PHASE-TRANSITIONS IN M-THEORY AND F-THEORY

Phase transitions are studied in M-theory and F-theory, In M-theory co mpactification to five dimensions on a Calabi-Yau, one can find topolo gy-changing transitions similar to those seen in conformal field theor y, but the non-geometrical phases known in conformal field theory are absent. At boundaries of moduli space where such phases might have bee n expected, the moduli space ends, by a conventional or unconventional physical mechanism, The unconventional mechanisms, which roughly invo lve the appearance of tensionless strings, can sometimes be better und erstood in F-theory.