Rosbags saving empty messages

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I am trying to record data from data from a Razor 9DOF imu, Hokuyo LIDAR and a ZED camera. The code compiles and runs fine but it is saving empty messages to the rosbag file. I have tried printing data in at different points and it wont. Can you tell me what my code is missing and why the messages are empty? I am running ROS kinetic 1.12.14 on Ubuntu 16.04 on an Nvidia Jetson TX2.

#include <iostream>
#include <ros/ros.h>
#include <rosbag/bag.h>
#include <sensor_msgs/Imu.h>
#include <sensor_msgs/LaserScan.h>
#include <sensor_msgs/Image.h>

sensor_msgs::Imu    imu_msg;
sensor_msgs::LaserScan scan;
sensor_msgs::Image msgr;
sensor_msgs::Image msgl;

   void imuCallback(const sensor_msgs::Imu &imu_msg)
  {

  }

  void scanCallback(const sensor_msgs::LaserScan::ConstPtr& scan)
  {

  }

  void imageRightRectifiedCallback(const sensor_msgs::Image &msgr) {

  }

  void imageLeftRectifiedCallback(const sensor_msgs::Image &msgl) {

  }


int main(int argc, char** argv){

    ros::init(argc, argv, "talker"); 
    ros::NodeHandle n; 

    rosbag::Bag bag; // create a bag named bag 
    bag.open("data.bag", rosbag::bagmode::Write); //open bag and create a file to save to.


    ros::Subscriber imu_sub = n.subscribe("imu_data", 1000, imuCallback); // subscribes to the imu and start the callback function
    ros::Subscriber sub = n.subscribe<sensor_msgs::LaserScan>("/scan", 1000, scanCallback); // subscribes to the lidar and start the callback function
    // subscribes to the zed and get the right image and start the callback function
    ros::Subscriber subRightRectified = n.subscribe("/zed/right/image_rect_color", 10, imageRightRectifiedCallback); 
    // subscribes to the zed and get the left image and start the callback function
    ros::Subscriber subLeftRectified = n.subscribe("/zed/left/image_rect_color", 10, imageLeftRectifiedCallback); 
   while(ros::ok()){
       bag.write("imu", ros::Time::now(), imu_msg); // sending msg into the bag
       bag.write("lidar", ros::Time::now(), scan);
       bag.write("ZED_RightImage", ros::Time::now(), msgr);
       bag.write("ZED_LeftImage", ros::Time::now(), msgl);



        ROS_INFO("Recording data...");
        ros::spinOnce();
    }
    bag.close();
    return 0;
}