# Gravity only illusion due to expansion of universe?

1. Oct 2, 2008

### Gerenuk

Hi,

I read some earlier threads, but didn't quite understand them. But they gave me an idea:

Maybe there is not gravitational force, but just the space expands in a way as to create the illusion of an attractive force, i.e. things accelerating towards each other?

Are there such ideas?
I once knew the very basics of GR, but thats a while ago :)

Gerenuk

2. Oct 2, 2008

### Vanadium 50

Staff Emeritus
This is not possible, as it predicts attraction to be independent of mass.

3. Oct 2, 2008

### Mentz114

I just fell down the stairs and my broken leg is not an illusion ! ( Oh no, sorry, I merely had an illusion that I fell down the stairs. I'm fine now).

There are models of gravity (on the fringes of accepted physics) that say this.

I thought gravitational acceleration is independent of mass as in the equivalence principle.

4. Oct 2, 2008

### Ich

There are such ideas, usually inconsistent crackpottery. There is one exception, GR, which states that a stationary observer near a mass actually is accelerating outwards. This idea is formulated consistently in 4 dimensions. Acceleration is defined as a local concept and does not necessarily imply changing distances.

5. Oct 2, 2008

### ManDay

He said attraction, not acceleration (I admit both starts with an A and ends in an N, though).

6. Oct 2, 2008

### Mentz114

I don't see the difference in this context. Attraction=force which causes acceleration, and they are independent of mass in for the gravitational field.

7. Oct 2, 2008

### MeJennifer

Acceleration is obviously not independent of mass.

Two heavy masses will come together quicker than two light masses.

8. Oct 2, 2008

### Mentz114

MJ,
the strength of the field depends on the mass of the source. The acceleration imparted by the field is independent of mass, and depends only on field strength.

If Vanadium 50 is talking about gravitational collapse, let him say so.

M

9. Oct 2, 2008

### Vanadium 50

Staff Emeritus
Exactly - and this idea does not predict that.

10. Oct 2, 2008

### MeJennifer

Experiment 1

Take two equal masses of mass M in empty space one meter apart and time how long it takes before they come together.

Experiment 2

Take two equal masses of mass M/2 in empty space one meter apart and time how long it takes before they come together.

Are you suggesting that both times will be the same? If so you are completely wrong.

11. Oct 2, 2008

### Mentz114

OK, Vanadium, you didn't make that clear in your first post.

MeJennifer, I repeat

"the strength of the field depends on the mass of the source. The acceleration imparted by the field is independent of mass, and depends only on field strength."

In your example, the influence of body A on body B depends only on A's mass, and vice-versa.

Your example in no way contradicts that.

M

12. Oct 4, 2008

### yogi

you can arrive at a plausible value for the gravitational constant based upon expansion of the Hubble sphere. For simplicity, take the Hubble sphere as dilating at a constant radial rate c so dV/dt = 4c(pi)R^2 and therefore the volumetic acceleration d^2V/dt^2 for constant radial dilation is 8(pi)(c^2)R ...now apply Gausses' theorem to make a volume to surface transformation ...this leads to division by the effective area which for a sphere is 4(pi)R^2 and therefore the isotropic acceleration is 2(c^2)/R

Multiply by the inertia to get the gravitational force

13. Oct 4, 2008

### granpa

while what has been said about gravity isnt exactly wrong nevertheless there is no reason to see gravity as being any different from other forces. replace mass with change and reverse the direction of the resulting force and you simply have the electric field. (excluding relativistic effects like time dilation of course.

14. Oct 5, 2008

### Mentz114

I have to disagree with that. Attempts to make gravity theories analogous with EM founder because gravitational 'forces' are independent of mass, unlike EM, where charge and mass are not the same thing. The acceleration of a charged particle in an electric field is F/m, but the acceleration from the gravitational field does not depend on m, only field stringth.

15. Oct 5, 2008

### granpa

replace mass with charge and reverse the direction of the resulting force

independent of mass? thats like saying that the electric force is independent of charge.

the gravitational force is proportional to m1*m2 in exactly the same way that the electric force is proportional to q1*q2

and the gravitational field of an object is proportional to its mass just as an electric field is proportional to its charge.

the biggest difference is that masses attract while like changes repel.

the fact that you have an additional mass term in your electric interactions no doubt makes the math very different but fundamentally the forces themselves are similar.

Last edited: Oct 5, 2008
16. Oct 5, 2008

### Mentz114

granpa,
I don't agree with that. Gravity is nothing like the electric field. Consider this - two different masses with the same charge move in an electric field. They will have different accelerations. Two different masses moving in a gravitational field will have identical accelerations. That is why I say that the effect of the field is independent of the mass of the body being influenced.

Also, there is only one sign on the 'mass charge'.

This is sufficient to indicate that the two fields are very different in nature ( and I haven't even gone in the quantum side of things).

M

17. Oct 5, 2008

### granpa

nothing that you just said contradicts anything I said an post 15. yet you come to the opposite conclusion. i think we will just have to disagree. have a nice day.

18. Oct 5, 2008

### malawi_glenn

Look at how different the field equations for gravity and EM are granpa, there are more things that they don't have in common then what they do have in common.

19. Oct 5, 2008

### Mentz114

granpa:
I suppose if you persist in thinking of gravity as the Newtonian theory only, it makes no difference. Seriously, you're onto a loser if you think the electric field is fundementally like the gravitational field.

Here's another one - how do you define free-fall in an electric field ? You can't. There is no transformation that takes away the force, unlike gravity, which is not a force.

Nice day to you too.

M

20. Oct 5, 2008

### MeJennifer

General relativity, unlike Newtonian gravity, is a non-linear theory.

You cannot isolate the influence of A on B and vice versa. Both A and B influence spacetime. Only the combined influence determines the dynamics of A with respect to B and vice versa.

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