Human understanding of light has wavered over the centuries. Some famous philosopher/scientists, including Rene Descartes, insisted that light consists of waves; others, including Isaac Newton were convinced that light consists of particles. In the twentieth and twenty-first centuries, most scientists who deal with the physics of light acknowledge that light is both wave and particle. The particles, called photons, also have wave-like qualities. Moreover, electrons also possess the same paradoxical wave-particle duality. Even protons and neutrons, consisting of quarks, appear to have wave-particle duality. Therefore, everything in the material world rests upon the paradox that the component parts of every item are, at the same time, tiny particles of matter and also waves of energy.
One result of this paradox is that knowledge is limited about each particle. For example, no one can know the precise position of a particle and also how it is moving. This principle was first enunciated by a scientist named Heisenberg and is called the “Heisenberg uncertainty principle.” One famous scientific joke involves a police officer pulling over a car driven by Dr. Heisenberg. When the officer asks the doctor the standard question, “Sir, do you know how fast you were going?” Dr. Heisenberg replies, “Please don’t tell me, because if you do, I’ll never figure out where I am.”
By the way, there is also a Salvageable uncertainty principle. Ask me what that principle says, and I will answer, “I’m not sure.”
Larger material items, made out of enormous quantities of protons and neutrons and electrons, generally follow rules of geometry and physics that make sense to the average human mind. A police officer’s radar gun accurately measures the speed of a moving car. That car might be shown, by the radar gun, to be traveling seventy miles an hour. That measurement does not prove that an hour ago the car was seventy miles away. Until a few minutes ago, the car might have been sitting in a parking lot only a few miles away. But, for large material objects, we can account for both the speed and the location of that object and can accurately report both statistics at any given moment.
Philosophically, though, the motion of a material object and its location remain a puzzle. Greek philosophers more than twenty-four centuries ago were already asking how any object could move through an infinite number of points in a finite time. Dividing time into an infinite number of punctiliar moments does not solve the philosophical quandary. We can observe an object at rest and can measure its size and describe its location. We can observe an object in motion and determine its speed and direction. Trying to gather all that information at the same time seems as though it should be easy, but problems remain. As we begin measuring size and location and speed in appropriate units, we are forced to make statements that are philosophically untenable. The car that is moving seventy miles an hour does not disappear from the highway this instant and reappear seventy miles away an hour later. Assuming that its speed and direction do not change, it will be present on every bit of paved highway between here and its destination at some point during the next hour. Chopping the highway into miles, feet, inches, or any other unit—while also chopping time into hours, minutes, and seconds, or any other unit—leaves the location of the car between those identified units a mystery. If, for example, we film the car at a rate of twenty-four frames per second, each frame will show the car at a different location on the highway without any explanation of how the car traveled from one point to the next point, since an infinite number of points exists between those two points.
Aside from that problem, the car in each frame of the film is not the same car. The car constantly changes. From instant to instant, it burns a tiny bit of gasoline. Its tires rotate, and tiny bits of rubber from the tires (perhaps mere molecules) separate from the tires. From time to time, dirt and insects are added to the windshield and other parts of the front surface of the car. Take the same car at any two points along its journey and compare its description; one will see that it is not the same car. Tiny changes have occurred to make the car slightly different as it travels down the highway and also travels through time from past into present and on into the future.
We are all like that car. We change continually. None of us is the same person who woke up this morning. We have breathed air in and out of our lungs, and some of that air has been taken into our body to be used by our cells; other air that was in our bodies has left our bodies. We eat, we drink, and we use the bathroom. We wash, removing dead skin cells from the surface of our bodies. Sometimes we cut our hair or trim our nails. Even our minds change as we experience and remember new events every instant of our waking lives (and also while we sleep). You are not the same person you were when you were a child. You are not the same person you were ten years ago. You are not the same person you will be ten years from now.
On an atomic and molecular level, we change constantly. On a cellular level, we change constantly. In other ways, we continually change while we travel the timeline of our lives. Yet, as we view that timeline from outside of time, we also perceive continuity. Because that timeline is unbroken, we are able to describe ourselves as the same person through the years and over the course of a lifetime. In the same way, a car remains the same car in spite of the many changes that happen to it—a new tank of gas, an oil change, new tires, replacement of damaged body parts, replacement of damaged engine parts. Over twenty years, every piece of a car could be replaced, but legally and philosophically it remains the same car. The philosophic implications of continuity as we change are enormous. J.