As humans, we measure time via change. For example, the second hand of a clock changes positions in a predictable manner that allows us to determine how many seconds have elapsed. However, imagine if the second hand stopped. Has time stopped? Of course not. We know that time is still continuing, even though our clock has stopped. As trivial as this example is, it points out one important aspect of time. It is not connected to change. Even if we were in a totally dark isolated room, our minds would still be aware that time is continuing. However, some may argue our minds are changing from one state of consciousness to another, and that is why we are aware of the passage of time. So let us remove all minds and consciousnesses from the universe. Does time still exist? To address this question, let us understand entropy. Entropy may be defined as a thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.Some physicists would argue that the entropy of the universe is always increasing, thus always changing. Therefore, the logic proceeds, time exists because entropy is continually changing (i.e., increasing). However, at some point, the entropy of the entire universe will reach its maximum value. This means all energy has degraded to heat. I have termed this the “entropy apocalypse.” Some physicists refer to it as “heat death.” At this point, has time stopped? I would argue it has not stopped. Why? Because the heat of the universe continues to exist, even when entropy has ceased to increase. This brings us to an important point. If time is not a measure of change, then what is it?

I argue that time is actually a measure of existence, not change. To understand this, let us consider a mass’ movement in time. Essentially, we can define a mass’ movement in time as existence. Let us take a simple example to illustrate this definition. Pretend we are viewing a mass and recording its position, which is at rest. The mass is not moving in any of the spatial dimensions. However, at a later interval (let us pretend our wristwatch records an hour to have passed), we again view the mass and record its position. We observe the mass still exists, and the coordinates are identical to the first set of measurements. In effect, the mass has moved in time along with us. Since the mass and we are in the same frame of reference, we have every reason to believe the rate of the movement of the mass in time is equivalent to our rate of movement in time. If the mass did not move in time—for example, stopped moving in time—it would not be there at the recorded coordinates for the second measurement. We would say it ceased to exist. On this basis, we can assert existence equates to movement in time. In this case, both the mass and we, the observers, moved in time at the same rate.

If you think this is far fetched, consider what occurs when a clock moves close to the speed of light. A clock moving close to the speed of light will appear to run slower to an observer at rest (one frame of reference) relative to the moving clock (another frame of reference). In simple terms, time is not an absolute, but is dependent on the relative motion of the event and observer. It may sound like science fiction that a clock moving at high velocity runs slower, but it is a widely verified science fact. Even the clock on a jet plane flying over an airport will run slightly slower than the clock at rest in the airport terminal. Einstein predicted this time dilation effect in his special theory of relativity, and he derived an equation to calculate the time difference. If we apply this to elementary particles with a short decay time, they will actually exist longer (i.e., not decay) if they are accelerated close to the speed of light. Once again, we (the observers) continue to measure the existence of the elementary particle in the future. From our frame of reference it has not changed (i.e., decayed), but continued to exist. For completeness, I will also mention that time dilation occurs in the presence of a strong gravitational field. Therefore, a clock close to the Sun would run slower than a clock on Earth, because the Sun has a much greater gravitational field than the Earth.

Scientifically speaking, there is no consensus on the definition of time. Even though we humans typically use change to measure time in our everyday world, time dilation experiments suggest this is only a convenience. It works because we are not typically measuring entities traveling at the speed of light or in vastly different gravitational fields. However, to my mind, some of the simple examples presented above, along with time dilation experiments, suggest time is more closely aligned with existence, not change.