Contents

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 subject matter is so unusual.

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.

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. It conflates 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 used by this site because:

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:

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

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. For one thing, there are various versions it, as typically defined:

This is confusing to beginners. They expect that a principle should be something that doesn't change according to context.

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

An illustration:

Einstein was deeply conservative in the sense that he wanted to extend all of physics to the same group as used by Galileo/Newton. To preserve that group he was forced to replace the assumption of the invariance of time-intervals and distances with the invariance of c. This is an important point. The clearest way to make this point is to keep separate the two ideas of a group, and the invariants of that group.

Clock
The word clock is used on this site. But the word sensor is often used instead. Reasons:

Interval Shell
Surfaces of constant interval 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 interval, this site calls it the interval shell, or simply shell for short.

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

Photon Vector
This term is used instead of propagation vector or wave vector, because it's more descriptive.

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.