02.06.2013 Views

String Theory Demystified

String Theory Demystified

String Theory Demystified

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

CHAPTER 10 A Summary of Superstring <strong>Theory</strong> 193<br />

vice versa (or switch momentum and winding modes). The same holds for the two<br />

heterotic theories. This means that type II A and type II B are really the same theory,<br />

and the two heterotic theories are really the same theory.<br />

S-DUALITY<br />

The second big duality that has been discovered is S-duality. Remember that a coupling<br />

constant determines the strength of an interaction, and in string theory the dilaton<br />

fi eld determines the value of the coupling constant. <strong>String</strong> theories have different<br />

coupling constants that are weak or strong. By letting ϕ →− ϕ where ϕ is the dilaton<br />

fi eld, since the coupling constant is defi ned from g = exp ϕ , we see that we can<br />

transform a large coupling constant into a small one and vice versa, changing a strong<br />

interaction into a weak one and vice versa. This is what S-duality is about. S-duality<br />

brings type I superstring theory into the fold. That is, under S-duality<br />

• Type I superstring theory is related to heterotic SO(32) superstring theory.<br />

• Type II B is S-dual to itself.<br />

So, a strong interaction in Type I superstring theory is the same as a weak<br />

interaction in heterotic SO(32) theory, and vice versa. In other words, the two<br />

theories are really the same theory at different coupling strengths.<br />

Quiz<br />

1. T-duality relates string theories in which way?<br />

(a) It relates strong interactions in type II A theory to weak interactions in<br />

type II B theory.<br />

(b) It relates strong interactions in heterotic theory to weak interactions in<br />

type I theory.<br />

(c) It relates large and small distance scales and momentum and winding<br />

modes in two different theories.<br />

(d) It only relates large and small distance scales.<br />

2. The most signifi cant difference between superstring theory and bosonic<br />

theory is<br />

(a) Bosonic theory has 16 extra space-time dimensions.<br />

(b) Superstring theory eliminates tachyon states and incorporates fermions<br />

into the theory.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!