Recapitulation on general relativity
To recapitulate on general relativity:
- gravity and accelerated motion are indistinguishable – the equivalence principle;
- all observers, regardless of their state of motion, are on an equal footing and can proclaim that they are stationary and the rest of the world is moving about them, so long as they acknowledge the presence of a gravitational field. That is, the laws of physics are the same for all observers, those who claim to be at rest, those undergoing constant velocity relative motion and those in a state of complicated accelerated motion;
- space and time appear to warp and curve in the presence of mass or energy; the presence of mass causes the shape of space and time around it to warp;
- gravity is the warping of space and time; it consists of ripples and curves in the architecture and geometry of spacetime[1];
- gravity cannot outrun the speed of light, the “cosmic speed limit”. This overturned a basic principle of Newton’s theory of gravity[2];
- a gravitational field slows the passage of time;
- light passing close to a massive object appears to bend in response to spacetime’s curvature; the amount of bending increases as the object’s mass increases. This overturned another aspect of Newton’s theory;
- general relativity is most manifest when things are massive;
- from our limited three-dimensional perspective here on earth, many of these phenomena are not apparent; we have a myopic view of reality and divide space and time into different entities[3].
[1] Some commentators use terms such as ‘fabric of spacetime’ or ‘fabric of the cosmos’, which are apt to cause confusion. They are intended as no more than euphemisms or analogies used to describe the principles involved. I prefer the description “architecture and geometry” in lieu.
[2] Newton thought that if the sun were suddenly to disappear from the heavens, its loss of gravitational effect would be immediately apparent here on earth. However, Einstein said that nothing would happen here on earth (the sun would continue to shine, the earth would remain susceptible to the sun’s gravity) for the time it took for the gravity loss, travelling at the speed of light, to reach earth, approximately 8 minutes.
[3] Gleiser, 68.