**In what follows it will be shown through appropriate mathematical equations that the speed of light is not a constant of motion but is rather a physical variable that is dependent on the type of forces that act on it. This derivation gives clear evidence that light variable speed is in contradiction with the assumptions of the theory of relativity about the constancy of the speed of light. The derivation of theformula uses Einstein mathematical expression of the photon energy:**

** E=p*c**

**This expression of the energy that is associated with a photon particle shows clearly the constancy of the speed of light. By converting this expression of photon energy into an equation of forces that are acting on the photon one gets the following formula:**

** dE/dr = (dp/dr)*c **

**Where I did a differentiaition with respect to the displacement r.**

**The differentiation of energy with respect to r usually gives a form of force. In the classical laws of motion by Isaac Newton conservative forces are usually obtained from scalar potential by a differentiation process with respect to r. the formed force must be conservative in this case.**

**Therefore one gets the following expression:**

** F = (dp/dr)*c**

**Dp/dr has no physical significance. Therefore I will change the differentiation from r to t( the time) in order to get another form of force F1. Thus one gets: **

** F = F1*(dt/dr)*c**

**In this expression another new term was introduced which is the term dt/dr.**

**This term depicts the inverse of the velocity v of light. Thus the whole equation looks like this now:**

** F = F1*(c/v)**

**This equation shows clearly the dependence of the speed of light v on the magnitude of the forces in the equation. **

**This equation has dramatic consequences about our current ideas of the nature of light as a physical constant of motion of nature. **