Any change in the speed with which an object moves, or the direction
in which it moves.
Active Galactic Nucleus
The central portion of a galaxy which gives off unusually
large amounts of energy. These are thought to be powered by
supermassive Black Holes.
The height of the peak of a wave, measured relative to its
center. Equivalently, the depth of the trough of a wave.
Essentially, the "opposite" of a particle. Every type of
matter has a corresponding antiparticle, with the same mass
but opposite charge, for example. Other numbers describing
the particle will be reversed for the antiparticle.
An astrophysical theory of the beginning of the Universe.
It suggests that the Universe began in a very tiny region of
space, and exploded outward. Astrophysicists believe that
this occurred roughly 14 billion years ago. Other
astrophysical theories for the beginning of the Universe—
like the Braneworld theory—exist, though none is as
thoroughly studied and supported by the data as the Big Bang
model. Scientists have no idea what came before the Big
Essentially the opposite of the Big Bang, the Big Crunch is
one possible fate of the Universe. If the matter and energy
of the Universe are not moving outward quickly enough,
gravity could pull the Universe in on itself, collapsing it
in a final Big Crunch. It is not yet known whether this
will happen to our Universe.
A region of spacetime where the warpage of both space and
time (gravity) is so intense that nothing—even light—
can ever escape. Objects may fall in to the Black Hole, but
once they pass the Event Horizon, they can never escape
again. Most Black Holes believed to exist are thought to be
formed in the collapse of very large stars, or the collision
of stars or other Black Holes.
A type of particle with "integral angular momentum"—a
spin of 0, 1, 2, etc. Spin refers to an intrinsic quality
of all particles. Examples of fermions are photons (which
are the particles which give us light) and gravitons (which
give us gravity). The other type of particle is the
A division between two regions. Physicists frequently
analyze only a part of a larger system, so that they do not
need to keep track of everything. This usually simplifies
the analysis, but requires an understanding of what happens
at the boundary. Interesting computer simulations usually
require a boundary.
The state of a physical system at a boundary. Interesting
computer simulations usually require boundary conditions.
Objects which arise in string theory. They can have any
number of dimensions, and are usually imagined as existing
in a space with more dimensions than the brane itself has.
A four-dimensional surface—a "brane"—in a spacetime
with more than four dimensions.
The relationship between cause and effect. Typically, we
assume that a given event is a result of events that came
before it in time.
A specific type of Binary system in which both
members are compact (meaning they are White Dwarfs, Neutron
Stars, or Black Holes) and have roughly equal mass.
Cosmic Microwave Background
A long, heavy object from Quantum Field Theory or String
Theory, which is very thin. They may have been created in
the early life of the Universe, and would now stretch across
the entire Universe. No cosmic string has ever been
observed. They may or may not exist.
A mathematical device used by Einstein to keep the Universe
from falling in on itself. He later called this his
"greatest blunder", because it kept him from predicting
the expansion of the Universe. Astrophysicists now believe
there may be a use for the cosmological constant.
A type of matter which is found near other matter. It
cannot be observed directly (it is "dark"), but can be
noticed as a result of the pull of gravity from the dark
matter. Astrophysicists believe that more than half of the
Universe could be in the form of dark matter.
The branch of geometry which deals with curved surfaces and spaces.
Calculus is used to analyze the shape of these surfaces and spaces.
Differential geometry is a key tool used in the study of
A set of "tensor" equations Einstein devised to describe
how mass warps spacetime. The set of equations may be
written as G = 8 π T, where both
G and T each represent a set of ten
quantities. The G quantities represent the
warping of spacetime, while the T quantities—
the "Stress-Energy tensor" represent the mass.
An electric and magnetic disturbance that travels through
space like a wave. What we experience as light is an
electromagnetic wave. Electromagnetic waves therefore
travel at the Speed of Light. Other types of
electromagnetic wave range from Radio Waves and Microwaves,
through to X-Rays and Gamma Rays.
