After coming across this **amazing** video presentation of the physics double slit experiment on Reddit.com earlier today:

I was reminded of what I consider the best explanation of Einstein's special theory of relativity, and in particular the adage that "when objects travel near the speed of light, time slows down". The full explanation goes from page 47-51 of The Elegant Universe by Brian Greene.

What I find so remarkable about Professor Greene's explanation is the way in which it gradually sets up the punch line like a long and elaborate joke (minus the funny ending). Each step along his explanatory pathway is simple to the point of obviousness. Yet if you follow along closely you reach one of those 'Aha!' moments at the end where suddenly the entire concept becomes clear in your mind.

The crux of his explanation is that all things are constantly traveling in one of four dimensions - three of which are spatial and one is the time dimension. So an object that appears to be at rest in the three spatial dimensions is actually moving forward at the speed of light, but *only in the time dimension*. With this idea as the foundation, the remainder is easy to explain: When an object is traveling through one of the three spatial dimensions, it has to 'borrow' some of that speed from the time dimension. If you add up the total speed of an object through the 3 spatial and 1 time dimensions, that number always adds up to the speed of light.

When you apply this reasoning to a particle such as a photon of visible light - which travels through space at the speed of light - you can think of it as using up all of its speed to travel the three spatial dimensions. This leaves no remaining speed for it to spend in the time dimensions. Because of this, photons never grow old. They simply have no speed left to spare for the time dimension, so they do not experience passage of time. If photons wore wrist watches, the hands would never move.

To be honest, I'm not sure why I feel the need to constantly refresh my understanding of this concept. In fact I feel a mild sense of panic anytime I find myself unable to recall the explanation. It is of absolutely no practical value to me. I am sure that most real physicists would laugh at such a simple explanation and all of the details that it omits. Some part of me is waiting for the day when one of my four nieces/nephews asks the question "Uncle Jason, why is it that nothing can travel faster than the speed of light?" I'll then run to my bookshelf, re-read pages 47-51 again to refresh my memory, then walk them through it. When the light bulb of understanding appears, and a grin comes across their face, I'll remember why it was worth the effort.