Relativity

What do the letters in E=MC² stand for?
• ### Science Channel

1. An equation doesn't mean much if you're not sure what the variables represent. Though many people are familiar with the equation E=MC2, most of us don't really know what it means.

In this groundbreaking relativity equation, first demonstrated in a paper by Albert Einstein in 1905, the 'E' stands for energy and the 'M' stands for mass. The 'C' stands for the speed of light, which is in the ballpark of 186,000 miles per second (about 300,000 kilometers per second) when it travels through a vacuum [source: NASA]. So the energy contained in an object is equal to the mass of the object multiplied by the speed of light squared. One of the most important implications of this equation is that energy and matter are essentially interchangeable. In other words, a specific amount of mass correlates to a specific amount of energy, and because of the magnitude of the 'C' constant in the relativity equation, we know the transition from matter to energy releases an incredible amount of energy. It was this line of thinking, in fact, that led to the development of nuclear power and the atomic bomb.

If you're familiar with relativity, you know that mass affects time. As a moving object approaches the speed of light, time slows down for that object -- in fact, according to relativity theory, if an object were able to move at the speed of light, time would seem to stop completely [source: NASA]. Time also slows down the closer you are to a massive object. For this reason, we experience time at a slightly more sluggish rate on Earth than we do in orbit. You'd never notice it yourself, but ultra-precise clocks can illustrate the difference. The closer we are to the mass of the Earth, the slower time passes. A similar effect occurs when we stand next to a massive object such as an Egyptian pyramid, only on a scale that is billions of times smaller.

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