Why is Einstein's Gravity a Non-linear Theory?

Why is Einstein's gravity a non-linear theory?
- Dev (age 37)
Gwalior, India

Great question. 

The basic idea is that any form of energy is a source of the gravitational effect. That includes the energy in the gravitational field itself. So the expression for the field includes not just the sum of the effect of non-field sources but also the effect of the fields that the souces make, and then the effect of the fields those fields make, etc. 

This contrasts with electromagnetism, for which the fields themselves carry no charge, and thus do not appear as source terms. 

There's a very nice description online at einstein-online.info/en/sp: 

https://www.einstein-online.info/en/sp

Mike W.

(published on 12/27/2019)