Equivalence principle of general relativity holds even at gravitational extremes

The new paper claims to add to the confirmation of "the strong equivalence principle of general relativity."(0)

Reading Wikipedia to find which theories are (or were) the alternatives which assumed that principle not to hold:

"The strong equivalence principle": "The first part is a version of the weak equivalence principle" "The second part is the Einstein equivalence principle (with the same definition of "local"), restated to allow gravitational experiments and self-gravitating bodies."

"This is the only form of the equivalence principle that applies to self-gravitating objects (such as stars), which have substantial internal gravitational interactions. It requires that the gravitational constant be the same everywhere in the universe and is incompatible with a fifth force. It is much more restrictive than the Einstein equivalence principle."

"Einstein's theory of general relativity (including the cosmological constant) is thought to be the only theory of gravity that satisfies the strong equivalence principle. A number of alternative theories, such as Brans–Dicke theory, satisfy only the Einstein equivalence principle." (1)

So it seems that "Brans–Dicke theory" and similar are ruled out by this observation, that is that the following line from one Wikipedia page can't remain:

"At present, both Brans–Dicke theory and general relativity are generally held to be in agreement with observation."(2)

But note that the authors of (0) don't claim that the alternative is completely impossible, but that they have measured even bigger space where the "strong equivalence principle" holds, compared to all measured up to now:

"our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests"(0)

That is, of course, a success, as every scientific and measured increase of "known" is. Note that other "Solar System" tests mentioned in (0) are also very new, from this year. Also: the parameter is "a factor of almost a thousand smaller than that obtained from other strong-field tests"!

The article from Nature, of course more scientific that the OP on HN (currently: from discovermagazine.com), ends with:

"Although the [scalar–tensor] theories are not completely quashed, their hopes for validity have been made that much fainter."(4)

0) https://www.nature.com/articles/s41586-018-0265-1

1) https://en.wikipedia.org/wiki/Equivalence_principle#The_stro...

2) https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory

3) https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.34...

4) https://www.nature.com/articles/d41586-018-05549-4

  A gravitational constant that is the same 
  everywhere in the universe and is incompatible 
  with a fifth force.

The fallout of this ramification is that, gravity is some sort of ambient side effect of material presence, sort of like a shadow cast, more than an emission radiated.

As a constant, that means that its invariance is significant, in the same way the speed of light is significant. There is some externality pegging the phenomena we notice, at the value we observe. Perhaps some sort of Planck-level absolute fact, which is irreducible in the same way that the concept of color doesn't exist very much beneath 400 nanometers (violet/near-ultraviolet), since color is only an abstraction of our eyeballs and the language we use to describe our sensations.

> The fallout of this ramification is that, gravity is some sort of ambient side effect of material presence, sort of like a shadow cast, more than an emission radiated.

But then gravity changes would be felt instantaneously rather than propagating at the speed of light.

This was a measurement of the Nordtvedt parameter which characterizes the difference between a gravitational field and an accelerating reference frame. If there is no difference, as GR predicts, then the parameter is 0.

This experiment tests the idea that gravity itself gravitates: gravity imbues very massive bodies with gravitational binding energy, and therefore more inertia. If your inertia is increased due to gravity, do you nevertheless fall at the same rate? i.e. does gravity affect the energy that itself imbues? The conclusion is consistent with "yes."

http://www.scholarpedia.org/article/Nordtvedt_effect

That does not invalidate the concept. Gravity has not been found to be an incidental phenomenon measurable for anything other than matter's accumulation.

In the observable universe (regardless of theories and expressions penned on paper) gravitational forces are always induced by massive material objects, including the remnant artifacts of deceased stars. To suggest otherwise is to walrus the conversation.