True, you can always count on a photon to move at light speed all the time, but in a complex system, such as an organism like ourselves, elementary particles are organized into atoms, which are organized into molecules, which are organized into cells, which are organized into organs, which are organized into our body.
All of the quarks, electrons, bosons, and any other particles in one of your arms will move if a force is applied, such as the contraction of your biceps. This contraction is produced by energy from digestion, and controlled by another organ: the Brain. Therefore, this movement is controlled by forces known to us. So the real question is how physical laws can predict brain function.
This could prove the existence of a soul, due to the unpredictability of brain waves, BUT emotions, impulses, and thoughts can be 'somewhat' explained by chemicals in the brain, which react according to chemistry, which is made possible by physical laws. For example: When anyone who loves their mother is faced with the death of their mother, the emotion will almost always be sadness. This is a 'form' of predictability. So in reality, the brain is subject to chemical reactions, which are controlled by the laws of physics.
There is also the concept of temporal independance, which states that due to probability, all events now affect all things at all times, so when i type this P, it affects you tomorrow, alpha centauri, and even Moses. So this could also disprove the existence of free will.
In conclusion, free will is, as is said in the Matrix, an illusion. The complexity of all the systems involved in making a 'descision' are what creates the confusion. In reality, on the smallest levels, all of our actions are controlled by physical laws. It all depends upon the chemical makeup of your brain, your situation, and such and such. Sorry if I have offended any views of life! Now you feel miniscule and useless, which you are