Trespassing on Einstein's Lawn (61 page)

Arrow of time
The notion that time only moves in one direction, namely, forward.

Baryon
Any particle made of three quarks, including the neutrons and protons in the heart of every atom.

Big bang
The claim that the early universe was hot and dense, then expanded and cooled. That's really all there is to it.

Black hole
A region of spacetime where gravity is so strong not even light can escape.

Black hole information-loss paradox
When black holes evaporate via Hawking radiation and disappear from existence, what happens to the stuff that fell inside? If it escapes, Einstein's theories of relativity are wrong. If it doesn't escape, quantum mechanics is wrong. Neither relativity nor quantum mechanics is wrong.

Boson
A force-carrying particle whose quantum spin is an integer; examples are a photon, which carries the electromagnetic force and has spin 1, and a graviton, which carries the gravitational force and has spin 2.

Causal diamond
The entire region of universe that a given observer could have accessed in the past and can access in the future. Because it is formed by the intersection of an observer's past and future light cones, it is shaped like a diamond.

Census Taker
Leonard Susskind's hypothetical observer who lives in FRW space, the end product of any series of vacuum decays in eternal inflation. The Census Taker's light cone will continue to grow forever, allowing him to measure, in principle, any region of the universe—except himself.

Commutation
If A × B = B × A, then A and B commute. If A × B ≠ B × A, they don't. Commutation relations tell you whether order matters. Quantum uncertainty, for instance, tells you that, within a single reference frame, it matters whether you measure a particle's position first and then its momentum or its momentum first and then its position, because the more accurately you measure the first, the less accurately you can measure the second.

Conserved quantity
A quantity whose invariance is ensured by the laws of physics. All experiments suggest there are no nonzero conserved quantities in the universe.

Cosmic microwave background (CMB)
Remnant radiation from the big bang, composed of photons whose frequencies have been stretched to the microwave region by the expansion of the universe and now heat empty space to 2.7 Kelvin.

Cosmological constant
Originally a term in Einstein's equations of general relativity, it is now believed to be the energy inherent to the vacuum itself. If its value is positive, it exerts a negative pressure on the vacuum, causing space to expand at an accelerated rate. If its value is negative, it exerts a positive pressure, causing space to fold in on itself at every point.

D-brane
A membrane that from one point of view looks like a region of space where open strings can end, and from another point of view looks like an object that can move or form a black hole.

Dark energy
The unidentified force causing the expansion of the universe to accelerate. Most likely suspect: Einstein's cosmological constant.

Dark matter
A hypothetical form of matter that does not interact via the electromagnetic or strong nuclear forces, but whose gravity is thought to hold stars in place within galaxies.

Decoherence
The process by which quantum superposition states (and the interference patterns they produce) are quickly destroyed by interactions with their environment, which explains why we hardly ever see any simultaneously dead and alive cats.

Delayed-choice experiment
Wheeler's version of the double-slit experiment in which the observer's decision to either measure the interference pattern produced when a particle travels two paths or to measure which single path the particle takes, thereby destroying interference, occurs after the particle has presumably traveled either both paths or just one. In other words, the observer's choice of measurement determines the history of the universe
after
it has already occurred.

De Sitter horizon
In de Sitter space, the universe expands at an accelerated rate. Light can only travel a finite distance even given infinite time, because as the light crosses any given distance, the distance itself grows. For any inertial observer, there is a region of the universe from which light will never be able to reach him. The event horizon separating the accessible portion of the universe from the dark portion is known as a de Sitter horizon. It is observer-dependent, as it is relative to the observer's location.

De Sitter space
A space formed when a positive cosmological constant exerts an outward pressure, causing the expansion rate of the universe to accelerate, diluting the density of matter until the universe is empty except for the cosmological constant itself. In de Sitter space, every observer is surrounded by a unique horizon, so that no two observers in de Sitter space ever see the same universe.

Diffeomorphism transformation
A method of translating between misaligned points of view by introducing a force, like gravity.
To make a curve match up with a line, bend the paper.
This is the key tool of general relativity and an example of a gauge transformation.

Double-slit experiment
The classic, bat-shit crazy quantum experiment in which particles are fired at a screen that has two tiny slits in it. On the other side of the screen is a photographic plate to record where the particles land. When both slits are kept open, light fired at the screen creates an interference pattern of dark and light stripes on the plate, suggesting that light exhibits wave behavior. When single photons are fired, they will, one by one, build up the same interference pattern, a bizarre fact that seems to tell us the individual photons are traveling through both slits simultaneously. If a detector is placed at one of the slits to measure the photon's path, the photon will only travel through a single slit and the interference pattern disappears.

dS/CFT
A hypothesized analogue to AdS/CFT, in which the physics of our de Sitter universe is dual to a conformal quantum field theory living on the lower-dimensional boundary of the universe. Such a formulation, however, would describe a global universe that no observer can ever access, since any observer in de Sitter space is stuck in a finite region surrounded by an event horizon.

Duality
A one-to-one mathematical equivalence between two radically different physical descriptions.

Entanglement
A form of quantum superposition in which two systems are described by a single wavefunction, so that information about the system resides in neither system individually but in the correlations between them, which persist in spite of spatial separation.

