# Are we push or attracted to a planet?

1. Jun 21, 2010

### fcycles

Last year while thinking about it, I thought it will make more sense if we were pushed on our planet rather than being attracted to it. Mathematically, there is no change made to the current equation by reversing the force vector's direction!

While I am read "Reinventing Gravity" from John W. Moffat. I learn that Mr. Georges-Louis Le Sage in 1758 was proposing particules could exert pressure on a body. However, they don't mention why the idea was later consider to be wrong by Maxwell and Poincaré?

So, I am still not convince of the direction of the force? If we are push or attracted on Earth.

If I consider gravity has a space deformation, maybe my question make no sense?

Anyone could clarify it?

2. Jun 21, 2010

### Staff: Mentor

It is difficult, if not impossible, to make a "push" gravity model that is actually consistent with observation.

3. Jun 21, 2010

### K^2

It's all relative. In a coordinate system attached to Earth surface, there is a force of Gravity that pulls everything down. It is a fictitious force, however, resulting from non-inertial coordinate system choice.

In any inertial system, yes, it is Earth's surface that's accelerating outwards, pushing you up, rather than anything pulling you down.

4. Jun 22, 2010

### lilphil1989

Is this true?

Imagine a planet that is neither rotating, nor is it's centre of mass accelerating. Then the frame "attached" to the planetary surface WILL be inertial, but the downward gravitational force surely still exists?

5. Jun 22, 2010

### Staff: Mentor

Le Sage's theory of gravitation (Wikipedia)

6. Jun 22, 2010

### sophiecentaur

Can there be any doubt about the direction of the force? It must be towards the Earth. You either have to account for this force by the presence of the Earth and its mass (a pull force) or by the 'absence' of the Earth in every other direction and, somehow, that 'absence' produces a push force.
I know which description I'd put my money on.

Furthermore, the closer you are to the Earth, the greater the force - despite that there is less 'no Earth' around you.

7. Jun 22, 2010

### fcycles

Beeing closer to an object and seeing a force increase does not proof that the source of that force came from the object.

If you think that gravity has many sources caming from different directions. And that force react to matter in a way that it get reduce very slightly after traversing it. Then, considering a force on top of your head and one down below your feet (after passing throught the Earth), the result will be that you will be push down the Earth.

And if you think that sources in the universe are places in a way that they are not symetrical. That was my first guess to invalidate my hypothesis, you might argue that we will see things moving into one direction? Well, no... because it is relative and we are inside that system. We see things moving according to our position in space.

What maked me think this make more sense than having a force pulling you on Earth are:
- Gravity is the weakest force by a large margin;
- If we consider the source of gravity is the Earth and that gravitron is pulling object into the source... I cannot see how particules going into a direction will make an object move into the opposite direction (but rather the direction should follow);
- When we observe galaxy rotating, their speed does not match our current model;
- Base on the current physic models, there is an important missing mass in the universe;
- Everybody seem to take for granted that we are pulled into Earth (I doubt of things which are taked for granted without any proof, hehe!).

That was some clarification I wanted to add to my first post, just to explain better what I was thinking. But, in itself my thought does not convince me of anything... except that now I cannot say if we are pull or push on Earth.

K^2 bring an interesting statement... but I think like lilphil1989 that you will still move into the direction of the massive body. Being pull or push...? ;)

ps: I found the discussion really interesting... thank you all for your posts and I hope we can continue talking about it!

8. Jun 22, 2010

### sophiecentaur

I have to admit that my position on this is rather 'entrenched' but I always try to avoid the question "what is it really?". Things behave as they behave and some models can be used to make reliable predictions while others will let you down.
The 'attracted to' model seems by far the simpler model so that's what I will go with. What is 'really is' is, I think a meaningless concept.

9. Jun 22, 2010

Staff Emeritus
It's not your hypothesis. It's Nicolas Fatio's, and it's 380 years old. It's also, as the Wikipedia article points out, completely inconsistent with observation.

10. Jun 22, 2010

### K^2

No it's not. Surface of massive body is inherently non-inertial due to curvature in the space-time. That's where gravity comes from.

Gravity is always a fictitious force. If you have gravity, you have an accelerating frame of reference. And vice versa, if you found a locally inertial frame of reference, there is no gravity.

If you compute acceleration at Earth's surface, $a = \Nabla_u u$ for $u = (u_t, 0, 0, 0)$, you will get 9.8m/s² out away from center. Earth's surface is accelerating outwards.