A tiny particle usually found swirling around an atomic
nucleus, the electron carries the standard unit of negative
charge, which balances the positive charges in a nucleus.
The interaction between electrons and nuclei is responsible
for chemistry. Electrons can become detached from the
nucleus, when given enough energy, and become free.
Electrons are members of the particle class called
fermions, and are roughly 2,000 times lighter than
neutrons and protons.
A particle which is not made up of smaller particles. The
neutron and proton are both made up of smaller particles—
called quarks. On the other hand, quarks are not believed
to be made of anything smaller. Like quarks, electrons and
photons are also believed to be elementary particles.
A secondary circle centered on another, usually larger, circle. It's
center moves along the circumference of the main circle.
A surface—like the one surrounding a Black Hole—
enclosing a region of space from which nothing (even light)
can ever escape.
A rule in Physics which says that no two identical particles
can be in the same state (position, for instance) at the
same time. This principle only applies to fermions,
not to bosons. It is usually referred to as the
"Pauli Exclusion Principle", after its inventor Wolfgang
Extreme Mass-Ratio Inspiral
A particular type of binary in which there is a very large difference
in the masses of the two objects. Generally, this will
involve a super-massive Black Hole with a mass millions of
times that of our Sun, and a Neutron Star or Black Hole with
a mass roughly the same as our Sun.
Failure of Simultaneity
Simultaneous events are events in different places which
happen at the same time. It turns out that this concept
depends on how quickly one is moving. That is, if two
observers are moving relative to each other, they will not
be able to agree on the simultaneity of events. This is the
Failure of Simultaneity.
A type of particle with "odd half-integral angular
momentum"—a spin of 1/2, 3/2, etc.
Spin refers to an intrinsic quality of all particles.
Examples of fermions are electrons, neutrons, and protons.
The other type of particle is the boson.
First Law of Motion
The first of Newton's Laws of Motion, which says that moving
objects move in a straight line. Specifically, the Law says,
"An object at rest tends to stay at rest and an object in
motion tends to stay in motion with the same speed and in the
same direction unless acted upon by an unbalanced force."
See "Nuclear Fission".
The unexpected result that the Universe is not expanding so
slowly that it will clearly collapse back on itself in a
Big Crunch, nor expanding so quickly
that it will clearly keep expanding forever.
Instead, measurements show that the Universe is treading a
fine line between the two—the Universe is referred to as
"flat". Astrophysicists would not expect this to be the
case unless there were some cause for the Universe to tread
such a fine line. Finding this cause is the "flatness
The number of occurrences of something in a given period of
time. For a wave, this is might be the number of times the
wave peaks in one second.
See "Nuclear Fusion".
General Theory of Relativity
Einstein's version of the laws of physics, when there is
gravity. Building on the Special Theory of Relativity, this
theory generalizes Einstein's work so that the laws of
physics must be the same for all observers, even in gravity.
Einstein showed that gravity is best understood as a warping
of the geometry of spacetime, rather than as a pulling of
objects on each other. The crucial idea is that objects move
along geodesics—which are determined by the warping of
spacetime—while spacetime is warped by massive objects
according to the formula G = 8 π T
Essentially the "straightest path" in a curved space or
curved spacetime. This is the path followed by an object
with no forces acting on it. In the curved spacetime of
General Relativity, these paths may seem to be very curved
—even appearing as circles or ellipses, for example. A
geodesic is easily understood by looking at a very small
region around the object. Even in highly curved spacetime,
a small enough region will seem flat, so there is a natural
idea of a "straight path". By following short segments,
the whole geodesic is built up into one long path.
A gravitational disturbance that travels through space like
a wave. This type of wave is analogous to an
Electromagnetic Wave. Gravitational waves are given off by
most movements of anything with mass. Usually, however,
they are quite difficult to detect. Physicists are
currently working hard to directly detect gravitational
waves. Experiments like LIGO and LISA are designed for this
A problem with the simplistic Big Bang theory having to do with the
smoothness of the Universe. The Early Universe should have
been very random in terms of the temperature and density of
different parts of space. This randomness should not have
had time to distribute itself more evenly. Yet, this is
what is observed in the Universe. The theory of Inflation
solves this problem.
