I am jumping into this thread a bit late, but I have a few things to say.

I agree with everything you say, I just wanted to point out that while time may often be referred to as ``fourth'', time is usually the

*first* component of the Minkowski metric (the value at row 1, column 1 in the matrix).

They are very intimately related, see my comments below...

Time is concrete. In Special Relativity (which

*seems* to be correct, based on all our experiments) time is very intimately related to a spatial dimension.

The three spatial dimensions we are familiar with are all interchangeable through the physical process of ``changing your perspective'' (i.e. turning around maps the direction that used to be ``forward'' into the direction ``backward'', turns ``left'' into ``right'', etc.), and through the equivalent mathematical process called a

trigonometric rotation.

In a

*very* similar sense, space and time are interchangeable through the physical process of ``moving faster'', and through the equivalent mathematical process called a

hyperbolic rotation. Note the equivalent language; if the speed of light were an imaginary number (ignoring, of course, whether or not that even makes sense) there would be no difference between space and time!

Time is ``different'' than space, and because of that (or mathematically, because of the differences between trigonometric and hyperbolic functions) we can never

*completely* map space into time or vice-versa. But we can

*partially* map one into the other. That is the fundamental origin of the ``length contraction'' and ``time dilation'' that occur a relativistic speeds.

There connection between ``thermodynamic time'' and the time in ``space-time'' is very interesting but not completely figured out yet.

There

*is* definitely a difference between the time in ``space-time'', which is just an (arbitrary) coordinate, and the time in ``thermodynamic time'' which is more the expression of an object's

*age*. (And therefore closer to a

spacetime interval than just a difference in time.)

I think ``yes'', but how would we tell? If everything in the Universe changed by the same amount, would you be able to see anything different?

If you could somehow find a vantage point

*outside* the Universe maybe things would look different.