Hacker News Reader

Comment by twothreeone | Hacker News Reader

GR says spacetime is curved by mass, right. So what's the basis for explaining the curvature of space (which can be measured, e.g., LIGO) in MOND?

View on HN

MOND has nothing to say about the curvature of spacetime, since MOND is Newtonian (MOdified Newtonian Dynamics). It goes back to "F=ma and gravity is a force" and modifies the rules so that gravity grows weaker faster at a certain scale.

The fact that MOND fits a lot of the data troubled cosmologists, because they know that a General Relativistic theory is needed to explain pretty much the rest of gravity.

TeVeS is an extension to General Relativity that reduces to MOND in the non-relativistic limit. For comparison, General Relativity reduces to Newtonian gravity in the non-relativistic limit. The non-relativistic limit is when speeds and spacetime curvature are small.

How does MOND deal with the effects of time dilation and length contraction? Do we have to go back to Newton's time where there's a universal time?

I don't know if Newtonian gravity can be reconciled with Special Relativity. First thought is "no, that's why Einstein arrived at General Relativity." But I'm not in the field, so I don't know.

MOND is an effective theory, it only describes observations and doesn't put forward any explanation of what's really going on.

> GR says spacetime is curved by mass, right.

Wrong. GR says that gravitation can be modeled as acceleration.

General Relativity states that mass-energy curves spacetime, and objects follow the straightest possible paths (geodesics) through this curved geometry. The equivalence principle relates gravity and acceleration, but it's not the main description of gravity in GR.

Spacetime is model.

Reply on news.ycombinator.com