# A few questions about gravity

1. Jul 8, 2012

### AustinJones

please if your going to answer my questions with a formula explain it not just
something = z+r2

F=ma, true but i understand it better explained like; force equals the measured mass of an object multiplied by its rate of acceleration.

anyways, why are gravitational fields infinite?
How do we know they are infinite?

2. Jul 9, 2012

### Muphrid

Infinite in what way?

3. Jul 9, 2012

### K^2

I'm going to assume you mean infinite range. In classical gravity, gravitational field line can only start or terminate on a mass. So gravitational field will always extend out to infinity. This works because classical gravity is a low-energy approximation to General Relativity. And we don't really know why General Relativity holds. It's either just how the world works, or it's an approximation to an even more complex theory. But then we wouldn't know why the later holds either.

Anyways, we can't test this directly, of course. We cannot measure gravity from an object that's on the far side of the universe from here. But we can test whether the theory itself holds, and then we make an assumption that it holds at any range.

4. Jul 9, 2012

### AustinJones

Yes, the range of the gravitational field is by modern physics considered infinite.

I'm not saying its wrong i'm just saying i personally without a education in physics, don't like that concept.

Over time we have developed a lot of things to try and understand gravity
a gravitational constant, a pattern gravity fallows 1/r^2, theories like gravitons.
I feel like were missing something very simple which makes our calculations wrong in the big end results.

5. Jul 9, 2012

### K^2

While it's possible that long-range interaction somehow behaves differently, we have no good indication of that. On the contrary, we have very strong indication that General Relativity works. It gives extremely accurate predictions for certain phenomena. I don't want to discourage anyone from looking for alternative explanations, but they would need to make testable predictions that are distinct from GR, as well as explain why tested predictions of GR work so well.

Anything we know can be wrong. In fact, it's probably wrong. But ultimately, what we care about are predictions. If a theory makes predictions that work, we like that theory, and we are going to treat it as "this is how things work," even in situations where we know otherwise.

6. Jul 9, 2012

### K^2

While it's possible that long-range interaction somehow behaves differently, we have no good indication of that. On the contrary, we have very strong indication that General Relativity works. It gives extremely accurate predictions for certain phenomena. I don't want to discourage anyone from looking for alternative explanations, but they would need to make testable predictions that are distinct from GR, as well as explain why tested predictions of GR work so well.

Anything we know can be wrong. In fact, it's probably wrong. But ultimately, what we care about are predictions. If a theory makes predictions that work, we like that theory, and we are going to treat it as "this is how things work," even in situations where we know otherwise.

7. Jul 9, 2012

### Drakkith

Staff Emeritus
It isn't that we KNOW the range is infinite, it is that our observations have given us the means to develop theories and models that describe how gravity behaves. These models and theories tell us that the force of gravity falls off at a certain rate but never reaches zero. In the end I don't really see a big difference between being infinite and having a finite range. A finite range would certainly be so far away that it just doesn't matter. Consider that the center of the Virgo Galaxy Cluster is approximately 54 million light years away yet it's gravity affects our own galaxy and slows our recession down. So the range is already greater than 54 Mly, as part of the cluster is beyond that distance. Given the nature of cosmological models that describe the evolution of the universe over time it may matter if the range is significantly less than the diameter of the observable universe, but that's about the only thing I can think of where it would really matter.