the time dilation and the photon mass

i want to know how did scientists discover that the velocity of light is constant

i cant understand how did the mass of photon equal zero and at the same time the light beam make a curved line when it pass near a big mass this means that the photon has a mass

why did we see the light if the time for photons stopped as their velocity equal to c

if we go to the center of a planet of mass 8*10^23 and calculated the velocity by using the gravatational law and the third equation of motion we will notice that the velocity is more than the velocity of light the velocity of one kilogram body at a distance of one kilometer will be more than light velocity v>c

i asked my physics teacher in the school but he could not give me a suitable answer

please i want the answer

Easy Math Editor

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Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$