Space and Time
Aristotle and Galileo, two different systems
Maxwell theory of propagation and the ether
The meter is defined to be the distance traveled by light in 0.000000003335640952 second, as measured by a cesium clock. Equally, we can use a more convenient, new unit of length called a light-second. This is simply defined as the distance that light travels in one second. In the theory of relativity, we now define distance in terms of time and the speed of light, so it follows automatically that every observer will measure light to have the same speed (1 meter per 0.000000003335640952 second).
We must accept that time is not completely separate and independent of space, but is combined with it to form an object called space-time.
Space and time are now dynamic quantities: when a body moves, or a force acts, it affects the curvature of space and time - and in turn the structure of space-time affects the way in which bodies move and forces act.
Space and time not only affect but also are affected by everything that happens in the universe.
This revolution implied that the universe must have a beginning and, possibly, an end.