Equivalence principle of general relativity holds even at gravitational extremes
The new paper claims to add to the confirmation of &quot;the strong equivalence principle of general relativity.&quot;(0)<p>Reading Wikipedia to find which theories are (or were) the alternatives which assumed that principle not to hold:<p>&quot;The strong equivalence principle&quot;: &quot;The first part is a version of the weak equivalence principle&quot; &quot;The second part is the Einstein equivalence principle (with the same definition of &quot;local&quot;), restated to allow gravitational experiments and self-gravitating bodies.&quot;<p>&quot;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.&quot;<p>&quot;Einstein&#x27;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.&quot; (1)<p>So it seems that &quot;Brans–Dicke theory&quot; and similar are ruled out by this observation, that is that the following line from one Wikipedia page can&#x27;t remain:<p>&quot;At present, both Brans–Dicke theory and general relativity are generally held to be in agreement with observation.&quot;(2)<p>But note that the authors of (0) don&#x27;t claim that the alternative is completely impossible, but that they have measured even bigger space where the &quot;strong equivalence principle&quot; holds, compared to all measured up to now:<p>&quot;our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a <i>factor of ten</i> smaller than that obtained from (weak-field) Solar System tests&quot;(0)<p>That is, of course, a success, as every scientific and measured increase of &quot;known&quot; is. Note that other &quot;Solar System&quot; tests mentioned in (0) are also very new, from this year. Also: the parameter is &quot;a factor of almost a thousand smaller than that obtained from other strong-field tests&quot;!<p>The article from Nature, of course more scientific that the OP on HN (currently: from discovermagazine.com), ends with:<p>&quot;Although the [scalar–tensor] theories are not completely quashed, their hopes for validity have been made that much fainter.&quot;(4)<p>0) <a href="https:&#x2F;&#x2F;www.nature.com&#x2F;articles&#x2F;s41586-018-0265-1" rel="nofollow">https:&#x2F;&#x2F;www.nature.com&#x2F;articles&#x2F;s41586-018-0265-1</a><p>1) <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Equivalence_principle#The_strong_equivalence_principle" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Equivalence_principle#The_stro...</a><p>2) <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Brans–Dicke_theory" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Brans–Dicke_theory</a><p>3) <a href="https:&#x2F;&#x2F;journals.aps.org&#x2F;prd&#x2F;abstract&#x2F;10.1103&#x2F;PhysRevD.48.3436" rel="nofollow">https:&#x2F;&#x2F;journals.aps.org&#x2F;prd&#x2F;abstract&#x2F;10.1103&#x2F;PhysRevD.48.34...</a><p>4) <a href="https:&#x2F;&#x2F;www.nature.com&#x2F;articles&#x2F;d41586-018-05549-4" rel="nofollow">https:&#x2F;&#x2F;www.nature.com&#x2F;articles&#x2F;d41586-018-05549-4</a>