### Velocity Addition

Consider a truck (just for a change) moving with velocity *v*_{1} in the *x*-direction with respect to the
ground. Inside the truck a ball is thrown with velocity *v*_{2} with respect to the truck, also in the *x*-
direction. Call the frame of the truck *F*_{1} and the frame of the ground *F*_{2}. The question is this: what
is the velocity of the ball with respect to the ground? Under the Galilean transformations the answer is
intuitive and obvious: the ball is moving with velocity *v* = *v*_{1} + *v*_{2} with respect to the ground.
Things are quite different in relativity. We know that *v*, the velocity of the ball with respect to the ground
is given by *v* = , where the subscripts refer to the frame *F*_{2}. Since
*F*_{1} is moving with respect to *F*_{2}, we can use the lorentz transformations to write:

Δx_{2} = //Δt_{2} = |

Thus:

v = = |

However, we know that the velocity of the ball inside the truck is

*v*

_{2}= . Using this we can simplify our expression for

*v*:

v = = |

This is the velocity additional formula, and it is the true (so far as we know) equation for determining the relative velocities of moving objects. Note that when

*v*

_{1}< <

*c*and

*v*

_{2}< <

*c*, the equation reduces to the familiar

*v*

_{1}+

*v*

_{2}(as the correspondence principle would anticipate -- we hope that the Galilean form would continue to work for 'normal' speeds). This equation only applies when the speeds being considered are being measured in

*different frames.*Here, the speed of the ball is being measured in the frame of the truck and the speed of the truck is being measured in the frame of the ground. When the speeds are both measured in the same frame, the usual

*v*

_{1}+

*v*

_{2}formula still applies.

### Minkowski Diagrams

A Minkowski diagram or spacetime diagram is a convenient way of graphically representing the
lorentz transformations between frames as a transformation of coordinates. They are especially useful
for gaining a qualitative understanding of relativistic problems. We make a spacetime diagram by
representing frame *F* as the coordinate axes *x* (horizontal) and *ct* (vertical). We are
ignoring the *y* and *z* directions, since they are uninteresting. The plot of an object's *x*-
position versus time on the Minkowski diagram is called its worldline. Notice that light, traveling
one unit of *ct* for every unit of *x* will follow the line *x* = *ct*, inclined at a 45^{o} angle.

What do the axes of

*F'*, moving with velocity

*v*along the

*x*-axis of

*F*look like? Take the point (

*x'*,

*ct'*) = (0, 1). From the lorentz transformations we can find that this point transforms to (

*x*,

*ct*) = (

*γv*/

*c*,

*γ*). As shown in the angle between the

*ct'*and

*ct*axes is given by: tan

*θ*

_{1}=

*x*/

*ct*=

*v*/

*c*. Actually, the

*ct'*axis is just the worldline of the origin of

*F'*. The point (

*x*,

*ct*) = (

*γv*/

*c*,

*γ*) is a distance =

*γ*from the origin, so the ratio of units on the

*ct'*axis to those on the

*ct*axis is just this value, namely:

= |

This approaches infinity as

*v*→

*c*and is one if

*v*= 0. Similar analysis shows that the

*x'*axis is an equal angle from the

*x*-axis and that the ratio of units is also equal (see ). Thus, the faster

*F'*relative to

*F*, the more its coordinates are squished towards the

*x*=

*ct*line.

The advantage of a Minkowski diagram is that the same worldline applies to both sets of
coordinate axes (that is, to *x* and *ct*, as well as to *x'* and *ct'*). The Lorentz transformation is
made by changing the coordinate system underneath the worldline rather than the worldline itself.
In many situations this allows us to visualize the perspectives of the different observers more easily. If we
had a very detailed and accurate Minkowski diagram we could use it to read off the values for *Δx*, *Δct*, *Δx'*, and *Δct'*. To find the spacetime coordinates of an event in *F*, one
can read the value off the *x* and *ct* axes; to find the coordinates in a moving frame the *x'* and *ct'*
axes corresponding to the appropriate velocity can be constructed (using the angle formulas explained
above), and the value read off using the units derived for *x'* and *ct'*, above.