by Bruce E.R. Thompson.
Here is a question for you: why does time move forward?
This may sound like one of those philosophical imponderables like, how can nothingness exist? Or what is the right way to distinguish between right and wrong? Or if God is omnipotent, can God make a stone so heavy that even God can’t lift it? But it may be a serious question. In physics, the mathematics of time suggests that past and future are just as symmetrical as left and right, and just as interchangeable. Particles that spin clockwise when seen from the top will appear to spin counterclockwise when seen from the bottom. Likewise, if time moves forward a particle that is moving to the left and spinning counterclockwise will appear to be moving right and spinning clockwise if time moves backward. If we were to record a collision between two particles, our recording would make just as much sense when played in reverse as when played forward. All the interactions described by physicists, including time, would seem to be completely reversible. It is just as sensible to talk about particles moving both forward and backward in time as it is to talk about them moving up and down or left and right in space. So, why does time move only forward?
The curious answer that some philosophers—beginning with St. Augustine—give to this question is that time moves forward because we perceive events in that order. We do not remember the past because the past happened first; rather, the past happened first because we remember it. This may seem like a counter-intuitive idea, but it must be right. Let me explain why.
Let’s make a movie of an event in the world—not at the subatomic level, but at a grosser level, where we can see the familiar objects that make up our world. Let’s make a movie of a glass vase falling off a table and shattering into thousands of pieces. Now let’s run the film backward so that we watch the thousands of shards of glass leap off the floor, assemble themselves into a vase, and jump onto the table. Naturally, we can tell immediately that the film is running backward. At a subatomic level all the interactions we are seeing are perfectly possible. But their combination in just the required order to assemble a vase is so highly improbable as to count as statistically impossible. Time moves in the direction designated by probability. Time moves in the direction of increasing entropy: from order toward chaos, never the reverse.
Imagine you have a new car, fresh off the assembly line. It is in as highly ordered a state as it is ever likely to be in. Its paint is without scratches; its body without dents; its machinery in perfect working condition. Because there are many more disordered states than ordered stated, the car can only fall into disrepair as time moves forward. In the course of its life the paint will be scratched, the body will be dented, the machinery will wear out and fail. The record of all these scratches, dents, and failings will be preserved. The fender and body of the car will “remember” all its run-ins with trees, walls, and other cars by preserving evidence of those run-ins as scars etched upon its surface.
The memories recorded in our brains operate similarly. Our memories are the scratches and dents that our brains preserve as evidence of our encounters with the world. Like the fender of a car, our brains pass from a state of having no memories to a state of being scratched and dented with so many memories that the older ones become occluded by the newer ones.
Now, imagine how the world would appear to us if time were moving backward. By hypothesis, the events of the future would be happening “first,” and the events of the past would be occurring “later.” The shards of glass would assemble themselves into a vase and leap onto the table. The scratches and dents in the car’s fender would disappear as each encounter with a tree or a wall or another car had the effect of erasing a blemish that had previously been there. Eventually, after enough collisions, the car would be shiny and new.
But, by the same token, our brains would also be moving from a state of having memories to a state of lacking them. We would begin life in old age with a more or less comprehensive “memory” of the future, and our memories would slowly be expunged as we walk “backward” through life. Each encounter would cause us to “forget” something we had previously known. But since we would remember the “future,” we would perceive the future as that which had already happened. The “past,” by contrast, would be a deep mystery. We would, in short, experience the world exactly as we do now.
Indeed, for all we know, time is running backward. Or worse, maybe it is running sometimes backward and sometimes forward, switching randomly from one to the other. It might as well be, for all the difference it would make to us. It is not because the past happened first that we remember it; rather, we say that the past happened first because our memory of something places it in the past. The past is the past because evidence of it is etched into the fabric of the world—including into our brains. The past is the past because we remember it, not because it “happened first” in some grand cosmological sense.
This analysis of the nature of time answers the physicist’s question: why is time the only variable in physics that is not reversible? The answer is that it is reversible. But, if time does sometimes reverse itself, we would never know it. There is no scientific test that could reveal that time was moving backward, even if it was. That is because the test itself would be taking place backward in time. So, if time were moving backward, that fact would have no practical effect on the results of the experiment.
Is there a moral to this? I think not. A determinist might infer that our own actions are out of our control because both the past and the future have “already happened.” But that is bad logic. If time is running backward, then causality is also running backward. Remember that causality is probability: the relentless march of entropy to move from order to chaos. Hence, if time is running backward, then entropy is also running backward from chaos to order. Thus, the shards of broken glass will reassemble themselves into a vase which then seems to defy gravity, which is also running backward—because even Einstein’s laws of relativity are running backward—to leap onto the table before coming to rest. If time is running backward, then, rather than the past leaving its mark on the future, the future would leave its mark on the past. My choices would affect what has already happened rather than affecting what has yet to happen. Hence, the morality of my choices would still matter. I would still need to judge my actions by their reasonably predictable consequences. The only difference is that those consequences would be in the “past” rather than the “future.” But moral problems concerning right and wrong, how we live our lives, and so on, would not be affected in the least.