 
Summary: © 2005 Richard Conn Henry1
Where does E = mc2
come from?
Richard Conn Henry, The Johns Hopkins University, Baltimore MD
he location of an eventone beat of your heart, sayis given by its three space
coordinates, and its time coordinatebut, if we want to use the same units for all of those
4 numbers we will require a units conversion factor: if there are c centimeters in one
second, the conversion factor is c cm/sec, and the location of a heartbeat is given by the set of
numbers (x, y, z, ct) , where all four quantities are now in units of centimeters.
Of course we don't yet know the value of c, but  we would like to find that out!
The famous Pythagorean Theorem states that ds2
= dx2
+ dy2
for a right triangle, where
dx designates a length in the x direction. Let us GUESS that for two heartbeats, that are
separated in our (at least apparently) fourdimensional universe by dx, dy, dz, cdt( ), the
Pythagorean Theorem expands to:
ds2
= dx2
+ dy2
