I apologize for the seemingly random nature of this post. I started with just the premise that “time is not an illusion” and ended up speaking about entropy.
Time is just an arbitrary means to quantify a system’s entropy. Everything decays. Everything moves from new to old to degeneration into its fundamental components. New represents no entropy. Old represents more entropy than new. Degeneration represents a system’s reaching 100% entropy which means the system, say a brick, has collapsed and is no longer cohesive as a system. Time, then, represents the gradual increase of entropy until the system collapses into disorder (degeneration). Of course, everything is a system within a system. That brick is comprised of atoms where are comprised of smaller particles which are comprised of smaller elements still which are, even them, comprised of smaller elements (strings?). Each of those smaller elements is, in itself, a system with an entropy characteristic.
Of course, there are physical systems and non-physical systems (like a crowd at a political rally) but they all have entropy where entropy is both measurable energy and non-measurable energy. That political rally, as individual people get fired up from what the politician says, has a non-measurable energy. But it is energy none-the-less. And when that energy increases the entropy of the system (the crowd of people), the system may break down into chaos (people yelling and screaming and fighting). Call in the police, throw some tear gas, and entropy is reduced albeit in a painful way.
Time, then, is not an illusion. It is the progression of any system, whether sub-atomic or a crowd of people, towards 100% entropy. The “time” it takes for that to happen is dependent on a variety of factors that are specific to the system in question but, regardless, the system progress. And, in some cases, may ebb and flow as energy is added or removed thereby balancing, reducing, or increasing entropy. Time is just a word humans use to describe the movement of a system along a vector of entropy, even if we don’t recognize it that way. Our quantification of it is purely subjective. Here is another example: a place of work is a system that operates non-chaotically when people arrive according to a specific schedule. If people don’t arrive according to that schedule (which can be measured by a clock or by the passing of the sun) then the business fails to operate optimally. This increases the entropy of the business which, in turn, can eventually lead to a breakdown (i.e., orders not fulfilled). The best illustration of this is a union strike. That business is a system that devolves into chaos (breaks down) when there is too much entropy (people not arriving at the “time” they are supposed to). Of course, the individual people within that system are also systems in themselves and may breakdown due to increased energy (i.e., the stress of the job, the stress of resolving the union strike, etc.).
When we say the “passing of time” what we are really describing is the system (the world, our family, our life) increasing or decreasing in entropy. The doomsday clock is a great example of this. It describes a world that is hovering close to a dangerous level of entropy. Climate change is another. The human race has continued to add elements to the environment that increase its entropy. As such, the ecological balance is upset and the system is degrading (and will continual to degrade until the extra elements can be removed or countermanded at which time, energy will be released and entropy will return to a more balanced state).
Time equals the ebb and flow of entropy and while time is a linear progression of that entropy from one state of the system to another, entropy is not a linear flow but, rather, increases and decreases. Of course when the system reaches a critical entropy, it can break down and be unrecoverable as a system (even though the smaller systems within it may survive as is).
What if, instead of the arbitrary measurement of time we use, we measured existence in levels of entropy. If you could divide entropy measurement into a granularity at, say, the Planck scale, then concepts like “time travel” would be more practical as it would just be about committing the entropy of a system to a certain level. If, at a quantum understanding, the universe exists at all entropic levels simultaneously, then changing the entropy of a system while we are standing in the system would mean we return to that “time”.
Something to think about some more.