Einstein considered the Einstein -- Maxwell -- Dirac system by 1930 .
In fact , in 1929 Weyl wrote Einstein that any unified theory would need to include the metric tensor , a gauge field , and a matter field .
Einstein considered the Einstein         system by 1930 .
In fact , in 1929 Weyl wrote Einstein that any unified theory would need to include the metric tensor , a gauge field , and a matter field .