I recently completed reading this book by Martin Gardner. These are my notes/takeaways from the book (for quick future reference)
Special Relativity: Classical Theory (Newton’s) states that if you are on a uniformly moving object, there is no mechanical experiment by which you can prove that you are moving. Einstein took it to the next step, saying that there’s no optical (or electromagnetic) experiment by which you can prove the same.
Fundamental Postulates of SR : 1. There is no way to tell whether an object is at rest or in uniform motion relative to a fixed ether. 2. Regardless of the motion of its source, light always moves through empty space with the same constant speed.
Inferences of SR: 1. There’s no way ether wind or ether motion could be detected (if at all it’s present). 2. Light travels at the same constant speed relative to a uniformly moving observer. 3. e=mc^2
General Relativity:
GR is an extension of SR. There’s no experiment of any sort by which an observer in any sort of motion, uniform or non-uniform, can prove whether he is moving or at rest.
To put it simply, if you are traveling by a car saying that your car is traveling at 60mph in one direction is the same as saying that the entire cosmos is traveling in the opposite direction at the same speed (car is now fixed).
Inferences of GR: 1. Gravity and inertia are one and the same. (Galileo’s experiment: 2 objects of different weight fall with same acceleration because of the opposing effects gravity and inertia). 2. Gravity is the curvature of space-time line due to presence of massive objects (Objects travel through the 4-D Space-time in a straight line, which is projected as the difference in shape, size or time to an observer).
Twin paradox: At speeds closer to that of light, time runs slower. This would cause a twin traveling in a space vessel at speeds closer to that of light to be much younger when compared to his twin who is at rest in his home. (But per GR , the twin at home could also be considered to be at motion along with the cosmos relative to the space vessel. But I’m not sure of the physical laws that apply when his motion is along with the entire cosmos !!)
Applications of Relativity (that I know of):
1. Models of the universe 2. Explanation for quasars, pulsars and blackholes 3. GPS 4. Further exploration of Quantum mechanics (The 2 theories can’t co-exist in a single model per EPR paradox) 5. Large particle collider equipments (like the one at CERN)
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