How to solve Unable to read sdf file 'empty.world' ?

edit

I run

roslaunch pr2_gazebo pr2_empty_world.launch

It shows:

sam@sam:/opt/ros$ roslaunch pr2_gazebo pr2_empty_world.launch 
... logging to /home/sam/.ros/log/35026a8e-4ae6-11e3-9bc0-20cf30a23845/roslaunch-sam-15560.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://sam:33606/

SUMMARY
========

PARAMETERS
 * /base_controller/bl_caster_l_wheel_joint/d
 * /base_controller/bl_caster_l_wheel_joint/i
 * /base_controller/bl_caster_l_wheel_joint/i_clamp
 * /base_controller/bl_caster_l_wheel_joint/p
 * /base_controller/bl_caster_r_wheel_joint/d
 * /base_controller/bl_caster_r_wheel_joint/i
 * /base_controller/bl_caster_r_wheel_joint/i_clamp
 * /base_controller/bl_caster_r_wheel_joint/p
 * /base_controller/bl_caster_rotation_joint/position_controller/d
 * /base_controller/bl_caster_rotation_joint/position_controller/i
 * /base_controller/bl_caster_rotation_joint/position_controller/i_clamp
 * /base_controller/bl_caster_rotation_joint/position_controller/p
 * /base_controller/bl_caster_rotation_joint/velocity_controller/d
 * /base_controller/bl_caster_rotation_joint/velocity_controller/i
 * /base_controller/bl_caster_rotation_joint/velocity_controller/i_clamp
 * /base_controller/bl_caster_rotation_joint/velocity_controller/p
 * /base_controller/br_caster_l_wheel_joint/d
 * /base_controller/br_caster_l_wheel_joint/i
 * /base_controller/br_caster_l_wheel_joint/i_clamp
 * /base_controller/br_caster_l_wheel_joint/p
 * /base_controller/br_caster_r_wheel_joint/d
 * /base_controller/br_caster_r_wheel_joint/i
 * /base_controller/br_caster_r_wheel_joint/i_clamp
 * /base_controller/br_caster_r_wheel_joint/p
 * /base_controller/br_caster_rotation_joint/position_controller/d
 * /base_controller/br_caster_rotation_joint/position_controller/i
 * /base_controller/br_caster_rotation_joint/position_controller/i_clamp
 * /base_controller/br_caster_rotation_joint/position_controller/p
 * /base_controller/br_caster_rotation_joint/velocity_controller/d
 * /base_controller/br_caster_rotation_joint/velocity_controller/i
 * /base_controller/br_caster_rotation_joint/velocity_controller/i_clamp
 * /base_controller/br_caster_rotation_joint/velocity_controller/p
 * /base_controller/caster_names
 * /base_controller/caster_position_pid_gains/d
 * /base_controller/caster_position_pid_gains/i
 * /base_controller/caster_position_pid_gains/i_clamp
 * /base_controller/caster_position_pid_gains/p
 * /base_controller/caster_velocity_filter/name
 * /base_controller/caster_velocity_filter/params/a
 * /base_controller/caster_velocity_filter/params/b
 * /base_controller/caster_velocity_filter/type
 * /base_controller/caster_velocity_pid_gains/d
 * /base_controller/caster_velocity_pid_gains/i
 * /base_controller/caster_velocity_pid_gains/i_clamp
 * /base_controller/caster_velocity_pid_gains/p
 * /base_controller/fl_caster_l_wheel_joint/d
 * /base_controller/fl_caster_l_wheel_joint/i
 * /base_controller/fl_caster_l_wheel_joint/i_clamp
 * /base_controller/fl_caster_l_wheel_joint/p
 * /base_controller/fl_caster_r_wheel_joint/d
 * /base_controller/fl_caster_r_wheel_joint/i
 * /base_controller/fl_caster_r_wheel_joint/i_clamp
 * /base_controller/fl_caster_r_wheel_joint/p
 * /base_controller/fl_caster_rotation_joint/position_controller/d
 * /base_controller/fl_caster_rotation_joint/position_controller/i
 * /base_controller/fl_caster_rotation_joint/position_controller/i_clamp
 * /base_controller/fl_caster_rotation_joint/position_controller/p
 * /base_controller/fl_caster_rotation_joint/velocity_controller/d
 * /base_controller/fl_caster_rotation_joint/velocity_controller/i
