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The term **relativity** is used for two theories developed by Albert Einstein
that accommodate the apparent constancy of the speed of light,
even to someone traveling fast in the same direction as the
light beam.
The constancy was evident in the **Michelson-Morley experiment**, which
strove to find the velocity of Earth as compared to **ether**,
the name given to whatever substance has the waves that we perceive
as electromagnetic radiation. The inability to measure such a velocity by such
means was a chink in the Newtonian **paradigm**, i.e.,
the laws of gravity and motion summarized by Isaac Newton.

The Lorentz transformation (aka **Lorentz transform**)
was a mathematical attempt to explain the apparent constancy through
effects of motion on the dimensions of objects. Einstein showed the
transform was consistent with a model of nature that matched
experiment, but at the cost of throwing away our preconceived notion of
**simultaneity**, events occurring at the same instant in time: that
whether two events are simultaneous depends on the relative motion
of observers. The theory demands the Lorentz transform rather than
the **Galilean transformation** (aka **Galilean transform**), which
matches our intuitive senses and everyday experiences, but at typical
everyday speeds (aka **non-relativistic speeds**), the two converge.
(The adjective **relativistic** is often used specifically to specify
a regime in which relativity plays a significant role, particularly,
involving speeds near **c**, the speed of light in a vacuum).

This was termed *relativity*, then later **special relativity** when
Einstein developed general relativity, which extends it, also modeling gravity.

Relativity shows how even though you and your surroundings may be
in motion, that motion isn't evident unless you can observe something
moving relative to you. In addition to Einstein's conceptions, the
term *relativity* is also used for earlier explications of this concept, that
didn't take into account factors evident more recently when Einstein
tackled the problem, e.g., **Galilean relativity**.

While Einstein's relativity gives up the concepts of absolute time, simultaneity, and distance, identical in all frames of reference (i.e., the same no matter what your speed and direction while you measure), it does yield a minimal time or distance between events:

**Proper time**is a minimum possible time between two events, which a pair events have only if there is a*frame of reference*in which they are in the same place:*proper time*, is time measured in that frame.**Proper distance**is the minimum possible distance between two events, which a pair events have only if there is a*frame of reference*in which they take place at the same time:*proper distance*, is distance measured in that frame. It may be referred to as**proper length**, e.g., if speaking of an object stretching from one event location to the other.

barycenter

dark energy

Einstein-de Sitter model

fine structure

frame of reference

general relativity (GR)

gravitational wave (GW)

hydrodynamics

light cone

Lorentz transformation

mass

mathematical field

metric

numerical relativity (NR)

partial differential equation (PDE)

quantum mechanics (QM)

radial velocity (RV)

redshift (z)

relativistic energy

relativistic invariance

relativistic momentum

relativistic speed

radiation hydrodynamics (RHD)

rotation period

spacetime

spacetime diagram

speed of light (c)

strong-field gravity

time dilation

time standard

worldline

wormhole