Hypothesis Overview

Modified Newtonian Dynamics (MOND) is a hypothesis advanced by Mordehai Milgrom (Milgrom, 1993) in order to explain the anomalous rotation of spiral galaxies. Many such galaxies do not appear to obey Newtons’s law of gravitation attraction, which in this case is nearly identical with Einstein’s law of gravitation (General Relativity, GR) applied to slowly moving bodies in weak fields. Milgrom proposed a change to Newton’s law, wherein the force of gravity is stronger than would otherwise be expected at low acceleration, which correctly models the rotation, effectively explaining the anomalies. More detailed observational support for the hypothesis comes from Ricardo Scarpa (Scarpa, 2006) at the European Southern Observatory. The MOND hypothesis has since been integrated into a modified form of General Relativity by Jacob Bekenstein (Bekenstein, 2004) called TeVeS (Tensor Vector Scalar).

The existence of cold dark matter (CDM) is a competing hypothesis designed to explain the same anomalies. Although the cold dark matter hypothesis is presently favored, the issue has not yet been decided (see for example Angus et al, 2007). MOND is the more parsimonious of the two, since it has only a single universal constant unknown, namely the threshold acceleration at which the force of attraction departs from that predicted by Newton’s law. By contrast, the distribution of proposed dark matter must be tailored ‐ `fine‐tuned’ (Bekenstein, 2004) ‐ to fit the observation in each individual case, e.g. for each anomalous galaxy.

Experiment Overview

The Cavendish balance being used at ETI to confirm or deny the hypothesis is designed to measure tiny forces, small enough to be in the MOND regime, if it exists. If the MOND hypothesis is true, we expect the balance to behave according to when the forces are sufficiently weak, and according to otherwise. If the MOND hypothesis is not true we expect it to behave according to at all times, for all forces.

If MOND is ‘true’, the consequences will be profound. Two of the biggest are:

• Dark matter may then turn out to be an unnecessary hypothesis, radically changing our view of the evolution of the universe.
  
• For various reasons, confirmation of MOND increases the likelihood that gravity is the result of an EM-type force.