A brief period shortly after the Big Bang during which the
Universe expanded very rapidly. The theory of Inflation is
necessary to make the theory of the Big Bang agree with
Physicists frequently analyze a physical situation starting
at some moment in time, and ignore what happened before that
moment in time. To do this, however, they need to
understand what was happening at that initial moment. That
is, they need Initial Data. This is very similar to using
Boundaries, which requires having Boundary Conditions.
The gradually-shrinking orbit of a binary system. As the pair of
stars in the binary orbit each other, they give off energy
in the form of gravitational waves. This lost energy draws
them closer in their orbit—eventually resulting in a
A phenomenon which can occur any time there is any type of
wave, which amounts to two waves canceling each other out.
If two different waves meet at the same place, and one would
hit its peak while the other would hit its trough, the waves
will cancel, and there will be no disturbance.
Alternatively, if both waves would hit their peaks at the
same time, the waves will boost each other, so that there is
a greater disturbance.
A scientific device which makes use of the Interference of
waves—typically, light waves. This type of device can
measure changes in length with extraordinary precision, and
forms the basis of modern gravitational wave detectors.
The distance traveled by light in one year. This is roughly
1013 kilometers, or 6 × 1012 miles.
Loop Quantum Gravity
The portion of the Inspiral of a binary system in which the
individual objects are highly distorted, and their orbit is
changing rapidly. This portion is not well-understood, and
must be simulated using Numerical Relativity.
A set of numbers which encodes information about the
geometry of space or spacetime. Together with an
understanding of how that information is encoded (using
Coordinates) everything about the geometry can be
A type of particle which has no charge and an extremely
small mass. It is a Fermion, and is extremely difficult to
stop or to detect. Nonetheless, they are produced in large
numbers. The Sun, for example, sends 30 million neutrinos
through every square inch of the Earth every single second.
They are so hard to stop, however, that if a neutrino were
sent through a solid light year of lead, it would still have
a 50:50 chance of flying right through without stopping.
One of the particles in an atomic nucleus. These
particles have no electric charge, but they hold together
the protons (positive particles in a nucleus), and account
for roughly half of the particles in the nucleus. Neutrons
are fermions, and are believed to form the majority of the
matter in a neutron star.
A type of star which is very old, having cooled off and
stopped nuclear fusion reactions. When gravity pulls the
star down on itself, the electrons and protons are squeezed
together, leaving just neutrons. The star is then supported
against gravity by "neutron degeneracy pressure"
(no two neutrons can be in the same place at the same time).
These are produced when a star is too heavy to be a white
dwarf, but not heavy enough to turn into a
Newton's First Law of Motion
See "First Law of Motion".
A physical process which takes a nucleus of a heavy element
(like Uranium or Plutonium, for example) and breaks them
down into two or more smaller nuclei. This process releases
large amounts of energy. It is sometimes called "Splitting
the Atom". This is used in many modern power plants to
generate electricity by heating water with the energy
released. The two or more smaller nuclei which are produced
are frequently toxic, and nearly always radioactive, which
makes this method of producing electricity controversial.
Fission is also the method used in simple nuclear weapons.
A physical process which takes light elements and combines
them into a heavier element. An example is the fusion of two
Hydrogen atoms to form a Helium atom. This is the process
which gives stars (including our Sun) heat and energy so that
they shine. Though fusion is not yet used to produce
electricity, scientists are working on this method, which
will provide a nearly-inexhaustible source of cheap
electrical energy, with little of the pollution or danger of
nuclear fission. Fusion is also the force behind some very
powerful nuclear weapons—the "H-Bomb".
