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
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.
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.
- it's already familiar to people
- it has the appropriate connotation
The term grid (or sometimes sensor grid) is used here instead of frame of reference.
- it has a better connotation. A frame of reference needs to be a grid or lattice of sensors, in
order that measurements are all local.
- it has a stronger visual connotation
- it feels more down-to-earth, and less abstract
it's a lot shorter. The fundamental ideas of a subject are used again and again when exploring the subject.
Given a choice, it makes sense to prefer short terms for those that are used most frequently.
Rules of Physics
Here, the more conservative term rules of physics is used instead of laws of physics.
- law connotes immutability, while rule connotes something that's open to change
- law connotes irrefutable truth, while rule does not
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.
- Galileo's version
- Einstein's version for special relativity
- Einstein's version for general relativity
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.
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.
- Galileo's version of the principle of relativity is taken as applicable to the full Lorentz group.
- Einstein's version of the special principle of relativity is also taken as applicable to the full Lorentz group.
But authors then tack on, as a sort of addendum, the assertion of the invariance of c.
This is mixing together two different things. The Galilean statement of the principle makes no
explicit mention of the invariance of time or distance, so this isn't symmetric; the term is not being
treated the same in both cases.
The word clock is used on this site.
But the word sensor is often used instead.
- sensors need to detect events. Clocks can't do that.
- sensors need to communicate with sensors in other grids (sharing time readings, for example). Clocks can't do that.
- sensors can have arbitrary features added at will, when needed. Clocks can't do that.
- in the age of omnipresent computers, the idea of a sensor is just as familiar to
most people as that of a clock.
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.
This term is used instead of Lorentz transformation.
- it's more descriptive
- it's more useful to the reader to be reminded of the physics, instead of the name of equation's originator
This term is completely avoided here.
The mass is an invariant quantity.
This term is used instead of propagation vector or wave vector, because it's more descriptive.
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.