Viewing a single comment thread. View all comments

headlessplatter t1_j2xhjm7 wrote

Could Modified Newtonian Dynamics (MOND) be tested by measuring the gravitational effects of Jupiter on our moon? As I understand, Newtonian Dynamics suggests the force of gravity decays by the square of distance, but MOND says it decays somewhat more slowly over great distances. Since our moon is rather reflective, it seems to me we could probably bounce a laser off of it, or if necessary even place a mirror on the moon to facilitate this test. Then, I imagine we could measure its distance with very high precision. If this is feasible, then I imagine we might be able to detect the subtle gravitational effects of other planets on our moon as their distance to our moon changes. (I don't really believe this would work, or else astronomers would probably have tried it, but I'd be curious to know which aspect of this proposed experiment renders it completely infeasible.)

2

mfb- t1_j2xqps2 wrote

The Moon is about as reflective as asphalt, but various missions have left retroreflectors at their landing sites which are routinely targeted with lasers. They can measure the distance with a precision of a centimeter or so, depending on the experiment and targeted retroreflector.

Jupiter, Venus, Mars and Saturn and sometimes even larger asteroids need to be considered in these experiments, see e.g. this publication for a discussion. It's an extremely precise test of our understanding of gravity and the Solar System. Unfortunately it doesn't test MOND directly because a small acceleration from one specific object isn't enough (otherwise you could just say Earth is a billion objects providing a small acceleration each), you need a small acceleration overall. The Moon is not in such a place.

3