In every argument, there are hidden assumptions. The more deeply they are buried, the longer it takes to reveal them. Newton had assumed that time is absolute: all observers could synchronize their clocks and, no matter how they moved around, their clocks would always agree. He had also assumed an absolute notion of space. Different observers might occupy different positions and move at different velocities, but they would always agree on the relative positions of objects and the distances between them.
It took Einstein to realize that these two very reasonable assumptions — of absolute time and space — were actually incompatible with Maxwell’s theory of light. The only way to ensure that everyone would agree on the speed of light was to have them each experience different versions of space and time. It isn’t that that the measurements of space and time are arbitrary. On the contrary, there are definite relations between the measurements of space and of time made by different observers. Everyone’s viewpoint is different, but they are all equally valid.
The different versions of space and time experienced by different observers mix up their space and time coordinates. Such a mixing is impossible in Newton’s theory, because space and time are entirely different quantities. One is measured in metres, the other in seconds. But once you have a fundamental speed, the speed of light, you can measure both times and distances in the same units: seconds and light seconds, for example. This makes it possible for space and time to mix under transformations. And because of this mixing, space and time can be viewed as describing a single fundamental entity, called “spacetime.”
The unification of space and time in Einstein’s theory, which he called “special relativity,” allowed him to infer relationships between quantities which, according to Newton, were not directly related. One of these relations became the most famous equation in all of physics.
Excerpt from pages 111-112 of The Universe Within © 2012 Neil Turok and CBC.
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