Historical Background 97eral relativity. An interested reader can follow up on some of the topics in
Problem 2.2, page 414, and in Section 3.
2.4.1 Historical Background
As one legend puts it, back when he was a little boy, A. Einstein was curious
about two questions: (1) What happens if you run after a ray of light and
try to catch it? (2) What happens if you are in a moving elevator and
suddenly somebody turns off the gravity? The answers to these questions
later became the theory of special and general relativity.
We mentioned earlier that both the "relative" and "absolute" points
of view had proponents from the early years of modern science. In 1707,
the Irish bishop George Berkeley disputed the concept of absolute space
as described in Newton's 1687 book Principia. Newton contended that an
absolute rotation can be determined experimentally by measuring the effect
of a centrifugal force as in his famous experiment with a rotating bucket of
water in which the surface of the rotating water assumed a concave shape.
Hence, Newton argued, there is no need for a second body relative to which
the rotation is measured. Berkeley disputed this on philosophical grounds,
saying that "motion cannot be understood without a determination of its
direction, which in turn cannot be understood except in relation to our or
some other body." Absolute space, he claimed, is unobservable. If a body
is the only one in the universe, it is meaningless to speak of its rotation,
and we need the existence of other bodies, relative to which the rotation
can be measured. We now know mathematical formulas supporting this
argument: for example, the centrifugal force -mw x (a; x r) in (2.1.47) on
page 66 involves the rotation vector u> that must be defined in an inertial
frame. For more about the dispute between Newton and Berkeley, see the
book The Physical Foundations of General Relativity by D. Sciama, 1969.
The propagation of light was more difficult to understand. Up until the
late 19th century, it was assumed that light propagated through the aether
(or ether), a hypothetical substance believed to be filling all absolute space
and serving as the medium for the propagation of electromagnetic waves.
In 1887 in Ohio, two American physicists, ALBERT ABRAHAM MICHELSON
(1852-1931) and EDWARD WILLIAMS MORLEY (1838-1923) conducted an
experiment aimed at measuring the velocity of the Earth relative to the
aether.
The relative propagation speed of other wave disturbances in a medium,
like sound and water waves, depends on the velocity of the source. If the