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Nevertheless, physicists realized that their work was far from complete, and that the standard model left a great many questions unanswered. We have described these questions in some detail elsewhere (such as the pages listed at the top).

It was seen that, at the same time, a number of both the key successes as well as the chief shortcomings were to be found in the way that fundamental forces were unified. Here unification means, specifically, that two (or more) forces previously considered distinct can actually be described by the same equations. And, further, that these equations are invariant under symmetry operations that exchange distinct fundamental particles. That is, as far as the equations are concerned, an electron and a neutrino (for instance) behave substantially the same.

One of the primary entries in the success column for the standard model is the unified theory of the electroweak force. Yet this same theory illustrates some of the shortcomings. The symmetry between the forces is broken because the electromagnetic force and the weak force don't have the same strength and because otherwise similar particles (such as electrons and neutrinos) have quite different masses. Further, the unification itself isn't as seamless as it could be. One of the key parameters of the theory – the electroweak mixing angle which describe how the forces combine – is not specified by the theory, but instead can be determined only by experiment.

So. The standard model showed that two seemingly distinct forces could be successfully unified in a single, elegant mathematical theory. But at the same time, physicists still had a lot of explaining to do, in terms of how to clean up the unification of the electromagnetic and weak forces, and then to go further and add the strong force into the mix

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