Special Relativity

Think about accelerating the rocket in the above example. To accelerate the rocket (or anything else) an outside force must push on it. As the speed increases, the mass appears to increase as seen by outside observers including the one supplying the force (the one doing the pushing). As the mass increases, the force required to accelerate the rocket also increases. (It takes more force to accelerate a refrigerator than a feather.) As the speed approaches the speed of light the mass and hence the force required to accelerate that mass approaches infinity. It would take an infinite force to accelerate the object to the speed of light. Because there are no infinite forces no object can travel at the speed of light. An object can be accelerated arbitrarily close to the speed of light, but the speed of light can not be reached. Light can travel at the speed of light only because it has no mass. The speed of light is the ultimate speed limit in the universe.

This famous equation means that matter and energy are interchangeable. Matter can be directly converted to energy, and energy can be converted to matter. The equation, E=mc², is then a formula for the amount of energy corresponding to a certain amount of matter. E represents the amount of energy, m the mass, and c the speed of light. Because the speed of light is very large a small amount of matter can be converted to a large amount of energy. This change from matter into energy takes place in nuclear reactions such as those occurring in the sun, nuclear reactors, and nuclear weapons. Nuclear reactions release so much energy and nuclear weapons are so devastating because only a small amout of mass produces a large amount of energy.