torsdag 25 juli 2019

Speed of Gravity? Newton or Einstein?


Tom Van Flandern (1940-2009) was a free-thinking physicist who with perplexion made the observation (along with Laplace and also Newton of course) that the Earth on its path around the Sun at every instant in time accelerates in the direction of the actual position of the Sun, which is about 20 arc seconds ahead of the position of the Sun as seen in the sky from the Earth, because of the 8 minutes it takes for light to travel the distance from the Sun to the Earth. See also this review of Van Flandern’s work.

This observation is in accordance with Newtonian gravitation, which is assumed to propagate with infinite speed. If gravitation propagated with the speed of light, the acceleration would be instead in the direction of the visible Sun, but this is not what is observed (because it would be unstable).

I have discussed this observation in various posts with conclusion that the connection between mass density $\rho (x,t)$ and gravitational potential $\phi (x,t)$ as given by Poisson's equation in Newtonian gravitation
  • $\Delta\phi (x,t)=\rho (x,t)$      
with $\Delta$ the Laplacian with respect to a space coordinate $x$ and $t$ being a time coordinate, is to be interpreted as a relation where mass $\rho (x,t)$ somehow is "created" at $x$ at time $t$ by the local operation of differentiation through the Laplacian $\Delta$ acting on the gravitational potential $\phi (x,t)$. 

This is different from the standard interpretation where instead the presence of mass $\rho (x,t)$ at a specific point in space at time $t$ contributes to $\phi (x,t)$ for all points $x$ somehow through instant action at distance. Like Tom Van Flandern I view instant action at distance as physically impossible, while local instant action may be physical. The creation of mass from gravitational potential through the Laplacian thus may be possible, while its detailed physics remains to be discovered...

In any case the observation of the acceleration of the Earth towards the actual position of the Sun is only compatible with a speed of propagation of gravitational waves (if they exist), which is much bigger than the speed of light. This observation is in accordance with Newton's mechanics (with both the new and old interpretations of the mass-potential connection), but not with Einstein's mechanics.

What is your conclusion concerning who describes physics of gravitation best? Newton or Einstein? Be careful when you look at the Sun for answer.

The current wisdom among physicists is that despite the above Earth-Sun observation, for sure there are gravitational waves because Einsteins so says, waves which propagate with the speed of light and that these waves can be detected, not gravitational waves from the Sun, but from distant mergers of black holes and stuff. Do you buy this?

Sorry to say Tom passed away in 2009, but his ideas live.

PS Of course there is a cover up suggesting that also in Einstein's mechanics does the Earth accelerate in the direction of the current position of the Sun, even if the speed of gravitational waves is the finite speed of light,  because there is a subtle cancellation of the effect of the 8 minute delay from another effect, a most happy and welcome cancellation which allows a stable observable planetary system not only according to Newton but also for Einstein. But why Einstein if Newton explains what is observed? No wonder that Einstein begged for pardon in his: "Newton, forgive me!".

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