Editing Special relativity (section)

=== Reference frames and relative motion ===

[[File:Frames of reference in relative motion.svg|thumb|right|300px|Figure 2–1. The primed system is in motion relative to the unprimed system with constant velocity ''v'' only along the ''x''-axis, from the perspective of an observer stationary in the unprimed system. By the [[principle of relativity]], an observer stationary in the primed system will view a likewise construction except that the velocity they record will be −''v''. The changing of the speed of propagation of interaction from infinite in non-relativistic mechanics to a finite value will require a modification of the transformation equations mapping events in one frame to another.]]

[[Frame of reference|Reference frames]] play a crucial role in relativity theory. The term reference frame as used here is an observational perspective in space that is not undergoing any change in motion (acceleration), from which a position can be measured along 3 spatial axes (so, at rest or constant velocity). In addition, a reference frame has the ability to determine measurements of the time of events using a "clock" (any reference device with uniform periodicity).

An [[Event (relativity)|event]] is an occurrence that can be assigned a single unique moment and location in space relative to a reference frame: it is a "point" in [[spacetime]]. Since the speed of light is constant in relativity irrespective of the reference frame, pulses of light can be used to unambiguously measure distances and refer back to the times that events occurred to the clock, even though light takes time to reach the clock after the event has transpired.

For example, the explosion of a firecracker may be considered to be an "event". We can completely specify an event by its four spacetime coordinates: The time of occurrence and its 3-dimensional spatial location define a reference point. Let's call this reference frame ''S''.

In relativity theory, we often want to calculate the coordinates of an event from differing reference frames. The equations that relate measurements made in different frames are called ''transformation equations''.