The central part of an atom, which contains Neutrons and
Protons. Electrons are usually found around the Nucleus.
Strictly speaking, this is the only part of an atom involved
in Nuclear Reactions (Fission or Fusion).
The branch of Relativity research which deals with
simulating the development of Spacetime, using computers.
This is believed to be the only possible way to understand
things like the merger of two Black Holes.
A person or piece of equipment that measures something in
physics. Frequently, we speak of an observer measuring time
or a distance in a particular place.
A measure of distance which is roughly 3.26 Light Years.
The length of time between two events. For a wave, this is
usually the length of time it takes two successive peaks to
pass a given point. This number is simply 1 divided by the
Frequency of the wave.
1. For a wave, the position of any particular feature of the
2. For matter, a distinct form of a substance, such as solid, liquid,
A change of the state of matter from one phase to another, such as
the transition from liquid water to solid ice.
An Elementary Particle which carries the energy of light.
The photon is a Boson, and has no mass. It always moves
at the Speed of Light.
(Pronounced as "pie".) An important number in geometry.
This is defined to be the ratio of the circumference of a
circle to the diameter of that circle, in flat space. π
is an irrational number, which means that its exact value
cannot be written down, though it can be calculated as
precisely as necessary. Its value is approximately
Powers of 10
In order to write very large, or very small numbers
efficiently, scientists use a system of writing in powers of
10. For example, the number 108 represents 10 multiplied
by itself 8 times, which is a 1 with 8 zeroes after it:
100,000,000. Thus, rather than writing 300,000,000, a
scientist will write 3 × 108. Similarly, a number
like 10-21 represents 0.1 multiplied by itself 21
times, which is a 1 with 21 zeroes in front of it (including
the one before the decimal point): 0.000000000000000000001.
One of the particles in an atomic nucleus. These are
electrically positive particles which attract electrons to
the atom. Protons are fermions, and are very similar to
neutrons, except that they have electric charge, and a
slightly higher mass.
A neutron star with a very high rate of spin, and very
intense magnetic fields. The pulsar gives off beams of
radiation along its magnetic poles. If these poles are not
aligned with the spin poles, the beam will sweep around like
the beam of a lighthouse.
Quantum Field Theory
Quantum Field Theory in Curved Spacetime
A theory which attempts to incorporate ideas from Quantum Mechanics
and General Relativity. This theory is needed when the
quantum behavior of objects is important, and there is
extreme curvature of spacetime.
A modern physical theory which is vital to describing
extremely small objects, like electrons around an atom.
Though this theory also applies to larger objects, its
effects become very similar to those of Newton's theory—
which is typically much easier to use and understand. One
of the most important ideas in Quantum Mechanics is the
An Elementary Particle which makes up the Neutron and the
Proton, as well as many more exotic particles. These
particles are Fermions, and have charges of either 2/3 as
much as an electron's charge, or -1/3. They come in
"flavors" of up, down, strange, charmed, bottom, and top,
as well is in "colors" of red green and blue. The
"flavor" and "color" are just fanciful names given to
describe intrinsic properties of these particles—similar
to charge, mass, or spin.
A very dense and very bright object seen in the distant Universe.
Quasars are believed to be powered by Black Holes.
A theory of the Universe which postulates that we live in a
five-dimensional spacetime, though we are confined to a
four-dimensional Slice of it. This model is still being
explored as an accurate description of the Universe in which
The portion of an Inspiral, following the Merger, when the two
objects have combined into one. During this brief period,
the combined object will settle down by giving off
A point at which the curvature of spacetime becomes
infinite. Singularities can form—for example—when too
much matter is squeezed into a region which is too small.
Singularities are found at the center of a Black Hole, at
the beginning of the Universe in the Big Bang, and at the
end of the Universe (if it ever comes) in the Big Crunch.