 * /base_controller/fl_caster_rotation_joint/velocity_controller/i_clamp
 * /base_controller/fl_caster_rotation_joint/velocity_controller/p
 * /base_controller/fr_caster_l_wheel_joint/d
 * /base_controller/fr_caster_l_wheel_joint/i
 * /base_controller/fr_caster_l_wheel_joint/i_clamp
 * /base_controller/fr_caster_l_wheel_joint/p
 * /base_controller/fr_caster_r_wheel_joint/d
 * /base_controller/fr_caster_r_wheel_joint/i
 * /base_controller/fr_caster_r_wheel_joint/i_clamp
 * /base_controller/fr_caster_r_wheel_joint/p
 * /base_controller/fr_caster_rotation_joint/position_controller/d
 * /base_controller/fr_caster_rotation_joint/position_controller/i
 * /base_controller/fr_caster_rotation_joint/position_controller/i_clamp
 * /base_controller/fr_caster_rotation_joint/position_controller/p
 * /base_controller/fr_caster_rotation_joint/velocity_controller/d
 * /base_controller/fr_caster_rotation_joint/velocity_controller/i
 * /base_controller/fr_caster_rotation_joint/velocity_controller/i_clamp
 * /base_controller/fr_caster_rotation_joint/velocity_controller/p
 * /base_controller/max_rotational_acceleration
 * /base_controller/max_rotational_velocity
 * /base_controller/max_translational_acceleration/x
 * /base_controller/max_translational_acceleration/y
 * /base_controller/max_translational_velocity
 * /base_controller/publish_tf
 * /base_controller/state_publish_rate
 * /base_controller/timeout
 * /base_controller/type
 * /base_controller/wheel_pid_gains/d
 * /base_controller/wheel_pid_gains/i
 * /base_controller/wheel_pid_gains/i_clamp
 * /base_controller/wheel_pid_gains/p
 * /base_hokuyo_node/frame_id
 * /base_hokuyo_node/intensity
 * /base_hokuyo_node/max_ang
 * /base_hokuyo_node/min_ang
 * /base_hokuyo_node/port
 * /base_hokuyo_node/skip
 * /base_odometry/base_footprint_frame
 * /base_odometry/base_link_frame
 * /base_odometry/caster_calibration_multiplier
 * /base_odometry/caster_names
 * /base_odometry/cov_xrotation
 * /base_odometry/cov_xy
 * /base_odometry/cov_yrotation
 * /base_odometry/ils_max_iterations
 * /base_odometry/odom_frame
 * /base_odometry/odom_publish_rate
 * /base_odometry/odometer_publish_rate
 * /base_odometry/publish_tf
 * /base_odometry/rotation_stddev
 * /base_odometry/state_publish_rate
 * /base_odometry/type
 * /base_odometry/verbose
 * /base_odometry/wheel_radius_multiplier
 * /base_odometry/x_stddev
 * /base_odometry/y_stddev
 * /diag_agg/analyzers/joints/expected
 * /diag_agg/analyzers/joints/path
 * /diag_agg/analyzers/joints/startswith
 * /diag_agg/analyzers/joints/type
 * /head_traj_controller/gains/head_pan_joint/d
 * /head_traj_controller/gains/head_pan_joint/i
 * /head_traj_controller/gains/head_pan_joint/i_clamp
 * /head_traj_controller/gains/head_pan_joint/p
 * /head_traj_controller/gains/head_tilt_joint/d
 * /head_traj_controller/gains/head_tilt_joint/i
 * /head_traj_controller/gains/head_tilt_joint/i_clamp
 * /head_traj_controller/gains/head_tilt_joint/p
 * /head_traj_controller/joints
 * /head_traj_controller/type
 * /l_arm_controller/gains/l_elbow_flex_joint/d
 * /l_arm_controller/gains/l_elbow_flex_joint/i
 * /l_arm_controller/gains/l_elbow_flex_joint/i_clamp
 * /l_arm_controller/gains/l_elbow_flex_joint/p
 * /l_arm_controller/gains/l_forearm_roll_joint/d
 * /l_arm_controller/gains/l_forearm_roll_joint/i
 * /l_arm_controller/gains/l_forearm_roll_joint/i_clamp
 * /l_arm_controller/gains/l_forearm_roll_joint/p
 * /l_arm_controller/gains/l_shoulder_lift_joint/d
 * /l_arm_controller/gains/l_shoulder_lift_joint/i
 * /l_arm_controller/gains/l_shoulder_lift_joint/i_clamp
 * /l_arm_controller/gains ...