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Kindle Notes & Highlights
First printing, January 2010
Copyright © 2010 by Sean Carroll
ISBN: 9781101152157
Version_7
PROLOGUE
This book is about the nature of time, the beginning of the universe, and the underlying structure of physical reality.
TIME SINCE THE BIG BANG
A major theme of this book is that the arrow of time exists because the universe evolves in a certain way.
If everything in the universe evolves toward increasing disorder, it must have started out in an exquisitely ordered arrangement.
WHAT WE SEE ISN’T ALL THERE IS
Increasingly, scientists are taking seriously the possibility that the Big Bang is not really a beginning—it’s just a phase through which the universe goes, or at least our part of the universe. If that’s true, the question of our low-entropy beginnings takes on a different cast: not “Why did the universe start out with such a low entropy?” but rather “Why did our part of the universe pass through a period of such low entropy?”
I’ll 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. The universe we see is a tiny sliver of an enormously larger ensemble, and our particular journey from a dense Big Bang to an everlasting emptiness is all part of the wider multiverse’s quest to increase its entropy.
When we aren’t sure of the final answer, it behooves us to ask the question in as many ways as possible.
THERE WILL ALWAYS BE SKEPTICS
Many dramatic-sounding statements are contained herein, but I’m going to be as careful as possible to distinguish among three different types: (1) remarkable features of modern physics that sound astonishing but are nevertheless universally accepted as true; (2) sweeping claims that are not necessarily accepted by many working physicists but that should be, as there is no question they are correct; and (3) speculative ideas beyond the comfort zone of contemporary scientific state of the art.
It’s sometimes helpful to let our imaginations roam, even if our ultimate goal is to come back down to Earth and explain what’s going on in the kitchen.
PART ONE
TIME, EXPERIENCE, AND THE UNIVERSE
1
THE PAST IS PRESENT MEMORY
Even if we can’t easily articulate what exactly it is we mean by “time,” its basic workings make sense at an intuitive level. Like a Supreme Court justice confronted with obscenity, we know time when we see it, and for most purposes that’s good enough.
WHAT WE MEAN BY TIME
Time comes in three different aspects, all of which are going to be important to us.
1. Time labels moments in the universe. Time is a coordinate; it helps us locate things.
2. Time measures the duration elapsed between events. Time is ...
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3. Time is a medium through which we move. Time is the agent of change. We move through it, or—equivalently—time flows past us, from the past,...
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1. Time labels moments in the universe
“Time is Nature’s way of keeping everything from happening at once.”
We experience a degree of continuity through time—if the cat is on your lap now, there might be some danger that she will stalk off, but there is little worry that she will simply dematerialize into nothingness one moment later. This continuity is not absolute, at the microscopic level; particles can appear and disappear, or at least transform under the right conditions into different kinds of particles. But there is not a wholesale rearrangement of reality from moment to moment.
The world line of an object is just the complete set of positions the object has in the world, labeled by the particular time it was in each position.
Finding ourselves
What we call the “universe” is just the set of all events—every point in space, at every moment of time.
This is such a useful concept that we will often treat the whole collection, every point in space at every moment of time, as a single entity called spacetime.
2. Time measures the duration elapsed between events
The key to measuring time is synchronized repetition—a wide variety of processes occur over and over again, and the number of times that one process repeats itself while another process returns to its original state is reliably predictable.
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 or pressure changes, the crystal will vibrate the same number of times for every one rotation of the Earth.
If you really wanted to, you could reinvent the entire superstructure of physics in a way that completely eliminated the concept of “time,” by replacing it with elaborate specifications of how certain things happen in coincidence with certain other things.
Slowing, stopping, bending time
Twisty paths through spacetime
We can invert directions in space without doing damage to how physics works, but all sorts of real processes can happen in one direction of time but not the other.
3. Time is a medium through which we move
The view from nowhen
And what do we see, when looking down from nowhen? 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.
In the philosophical literature, this is sometimes called the “block time” or “block universe” perspective, thinking of all space and time as a single existing block of spacetime.
2
THE HEAVY HAND OF ENTROPY
THROUGH THE LOOKING GLASS
THE ARROW OF TIME
Why is it like that? Why do we live in a universe where X is often followed by Y, but Y is never followed by X?
Whenever we disturb the universe, we tend to increase its entropy.
