# Why the speed of light?

Hey all, we are all familiar with the equation $$E=mc^2$$
Where energy equals mass times the speed of light squared...

My question is...why? I mean, of course it works, but why the speed of light?

My theory so far has been that since light is pure energy, its speed is used as a sort of constant when it comes to measuring energy. But...why the speed? Why not some other property. I dunno, its jsut causing some cognitive dissonance right now, It seems like speed shouldn't be in there.

Please don't go out of your way to tell me "oh yes it should, because that equation works"...I'm not making any claism that its false at all.

## Answers and Replies

selfAdjoint
Staff Emeritus
Gold Member
Dearly Missed
It comes out of relativity, where the speed of light plays an important role. The covariant formula for energy in relativity is $$E^2 = p^2c^2 + m^2c^4$$ where p is the magnitude of the momentum and m is the invariant mass. If you take the momentum to be zero, meaning you are in the rest frame of the mass you are working on, then the famous formula results when you take square roots.

and it also predicts the existence of antimatter, E=+-mc^2

and it also predicts the existence of antimatter, E=+-mc^2
Just how would that equation have anything to-do with anti-matter.

Oracle

What if mass is varying? E*dE=mc^4*dm and then integrate?

Ebolamonk3y said:
What if mass is varying? E*dE=mc^4*dm and then integrate?

You mean Integrate E*dm = mc^2 dm?

Blistering Peanut said:
and it also predicts the existence of antimatter, E=+-mc^2

I think you mean negative energy.

PrudensOptimus said:
You mean Integrate E*dm = mc^2 dm?

Integrate the original function.