An Illustrated Guide to Relativity


This class is based on the book, An Illustrated Guide to Relativity,
by Tatsu Takeuchi, of Virginia Polytechnic Institute and State
University, a delightful book that uses simple space-time diagrams to
visualize and teach the basic features of special relativity. This is
done so well that the material can, in principle, be learned directly
from the figures and annotations without referring to the main text
at all.

Online Resources

Review Street Lamps Problem    

Inertial Frames of Reference


   In physics, an inertial frame of reference (also inertial reference
   frame or inertial frame or Galilean reference frame) is a frame of
   reference that describes time and space homogeneously, isotropically,
   and in a time-independent manner.

   All inertial frames are in a state of constant, rectilinear motion
   with respect to one another; they are not accelerating in the sense
   that an accelerometer at rest in one would detect zero acceleration.
   Measurements in one inertial frame can be converted to measurements
   in another by a simple transformation (the Galilean transformation in
   Newtonian physics and the Lorentz transformation in special
   relativity). In general relativity, in any region small enough for
   the curvature of spacetime to be negligible one can find a set of
   inertial frames that approximately describe that region

1. Frames of Reference

2. Inertial Frames

3. Laws of Physics in Inertial Frames

4. Newton's Second Law

By A. Einstein
June 30, 1905

I just want to read to you the first two paragraphs of
Einsteins 4th paper 1905 Paper
   It is known that Maxwell's electrodynamics--as usually
   understood at the present time--when applied to moving
   bodies, leads to asymmetries which do not appear to be
   inherent in the phenomena. 

   Take, for example, the reciprocal electrodynamic action
   of a magnet and a conductor. The observable phenomenon
   here depends only on the relative motion of the conductor
   and the magnet, whereas the customary view draws a sharp
   distinction between the two cases in which either the one
   or the other of these bodies is in motion. For if the
   magnet is in motion and the conductor at rest, there
   arises in the neighbourhood of the magnet an electric
   field with a certain definite energy, producing a current
   at the places where parts of the conductor are situated. 

   But if the magnet is stationary and the conductor in
   motion, no electric field arises in the neighbourhood of
   the magnet. In the conductor, however, we find an
   electromotive force, to which in itself there is no
   corresponding energy, but which gives rise--assuming
   equality of relative motion in the two cases
   discussed--to electric currents of the same path and
   intensity as those produced by the electric forces in the
   former case.

   Examples of this sort, together with the unsuccessful
   attempts to discover any motion of the earth relatively
   to the "light medium," suggest that the phenomena of
   electrodynamics as well as of mechanics possess no
   properties corresponding to the idea of absolute rest. 

   They suggest rather that, as has already been shown to  (1)
   the first order of small quantities, the same laws of
   electrodynamics and optics will be valid for all frames
   of reference for which the equations of mechanics hold
   good. We will raise this conjecture (the purport of which
   will hereafter be called the ``Principle of Relativity'')
   to the status of a postulate, 

   and also introduce another postulate, which is only     (2)
   apparently irreconcilable with the former, namely, that 
   light is always propagated in empty space with a definite
   velocity c which is independent of the state of motion of
   the emitting body. 

   These two postulates suffice for the attainment of a
   simple and consistent theory of the electrodynamics of
   moving bodies based on Maxwell's theory for stationary

   The introduction of a "luminiferous ether" will prove
   to be superfluous inasmuch as the view here to be
   developed will not require an "absolutely stationary
   space" provided with special properties, nor assign a
   velocity-vector to a point of the empty space in which
   electromagnetic processes take place.

   And, of course the paper goes on to develop the ideas
   and make his case...