One of the biggest conceptual pitfalls one faces when investigating the truth about reality is the tendency to picture oneself as something essentially separate from the rest of the world. (Another one is picturing self and world as static entities instead of active and evolving). The lesson of naturalism is that the world is an evolving network of related happenings which share the same fundamental character. We humans are embedded in this network.
It follows that it is incorrect to study the rest of the universe as if one was a truly separate and objective observer. We can’t look at the world from the outside. Our perspective is necessarily a view from the inside.
It is interesting to see how this notion of outside and inside viewpoints has played out in the arena of physics.
While Newton understood that motion only has meaning relative to something else, he still assumed one could measure things against an absolute backdrop of coordinates in space and time. The backdrop represented a God’s-eye view of the world. Einstein corrected this picture with relativity. Space and time became intertwined with matter and energy and could no longer be considered separately. There is no “correct” absolute frame of reference – it only makes sense to talk about the collection of reference frames within the world. It seems reasonable that a theory which turned out to give the correct explanations for the phenomena we observe at the cosmological scale had this conceptual feature – after all, our perspective is necessarily a parochial one inside the world.
Interestingly, the other 20th century revolution in physics, quantum mechanics, performs its calculations against a backdrop of time external to the theory. Given that the genesis of the theory was its success in explaining the behavior of the atom in a laboratory setting, this may not seem like a big deal. The measurements of atomic and subatomic phenomena obviously occur in the time frame experienced by the scientist in the lab.
However, in trying to reconcile general relativity and quantum mechanics into a new theory, the conceptual clash between a relational theory with no background space/time and another one which depends on a fixed background makes for a challenge. The work on a new theory of quantum gravity has taken many forms so far. Theorists in the largest part of the field, the various forms of string/M theory, have mostly deferred the issue of background independence so far, thinking the theories are rich enough that a background independent version will emerge in time. Others take the issue more seriously upfront, like the physicists working on loop quantum gravity.
I’ve been very interested to read some papers which preliminarily attempt to describe the universe in a way which is both quantum mechanical and background independent. These ideas implement the relational interpretation of quantum mechanics and are referred to as theories of relational quantum cosmology. I came across this last term in an appendix to Lee Smolin’s essay on the state of progress in quantum gravity theory (see the papers he references, for instance this one by Fotini Markopoulou. As far as I can tell, the idea is that the universe is a collection of micro-level space-time regions (observers) whose boundary interactions follow the rules of quantum mechanics. You build a cosmology from the bottom up, rather than assume there is a top-down quantum description of the entire universe. Time is a local, not a global phenomenon: relational, not absolute.