Mystery of Inertia — knowing vs understanding

Most people think they know what inertia is, but they definitely don’t understand it and surprisingly, many would use the same set of words/language to articulate it.

I wouldn’t be surprised if what you recall one from the below standard definitions :-

1. Inertia is an objects tendency to resist any change in motion, or
2. An object will continue to be in a state of rest or in uniform motion unless an unbalanced force acts on it

This only goes to say how well the definition is internalised and imprinted in our understanding of the Law of Inertia.

Source — toondoo.com

But beyond the definition, there is a big gaping foundational issue.

1. Why do objects tend to resist the change in motion.
2. How do they know that they need to oppose a force that is going to change their state of motion ?
3. And why is the measure of the tendency related to the Mass of the object ?

Spent some time scraping the internet to understand if it was just me who was curious to know the underlying mechanism of inertia. But somewhere in the back of my mind the backbencher still thinks that it’s not even a question perhaps.

Was pleasantly surprised to discover there are other people have have asked this question and the fact that Einstein also had to go through this question as he moved from special theory of relativity to general theory of relativity. But bottomline — the mechanism is still unknown. There are multiple theories that try to explain but there doesn’t seem to be a consensus on what it could really be. Some of the interesting ones that I found :-

1. General relativity — objects follow spacetime geodesics and any deviation from it will be met with resistance aka Inertia. And the matter distribution (even of the distant galaxies) of the universe will dictate local spacetime curvature. Counter question that i had — would there be no inertia of an object if the universe was empty ?
2. Special relativity — When an object experiences acceleration, various part of the body experience time at different rate and there is a temporal stress that is generated in the body — which is manifested as inertia- unwillingness to leave the geodesic. Counter question — How is the measure of the stress dependent on the mass of the object ?
3. Spacetime warping resistance — When objects move from one spacetime location to another, they also tend to warp a different spacetime. When the object is moving at a constant velocity, the de-warping and warping happens simultaneously and is balanced. But when object is accelerated there is a resistance caused by the stress from the uneven / unbalanced spacetime warping. And the measure is defined by the strength of the warping — mass.

This exercise was particularly joyful because I had a question that I hadn’t asked myself or the teachers who taught me. Had accepted borrowed understanding without being genuinely curious about it. Goes to say that there are so many more important fundamental questions to seek answers to and our assumptions themselves are so fundamental that it behoves us to still be curious about them.