# Is G really a constant?

1. Aug 15, 2004

### Gonzolo

Hi, some researchers have wondered whether the G in F(M,m,r)=GMm/rr was a true constant. I believe an associated problem is the motion of stars on the edge of our galaxy (there are either too fast or too slow...). A nonconstant G was then proposed to explaine this motion. This may relate to dark matter too, I 'm not sure. If someone can brief me of the current state this problem, I'd appreciate it.

My related question is more mathematical : What is (are) the proposed alternative(s), for F(M, m, r)? Can we make somekind of an expansion to F so that other terms G2, G3, ...Gn, are introduced? Experimentally (according to fits), what does F look like for these far away stars?

2. Aug 18, 2004

### Dark Knight

G is constant...

because the relationship that relates the force with the mases over the radius squared is constant...and these is noway to incraese the force as you said, since an increase is the force will automatically increase the other side...

3. Aug 19, 2004

### Nereid

Staff Emeritus
Gonzolo, you may be thinking of MOND, an idea which has had remarkable success in matching the rotation curves of galaxies (esp the outer parts). It does not involve a variable G.

There are various cosmological theories which have G varying, over cosmological times and distances, and IIRC a PF member (Garth) has posted in Theory Development with his SCC that involves a varying G.

MOND was developed partly because Milgrom didn't like the consensus view of the rotation curves of galaxies (that view is that they can be accounted for in terms of 'dark matter' - mass which interacts via gravity but nothing else - in the halos of those galaxies). The current state of observations is that dark matter is 'seen' in many places, and that the observations can be accounted for consistently with this dark matter; unfortunately, we are no wiser regarding what the dark matter is!

4. Aug 19, 2004

### Chronos

Not much of a choice. Neither dark matter or MOND are very elegant, just better than the alternatives so far as I know. A non constant G would really be messy.

5. Aug 22, 2004

### Gonzolo

That must be it, thanks. I agree that neither are very elegant, but this dark matter thing is getting on my nerves. I'll check out this MOND. In non-linear optics, the fonction $$P= \chi E$$ is simply expanded in a Taylor's series and all makes sense and is elegant. I thought a similar thing would be possible for F(G,r,r), but I still have much homework to do in the area.

6. Aug 25, 2004

### Garth

In SCC mass also varies in order to 'absorb' gravitational potential energy and to locally conserve energy, (there is a thread on the subject), and cosmologically mass increases expontentially with cosmolgical time.
In both cases in SCC G is inversely proportional to m so GM is constant. All experiments to measure either G or M actually measure GM, to measure G we need to know M (such as the mass of a mountain), and to measure M (such as the mass of the Sun) we need to know G! So these measurements are not able to detect the local or cosmological variation in either G or M.

Garth