Time is a strange effect in the universe. What exactly is time, and how does it arise?
One way of looking at time is to consider to be the fourth dimension: after the usual three of length, width and depth. Hence we have the concept of spacetime. This is most famously put forward by Einstein in special relativity, though the roots of that predated even him.
At our personal level of perception, time is a physical reality. Everything else around us also seems to exist in the same time-frame as ourselves. For example, we stretch out our arm to shake the hand of someone else, and there really is a someone else there with whom we can interact.
But relativity says that is an illusion, that time progresses differently in various places. This is called time-dilation, and the effect is real: time passes slower where gravity is stronger or acceleration is greater.
In our recent paper we explore time, using the cordus conjecture. What emerges is a novel and useful way of thinking about time. Cordus suggests that at its most basic level time originates with the frequency cycles of the particules of matter and photons. Thus time is locally generated, and cordus rejects the idea of an absolute clock. The forward arrow is only applied to the ticks of time when irreversibility arises.
The paper explains how the irreversibility arises, in terms of the interaction between two volumes of matter and the statistically impossibility of returning all particules in the system to their original positions and states. Thus decoherence, irreversibility, entropy, cause-and-effect, and the arrow of time all arise at the same discontinuity in physics. The interconnectedness of matter, via its fields, creates a patchwork of temporal cause-and-effect.
Thus human perceptions of time are a construct, with all the potential for illusion that implies, founded on a real physical principle of temporal causality.
However that is really only a convenience, becuase the first three are spatial (geometric) dimensions, whereas time does not have the same units.
About time: What is it? New Scientist
About time: Why does time’s arrow fly only one way? New Scientist
About time: Is time travel possible? New Scientist
Limits of Coherence (cordus.wordpress.com)