From Eternity to Here

by Sean Carroll

A major theme of this book is that the arrow of time exists because the universe evolves in a certain way.

of irreversible processes: something called entropy, which measures the “disorderliness”

everything in the universe evolves toward increasing disorder, it must have started out in an exquisitely ordered arrangement.

argue that the most sensible model for the multiverse is one in which entropy increases because entropy can always increase—there is no state of maximum entropy. As a bonus, the multiverse can be completely symmetric in time: From some moment in the middle where entropy is high, it evolves in the past and future to states where the entropy is even higher.

John Archibald Wheeler, an influential American physicist who coined the term black hole, was once asked how he would define “time.” After thinking for a while, he came up with this: “Time is Nature’s way of keeping everything from happening at once.”

Time isn’t just a label on each instance of the world; it provides a sequence that puts the different instances in order.

The history of an object (a cat, a planet, an electron) through time defines its world line—the trajectory the object takes through space as time passes.

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. (That’s 32,768 vibrations per second, 3,600 seconds in an hour, 24 hours in a day.5) The reason why quartz watches are reliable is that quartz crystal has extremely regular vibrations; even as the temperature

clocks—in which no processes repeated themselves a predictable number of times relative to other repeating processes—would be a scary universe indeed.6

Every time one day passes, the Earth rotates once about its axis, a pendulum with a period of 1 second oscillates 86,400 times, and a quartz watch crystal vibrates 2,831,155,200 times.

heartbeats, electrical pulses, digestion, rhythms of the central nervous system. We are a complicated, interconnected collection of clocks.

because the duration elapsed along two trajectories connecting two events in spacetime need not be the same.

a straight trajectory between two events in spacetime describes the longest elapsed duration.

We don’t see anything changing with time, because we are outside of time ourselves. Instead, we see all of history at once—past, present, and future.

It’s like thinking of space and time as a book, which we could in principle open to any passage, or even cut apart and spread out all the pages before us, rather than as a movie, where we are forced to watch events in sequence at specific times.

least as early as Virgil’s Aeneid. But to really jar readers out of their

temporal complacency, you want to have some of your characters experience time backward. The reason it’s jarring, of course, is that all of us

THOMASINA: When you stir your rice pudding, Septimus, the spoonful of jam spreads itself round making red trails like the picture of a meteor in my astronomical atlas. But if you need stir backward, the jam will not come together again. Indeed, the pudding does not notice and continues to turn pink just as before. Do you think this odd?

SEPTIMUS: No. THOMASINA: Well, I do. You cannot stir things apart. SEPTIMUS: No more you can, time must needs run backward, and since it will not, we must stir our way onward mixing as we go, disorder out of disorder into disorder until pink is complete, unchanging and unchangeable, and we are done with it for ever. This is known as free will or self-determination.23

Whenever we disturb the universe, we tend to increase its entropy.

Physical systems evolve toward a state of equilibrium—a

The trick, though, is that we can only decrease the entropy of one thing by creating more entropy elsewhere. We

Maybe send time to another universe?

But we now understand that such atoms are not indivisible; they consist of electrons orbiting the atomic nucleus, and the nucleus is made of protons and neutrons, which in turn are made of different combinations of quarks.

In an isolated system entropy tends to increase, because there are more ways to be high entropy than to be low entropy.

Boltzmann equation

So the arrow of time isn’t just about simple mechanical processes; it’s a necessary property of

the existence of life itself. But it’s also responsible for a deep feature of what it means to be a conscious person: the fact that we remember the past but not the future. According to the fundamental laws of physics, the past and future are treated on an equal footing, but when it comes to how we perceive the world, they couldn’t be more different. We carry in our heads representations of the past in the form of memories. Concerning the future, we can make predictions, but those predictions have nowhere near the reliability of our memories of the past.

That distinction between the fixedness of the past and the malleability of the future is nowhere to be found in the known laws of physics.

The punch line is that our notion of free will, the ability to change the future by making choices in a way that is not available to us as far as the past is concerned, is only possible because the past has a low entropy and the future has a high entropy. The future seems open to us, while the past seems closed, even though the laws of physics treat them on an equal footing.

When we say space is expanding, we mean that more space is coming into existence in between galaxies. Galaxies themselves are not expanding, nor are you, nor are individual atoms; anything that is held together by some local forces will maintain its size, even in an expanding universe.