Can We Measure The External Field Effect On Earth?

Color me skeptical. 

The External Field Effect can be understood as basically a swamping of a second order MOND effect by first order Newtonian effects, so I'm very doubtful that it could be measured experimentally in the solar system anywhere near Earth. Also, a 0.1 fm precision measurement, i.e. a fraction of the size of a proton or neutron, starts to run into definitional issues about what the object your measuring even is due to quantum mechanics and the parton makeup of hadrons. And, the uncertainty regarding the functional form of a MOND interpolating function further muddies the waters and makes any measurement model dependent.

Despite compelling evidence, the absence of a confirmed dark matter particle has sustained interest in modified gravity as an alternative explanation for the observed phenomenology. One prominent example is Modified Newtonian Dynamics (MOND), which predicts that the internal dynamics of a system depends on the external gravitational field in which it is embedded. This so-called External Field Effect violates the strong equivalence principle (SEP) and is absent in canonical mechanics, making it a promising avenue for experimental tests of modified gravity. 

Motivated by this, we investigate the dynamics of two spherical masses arranged such that their symmetry axis is either parallel or orthogonal to the local gravitational field. We derive solutions describing the internal dynamics of such systems in both strong uniform and radial external fields. In particular, for a radial external field, if the non-relativistic gravitational field is free to have non-vanishing curl, we find that the mutual attraction of the masses in the perpendicular configuration is not strictly aligned with their symmetry axis. It acquires a small transverse component, even when the external gravitational field is everywhere balanced by non-gravitational forces. 

Using these solutions, we determine the spatial and temporal sensitivities required to distinguish the two configurations and systematically assess experimentally relevant effects, including air drag, object size, and surface interactions. As an example, detecting the prediction of the simple MOND interpolating function requires a spatial sensitivity of order 0.1 fm for sub-millimeter masses evolving over approximately 30 minutes. Such times may be achievable with levitated particles or in space-based environments. Experiments operating at lower resolutions are also interesting as independent tests of SEP and place constraints on modified-gravity theories.

Ankit Kumar, et al., "Probing the Strong Equivalence Principle through the External Field Effect. How Do Two Masses Fall?" arXiv:2607.10247 (July 11, 2026).

A Central Black Hole's Rotation Direction Doesn't Determine A Galaxy's Rotation Direction

While I didn't have strong expectations one way or the other, this paper's conclusion is potentially important to understanding galaxy formation, and tends to disfavor a purely accretion hypothesis. 

In contrast, the fairly strong correlation between central black hole size and galaxy size tends to argue for a very important role of central black holes in the galaxy formation process, because black holes only make up ca. 1% of a galaxy's mass.

The paper also makes notable observations about other aspects of spiral galaxy geometry.

We compare the apparent directions of rotation in the plane of the sky of active galactic nuclei (AGNs) and their host galaxies. The direction of rotation of the galaxy was inferred from the direction of the spiral arms, while the direction of rotation of the AGN was inferred from spectropolarimetry, where the change in relative polarization position angle (PA) across broad lines is believed to be caused by equatorial scattering. The numbers of co-rotating and counter-rotating AGNs are equal. 

Studies of the relative position angles of radio jets have implied that there is a "zone of avoidance" where jets avoid being in the plane of disk galaxies. We point out that bi-conical narrow-line-region outflows also avoid the plane of the host galaxy. 

The equal numbers of co-rotating and counter-rotating AGNs exclude the hypothesis that the "zone of avoidance" is due to a lack of large tilts of the black hole rotating axis relative to the host galaxy rotation axis. Our results imply that the relative orientations of spin axes are random, at least for the black hole mass range we consider. 

We propose that changes in the broad-line polarization PA with wavelength that do not closely follow the predictions of the simple equatorial scattering model are a consequence of the scattering dust being clumpy. We note a couple of cases of possible changes in PA over several years, which, if real, could be due to motions of the dust clumps or changing anisotropy of the continuum emission.

Loren Gigi, C. Martin Gaskell, "The direction of rotation of supermassive black holes is unrelated to the direction of rotation of the host galaxy" arXiv:2607.06902 (July 8, 2026).