Setup: Four objects are moving at different constant velocities in along the vertical axis.

Observations:

• The space-time diagram is a straight line for each of the objects as it is for any object moving at a constant speed.
• The object moving in a direction that is opposite to the direction chosen as positive has a negative slopes - it is moving backwards.
• The faster the object is moving, the larger the slope of the line. The ultimate limit of the size of the slope occurs when the object's speed approaches the speed of light.
• The object is not actually moving along the space-time path, its world line. It is only moving in physical space. See Space-Time diagram for more details.