Ever since high school physics, I have been intrigued, even awed, by the properties of light. Light provides at least two metaphors for the nature of God. In the first metaphor, the wave-particle duality of light teaches the concept of complementarity. Depending on the type of experiment used to observe the behavior of light, the human observer can show that the properties of light can be explained as waves or particles. These two different qualities are in apparent conflict, yet they are both true. In the same way, the principles of free will and determinism are both true when applied to our understanding of human behavior under the providence of a sovereign, omnipotent God. Likewise, the hypostatic union, the term describing the complementary attributes of Christ as fully human and fully divine, can be similarly understood.
In the second metaphor, the unique property of the speed of light as a universal constant in all reference frames as expressed in the theory of special relativity provides another illustration of God's nature. Einstein proposed, and many scientists have since confirmed, that the speed of light in a vacuum is a constant, regardless of the relative speed of the observer. Likewise, God's standards and character are constant and unchanging—He is immutable. Furthermore, from the theory of special relativity, we know that an object of finite rest mass will increase in mass, decrease in length, and experience a dilation (slowing) in the elapse of time as seen by an external observer who is at rest. In fact, according to this theory, an object with mass cannot achieve the speed of light, because, if it did, its mass would become infinite, its length would become zero (invisible), and time would stand still, making it eternal. By analogy, John's statement that "God is light" has new meaning in light of the theory of special relativity, providing insight into God's infinite, invisible, and eternal nature.
The apostle John makes the simple statement that "God is light" in 1 John 1:5. It thrills me to think that the Holy Spirit inspired John to pen those words knowing that it would take almost two millennia before mankind would fully appreciate their significance.
