Newton's model of gravitation is Poisson's Equation:
- $\Delta\phi (x,t)=\rho (x,t)$ (PE)
where $\rho$ is
mass density, $\phi$ is
gravitational potential depending on a 3d
Euclidean space coordinate $x$ and a
time coordinate $t$. It is the most remarkable mathematical model of all of classical physics allowing precise computational simulation/prediction of effects of gravitational forces in complex systems, as demonstrated in the
Millennium Run tracing the evolution of the matter distribution of the Universe over 2 billion light-years using 10 billion particles interacting by (PE). There is massive evidence that (PE) captures gravitation to experimental precision in all cases of any practical interest.
In the modern phyics of today PE has been displaced from 1st to 2nd place by Einstein's Equation EE, despite the fact that PE covers all cases of practical importance.
This degradation of PE took a long time become the official truth of modern physics. Einstein presented EE in 1915 which was met with criticism as non-physics in a form of complicated mathematics, and it was only after Einstein's death in 1955 that EE gradually came to replace PE as the true model of gravitation of modern physics.
The key argument used to put PE into 2nd place was that PE could be viewed to require instant-action-at-distance in the sense that a change of $\rho (x,t)$ at a certain point $x$ at time $t$ would instantly affect the value of $\phi (y,t)$ for all points $y$, since (PE) as a differential equation in space represents $\phi$ in terms of $\rho$ in terms of a global summation process according to the formula with the same $t$ on both sides, thus without time delay:
- $\phi(y,t)=-\frac{1}{4\pi}\int\frac{\rho (x,t)}{\vert x-y\vert}dx$
Since instant-action-at-distance appeared to be in conflict with Einstein's Special Theory of Relativity SR, which was accepted before his General Theory of Relativity with EE, leading physicists decided to displace PE to 2nd place in modern physics.
Let us now take a fresh look at (PE). We see a linear partial differential operator with constant coefficients as the Laplacian $\Delta$ connecting $\phi$ and $\rho$ which can be viewed in 3 ways:
- Differential equation $\Delta\phi =\rho$ with $\rho$ as cause and $phi$ as effect derived by global integration.
- Specification $\rho =\Delta\phi$ with $\phi$ as cause and $\rho$ as effect derived by local differentiation.
- Simply a coupling of equal parts without cause-effect.
Here 2 corresponds to local-instant-action which is compatible with SR, if that is the concern.
Here 3 connects to the Pre-Established Harmony of Leibniz as a Deep Truth. It means that $\phi$ and $\rho$ are locked to each other with the Laplacian as a linear relation of simple mathematical form the same everywhere. Such a relation can be read without causation as in 3 and then in particular without demand of instant-action-at-distance. It also makes sense from computational point of view since the computational complexity of (PE) scales linearly with number of spatial mesh points.
In discrete form (PE) takes the following form in 1d with $dx$ a space step:
- $\phi (x+dx,t)-2\phi (x,t)+\phi (x-dx,t)=dx^{2}\rho (x,t)$
locking $\phi$ to $\rho$ at common time $t$ by a simple linear relation which can be read both ways.
Let me list virtues of (PE) and Newton motivating back to 1st with (EE) and Einstein 2nd:
- Generality.
- Simplicity.
- Minimal computational complexity.
- Compatibility with Quantum Mechanics.
- Understandable to a wide audience.
As a young patent clerk in Bern in 1905, Einstein took on a role to overthrow principles of classical Newtonian physics at the same time cubism and atonal music emerged as challenges to classical painting and music. This was the emergence of modernity at the turn of the century triggered by an explosion of new technology. Today we see a return to tonal music and figurative painting as post-modernity, and so a return of Newton may also take place after an aberration into Einstein.