Newton laid the foundations of classical mechanics in the 17th
century. His monumental Principia
enunciated simple laws of motion. These laws, when augmented by Newton's law of
gravitation, dominated physics for the next 200 years. He assumed the existence
of absolute space; space that is
immovable and similar everywhere and at all times. He also presumed that
gravitational interaction between any two bodies is instantaneous; that is, its speed of travel is infinite.
Maxwell’s theory of classical electrodynamics, formulated in 1864,
extended Newtonian classical mechanics to account for the motion of charged
particles in electric and magnetic fields. Modern communication technology and
information technology and much else is governed by the celebrated Maxwell
equations.
The
beginning of the 20th century saw the sharpening of challenges to
the existing edifice of theoretical physics based on Newton’s laws and
Maxwell’s equations. The first challenge was to the concept of ether, a hypothetical medium assumed to
be necessary for the propagation of electromagnetic waves. From such a
presumption it followed that, observed from the Earth, light from different
extra-terrestrial sources must travel at different speeds. But Michelson’s
famous interferometric experiment showed in 1887 that the observed speed of
light does not depend on its direction with respect to the direction in which
the Earth is moving. Thus the postulation of ether as a medium at rest in the
universe did not serve any sensible purpose, and the idea was abandoned. Electromagnetic
waves travel in empty space; there is no ether anywhere.
Einstein's special theory of relativity is based on two postulates:
1. The laws
of physics are the same for all observers in uniform (i.e. constant-velocity) motion relative to one another; this is the PRINCIPLE OF RELATIVITY.
2. The speed of light in vacuum is the same for all observers, regardless of their relative motion or
of the motion of the source of the light.
The theory
has some remarkable consequences, most of which have been confirmed by
experiment:
RELATIVITY OF SIMULTANEITY: Two events, simultaneous for one observer, may
not be simultaneous for another observer if the observers are in relative
motion.
TIME DILATION: Moving clocks are measured to tick more slowly than an
observer's 'stationary' clock, as confirmed by the muon-lifetime experiment
mentioned above. [For the same reason, GPS technology involves having to make
substantial time corrections for coordinating the various clocks.]
LENGTH CONTRACTION: Objects get shortened in the direction they are
moving w.r.t. the observer.
MASS-ENERGY EQUIVALENCE: E = mc2;
thus energy (E) and mass (m) are interconvertible, related by the
square of the speed of light in vacuum (c).
FINITENESS OF MAXIMUM POSSIBLE SPEED: No physical object, message or field can travel
faster than the speed of light.
However, predictions of the theory become significant only for speeds
comparable to the speed of light, which is ~3 x 108 meters/second.
Unlike the Newtonian concept of absolute time which does not change from
one frame of reference to another, space and time cannot be distinguished in
this theory, and must be treated as a unified spacetime. Two observers
moving relative to one another at a constant velocity would observe the
same laws of Nature in action. One of these observers, however, might record
two events on distant stars as having occurred simultaneously, while the other
observer would find that one had occurred before the other. Simultaneity does
not exist for distant events. In other words, it is not possible to specify
uniquely the time when an event occurs, without a frame of reference. The 'distance'
or 'interval' between any two events can be described only by means of a combination
of space and time, and not by either of them separately. The spacetime of four
dimensions (three for space and one for time) in which all events occur is the spacetime
continuum.
Relativity
became the premier guiding force in twentieth-century thought and art also: No
independent absolute value exists, but rather truth is meaningful or
significant only in a given context and time. Ditto for moral decisions. The
relativity idea had a profound effect on artists, authors and musicians, and many
new styles of literature, art, and music emerged in the early twentieth
century.
http://en.wikipedia.org/wiki/The_Persistence_of_Memory
The above
painting ('The Persistence of Memory') is by Salvador Dali. As Dawn Ades
wrote about it: 'The soft watches are an unconscious symbol of the relativity
of space and time, a Surrealist meditation on the collapse of our notions of a
fixed cosmic order'. Time seems to have stopped; a time interval of, say, one
second, will extend to infinity if an identical watch were traveling at the
speed of light.
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