Why is "c" used to define speed of light in the equation E=mc2???

Because the speed of light (SOL) in a vacuum is ALWAYS constant and constant = c.





So what does Einstein mean about SOL always being constant?





Well you see time is relative. You cannot express what the speed of an object is in space unless you make it relative to another object. Even then, you can't prove which object it is that is actually moving! But instead, lets take a beam of light which is travelling at say 300 000 000 metres per second towards you. You suddenly decide that you want travel at say 100 000 000 m/s in the direction that the beam of light is coming from. If you were now to measure the speed of that light beam, you would expect it to be travelling at a speed of 400 000 000 m/s towards you, but it's not! It's still travelling at 300 000 000 m/s! Same as if you were trying to catch up with a beam of light, you can't, because no matter how fast you go, that beam of light will always be travelling at 300 000 000 m/s away from you. How is this possible? Well, it's like Einstein said, time is relative and the speed of light is constant - not the other way around!





Do a google search on the Michelson-Morley experiment!








Just to be a little more precise, the speed of light is always and exactly 299,792,458 metres per second or 1,079,252,848.8 km/h in a vacuum.

Why is "c" used to define speed of light in the equation E=mc2???
Because Einstein wanted to make a pun. Can't you c?
Reply:This is as c=constant, as in a vaccum, the speed of light is a constant speed.
Reply:By convention, the speed of light has been assigned the letter "c" in the same manner that "pi" = 3.14159.........
Reply:Because C = a constant , ie ;light has a fixed speed of 186,000 miles per sec., thus C= the speed of light in the equation.
Reply:300,000km/hr is the speed of light in Vacuum


That is the maximum possible speed as per relativity.
Reply:Speed of light is denoted by the letter c for the Latin celeritas which means "swiftness" or "speed".





C is defined by ISO 31-5 which is the part of international standard ISO 31 that defines names and symbols for quantities and units related to electricity and magnetism.