The Nature of Consciousness (Copyright © 2013 Piero Scaruffi | Legal restrictions )
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These are excerpts and elaborations from my book "The Nature of Consciousness"
 General Relativity: The Illusion of Gravity Newton explained how gravity works, but not what it is. Einstein’s  Gerneral Relativity Theory (“The Foundation of the Generalised Theory of Relativity”, 1916) is ultimately about the nature of gravitation, which is the force holding together the universe. Relativity explains gravitation in terms of curved space-time, i.e. in terms of geometry. The fundamental principle of this theory (the "principle of equivalence") is actually quite simple: any referential frame in free fall is equivalent to an inertial reference frame. That is because if you are in a free fall, you cannot perceive your own weight, i.e. gravity. Gravity is canceled in a frame of reference that is free falling, just like the speed of an object is canceled in a frame of reference which is moving at the same speed.  Newton had introduced two kinds of mass: the gravitational mass and the inertial mass. The gravitational mass of a body determines the power of gravitational attraction. The inertial mass of a body determines how strong a force has to be to move that body, or, conversely, how fast the body will accelerate when subject to a force. In theory, the two masses could be completely different. In practice, they are the same. That’s why a uniform acceleration and standing still in a gravitational field turn out to have the same effect. This means that the acceleration generated by a gravitational field does not depend on the mass, as Galileo famously proved dropping balls of different material from the Leaning Tower of Pisa. This is unique to gravitation: in the case of any other force, the mass of the body determines the acceleration of the body. Since gravity is canceled when you are in free fall, the world around you will still behave the way it behaves in an inertial frame of reference. For example, if an object is falling while you are falling, the object will appear to be standing still. If you can't see what is going on outside, and all you can measure is the 9.8 m/sec2 acceleration of an object that you let fall, you can't decide whether you are standing still, and subject to Earth’s gravity, or you are accelerating in empty space. All you observe is an acceleration of 9.8. If you are still, that's the acceleration you expect for any falling object. If you are in a rocket that is accelerating upwards at 9.8, that's the acceleration you expect for any object left behind. The effect is the same. Someone in a free fall does not feel her own weight. If you are inside a box that is falling to the Earth, you feel like you are floating in empty space far away from any planet. Viceversa you feel your weight when you are at rest on the Earth and when you are on a space rocket that is being accelerated in empty space (that is accelerating at a rate equal to that of the gravitational field). In fact, if you are locked inside a box, and cannot see outside and hear no sound, there is no experiment that you can perform to find out whether you are at rest on Earth (in a gravitational field) or accelerated in empty space (where there is no gravitational field), whether you are floating in empty space or free falling towards the Earth. If there is equivalence between being in a gravitation field and being accelerated in empty space, this must be true also for light, but this means that light beams must be bent in a gravitation field: a light beam inside the accelerating box looks slightly bent by its occupants, and the principle of equivalence now requires that it must be bent also for the occupants of the box at rest on Earth. It is more logical to assume that the light beam is moving in a straight line. This implies that the bending must due to the space in which the light beam travels, i.e. that space is curved by the gravitational field. Planet don't fall in a straight line, they turn. Einstein made the logical inference that they must be falling in a straight line. What makes the straight line turn into an orbit is that space is not flat. This, however, is true only "locally". The Earth and Mars are both attracted to the Sun but the direction is usually different. Globally, the acceleration required to cancel gravity is different in different places. This means that each point in space must have its own "distortion" so that the principle of equivalence still holds. And therefore gravity must be a property of spacetime. This also explains why gravity acts instantaneously, not at the speed of light like electromagnetism. Back to the beginning of the chapter "The New Physics" | Back to the index of all chapters