problem*.

5.  ${\rm \bf Quantum\,\,\, Mechanics}\bigoplus
    {\rm \bf Relativistic\,\,\, Field\,\,\, Theory}
      \stackrel{{\rm\bf  Many\,\,\, people}}{\Longrightarrow}
      {\rm\bf  QFT}$

    A difficult but fruitful marriage.

6.  ${\rm \bf Weak\,\,\, interactions}\bigoplus{\rm \bf  Electromagnetism}
      \stackrel{{\rm \bf Glashow,\,\,\, Salam,\,\,\, Weinberg}}{\Longrightarrow}
      {\rm \bf EW\,\,\, interaction}$

    In this case, two Relativistic QFTs are unified.

    -   Unification is achieved by an increase of *local* (Yang-Mills-type) symmetry, from $U(1)$ to $SU(2)\times U(1)$.

    -   The symmetry is *spontaneously broken* by the Higgs mechanism: choice of vacuum by energetic reasons (minimization of the [*ad hoc*]{} Higgs potential). (This is the main difference with Kaluza-Klein and other theories including gravity in which different vacua are associated to different spacetimes and, therefore, different definitions of energy that cannot be compared.)

    -   The spontaneous breaking of the symmetry renders the model renormalizable.

    -   The symmetry is restored at high energies.

    -   New massive particles are predicted associated to the enhanced symmetry (gauge bosons, found) and a new massless spin-0 particle is also predicted (Higgs boson, not yet found).

    This model, part of the Standard Model of Particle Physics, has had an extraordinary success and most unification schemes of relativistic QFTs have followed the same pattern. In particular

7.  ${\rm \bf Electroweak\,\,\,  interaction}
    \bigoplus{\rm \bf Strong\,\,\, interactions}
      