Albert Einstein's Birthday
Today is Albert Einstein's birthday, let's celebrate by reviewing his groundbreaking discovery: General Theory of Relativity.
There it is:
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General Relativity
The principle of equivalence
Suppose there are a person in a box of no window and person on the identical box on the surface of the Earth. We cannot tell the difference between the box in the outer space with gravity upward and the the box on the surface of the Earth stationary with gravity downward. (That is not really true as the latter part is the that the gravity may vary due to altitude that cause by tidal effect of the Moon, let's get thing simple)
This the equivalence Principle that all objects has no physical difference observable in the same acceleration frame of reference and in the gravitational field.
Another principle is mass can twist spacetime that light bends in the gravitational field, just like an object put on the trampoline.
The massive the object is, the greater is the degree of curvature caused to the fabric of the universe, such that light travel not in straight line when it is near the gravitational field of the massive celestial bodies.
It has been proved by the experiment observing the eclipse of the Sun. At the moment, we know that Mercy is behind the Sun in front of the Earth. The scientists could still observe the objects due to the gravitational field of the Sun that bent the light reflected from the Mercury so the trajectory of the light become curved.
This finding overturn our understanding of the universe utterly. From Newton's law of gravitation, the force includes mutual attraction of the two objects with mass. But in this observation, as we all know, photon does not occupy any mass. Therefore, matter does not act on the mass, instead, matter tells how the spacetime should curve while spacetime tells how a matter should move.
Let's consider the degree of the curvature as steepness of a mountain, and now we upside down the trampoline. Initially, we stand at the top of the mountain, in a 3 dimensional coordinate system, x, y,ø. As we move along the mountain, there is a change in ø with respect to x or y.