metabolism—breaths, heartbeats, electrical pulses, digestion, rhythms of the central nervous system. We are complicated, interconnected collection of clocks. Our internal rythms are not as reliable as a pendulum or a quartz crystal, they can be affected by external conditions or our emotional states, leading to the impression that time is passing more quickly or more slowly. But the truly reliable clocks ticking away inside our bodies—vibrating molecules, individual chemical reactions—aren’t moving any faster or slower than usual.
The most obvious, blatant, unmistakable difference between time and space is that time has a direction, and space doesn’t. Time points from the past toward the future, while all directions of space are created equal.
3. Time is a medium through which we move.
Nine times out of ten, a physicist will say something related to one of the first two notions above—time is a coordinate, or time is a measure of duration, whereas a non-physicist will say something related to the third aspect: time is something that flows from past to future.
As St. Augustine said in his Confessions, the present considering the past is memory, the present considering the present is immediate awareness, the present considering the future is expectation.
Complement From Eternity to Here with Why does time go forward?