“People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion. Albert Einstein
Our experiences are rooted in the continuous ‘flow of time"; there is no choice. We cannot stop and rest in any moment. Time is an abstraction but change is real. The apparent flow of time is undifferentiated and when we are not thinking about the negative aspects of change, the flow of events is implicit, natural and mostly interesting. Animal bodies are governed by a series of sequencers whose functions and rhythms overlap. Time is implicit in the body and cannot be separated as an independent something or other. Life time is measured by how many times you fall asleep and then awake. Life time is measured by how many times your hunger overrides other activities and you must eat. Life time is measured by how often thirst overrides other activities and you must drink. Life time is measured by seasons and how often the sun reaches its zenith and then falls back below the horizon. Life time is measured by the moon and the great movements of the oceans that follow the moon's path in orbit around the earth. Life time is manifest by growth and development, by ovulation and menstrual cycles, and by babies gestating in wombs. Lifetimes are measured by growing, learning, aging, senescence and death.
A clock is a time-measuring device. But what is the clock really measuring? The more deeply you ponder the question, the less obvious is the answer. Everything changes continuously. Everything is impermanent. The clock is an event in the present that correlates with other events. The clock is not separate and different. In Einstein’s four-dimensional spacetime, time is space-like and space is time-like. Events in spacetime all have four dimensions and each dimension defines the other. Einstein considered that spacetime was relative to the observer’s frame of reference and there was no absolute time except that the speed of light is constant in the universe. If you accelerate away from a stationary observer, your clock slows down relative to the observer’s clock. The faster you travel, the slower your clock marks time but you do not notice; you do not perceive your clock as marking out the intervals more slowly and presumably your sense of duration does not change. Time remains constant in every space-time frame but is relative to the observer's position. You can imagine the space traveler existing in a different time zone, but you cannot have his or her experience.
When people invent stories about time travel, they confuse time and space dimensions. Time is understood and expressed metaphorically. The time metaphor is a journey that has a beginning and an end. The progress of the journey is marked by episodes and waypoints. A ‘point in time” is a curious reference that suggests that change can be interrupted and isolated. But, the really real is flux. There are no points in time. Humans have numerous measuring and recording devices that seem to fix events in the past as if the past were real and time was an independent entity.
You could argue that past time is a reasonable fiction that allows humans to understand their path as a journey through spacetime. Or you could argue the journey metaphor is wrong and misrepresents reality in every possible way. The past is not real. There is no future… only a present. The present is a feature of the human mind.