The Parallel Postulate

We have viewed Euclidean geometry as an axiomatic system. In introducing axiomatic systems, we studied three-point geometry, four-line geometry, and Fano's geometry. Recall the two ingredients of an axiomatic system:

We have seen the axioms that are also true in Euclidean geometry as well as the axioms that are invalid in Euclidean geometry.

Now, we are going to study some geometries, other than finite geometries, different from the Euclidean geometry. First, we want to take a closer look at the foundation of Euclidean geometry.

With 23 definitions and several implicit assumptions, Euclid derived much of the (planar) geometry from five postulates (or axioms):

  1. A straight line may be drawn between any two points. (Given two distinct points, there is always a line passing through the two points.)
  2. A piece of straight line may be extended indefinitely.
  3. A circle may be drawn with any given radius and an arbitrary center.
  4. All right angles are equal.
  5. If a straight line crossing two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if extended indefinitely, meet on that side on which are the angles less than two right angles.

Postulates 1 and 3 are based on the geometric construction. Postulate 2 illustrates a common belief that straight lines may not terminate. A right angle was defined in Definition 10 by Euclid as an angle that equals its adjacent angle. So, Postulate 4 is justified by our belief that no matter where two perpendicular lines are drawn, the angle they form is one and the same and is called right.

Compared with the first four postulates, Postulate 5 is much more complicated. It is conjectured that Euclid himself had mixed feelings about the fifth postulate as he avoided using it until Proposition I.29 in his Elements. The fifth postulate, also known as the Parallel Postulate, attracted immediate attention. Many substitutes were suggested and many felt it is a consequence of the other postulates. Now, we know that the fifth postulate is independent of the other postulates and it cannot be derived from the other postulates.

Here's a short list of equivalent statements to the fifth postulate:

  1. (Playfair's axiom): Through a point not on a given line, exactly one line can be drawn in the plane parallel to the given line.
  2. There exists a pair of similar non-congruent triangles.
  3. For any three non-colinear points, there exists a circle passing through them.
  4. The sum of the interior angles in a triangle is two right angles.
  5. Straight lines parallel to a third line are parallel to each other.
  6. There is no upper bound to the area of a triangle.
  7. Pythagorean theorem.

Euclid's set of axioms (postulates) is not complete. One example is about betweenness (other examples are contained in our last project, Project III). The following is another example of a statement that is independent and consistent with the five postulates:

Pasch's axiom: A line entering a triangle at a vertex must intersect the opposite side, and a line that intersects one side of a triangle at a point other other a vertex also intersects a second side.

Many modern sets of axioms were devised. We will use the one set up by the great French mathematician David Hilbert in 1899 (100 years ago!) in his book Foundations of Geometry (English edition is still available). Finally, here is the definition of Euclidean geometry (after we talked about it so many times!):

Undefined terms:

Hilbert's axioms: