Escher questions

1) When did Escher live ? What developments were taking place in science at that time ?

2) What is the name for a tiled pattern of the kind Escher liked to draw ?

3) Notice Escher's people, the ones who inhabit those odd little worlds where water appears to flow uphill and downhill. Why are so many of them faceless ?

4) Which of the drawings might be about the equivalence of mass & energy ?