Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Mass Into Energy

Einstein showed that mass itself could be converted to and from energy, according to his celebrated equation E = mc2, in which c is the speed of light. We thus speak of mass as simply another form of energy, and it is valid to measure it in units of joules. The mass of a 15-gram pencil corresponds to about 1.3 1015 J. The issue is largely academic in the case of the pencil, because very violent processes such as nuclear reactions are required in order to convert any significant fraction of an object's mass into energy. Cosmic rays, however, are continually striking you and your surroundings and converting part of their energy of motion into the mass of newly created particles. A single high-energy cosmic ray can create a shower of millions of previously nonexistent particles when it strikes the atmosphere. Einstein's theories are discussed in book 6 of this series.

Even today, when the energy concept is relatively mature and stable, a new form of energy has been proposed based on observations of distant galaxies whose light began its voyage to us billions of years ago. Astronomers have found that the universe's continuing expansion, resulting from the Big Bang, has not been decelerating as rapidly in the last few billion years as would have been expected from gravitational forces. They suggest that a new form of energy may be at work.

Discussion Questions

A I'm not making this up. XS Energy Drink has ads that read like this: All the Energy ... Without the Sugar! Only 8 Calories! Comment on this.

Last Update: 2009-06-21