Question

Why speed of light is 3×10^8 m/s

Answer 1

"The speed of light in empty space- where atoms do not delay its travel- is 186,282 miles (299,792 kilometers) per second." "Light, Electromagnetic Theory of." ... "… regardless of the motion of the observer, the speed of light always came out to be the same value, c = 3 × 108 m/s, with respect to the experimenter."

Answer 2

Speed of light is a universal constant and is a fundamental characteristic of light or photon. There is no argument as to why the speed of light is 3*10^8 m/s. The only thing we can say is that if the speed of light was more or less than this, then our concepts and theories would have to be modified for the same. Since our theories are valid and have been both mathematically and practically confirmed and are correct thus the speed of light is also correct. It is like we assume something and using that assumption we prove a theory if the proof for theory is valid then the assumption is valid.

To understand what dramatic change can happen if try to change the most fundamental thing. Let us take the mass of an atom, it is of the order 10^-27 kg if it were to be changed to 10^-26 then the entire universe would be ten times heavier and then the constants of the entire universe would change.

The speed of light and mass of an atom, stuff like this are fundamental values and are the reason for our existence. A slight change in any of these numbers can have effects that can not be calculated or comprehended.