A Brief History of Time | Chapter 2: Space and Time | Summary.
Updated: Jan 15
What is time? How is time measured for the universe? What is space? Well, there have been attempts to answer these questions for ages. Both Aristotle and Newton believed in absolute time. Ole Christensen Roemer made the first attempt to measure the speed of light through Jupiter’s moon eclipse but there were certain inaccuracies in his observations. A proper theory of the propagation of light began to take shape when Maxwell proposed the concept of electromagnetic waves. But then there arose a difficulty in determining the “relative” matter with respect to which the speed of the light could be measured. Thus, it was assumed that ether was present everywhere even in empty spaces and light’s speed was measured with respect to it. In 1887, Michelson and Edward compared the speed of light in the direction of earth’s motion with that at the right angle and found them to be the same, contradictory to what was believed that the speed in the former case would be greater than that in the latter one. In 1905 Einstein, pointed out that the whole idea of an ether was unnecessary, providing one was willing to abandon the idea of absolute time. Moreover, the fundamental postulate of the theory of relativity was that the laws of science should be the same for all freely moving observers irrespective of their position and velocities.
Maxwell’s equation predicted that the speed of light should be the same irrespective of the speed of the source. Light spreading out from an event form a 3D cone in a 4D space-time.
For every event in space-time, we can construct a light cone and since the speed of light is same at every event and in every direction, all light cones will be identical and thus all will point in the same direction. This means that the path of an object through space and time must be represented by a line that lies within the light cone at each event on it.
The special theory of relativity was inconsistent with Newtonian theory of gravity because according to it, any change in the mass or distance between the objects will instantly change the force between them which means gravitational effects should travel with infinite velocity instead of at or below the speed of light. Therefore, Einstein proposed the general theory of relativity and made the revolutionary suggestion that gravity is not a force like other forces but is a consequence of the fact that space-time is not flat. It is curved or wrapped by the distribution of mass and energy in it. Bodies are not made to move on curved orbits by a force called gravity, instead, they follow the nearest thing to a straight path in a curved space called a geodesic.
The mass of sun curves space-time in such a way that although the earth follows a straight path in a 4D space-time curve, it appears to move along a circular path in 3D space. Extending the same reasoning, light must also follow geodesics in space-time. Since space is curved, light no longer appears to travel in a straight line in space and hence light rays do not get reflected near the sun due to its mass. Another prediction of general relativity is that time should appear to move slower near a massive body because there is a relationship between the energy of light and frequency (E=hf).
To conclude, Newton’s law of motion put an end to the idea of absolute position in space and the theory of relativity gets rid of absolute time.
[ A chapter’s summary from the great book A Brief History of Time -By Stephen Hawkins].
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