But as we move through the Landscape, from one location to another, strange things happen. The building blocks change places with composite objects. Some particular composite shrinks, behaving in a simpler and simpler manner, as if it were becoming an elementary building block. At the same time the original building blocks start to grow and show signs of having the structure of composites. The Landscape is a dreamscape in which, as we move about, bricks and houses gradually exchange their role. Everything is fundamental, and nothing is fundamental.
What are the basic equations of the theory? Why, they are the equations governing the motion of the basic building blocks, of course. But which building blocks—open strings, closed strings, membranes, D0-branes? The answer depends on the region of the Landscape we are momentarily interested in. What about the regions intermediate between one description and another? In those regions the choice of building blocks and defining equations is ambiguous. We seem to be dealing with a new kind of mathematical theory in which the traditional ideas of fundamental versus derived concepts is maddeningly elusive. Or perhaps ’t Hooft is right, and the true building blocks are more deeply hidden. The bottom line is that we have no clear idea how to describe the entire mathematical structure of String Theory or what building blocks, if any, will win the title of “most fundamental.”
Still, I hope that the principles of String Theory, or whatever underlies it, will have the elegance, simplicity, and beauty that theorists hunger for. But even if the equations satisfy every esthetic criterion that a physicist could hope for, it does not mean that particular solutions of the equations are simple or elegant. The Standard Model is so complicated—with thirty apparently unrelated parameters, unexplained replication of particle types, and forces whose strength vary all over the map—that the String Theory version of it will almost certainly have Rube Goldberg complexity and redundancy.
For my own tastes, elegance and simplicity can sometimes be found in principles that don’t at all lend themselves to equations. I know of no equations that are more elegant than the two principles that underpin Darwin’s theory: random mutation and competition. This book is about an organizing principle that is also powerful and simple. I think it deserves to be called elegant, but again, I don’t know an equation to describe it, only a slogan: “A Landscape of possibilities populated by a megaverse of actualities.”
And what about the biggest questions of all: who or what made the universe and for what reason? Is there a purpose to it all? I don’t pretend to know the answers. Those who would look to the Anthropic Principle as a sign of a benevolent creator have found no comfort in these pages. The laws of gravity, quantum mechanics, and a rich Landscape together with the laws of large numbers are all that’s needed to explain the friendliness of our patch of the universe.
But on the other hand, neither does anything in this book diminish the likelihood that an intelligent agent created the universe for some purpose. The ultimate existential question, “Why is there Something rather than Nothing?” has no more or less of an answer than before anyone had ever heard of String Theory. If there was a moment of creation, it is obscured from our eyes and our telescopes by the veil of explosive Inflation that took place during the prehistory of the Big Bang. If there is a God, she has taken great pains to make herself irrelevant.
Let me then close this book with the words of Pierre-Simon de Laplace that opened it: “I have no need of this hypothesis.”
A Word on the Distinction between
Landscape and Megaverse
The two concepts—Landscape and megaverse—should not be confused. The Landscape is not a real place. Think of it as a list of all the possible designs of hypothetical universes. Each valley represents one such design. Listing the designs one after another, like names in a telephone book, would not capture the fact that space of designs is multidimensional.
The megaverse, by contrast, is quite real. The pocket universes that fill it are actual existing places, not hypothetical possibilities.
Note on Terminology
When I first began to write this book, I encountered a problem of terminology that I’m still wrestling with. I didn’t know what to call the new vastness that is replacing the old concept of universe. The term that was (and is) most common is multiverse. I have no objection to multiverse except that I just don’t like the sound of it. It reminds me of multiplex cinemas, which I try to avoid. I experimented with a number of other possibilities, including polyverse, googolplexus, polyplexus, and googolverse, without success. I eventually settled on megaverse, knowing full well that I was committing the linguistic crime of combining the Greek prefix mega with the Latin verse.
After deciding to use the term megaverse, I looked it up on Google and found that I was far from the first to use it. I got 8,700 results for megaverse. On the other hand, the same technique applied to multiverse got 265,000 results.
Finally, I should add that some of my best friends are users of the term multiverse, and so far, we haven’t come to blows over it.
Glossary
Absorption lines—Dark lines superimposed on a rainbowlike spectrum of colors. The dark lines are due to absorption of certain colors by gas.
Anthropic Principle—The principle that requires the laws of nature to be consistent with the existence of intelligent life.
Antiparticle—The twin of a particle that is identical except with opposite electric charge.
Boson—A type of particle not constrained by the Pauli exclusion principle. Any number of identical bosons can occupy the same quantum state.
Broken Symmetry—An approximate symmetry of nature that for some reason is not exact.
Calabi Yau manifold—The special six-dimensional geometries that String Theory uses to compactify the extra dimensions of space.
Calabi Yau space—Same as Calabi Yau manifold.
Charge conjugation symmetry—A (broken) symmetry of nature under which every particle is replaced by its antiparticle.
