Continuing the discussion from Gravity is Not Intuitive:
So, I’m going to take @PdotdQ’s invitation to explore this topic a little bit for those who are interested (since I find it quite interesting).
The basic question here is: what does science tell us about the nature of time? There’s a big philosophical debate about what the temporal aspect of reality is like, and the accepted belief among most scientists and philosophers who think about this issue seems to be that science has come down pretty decisively on one side of the matter. But that conclusion may be premature.
One focus of that big philosophical debate is about, essentially, what is now? Is the present moment an objective feature of reality - and therefore the passage of time happens really and objectively? (This thesis is called the A-theory of time.) Or is the present moment merely subjective - there is nothing metaphysically privileged about the present, and now is merely an indexical word like here? (This is the B-theory of time.)
Science steps into this debate in a major way with Einstein’s theories of special and general relativity. The reason for this is that, with a metaphysically privileged present, the A-theory requires that there is absolute simultaneity - when something is happening here and now, there is an objective fact of the matter about whether something that is happening somewhere else is also happening now, or whether instead it is happening earlier or later.
But Einstein’s relativity theories imply the relativity of simultaneity - or at least, if there is such a thing as absolute simultaneity, relativity theory implies that we can never ever detect it. Absolute simultaneity is superfluous, according to relativity physics (and a strand of philosophy that was popular, back in Einstein’s day), and thus we should discard it from our theories. So it is commonly believed that science says B-theory wins, A-theory loses, and that pretty much settles it.
This is a case where I think the common belief is, in fact, incorrect.
First, the inference from “we can’t detect it” to “we must discard it from our metaphysics” isn’t a given and has to be justified. The philosophy that was popular in Einstein’s day - called “logical positivism” - took it to be justified because this philosophy actually held that statements were meaningless unless they could be logically or empirically verified. Unfortunately for logical positivism, that criteria of meaning can’t be logically or empirically verified, so it’s a little self-refuting. So belief in absolute simultaneity may still be justified, after all.
Second, absolute simultaneity is not actually in contradiction with relativity physics - all that the physics implies is that absolute simultaneity (if it exists) is not the same as apparent simultaneity (simultaneity operationally determined by synchronizing clocks via light signals). Absolute simultaneity can be represented in relativity physics as a particular slicing (or foliation) of 4-d spacetime into many 3-d spaces (the leaves of the foliation) ordered in time. We can’t detect exactly what that slicing is, but it might still be there.
This means that physics alone can’t decide between the A-theory and the B-theory - it can only provide one more philosophical consideration that has to be weighed against others.
Third, relativity physics may not be the last word from science on the nature of time. Though it isn’t usually recognized as such, there’s a good case to be made that Bell’s theorem does more than just show that quantum mechanics is weird - it shows that quantum mechanics is in outright contradiction with relativity physics, and that there are causal influences in nature that must violate the speed-of-light limit in order to produce the (experimentally well-confirmed!) predictions of quantum theory.
So it may very well turn out that quantum physics - and a quantum theory of gravity, in particular - will provide a natural slicing of spacetime into 3-d spaces evolving over time which is simply hidden at the observational level. (Pilot-wave theories are a candidate for such.) Which would flip the support of science from the B-theory to the A-theory; or at least turn the focus of the debate back towards more traditional philosophical considerations.
So that is a little introduction to this topic. I would enjoy discussing it here if anyone wants to do that. Or if (for some reason) you really just want to read more of what I have to say about this subject, I do have some posts about it on my blog (specifically, this post and the three immediately following it).