The notion of the Present also involves the notion of
. All of the events simultaneous
event are part of the Present. But, as Einstein showed, the notion of simultaneity is a
relative one. Let us look at Einstein’s Special Theory of Relativity (STR).
First, some background. Newton founded his dynamics upon the twin notions of absolute space and
time. As he says in the Scholium, “Absolute, true, and mathematical time, of itself, and from its own
nature flows equably without regard to anything external, and by another name is called duration:
relative, apparent, and common time, is some sensible and external (whether accurate or unequable)
measure of duration by the means of motion, which is commonly used instead of true time; such as
an hour, a day, a month a year.” He says similar things about absolute space. The postulation of
absolute space and time bothered many of his contemporaries, insofar as both are invisible and
undetectable. But Newton needed them in order to found his principles of motion, and his theory of
universal gravitation. A century and a half later, Maxwell, building upon the physical insights of
Michael Faraday, found the laws that unified electricity and magnetism. Among other things, these
laws indicated that light was an electro-magnetic field propagating as a wave through space. On
Maxwell’s view, a propagating wave required a medium, as a sound wave requires air, so the
was proposed to serve that role. The aether was thought to fill all of space, and
to be at rest relative to absolute space. After a time, the aether came to be identified with absolute
space. But if the aether is at rest, and the earth moves through it on its journey around the sun, then
effect of this motion on light should be observable. In a famous set of experiments in 1887 in
Cleveland, Ohio, Michaelson and Morley attempted to measure the velocity of the earth through the
aether. They found no evidence of any such motion. Their “null result” caused quite a stir.
Einstein begins, in his 1905 paper “On the Electrodynamics of Moving Bodies”, with two
assumptions, or postulates.
The Principle of Galilean Relativity, which states that the laws of nature are the same in any
inertial reference frame.
The velocityof light is the same in everyinertial reference frame, independent of the velocity
of the light source.
The justification of the first comes from, among other things, “.
..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.” The second comes from the results of manymeasurements over manyyears. He then addresses
one of Poincaré’s questions, viz., how do we establish the simultaneityof distantlyseparated events.