Motion, Space, Mass and Causality

Updated on January 28, 2026 at 4:00 PM

Toward a Relational and Causal Interpretation of the Cohesion of the Physical World

General relativity profoundly renewed our understanding of gravity by showing that it is not a force in the Newtonian sense, but a manifestation of the structure of space-time. This intuition of Einstein is correct. However, the geometrization of gravity, while correctly describing the form of trajectories (geodesics), leaves open a fundamental question: why does a body actually move along such a trajectory?

Saying that a body “follows a geodesic” describes a geometric constraint, not an actual cause. A structure, however coherent it may be, does not act by itself. It is therefore necessary to clearly distinguish the form of motion from its effective actualization.

This distinction makes it possible to remove a frequent conceptual confusion, which consists in implicitly identifying the actual cause of motion with the existence of a force. This assimilation, inherited from a mechanistic view of the world, prevents a proper understanding of the deep meaning of the principle of equivalence brought to light by Einstein. Within a relational conception of space and motion, every motion indeed involves an actual cause, but this cause is not necessarily a force. A force appears only when there is an opposition between two tendencies. In other words, force is not the foundation of motion, but the expression of a constraint.

Initial motions correspond to the constitution or expansion of the reference space itself. They are not displacements within a pre-existing space, but are foundational to relational space. At this level, there is neither inertia, nor force, nor speed limit.

Composite motions are motions of bodies within this already constituted reference space. It is at this level that inertia, geodesic trajectories, and forces appear, when the actualization of motion is impeded.

Gravity provides a particularly illuminating example. When a body is in free fall, no force acts locally upon it: it is in an inertial state. Yet this motion is not devoid of cause. It results, in my perspective, from the actual action of the motive principle, which simultaneously takes into account several relational aspects: the global relation of the body to the whole reference space, and its particular relation to other bodies. As long as this motion is not impeded, gravity does not manifest itself as a force.

By contrast, when the motion is hindered—by the ground, a support, or any other constraint—a force appears. This force then expresses the opposition between the tendency of the body to continue its inertial motion and the opposite tendency imposed by the constraint. Gravity becomes a force precisely because it is impeded.

Only by clearly distinguishing actual cause from force can one understand how a motion may be causally determined in the absence of any local force, in accordance with the principle of equivalence. Inertia is then neither a simple balance of forces nor an absence of causality, but the positive result of the immanent and non-mechanical action of the motive principle, integrating several relational aspects of reality simultaneously.

Proper mass expresses the intrinsic unity of a body. Gravitational mass is the effect of this unity on the surrounding space. Directional mass is the oriented actualization of this gravitational structure when a motion unfolds.

Inertial mass integrates these different aspects as a global expression of the body’s relation to space. Momentum is the dynamic mediation through which a directional contraction of space is actualized as motion. Force is the expression of a contraction of space that is prevented.

The speed limit is linked to interaction with the spatial configuration. The photon, in interaction with space, thus possesses a relational proper mass, which is a necessary condition for the very existence of a speed limit.

Each body, through its intrinsic unity, is a mediator of the action of the motive principle ensuring the cohesion of the physical world.