Your physicist is wrong.
Yes part of that expansion pushes on the forward part of the combustion chamber, but part of that expansion will also push on the rear part of the combustion chamber. Newton's Law, equal and opposite and all that.
A jet engine creates an area of high pressure at the rear/exhaust end, effecting a low pressure area at the front end. Everything moves from high to low pressure (like a wing) and voila! 'Tis a simple explanation as I haven't had any caffiene yet.