Special relativity is one of the two cornerstones of twentieth
century physics (the other is quantum mechanics). It makes a fundamental
break with Newtonian mechanics, drastically changing our conceptions
of the physics of space, time and their relation. It was published
in 1905 by Einstein as a complete theory (compare this with the
complex development of quantum mechanics by dozens of physicists
over a thirty year span, 19001930).
According to special relativity, the threedimensional space
of our ordinary experience (x, y, z) and time, say as measured
by a watch, are combined into a single fourdimensional system
called "spacetime". In three dimensions, we measure
the distance r between points by the usual Pythagorean measure:
x^{2} + y^{2} + z^{2} = r^{2}.
In spacetime, we measure the "interval" or fourdimensional
"distance" τ between points:
c^{2}t^{2}  ( x^{2} + y^{2} +
z^{2} ) = c^{2}τ^{2}.
The minus sign in this equation is of critical importance, for
it leads to the socalled "spacetime paradoxes" such
as the twin paradox, length contraction, time dilation, and so
on. These paradoxes, in turn, expose the "relativity"
of separate space or time measurements, but they point to an underlying
"absolute" or unchanging property of spacetime: the
invariance of the spacetime interval. According to Einstein and
others relativity "spatializes time", making our experience
of the passage and flow of time an illusion. This view is often
called the "block universe" view. Other scholars, though,
argue instead that relativity "temporalizes space" and
is entirely consistent with a "flowing time" view of
the world as given in ordinary human experience.
Other results include the equivalence of mass m and energy
e, given by the famous expression, E = mc^{2}. This does
not mean that matter is reduced to energy but that mass and energy
are interchangeable properties of matter. Another result is that
no energy or information can be transmitted faster than the speed
of light, c, approximately 30,000,000,000 cm/sec. This leads to
the "lightcone" structure surrounding each event O
in spacetime. Events in the past which can influence event O lie
inside what is called the past lightcone of O. Events which O
can influence in the future lie inside what is called the future
lightcone of O. Events which can neither influence O nor be influenced
by O lie outside the lightcone at O, and are referred to as lying
in the "elsewhen" of O. The elsewhen is like
the present in ordinary experience, except it is `thick', being
a volume of spacetime, not a surface. Each observer moving through
O singles out a different slice or surface through the elsewhen
and considers this to be his or her present. The present is ambiguously
defined in relativity; its meaning depends on the relative motion
of the observer. Physicists use the term "locality"
to refer to all events which fall within the past or future lightcone
of E: a nonlocal interaction, or superluminal interaction, would
be one transmitted faster than light, contradicting all available
data to date. Another way to say this is that the order
of events in the past and future light cones of O is the same
regardless of the motion of an observer with respect to the event
O. The order of events in the elsewhen of O is entirely dependent
on the motion of the observer moving through O. Physicists say
that relativity preserves causality in that events which are causally
related according to one observer are causally related according
to all, and events which are not causally related to one observer
are acausal to all.
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 Contributed by: Dr. Robert Russell
