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## Main Question or Discussion Point

I just read the following passage on unification from Howard Georgi:

"The SU(2) x U(1) theory is not particularly beautiful. It is often called

a unification of the weak and electromagnetic interactions, but, in fact,

the unification is partial at best. The problem is the U(1) charge....

[T]his is a charge that commutes with all the other weak and colour

charges, so group theory tells us nothing about it. In particular, be-

cause of the U(1), the theory gives us no explanation of the striking fact

of electric charge quantization."

But I thought the fact that charge has a U(1) symmetry DOES explain why charge is quantized? John Baez seems to say as much here: http://math.ucr.edu/home/baez/qg-spring2003/hypercharge/

Can anyone tell me what it is that Georgi has in mind? Why doesn't the U(1) symmetry of electric charge explain why it is quantized? (And hence what's wrong with Baez's argument?!) Thanks!

"The SU(2) x U(1) theory is not particularly beautiful. It is often called

a unification of the weak and electromagnetic interactions, but, in fact,

the unification is partial at best. The problem is the U(1) charge....

[T]his is a charge that commutes with all the other weak and colour

charges, so group theory tells us nothing about it. In particular, be-

cause of the U(1), the theory gives us no explanation of the striking fact

of electric charge quantization."

But I thought the fact that charge has a U(1) symmetry DOES explain why charge is quantized? John Baez seems to say as much here: http://math.ucr.edu/home/baez/qg-spring2003/hypercharge/

Can anyone tell me what it is that Georgi has in mind? Why doesn't the U(1) symmetry of electric charge explain why it is quantized? (And hence what's wrong with Baez's argument?!) Thanks!