More scientifically, inertia is the principle that an object will maintain its state of motion until acted upon by unbalanced forces, i.e. two or more forces that push against the object in such a way that they do not balance out. Of course, if the forces balance out, the object is not going to change where it is currently going.

An example of balanced forces is when you are sitting inside a car that is traveling at a constant speed along a flat piece of road. The only forces acting on you at that point is the Earth pulling you down onto the seat and the seat pushing up against you. Relative to the Earth, you maintain your motion for as long as the car does not accelerate or decelerate.

Mach's Principle and Einstein's Geodesics

An interesting point about inertia is that nobody is quite sure what its origin is. We take it for granted that it's a property of objects with mass, but how does an object 'know' how it is moving, so that it can resist efforts to change that movement? Both Galileo and Newton thought that an object somehow 'knows' its movement relative to distant matter. In 1863, Ernest Mach published "Die Machanik" in which he formalized this argument of his forerunners. Einstein was greatly influenced by it. In 1918 he named it "Mach's Principle".

Einstein's general relativity took the definition of inertia a step further by showing that all objects move through a continuum called 'spacetime' along spacetime geodesics. This simply means that objects take the path of least action (effort) through spacetime, unless acted upon by forces. The matter of the Universe defines the gravitational field and the gravitational field defines the geodesic paths through spacetime.

The present scientific view is that the distant galaxies emitted virtual gravitons that traveled through space for billions of years before being absorbed by your body as you're sitting in the car. The resistance of your body to acceleration is wholly or partially due to the interaction between your body and virtual gravitons that were originally emitted billions of years ago by 'living' and long 'dead' stars in distant galaxies. Is that weird, or what?

Time-stress the Origin of Inertia?

Presently there is another view that is slightly controversial, but far less weird. It says that when you move freely in empty space, the various parts of your body all record time at the same rate. A scientist would say that 'the temporal (time) relations' between all parts of your body remain constant, or that there is no 'temporal stress' on your body.

As soon as you are being accelerated, the temporal relations are disturbed and the various parts of your body need to run at their own times. This creates temporal stress inside the body and the body's reaction is to resist this disturbance so that the stress is reduced; hence it shows inertia ? it wants to follow its spacetime geodesic with the minimum effort and stress.

The beauty of this hypothesis is that it only requires Einstein's rather easy to understand special theory of relativity and not the quite formidable general theory of relativity. Special relativity is mostly about how relative movement influences relative time.

Was Einstein Right?

We know that Einstein was right with his special theory of relativity. It has been proven beyond the shadow of a doubt. It may however be that Einstein was wrong on the origin of inertia. Maybe inertia has nothing to do with Mach's principle and the matter of the Universe at large, but just with what's going on within every object's temporal relations.

At a 'feet-on-the-ground' level, we are almost continually under stress from the force of gravity and hence our bodies are under constant temporal stress. Yes, the time in our heads do run faster than the time in our feet (unless we are upside down, of course). Add to this the inevitable time-stresses of modern society and it is no wonder that we are all somewhat 'stressed-out'!