Commentary

The Lure of Natural Design

Last week I discussed the shortcomings of intelligent design arguments for the existence of God. An intelligent design argument asserts that if we cannot explain a phenomenon through our understanding of nature, then an intellect must be guiding the phenomenon. A common intelligent design argument from ancient philosophy is that the perfect regularity of planetary motion is only possible if a divine intelligence is guiding that motion. This example shows the flaw in intelligent design: our inability to understand something may indicate something other than the action of God, and in this case that something is a poor understanding of physics.

Intelligent design is not the only design argument one encounters, and proof of God's existence is not the only goal in mind. Within the scientific community there is a strong school of “natural design” arguments for particular fundamental theories of cosmology. Examples of these arguments are the “flatness problem” and the “horizon problem.”

The flatness problem refers to the rate at which the universe is expanding. The rate of expansion is nearly counteracted by the gravitational deceleration. If the universe were slightly more dense, its expansion would have stopped by now, and a collapse would be underway, but if the universe were slightly less dense, it would be in a coasting phase, expanding forever at a constant rate.

The horizon problem refers to the observation that the universe looks the same in all directions, despite the fact that under the Big Bang theory, the various parts of the universe that we see are causally disconnects. For example, two regions at opposite ends of the visible universe can only interact physically on a time scale longer than the light travel time between the two regions, but the light travel time is longer than the current age of the universe.

Many scientists believe that a physical explanation for these characteristics of the universe must exist. The reasoning is similar to the reasoning in the intelligent design argument: the perceived problem implies an action of nature, which is similar to the assertion than the perceived problem implies the action of intelligence. Another option, that there is no explanation for the observations, is ignored.

This point goes to the heart of the scientific enterprise. The fundamental theories of physics are often described as laws, as if the universe itself follows the equations we write. In fact, the fundamental theories of physics are descriptions. We can give explanations of the workings of the world in terms of these descriptions, but the descriptions themselves have no explanation. The planets move in nearly Keplerian orbits because of the inverse-square law of Newtonian gravity, but this inverse-square law cannot be explained.

A popular story in astrophysical circles is about the turtle theory of cosmology. An astrophysicist gave a popular talk on astronomy. After the lecture, he was approached by a woman from the audience. “You are totally wrong about us standing on a ball floating in space,” she said. “We are standing on the back of a turtle!” The astrophysicist replied “but if we are on a turtle's back, what is the turtle standing on?” The woman replied “oh, he's standing on another turtle. Its turtles all the way down!”

At times, the attempt to explain the world as it is produces theories that seem to be turtles all the way down. At some point we must stop and simply accept that the world is as it is, with no explanation of why; the only question is where to draw this line.

Both the horizon problem and the flatness problem focus on characteristics that extend throughout the universe within the bounds set by the microwave background and primordial nuclear synthesis. Perhaps these bounds will be pushed to earlier times through observations of neutrinos and gravitational waves. At some point, however, the early universe becomes observationally inaccessible, and the conditions we find at this observational boundary cannot be explained, because no theory that explains them can be tested. We can only treat the universe at this boundary as a description.