- #36
TalonD
- 182
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The OP's title of this thread asks if gravity is due to the expansion of the universe. That's something I would be interested in responding to, but I think my post would be removed as being too speculative.
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Oh I see what you're getting at. The cannonball would pull the Earth toward it faster than the feather would, making for a slightly shorter delay before contact. (You'd have to do the two tests sequentially, rather than simultaneously.)TalonD said:What I learned in grade school is that they would both hit the ground at the same time without any air resistance. Remember Galileo's famous experiment? Even the astronauts demonstrated it on the moon. But now that I'm all grown up with a higher education, it suddenly occurs to me after reading the previous post that the cannonball would hit first. And I am assuming it is because the cannonball would have it's own gravitational field which is stronger than the gravitational field of the feather, is that right?
DaveC426913 said:Oh I see what you're getting at. The cannonball would pull the Earth toward it faster than the feather would, making for a slightly shorter delay before contact. (You'd have to do the two tests sequentially, rather than simultaneously.)
Yes. This might be easier to visualize of you substituted a moon for a cannonball.
In an "Earth-feather system", your total mass (and thus your total gravitational attraction) is equal to 1 Earth + 1 feather.
In an "Earth-moon system", your total mass (and thus your total gravitational attraction) is equal to 1 Earth + 1 moon.
It becomes intuitively obvious now that the Earth-Moon system should make contact in less time.
but you seem to have gone most of the way towards addressing it.Metz said:In your example, the influence of body A on body B depends only on A's mass, and vice-versa.
yogi said: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
Gerenuk said: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?
Vanadium 50 said:This is not possible, as it predicts attraction to be independent of mass.
Gerenuk said: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?
atyy said:How about construing "expansion of space" more broadly as the "metric"? Then given a metric, the field equations (and equations of state) give the stress-energy-momentum distribution.
...large velocities affect masses differently from electric charges. Whereas a bodies electric charge has the same value for all observers, it's mass depends on its speed relative to the observer...Because the magnitudes of the sources of gravitation depend so much on the frame of reference in which they are measured, the resulting field is bound to be more complex than the electromagnetic field...Einstein concluded the gravitational field was probably a...tensor field...
Though charge density is not Lorentz invariant....large velocities affect masses differently from electric charges. Whereas a bodies electric charge has the same value for all observers,...
Any metric is written in a specific coordinate system and represents the underlying spacetime geometry as described by that specific coordinate system. You can transform the metric to another coordinate system and get a different "reference frame" that describes the same spacetime. Neither is preferred or in any way describes a different spacetime.Phrak said:The Friedmann-Lemaître-Robertson-Walker (FLRW) metric
[tex]\Large{c^2 d\tau^2 = c^2dt^2 - A(t)^2 d \Sigma^2}[/tex]
This looks suspiciously as if it gives preferencial treatment to particular inertial frames, nominally at rest with the cosmic background radiation, perhaps. Does anyone know?
DaleSpam said:Out of curiosity, since the universe is expanding isotropically how could that be the explanation for gravity which points down rather than being isotropic?
DaleSpam said:Any metric is written in a specific coordinate system and represents the underlying spacetime geometry as described by that specific coordinate system. You can transform the metric to another coordinate system and get a different "reference frame" that describes the same spacetime. Neither is preferred or in any way describes a different spacetime.
DaveC426913 said: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.
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Sounds like a traditional "push gravity".neopolitan said:Expansion of the universe may be isotropic (specifically in all directions), but it is not smooth. ... If concentrations of mass-energy did resist expansion, then gravity would then "point" along a line of increasing concentration, ie towards the (other) mass.
This kind of arbitrary function is fairly common and allows you to easily define a family of coordinate systems simply by choosing different functions. http://arxiv.org/abs/gr-qc/0311038" of the same thing for the Schwarzschild spacetime. In this paper the free parameter is a function of the radius instead of time, but it amounts to the same thing.Phrak said:It's the appearance of the A(t) term that looks questionable, as if space has a preferred coordinate system in which it expands isotropically. I suppose it depends upon what motivates its inclusion. Is it ad hoc, to explain the expansion of the Universe; does it require a cosmological constant added to the Einstein tensor?
DaleSpam said:Sounds like a traditional "push gravity".
Now it sounds like GR.neopolitan said:What I described was not at all like Le Sage's push gravity, which seems to introduce more forces. ... I was thinking of whether the force could be an illusion in entirety, not replaced by another, rather more counter intuitive force.
GR already describes that well.neopolitan said:For me the first step is to realize that the universe is expanding, but not all of it ... why is that?
DaleSpam said:I just fail to see (1) how this idea relates to established theories (2) what the motivation for this idea is.neopolitan said:For me the first step is to realize that the universe is expanding, but not all of it ... why is that?
Then in a following paragraph I said:Is it so unreasonable to ponder what would have happened if Hubble's work had come earlier than Michelson and Morley, and we knew that the universe was expanding but not that there was something odd about some of our late 19th century assumptions which included aether?
it might be worth the effort if we get the whole picture - well, maybe more of the picture or a different perspective on the same piece of the picture we already have