An Introduction to String Theory

Slide 17 of 37
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[First and foremost, I have to give credit where credit is due: the expression for the Lagrangian of the Standard Model on the left side of this slide was laboriously typed by Thomas D. Gutierrez (based on an appendix of the book Diagramatica by Veltman). The image above is an adaptation of the Postscript file on his website at nuclear.ucdavis.edu. He comments that if you squint at it long enough, it might start to look like a bunny.]

Particle physics, as we now understand it, is described by the "Standard Model". The Standard Model is defined by a Lagrangian that includes both the QED that we've seen (describing electromagnetism) and also many other pieces necessary to describe the nuclear forces and the other particles observed in nature (quarks and leptons).

The first point to make is simply a comparison of the fundamental equation of the Standard Model (its Lagrangian) with the fundamental equation of General Relativity. In their hearts, most theorists just don't believe that the Ultimate Truth^TM could be as complicated and ugly as the Standard Model seems to be. Marginally more importantly, the ugly form of the Standard Model seems to include a number of subtle but significant patterns that the theory leaves totally unexplained.

This may already be enough of a "failure" of existing physical theory to inspire idealists (like me) who like to ask "Why?". But pragmatists could easily say "Tough luck", and since this is science rather than art, the pragmatists are probably on the right track. So we'll look for better reasons.