Environmental Engineering Reference
In-Depth Information
5
The Need for a New
Theory of Space and Time
5.1 SPACE AND TIME REVISITED
Perhaps the most astonishing idea underpinning Einstein's Special Theory of
Relativity is the rejection of the assumption that both space and time are absolute.
Since the whole of Part I of this topic was built on such an assumption, it means
we will have to start all over again. Of course that is not to say that Newton's
theory is useless, for whatever Einstein's theory says, it had better be experimen-
tally indistinguishable from Newton's theory for a very wide range of phenomena.
Before we attempt to figure out what Einstein's theory actually says, we should
first be very clear on what exactly it means to say that space and time are absolute.
Intuitively, absolute space means that we can imagine a gigantic fixed frame of
reference against which the positions of events can unambiguously be determined.
Of course the actual co-ordinates of an event will depend upon where the origin
of the reference frame is
1
but its position vector will nevertheless specify a unique
position in absolute space. Absolute time is also very intuitive. We can imagine the
Universe being filled with tiny clocks all synchronised with each other and ticking
at exactly the same rate. The time of an event can unambiguously be measured
by the time registered on a clock located close to the event (these are imaginary
clocks so we don't worry too much about the fact it isn't really practicable to put
clocks everywhere). Again, although the actual time of an event will depend upon
when we set the clocks to zero it still specifies a unique moment in time. The
consequences of absolute space and time are clear: there is no argument about how
long a body is (it is the distance between two points in absolute space), or whether
1
They'll also depend upon the orientation of our axes and on our choice of co-ordinate system (e.g.
cartesian or spherical polar).
