Milgrom's Comet


Mordehi Milgrom in the early 1980’s developed an alternative theory to explain the behavior of galactic rotation curves informally known as Modified Newtonian dynamics, less commonly known as Milgrom’s law.

MoND had been more or less kept on a shelf by the broader scientific community and it’s no secret it fails to explain the large scale structure of the universe, yet it doesn’t go away because it does make very precise predictions which have been shown true by observation, far more accurately in fact than dark matter theory on the galactic scale.

In simple terms Milgrom’s law proposes the force of gravity changes from an inverse square of the distance between two objects into just the inverse of that distance at a point relative to size and speed of the objects involved.

This may seem counter intuitive on the surface as there is no obvious mechanism to change the force of gravity, but this behavior of differing field strengths over distance is not unknown to science. In fact the only analogous energy field in the universe, the far stronger magnetic field, experiences precisely this kind of loss in the near and far field ranges. In the near field power over distance is measured by the inverse of the cube of the distance between two points, in the far field it is measured by the inverse square.

Milgrom basically says gravity does exactly that, and when the equations are run you get perfectly spun spiral and elliptical galaxies. When put in those terms it seems so symmetrical with standard theory to me it’s hard to imagine why there has been such resistance to look at trying to develop an underlying theory unless you know more of the story.

One reason it the idea of Milgrom’s law gathers dust on the shelf is it’s hard to prove. To do that we would need an analogous gravitational system that we could measure the speed of the furthest orbiting objects accurately. For us our best bet is our solar system, and that would mean measuring the speed of Oort cloud objects we can’t yet really see. Maybe one day we can dream up a mission to illuminate and survey these distant objects in the Oort cloud. I believe we would learn much more from that than we would dropping several more tons of ultra pure water down another mineshaft.

We can measure the speed of some Oort cloud objects though, those which sweep into the inner solar system, the comets. Some comets will take trajectories which represent little gravitational interaction to explain their velocities, though initial interactions can’t be ruled out, those velocities can none the less be compared to predictions made using Milgrom on similar bodies on similar long period orbits.

If there is any symmetry at all, whatsoever between those predictions and comet velocities, it would be evidence that maybe MoNd isn’t just a fringe idea, and might very well be on to something after all.

Milgrom’s comet would throw serious doubt on the mysterious and elusive dark matter particle if we found it. If any such symmetry existed between MoNd and the orbital velocities of comets or Oort cloud objects it would be a stunning validation of an idea which for years has clung to outside edges of mainstream thought and debate.

Perhaps it’s time we take another look…


I believe that a fair number of MOND-based hypotheses were killed off by the observations of the recent neutron star merger. Admittedly, there are some that survive, but they appear to be an ever decreasing number. Every test of GR seems to confirm it to very high precision.
This article seems to do a good job of summarising the current state of affairs;


The fundamental assumption that prevents science from taking Milgrom seriously is the idea this needs to replace relatively. It does not, they are complimentary equations which on balance describe the whole behavior of a gravitational field.

The recent studies which attempt to eliminate MoND also rely heavily on fundamental assumptions to draw their conclusions even though the process was both rigorous and scientific the results pointed in a direction but were far from conclusive.

Einstein was limited by the extent of our ability to make observations at the time his theories and ideas were developed. We have no idea how he would have responded to the mystery of galactic rotation curves, but I feel he would have given Milgrom’s ideas which point to a duality of the gravitational field similar to the duality experienced by the magnetic field more weight than a magic particle designed to keep his ideas intact.

The jury is yet very much out…