diff --git a/game_definitions.cpp b/game_definitions.cpp
index d30e1e5..88043ec 100644
--- a/game_definitions.cpp
+++ b/game_definitions.cpp
@@ -1,5 +1,7 @@
 #include "game_definitions.h"
 
+#include <math.h>
+
 namespace GameDefinitions {
 
 MenuItemEnum & operator++ (MenuItemEnum & m) {
@@ -26,5 +28,17 @@ MenuItemEnum & operator-- (MenuItemEnum & m) {
     }
 }
 
+double LorentzFactor(double v) {
+    double lf;
+    double r = v / kSpeedOfLight;
+    if (r > 1.0) {
+        lf = kEpsilon;
+    }
+    else {
+        lf = sqrt(1.0 - r*r);
+    }
+    return lf;
+}
+
 } // namespace GameDefinitions
 
diff --git a/game_definitions.h b/game_definitions.h
index 7e3a0ac..c62bfcd 100644
--- a/game_definitions.h
+++ b/game_definitions.h
@@ -3,7 +3,9 @@
 
 namespace GameDefinitions {
 
+const double kEpsilon = 0.001;
 const double kGravityConstant = 0.7;
+const double kSpeedOfLight = 180.0;
 
 enum OrbitDirEnum {
     od_Clockwise,
@@ -27,6 +29,8 @@ enum MenuItemEnum {
 MenuItemEnum & operator++ (MenuItemEnum & m);
 MenuItemEnum & operator-- (MenuItemEnum & m);
 
+double LorentzFactor(double v);
+
 }  // namespace GameDefinitions
 
 #endif  // GAME_DEFINITIONS_H_
diff --git a/space_ship.h b/space_ship.h
index 8a94535..fbd7466 100644
--- a/space_ship.h
+++ b/space_ship.h
@@ -36,7 +36,7 @@ class SpaceShip {
     double density_;
     double thrust_force_;
     double moment_;
-    
+
     b2Vec2 position_;
     b2Vec2 velocity_;
     b2Vec2 gravity_;
@@ -46,7 +46,7 @@ class SpaceShip {
 
     float color_[3];
 public:
-    SpaceShip() 
+    SpaceShip()
     : engine_(0)
     , radar_(0)
     , kMaxHullStrength(10.0)
@@ -152,10 +152,6 @@ class SpaceShip {
     // Begin -- Handlers for Engine system.
     void hndThrustOut(double value) {
         thrust_force_ = value;
-
-        BD_Scalar thrust;
-        thrust.value = thrust_force_;
-        DATABUS.Publish(db_PlayerThrust, &thrust);
     }
     void hndMomentOut(double value) {
         moment_ = value;
@@ -172,7 +168,9 @@ class SpaceShip {
         physics_body_->ApplyTorque(moment_, true);
         // Get velocity for devices.
         velocity_ = physics_body_->GetLinearVelocity();
-
+        double speed = velocity_.Length();
+        double lf = GameDefinitions::LorentzFactor(speed);
+        physics_body_->GetFixtureList()->SetDensity( density_ / lf );
         // Get position.
         position_ = physics_body_->GetPosition();
         angle_ = physics_body_->GetAngle() * 180.0 / M_PI;
@@ -192,6 +190,10 @@ class SpaceShip {
         s.value = angle_;
         DATABUS.Publish(db_PlayerAngle, &s);
 
+        BD_Scalar thrust;
+        thrust.value = thrust_force_ * lf;
+        DATABUS.Publish(db_PlayerThrust, &thrust);
+
         engine_->Update(delta_time);
         radar_->Update(delta_time);
     }