A ‘60s dictionary definition:  Inertia.  A property manifested by all matter, representing the resistance to any alteration in its state of motion.  Mass is the quantitative measure of inertia.  

A similar definition is still in the Oxford dictionary but you would be hard pressed to find such a simple and clear account in today’s physics literature.     

Isaac Newton and Ernst Mach recognized and characterized inertia, then Einstein gave it special status e.g. who could forget “inertial reference frames”?   Despite all of the attention however, nobody has ever explained exactly what inertia is and how it works.

“Why shouldn’t a car keep rolling on level ground all by itself if there is little friction to stop it?”  That question once floored me because it takes a lot of work to push a car and get it moving in the first place!  A better question would be, “why is it not a continuous struggle to push a car; why does the initial resistance magically dissipate?”

Equally mysterious is why external force is again required to stop the car.  What force continues to make the car go that requires a definite counter force?”

Nothing is ever really at rest in the absolute sense and we couldn’t tell if it was.  We are moving through space as the earth turns and orbits the sun etc.  These things are not noticed because the motion is relatively constant without any acceleration that can be felt.  While sitting in your chair you have established equilibrium in your gravity links to earth and millions of stars.

In Anim. 1 above, a ball is sitting on a table and has millions of gravity links pulling it in all directions.  Most pulls cancel except for a slightly higher concentration to/from earth, on the bottom.

Links are continually replaced as one body eclipses another or some links get swept into collisions with others crossing their paths.

Even though the links come and go, there are millions on the ball at any given instant.  Any attempt to move the ball will instantly bring the attached ‘side’ links up against others crossing their paths.  One or the other must be sheared as the body moves, and this obviously requires work.  

Inertia resists acceleration because of gravity links that must be sheared.

Pre-existing links to everything near or far is the only explanation for why inertia comes into effect instantly, anywhere, and in any direction.

Note how ‘side’ links are continuously sheared and replaced.  Here’s the secret however; as soon as the ball moves it picks up extra links in ‘front’. 

This increased probability of forming gravity links in ‘front’ is analogous to running in the rain.  You will get equally wet whether you run or walk in the rain   

for the same duration.  If you run however, you will get wetter in front than elsewhere because you intercept more raindrops that would otherwise have fallen ahead of you.  The extra pulls in ‘front’ defeat the initial resistance of ‘side’ links and continue to shear them.  

The extra pulls in ‘front’ are from anything including the distant stars.  This is the previously unacknowledged force that continues to make things go while apparently unaided.

The ball would keep rolling indefinitely were it not for friction and wind resistance.  If we try to accelerate the particle some more, we have to shear even more ‘side’ links per second, which requires more work.  Once an incremental increase in speed is achieved, the particle establishes and maintains a running total of even more links in ‘front’.  

The additional ‘front’ links handle the additional shearing load, and a new equilibrium is reached.  The particle will tend to continue its motion at the new velocity.

Finally, the end block counters the extra ‘front’ links and stops the ball.  With the ball no longer moving, the extra ‘front’ links are quickly lost to attrition and equilibrium is restored.

These forces are also easy to understand in terms of energy links.  Consider for example, swinging a ball on a string around your head.  At any moment, ‘straight ahead’ for the ball is a line tangent to its circular path.  This is where the ball will go if released because of extra links on its ‘front’ pulling it to the stars ahead.  If you do not release the ball then you have to keep pulling it into the circle and away from its natural path.

As you swing the ball, you are accelerating it towards the centre of the circle from an infinite number of points on its circumference.  The outward pull you feel is resistance to your accelerations repeatedly shearing additional ‘side-lines’ on the ball.  Whether the ball is released or not, these centrifugal/centripetal effects are all due to the ball ‘hanging onto the stars’.

This would be a shock to Newton because it is not evident in everyday experience.  Inertia does not have the synergy and two-way reinforcement as in moving electric and magnetic fields.

Continued motion of a body does tend to collect more gravity links in front to maintain its motion but there is no mechanism to replace a lost link, however rare. 

In 1980, John Anderson of JPL noticed that Pioneer 10 & 11 spacecraft were inexplicably slowing down.   Nearly ten years after launch, they were nearing the edge of the solar system.

They checked for spacecraft malfunctions, their own calculations, radio beams, solar radiation, and solar wind etc.   Decades later, now well beyond any solar effects, nothing had changed.  The slow down is miniscule but after 30 years it adds up, leaving the spacecraft about 248,000 miles short.  For more details check the link immediately below.

Scientists are still scratching their heads over this one and resist ‘new science’.  It should now be obvious from these pages that inertia will gradually deteriorate, albeit extremely slowly.