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Union University Department of Physics

The Science Guys

Science Guys > May 2000

May 2000

Einstein was voted the person of the century at the end of 1999. Exactly what does his famous equation E = mc2 mean?

It was July 16, 1945. An expectant community of scientists, engineers, and military observers watched the predawn skies near Alamogordo, New Mexico. A great light filled the skies and a superheated shock wave rushed out from ground zero at hundreds of miles per hour, splintering houses in a fraction of a second. The 100 ft. steel tower on which the device was mounted was vaporized, and the sand near Ground Zero was fused to radioactive silica glass. A saddened J. Robert Oppenheimer, head of the scientists involved in the Manhattan Project, saw the spectacle and quoted Shiva from the Bhagavad Gita: "I am Death, the Destroyer of worlds." The nuclear age had just been born with the test explosion of the first atomic bomb. From where did all that energy originate? Einstein provided the answer with his famous equation,

E = m c2.

In the equation, E = m c2, E stands for energy, m stands for an object’s mass, and c2 represents the speed of light ( 186,000 miles per second ) multiplied by itself. Think of mass as simply the quantity of matter present. Energy is a tougher concept, but it is okay to think of it as a property of heat or light. The equation is known as the mass-energy equivalence relationship. Before Einstein’s radical thoughts, mass and energy were thought to be very different things. Today, physicists see that there is no fundamental difference between mass and energy. To quote Einstein himself in his 1905 article on relativity published in the Annalen der Physik; "The mass of an object is a measure of its energy content." One of the basic laws in physics is the conservation of energy law (the amount of energy in the universe remains constant, only changing in form). In working out his theory of relativity, Einstein found that mass must be related to energy, in essence, mass is "congealed energy." This is certainly a surprising result.

For example, this relationship indicates that if you lift an object from a table and increase its height, you have increased its energy, and its mass increases by a very, very tiny amount. Theoretically, a baseball has more mass when thrown than when sitting motionless. Thrown by a major league pitcher a baseball would have about 0.000 000 000 000 007 kg more mass than when it is resting - an amount that is undetectable by today’s standards.

What relevance does this have for us today? This amazing equation makes nuclear power possible as well as, unfortunately, nuclear missiles. In both of these applications matter is directly converted- or "uncongealed"- into energy. In the case of a nuclear power plant the conversion is done slowly, while in the case of a bomb the conversion is allowed to run amok and proceed almost instantaneously. In either case, fantastic amounts of energy are released when even tiny amounts of matter are converted to energy.

Of course our Creator has made most effective use of this relationship between mass and energy. Humans benefit from this process each day of their lives. The direct conversion of mass into energy is accomplished every moment within the heart of our Sun, and thanks to that fact our planet is bathed continuously with life-giving energy. So, the next time you enjoy the sunshine, ponder the fact that approximately 9,000,000,000 pounds of solar material are being converted into energy each second in the depths of our Sun! A small portion of that energy reaches the Earth as light and heat. We are thankful to Einstein for revealing to mankind the equation that allows scientists to understand how the sun produces energy, that is, by changing mass into energy.