Time is not a tangible. It is neither a field nor a fabric. It is nothing more nor less than a measurement of change. As subject 'X' changes from state #1 to state #2, subject 'Y' changes from state #3 to state #4 or as subject '1' changes from position #A to position #B, subject '2' changes from position #C to position #D. It is simply the comparison of two (or more) sequences. It is no different than measuring distance with a ruler, but instead of evaluating an unknown length against a standard unit, you are measuring the quantity of change occurring within some procedure against the progress of some standard event - like a rotation of the planet or the vibration of a cesium atom.

Mathematically it is convenient to consider time as a 'dimension' but other than the physical changes it measures, it has no separate existence or reality of its own. A dimension is defined as a relative position in space. To call time a dimension is to be intellectually lazy.

The concept of time has been highly over-mystified due to the illusion imposed upon our thinking by the phenomenon of memory. By engaging our memories, we can conceptually revisit past events. Time is nothing more than change, and though some changes can be reversed, to actually go back in time it would be necessary to find a means to halt all processes in the Universe and then successfully apply sufficient force to reverse them throughout the cosmos. Even such an extraordinary procedure would not reverse the course of time. Whatever process was engaged to controvert time would also have to CEASE acting at the same temporal point it began - for if it progressed, so would time.

Time is, of course, relative and there seems to be a limit to the rate at which things can change. This limit seems to be somehow related to the speed of light. Velocity through the medium of space is an external change. As objects move, they move away from that part of the Universe which is behind them and toward the part which is in front of them. This is motion relative to the Universe. At relatively low speeds, this motion doesn't seem to impede the rate of internal change of a body. At 60 miles per hour, a clock seems to keep perfect time, but as it approaches the speed of light, it seems internal change slows and the clock would only tick a few seconds per hour. This natural limit seems to force a trade off between external change and internal change and time seems to slow to a crawl. Interesting, isn't it?