Many scientists believe—though there is no solid evidence
—that all singularities in the present Universe lie hidden
behind Event Horizons. This is the "Cosmic Censorship
A theorem (a mathematically proven fact) which shows that a
singularity must exist under certain circumstances. For
example, physicists have shown that the Universe must
have begun with a singularity.
A concept in physics which merges our usual notion of space with our
usual notion of time.
Special Theory of Relativity
Einstein's version of the laws of physics, when there is no
gravity. The two fundamental concepts in the foundation of
this theory are equality of observers, and the constancy of
the speed of light. The first of these means that the laws
of physics must be the same, no matter how quickly an
observer is moving. The second means that everyone measures
the exact same speed of light. This theory is useful
whenever the effects of gravity can be ignored, but objects
are moving at nearly the speed of light. It has been
successfully tested many times in particle accelerators, and
orbiting spacecraft. For objects moving much more slowly
than light, Special Relativity becomes very nearly the same
as Newton's theory, which is much easier to use.
For a wave, the speed of a particular point (such as its crest).
Speed of Light
A constant of Nature. This speed is precisely 299,792,458
meters per second, or roughly 670,616,629 miles per hour.
One of the most unusual discoveries of science has been the
fact that all Observers measure light as moving at exactly
this speed, even if those observers are moving relative to
each other. This fact is one of the basic ingredients in
Einstein's Special Theory of Relativity.
An intrinsic property of particles. (That is, a property
which does not change. Mass and electric charge are
examples of intrinsic properties.) Spin is related to the
usual notion of spin, though it is a little more difficult
to understand. Spin comes in units of 1/2, so that a
particle may have a spin of 0, 1/2, 1, 3/2, and so on.
A particle's spin determines whether it is a Fermion or a
Pertaining to a process involving a randomly-determined sequence of
The fundamental object in String Theory, which replaces the
notion of a particle in standard Quantum Mechanics. Rather
than being a simple point-like object, fundamental particles
become tiny strings or loops. The vibrations of these
strings result in various properties like spin.
A theory of physics taking the String as its fundamental
object. This theory attempts to solve problems in standard
Quantum Mechanics and Quantum Field Theory. It actually
predicts the existence of gravity.
Another name for a "Supersymmetric String", which is
usually referred to simply as a String.
Violently exploding stars which shine very brightly for days
or weeks. They occur when the fuel for nuclear reactions is
used up, and a star cools. Gravity pulls all the matter
down toward the star's center. If this happens quickly,
nuclear reactions may suddenly begin again, detonating the
star in a nuclear explosion.
The principle of Quantum Mechanics—as well as Quantum
Field Theory and String Theory—which says that an
observer can never know both the position and velocity of a
particle with perfect precision. Specifically, the more
certain an observer is of the position, the less certain
that observer must be of the velocity, and vice-versa.
The energy that is present even in otherwise empty space.
This energy has been measured to exist (in the "Casimir
Effect"). Whereas matter causes the expansion of the
Universe to slow down, vacuum energy actually causes the
expansion to speed up.
A particle which cannot be directly detected, but is assumed
to exist due to its indirect effects on real particles.
Virtual particles can form in pairs from the vacuum of
The distance between two neighboring peaks or troughs of a
A type of star which is very old, having cooled off and
stopped nuclear fusion reactions. A white dwarf is
supported by "electron degeneracy pressure" (no
two electrons can be in the same place at the same time).
These are produced when a star is not heavy enough to turn
into a Neutron Star or a Black Hole.
A tube in spacetime which connects two widely separated
places in the Universe. Wormholes could provide the
possibility of time travel, though they would probably be
very unstable, and only exist for a short period of time.
A type of light—or electromagnetic wave—which is
invisible to the naked eye. X-Rays are much more energetic
than the light we see. They can penetrate skin very easily,
for example. In the doctor's or dentist's office, X-Rays
are detected on a photographic plate, allowing us to see
inside the body. Like all other forms of electromagnetic
radiation, X-Rays travel at roughly 300,000,000 meters per
second (186,000 miles per second).