A friend wanted to know which force causes thought? It is the laws of inertia. Kanada describes three forms of inertia (संस्कार). Prashastapaada has elaborated on this in his Compendium on Properties of Matter (पदार्थधर्मसंग्रह). The first of course, is inertia of motion (वेग). The second is thought (भावना) and the third is inertia of restoration (स्थितिस्थापक), which is generally referred to as elasticity. Inertia is generally defined as the resistance of any physical object to any change in its state of motion. This includes changes to its speed, direction, or state of rest. But it is also the tendency of objects to keep moving in a straight line at constant velocity. Generally, application of force moves an object. But what happens when the force ceases to apply?
Since force causes motion, cessation of force should imply cessation of motion. However, since the bodies are seen to move even after the force ceases, there must be something that moves an object after the force ceases to operate. This is called by Kanada as counter-force (प्रतियत्न), as it counters the normal effect of force or the absence of it (which should have normally made the object to be at rest). This counter-force (प्रतियत्न) is called inertia (संस्कार). There is nothing as inertia of rest. Rest is the condition of mutual cancellation of all forces acting on a body. This includes inertia, which also moves bodies. Rest gives the particle or the body its position. When it continues the displacement of objects fully from its position, the inertia is called inertia of motion. When it displaces partially but not completely from its position, the inertia is called inertia of restoration (स्थितिस्थापक). It continues to operate as long as the force continues to operate, by generating a counter-force (प्रतियत्न). Hence, it is a type of inertia.
Perceptions like "seeing, hearing, touching, etc", are generated when an external impulse interacts with its respective sense organs, which can be linked to the fundamental forces of nature (like seeing is related to electromagnetic - उपयाम, taste to weak interaction वहिर्याम, touch to another variant of weak interaction यातयाम, smell to strong interaction and sound to gravitation उद्याम). These five can be related to alpha, beta, delta, theta mind waves and their gamma coupling (नित्यगति, सम्प्रसादगति, यज्ञगति, साम्परायगति, उरुगायप्रतिष्ठा). Thought (भावना) is generated when we perceive the images of not objects directly reaching our senses, but the direct perception of something triggers indirect perception by drawing from memory (स्मृति) the earlier perceptions or realizations (प्रत्यभिज्ञा). Prashastapaada has elaborated on how these are generated and continued and how these are destroyed.
Thought is electric as it is related to impulses that travel at the speed of light. It is chemical, as it acts like ATP in the potassium-sodium pump. It is processed in the brain like the information in the CPU of a supercomputer. So there is no doubt that it travels at the speed of light. The question is, whether it exceeds the speed of light.
In the case of thought, the external impulse is received, but may or may not be processed till our mind is connected to it. We may be looking without seeing. We may be hearing without listening. This proves that at any one time, only one channel opens out to carry the external impulse to the brain. Though the nerve cells and neurons that carry and process the external impulse are in position, they need some agency (in addition to the vital energy that only can move the information through the cells) to activate them. The agency that activates the nervous system for any response is mind. But what is it?
Energy appears or manifests itself as motion (गति) when an equilibrium (स्थिति) is broken. Energy in its equilibrium position is called potential energy. New energy is neither created nor the potential energy destroyed. The conversion from potential to kinetic (गति) is due to destabilization in the state of equilibrium (स्थिति) of the potential energy, which leads to a conjugate of forces in a given direction and its opposite effect in another direction. This need not be in the opposite direction in every case, but its effect can be non-linear. The point from where the equilibrium breaks is called mind (मन). It can be in any living being or inert objects or the universe itself as a whole (when it is called श्ववस्यस्).
We have measured speed of light within the solar system only. Light is known to vary its speed according to the density of the medium, which is evident in the time lag in GPS, which is wrongly attributed to time-dilation, though it is actually refraction. Voyager 1 has shown that the inter-stellar medium is sparser than that within the heliosphere. Thus, light should be moving at a faster rate than the presently measured value of c. We do not know the density of the inter-galactic medium. But it is reasonable to assume that it would be sparser, which translates to the speed of light increasing further. Thus, in the case of the universal mind (श्ववस्यस्), it must be the fastest. That is the reason why Yudhisthira called mind the fastest thing in the universe.