John,
You wrote, "I certainly agree time is simply a measure, 'mathematical convenience,' if you will, but why then is spacetime 'physically real?'"
Einstein explained this in *The Meaning of Relativity.* In his introduction to general relativity, seeing that Minkowski had successfully obviated the independence of time and space, Einstein wrote:
"All of the previous considerations have been based upon the assumption that all inertial systems are equivalent for the description of physical phenomena, but that they are preferred, for the formulation of the laws of nature, to spaces of reference in a different state of motion. We can think of no cause for this preference for definite states of motion to all others, according to our previous considerations, either in the perceptible bodies or in the concept of motion; on the contrary, it must be regarded as an independent property of the space-time continuum. The principle of inertia, in particular, seems to compel us to ascribe physically objective properties to the space-time continuum. Just as it was necessary from the Newtonian standpoint to make both the statements, tempus est absolutum, spatium est absolutum, so from the standpoint of the special theory of relativity we must say, continuum spatii et temporis est absolutum. In this latter statement absolutum means not only 'physically real,' but also 'independent in its physical properties, having a physical effect, but not itself influenced by physical conditions.'"
As I have tried to get across for years, to you and others who want to sneak a preferred reference frame into your ideas -- it doesn't work. The physics is against you.
You write, "As for temperature, if you were to pour hot water into cold water, it would be quite simple to guess what the temperature will be in the future."
You think so, eh? Try doing it with numbers. Try to understand the role of Euler's constant e. Then try doing it without boundary conditions. This is the trouble, John, with thinking that physical laws, a la Aristotlean logic, are intuitive (against all evidence that most of what we actually know objectively is counterintuitive) -- you think that because equilibirum thermodynamics in simple systems like coffee and cream just tastes right, the universe must be that simple in general. Throughout nature, though, we actually encounter nonequilibrium thermodynamics (by which everything that we call "life" functions), adiabatic heating and cooling, and other complex relationships.
"As for space and temperature, can you propose temperature independent of volume?"
You bet I can. But why should I expend my own energy when the masters have already covered it? Einstein writes immediately after the preceding: "As long as the principle of inertia is regarded as the keystone of physics, this standpoint is certainly the only one which is justifed. But there are two serious criticisms of the ordinary conception. In the first place, it is contrary to the mode of thinking in science to conceive of a thing (the space time continuum) which acts itself, but which cannot be acted upon. This is the reason why E. Mach was led to make the attempt to eliminate space as an active cause in the system of mechanics. According to him, a material particle does not move in unaccelerated motion relatively to space, but relatively to the centre of all the other masses in the universe; in this way the series of causes of mechanical phenomena was closed, in contrast to the mechanics of Newton and Galileo. In order to develop this idea within the limits of the modern theory of action through a medium, the properties of the space-time continuum which determine inertia must be regarded as field properties of space, analogous to the electromagnetic field. The concepts of classical mechanics afford no way of expressing this. For this reason Mach's attempt at a solution failed for the time being."
No space = no volume. Motion is purely relative in Mach's mechanics.
"And where did I say time is independent of space?"
Ah, so now you are going to say that not only is time not physically real, space is also not physically real, and therefore and certainly, spacetime is not physically real? As I have been asking you all along, John, just where is the physics in your idea of the universe?
"What are more basic physical situations than the cosmic background radiation and vacuum fluctuation, both of which would seem to be thermodynamic conditions, than spacetime ones?"
Listen closely to Einstein for the definition of "physically real" -- " ... independent in its properties, having a physical effect, but not itself influenced by physical conditions." CBR is believed to be the dying embers of a big bang event, and thus dependent on physical conditions. Quantum fluctuations are dependent not only on a purely mathematical hypothesis of "equally likely" events, but on the condition of extreme energy densities.
Tom