I'm really glad we have people here who can comprehend these complex concepts. I'm not particularly one of those people.
In layman's terms can you explain to me what constitutes "up" and "down"? I don't believe in Newtonian gravity, and I feel there is adequate cause to believe density and pressure are enough to explain the phenomenon. But I'm still at a loss to explain why down is perpendicular to the plane.
You claim "there is adequate cause to believe density and pressure are enough to explain the phenomenon". This is simply not true, it does not begin to explain "the phenomenon". Pressure acts all around an object, not just on top, so cannot cause a "downward" force. Also measure the weight of objects under a very low pressure in a "vacuum chamber" and the weight
increases, yet we take an object to a high altitude where the pressure is also much much lower and the weight
decreases!As well as this, attractive forces can be measured between masses on the earth's surface. The force is very small, because gravitation causes only a very
slight attraction. The earth has a mass of almost 6×10
24 kg (or 6,000,000,000,000,000,000,000,000 kg) and it attracts a 1 kg mass with a force of only 9.81 N (or 1 kg weight). Hence test masses of a few 100 kg still cause of minute forces.
Nevertheless the experiment has been performed hundreds of times since the "Cavendish Experiment" (1797–98).
Once an experiment is performed getting a certain result
other scientists will repeat the experiment. If the
experiment is not repeatable it is rejected as either a fluke of based on a wrong premise.
Cavendish was the first to perform the experiment (using a torsion balance apparatus constructed by geologist John Michell[1])
which lead to a determination of the Universal Gravitational Constant G.
Many other experimenters performed similar experiments numerous times[2] to verify this and improve the accuracy.
Modern results of these experiments differ from the results from Cavendish's experiment by less than 1%, and agree among themselves by better than 0.03%. It is a difficult experiment and there are still some unanswered questions,
but Cavendish has certainly been vindicated!
No-one will pretend that all questions regarding gravity have been answered. Einstein took it a step further further with his General Relativity explaining it as a distortion of space-time by the presence of mass and energy. This explained a number of anomalies that had been noted earlier (eg in the anomalous rate of precession of the perihelion of Mercury's orbit), but so far there is no link between Einstein's GR and Quantum theory.
But, on the mundane level of gravity on earth experiments such as Cavendish's have surely demonstrated that
massive objects do attract with a force given by Newton's law of gravitation[3].
[1] John Michell died before he could perform the experiment himself.
[2] One reason for the numerous repetitions is that, partly because of the forces to be measured, high accuracy is difficult to achieve.
[3] There is a proviso in this. The objects cannot be too massive (like a huge sun!) or move at too high a velocity (compared to the velocity of light). If these are not satisfied we get into the realm of General Relativity.