# Did Einstein know about Laplace's query into whether gravity is instantaneous?

Here it states,

In 1776, Laplace published a memoir in which he first explored the possible influences of a purported luminiferous ether or of a law of gravitation that did not act instantaneously. He ultimately returned to an intellectual investment in Newtonian gravity.

Is it known whether or not Einstein was aware of Laplace's memoir (and the calculations within) when he himself considered the propagation of gravity as not being instantaneous?

More specifically, was Laplace's "latency problem" merely an artifact of his approach? Or was it pointing to something deep about gravity that was explained by general relativity? Was it influential on Einstein? Or was it already obsolete by the time of Einstein?

Thanks!

• Laplace's assumptions were obsolete by Einstein's time, and, yes, there were better variants offered later by Lorentz and Gerber that Einstein was aware of. Feb 26 at 0:55
• Does this answer your question? What 19th century developments contributed to the General theory of Relativity? Feb 26 at 0:55
• It answers my second question, certainly! Thank you. But it prompts me to ask a more precise question that (I think) is not addressed in your comprehensive answer but is at the core of my first question above: was Laplace's "latency problem" merely an artifact of his approach? Or was it pointing to something deep about gravity that was explained by general relativity? The paragraph on this is a bit scant.... because I just wonder if it was influential on Einstein. Or was it kind of swept under the rug by Poincare and Einstein pretty much started from there? Or did I misunderstand something? Feb 26 at 15:34
• It was an artifact of Laplace's assumption that the inverse square law is modified simply by taking the force to be directed towards the position retarded according to the relative velocity with the distance computed accordingly. In Gerber's and Lorentz's theories gravity was modeled on electromagnetism, so they were (implicitly) relativistic, and that is what was discussed by then current authors like Mach and Poincare. Laplace's version was not even discussed due to its absurd consequences. Feb 26 at 18:51
• That makes much sense! Thank you. If you'd like, I can edit my question and I'll gladly accept your answer if you write it up briefly. Feb 26 at 22:29

Laplace in 1799 introduced velocity dependence in a particular way, modifying the inverse square law so that the force was directed towards the position retarded according to the relative velocity $$v$$ and the distance was computed accordingly. The consequence of that was that the correction to the Newtonian force was of the order $$v/c$$, and, according to Laplace's calculations, the planets would fly off of their orbits in a hurry if gravity propagated at the speed of light. It had to be at least $$7×10^6$$ times faster than light to match observations.
By the turn of 19th century velocity dependent gravity was instead modeled on Maxwell's electromagnetism, see What 19th century developments contributed to the General theory of Relativity? Gerber in 1898 proposed such a theory in a paper deemed unintelligible, and Lorentz proposed a coherent one in 1900 based on Mossotti's 1830 idea that electric attraction and repulsion do not balance each other exactly and the difference is gravity. They were (implicitly) special relativistic since the Maxwell equations are Lorentz invariant, and in them the correction was of the order $$v^2/c^2$$. So Laplace's fly off problem did not arise, but they still did not predict the correct precession of the perihelion of Mercury, and did not satisfy the more general relativity principle that Einstein favored. It is these theories that were discussed by contemporary authors like Mach and Poincare, and that Einstein was familiar with. Laplace's theory was not discussed at the time due to its absurd consequences.