code: function( cfg, fn ){
  
              var self = this;
  
              if ( Physics.util.isFunction( cfg ) || Physics.util.isArray( cfg ) ){
                  fn = cfg;
                  cfg = {};
              }
  
              this._meta = {
                 // statistics (fps, etc)
                 fps: 0,
                 ipf: 0
              };
              this._bodies = [];
              this._behaviors = [];
              this._integrator = null;
              this._renderer = null;
              this._paused = false;
              this._warp = 1;
              this._time = 0;
  
              // set options
              this.options = Physics.util.options( defaults );
              this.options.onChange(function( opts ){
  
                  // set timestep
                  self.timestep( opts.timestep );
              });
              this.options( cfg );
  
              // add integrator
              this.add(Physics.integrator( this.options.integrator ));
  
              // apply the callback function
              if ( Physics.util.isFunction( fn ) ){
  
                  execCallbacks([ fn ], this, [this, Physics] );
  
              } else if ( Physics.util.isArray( fn ) ){
  
                  execCallbacks(fn, this, [this, Physics] );
              }
          }
docs: internal, see: new Physics.world
  Physics.world#init( [options, fn(world, Physics)] )
  - options (Object): configuration options (see constructor)
  - fn (Function|Array): Callback function or array of callbacks called with this === world
  
  Initialization