Time is an oddity. In mystic thought, it is often cyclical. In classical Newtonian physics it was thought of as a steady march onward. In Einstein’s universe, it is wrapped up with the fabric of spacetime and can dilate in unexpected ways.
One thing we know about time is that it waits for no one. Time marches on (although not in a steady way, thanks to Einstein) and we experience it passively. Without any effort on our part, future becomes present and recedes into the past, even while we sleep. We can’t feel the passing of time like we feel the wind against our face, although change is happening imperceptibly, and after a lifetime, we realize that its passing has ravaged our fragile bodies.
Where did the time go, we ask? As if it is a thing that moves.
Our human experience of time is very subjective and unreliable. When we are having fun, time vanishes, which seems rather unfair, as if the world is conspiring to cut short our enjoyment and prolong our agony. We much prefer enjoyable experiences to take place in the present or the future, and consign our bad experiences to the past. Most of what motivates us in life is geared to trying to make this happen.
Time isn’t something we can measure easily. We can’t put physical markers on it. To grasp time, we must remember past events, and attach labels to those events. But those events vanish in an instant and no longer exist. We must trust to our memory, or written records, to recall them.
Time is something that we must measure indirectly and relatively. There is no start time or standard time that we can refer to. The only way to measure time is by intervals between regular repeating events. The rising of the sun on the horizon is one of those events. We can call it a day. We can subdivide it according to our whim into hours and minutes, and combine days into weeks, months and years. But the length of a day isn’t a constant. The sun rises and sets at different times throughout the year. The number of days in a year isn’t a whole number. Even the rate of rotation of the Earth is slowing inexorably as the Moon pulls it towards an eventual halt, making the day longer by a millisecond or so every century. Time based on such crude measures would be hopelessly inaccurate, except for simple purposes.
To measure time more accurately, we need a regular repeating event that never changes. Atomic clocks, based on energy level transitions of caesium atoms, measure time more accurately than any astronomical observations ever did, with an accuracy of one second every billion years. This is the best we can do with our current technology.
Yet still we are stuck with a definition of time as “what a clock measures.” It sounds like the opinion of a three-year-old, yet it’s not a bad definition, apart from being circular (pun intended.) After all, what is a clock, other than a device for measuring time?
Can science not do any better than this?
According to Einstein, time is what separates events in space. And Einstein wasn’t being vague when he used the word event. Events are not just arbitrary human-invented entities like “my birthday” or “the battle of Hastings in 1066”. They are also the fundamental interactions between elementary particles that make up the matter in this universe. If a photon is absorbed by an electron, or if two electrons scatter off each other, that is an event, and it is observable.
What about the direction of time? Some people have suggested that it is the irreversibility of physical events that give time its direction. For example, a hot object will cool, never spontaneously heat up, unless we apply some kind of heat source. It is argued that this arrow of time, defined by the scientific measure called “entropy” (a precise definition of the amount of disorder in a physical system) is what makes time go forward, rather than back. Entropy always increases.
But even reversible events have a before and after, so I don’t think that can be true. If the universe ever stopped expanding and started to contract again, entropy would continue to increase. We wouldn’t all reappear, running our lives in reverse order, from death to birth. The direction of time is real, not a perception.
The microscopic events that map out the passage of time are real and objective, and so time must be too. We can measure it with clocks, and define how it appears to different observers using Einstein’s theory. And that, however unsatisfactory, is probably as close as we can get to grasping what it is. At the present time, anyway. In the future (whatever that is), who knows?