Imagine the planet Earth floating in space, lonely with a backdrop of distant stars and perhaps the sun, our mental camera planted firmly upon the moon.
To visualize the dimension of time, imagine the electromagnetic radiation being emitted and reflected by the earth and everything on it, including the visible light you see it as now, coming at you (as chance would have it) at the speed of light. What you are picturing, from your vantage point, that light is 1.255 seconds old. Similarly if your mom were standing on Earth looking up at the moon, the light she would be receiving would be 1.255 seconds old.
Now imagine all of Earth's light that it emits in one instant, all expanding spherically outward like a 3 dimensional wavefront. In a very real sense, that "wavefront" is literally that "instant" travelling out to influence the universe. Imagine you have a network of red laser beams all around the world, ready to fire together. You click your remote control to send the signal to fire. Imagine that signal emnating from your remote in the same way, radiating outwards over a full second (the wavefront is one lightsecond thick, by the time it finishes sending, it only has 0.255 seconds left to cover) and one section of it travelling towards the Earth. Then after a 10 second delay, all the lasers fire in a synchronized fashion, blasting red lasers out towards the universe. That moment of the lasers firing is now travelling throughout the universe at the speed of light. Meanwhile the rest of the universe also flows in towards the Earth continuously. Each second of the Earth's future is arriving in the Earth's "now" from the past of the rest of the universe, in a circular fashion, and flowing into the earth's past (the future for the rest of the universe). The easiest visual analogy to think of would be to imagine putting a narrow ring around a very wide fabric tube and pulling the tube through the ring.
The fabric contracts in "from the future" to pass through the ring as you slide it in one direction, and becomes the past as it slides in the direction away from which you are moving.Q
See, the thing is that the speed of light isn't just the speed at which massless particles appear to travel in all frames of reference. The speed of light is actually the speed of
causality itself, hence the constant "c". Not only can nothing communicate faster than that speed, that is also very genuinely "how fast things happen", it is literally the speed of reality.
So standing on the moon, you are actually genuinely "1.255 seconds away" from the earth, even if it would take you longer to actually get there on a rocket or something, because your reflected and emitted light would continue to affect things at that speed. So imagine that flow in and out in 3 dimensions to be that 4th dimension of time. "Out" goes your past and "in" flows the future.
