Only in theory, the project is possible.

In theory it is possible, but in practice it is very difficult.

The theory is not accepted.

This is an important theory.

It is that Emmet's theory is compatible with previous theories in physics that is of most significance.

He's not talking theory.

The results will not contradict the theory.

The theory is generally accepted.

This theory will bear examination.

The theory is not accepted yet.