Newton's first law states that the inertia of a material body is what keeps a material body in constant motion.
That suggests that inertia and mass are real and that motion is elusive.
And motion relativity bolsters such thinking.
Motion reality
Here is an interpretation of light that to the knowledge of this writer has not been mentioned in science prior to March 9 2019 when the following paragraph has first been published.
The light emitted by our Sun is moving from Sun to Earth; and science teaches that light has no mass; the sunlight, which is moving at constant speed and occurring without mass, is pure motion; light is proof that constant motion is a physical entity, which needs neither inertia nor mass to occur.
Furthermore, looking at an object you do not see in space, between you and that object, the sunlight that highlights that object; light's motion just as any motion, such as material objects' motion, is invisible; one sees only the effects of motion.
Motion, which has no mass and is invisible, is a physical entity; just as physical matter, motion is an integral part of reality.
In this new interpretation, motion departs drastically from our human and scientific concept of speed expressed in miles per hour, in which neither the miles nor the hour is contributing. This new interpretation highlights the fact light's motion is actually self-propelling; compared to the speed's inert or lifeless mathematical definition, motion is alive.
Unprecedented generalization of Newton's law ... gravimotion's interpretation of Nature
Newton's first law: Any object in constant linear motion remains in constant linear motion unless acted upon.
is rewritten here: Any constant linear motion continues acting in constant linear motion until any external motion comes and modifies it.
The 2 words " object in " have disappeared and the word remains becomes continues acting. This is an unprecedented generalization of Newton's law; it justifies the fundamental fact mentioned above: light's constant linear motion mandates the physical occurrence of motion whether moving mass or not.
Our highly intuitive inertia human concept, which has no measuring unit in science, becomes alive and active that is the opposite of being inert; matter's inertia that is matter's very reality is occurring under motion.
The controversy
Physics and gravimotion are two different languages. In physics light waves are not in motion, motion applies to objects of material mass only; light is a flux and above light's motion has no meaning in physics.
There is a middle ground though. Using physics' words, gravimotion's motion is interpreted as energy on the move; even though not on the move, light is energy in physics. Our human concept of energy is the link existing between gravimotion's and physics' interpretations of Nature.
Nevertheless the 300,000 km/sec light speed and light's energy not on the move represent a conflict known as light duality in physics that is a controversy simply denied in gravimotion interpretation.