This is a continuation of previous posts on the gravitational potential $\phi (x)$ as primordial giving mass density $\rho (x)$ to matter by differentiation as instant local action
- $\rho (x) =\Delta\phi (x)$ (1)
where $\Delta$ is the Laplacian differential operator with respect to a 3d space coordinate $x$. It is assumed that $\rho (x)$ is non-negative expressing absence of negative mass density.
This is a non-standard view to be compared with the standard view with mass density primordial from which the gravitational potential is created as solution to the differential equation $\Delta\phi =\rho$ which is a global summation process appearing as instant action at distance.
Both views, referred to as classical Newtonian gravity and new Newtoninan gravity formally express Newtonian gravity but they represent different physics. The big seemingly insurmountable problem of oldNewton since the days of Newton, is the physics of instant action at distance, which does not appear in newNewton.
Further, newNewton opens to a distinction between visible matter and invisible matter or dark matter, through the smoothness or $\phi(x)$ that is the size of $\Delta\phi (x)\ge 0$:
- $\phi (x)$ produces visible matter where $\Delta\phi (x)$ is large.
- $\phi (x)$ produces invisible dark matter where $\Delta\phi (x)$ is small-moderate.
What does F=ma look like in terms of phi and dphi/dt ?
SvaraRaderaIf F is gravitational force, then F = grad phi and m is gravitational mass. If F is inertial force or other force, then m is inertial mass = gravitational mass and a is the resulting acceleration.
SvaraRaderaDen här kommentaren har tagits bort av skribenten.
SvaraRaderaBut at least a priori `a` means d²u/dt² where u is some position vector as a function of time R -> R³, and `phi` : R³ -> R (or at best (R x R³) -> R) doesn't immediately give you such a position vector. So I am wondering how to rephrase F = ma in a situation in which `a` can only be indirectly accessed. If possible, I am looking for an equation of the form d²phi/dt² = A(phi, dphi/dt) where A is an operator of type ((R³ -> R) x (R³ -> R)) -> (R³ -> R) involving nabla.
SvaraRaderaDen här kommentaren har tagits bort av skribenten.
SvaraRaderaTo be clear, I am assuming that the mass m generated by phi is also the mass upon which the force of gravity F (also generated by phi) is going to be acting.
SvaraRaderaIn order to demonstrate a solution to the action-at-a-distance problem, shouldn't you show how to write an equation for the time evolution of phi, using only local data i.e. space and time derivatives?
SvaraRaderaGood question! This is discussed in chap 32 of Computational Thermodynamics under Upcoming Books in the left menu.
SvaraRaderaI see, so you postulate another quantity `m` which is something like the velocity/momentum, and the first equation of (32.1) expresses that the mass density changes according to the divergence of this flow of mass, while the next equation says how this flow changes so I guess it is somehow "F = ma". Will need to read this, thanks!
SvaraRadera