Compactification—The rolling up of extra dimensions of String Theory into microscopic spaces.
Cosmic microwave background (CMB)—The electromagnetic radiation left over from the Big Bang.
Cosmological constant—The term that Einstein introduced into his equations to counter the effect of gravitational attraction.
Coupling constant—The constant of nature that determines the probability for an elementary event.
D-brane—The points or surfaces where the strings of String Theory are allowed to end.
Density contrast—The variations of energy density in the early universe that eventually evolved into galaxies.
De Sitter space—The solution to Einstein’s equations with a positive cosmological constant. De Sitter space describes an expanding universe in which space clones itself exponentially.
Domain wall—The boundary separating two phases of a material such as water and ice.
Doppler shift—The shift in the frequency of waves due to the relative motion of the source of the waves and the detector of the waves.
Electric field—The field surrounding charged particles at rest. Along with magnetic fields, electric fields are composed of electromagnetic radiation such as light.
Electron—Elementary charged particle that makes up electric currents and the outer parts of atoms.
Emergent—Refers to properties of matter that manifest themselves only when large numbers of atoms behave in a collective or coordinated manner.
Eternal Inflation—The exponential cloning of space that spawns bubbles, which populate the Landscape.
Exchange diagram—A Feynman diagram in which a particle such as the photon is emitted by one particle and absorbed by another. Such diagrams are used to explain the forces between objects.
Fermion—Any particle that is subject to the Pauli exclusion principle. This includes electrons, protons, neutrons, quarks, and neutrinos.
Feynman diagram—Feynman’s pictorial way of explaining the interactions among elementary
particles.
Field—An invisible influence in space that affects the motion of objects. Examples include the electric, magnetic, and gravitational fields.
Fine structure constant (0.007297351)—The coupling constant governing the emission of a photon by an electron.
Flux—One of the many components of a string compactification. A flux is similar to a magnetic field except oriented along the compact directions of space.
Glueball—Composite particles made out of collections of gluons and having the structure of closed strings.
Gluon—Particles whose exchange account for the forces between quarks.
Gravitational waves—Disturbances of the gravitational field that propagate through space with the speed of light.
Graviton—The quantum of the gravitational field. Its exchange accounts for the gravitational force.
Harmonics—The patterns of vibration of a string, such as a guitar string.
Heisenberg Uncertainty Principle—The principle that says that it is impossible to determine both the position and momentum of any object.
Higgs boson—The quantum of the Higgs field.
Higgs field—The field in the Standard Model whose value controls the masses of elementary particles such as the electron and quark.
Holographic Principle—The principle that says that a region of space can be completely described by degrees of freedom on its boundary, with no more than one degree of freedom per Planck area—an area equal to one square Planck length.
Homogeneous—Everywhere the same; completely smooth and without variation from point to point.
Horizon—The point of no return at which an observer would be receding with the speed of light. It applies to both black holes and to rapidly inflating cosmic space.
Hubble constant—The constant appearing in Hubble’s Law.
Hubble’s Law—The law stating that the recessional velocity of galaxies is proportional to their distance. It can be expressed as an equation: V = HD, where V is velocity, D is distance, and H is the Hubble constant.
Inflation—The rapid exponential expansion of space that ironed out all the wrinkles and created a large, smooth universe. Inflation has become the standard theory of the early universe.
Isotropic—The same in every direction.
Joule—An ordinary unit of energy. The quantity of energy needed to heat one gram of water 1 degree centigrade.
Landscape—The space of possible vacuums (environments) allowed by fundamental theory. In practice, the space of vacuums of String Theory.
Long range—Refers to forces that reach out over long distances to pull or push objects. Gravity, electric, and magnetic forces are long range.
Magnetic field—The relative of the electric field which is created by charges in motion (currents).
Matrix Theory—The underlying mathematical framework for M-theory.
Megaverse—The huge vastness of pocket universes.
Mode of oscillation—Same as harmonic.
Moduli—The parameters that determine the size and shape of the compact directions of space, particularly in String Theory.
MRI machine—Medical imaging machine utilizing a space with a large magnetic field in it.
M-theory—The eleven-dimensional theory that unifies many of the diverse String Theories. M-theory has membranes but no strings.
Neutrino—The “ghostly” particle emitted by a neutron, along with an electron, when the neutron decays and becomes a proton.
Neutron—One of two particles composing the nucleus. The neutron is electrically neutral.
Non-abelian gauge theory—A class of quantum field theories that form the basis for the Standard Model of particle physics.
Nucleon—Proton or neutron.
Pauli exclusion principle—The principle that says that no two fermions can occupy the same quantum state.
Photon—Quantum of the electromagnetic field. The basis for Einstein’s particle theory of light.
Planck length or Planck distance—The natural unit of length determined by Planck’s constant, Newton’s gravitational constant, and the speed of light. It is about 10-33 centimeters.
