Thanks for down learning the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.uk.

I hope you enjoy the program.

Hello, Einstein left us with his theory of general relativity, which explained how gravity works on the scale of stars and galaxies and the universe itself.

And Schretinger left us with the equation that explained the mechanics of the tiny quantum

realm. Both theories work to wonderful effect in their own worlds. But, and this is the sticking

point, gravity is strangely absent from the quantum realm

and planets behave nothing like particles. The enigma for scientists throughout most of the

last century is that as they're currently formulated, general relativity in quantum mechanics cannot both be right.

The history of 20th century physics has been a struggle to find a way to unite them, to find

what has become the holy grail of modern physics, the grand unified theory.

With me to discuss the dilemma of modern physics and the quest for its solution is Brian Green,

professor of mathematics and physics at Columbia University, and author of The Elegant Universe, Superstrings,

Hidden Dimensions and the Quest for the Ultimate Theory.

I'm also joined by the Astronomer Royal Professor Sir Martin Reese, whose work just six numbers, also tackles this problem.

Brang, can we begin by explaining why Einstein's theory of general relativity can't be incorporated

into the current understanding of quantum mechanics.

Sure, the basic idea in Einstein's theory of gravity general relativity,

strangely enough is that the fabric of space itself is connected with the force of gravity. It's kind of a hard idea to

imagine but it's as though the fabric of space bends and warps and in that way

communicates gravity. The only thing we need to know though is that the curves in space from his theory are viewed as gentle, gentle curving geometry.

But at the other end of the spectrum in the quantum realm, we learn that microscopically the universe is a jittery, frenzy, turbulent arena,

very different from the gentleness of Einstein's theory.

...