The concept of inertia is very old and it was well defined in the Newton's first law. It guarantees that without any external force there are inertial systems. This condition is out of gravitation and quite natural for electromagnetism where special relativity is its natural reference from a physical observer. Therefore the inertia is one concept which cannot joint both interactions.

Many thanks Preston Guynn . I'll be taking a look at the articles.

It took a number of my own papers to realise the problem with inertia. Inertia as a concept is really a testimony of our initial need to push-pull objects around us, to feel our way through the basics of physics. Yet when it comes to a "unified field theory", everything, all motion, must be accounted for from that basis, and not our simple understanding of inertia.

All evidence suggests that Inertia as a simple concept on the surface of reality is fine, yet it won't take us to a unified field theory, it's logically impossible; the issue is, with a "unified" field theory, the presumption is that such would be (a unified field theory) the basis of all motion in reality between objects, and thus the idea of "inertia" must be superseded, as "inertia" describes the "unwillingness" of a body to change its motion, and thus there is at some point from the gross-scale of bodies in motion (as per what inertia describes) to the field forces a level where the concept of inertia breaks down. Otherwise inertia, and with what you present, is valid.

Stephen Jarvis

It is undoubtedly confirmed without any single conflict with experiment that gravitation and inertia, either inertia is measured by mass or as local inertial frames, are equivalent through Einstein equivalence principle , which links gravity to inertia or vice versa and is realized by the physical geometrodynamical structure of spacetime described in general relativity. Thus, locally, gravity and inertia cancel each other since they are the same and characterized by spacetime metrics.

As a good consequence of that local equivalence, any classical or quantum law to be locally physical describing any interaction (weak with or without “electro-”, electromagnetic, strong, or gravitational) must be either Lorentz covariant in form or Lorentz invariant or scalar or tensor of rank zero in value, thus the same in all inertial frames.

Another happy and great consequence is that electromagnetism and Maxwell’s equations, which can be derived from special relativity, were revisited by Einstein and tensorially reformulated in such a way that both the electric field and the magnetic field have been clearly revealed to be frame-dependent thus unphysical in the sense that they are not Lorentz-invariant quantities. Thus, the vanishing of the electric field or magnetic field in one frame does not necessarily imply its vanishing in another inertial frame.

Issam Mohanna, many thanks for your reply.

I think what I am explaining here is the idea that "Imagine being asked to survey farmland by picking up each grain of dirt, each rock, small and large, and measuring its potential trajectory when thrown from one end of the field to the other at varying speeds and trajectories, and with the aim of measuring the whole field through big-data dirt analysis, how it is all held together, based on a theory of the collective inertial trajectories and how gravity would (in the theory based on all those trajectories) keep the field together".

That’s physics using inertia theory. Of course “inertia” complements gravity, it must, as "inertia" is only a "way" of measuring mass and thus gravity, yet the end of that theory of that "way" of measuring mass and thus gravity is an understanding of the field forces separating gravity from electrodynamic field forces, from “electromagnetism”, simply because the “way” mass was used with “gravity” held the premise, the “way”, as “inertia”, as a change of the natural state of the mass in question. In initial physics theory century past that’s fine, yet as the theory becomes more involved with the field forces, it becomes apparent that “inertia” becomes insufficient as a “way” to explain mass and thus gravity, simply because an "ultimate” theory doesn’t complement the idea of “changing” mass and it’s location (as per inertia) yet “abiding by” the more fundamental field forces at play. The real question is “what is the underlying field force central to time and space relevant to “mass” and thus “gravity”. It’s not “inertia”, by the very concept of “inertia” it can’t be.

One thing is inertia and another is the inertial mass involved in the equivalence principle of General Relativity. The key difference between gravitational mass and inertial mass is that gravitational mass is measured under gravity, whereas inertial mass is measured under any force.

Mass is a property of a substance as well as its resistance towards acceleration. There are two types of mass as gravitational mass and inertial mass, which give the type of force acting on the mass. Gravitational force is the reason for acceleration due to gravity, and we can measure the mass of an object that moves due to gravity.

One thing is General Relativity and another Gravitation Interaction.

Daniel Baldomir , yes. I agree, "gravitational mass is measured under gravity, whereas inertial mass is measured under any force". The "question" though is "does the underlying field forces of time and space, gravity and electromagnetism, convene to ultimately disrupt each other, and more to the point, "act to disrupt everything, throw everything out of whack, as what "inertia" prescribes", which is what happens when a unified field theory is lacking.

