A Note on Sources

This question before us is this: is it possible to affect angular momentum (or "spin" at the micro-physical, i.e. QM level) so that it can be transformed into the macro-physical linear momentum? To understand the problem and make suggestions and generate ideas regarding a research program, we need to have an understanding of what is meant by "spin" in QM (it's not what most people think) and QM processes in general. In short, we need to be able to think in QM terms. This is not an easy thing to do. It been some 85 years since the Quantum Revolution of the mid-1920's and I would say that even most working physicists have not managed the trick. Indeed, I would say that most working physicists actively discourage such attempts. Work with the mathematical formalism and go away is the attitude.

The same is true for General Relativity (which has been around about a decade longer, 95 years). Both QM and GR are vital to our story so I will say right off that we will most likely not have a definitive answer to the question until we have a theory of Quantum Gravity as powerful and as workable as QM and GR are in their respective and highly distinct domains. This may take a while. Given the current state of the world it may be a decade, a century, or never. And I do mean never, at least as far as the human race is concerned. Perhaps advanced AI entities will knock off such an ideal theory on a lazy afternoon . . .

In the meantime, I will do my best and hope some inspiration may result from my efforts.

Here were the books that guided me:

For clearing up conceptual issues regarding GR and QM, Quantum Gods (Victory J. Stenger), Quantum Gravity (Carlo Rovelli), and Particle or Wave (Charis Anastopoulos) were all helpful. The latter book in particular is just a marvel of depth and exposition, one of the few "popular" books on physics that does not hesitate to go in deep, guiding the reader carefully each step of the way from marvel to marvel. It is easily the best book on the physics of QM I have ever read and an unstoppable candidate for best science book of 2009.

Now this one may seem a bit odd but I urge you to give this a try, certainly before tackling Particle or Wave, which the course nicely complements: I'm speaking of the Teaching Company course Quantum Mechanics: the Physics of the Microscopic World. The teacher is Professor Benjamin Schumacher and he does an amazing job over 24 half-hour lectures in summarizing what we know about QM from both the standpoint of physics and information (Schumacher's specialty). Schumacher (of Kenyon college) is a wonderful teacher and like all great teachers is both comfortable with and in awe of his subject. I honestly believe everyone, of whatever degree of knowledge and interest, can learn from this course.

Finally, a caution. To come to a definitive answer to the question, as noted we need to have a theory of quantum gravity. A problem, likely the problem, is that there are huge conceptual difficulties in doing this. The world of QM is one of a static spacetime background (ok, with special relativity added but the addition of SR really doesn't change anything essential) while in the world of GR, spacetime is dynamic. Quoting Carlo Rovelli: "It is as if we have observed in the ocean many animals living on an island . . . Then we discover that the island itself is in fact a great whale. So the animals are no longer on the island, just animals on animals. Similarly, the Universe is not made up of fields on spacetime: it is made up of fields on fields."

This makes for grave conceptual difficulties, indeed. Just how do we think about such a thing, and how can we possibly merge it with QM with its static spacetime? My own hope is that loop quantum gravity (Rovelli's specialty) is the approach that will work, but it is impossible to say.