How Are Energy And Matter The Same?

Mass is frozen energy. Present day particle physics could not be where it is without Einstein's famous equation connected mass, energy and the speed of light. protons can be made be collide with each other and produce other particles. And that's what they did at CERN in 2012 to isolate the Higgs Boson. Protons have mass. It is possible even to collide massless particles and produce other particles provided mass less ones had enough energy by speeding them up in a collider. when referring to the Higgs Boson discovered at CERN , its mass is always expressed in GeV ( Giga electron Volts), a measure of energy.

So Einstein invented the idea that energy equaled the mass of an object multiplied by some constant. I'd like to know how he determined that constant to be the speed of light squared. It's almost as though he pulled the constant c-squared out of thin air. Pretty amazing thing no matter how he did it. Peter Price