## Tuesday, November 29, 2011

### Jim Hartle

Here I comment on a paper by Gell-Mann and Hartle, [link].

First of all, I was Jim's T.A. for his Quantum Mechanics class at UCSB in the late 70s. That was good.

I have read Gell-Mann and Hartle's ideas on "histories", to tell you the truth, I never got their point. Now I have my own interpretation. Once I was visiting Jim after I got my degree under Bob Sugar, and he was surprised that I was interested in fundamental issues. Then I had an idea about fractal paths, in something I was calling a geometric theory of mass. Since nothing came of it, not much to report here.

Neverhteless, I look at these works with new eyes.

Murray and Jim want to give a consistent mathematical theory of , as they put it, "Decoherent Histories Quantum Mechanics with One “Real” Fine-Grained History". The key word is Real. Implying also, I suppose, a unique description.

In my new view, which I have been presenting in this blog, all I have to look for, is how this description fits data, all else be damned!

It is helpful if it also is a nice "History".

We love to tell stories.

"Quantum mechanics can be viewed as a classical stochastic theory of histories with extended probabilities and a well-deﬁned notion of reality common to all decoherent sets of alternative coarse-grained histories".

Doesn't this sound like coming full circle?

To me it sounds like David Bohm, and Luis de la Peña Auerbach.

Also I can see Dick Feynman here:

"To describe these preferred variables we assume a particular Lorentz frame and let t be the time coordinate of that frame. We denote the preferred variables by qi or just q for short. For particles i might be x, y, z and a particle label. For ﬁelds i would include the label $$\vec{x}$$ of the spatial point. We denote the conﬁguration space spanned by qi by $$\mathcal{C}$$. A ﬁne-grained history is a path q(t) in $$\mathcal{C}$$ that we assume to be single-valued — one and only one value of q for each t. The set of all ﬁne-grained histories between an arbitrary pair of times t0 and tf is the set of all such paths {qi(t)} between these two times. They are continuous but typically non-differentiable"
In modern times we could call these continuous and non-differentiable paths, fractals. I also see Paul Adrien Maurice Dirac:

There is something interesting in Murray's and Jim's world. God not only plays dice, but S/He also bets!