What are the limitations of the Einstein field equation?

[aditya23456]

what does this statement mean..?actually complete statement is--
" Einstein field equation is not very choosy about what kinds of states of matter or nongravitational fields are admissible in a spacetime model. This is both a strength, since a good general theory of gravitation should be maximally independent of any assumptions concerning nongravitational physics, and a weakness, because without some further criterion, the Einstein field equation admits putative solutions with properties most physicists regard as unphysical, i.e. too weird to resemble anything in the real universe even approximately"
Can anyone please explain it physically

 

[Dale]

Basically, the EFE allows systems of matter which we have never observed. For instance, the EFE are perfectly happy with negative energy density, but since mass has energy that would imply particles with negative mass. Since we have never seen such particles most scientists think that such solutions to the EFE are non-physical.

To avoid this problem, the EFE are usually supplemented with one or more energy conditions which constrain the set of admissible solutions to a set which are more reasonable given the types of matter systems that we know about:

http://en.wikipedia.org/wiki/Energy_condition