Time, as we experience it in our everyday lives, has a direction. The future becomes the present moment, and what was the present moment for a fleeting instant is relegated to the past. The question remains, what exactly is it that causes the flow of time? What is it that separates past from future? That allows you to clearly see what is, vs what was and what will be? What is it that makes time move forward?
Something to look for when trying to pin down an arrow of time are phenomena that are time-asymmetric. This means that as time passes, they only happen in one direction. Someone viewing the phenomenon will clearly be able to tell, comparing several states, which state happened in the past, the present, or the future.
The most common phenomenon that is used to define an arrow of time is entropy. This is a quantity that reflects the state of disorder of a system. As anyone who has had to clean their room knows well, rooms, as well as everything else, tend to get more disordered over time. A room will inevitably go from a clean state to a messy state over time if no work is done to clean it. This is because there are many, many more ways for a room to be messy than for a room to be clean. This flow from organization to disorganization, from order to chaos, is one fairly definite way to tell if something is in the future or the past. The arrow of time as defined by entropy is saying “The future is definitely going to be in a more disordered state than in the past.”
Below a diagram illustrating the arrow of time as defined by entropy. A set of balls begin in an ordered state, and after time passes and they can move, they enter an increasingly disordered state.
Is it intuitively obvious to tell which state was in the future, and which was in the past?
There is another way that an arrow of time can be defined. It may even be more fundamental than the arrow of entropy. It can be thought of as the arrow of energy. Physically unimpeded matter will flow along an energy gradient from past to future, from a high energy state to a low energy state, as time passes, if no energy is added. This is simply a byproduct of the law of conservation of energy, and how energy works. Energy cannot be created or destroyed, so unless outside energy is added to a system, it will only be able to lose energy over time. It can only move to states of equal or lower energy. I believe that this is very closely connected with how and why we perceive time.
Think about a ball resting precariously on the edge of a valley. With a small gust of wind, the ball is pushed over the edge. It rolls down the slope, then comes to a stop at the bottom of the valley. In this case, the earth’s gravity provides an energy gradient, so something higher up has more energy than something lower down. When the ball rolls down into the valley, it is moving from the high energy state at the top of the valley to the low energy state at the bottom. Because no energy was added to the ball, the only way this could happen is if the ball was at the top of the valley in the past, and rolled down to the bottom of the valley in the future. Because this is how we experience reality intuitively, it is incredibly obvious. The ball was at the top of the valley first, then it rolled down, then it came to a stop at the bottom. Because the ball must flow along this energy gradient through time, we can order it temporally by its location in the energy gradient.
The flow along an energy gradient from high to low energy is SYNONYMOUS in our minds with the flow of time. We intuitively understand this causal link, as it is a fundamental part of how our world works, and therefore how our minds perceive and order the world.
The ENERGETIC arrow of time is saying, “THE FUTURE IS DIFFERENT FROM THE PAST, BECAUSE IN THE FUTURE A SYSTEM WILL BE AT A LOWER ENERGY STATE THAN IN THE PAST.”
A jug of water flowing into a basin can again be used to exactly predict the temporal order. The water in the higher jug, if allowed to flow, must always flow into the lower basin as time passes. It cannot gain water through time, because that would involve water flowing against the energy gradient of gravity. This flow must occur along with the flow of time. It may actually be synonymous with the flow of time.
In a very real way, something that does not move across an energy gradient does not really experience time. Imagine a rock embedded in a cliff for a thousand years. Because this rock is never moved, or touched, or effected in any way, the passage of time may as well not be happening from the perspective of the rock. For the rock, there is no meaningful difference between past, present, and future. It is obvious that a motionless rock does not move through an energy gradient through time, but is it possible for something be moving through space, yet not moving through an energy gradient?
Consider the case of a pendulum that moves back and forth. A perfect pendulum that doesn’t lose energy. As the pendulum goes up, stops, and goes back down energy is transferred back and forth from kinetic to potential, but the total amount of energy does not change. This means that when the pendulum goes back up against gravity, it rises exactly to where it started, but no higher. The pendulum contains the exact same amount of energy in total throughout its entire arc of motion.
The pendulum is moving in space, going back and forth, but the amount of energy that it has in total is staying the same. Its swinging will not change at all unless it gains or loses energy. This has interesting implications when it comes to time.
This implication can be understood by asking this question: Can you tell WHEN in time the pendulum is from its position? Which position happened before the other, or after the other? The answer is no, because the pendulum goes through the same exact motions in space at many different times. Any given position of the pendulum could have happened during any one of the pendulum arcs in the past or future. There is therefore no meaningful way to tell which position is in the past, present, or future. In other words, there is no way to meaningfully order them temporally. The diagram illustrate this by showing that there are many different correct answers for the possible temporal order of the states of the pendulum.
This means that for the pendulum, there isn’t really a strongly defined arrow of time. The motion of the pendulum is also symmetrical through time, meaning that if it was played backwards, it would behave in the same way, swinging back and forth in the exact same way. From the perspective of this pendulum, the arrow of time doesn’t really have a meaning. For the pendulum, forward in time is the same as backward in time, past, present, and future contain the exact same repeated motion. They are meaningless to the pendulum. Just like the motionless rock, this is because the pendulum has an unchanging amount of energy. It is motionless in terms of energy, even though it changes position through time.
Now imagine something changes. The pendulum begins to lose energy. Because of this, every swing is slightly lower than the one before. The pendulum is gradually going from a higher energy state to a lower energy state. Now that the pendulum’s energy is changing, the order of each swing in time can be easily determined. The pendulum must have been swinging higher in the past, lower in the present, and yet lower in the future. Because the pendulum is now travelling along an energy gradient, the concepts of past, present, and future become meaningful. The arrow of time for the pendulum now matters. It is no longer symmetrical, and it moves in only one direction, from past to future, because the pendulum must always lose energy through time.
I believe that the flow of time is intimately connected with the flow of matter along energy gradients. For all intents and purposes, in our minds and in physical reality, this is what gives time meaning. This means that for the passage of time to matter, some kind of flow along energy gradients must be possible. This of course means there must be energy gradients available along which matter can flow. If there was no energy gradient, there could be no meaningful concept of time.
Since time is produced by the flow of matter along energy gradients, how do you then measure it? It must be referenced to something, but what?
Matter that is flowing along energy gradients can be referenced to matter that, like the pendulum, is in motion physically but not energetically.
We already measure and define time in our world in this way. We use pendulums and springs in clocks, and more recently oscillating atoms, as a stable reference to be able to define time in our world. By having this reference, a specific rate of “time” can be established. What this is really doing is comparing the constantly flowing energy and matter of our world with a perfectly stable energy state. This is like a rock on shore being used to measure the current in a flowing river. The stable energy state of an oscillator can act as a reference for the flowing energy state that gives time meaning.