# Magnetic field

1. Apr 13, 2004

### kurious

I read somewhere that a spinning mass with no charge creates a mass version of a magnetic field.Is this true?

2. Apr 13, 2004

### turin

It depends on how rigorous you want to be. Firstly, I suggest posting this to the Relativity forum, where you have a better chance of attracting people who know the answer and are interested enough to answer. I will tell you what I have read/heard.

There is a manifestation/form of gravitational interaction called "gravitomagnetism." I have seen no reason to doubt its existence. It follows naturally from a first order approximation of Einstein's equation. That is, the zeroth order approximation gives you Poisson's equation for Newtonian gravity (basically the F ~ r-2 law), and the first order approximation is one order more accurate (so it includes more detail, like getting the kinetic energy from expanding the relativistic energy term of a particle). This first order approximation is also sometimes called the PN1 (Post Newtonian to first order) or the Maxwell approximation (I think).

Strictly speaking, this is not exactly like the magnetic field, even in analogy. In fact, it is only like this in PN1. Any higher (PN2, PN3, etc.) and you lose the analogy, so I've heard/read.