# How do we know that gravity is an attractive force?

1. Dec 12, 2007

### Meatbot

Couldn't the same effects be seen if the vacuum was anti-gravitational and mass shielded it? Then the vacuum would push masses into the gravitational holes generated by the shielding. How would you tell the difference between that and the current idea, if you even could?

2. Dec 12, 2007

### Chris Hillman

How do we know that gravity is an attractive force?

Not be flippant, but: drop something and watch how it moves wrt the most massive object in your immediate neighborhood.

If you want a serious answer, you should say what theory you are asking about. For example, in gtr, the Raychaudhuri formula is what you want. And what do you mean by "suppose the vacuum is anti-gravitational"? What do you mean by "mass-shielding"?

3. Dec 12, 2007

### Meatbot

-- I mean how do we know that the dropped item isn't pushed to the ground instead of pulled?

-- Suppose that vacuum repels mass (and itself?).

-- Mass would negate the vacuum anti-gravity. The more mass, the less the vacuum would repel. This would generate areas around mass where the antigravity was less. Then mass would be pushed into other mass instead of attracted to it.

OR something like that. I can't quite get my mind around it totally but maybe you can see what I'm getting at. Basically how do you make gravity a push instead of a pull and still reproduce what we see around us. I think this is related to the idea of a universe filled with liquid where the bubbles would move toward each other.

Last edited: Dec 12, 2007
4. Dec 12, 2007

### p4h

Not sure if I'm qualified to answer, but the above would not make sense in my head. If matter is pushed to the ground, then it's not earth's gravitational field but something outside of earth. And if this is the case, then there'd have to be even amounts of force pushing all around earth, as we don't have shifts in gravity. By this you have already falsified your statement, as equal force to push on all sides of earth, would make it impossible for the moon for example, to have less gravity than us.. Not sure if I understood it correctly though.

5. Dec 12, 2007

### Integral

Staff Emeritus
Wow, you might be on to something.

Now all you need do is create a mathematical model based on your idea. Of course your model will have to be at least as successful as even Newtons gravitation and should be able to match GTR also.

Let us know when you are done.

6. Dec 12, 2007

### Meatbot

--Right, I'm asking what would happen if it was the vacuum that was pushing.

-- I'm saying that the moon would create a smaller "hole" in the vacuum antigravity than the earth would. So things wouldn't be pushed into it as easily. Basically, the vacuum anti-gravity woud have a HARDER time pushing things into the moon because you would have more anti-gravity between the two objects than you would have between the object and the earth.

7. Dec 12, 2007

### Meatbot

-- I'm not saying I think this is true. I have no idea. I'm acknowledging that most of you you know more about it than me. I want you to tell me why this scenario doesn't work. No need to get an attitude. Plus, I don't know the math well enough to create a model. That's for you guys to handle.

Last edited: Dec 12, 2007
8. Dec 12, 2007

### p4h

Well that CAN make sense, but I'm not totally sure how you'd explain our solar system then?

9. Dec 12, 2007

### Meatbot

---I'm not either, other than that I don't see how you'd be able to tell the difference. Wouldn't things operate exactly the same way? I don't know....

But if this is true, then a black hole does not have infinite gravity at the singularity, instead it has 100% vacuum anti-gravity shielding - a finite amount. Isn't that a useful idea to consider?

10. Dec 12, 2007

### p4h

Well again, I'm not sure I'm qualified to debate this, but it just goes against some core things in my head.

How would the Earth get affected by the Sun, if it was not the Sun that made the gravity?

11. Dec 12, 2007

### Meatbot

Because in the direction of the sun there is less vacuum anti-gravity (because the sun is shielding it). That means that there is more anti-gravity in the direction away from the sun. The imbalance would push you toward the sun. I would think you'd get a orbit just like you would with current theory. It's pushing you around a hole essentially. Anyone see where this is wrong?

12. Dec 12, 2007

### p4h

Good arguement, though would it still produce the same effects if you look at the curvature of space around a planet?

13. Dec 12, 2007

### Astronuc

Staff Emeritus
Well then, why does a vacuum chamber collapse, if poorly constructed, with a vacuum on the inside and atmospheric pressure on the outside?

If the vacuum was 'pushing' from the outside, then the atmospheric pressure would not decrease with elevation or height from the earth's surface.

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14. Dec 12, 2007

### geoffc

Did anyone mention that it might have something to do with a spin 2 particle?

15. Dec 12, 2007

### Dr. Keith

That is a very interesting idea you have there Meatbot. I can see how that would work in some cases.

However, not only would it be hard to explain why a vacuum would collapes, but how does it account for the density of the object?

for example, 2 objects can have the exact same volume which would shield the exact same amount of gravity if your theory was correct. But, we observe that these 2 objects can have very different densities and will result in very different gravitational attractions.

Black holes for instance.

16. Dec 12, 2007

### pervect

Staff Emeritus
It sounds to me like you are proposing what's known as Le Sage's theory of gravitation. This has well known problems in that the theory predicts drag which does not occcur.

Feynman, in particular, looked at this sort of theory. From Feynman as quoted by the Wikipedia:

Feynman, R. P. (1967), The Character of Physical Law, The 1964 Messenger Lectures, pp. 37-39, ISBN 0-262-56003-8

Last edited: Dec 12, 2007