Editing Creating the Simulation Environment (section)

===The World===

The <span style="color:red">'''world'''</span> is home to all our physics process. Graciously, Geant's developers have implemented and included known materials and physics processes to be used inside of these virtual <span style="color:red">'''worlds'''</span>. Seen in the diagram above, the <span style="color:red">'''world'''</span> can be thought of as Luter Hall which houses many different rooms (<span style="color:blue">'''envelopes'''</span>) within it. Since Luter Hall is our fictitious creation, ideally we would like to model it closely to a real world building that is filled with air. But we are at liberty to abstract our  <span style="color:red">'''world'''</span> in any way to augment the current experimental configuration. For Luter Hall to exist we need three essential components:   

*'''G4VSolid''': To describe a shape, we use a solid volume. The geometry of choice is up to the user. Here we chose to use a G4Box object.  
*'''G4LogicalVolume''': To characterize a volume's properties we use a logical volume. It includes the geometrical properties of the solid and adds physical characteristics: the material of the volume; whether it contains any sensitive detector elements; the magnetic field; etc.
*'''G4PVPlacement''': To position the volume, create a physical volume. This places a copy of the logical volume inside a larger containing volume that includes the coordinate system of the <span style="color:red">'''world'''</span>.

Below is an example on how to generate the <span style="color:red">'''world'''</span> volume in the ''Detector Construction Class'':

  G4double world_sizeXY = <SCALE_FACTOR*envelope_sizeXY>;
  G4double world_sizeZ  = <SCALE_FACTOR*envelope_sizeZ>;
  G4Material* world_mat = nist->FindOrBuildMaterial(<"MATERIAL">);
  
  G4Box* solidWorld =    
    new G4Box("World",                       //its name
       0.5*world_sizeXY, 0.5*world_sizeXY, 0.5*world_sizeZ);     //its size
      
  G4LogicalVolume* logicWorld =                         
    new G4LogicalVolume(solidWorld,          //its solid
                        world_mat,           //its material
                        "World");            //its name
                                   
  G4VPhysicalVolume* physWorld = 
    new G4PVPlacement(0,                     //no rotation
                      G4ThreeVector(),       //at (0,0,0)
                      logicWorld,            //its logical volume
                      "World",               //its name
                      0,                     //its mother  volume
                      false,                 //no boolean operation
                      0,                     //copy number
                      checkOverlaps);        //overlaps checking