…i.e., if it didn’t really exist as a real phenomenon, but was the result of a local reduction in some other all-pervasive force, would we be able to tell the difference?
I’ll explain myself.
The phenomenon we call ‘suction’ isn’t a real one; reduced pressure doesn’t ‘suck’ fluids up a tube, it is the (greater)pressure everywhere outside the tube that pushes, or squeezes the fluid up into the partially evacuated tube, as the reduced pressure inside is unable to stop it.
OK, now suppose that gravity wasn’t a force of attraction exerted by objects with mass, but rather a force of repulsionexerted by empty space - objects with mass occupy space that would otherwise be empty (and ‘pushing’) and so the empty space in all other directions is able to push two objects with mass together - as they get closer, there is less and less empty space between them, pushing them apart and so the space on the outside of the system encounters less and less resistance.
Is there any way that this could be the case (Note, I’m not seriously proposing this idea) and not be distinguishable in any way from gravity being an attractive force?
Then what? Density is still a problem, isn’t it? The empty space inside an object would push equally in all directions, so it would be just as if there was no empty space inside objects at all.
I do see some another problem though. Experiments show that acceleration due to gravity increases with the square of the distance. According to your new theory, it would increase with the amount of empty space above an object (and then you’d have to either subtract or divide by the empty space under an object, not sure which). So in that sense, it wouldn’t fit the data.
It’s an interesting thought however. I hope there’s some way to make it fit the data closer; we could have a whole new theory on our hands!
I’m suggesting that the force would be able to act at a distance and that inside a dense object, more (of what would be empty) space is occupied by fundamental particles, so the brass ball excludes more empty space, so the ‘push’ is attenuated more than from the balloon which is filled with a fairly insubstantial gas leaving more room for empty space.
The inverse square thing is a serious hurdle and I think it might be the significant factor that makes it possible to tell the difference.
Suppose the universe is flat, matter consists entirely of point particles of fixed mass (“protons” and “neutrons”), and gravity is a force atracting every particle A to every particle B with magnitute mmG/r^2 (where m is the mass of any particle). I think this is equivalent to gravity being a repulsive force on a particle from every point of vacuum magnitude mmG/r^2, as most points cancel out, but wherever there is a particle, a point of vacuum on the opposite side is uncancelled, having the same effect as if that particle exerted an attractive force.
What if particles have volume? I think the same reasoning works, assuming a repulsive force related to the constant density.
However, this works because all the information about gravity is contained in where the particles are (ie. where the vacuum isn’t) as all particles are the same. I think this is unworkable if you introduce electrons of anything other than the same density, as then the vacuum can’t ‘know’ how much force it should be exerting, as the vacuum could be the same opposite a proton, or a lot of electrons.
I have no idea how this would work with relatavistic gravity.
Let me start this reply by saying that MT’s suggestion is far from absurd. However physicists who have pursued it have not been able to make the equations of gravity work.
Let me begin the explanation by mentioning a little-known analogous phenomenon that is well-understood. Did you know that docks attract ships? That is, if you bring a ship up near a dock and can somehow get rid of wind, waves, tides,…, the dock will attract a ship. You could actually try this out with a toy boat in a basin, if you like. The explanation is very simple. A ship in open waters is being constantly bombarded on all sides by water molecules (Brownian motion). Now bring it up to the dock. There are fewer bombbardments from the dockside and the ship gradually closes with the dock.
So some physicists wondered if gravity could be similarly explained. Could space be filled with some kind of dark metter that is constantly bombarding us from all sides and could the “force of gravity” be merely the shielding effect of mass? In this view density would simply be measure of the opacity to these particles? Attractive idea (no pun intended), but they could not get the math to work out. I don’t know any of the details.
MonkeyMensch’s explanation is too simple, since it ignores the possibility that various kinds of matter are more or less opaque to these particles (as I said above). Think of the shadow of a clouded window, not a solid piece of wood.
What Chronos said in the cited thread is that you cannot make such a theory relativistically valid–the same in every reference frame. In other words, it might work for Newtonian gravity, but is inconsistent with general relativity. And that is something I believe correct. "Tis a shame, really.
If I read this correctly he in fact said that the argument shows that shadow pressure is inconsistent with either Newtonian or Relitavistic gravity, since it can’t be made reference-frame invariant which both are.