1.

String theory has the potential to show that all of the wondrous happenings of the universe . . . are reflections of one grand physical principle, one master equation.

This quotation summarizes what Greene means when he commends the “elegance” of physics. Greene finds it impossible to believe that the universe operates along incompatible frameworks. String theory has set out to show that the “frantic dance of subatomic quarks” and the “stately waltz of orbiting binary stars” are intimately interconnected—that is, that they are facets of the same coherent set of laws. Greene frequently employs musical language like “frantic dance” and “stately waltz” to convey the harmonizing power of superstring theory. To describe their constant vibrational patterns, he compares the strings of superstring theory to those on a violin. The entire structure of the universe can be imagined as an infinite number of instruments joining together in what Greene calls a “cosmic symphony.”

Greene never dismisses the importance of aesthetic coherence in both art and nature. And it is the evidence of the “one grand physical principle, one master equation” that first attracted him to string theory. For Greene, “elegance” describes the inexhaustible complexity of the universe arising from a single theory. Only string theory has attempted to unify the “primordial fireball of the big bang” with the “majestic swirl of heavenly galaxies.” As far as Greene is concerned, the ambition to find coherence is itself sufficient reason to examine string theory in greater depth.

2.

[I]ndividuals who are moving with respect to each other will not agree on their observations of either space or time.

The history of the twentieth century—in psychology, literature, and psychoanalysis—has been a series of revolutions of subjectivity. Greene begins his discussion of string theory by contextualizing it within this modern framework. Einstein’s theories of relativity were groundbreaking not just for the physical truths they unveiled but for what they revealed about individuals’ power to determine the “truth” of their experiences. Historically, science, like religion, proclaimed that one point of view was right and the other wrong. Einstein’s theories of relativity undid the simplicity of that formulation and replaced it with an all-pervading subjectivity. There is no longer one correct answer or measurement for the motion of objects in space.

And yet, even as it emphasized the significance of different perspectives, relativity also demonstrated the deficiencies of human intuition, as Greene points out above. It is by no means intuitively obvious that everyone experiences time differently. Indeed, the notion undermines everything humans believed about distance and time and motion. Relativity shows that many movements in the universe far exceed humans’ capacity to apprehend them in day-to-day experience—or even to make sense of them. In special relativity, this hard-to-comprehend motion is the speed of light, which never changes. In general relativity, the tricky motion is gravity.

3.

Most physicists find it hard to believe that, at rock bottom, our deepest theoretical understanding of the universe will be composed of a mathematically inconsistent patchwork of two powerful yet conflicting explanatory frameworks.

Greene cannot emphasize this point enough.
He, and many physicists before him—Einstein being the prime example—simply
cannot accept the idea that the universe is framed by incompatible
sets of laws. The search for a “grand unified theory” or a “Theory
of Everything” is the attempt to resolve this inconsistency. String
theorists believe that, “at rock bottom,” only one theory can reveal
the *real* inner workings of the universe.

Immediately before this passage, Greene describes quantum foam, the violent fluctuations that occur in a “rather esoteric realm of the universe.” It is only at extremely short-distance scales that quantum fluctuations occur, but even ultramicroscopic turbulence overturns Einstein’s conception of space as a smooth geometrical surface. General relativity works on the large scale, but not at all on the small scale. The existence of quantum foam fundamentally undermines the central principle of general relativity, which presupposes a smooth fabric of space.

Einstein was one of the first physicists to suspect that there was, as Greene says, “an essential flaw in our understanding of the physical universe.” But though he labored for thirty years to uncover it, Einstein never arrived at a satisfactory one-theory solution. In the years after his relativity and Bohr’s quantum mechanics emerged, physicists tended to study either large-scale or small-scale phenomena, seldom both. Superstring theory, Greene argues throughout the book, is the first attempt to articulate a mathematically sound theory for both.

For all its advances, however, physicists continue to debate the merit of superstring theory. Many think the inconsistency is not “worth worrying about,” and contentedly study either the laws of quantum mechanics or the laws of general relativity without endeavoring to synthesize the two “conflicting explanatory frameworks.” It is only as the mathematical underpinnings of superstring theory slowly fall into place that more and more conservative physicists are coming to accept it.

4.

String theory alters this picture radically by declaring that the “stuff” of all matter and all forces is the same.

In string theory, matter and all its various properties are exactly the same thing: vibrating strands of string. Differences in particles, properties, and nuclear forces reflect the different vibrational patterns of these strands. Just as the same strings of a violin or guitar can produce a tremendous quantity of different notes, depending on how they are plucked, the strings of superstring theory can generate all the multiplicity of the universe, depending on how they vibrate. Figuring out how these vibrations work, and the configuration of the strings, is an immense task, but the premise—that the entire universe has the exact same base ingredient—is thrillingly simple.

String theory is still in a state of constant flux. Though many physicists now agree that the universe is not composed of zero-dimensional dots, they continue to speculate about geometric possibilities other than strings. With the development of M-theory, they are studying refinements of the eleven-dimensional model by predicting the existence of two-dimensional Frisbee-like branes and even three-dimensional blobs, both of a size no one has yet determined.

5.

At the opposite extreme, we have also seen that our universe may merely be one of the innumerable frothing bubbles on the surface of a vast and turbulent cosmic ocean called the multiverse.

String theory, like the development
of twentieth-century physics, is a study in extremes. In seeking
to articulate a single law that explains both the largest galaxy
and the smallest component of a quark, string theorists have come
across other arresting prospects that go far beyond their earlier
hypotheses. At the end of *The Elegant Universe*,
Greene introduces some of string theory’s more recent discoveries,
and its prospects for future ones.

Edward Witten’s M-theory, which was introduced four years before
the publication of *The Elegant Universe*, has revealed
some bizarre cosmic behavior. Perhaps, as Greene mentions in this
passage, the universe as we know it is just a tiny “frothing bubble
on the surface of a vast and turbulent cosmic ocean called the multiverse.” Perhaps
what string theorists thought were the tiniest components of matter
also possess macroscopic properties. Perhaps the reconciliation
of quantum mechanics and general relativity will yield revelations
far more dramatic than anyone understands right now.

Special relativity hinges on the constancy of the speed of light, but for the moment, string theory lacks an organizing framework of this sort. Until that principle comes to light, string theory remains a subject of much uncertainty and debate. The next superstring revolution, Greene suggests, will require another “leap in our understanding of the universe.” Until then, string theorists can only continue exploring varied and complex speculations about the cosmos.

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