Figure 4
Fig. 4. In the type I string framework, our universe contains (besides the three known spatial dimensions; denoted by a single blue line) some extra dimensions (dII = p-3) parallel to our world p-brane (green plane), along which the light described by open strings propagates, as well as some transverse dimensions (yellow space), where only gravity described by closed strings can propagate. The longitudinal extra dimensions have a string size of about 10-18 m, while the size of the transverse dimensions varies between 10-14 m and a fraction of a millimetre.