Breaking it Down
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How Small Can We Go?
What I wish to do in this section is play a reductionist game with nature. To what extent can we continue reducing the size or complexity of a system? We do not know for sure, but we suspect that there are elementary particles (field quanta) beyond which we cannot subdivide nature.
Beginning with the macroscopic realm, we can subdivide materials until we reach the level of a single molecule. The single molecule is the smallest we can go if we wish for the material to retain most of the physical properties of the bulk material. There is a huge range of molecular sizes since some molecules only have two atoms - like most diatomic gases for instance - and other molecules have thousands of atoms or more like most biological molecules. So sizes of molecules range from around across.
Atoms are the smallest form of neutrally-charged and stable matter. As you likely know already, they are listed in the periodic table and are often called elements. The smallest is hydrogen and the largest stable element is lead-208. The roughly spherical atoms have diameters in the range from .
Electrons, which form the outermost layers of atoms are fundamental entities often called fundamental particles. They are fundamental because they cannot, as far as we know, be broken into anything smaller. We have attempted to measure the size of electrons and seem to always come up empty-handed. Depending on the definition of "size", electrons are at the very largest .
The nucleus of most atoms consist of protons and neutrons, both of which can be further subdivided, and are therefore not fundamental. They are referred to as hadrons - which is another way of saying they are made of quarks. Both protons and neutrons are made of quarks, which are particles that make up such hadrons. The proton and neutron both have diameters on the order of .
We will discuss more about these fundamental particles after first discussing nuclear processes in the next few sections.
What is Size Anyway?
Since we have discussed the fact that all entities in nature - including the aforementioned particles - are really waves (or field quanta), we should have a discussion about size. By size you should understand something related to the range of the interaction, but it's not so easy to define by range. After all, every electron in the universe has an infinite range of interaction. It's electric field just drops off with distance squared. So if we equated the size of an electron with the range of its interaction, we'd have to call it infinite.
Really what it comes down to is this: When something like a nucleus interacts differently with other things depending on how close they get, then we can measure the distance where the behavior changes and decide that we have gotten close enough to literally be inside the other particle.
Experiments that do this are called scattering experiments. Nuclear scattering experiments aim beams of particles, such as alpha particles (little Helium nuclei), at other larger nuclei and measure how the beamed alpha particles get deflected by the interactions or collisions with the nuclei. Since both are positive, an alpha particle gets repelled more and more strongly the closer it gets to the nucleus. If you shoot the alpha particle with enough energy, however, it can enter into the nucleus and get to a place where the repulsion actually decreases, just as gravitation (albeit attractive) decreases inside the earth. In fact, given enough initial kinetic energy, the alpha particle can pass right through a nucleus without any deflection in its path. There is nothing fundamentally hard to hit - just invisible fields that exert forces on one another.