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It is undeniable that spacetime is medium for gravitational waves propagation as first confirmed by LIGO experiment having elasticity and compressability.

However, for electromagnetism space has the total opposite behavior effectively portrayed as an analogue of a perfect incompressible fluid in the worst case and as superfluous in the best case, not needed for EM waves propagation at all.

Seems to me that the definition of vacuum free space for these two different phenomena gravity and EM suffers from bipolar disorder and is schizophrenic at least.

Additionally, I find it troublesome how these different phenomena allegedly one of propagating without a medium (i.e. EM) share the same speed of propagation c inside the vacuum which c is clearly an intrinsic physical property of free space c=1/sqrt(μ0ε0) according to electromagnetism theory?

So what is the resolution of these two conflicting descriptions about space?

Is vacuum space a medium of energy propagation or not? It can't be both?

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  • $\begingroup$ Out of curiosity what makes you think that these two things should behave the same? Physics does not tell us anything about how the world "is", it really only predicts how it will behave. Do you have some argument for why EM waves and gravitational waves should be mathematically similar? $\endgroup$
    – Charlie
    Commented Oct 21, 2023 at 19:23
  • $\begingroup$ IMO "EM waves don't need a medium to propagate" is an old relic of the past that is now proven obsolete and should be neglected in the future. This was before the quantuim vacuum era of research that proves that vacuum free space id far from empty and nothing and that has ZPE quantum foam noise field background that by definition makes ZPE of vacuum a medium for EM and all quantum fields. Moreover, it was never proven theoretically that EM waves can exist without the vacuum. Which that alone makes the vacuum a medium of EM waves. $\endgroup$
    – Markoul11
    Commented Oct 21, 2023 at 19:31
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    $\begingroup$ not needed for EM waves propagation at all Where would EM fields live if not in spacetime? Both gravitational waves and EM waves propagate through spacetime. They are simply waves in different fields that live in spacetime: the EM field vs. the metric field. I don’t understand this question at all. The word “medium” is a red herring, and the notion that a quantum vacuum is required for classical EM waves to propagate is not mainstream physics. $\endgroup$
    – Ghoster
    Commented Oct 21, 2023 at 19:40
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    $\begingroup$ Furthermore, the notion that $c$ somehow comes from EM theory is completely wrong. An invariant speed is part of the geometry of spacetime. EM, like everything else, happens to be compatible with Special Relativity, so (when you use ugly SI units) it is hardly surprising that $c$ is related to silliness like $\epsilon_0$ and $\mu_0$ (which have no physical significance and don’t even exist in better unit systems). $\endgroup$
    – Ghoster
    Commented Oct 21, 2023 at 19:45
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    $\begingroup$ Maybe somebody can explain to me why this question has already been downvoted 4 times? What is so offensive about this question? Please, somebody, do explain. @Ghoster is right that it is totally wrong to think that $c$ comes from EM theory, fine. If Markoul knew it better he would not be asking; he is quite confused, no doubt but I thought the reason for this platform is to learn something from others who know it better and willing to share their knowledge. Thank you. $\endgroup$
    – hyportnex
    Commented Oct 21, 2023 at 23:49

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So what is the resolution of these two conflicting descriptions about space?

I think the the issue here is that there isn't really a conflict.

EM is described in three different important (and still used) theories : Maxwell's theory, Special and General Relativity and Quantum Electrodynamics (QED). These models described different contexts and details of electromagnetism. That they have commonality is remarkable, because there's no way Maxwell or Einstein could have foreseen later theories.

Gravity has been modeled using Newton's theories and later general relativistic theories. Many attempts have been made to develop a quantum field-like theory explaining gravity but we have not yet developed such a theory that ticks all the required boxes.

There's no conflict here, they're simply different models of different levels of understanding of physically observed phenomena. They're allowed to be different.

Spacetime isn't considered a medium in modern physics. It was an idea Maxwell had no knowledge of when he developed his brilliant theory. Field theory is typically taught as two different things : classical and quantum. General relativity is, despite the association with modern physics, considered a classical theory - there's no inherent conception of quantum fields in general relativity.

So you're comparing apples with oranges.

They're simply different theories encompassing different levels of understanding.

Is vacuum space a medium of energy propagation or not? It can't be both?

There isn't really a single idea of vacuum/spacetime.

Again this apples and oranges because these theories were all developed under different models of spacetime. Strictly speaking Maxwell worked using a pre-relativistic understanding of what we now call spacetime. It's remarkable indeed that his theoretical model was so good that it "lit the way" for later theorists (including Einstein) to recognize that it required accepting the concept of spacetime as we now understand it to fully "fit" Maxwell's theory into a cogent larger picture.

So we're not comparing like with like and there is no single model of spacetime at work here.

Vacuum is an even more complex concept when you reach quantum field theories. Again, it would be naive to think that these theories share a single model of vacuum.

There's no conflict as I see it, just that you're comparing apples and oranges.

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