Special Relativity

Terms Used, and Terms Avoided

Language and thought are deeply entangled with each other. In order to think clearly, your language needs to be crystal clear. This is true in all subjects, and it's especially true in special relativity, where the ideas contradict old ways of thinking.

The terms used on this site are sometimes non-standard. The reason is that, in the opinion of the author, the alternative terms do a slightly better job at what terms are supposed to do - communicate ideas as clearly, simply, and accurately as possible, without ambiguity. In addition, the target audience for this site are those unfamiliar with special relativity, not experienced physicists.

From Space and Time in Special Relativity, by David Mermin:

Language can be enormously confusing when it is used to discuss relativity, because words and even grammar often introduce physical assumptions into what we say with such subtlety that we fail to realize that the assumptions are present.

...misunderstanding is continually generated by commonplace, incorrect notions that are often implicit in the very language we use (a pre-relativistic structure) and are therefore particularly difficult to recognize.

Observer
This term is used nowhere on this site. It's much too ambiguous and should be abandoned. It connotes a single entity or person, when it often should connote just the opposite. If used indiscriminately, it can conflate two ideas that need to be separate: the idea of a grid of multiple sensors, and the idea of a single sensor viewing its past light cone. The idea of a grid (frame of reference) is of fundamental importance, and needs to be absolutely clear, with no fuzziness or ambiguity.

History
The terms history and world-line are synonymous. The term history is preferred here because:

it's already familiar to people
it has the appropriate connotation

Wikipedia (at the time of writing) defines world-line as follows: "In physics, a world line of an object (approximated as a point in space, e.g., a particle or observer) is the sequence of spacetime events corresponding to the history of the object." The best technical terms have definitions that read like tautologies: the term is already the simplest way of expressing it.

Grid
The term grid (or sometimes sensor grid) is used here instead of frame of reference. Reasons:

it has a better connotation for a lattice of sensors
it's strongly visual
it feels more down-to-earth, and less abstract
it's shorter

Note that Robert Wald in General Relativity uses the word grid multiple times, in an informal manner.

Rules of Physics
Here, the more conservative term rules of physics is used instead of laws of physics. Reasons:

law connotes immutability, while rule connotes something that's open to change
law connotes irrefutable truth, while rule does not

Speed Limit
The term speed limit is almost always used instead of speed of light. The speed limit c is the speed of any massless particle - photons, massless neutrinos, and gravitons. It isn't solely the speed of photons. The most important thing about c is that it's a speed limit for all signals, not just for light. Referring to this fundamental entity as the speed of light is a historical accident, and, in the opinion of the author, probably a bad habit that reduces understanding of the physics.

Principle of Relativity
Somewhat strangely, this term is ignored here. There are various versions of it, as typically defined:

Galileo's version
Einstein's version for special relativity
Einstein's version for general relativity

This is confusing to beginners. They expect that a principle shouldn't change according to context.

Secondly, as used by some authors, the term often mixes two separate ideas that are best kept separate:

the group of coordinate transformations for which a given set of rules is meant to be invariant/covariant.
the actual invariants themselves.

Clock and Sensor
The word clock is used on this site. But the word sensor is often used instead, because sensors can do more than clocks:

sensors can detect events.
sensors can communicate with sensors in other grids (sharing time readings, for example).
sensors can have arbitrary features added at will, when needed.

In the age of omnipresent computers, the idea of a sensor is just as familiar to most people as that of a clock.

Interval Shell
Surfaces of constant s² with respect to an event trace out a hyperbola (or hyperboloid of revolution) in space-time. Like the light cone, this is an important idea, and it appears repeatedly. It deserves its own term. Instead of referring to it as the hyperbola of constant s², this site calls it the interval shell, or simply shell for short.

Boost Transformation
This term is used instead of Lorentz transformation. Reasons:

it's more descriptive
it's more useful to the reader to be reminded of the physics, instead of the name of originator of the transformation
the precise meaning of Lorentz transformation varies between authors

Relativistic Mass
This term is completely avoided here. The mass is treated as an invariant quantity.

Superluminal Motion
This term is avoided here. Careful authors use the term apparent superluminal motion. But some authors aren't so careful, and truncate this to just superluminal motion. Consequently, people think that the speed limit rule is being violated, when of course it isn't.