Thanks for downloading the NRTIME podcast. For more details about NRTIME and for our terms of use, please go to bbc.co.uk forwardslushradio4.

I hope you enjoy the program.

Hello, at the end of the last century, brave voices were predicting that all the big questions of physics were on the verge of being answered by a theory of everything.

The disparity between the physics of the very small would finally be reconciled with a very large, and the four forces of nature would finally be united with a single set of equations.

It was suggested that with such a theory, we might solve the riddle of black holes, unlock the secrets of the Big Bang, probe other universes, and even uncover the mystery of traveling through time.

Now it's 2004 and the clock is still ticking. Stephen Hawking or once said that with a theory of everything, quote, we would know the mind of God as change his mind, and now says that it may not be possible after all.

So what are the prospects for a theory of everything? Why do we need one? How do we get one? And what would it mean if we did?

With me to discuss, this subject is Brian Green, professor of physics and mathematics at Columbia University and author of the fabric of the cosmos.

Val Gibson, particle physicist from the Cavendish laboratory and fellow of Trinity College Cambridge, and John Barrow, author of constants of nature and professor of mathematical sciences at the University of Cambridge.

Brian Green, can you start by outlining for us the central problem for modern physics? Why is it the physics of gravity and the physics of the quantum really don't fit?

Well, that is the essential problem, trying to put together the two main theories of physics that were developed during the 20th century, the physics of gravity, general relativity, and the physics of the micro-al quantum mechanics.

And indeed, both work extremely well in their own domain, but attempts to put them together into a single theory that would describe the big and the small has been very, very difficult.

And I think one way of understanding why it's so difficult is to recognize that the central core of each theory is so different from the core of the other.

General relativity in a nutshell describes space and time as a smooth geometrical structure, sort of like a rubber sheet that can bend and warp, but it's nice and gentle, a nice gentle curving geometry.

Quantum theory, though, at its core has something called the uncertainty principle, which says that there's always a certain amount of fuzziness, quantum jitter associated with anything in the micro world.

So you have this jittery character of the micro world versus this gentle character of the macro world, so putting them together into a single theory requires meshing those two contradictory views of the universe.

That's why it's been so hard.

And when was it realized that it was a problem?

It's been recognized for about 70 years that there's an issue.

Certainly when people first developed quantum mechanics that was in 1920s and 1930s, they were only seeking to apply it to molecules, atoms, and subatomic particles, and they found fantastic success.

But the theory of gravity that Einstein had developed by 1915 was out there waiting to be joined together with quantum theory, little by little people took the problem seriously, but the more they investigated, the more difficult it looked to actually combine them.

Is this stopping things happening? The fact that these two theories, as it were, don't gel.

...