- What exists?:
The question "What exists?" is important for physics because the universe and its constituents exist and physics is supposedly the study of those things. So, if you can figure out why things exists and what the properties of those existent entities are, this should give you a glimpse into fundamental physics. I call this a metaphysics-to-physics approach. But, to do that, physicists will need to move beyond the shut up and calculate mindset.
I think that a thing exists if it is a grouping. A grouping ties zero or more things together into a new unit whole and existent entity. An example of tying zero things together is the empty set. This grouping is manifested as a surface, or boundary, that defines what is contained within, that we can see and touch as the surface of the thing and that gives "substance" and existence to the thing as a new unit whole that's a different existent entity than any components contained within considered individually. This surface or boundary doesn't have some magical power to give existence to stuff. But, it is the visual and physical manifestation of the grouping into a new unit whole or existent entity. The grouping idea isn’t new. Others such as Aristotle, Leibniz, etc. have used the words “unity” or “one” instead of “grouping”, but the meaning is the same. After all, what does a grouping into a new unit whole do if not create a unity or a one? Philosophers often make ontology way too hard, I think. If there’s a grouping that ties stuff together into a new unit whole, a thing exists where ever (inside the mind or outside the mind) and whenever that grouping is present. I don’t think it needs to be any harder than that.
Some examples of groupings are 1.) the grouping together of paper and ink atoms to create a new
unit whole called a book that's a different existent entity than the atoms considered individually. Here, the grouping is manifested in the surface of the book; 2.) the grouping together of previously unrelated elements to create a set, with the grouping being manifested mentally as the braces around the set signifying that the list of elements creating the new unit whole is complete; 3.) the grouping of no elements at all, or “nothing”, to create the empty set; 4.) the mental construct the mind labels "concept of a car" is a grouping together of the other concepts tires, chassis, steering wheel, use for transportation, etc. Here, the grouping is manifested in the mind as the top-level label "concept of a car" under which the mind ties other headings together into one; 5.) the mental concept the mind labels “1+1=2” is a grouping formed during learning when a child uses his or her power of thought to group together the sub-concepts of one object in location A, another object in location B, moving of the objects together to be in the same location, C, and calling this new set “two”, and labeling this process with the labels of "1+1=2" and “addition” as provided by a parent or teacher; and 6.) Even a property, like redness, is a mental grouping (i.e., inside the head) labeled “redness” that ties together the subconcepts: mental images of firetrucks, apples, etc. with memories of the emotions and physiological sensations felt when seeing firetrucks and apples. These examples illustrate various cases where the grouping is caused by something internal to the grouping (book caused by chemical bonds) or external to the things being grouped (mental constructs caused by the power of thought). Outside-the-mind groupings are manifested as physical surfaces. Inside-the-mind grouping are manifested as mental labels like "concept of a car", "1+1=2", "redness" that tie together sub-concepts into a new unit whole.
A corollary of this is that a thing exists only where (inside a mind or outside a mind) and when the grouping is present.
One nice feature of the grouping idea is that it can be used to answer the question “Why is there something rather than nothing?”. This answers the question of what exists at the most fundamental level and also leads to ideas on the emergence of spacetime.
- What is an observer?
If the grouping idea is correct, then an observer is a force or situation that causes a grouping (i.e., a new existent entity) to come into being so it can be observed by measurement.
- Do non-physical entities, like the world in which the Escher painting is possible, exist?
Yes, but only in the mind of the person thinking about this. This is where the groupings that are mathematical and logical constructs and the grouping that is the world in which the Escher painting is possible exist - in the mind of those thinking about them. To suggest, as many scientists do, that mathematical and logical constructs exist in a mind-independent Platonic realm is just another kind of religious faith. It could be right, but it can’t be discussed in a rational, non-faith way.
- How can time emerge from a realm with no time?
In regard to how we can ask how time emerges from a situation with no time since time emergence would seem to itself be a time-consuming process. Aquinas had a good answer for this, with which I agree. I think the situation with no-time and the situation in which time exists are the same situation, just thought about in different ways. When we switch between these two different ways of thinking, the mind mistakes this for a temporal process. So, there really is no emergence of time, it’s just switching between two different ways of thinking about the same situation.