11. Jun 22, 2010

### DrGreg

Just in case lilphil1989 is confused, K^2 is talking about Einstein's General Theory of Relativity. According to Newton's Law of Gravity, lilphil1989 is correct, but Einstein's theory, which superseded it, takes a different view: the inertial objects are the ones that are falling freely under gravity.

12. Sep 13, 2010

### Barwick

Sorry to dig this one out of the depths, but how exactly is a push theory of gravity "completely inconsistent with observation"?

There are a lot of things out there that would be explained by a push theory of gravity.

Similarly, pull gravity: A completely undiscovered force field, magically generated by "mass" of an object, that pulls on an object at a distance through vacuum, without any known interaction between those two objects. It seems ridiculous to me to claim that gravity "must" be a pull interaction between two objects.

13. Sep 13, 2010

Staff Emeritus

14. Sep 13, 2010

### pallidin

That's easy. Show me even ONE observation that indicates otherwise.

15. Sep 14, 2010

### Barwick

I may be too dumb to understand because I read that jtbell linked, and couldn't find anything where it was "completely inconsistent with observation". That's very very possible, I'm not exactly a PhD when it comes to physics, and it's been so long that I probably couldn't do the simple math to derive functions for mechanical interaction of bodies without referencing Google.

On the other hand, there have been experiments where neutrons were isolated from all outside sources of energy, except for obviously gravity. They observed their behavior and the neutrons didn't accelerate at constant velocities, but rather in discrete jumps.

A pull type of gravity would result in a constant, uniform movement of mass in the direction of the other mass.

A push type of gravity (be it from a particle or photons) would have a probability of coming into contact with the mass being "affected" by gravity (the one being observed), and thus result in discrete changes in velocity, rather than constant changes.

16. Sep 14, 2010

### pallidin

There is no such thing as "push gravity"
Doesn't exist.
It would be an oxymoron on conceptual terms.

Perhaps those whom wish to believe in it believe also in a 9/11 conspiracy. Who knows.

17. Sep 15, 2010

### Barwick

Ok, not to turn this into an argument, but now we have a proof by hand waving, and name calling?

18. Sep 15, 2010

### pallidin

Yeah, my bad. I'm usually more professional. Sorry.
Anyway, for any theory that counters accepted theory, one must provide "acceptable" evidence that the new theory is worthy of further thought.

What would be interesting is if you could use your undoubtedly creative mind to design an experiment that supports this theory. Not that you have to build it, as it might be prohibitively expensive for an individual, but properly propose it for others to MAYBE take it up.

19. Sep 15, 2010

### brainstorm

what is the point of threads like this? anyone who posts anything that considers a model of gravity that relies on pushing instead of pulling will be accused of speculation and disciplined according to forum rules.

20. Sep 15, 2010

### cesiumfrog

That Le Sage theory is really cute. Wonder if virtual particle mediated attraction can be re-expressed in this kind of way..

21. Sep 15, 2010

### planck42

The reason that gravity doesn't push us away from Earth is due to the anthropic principle; if it did, we couldn't exist!

22. Sep 15, 2010

### cr45

I was trying to think of a single sentence that could somewhat explain gravity to a layman. Anyone have any suggestions? This is the best I could come up with:

The bigger something is, the more likely another thing will fall towards it.

23. Sep 16, 2010

### Drakkith

Staff Emeritus
I'd say its more like "The bigger something is, the faster another thing will fall towards it."

24. Sep 16, 2010

### cesiumfrog

The more massive something is, the more it pulls on everything else?

25. Sep 16, 2010

### Drakkith

Staff Emeritus
Yes. Something like a black hole exerts an enormous pull on everything around it, while something like the moon is so less massive that it only causes us to weigh 1/6th of what we normally do on earth.

Also, the denser something is, the stronger its gravity is. If i compacted the earth into the size of a baseball, the gravity at say 1 foot away from it would be enormous. Way more than what we have on the surface of the earth now. But if i were in orbit of the earth when it is compressed, i wouldn't notice a difference in the gravitational pull.

This is how a black hole can form out of supermassive stars. Once the star cannot fuse any more elements at its core, there is no more outward pressure to counteract gravity. The rest of the star starts to collapse inwards towards the core and in a complicated series of events, the star explodes in a supernova and leaves behind a very very dense core of material in the form of a black hole. Theoretically i could compress the earth so much that it would form a black hole if i had the means.