This is an example of unification of fundamental concepts (not interactions), although it is strongly related to our previous example because the increase in symmetry (from Galileo to Poincaré) is the same and the underlying mechanism is similar (if space and time are different aspects of spacetime, there must be transformations that take space into time and vice-versa). It is important to observe that the new symmetry is only apparent at high speeds, but it is never broken.

3.  ${\rm\bf  Waves}\bigoplus{\rm \bf Particles}
    \stackrel{{\rm \bf de Broglie}}{\Longrightarrow}
    {\rm\bf Quantum\,\,\, particles}$

    This unification of two entities always believed to be distinct is required (and led to) Quantum Mechanics. It is, to this day, mysterious, perhaps because it is different from the other instances of unification: in this case there seems to be no underlying symmetry group transforming particles into waves and vice-versa.

4.  ${\rm\bf  Gravity\,\,\,     (GR)}\bigoplus{\rm\bf 
        Electromagnetism}\stackrel{{\rm\bf  Kaluza,\,\, Klein,\,\,
          Einstein}}{\Longrightarrow}
      {\rm \bf Higher-dimensional\,\,\, gravity}$

    $$g_{\mu\nu},  A_{\mu}
    \,\, \longrightarrow \,\,
    (\hat{g}_{\hat{\mu}\hat{\nu}})\equiv
    \left(
      \begin{array}{c|c}
    k^{2} & A_{\nu} \\
    \hline
    A_{\mu} & g_{\mu\nu} \\
      \end{array}
    \right)$$

    This attempt was unsuccessful (it was, may be, too early) but introduced many new ideas that have stayed around until now. In this theory there is also an 