Learn more physics!
How did Einstein figure out his famous formula, E=MC2
- coltyn (age 12)
The basic idea is this. He wanted to preserve the idea of conservation of momentum in the new physics where time intervals were different according to different observers. (We can explain that if you're interested.) He then showed that as an object lost energy by radiating light its momentum could only be conserved if the lost mass was equal to the lost energy/c^2. That just required knowing the energy/momentum ratio for light, which was known from classical electromagnetism. The basic argument uses no fancy math.
There's some discussion in lecture 11 of the course I taught last semsester:http://online.physics.uiuc.edu/courses/phys419/spring08/
(published on 07/28/08)
Follow-up on this answer.