Contravariant components of the stress-energy tensor. Stress–energy tensor
World line In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" (such as an orbit in space or a trajectory of a truck on a road map) by the time dimension, and typically encompasses a large area of spacetime wherein perceptually straight paths are recalculated to show their (relatively) more absolute position states — to reveal the nature of special relativity or gravitational interactions. The idea of world lines originates in physics and was pioneered by Hermann Minkowski.
Twin paradox In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as traveling, and so, according to an incorrect naive application of time dilation, each should paradoxically find the other to have aged more slowly.
Time dilation explains why two working clocks will report different times after different accelerations. Time dilation
The speed at which light propagates through transparent materials, such as glass or air, is less than c.
Principle of relativity In physics, the principle of relativity is the requirement that the equations describing the laws of physics have the same form in all admissible frames of reference.
Relativity of simultaneity Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.
In non-relativistic classical mechanics, the use of Euclidean space instead of spacetime is appropriate, as time is treated as universal and constant, being independent of the state of motion of an observer. In relativistic contexts, time cannot be separated from the three dimensions of space, because the observed rate at which time passes for an object depends on the object's velocity relative to the observer and also on the strength of gravitational fields, which can slow the passage of time. Until the beginning of the 20th century, time was believed to be independent of motion, progressing at a fixed rate in all reference frames; however, later experiments revealed that time slows at higher speeds of the reference frame relative to another reference frame.
The invariant mass, rest mass, intrinsic mass, proper mass, or (in the case of bound systems or objects observed in their center of momentum frame) simply mass, is a characteristic of the total energy and momentum of an object or a system of objects that is the same in all frames of reference related by Lorentz transformations. If a center of momentum frame exists for the system, then the invariant mass of a system is simply the total energy divided by the speed of light squared. Invariant mass
Different aspects of "frame of reference" The need to distinguish between the various meanings of "frame of reference" has led to a variety of terms. Frame of reference
The dark blue vertical line represents an inertial observer measuring a coordinate time interval t between events E1 and E2. Proper time
Proper length In relativistic physics, proper distance is an invariant measure of the distance between two spacelike-separated events, or of the length of a spacelike path within a spacetime. In contrast, proper length or rest length refer to the length of an object in the object's rest frame, which is not necessarily the same as the proper distance between two events. The measurement of lengths is more complicated in the theory of relativity than in classical mechanics.
Gravitomagnetism Gravitoelectromagnetism, abbreviated GEM, refers to a set of formal analogies between the equations for electromagnetism and relativistic gravitation; specifically: between Maxwell's field equations and an approximation, valid under certain conditions, to the Einstein field equations for general relativity.
Gravitation, or gravity, is a natural phenomenon by which all physical bodies attract each other.
Minkowski space In theoretical physics, Minkowski space is often contrasted with Euclidean space.
Inertial frame of reference
Geodesic (general relativity)