If I can ask, are you suggesting gravity and mass cannot be connected by your suggestion that mass has an inherent quality of itself against acceleration, because that's "inertia" (and apologies for not answering your first statement above in this discussion as I was unsure as to your leading idea in that remark)?

Dear Stephen,

When you write:

SJ> If I can ask, are you suggesting gravity and mass cannot be connected by your suggestion that mass has an inherent quality of itself against acceleration, because that's "inertia"

No, I'm not suggesting such a thing. Gravitation is conneted with energy-momentum tensor and therefore with mass, as Einstein's equations say. But this is not more than one special case of inertial motion, notice that gravitation has a constant acceleration which is fundamental for the equivalence principle, while the inertia can be given to any acceleration or force. This distinguish clearly the different concepts that some people confuss.

Yes, which makes the idea of "inertia" a broad-spectrum "manner" of examination, does it not? "Inertia" applies to anything, can apply to anything. It's a "manner" of examination. The "manner", that "manner" of inertia, is in question, as ultimately it represents an idea that challenges more fundamental properties of time and space "we" as humans are not naturally conscious of. We are conscious of push-pull, sure, and so we determine ideas of "work", "effort", and "force" in our own physical structuring within that reality we are trying to understand in such a manner, "yet"......there is more, is there not, more than "push-pull inertial logic to the explanation of time and space? There clearly is.

Inertia is a "concept" of "resistance" a body prescribes by our observation of that resistance we physically force ourselves to fumble through space and time with, "and then" construct theories further with that fumbling. "Yet" we are given hints on that journey with which we can be more intellectual "with" as opposed to physical, as per our approach to the primary axioms we are seeking to calculate.

Is inertia a "field force"? Seriously, no.

Does "inertia" exist between the fundamental field forces?

Seriously, no.

If it did, it would be its own field force separate to everything.

Yet, it was our start point in the middle ground of examining reality, was, and is, it not?

Inertia is like a natural flaw we have in being human....we strive, we work, we use force....yet that is a subjective reference of control. Reality exists, and here we are examining what exists from our inertial reference we seem to be applying to reality, to time and space. Its a natural thing for us to do, sure.

Inertia is an archaic terminology that could mean any of (1) Inertial mass, (2) Kinetic energy, or (3) Momentum. So why use the term? It can't be inserted into equations until we have identified which of the above three concepts we are referring to. As regards the suggestion that this vague term inertia might be used to unite gravity and electromagnetism, I can't see how.

There is however a distinct parallel between the inertial forces and the electromagnetic forces, but not gravity. Gauss's law does however provide a unifying link between gravity and electrostatics.

A good place to start in order to investigate this matter further would be with the Coriolis force, F = 2mv×ω, and the EM force F = qv×B. A comment is made on this topic at the very end of this article, https://www.researchgate.net/publication/334376750_Faraday's_Law_of_Electromagnetic_Induction

I agree. The problem is that with GR and SR we consider the manner of physics equations abiding by the idea of "inertia", and thus thereof the entire gambit of modern physics theory, the standard model (SM). And so we get to all the problems outlined in the paper: Preprint The Physics Chimera

As I said above, "Inertia is a "concept" of "resistance" a body prescribes by our observation of that resistance we physically force ourselves to fumble through space and time with, "and then" construct theories further with that fumbling. "Yet" we are given hints on that journey with which we can be more intellectual "with" as opposed to physical, as per our approach to the primary axioms we are seeking to calculate."

There must come a point in physics theory in reaching the core axioms of time and space where inertia as a concept must "not" be consider as a manner of our examination, our equations, yet to do that the idea of relativity needs to be then "rewritten" in accordance with those new axioms. Is that an incorrect suggestion, and if so why?

There are many people who would argue that Einstein's theories of relativity need to be abandoned altogether, and I'm one of them. See this article here,

Article Newton's Cradle Disproves Einstein's Theories of Relativity

Frederick David Tombe , yes, Tensors, elasticity, etc etc, they are words to build bridges between concepts that don't exactly marry up other than using those "pliable" descriptors. That's the problem with physics, all those words, one "fix" to another. It's great as a way to build bridges, yet ultimately all those words must be abandoned when an "entire" understanding, a pure statement of the field forces using purely the ideas of time and space, is reached.

Dear all, I've added a new paper to better elaborate on the idea of inertia and space:

Preprint Space, and the Propagation of Light

The topic question remains the same, and the above paper aims to address how inertia found itself embedded in the theory of "space" re. relativity theory.