Nothing: A Very Short Introduction by Frank Close & Ray Chase

Author:Frank Close & Ray Chase [Close, Frank & Chase, Ray]
Language: eng
Format: epub
Tags: Politics & Social Sciences, Philosophy, Metaphysics, Movements, Existentialism, Science & Math, History & Philosophy, Physics
Amazon: B00H2DS3YI
Publisher: Audible Studios
Published: 2009-06-24T22:00:00+00:00

Chapter 6

The cost of free space

Curved space time

Einstein had created his theory of special relativity by means of thought experiments involving electromagnetic radiation, light. Following this he did the same for gravity, which led him to general relativity.

Einstein had come to his original theory of special relativity under the assumption that there is no absolute state of rest. His general theory came from the thought that there is no absolute measure of force and acceleration. First, consider the problem for relativity when gravity is involved. Special relativity was based on the axiom that the velocity of light is a universal constant. Light has energy and as gravity acts not just on mass but also on energy in all its forms, then a light beam should be deflected by gravity as it passes near to a large object such as the Sun. As gravity fills the universe and light beams are being disturbed always, the principles of relativity, which assumed that light travels in straight lines at constant velocity, seemed only to be able to survive if gravity could somehow be turned off. Einstein realized this was a problem very quickly and it would take him ten years to solve it completely. His great insight came when he realized that gravity is effectively switched off for the case of a body that is in free fall. This means that there is no net force acting on the object and hence it moves at a constant velocity.

A lump of falling rock has no weight. If you catch it, what you perceive as weight is the force that you have to apply to stop it falling to ground. It is the solid floor that stops us falling to the centre of the Earth. It is the resistance of the floor, the force that it exerts to prevent our fall, that we feel as our own weight. Were the floor and the earth just vapour, we would fall to the centre of the planet, weightless.

This was the starting point of another of Einstein’s thought experiments.

Suppose that you are inside a cabin that is in free fall, and there are no windows to look out of. This could be a broken elevator or, less traumatic, a satellite orbiting the Earth. In the latter case the satellite and you are in free fall but also travelling ‘horizontally’ at such a speed that the Earth’s curvature makes the ground fall away from you at the same rate as you are falling towards it. In either example, in your immediate surroundings there would be no sense of any gravitational force. For example, if you let go of a ball, it will be pulled towards the Earth by gravity at exactly the same rate as you are, and so will remain stationary relative to you. Astronauts appear to float in their cabin for the same reason; they and the satellite are ‘falling’ at the same rate. Although we happen to know that the astronauts are falling in the gravitational

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