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E=MC^2, so energy and mass are the same. Mass is the measure of inertia. Inertia is the resistance to change.

The more energy something has, the more inertia it has (it is easier to stop a ball going 1 MPH than a ball going 99 MPH. However, in any observable universe, would you need to have "resistance," otherwise it would be impossible to interact with it? For example, inertia can be demonstrated in the fact that if you are typing your response, your hand does not go through the keyboard (and you can see it, so the light must be affecting you and the keys must be able to reflect light).

Thus, if there was no inertia, then you would not perceive anything, so the universe would not exist to you. Thus, if you have a universe, do you have inertia?

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  • Could God make a universe without inertia? I think most would not be ready to restrict God as you have.
    – J Kusin
    Oct 25, 2021 at 17:40
  • This doesn't seem a good or useful question. The Higgs & gravitational field interactions give inertia. As far as we know there are 19 'free' parameters resulting in our en.wikipedia.org/wiki/Fine-tuned_universe The en.wikipedia.org/wiki/Anthropic_principle suggests it may just be observer bias meaning they have to be at or very close to these values. Could there be other sets, with other observers? The space of 19 variables is HUGE, so probably. Can we contact them? No. We don't understand our own universe, how can we hope to grasp others? & so many unknowns..
    – CriglCragl
    Oct 25, 2021 at 20:06
  • No, one can have observable universes where the notion of inertia does not even make sense. For example, universe where every object instantaneously transfers all information about itself to every other object.
    – Conifold
    Oct 25, 2021 at 20:39
  • The possibiliy to interact with an object like typing on a keyboard has nothing to do with inertia. IIRC it's the repulsive electric force between the electrons around your hand's atoms and the keyboard's, along with the covalent liaisons between each object's atoms that prevent your fingers going through the keys. It has nothing to do with mass. One huge problem I can see in a world without inertia is that stable orbits become impossible. So the universe could exist, just not us.
    – armand
    Oct 25, 2021 at 22:41
  • Does an empty universe count as observable?
    – kutschkem
    Oct 26, 2021 at 7:52

4 Answers 4

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Inertia is specifically a property of massful things. It is not obligatory for observation to occur. For instance, it was long believed that light could not transfer momentum at all. Only recently did we realize that it could transmit small amounts of momentum from one object to another. Our concepts of the world did not fall apart due to this detail, so clearly there are conceptual worlds which do not need this property.

The real question is what laws of physics you keep, and whether those particular laws are consistent with "there is no inertia."

And, just thinking outside the box, I can see no reason why inertia is obligatory in a idealist world (where all that exists is thought).

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No. In the sense that you are using inertia

However, in any observable universe, would you need to have "resistance," otherwise it would be impossible to interact with it?

which is that inertia=whether something is real or not, then you cannot have a real universe without having a real universe. God is another topic, and I believe that God could make an inertialess universe, but in any universe that we could interact with, inertia must exist.

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  • Welcome to PSE.. Just an observation: '...you cannot have a real universe without having a real universe'. I see the point you are making but this seeming tautology might throw the reader. Perhaps rephrase.
    – Geoffrey Thomas
    Oct 27, 2021 at 7:53
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As pointed out by Cort Ammon, inertia is a property possessed by (commonplace) massive objects and some other things, like photons.

But please note there is a difference between inertia (a scalar quantity measuring the particular object's resistance to changes in velocity) and momentum (a vector quantity measuring a conserved quantity in interactions with other massive objects (as for instance in collisions).

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I can see it happening a few different ways so yeah I would say so .... imagine a universe that has been omni quantum locked...Simply by the placement and Gathering of certain particles. It is truly not that far-fetched at all.

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