Physicists have been working for the past 20 years on string theory, which would theoretically unify Einstein's general theory of relativity and quantum theory.

But physicist Stephen Hawking now says that, while researchers have made enormous progress, the physics community is no closer to reaching its goal than it was when it started.

Hawking spoke this morning at the Strings '99 conference at the Max Planck Institute in Potsdam, Germany, according to the BBC.

During his speech, Hawking said he now thinks it could take another 20 years to prove string theory. But even if scientists accomplish their goal, the theory may not have a solution that works all the time, he said.