## onsdag 17 februari 2016

### Gravitation, Motion, Programming with Codea and Zeno's Paradox

Fresh reprinting of the world at each new time instant.

The experience with the programming platform Codea which I am reporting on Matematik-IT  connects to an earlier view exposed on The World as Computation on the connection between matter and gravitation described by Newton's gravitational equation
• $\Delta\phi =\rho$,            (1)
where $\phi (x,t)$ is gravitational potential and $\rho (x,t)$ mass density depending on a Euclidean space coordinate $x$ and a time coordinate $t$, and $\Delta$ is the Laplacian differential operator.

The conventional way of viewing (1) is to think of mass density $\rho$ as primordial which generates a gravitational potential $\phi$, and corresponding gravitational force $\nabla\phi$, through the integral
• $\phi (x,t) =\frac{1}{4\pi}\int\frac{\rho (y,t)\, dy}{\vert x-y\vert}$      (2)
with apparent (instant) action at distance expressed by the global nature the integral. The trouble with this view is that the physics of (instant) action at distance, supposedly being transmitted by graviton particles (appearently traveling at infinite speed), is still completely open despite centuries of deep thinking by deeply thinking physicists. The idea of matter density as primordial may come from primitive thinking that what we can see/touch, must come first and what we cannot see/touch, must be secondary.

But there is another way of thinking, maybe less primitive, which is to view instead gravitational potential $\phi (x,t)$ as primordial and mass density
• $\rho (x,t) =\Delta\phi (x,t)$      (3)
as being produced by the local operation of differentiation. With this view, there is no action at distance to explain.  What asks for understanding is the physics of (3).

Viewing the world as computation then connects to the way Codea works with the screen being freshly redrawn 60 times a second from the code under function draw()....end, without storing anything previously written on the screen.  This is the same way we perceive the world with our eyes with a fresh image at each new instant.

With this perspective we can think of (3) as the computer code which at each instant draws the world in a new configuration with new mass density $\rho (x,t)$  from a gravitational potential $\phi (x,t)$ which is changing in time according Newtonian mechanics.

In the same way as motion is exhibited by Codea by a sequence of fresh images coded under function draw()...end, the motion of matter we perceive would be a result of fresh reprinting of the world at each new instant according to the code (3).

The alternative view suggests a solution to Zeno's paradox, still unsolved after 2500 years, with the arrow being reprinted at each new instant in time giving the appearance of motion as change of position with time. Think of that!

Note that it Einstein's equations in nearly flat Minkowski space-time reduces to a wave equation variant of Newton's equation of the form
• $-\frac{1}{c^2}\frac{\partial^2h_{\alpha\beta}}{\partial t^2}+\Delta h_{\alpha\beta} =\frac{16\pi G}{c^4}T_{\alpha\beta}$,
where $h_{\alpha\beta}$ is metric perturbation and $T_{\alpha\beta}$ stress-energy. This equation allows waves traveling at the speed $c$ of light as "ripples of the fabric of space-time" in the common mysterious jargon of relativity theory, but the presence of the factor $c^4$ make such waves vanishingly weak. In short, there seems to be little reason to expect a wave equation variant of (1) to have physical significance, which can be seen as support of the alternative view.

#### 2 kommentarer:

1. The hypothesis that the gravitational potential is some primal quantity and changes by Newtonian mechanics clearly breaks down on the scale of atoms as one example.

2. 