The “rubber sheet distorted by heavy balls” demonstration is how most people visualise gravity.
However, there may be another way of looking at it that could prove fruitful.
Standing on an inflatable mattress (as you do) I noticed that where your feet are (the gravity wells) is indented, just like the rubber sheet, but between your feet and around, the landscape rises above the mean - unlike the rubber sheet in the demonstration, which is usually fixed to a rectangular frame. In other words, it also bulges UP.
You could choose to:
a) measure the attractive pull of gravity from the 'hilltop' towards the 'valley' floor, as conventionally - or, you could choose to b) imagine that gravity is a purely repulsive force and measure the 'push' of the hilltops towards the valleys. Or - a hybrid model - c) you could take as your baseline the flat geometry of empty space, and have hills of negative, repulsive gravity (-) and valleys of attractive gravity (+).
Gravity, unlike all other forces, is apparently only attractive. However, it could be that we never experience negative gravity as we are at the bottom of a gravity well, sitting on a planet. Could we ever experience it somewhere else?
Applying this "mattress" model, were we to travel to a region of the Universe where negative gravity prevailed - the top of the hill (see diagram above) - it would be empty of most matter, which would have rolled down the hill into the valleys. If we were to measure the force of g at the top of the hill, we still would not get a negative reading, as the result would be consistent with an attractive force exerted upon us from all sides.
My question is this: would it be possible to differentiate between these three models by observation, or are they matematically and observationally equivalent? The diagram indicates that c) - the mattress model - possibly has a steeper gravity gradient. Might this be equivalent to a dark matter model, and so offer a plausible alternative without the need for new exotic particles that exert an additional inward force?
Some further observations and questions:
The shape of the intergalactic "voids" certainly suggests bubbles with an outward pressure. Does purely attractive gravity explain these just as well?
if gravity exhibits the hills and valleys of a wave, could it be considered extremely long wavelength electromagnetic radiation, huge standing waves with resonances of the order of the simple harmonics of the 'width' of the universe? Even if they are not standing waves, moving at the speed of light they would still have a cycle of the order of billions of years. Might the large-scale pattern of galaxy distribution be related to the interference pattern these waves might generate?
The associated particle, the graviton, would thus be similarly huge. Might this preclude it's detection?
Does the spherical rather than elliptical distribution of dark matter around a galaxy also indicate that c) is a more plausible model?
I'd love to hear opinions on all of the above.
