You should readily understand how a system with very little mass has the potential to release a phenomenal amount of energy (in E=mc², c² is an enormous number). In nuclear fission, an atom splits to form two more atoms. At the same time, a neutron is released. The sum of the new atoms' masses and the neutron's mass are less than the mass of the initial atom. Where did the missing mass go? It was released in the form of heat - kinetic energy. This energy is exactly what Einstein's E=mc² predicts. Another nuclear event that corresponds with Einstein's equation is fusion. Fusion occurs when lightweight atoms are subjected to extremely high temperatures. The temperatures allow the atoms to fuse together to form a heavier atom. Hydrogen fusing into helium is a typical example. What is critical is the fact that the mass of the new atom is less than the sum of the lighter atoms' masses. As with fission, the "missing" mass is released in the form of heat - kinetic energy.

One often-misinterpreted aspect of the energy-mass unification is that a system's mass increases as the system approaches the speed of light. This is not correct. Let's assume that a rocket ship is streaking through space. The following occurs:

- Energy must be added to the system to increase the ship's speed.
- More of the added energy goes towards increasing the system's resistance to acceleration.
- Less of the added energy goes into increasing the system's speed.
- Eventually, the amount of added energy required to reach the speed of light would become infinite.

In step 2, the system's resistance to acceleration is a measurement of the system's energy and momentum. Take notice that in the above 4 steps, there is no reference to mass. Nor does there need to be.

Next, we’ll look at why simultaneity between two events cannot happen in the world of special relativity.