Yeah, but who controls the chemical reactions when it comes down to descion making? Who sets the chain of reactions in motion? An electrical signal? From where - where exactly does the whole process begin?
Since so many factors are included in even one small decision, it's almost impossible to answer that question in practise. But what we do know, is that it's all just molecules reacting. The exact how and where isn't important, the basic premise is that molecules reacting in our body determine what we do and think. And what molecules do is set by physical laws. With neurotransmitters, signals are being transmitted through the nerves, they trigger the setting free of other enzymes, which start a cascade in the cell through receptors etc... you know the drill. The fact that in one decision, there are thousands or even millions of these processes going on, doesn't mean that the outcome couldn't be predicted in theory. There is not one part of it that is free of the laws of physics, and it is a fact that anything governed by the laws of physics can be calculated, and has only one possible outcome when given the correct starting information (thus the input a body would get from its environment by its senses). Thus, once at a certain point in time, the input is defined, and the processor, the body, is defined, the outcome is DETERMINED. No free will. No intersections. Just one road to take.
Science hasn't even begun to comprehend the complexities of the human brain. All i know is that i can control what i do. Science cannot predict what i will think - and hence could never predict what i will do.
It doesn't matter that we can't predict yet at this point what you do. What does matter is, that if we have uncovered all the processes that take place in the body, (which are after all physical and governed by set laws which we have mathematical and physical equations for) and we had an infinitely complex processor that could create a mathematical equation to model everything happening in a human body, with the correct environmental information, that we COULD predict what you would do. And if we can do that, it means that you have no free will.
Could science have predicted that after a think, i would decide to....pick up one of my speakers? A random thing - no reason for doing so - yet i did it.
In theory this is possible, not yet at this point though, since like you said we don't understand enough of the processes yet. In practise it will probably never happen either because the processes are incredibly complex, and understanding them all, and being able to create a mathematical equation that contains them all, is nearly impossible. But that doesn't change anything to the fact that it is theoretically possible to determine everything you do, if we had enough information and the ability to create an incredibly complex equation.