If someone is in the middle of space, in a vacuum, with no gravitational forces acting on them (I know this impossible, but just accept it), and he moves his hand towards his head, where is the equal and opposite reaction force?
The forces would be the muscles inside your arm contracting/extending depending what muscle it is. I'm sure there are also movements in your body which don't require pairs of muscle, so solving that may be more simple.
The person in question will due to the vacuum surroundings not create any net movement (of the body as a whole). Any isolated movement will be counteracted by an opposite proportional movement according to Newton's third law. If the person moves one hand towards the head, the rest of the body will move ever so slightly towards the direction of the feet, such that the entire body as a whole experiences no net movement around the centre of mass and thus exhibits an intrinsic net force of zero. It is possible to split up each possible movement in vacuum into muscular forces, calculate their sum, and find this sum to be zero.
