What is the fine-grained topic of the following text: While we have known for fifty years that protons and neutrons are made of quarks and gluons (represented in the figure to the left as spheres with arrows and springs, respectively), we are just beginning to learn how to image the structure and interactions inside protons and neutrons that are at the femto-scale, a million times smaller than the nano-scale of modern micro-electronics. Only return the topic.
Imaging the structure and interactions inside protons and neutrons at the femto-scale.