Einstein deduced from how our world behaves that the speed of light is a maximum but he didn’t explain why light has that speed and no other and saying that light goes at light speed because it has no mass doesn’t explain why there is a maximum speed at all. Why not the speed of light plus one? What sets the speed limit of our universe? The current view, after almost a century of consideration, is that:
“… the speed of light is a constant because it just is, and because light is not made of anything simpler.” (Laughlin, 2005) p15
Yet “because it just is” has never been a very satisfactory answer in science. The speed of a wave depends on the medium it travels through not the wave itself, so the speed of light should be defined by the space that physical realism calls nothing. If light is a wave of processing passed on a network, it must move at a finite rate if the quantum network cycles at a finite rate as our computers do. Yet while a 5GHz computer runs 5,000,000,000 cycles per second, the quantum network cycles at an astonishing 1045 times a second! If light is processing passed from one node to the next each cycle, its speed follows from the cycle rate and the node-to-node distance of the network, so what we call the speed of light is really the speed of space.
Yet the speed of light isn’t constant, as light slows down in water. When light moves in water, we say the medium is water, and if it moves in glass, we say the medium is glass but if it moves in space, we call it a wave of nothing! In quantum realism, whether light travels through glass, water or space, its medium is always the quantum network. Light slows down in water not because its medium is water but because the quantum network slows down when it has to process matter and the speed of light is defined by the speed of the quantum network. Light slows down in water for the same reason that a computer game slows down under load and it slows down due to gravity for the same reason.
The quantum network runs at different rates depending on load but photons still keep in a strict sequence, one behind the other, like the baggage cars of a train driven by the same engine. Each node passes on the photon it has then accepts another in the line. If the engine slows down under load near a massive star, photons go slower but still keep the same order, so in gravity lensing, photons from a cosmic event arrive on earth at different times by different paths but still arrive in lock-step order. This maintains causality, as if one photon could overtake another one might see an object arrive before it left! Causality requires photons to stay in sequence and the quantum engine rigorously maintains this.
Note: The speed of light c = LP/TP, where LP is a Planck length of 1.616×10−35meters and TP is Planck time of 5.39 × 10−44 of a second. This gives the speed of light as 299,792,458 meters per second (see here).