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**These are excerpts and elaborations from my book "The Nature of Consciousness"**

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/sec 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 |