# Gravity only illusion due to expansion of universe?

by Gerenuk
Tags: expansion, gravity, illusion, universe
P: 15,294
 Quote by TalonD 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 gravitional field of the feather, is that right?
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.
P: 645
 Quote by DaveC426913 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.
I wanted to reply to Metz' earlier comment:
 Quote by Metz In your example, the influence of body A on body B depends only on A's mass, and vice-versa.
but you seem to have gone most of the way towards addressing it.

The influence is, of course, related to the mass of the total system. We usually are talking about systems like "Earth" and "feather" where M>>m and we can approximate and simplify by saying (M+m)=M.

The question then arises, is the OP's original question related to crackpottery if we consider the mass of the total system, ie the whole universe, rather than just considering local (and open) subsystems. Naturally, we would need to carefully consider what we would treat a mass and I would suggest that we look at a mass as a concentration of energy (or concentration of mass-energy, if you prefer) rather than a point mass notionally located at a body's centre. A body's concentration of mass-energy falls away as you move away from that body (the rate at which the concentration falls away is easily calculated).

If you can visualise the effects of two concentrations of mass-energy in an expanding universe, accept that concentrations of mass-energy would resist expansion in proportion to the concentration of mass-energy (if they didn't the gravitational "illusion" would never eventuate) and do the sums, you will find that the effects would be the same as gravity (and G would be related to a coefficient of "expansion resistance" which would in turn be inversely related to the speed of light squared. If planck units are used, the coefficient of "expansion resistance" resolves back to unity).

The equations are available, but as it is not considered to be mainstream physics this is not the correct forum for providing them or even posting links to them.

Even if it were mainstream physics, you would still be left with the questions: what causes the expansion? (an answer is available, but may not be mainstream) and what causes the resistance to expansion of concentrations of mass-energy? (Although, to be fair, without gravity or some similar phenomena which leads to the "illusion" of gravity, the universe would be smooth and there would be no lumpy bits like ourselves to ponder the question.)

cheers,

neopolitan
P: 1
 Quote by 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
Hi Yogi:
I did not undestand the last line ' multiply by inertia.....
P: 7,405
 Quote by Gerenuk 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?
 Quote by Vanadium 50 This is not possible, as it predicts attraction to be independent of mass.
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.
P: 4,513
 Quote by Gerenuk 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?
This idea may not be as dismissable as it seems. In some form it might be found to be equivalent to general relativity, or similar to general relativity.
Mentor
P: 14,656
 Quote by atyy 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.
That's a pretty broad way of looking at it, as under this definition space isn't necessarily expanding at all, and indeed can be contracting, and you'd still have gravity.
PF Patron
P: 5,495
Here's what Peter Bergmann (a student of Einsteins) had to say about gravity and electromagnetism in THE RIDDLE OF GRAVITATION (1992) :

 ....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...
 Mentor P: 15,586 Out of curiosity, since the universe is expanding isotropically how could that be the explanation for gravity which points down rather than being isotropic?
P: 4,513
 ....large velocities affect masses differently from electric charges. Whereas a bodies electric charge has the same value for all observers,...
Though charge density is not Lorentz invariant.
 P: 4,513 The Friedmann-Lemaître-Robertson-Walker (FLRW) metric $$\Large{c^2 d\tau^2 = c^2dt^2 - A(t)^2 d \Sigma^2}$$ 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?
Mentor
P: 15,586
 Quote by Phrak The Friedmann-Lemaître-Robertson-Walker (FLRW) metric $$\Large{c^2 d\tau^2 = c^2dt^2 - A(t)^2 d \Sigma^2}$$ 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?
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 prefered or in any way describes a different spacetime.
P: 645
 Quote by DaleSpam Out of curiosity, since the universe is expanding isotropically how could that be the explanation for gravity which points down rather than being isotropic?

