Space and Time Interconversion

There is a wonderful explanation, read the Annus Mirabilis papers published in the 1905 Annalen der Physik scientific journal. You can find it on line in several places, you will find answers there.

I suppose it depends on exactly what is meant by 'interchangeable'.

Space-time is a continuum that is woven together, but in a deep sense, space is still space and time is still time - they are not just different views of the same thing.

I suspect that the interchangeable idea stems from the relativistic fact that when you change the way that you move through space (change speed relative to some reference), your "speed of movement through time" also changes. Any increase in relative speed through space decreases "relative speed through time" - hence, in this sense space and time are 'interconverted', i.e., time movement is traded for space movement.

Another way in which this 'conversion' happens is purely conventional: we take the speed of light, multiply it by elapsed time and call it the distance that light traveled in (say) light-years. We can even choose our units so that space and time components have the same units, most commonly with both in cm or in meter. Still, this does not make space and time the same thing that can be freely exchanged.

-J