Entropy
A measure of the amount of information it would take to characterize every detail of a physical system.

Epistemology
The study of what is possible to know and how we may or may not know it.

EPR
A thought experiment conceived by Einstein, Boris Podolsky, and Nathan Rosen in an attempt to argue that quantum theory could not be a complete description of reality. If features, say spins, of two particles are correlated, then measuring the value of one seems to instantaneously decide the value of the other, no matter how far apart the particles are. EPR concluded that since relativity forbids instantaneous action-at-a-distance, there must be so-called hidden quantum variables that allow the particles to have well-defined values at all times, even prior to measurement. Bell's theorem has since shown the local hidden variables interpretation to be flawed, while Carlo Rovelli's relational interpretation of quantum mechanics resolves the EPR paradox by revealing the outcome of quantum measurements to be observer-dependent.

Equivalence principle
Einstein's happiest thought: there's no difference between an accelerated frame without gravity and an inertial frame with gravity. In other words, gravity is ultimately not real—it's a gauge force that accounts for the mismatch between reality as seen from an inertial frame and reality as seen from an accelerated frame.

Eternal inflation
According to the theory of inflation, the universe begins in a false vacuum that eventually decays, but, thanks to the uncertainty principle, it doesn't decay everywhere at the same time. When one region decays, it briefly expands faster than light, forming a bubble universe causally disconnected from the original vacuum. Remaining portions of the vacuum decay, too, forming other bubbles. The
false vacuum grows faster than it decays, so there is always more left over and it never stops blowing bubbles. Any feasible theory of inflation is bound to go eternal and create an infinite multiverse.

Event horizon
A surface in spacetime that light cannot cross. Horizons divide spacetime into causally disconnected regions.

False vacuum
A state that is temporarily stable but is not the lowest possible energy of the system. Given enough time it will decay, dropping down to its lowest energy state.

Fermion
A matter particle whose quantum spin comes in half integers, such as an electron, which has spin ½.

Fictitious force
A force that arises as a consequence of an observer's perspective. Because they are gauge forces, the four fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—are fictitious.

Friedmann-Robertson-Walker (FRW) space
A simple, homogeneous, expanding or contracting universe.

Gauge
A phase or reference frame.

Gauge boson
A particle that carries a gauge force.

Gauge force
A force that exists in order to account for mismatched descriptions between two reference frames. The electromagnetic force, for instance, exists so we don't confuse two different descriptions of the same electron—whose phase changes from one reference frame to another—for different electrons.

Gauge symmetry
All gauges—or reference frames—are created equal. None offers a truer version of reality than another.

General covariance
Einstein's key principle that there's no preferred way to slice spacetime into space and time—slice it any way you want and the fundamental laws of physics remain unchanged. In a world with both accelerated and inertial reference frames, getting general covariance to hold requires diffeomorphism transformations.

General relativity
Einstein's masterpiece, which puts inertial and accelerated observers on equal footing by introducing gravity, which bends spacetime in precisely the right way to align mismatched reference
frames and ensure that we don't mistake different descriptions of reality for different realities.

Global
A large-scale description that encompasses many light cones, larger than any single observer could ever see.

Gödel's incompleteness theorem
If a sufficiently complex mathematical system—one that is capable of making statements about itself—is consistent, it can't be complete. That is, it will contain statements that are fundamentally unprovable.

H-state
A state of infinite, unbounded homogeneity. Otherwise known as nothing.

Hawking radiation
When an event horizon is present, observers will no longer agree on whether space is empty or filled with particles. Those observer-dependent particles are called Hawking radiation.

Higgs boson
An excitation of the Higgs field.

Higgs field
A pervasive field that interchanges right- and left-handed particles, allowing them to have mass without violating gauge symmetry.

Hilbert space
A mathematical vector space for representing quantum states.

Holographic principle
All information necessary to reconstruct the physics of a given region of spacetime can be encoded on the region's lower-dimensional boundary. Or, equivalently, the total amount of information that can fit in a given region of spacetime must be less than one-quarter the area of its boundary in Planck units.

Horizon complementarity
You can describe the universe in terms of what's on one side of an event horizon or in terms of what's on the other side, but never both at once.

Inertial observer
An observer in uniform (as opposed to accelerated) motion. Goes by the name Screwed. Can be found falling into black holes.

Inflation
A brief period in which the universe expanded faster than light; occurred in the first trillionth of a second following its birth.

Inflaton
A hypothetical scalar field that existed in the first fraction of a second following the big bang. The inflaton was believed to have begun in a false vacuum state; its decay triggered the superluminal expansion of inflation.

Interference pattern
The pattern that arises when waves come together, their phases adding in places where they are aligned and canceling where they are misaligned.

Invariance
Sameness. A feature is invariant if it doesn't change from one reference frame to another.

Isomorphism
A one-to-one correspondence. For example, the scrambled information in a two-dimensional hologram is isomorphic to the three-dimensional image it projects.

It from bit
Wheeler's expression for the notion that a physical object, an “it,” is at bottom just a configuration of information, or “bits.”

Landscape
The set of 10
⁵⁰⁰
vacua described by string theory, which are formed by the various ways in which one can compactify extra spatial dimensions. Each vacuum corresponds to its own universe, with its own local laws of physics and its own value of the cosmological constant.