Which is larger - a quark or an electron?
Another very fundamental question with a very counter-intuitive answer. We all tend to ask such questions because our mental picture of subatomic particles is plain wrong. This is because the behavior of such particles do not resemble our daily experience at all. When we try to picture an electron, it turns out we mostly think of it as a 'very small steel ball'. But in fact, it is _not_ a steel ball at all, and it resembles nothing we have 'seen'.
Here is my partly physical and partly philosophical understanding: In order to be able to talk about a 'size', you need at least two particles. The proton for example, does indeed have a size, but this is because it has internal structure -- it is made up of other particles. The same goes for the atom, it consists of electrons and a nucleus. However, if you have a truly fundamental particle at hand, there is no way you can assign a size to it.
As far as we know (see http://www.physlink.com/Education/AskExperts/ae114.cfm), electrons and quarks are fundamental particles, thus have no size. The "as far as we know" part here is important, because it might turn out that these particles do have internal structure, and are composed of other, 'more fundamental' particles. Electrons have so far resisted all attempts to find a hint of internal structure. With quarks, there is the further trouble that free quarks do not exist in nature, making experiments to probe their internal structure (if any) more difficult.
So, finally, you may ask, which is larger - a quark, or an electron? Neither has a known size, they are apparently both point particles. If they are indeed both point particles, the question does not apply. If they are not point particles, the answer is that I do not know...
Yasar Safkan, Ph.D., Software Engineer, Noktalar A.S., Istanbul, Turkey
'There is no inductive method which could lead to the fundamental concepts of physics. Failure to understand this fact constituted the basic philosophical error of so many investigators of the nineteenth century.'