# E=mc2

Part of the Einstein exhibition.

The equation is as famous as the man who wrote it. Click on the image to learn about the equation.

In Einstein's first paper about energy and mass, E=mc2 doesn't actually appear anywhere—he originally wrote the formula as m=L/c2. What happened? Einstein was using "L" (for Lagrangian, a general form of energy) instead of "E" for energy. Later, he replaced "L" with "E," rearranged the variables, and the famous form of the equation emerged.

The implications of E=mc2 are profound. For centuries, scientists had considered energy and mass to be completely distinct and unrelated to each other. Einstein showed that in fact, energy and mass are different forms of the same thing. Einstein himself was surprised by the finding, calling it "amusing and enticing" and wondered "whether the Lord is laughing at it and has played a trick on me."

## The Power of a Penny

Einstein's equation shows that mass and energy are equivalent—so long as you multiply by the "conversion factor" of c2 (the speed of light multiplied by itself). This factor is huge: 90 billion kilometers2 per second2. So if you multiply a small amount of mass—say, the mass of a penny—by c2, you'll get a tremendous amount of energy.

If a penny could be converted entirely to energy, it would provide enough energy to power the New York City metropolitan area for at least two years.

## Reality Check

Converting a penny entirely to energy would require temperatures and pressures much greater than those found inside the Sun. So unfortunately, small coins are not a practical source of energy.

## E=mc Something?

Einstein's equation uses just three letters and one number. What do these symbols mean?

E = Energy
m = Mass
c = Speed of light
from the Latin term celeritas, which means "speed"
2 = Squared
when you "square" something, you multiply it by itself