*A Brief History of Time*).

## Absolute space/time

The ideas of absolute space and time came to the forefront of scientific debate only when challenged. Sir Isaac Newton’s laws of motion suggested that space was not absolute because objects do not have…
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## Anthropic principle

This philosophical principle arose in response to the question of why the conditions in the universe are just right to support life—whether it is sheer coincidence, or whether the universe has developed specifically to be…
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## Anti-particle

Each particle has its matching antiparticle. When the two collide, they annihilate each other—in the process creating energy that is then emitted into the universe.
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## Arrows of time

Stephen Hawking suggests there are three arrows of time. First, there is the thermodynamic arrow, which relates to entropy. This arrow points in the direction of disorder increasing—for example, whole glasses smashing into pieces…
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## Atom

The base component of matter, comprising a nucleus of neutrons and protons, which is orbited by electrons. The electromagnetic force holds the particles in the atom together.
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*A Brief History of Time*LitChart as a printable PDF.## Big bang

The big bang theory first came into being after Edwin Hubble discovered that all galaxies everywhere are rapidly moving away from one another. This means all matter most likely started off all in one place…
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## Black hole

A black hole is a localized singularity that forms from a collapsing star. Once the star uses up its fuel, its spent energy is not enough to balance its own gravity, and it begins…
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## Chandrasekhar limit

Subrahmanyan Chandrasekhar found the limit at which a star’s mass is too big for it to stabilize after it runs out of fuel. If the star is bigger than this limit, it will collapse under…
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## Cosmological constant

First created by Albert Einstein to make his theory of relativity work within a static universe, this was considered a kind of anti-gravity force that would hold everything together. When the theory of a…
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## Electromagnetic force

One of the four main types of forces in the universe, electromagnetism draws electrically charged particles toward or away from one another according to their charge. There are positive, neutral, and negatives charges. The electromagnetic…
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## Electron

A negatively charged particle that orbits the nucleus of an atom. Electrons stay on fixed orbits due according to their wavelengths.
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## Entropy

The second law of thermodynamics states that disorder, or entropy, tends to increase in any isolated system. For example, a box with a divide in the middle could have oxygen on one side and nitrogen…
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## Ether

When people still thought space was absolute—that is, that every observer anywhere could measure space in the same way regardless of their position or velocity— they needed to explain what light traveled through so…
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## Event horizon

The event horizon is the boundary of a black hole at which light, or anything else, cannot escape its gravitational pull. This is why why we cannot see black holes even though they glow. Jacob…
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## Friedmann model

Alexander Friedmann proposed models of the universe based on the idea it is uniform on a large scale and as such should look uniform from any given point. From his work came a variety of…
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## Graviton

The gravity force-carrying particle with spin 2.
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## Gravity

First put forward by Sir Isaac Newton, the theory of gravity states that all particles have an attractive gravitational force that draws them together. This operates over long distances, and although it is weak…
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## Imaginary numbers/time

If real numbers run on a left to right axis, imaginary numbers run up and down. They allow for negative answers to multiplication, for example -2 times -2 equals 4, but i2 times i2 equals…
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## Neutron

A neutrally charged particle in the nucleus of an atom.
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## Neutron stars

These stars are supported against their own gravity by the exclusion principle between neutrons, so they do not collapse into black holes. They are very small, cold, and dense. Certain kinds are called…
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## Newton’s laws of motion

Sir Isaac Newton overturned Aristotle’s idea that matter has a natural state of rest. Instead, he showed that forces act on objects to accelerate or change their velocity, not to start them moving in…
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## Pauli exclusion principle

Wolfgang Pauli put forward the idea that similar particles cannot be in the same place as one another while moving in the same direction. They repel each other, meaning particles of the same kind will…
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## Photon

A quantum of light, also seen as a force-carrying particle.
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## Proton

A positively charged particle in the nucleus of an atom.
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## Quantum

A quantum is a packet of energy that is always emitted in certain quantities. The rate at which hot bodies, such as stars, lose energy is finite. The higher the frequency of the wave, the…
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## Quantum mechanics

A theory of how small particles are formed and act based on the quantum principle (by which energy is emitted in certain packets, or quanta) and the uncertainty principle.
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## Quark

The most basic building blocks of particles that scientists have found yet. They come in different kinds and “colors,” which combine to form the particles in an atom.
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## Quasars

A region of a galaxy that is collapsing into a large black hole at the center. Also known as quasi-stellar objects, quasars shine brightly before they are sucked into the black hole.
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## Red shift

Edwin Hubble first noticed that the light given off by stars in distant galaxies is shifted toward the red end of the spectrum of light. Red light has a longer wavelength than other colors, indicating…
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## Singularity

A singularity is a space of zero size holding an infinitely dense amount of matter. Black holes are localized singularities. The big bang starts with and the big crunch ends in a singularity. It is…
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## Space-time

The three dimensions of space and the dimension of time come together under the general theory of relativity to create space-time. Events take place on a point in space-time.
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## Spin

Each particle (or anti-particle) is associated with a spin, which reflects the number of times one needs to turn the particle until it looks the same. For example, a single-headed arrow must complete one…
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## String theory

A relatively modern theory, the idea is that particles are not dots in space-time but rather waves of infinitely long, one-dimensional lengths, like strings. These strings can join together and separate but require the existence…
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## Strong nuclear force

One of the four major types of force, the strong nuclear force creates the particles within an atom by binding together quarks.
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## Sum over histories

Richard Feynman first came up with the idea that particles do not have one history, but rather have every possible history. This means scientists cannot say exactly how a particle traveled from A to B…
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## Symmetry

The laws of physics can be said to obey certain symmetries. Symmetry C is when the laws for particles are the same as their anti-particles, P is when the laws are the same in…
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## Theories of Relativity

Albert Einstein first suggested the theory of relativity in 1905, which states every observer has their own unique measure of time, though the laws of science and the speed of light are the same for…
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## Uncertainty principle

Werner Heisenberg proposed the uncertainty principle when he found it was impossible to accurately measure the position and velocity of a particle without that examination affecting the particle’s position or velocity. To see the position…
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## Unified theory of physics

Stephen Hawking’s life’s work was to find the unified theory of physics, which would successfully integrate quantum mechanics with the theory of relativity. Many have joined him on this quest, but so far…
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## Virtual particle

A particle that is too small to be seen directly but can be detected by its effects on other particles.
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## Wavelength

Imagine a ripple on a pond—there are peaks and troughs to the wave. The more powerful the ripple, the shorter the gap between the peaks, called the wavelength, and the higher the frequency of the…
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## Weak nuclear force

One of the four main forces, this is the nuclear force relating to radioactivity and particles of spin ½, which means they look the same only after two turns.
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## Wormhole

First proposed by Albert Einstein and Nathan Rosen in 1935, wormholes are small irregularities in space-time that allow short cuts to far-distant regions of the universe. It could be possible for an advanced civilization to…
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