This may sounds like a dumb question but I want to figure out the 'point' of them. I know that isospin was an attempt to describe the proton and nucleon as an isospin doublet and that hypercharge seems to me to be a nifty little relation between the electric charge and the isospin - but what is the point of them now that we know that we cannot describe the nucleon as an isospin doublet and that it is in fact constructed of quarks. Is it that: a) Before we new about quarks it seemed like a good idea. or b) We can indeed build a model based on isospin. I am learning about GUTS at the moment and specifically [tex]SU(2)_L \times SU(1)_Y [/tex] What is the significance of the hypercharge Y subscript in the [tex]SU(1)_Y [/tex]? It seems (to my admittedly ignorant mind) that hypercharge is nothing more than a nifty relation to charge so why does it get elevated to the subscript level [tex]SU(1)_Y [/tex] in GUTS??? Please help me understand the significance!