This is one of the most important questions in physics. As Victor Toth said “why and also how” questions tend to be questions that are I believe are improperly categorized as philosophical. This allows physicists to dodge the issue. The problem is Quantum Mechanics and Quantum Theory can never, ever tell us why or how anything works. So why and how questions are dismissed into the realm of philosophy and out of the realm of physics. This excuse is nothing more than a dodge tactic. Modern day QM advocates don’t want to deal with foundational questions. Why? Because they do not know the answer.
I believe there is a very precise and mechanical explanation that can fully explain why the velocity of light in free space is mostly constant. There are situations though where the speed of light slows down. The speed of light does slow down whenever it enters a denser medium. The denser the medium the more it slows down. That is a fact of our reality and I have seen this as an important clue. Obviously the velocity of light is determined by the density of the medium it is traveling within.
The big question is how or why does the photon move? Which then leads into why and how do all particles move? If we find out why the photon moves we will then understand why other particles like quarks and electrons move. The photon is just another type of electromagnetic object or particle type.
It is now time to introduce another clue that comes from James Clerk Maxwell. This equation in my opinion is one of the most important equations ever created. This equation was created by Maxwell and was based upon his physical concept of the wave of light that was traveling within a medium. In that time the medium was called the aether. Permittivity and permeability are properties of the medium of space. They can be thought as being the elasticity and density of the medium that light is traveling through. Since light is a transverse wave it would be sensible to assume that the medium of the so called empty space also called the vacuum must in reality be a continuous solid. Space exists and it is not empty! Many scientists today refuse to admit that space is a in fact a real physical object. Based upon what I have stated so far the above evidence provided clearly demonstrates that the aether medium is a quasi-elastic solid. This is what Lorentz assumed the aether medium to physically be. Lorentz and later on Einstein both agreed that an aether medium must exist.
So I have now established that part of the answer to why photons move has to do with an interaction between the photon and the medium that it moves within. The key to understanding particle motion is that in order for anything to move within a medium there has to exist a pressure gradient. Pressure differences cause motion.
Fluids move because of differences in pressure. The airplane and motor vehicle engines move because of expanding gases that create high pressure in the pistons. Airplanes have lift because of low air pressure on top side of wing and high pressure on bottom of wing. Water moves in a hose because of high pressure forcing water out of the open end. Air bubbles in water move up because of a pressure gradient in the medium of the liquid that pushed the bubbles upward. Clearly the solution of why particles move must have something to do with a pressure gradient.
The Slip-Wave
According to my theory all particles have a pressure gradient that exists within their internal particle structure. Inside each photon from front to back there is a pressure field that is established. The pressure field or wave starts with a high pressure and decreases toward the back of the positive particle of the particle. At the same time the second half of the photon which is the negative half repeats the pressure gradient with a high pressure to lower pressure. The pressure gradient happens as a result of the twisting vortex structure that is created by the particles electric field (the Coulomb Force). This twisted structure exists as a persistent structure that can only happen within a solid medium. This is more evidence that supports the existence of the aether.
The two pressure gradients inside the photon causes it to move forward. At the same time the density in these two regions decreases from front to back. The net result of this is that the spatial medium around the sides and back are higher pressure and so this higher pressure pushes the photon forward. The greater the permeability and permittivity of free space the slower the photon will move. Remember permittivity and permeability are like elasticity and density of free space. They cause a resistance to the motion of the particle within the medium. If permeability and permittivity could be reduced to zero it would be theoretically be possible for the photon to travel infinitely fast.
So the speed of light will remain constant as long as the permittivity and permeability remain constant. Therefore if we stretch space space it will become more tense and light will be able to travel faster. If light enters a gravitational field this will cause light to slow down. This happens because a gravitational field is equal to a gradient field contraction of the aether. Gravitational field will increase both permeability and permittivity of the space where ever there is a gravitational field.
The bottom line is that the speed of light is determined by the physical state of the medium and the structure of the photon. These two physical aspects determine the maximum speed of light. So technically speaking the speed of light is not constant.
As for why the Michelson & Morley experiment showed that light speed is independent of all other moving frames of reference is fairly simple to determine. The photons velocity is independent of all other motions because the light wave-particle has autonomous motion. This means that photons move by their own accord in perpetual motion. Its source of motion is not a result of any external force. Its source of motion is caused by its internal structure interacting with the medium it resides within. The Ether wind was not detected because of this fact. The photon has no mass because its is composed of a balanced charge. The fact that the photon has a balanced charge causes the photon to move in a straight line and have no acceleration. This type of motion will ensure the photon travels at the maximum speed and will not develop any mass. The mass of other particles is caused by the fact that they have unbalanced charges which causes accelerated motion (an angular momentum type). This type of motion causes a contraction of space around the particles. That type of motion gives those particles their rest mass. This also allows them to experience inertia. Photons are not subject to the Lorentz Transformations because they travel in a straight line at a constant rate of non-accelerated motion.
Finally the speed of light is constant in space as long as we do not bias space with a magnetic, gravitational or static electric field. Otherwise the speed of light is not constant.