Suppose a baseball team is traveling on a train moving at 60 mph. The star fastball pitcher needs to tune up his arm for the next day’s game. Fortunately, one of the railroad cars is free, and its full length is available. If his 90 mph pitches are in the same direction the train is moving, the ball will actually be moving at 150 mph relative to the ground. The law of addition of velocities in the same direction is relatively straightforward,

. But according to Einstein’s special theory of relativity, this is only approximately true and requires that

and

be small fractions of the speed of light,

≈ 3 ×

m/sec (or 186,000 miles/sec). Expressed mathematically, we can write

if

According to special relativity, the speed of light, when viewed from any frame of reference, has the same constant value

. Thus, if an atom moving at velocity

emits a light photon at velocity

, the photon will still be observed to move at velocity

, not

.

Our problem is to deduce the functional form of

consistent with these facts. It is convenient to build in the known asymptotic behavior for

by defining

. When

, we evidently have

, so

, and likewise

. If both

and

equal

,

. A few moments' reflection should convince you that a function consistent with these properties is

, which gives Einstein's velocity addition law

.