Gravity only illusion due to expansion of universe?

[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 that's a while ago :)

Gerenuk

 

[Vanadium 50]

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

 

[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.

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

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

 

[Ich]

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?

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.

 

[ManDay]

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

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

 

[Mentz114]

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

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.

 

[MeJennifer]

Acceleration is obviously not independent of mass.

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

 

[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

 

[Vanadium 50]

Acceleration is obviously not independent of mass.

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

Exactly - and this idea does not predict that.

 

[MeJennifer]

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.

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.

 

[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

 

[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

 

[granpa]

while what has been said about gravity isn't 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.

 

[Mentz114]

... 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.

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.

 

[granpa]

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

independent of mass? that's 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.

 

[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

 

[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.

 

[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.

 

[Mentz114]

granpa:

but fundamentally the forces themselves are similar.

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

 

[MeJennifer]

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

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.

 

[Jonathan Scott]

Another difference between gravity and electromagnetism is that as seen in a typical background coordinate system a static gravitational field changes the momentum but does not change the total energy (so it also changes the rest energy relative to those coordinates), but a static electric field changes the kinetic energy and total energy but doesn't change the rest mass.

However, there's still quite a lot in common between gravity and electromagnetism in the weak field non-relativistic case.

 

[granpa]

does an object in a gravitational field experience a force proportional to its gravitational mass (just like a charge in an electric field experiences a force proportional to its charge) or an acceleration proportional to the gravitational field (as the posters below suggest)? obviously it makes no difference unless gravitational mass is not equal to inertial mass.

if gravity does produce acceleration instead of force then it would be extremely difficult to tell if gravitational mass is different from inertial mass.

one also has to take self inductance into account.

 

[MackBlanch]

granpa,
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

Hmmm. Interesting, in an electric field, the smaller mass will have a greater acceleration, and emit EM waves of a higher frequency. In a gravitational field, they have the same acceleration and will emit the same frequency.

I think a good argument for them being incomparable is that charge is invariant and mass is not. You can increase the mass of an object by increasing its potential energy. If you squash a spring and tie it down, it will have a (quite negligible) mass increase. There is no analogous scenario for charge.

 

[Mentz114]

In a gravitational field, they have the same acceleration and will emit the same frequency.

A freely falling body does not radiate gravitational waves, as far as I know.

This is off-topic in this thread. Would you like to start a new thread in the 'Classical' forum ?

 

[Vanadium 50]

A freely falling body does not radiate gravitational waves, as far as I know.

Why not? The object and the object it's falling towards have a changing mass quadrupole moment. Why wouldn't it radiate?

 

[MackBlanch]

A freely falling body does not radiate gravitational waves, as far as I know.

This is off-topic in this thread. Would you like to start a new thread in the 'Classical' forum ?

I wasn't even touching on gravitational waves. Sorry for the confusion. I was hoping the "EM" would be implied on the second set of waves.

 

[DaveC426913]

the effect of the field is independent of the mass of the body being influenced.

Isn't this more to do with the fact that the greater attraction is perfectly canceled by the greater inertia?

A satellite of mass m will fall to Earth as a = F/m.
A satellite of mass 2m will fall to Earth as a = 2F/2m.

i.e. it's not that the effect is independent of the object's mass, its that the object's mass cancels out of the result.

I realize that this is a completely semantic argument, but it seems to clarify the topic at-hand.

 

[D H]

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.

Isn't this more to do with the fact that the greater attraction is perfectly canceled by the greater inertia?

A satellite of mass m will fall to Earth as a = F/m.
A satellite of mass 2m will fall to Earth as a = 2F/2m.

Both of you are ignoring that gravitation works both ways: object A attracts object B gravitationally, and object B attracts object A gravitationally. Suppose you measure the time for two masses each of mass M to collide after starting at some distance with a relative velocity of zero. If you perform the same experiment but with one of the masses now having mass m<<<M you will get a different (longer) time to collision.

 

[TalonD]

Acceleration is obviously not independent of mass.

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

I dissagree. Drop a cannon ball and a feather off the top of the Eifel tower (in a vacum) and see which hits the ground first.

 

[D H]

The cannon ball will (assuming we are talking about normal-sized cannon balls and normal-sized feathers). The difference in this case is immeasurably small.