Talk:Special relativity: kinematics

Professor Wolfgang Rindler literally wrote the textbook on special relativity, and he makes an appropriate and authoritative choice to write a Scholarpedia entry on the kinematical aspects of the theory. This is a fine article, written at a level suitable for a smart high-school student, or a beginning undergraduate student. No changes are required.

Reviewer B

This is an excellent introduction to special relativistic motion and phenomena (time dilation, simultaneity, etc.) The author's choice not to include a derivation of the Lorentz transformation seems at first to be strange but is in fact a good choice for keeping the article brief and focused on relativistic phenomena.

I list below a number of minor issues, but there is a something not so minor to be considered first. I think that the article would be better suited to the purposes of Scholarpedia if it were shorter, and I believe that there is a natural way to make it shorter: section 4, the section on space-time diagrams could be removed with little effect on the rest of the article. Space-time diagrams are a very nice tool for pedagogy in relativity, and in fact one excellent introduction, the text by Taylor and Wheeler, relies very heavily on spacetime diagrams as the fundamental teaching tool. But that is not the case in this article. Instead, space-time diagrams are mostly used to get an additional viewpoint on relativistic phenomena. That would be fine in a text in which brevity were not an important issue. I think that in this article it is not so fine.

Moreover, the space-time diagrams come at a cost in addition to length. In Section 4 the author deviates further from the heart of SR kinematics than elsewhere in this generally excellent article. There are, for instance, the "calibrating hyperbolae", which are interesting but in connection with the graphs, not with SR kinematics itself. This criticism, in my view, also applies sharply to the discussion of the active transformation, and to the last several paragraphs of Section 4.

My recommendation is to remove Section 4 from the main text. It could be made a sort of appendix, or perhaps with some expansion, as a separate article on "Space-time Diagrams." The special relativistic Doppler shift which, I believe, occurs only in this section would need to be moved to the previous or following section.

Minor points

• The article would benefit from more varied references.

As a good introduction suitable for high school students I recommend "Spacetime Physics" by Edwin F. Taylor and John Archibald Wheeler (W.H.Freeman, 1992).

An interesting and very unusual introductory approach is taken in the book "It's About Time: Understanding Einstein's Relativity" by N. David Mermin (Princeton U. Press, 2009)

• The concept of "proper" is used several times (proper time, proper

acceleration). It would be good if early in the article the general idea of a "proper" measurement (measurement in a momentarily comoving frame) were introduced.

• In the last paragraph of the first section, the article states "in

this and the following two articles". Does article here mean section, and this left over from an earlier version? Or are there supposed to be linked articles?

• In Section 2, in the paragraph starting "Henceforth SR became",

there is a discussion in which a small freely falling cabin becomes a SR approximation because it takes up little space, but really a SR approximation to curved space-time must be limited to a small region of space-time. The cabin will be a good approximation only for a limited time.

• This is a very picky point, but.. at the end of Section 2 there is

mention of relative acceleration of two SR local frames. This isn't really acceleration in the same sense as the acceleration that is discussed in Section 5 since there is no global SR frame. Rather, it is an increase in distance that is not linear in time.

• The 3rd paragraph of Section 3 would be made clearer if to the phrase

"will light up before B, since it is closer to the emission point in S'" were added the words "at the time the light signal is received".

• At the end of Section 4 it is stated that photons move with an

infinite proper acceleration. This is a cute point, but in an introduction to relativity it is more likely to confuse than to instruct.

User 3 (User:): Orbits vs. Geodesics.

"... gravitational orbits become inertial (straightest) paths in curved space-time."

Only idealized test particles and uncharged spheres of uniform density move on geodesics. The center of mass of a non-spherical or non-uniform rigid body is, in general, accelerated with respect to its instantaneously co-moving inertial frame. The planets do not move on exact geodesics, their harmonic coefficients perturb their orbits. The oblateness of the Earth, Sun, Jupiter Saturn, and the triaxiality of Mercury are all significant at today's level of accuracy.