John Wheeler - The Wheeler-Dewitt equation (76/130)

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American physicist, John Wheeler (1911-2008), made seminal contributions to the theories of quantum gravity and nuclear fission, but is best known for coining the term 'black holes'. A keen teacher and mentor, he was also a key figure in the Manhattan Project. [Listener: Ken Ford]

TRANSCRIPT: A spherical geon is, in principle, possible too, where some radiation is going round one circle, other radiation going around another circle, and these various circles with the various items of radiation, these circles are added at random. So you end up with a smooth, spherical distribution of energy. But, again, unstable. Did you hold out the hope that when quantum theory was brought into the picture that it might provide the necessary stability? Yes, I didn't see how to use quantum theory in the whole story. But it would be marvelous if quantum theory had led to some structure that would keep this thing from collapsing. But I don't see now any likelihood that that is the case. But at the time, I was hoping that this would be a model for elementary particles. Why dream up something new out of which to make particles when you already have in front of you electromagnetic radiation and gravitational radiation? But we are still struggling with the ultimate constitution of particles today. So I would not want to bet. Dealing with gravitation, I must say that it seemed to me the whole subject fell into order in a new way. There were no great mysteries except at the interface between gravitation and quantum theory. Quantum theory says that a physical system has a certain probability to be in this configuration or that configuration or another. But how would you talk of the probability of a space that's curved like this and a space curved like that, and so on? Where would you be standing when you were pontificating about this geometry? In which geometry would you be? It's a ticklish business. Fortunately, we could talk about these things in a seminar and I had a colleague, Valentine Bargmann, who had been an assistant of Einstein at one time. And he saw into some recent work by a colleague at the University of Rochester, saw enough to give some guidance into this. And another student of mine, named Edward Fireman saw how to operate at the semi-classical level, at the level where you almost can use classical concepts. And that was a step on the way to translating this question of the quantum theory of gravitation into a 'doable' form. It ended up with an equation which looked mostly symbolic. My Texas colleague, Bryce Dewitt, found a way to translate that symbolic equation into quite concrete mathematical terms, so today it's called a Wheeler-Dewitt Equation. But it's one thing to have an equation, another thing to solve it, and so another thing to interpret the solution. A colleague at Pennsylvania State University, Abhay Ashtekar, has found a lovely way to solve this equation. But we still haven't got a full insight into what the solutions mean and how to speak about them. That's a continuing enterprise. It's strange that the two greatest developments of theoretical physics, the quantum theory and relativity, should take so long to come into a union.