Physics Reference Libraries

Strange Matter -- Strange matter (also known as quark matter) is an ultra-dense phase of matter that is theorized to form inside particularly massive neutron stars (which are then known as "strange stars" or "quark stars"). It's theorized that when neutronium is put under sufficient pressure due to the gravitation of a large neutron star, the individual neutrons break down and their...

Spacetime -- In special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional manifold called spacetime (alternatively, space-time; see below). A point in spacetime may be referred to as an event. Each event has four coordinates (t, x, y, z). Just as the x, y, z coordinates of a point depend on the axes one is using, so...

Space -- The definition of space in physics is contentious. Various concepts used to try to define space have included: -- the structure defined by the set of "spatial relationships" between objects -- a manifold defined by a coordinate system where an object can be located. -- the entity that stops all objects in the universe from touching one another In classical physics,...

Event Horizon -- The event horizon is a boundary beyond which information will never reach an observer. An event horizon is a mathematical construct and not a physical object and a person passing through an event horizon will not notice any odd behavior. From an outside observer however, an object passing though an event horizon will appear redder and dimmer and will appear to freeze at the...

Escape Velocity -- An escape velocity is the minimum speed at which an object without propulsion can move away from a source of a gravitational field indefinitely if there is no friction. This definition may need modification for the practical problem of two or more sources in some cases. In any case, the object is assumed to be a point with a mass that is negligible compared with that of...

Electromagnetic Spectrum -- The electromagnetic spectrum describes the various types of electromagnetic radiation based on their wavelengths. Radio, representing wavelengths from a few feet to well over a mile, is at one end of the spectrum. Gamma ray radiation is at the other end: the wavelength of the harder types is so short, in the subatomic range, that we do not have instruments...

Electromagnetic Radiation -- Electromagnetic radiation is a combination of oscillating electric and magnetic fields propagating through space and carrying energy from one place to another. Light is a form of electromagnetic radiation. The theoretical study of electromagnetic radiation is called electrodynamics, a subfield of electromagnetism. When any wire (or other conducting object...

Doppler Effect -- The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves, that propagate in a wave medium, the velocity of the observer and the source are reckoned relative to the medium in which the waves are transmitted. The total Doppler effect may therefore...

Quintessence (Dark Energy) -- Quintessence or dark energy is a hypothetical form of energy postulated to exist in order to explain observations of an accelerating universe. This energy would act like a vacuum pressure, pushing things apart. Other attempts to explain these recent observations involve a non-zero cosmological constant, which has the same effect. Indeed, sometimes...

Cosmological Constant -- The cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is a value occurring in Einstein's theory of general relativity. The units of Λ are 1/second2; its value is unknown but believed to be positive based on recent observations. The constant is proportional to the energy density of the vacuum ρ, where π is Pi, G is the...