PRIDE CHEKERAthan 180°.
(3) In a flat space, parallel lines never meet and sum of angles of
a triangle is always 180°.
This geometry furnished by Gauss and Riemann became the
fundamental basis of general relativity.
Let's look at this gedanken experiment done by Einstein to set
the general theory of relativity.
Gedanken experiment of a laser beam in a lift
Let's say we have a lift with a laser fixed on one of its four
walls. In the first scenario, this lift is at rest on Earth's surface.
To a stationary observer, when the laser is switched on, it
emitted a straight beam of light which hits the opposite wall and
reflected back in a straight line again. This is something obvious
right? Yes, of course. Now, let's consider the same lift
accelerating in space; here, let’s say the laser beam was emitted
at a height of 1.5m above the floor of the lift, it will hit the
opposite wall at a height of about 1.4m above the floor of the
lift!
The lift had traveled a certain distance as it accelerates such that
the beam will not define a straight trajectory but a curved one to
hit the opposite wall at a lower height than that at which it was
emitted. When a body accelerates, relative to it, light is curved.
But, acceleration and gravity are equivalent, thus gravity bend