From the point of view of Special Relativity, time doesn’t flow. It’s just another dimension that for reasons we don’t understand we perceive in only a limited way and in only one direction. The order of occurrence for events that are relatively close in time can change depending on the position and relative motion of the observer.
In General Relativity, the four dimensions are called space-time dimensions and are not theoretically distinguished, although a common formulation of the equations can separate them.
A possible implication is that the future exists just as concretely as the past does.
---ANOTHER NOTE – General Relativity is probably not a complete theory. Quantum Mechanics is not yet reconciled with it and the evidence of what is called dark matter or dark energy is opening new questions. This means that the foundations for these philosophical interpretations are not really secure. However, we should be willing to work with what we have and see where it takes us, while being ready to adapt to new information. --
It seems clear that light occupies a special place in the physics of our universe. It is a constant of a sort. The speed of light is constant in all inertial reference frames and locally constant in all reference frames. (“Inertial” means not accelerating.)
When talking about light earlier I spoke of it as a particle. It’s also thought of as a wave. When Maxwell devised his electromagnetic field equations 150 years ago, it was all about waves.
Waves in and of themselves are interesting things. They move without anything actually moving much in the direction of the wave. Think of a long thin flag rippling in the wind. The waving cloth clearly has a motion from the flagpole to the tip of the flag, but the flag itself is equally obviously going nowhere, just moving side to side in a regular pattern to create the moving waves.
Electromagnetic waves, which light turns out to be, have another oddity. They wave without anything there to wave. No flag. And they dampen rather quickly compared to a flag because they spread out in all directions through space, which, of course, they do at the speed of light.
The wave theory of light is very attractive because it has the speed of light built into it. That’s one of the most remarkable results in the history of physics. Light speed has to do with the rate of propagation of the intertwined electric and magnetic fields as they alternately create each other through space. Maxwell’s discovery of this led directly to relativity. Relativity is built into his equations. Almost all of physics had to be rewritten after Einstein, but not Maxwell’s equations. They were already relativistic.
As a wave it’s easy to understand how light can have a rest mass of zero. At rest, nothing waves. Light doesn’t exist. Apply an electric or magnetic field under certain simple conditions, and presto, light. Zero to light speed in no time at all. Literally. This can only be possible if there is no mass involved. Oh, and no time also. Oh yes, no distance (space) either.
This jives with what we encountered earlier: time doesn’t exist for light. Neither does space. Or mass. From the point of view of light, light doesn’t exist—or anything else. For light, nothing exists or ever could or ever will. It’s not just zero existence, its null existence. Not even a possibility of a hint of a thought of a potential of existence. Existence itself doesn’t exist.
Really? But it has energy and momentum. As a particle or as a wave, light has momentum.
Like Athena from the head of Zeus, light springs full grown at birth, with momentum dependent on its energy. But from the point of view of a light beam, nothing has changed from its initial null-existent state before a field was applied to send it on its journey (in our view).
Maybe if we can figure out where this momentum comes from, we will actually know something about the origin of our universe, which is all about momentum.