"Time" has been traditionally missunderstood to be a physical phenomena. It is in fact only a system of measurement based on the the differences between two non-simultaneous events.

Traditionally we have viewed "time" as a lineal dimension in a fashion similar to the three dimensions of space. We generally see it as a continum consisting of a past, a future and with the present as the boundary between the two. This may be useful in our daily lives but is it an accurate view? Does "time" exist on some quasi-physical plane where travel back and forth is possible? Can there be conditions under which "time" does not exist?

Examining the various physical dimensions, we see that six feet "up" at the North pole is six feet "down" at the South pole, that a stone sinks to the bottom of a stream and that a horse cannot walk into a badger den. And when we wish to grow corn, we need to determine when to begin planting the seed for the best results. Thus we need to create references for linearity, mass and volume. And for "time". All of this is accomplished through the presence and use of our physical surroundings.

Now imagine a universe that is completely empty. In the absence of any reference there can be no kilometer, up or down, mass or gallon. Nor can there be an hour. Therefore, "time" is an artificial construct and in that sense, no different than any other system of measurement. This has serious consequenses for cosmologists, physicists and for our basic concept of the universe as a whole.

Days, months and years are measured by the rotation of the Earth and the orbits of the moon and the Earth. These relationships in turn mark the seasons. It is the motion of these bodies that mark "time". In fact, all elements of "time" are marked by motion. In an empty universe, there would be no "time" because there would be nothing by which it could be measured.

From the particles that make up the galactic clusters to the smallest sub-atomic particle, everything is in constant motion. If this motion suddenly stopped, "time" would stop. Stars would wink out, roses would no longer bloom and there would be no night or day, no next year or last night. All birth, growth, death and decay whether animal, vegetable or mineral, depend on this motion. And so does the measurement of "time".

This provides a simple explanation of time because it identifies the mechanism through which it can be understood.

I then define "time" as the difference between any two events. An event is defined as the fixed location of every particle within the universe relative to every other particle. "Time" is then measured by the difference traveled by the particles between any two identified events. (I use the term difference in lieu of distance intentionally.)

This poses a problem for Einstein and science fiction fans since it prohibits "time" travel. In order to travel back in "time", it would be necessary to determine the location, vector and velocity of all particles in the universe and then reposition them to some desired prior location. Similarly, travel to the future would require prepositioning the particles to some desired subsequent location. Conceptually this would be possible, but not as a practical matter since there could be no physical means of accomplishing this.

Could "time" travel be possible locally? Emphatically no! Anyone duplicating the traveler in H. G. Wells' "Time Machine" would find themselves many lightyears from Earth at journey's end.

What we experience as "time" is the product of the motion of every particle in the universe. But "time" itself is just a measurement. Like mile or gallon or gram, it only has meaning within the context of some discussion. "Time" travel makes for exciting science fiction, but nontheless, it remains fiction.