When you say gravity "points down" you are thinking of it as a force, I thought the whole idea of the OP was that it is an illusion. Expansion of the universe may be isotropic (specifically in all directions), but it is not smooth. It it were smooth, we'd not notice it because we are largely empty space ourselves. What we see expanding are the gaps between masses, or concentrations of mass-energy.

What then gets difficult to explain is why we see expansion between big lumps (galaxies) and not so much between smaller lumps (planets in the solar system). It could be that the universe would expand both isotropically and smoothly, if it weren't for mass (or concentrations of mass-energy). Concentrations of mass-energy seem to do something, it could be that they bend space or it could be that they resist expansion, the overall effect would be the same.

If concentrations of mass-energy did resist expansion, then gravity would then "point" along a line of increasing concentration, ie towards the (other) mass. Put two masses close enough to each other and you will see a line of maximum resistance to expansion linking them, because the centre of their combined mass (combined mass-energy) lies on that line.

How would this differ from "normal" gravity? Not much, it just wouldn't be a force, it would be a phenomenon - and you wouldn't, therefore, have a gravity field or gravitons. Gravity lensing would still happen if a photon passed through a region of resistance to expansion. Space would still seem to be bent.

Oh, and some physicists would be upset since it would play havoc with some pet theories.

cheers,

neopolitan
P: 4,513
 Quote by DaleSpam 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 prefered or in any way describes a different spacetime.
It's the appearance of the A(t) term that looks questionable, as if space has a prefered coordinate system in which it expands isotropically. I suppose it depends upon what motivates its inclusion. Is it ad hoc, to explain the expantion of the Universe; does it require a cosmological constant added to the Einstein tensor?
P: 1,162
 Quote by DaveC426913 Isn't this more to do with the fact that the greater attraction is perfectly cancelled 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. .
Hi---What you have said here seems, to me, to be completely logical and valid.
That the acceleration is always going to be a result of reciprocal interaction but if inertial mass is actually equivalent to gravitational mass, this shouldnt make any difference.
But subsequent posts seemed to question this so now I am curious as to which is correct.
Mentor
P: 15,586
 Quote by neopolitan 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.
Sounds like a traditional "push gravity".
Mentor
P: 15,586
 Quote by Phrak It's the appearance of the A(t) term that looks questionable, as if space has a prefered coordinate system in which it expands isotropically. I suppose it depends upon what motivates its inclusion. Is it ad hoc, to explain the expantion of the Universe; does it require a cosmological constant added to the Einstein tensor?
This kind of arbitrary function is fairly common and allows you to easily define a family of coordinate systems simply by choosing different functions. Here is an example 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.
P: 645
 Quote by DaleSpam Sounds like a traditional "push gravity".
What I described was not at all like Le Sage's push gravity, which seems to introduce more forces. I looked only very briefly at a site dedicated to Wright's push gravity - it was enough to glimpse a large number of the words spelt in capitals to realise that I didn't want to read any of it. I did notice that it was an aether theory, which was enough in itself to dissuade me from reading further.

If there are reputable push gravity theories, I may be interested, but it seems they replace gravity with a repulsive force which is not what the OP here seemed to be considering and certainly not what I was hypothesizing about - I was thinking of whether the force could be an illusion in entirety, not replaced by another, rather more counter intuitive force.

Try thinking of this: one consequence of relativity, the one that really messed with Einstein's head is that it implies expansion, he worked tirelessly to try to get around that and later described that effort as his greatest blunder (and that is ok, science benefits enormously from blunders). 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? Could we get from this universal expansion back to relativity and the understanding that aether is unnecessary, as opposed to from relativity to expansion? I think you should be able to.

For me the first step is to realise that the universe is expanding, but not all of it ... why is that?

It may be difficult to try the exercise properly, since there are huge temptations to take the shortcuts we know are there, but 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

cheers,

neopolitan
Mentor
P: 15,586
 Quote by neopolitan 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.
Now it sounds like GR.

 Quote by neopolitan For me the first step is to realise that the universe is expanding, but not all of it ... why is that?