More on this book
Community
Kindle Notes & Highlights
by
Brian Greene
Read between
April 20 - September 25, 2020
Copyright © 2011 Brian Greene
eISBN: 978-0-307-59525-6
Preface
CHAPTER 1
The Bounds of Reality
On Parallel Worlds
This book is an exploration of such possibilities, a considered journey through the science of parallel universes.
Universe and Universes
Varieties of Parallel Universes
After decades of closely studying quantum mechanics, and after having accumulated a wealth of data confirming its probabilistic predictions, no one has been able to explain why only one of the many possible outcomes in any given situation actually happens. When we do experiments, when we examine the world, we all agree that we encounter a single definite reality. Yet, more than a century after the quantum revolution began, there is no consensus among the world’s physicists as to how this basic fact is compatible with the theory’s mathematical expression.
The mathematics underlying quantum mechanics—or at least, one perspective on the math—suggests that all possible outcomes happen, each inhabiting its own separate universe.
The Cosmic Order
The subject of parallel universes is highly speculative. No experiment or observation has established that any version of the idea is realized in nature. So my point in writing this book is not to convince you that we’re part of a multiverse. I’m not convinced—and, speaking generally, no one should be convinced—of anything not supported by hard data.
My intention, then, is to lay out clearly and concisely the intellectual steps and the chain of theoretical insights that have led physicists, from a range of perspectives, to consider the possibility that ours is one of many universes. I want you to get a sense of how modern scientific investigations—not untethered fantasies like the catoptric musings of my boyhood—naturally suggest this astounding possibility.
For me, it is the depth of our understanding, acquired from our lonely vantage point in the inky black stillness of a cold and forbidding cosmos, that reverberates across the expanse of reality and marks our arrival.
CHAPTER 2
Endless Doppelgängers
The Quilted Multiverse
But even without the capacity to observe these realms, we’ll see that basic physical principles establish that if the cosmos is infinitely large, it is home to infinitely many parallel worlds—some identical to ours, some differing from ours, many bearing no resemblance to our world at all.
The Father of the Big Bang
General Relativity
There’s no rope tethering them together, no chain tugging the earth as it moves, so how does gravity exert its influence? In his Principia, published in 1687, Newton recognized the importance of this question but acknowledged that his own law was disturbingly silent about the answer.
(Einstein contended that warps in space and time don’t need a facilitator since they are gravity);
The Universe and the Teapot
The cosmological principle asserts that if the universe is examined on the largest of scales, it will appear uniform.
In the years since, astronomical observations have provided substantial support for the cosmological principle, but only if you examine space on scales at least 100 million light-years across (which is about a thousand times the end-to-end length of the Milky Way). If you take a box that’s a hundred million light-years on each side and plunk it down here, take another such box and plunk it down way over there (say, a billion light-years from here), and then measure the average overall properties inside each box—average number of galaxies, average amount of matter, average temperature, and so
...more
Einstein, who adhered to this cosmic perspective, found to his dismay that it was at odds with general relativity. The math showed that the density of matter and energy cannot be constant through time. Either the density grows or it diminishes, but it can’t stay put.
Because there’s no outward force to cancel the attractive pull of gravity, general relativity shows that the universe can’t be static. Either the fabric of space stretches or it contracts, but its size can’t remain fixed.
The eternal and static cosmos that Einstein expected would emerge from his equations was simply not there.
Taxing Gravity
When general relativity elevated space and time into dynamic participants in the unfolding of the cosmos, they shifted from merely providing language to delineate where and when things take place to being physical entities with their own intrinsic properties.
If the universe had a cosmological constant of the right size—that is, if space were endowed with the right amount of intrinsic energy—his theory of gravity fell in line with the prevailing belief that the universe on the largest of scales was unchanging.
The Primeval Atom
In 1929, using what was then the world’s largest telescope at the Mount Wilson Observatory, the American astronomer Edwin Hubble gathered convincing evidence that the distant galaxies were all rushing away from the Milky Way. The remote photons that Hubble examined had traveled to earth with a clear message: The universe is not static. It is expanding.
Lemaître and Friedmann were vindicated. Friedmann received credit for being the first to explore the expanding universe solutions, and Lemaître for independently developing them into robust cosmological scenarios.
The Models and the Data
The cosmological principle—the assumed homogeneity of the cosmos—constrains the geometry of space because most shapes are not sufficiently uniform to qualify: they bulge here, flatten out there, or twist way over there. But the cosmological principle does not imply a unique shape for our three dimensions of space; instead, it reduces the possibilities to a sharply culled collection of candidates.
The uniformity of the cosmos articulated by the cosmological principle substantially winnows the possible shapes for the universe. Some of the possible shapes have infinite spatial extent, while others do not.
Our Universe
If there is a lot of matter, gravity will cause space to curve back on itself, yielding the spherical shape. If there is little matter, space is free to flare outward in the Pringles shape. And if there is just the right amount of matter, space will have zero curvature.*
The mathematics shows that “just the right amount of matter,” the so-called critical density, weighs in today at about 2 × 10–29 grams per cubic centimeter, which is about six hydrogen atoms per cubic meter or, in more familiar terms, the equivalent of a single raindrop in every earth-sized volume.
The question then is whether each cubic meter would weigh more or less than six hydrogen atoms.
The result differs from what Einstein long ago proposed (since he posited a value that would yield a static universe, whereas our universe is expanding). That’s not surprising. Instead, what’s remarkable is that the measurements have concluded that the dark energy filling space contributes approximately 73 percent of the critical density. When added to the 27 percent of criticality astronomers had already measured, this brings the total right up to 100 percent of the critical density, just the right amount of matter and energy for a universe with zero spatial curvature.
Reality in an Infinite Universe
The closer to time zero you consider an infinite universe, the denser it becomes at every location, but its spatial extent remains unending.
Infinite Space and the Patchwork Quilt
Fifty-two cards can be arranged in different ways (52 possibilities for which card will be the first, times 51 remaining possibilities for which will be the second, times 50 remaining possibilities for the next card, and so on).
80 unvigintillion 658 vigintillion 175 novemdecillion 170 octodecillion 943 septendecillion 878 sexdecillion 571 quindecillion 660 quattuordecillion 636 tredecillion 856 duodecillion 403 undecillion 766 decillion 975 nonillion 289 octillion 505 septillion 440 sextillion 883 quintillion 277 quadrillion 824 trillion
Finite Possibilities
For instance, if the smallest increments that can be detected are a hundredth of a centimeter, then each centimeter offers not an infinite number of detectably different locations, but only a hundred. Each cubic centimeter would thus provide 1003 = 1,000,000 different locations, and your average bedroom would offer about 100 trillion. Whether the fly would find this array of options sufficiently impressive to keep away from your ear is difficult to say. The conclusion, though, is that anything but measurements with perfect resolution reduces the number of possibilities from infinite to finite.
According to quantum mechanics, there’s a precise sense in which there is a fundamental limit on how accurate particular measurements can be, and this limit can’t ever be surpassed, regardless of technological progress—ever. The limit arises from a central feature of quantum mechanics, the uncertainty principle.