Planck mass—The natural unit of mass determined by Planck’s constant, Newton’s gravitational constant, and the speed of light. It is about 10-5 grams.
Planck’s constant—Very small numerical constant that determines the limit in the simultaneous determination of position and momentum (Heisenberg Uncertainty Principle).
Planck time—The natural unit of time determined by Planck’s constant, Newton’s gravitational constant, and the speed of light. It is about 10-42 seconds.
Plasma—Gas that has been heated to the point where some or all of the electrons have been torn free of the atoms and are free to move through the material. Plasmas are good electrical conductors and are opaque to light.
Pocket universe—A portion of the universe in which the Laws of Physics take a particular form.
Positron—The electron’s antiparticle.
Principle of Black Hole Complementarity—The principle that allows two apparently contradictory descriptions of matter that falls into a black hole.
Propagator—The component of Feynman diagrams that represent the motion of a particle from one space-time point to another; also the mathematical expression that controls the probability for such a process.
Proton—The positively charged nucleon.
Quantum Chromodynamics (QCD)—The theory of quarks and gluons that explains the existence and properties of nucleons and nuclei. The modern nuclear physics.
Quantum Electrodynamics (QED)—The theory of electrons and photons. The basis for all atomic physics and chemistry.
Quantum field theory—The mathematical theory of elementary particles that originated by combining quantum mechanics with the Special Theory of Relativity.
Quantum jitters—The unpredictable fluctuating motion of particles or fields that derive from the principles of quantum mechanics.
Quark—The elementary particles that combine together, three at a time, to make up nucleons.
Reductionism—The philosophy that says that nature can be understood by reducing all phenomena to ultimately simple microscopic events.
Rube Goldberg machine—An overly complicated, inelegant solution to an engineering problem. Named after the cartoonist Rube Goldberg, whose cartoons depicted fantastic and silly Rube Goldberg machines.
Scalar field—A field that has a magnitude (strength) but no sense of direction. The Higgs field is a scalar; the electric and magnetic fields are not.
Short range—Refers to forces that do not reach out over long distances, i.e., forces between objects that act only when the objects are in contact or almost in contact.
Space-time—The four-dimensional world including time in which all phenomena take place.
Spectral lines—The discrete sharp lines in the spectrum of light that arise from atomic transitions in which an electron makes a quantum jump from one energy level to another and in the process emits a photon.
Standard Model—The currently accepted quantum field theory that describes elementary particles. It includes QED, QCD, and the weak interactions as well as phenomena involving the Higgs boson.
Supercooled water—Water that has been cooled below the freezing temperature but that has remained liquid.
Supernova—The final event in the life of certain stars that results in a collapse to a neutron star. At the same time an explosion sprays chemical elements into the surrounding space.
Supersymmetry—A mathematical symmetry relating fermions and bosons.
Symmetry—An operation that leaves the laws of nature unchanged.
Vacuum—A background or environment in which the Laws of Physics take on a certain form.
Vacuum energy—Energy stored in the quantum fluctuations of empty space.
Vacuum fluctuation—The jittery fluctuation of quantum fields in empty space.
Vacuum selection principle—A mathema
tical principle that would select a single unique String Theory vacuum out of all the diverse vacuums that the theory describes. Thus far, no such principle has ever been found.
Vector field—A field that has, in addition to a strength, a direction in space. The electric and magnetic fields are vectors.
Vertex diagram—The Feynman diagram depicting the elementary event in which a particle is emitted by another particle.
Virtual particle—A particle in the interior of a Feynman diagram. Not one of the particles that enters or leaves at the beginning or end of the process.
W-boson—One of the particles whose exchange gives rise to the weak interactions.
Weak interactions—Phenomena that are similar to the decay of the neutron.
Weinberg’s bound—The bound on the size of the cosmological constant that derives from the condition that galaxies could form in the early universe.
Yang Mills theory—Same as non-abelian gauge theory.
Z-boson—A close relative of the W-boson, also involved in weak interactions.
About the Author
Leonard Susskind grew up in the South Bronx, where he worked as a plumber and steam fitter during his early adult years. As an engineering student in CCNY, he discovered that physics was more to his liking than either plumbing or engineering. He later earned a PhD in theoretical physics at Cornell University.
Susskind has been a professor of physics at the Belfer Graduate School in New York City, Tel Aviv University in Israel, and since 1978 at Stanford University, where he is the Felix Bloch Professor of Physics. During the past forty years he has made contributions to every area of theoretical physics, including quantum optics, elementary-particle physics, condensed-matter physics, cosmology, and gravitation. In 1969 Susskind and Yoichiro Nambu independently discovered string theory. Later he developed the theory of quark confinement (why quarks are stuck inside the nucleus and can never escape), the theory of baryogenesis (why the universe is full of matter but no antimatter), the Principle of Black Hole Complementarity, the Holographic Principle, and numerous other concepts of modern physics.
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