The space-time interval (s) between two events can be calculated by using the space-time metric:

s² = (ct)² - x².

Because of the minus sign, s² may be positive, negative, or zero:

• If (ct)² > x², then the interval is said to be time-like. These events may be causally related; one may cause, or be caused by, the other.
• If (ct)² 2, then the interval is said to be space-like. These events cannot be causally related; neither event could cause the other.
• If (ct)² = x², then the interval is said to be light-like. These events may be traversed by a light ray.

The space-time interval between two events is invariant; that is, all observers will agree on it's value.

The graph on the left shows three events in the Home Observer's frame of reference, labeled A, B and C. The worldline of a second, Other Observer, is also shown (in blue). The graph on the right shows three events in the Other Observer's frame of reference, labeled D, E, and F. These are NOT the same events as A, B, and C. The worldline of the Home observer, as seen by the Other Observer, is also shown (in green). The ticks on all axes are drawn with the correct spacing. Drag events E, F, and G to their correct locations on the Home Observer's graph, on the left. Drag events A, B, and C to their correct locations on the Other Observer's graph, on the right. Home Other O O O O O O Check Check Complete the following tables, one for each observer. Be sure to include the correct signs. For example, Ax for B-A would be XB - XA. Home Observer Vother x = c Vother = Events Act) Ax As2 Type of Interval B-A B-C A-C E-D E-F. D-F Other Observer VHome,x = C PHome = Events Act) Ax As2 Type of Interval B-A B-C A-C E-D E-F D-F Check The graph on the left shows three events in the Home Observer's frame of reference, labeled A, B and C. The worldline of a second, Other Observer, is also shown (in blue). The graph on the right shows three events in the Other Observer's frame of reference, labeled D, E, and F. These are NOT the same events as A, B, and C. The worldline of the Home observer, as seen by the Other Observer, is also shown (in green). The ticks on all axes are drawn with the correct spacing. Drag events E, F, and G to their correct locations on the Home Observer's graph, on the left. Drag events A, B, and C to their correct locations on the Other Observer's graph, on the right. Home Other O O O O O O Check Check Complete the following tables, one for each observer. Be sure to include the correct signs. For example, Ax for B-A would be XB - XA. Home Observer Vother x = c Vother = Events Act) Ax As2 Type of Interval B-A B-C A-C E-D E-F. D-F Other Observer VHome,x = C PHome = Events Act) Ax As2 Type of Interval B-A B-C A-C E-D E-F D-F Check