## Why quantize gravity? – More than fifty years of debate

A few weeks ago I downloaded a paper by H.D. Zeh that I had noticed on SciRate. I finally read it and it highlights some extremely prescient ideas discussed at a conference at UNC-Chapel Hill in 1957. I’m less interested in critiquing the paper itself at this point (it could use some editing) than I am in discussing the issues it raises (so, in that sense, I think it is incredibly important). Before I get started I should mention also that it pointed out the fact that there were *two* DeWitts – Bryce *and* his wife Cécile! For whatever reason, I had never known this despite being an historian of cosmology.

So, to the paper then. In short, it analyzes and deconstructs aspects of the discussion centered around Feynman and relating to the transition between the quantum and classical worlds. Briefly, Feynman holds true to the “legend” that maintains that he viewed the superposition principle (double-slit experiment in some cases) as the definitive signature of quantum processes. Note that Schrödinger maintained that entangled states represented this signature. There are those that might say that these two phenomena are the same. In fact that is not entirely true since entangled states are specifically non-factorable while superposition states may or may not be. In short, there *are* classical/macroscopic superposition states but, as yet, no one has observed a classical/macroscopic *entangled* state. As for Feynman, he makes the point that there is nothing in quantum mechanics that expressly *prevents* interference in macroscopic masses, i.e. *could* classical superpositions really be washed out versions of quantum superpositions?

The conversation at the conference works up to a discussion surrounding the effect of amplification on quantum effects. Bohr maintained that the switch from quantum to classical occurred in the measuring apparatus where this amplification takes place. In the conference discussion, the apparatus is designed to include gravitational effects.

Two issues are highlighted by the discussion:

1. Decoherence is central to the understanding of the transition since it signals (or *can* signal) the transition (from quantum to classical) is occurring. However, decoherence is still not fully understood. In relation to this, I think the recent works by Fuentes-Schuller and her many collaborators on how gravity degrades entanglement are a major step in understanding what’s truly going on here.

2. Related to all of this is the notion of irreversibility (oh that pesky second law again). I had a short chat with Paul Kwiat at the March Meeting this year about my personal ideas in this regard (which are modeled on Eddington’s) and the basic idea is somewhat similar to Feynman’s, I think, though not quite the same. My idea (or Eddington’s) is basically this: as systems grow in size there are more and more configurations and the system becomes deterministic. Eddington gave an example based on flipping coins. The more coins you flip the harder it becomes to reverse the process perfectly. While this perhaps seems a bit naive on the surface, it at least highlights the fact that there is an undeniably *statistical* aspect to the transition from quantum to classical.

There’s quite a bit more in this paper that deserves slow digestion including some peripheral discussion of the Everett interpretation (and some of its problems), but it highlights the fact that we need to be thinking about entanglement, irreversibility, and gravity in conjunction with one another if we are to really understand the quantum/classical transition and perhaps finally quantize gravity (or discover we don’t need to).

Two final quotes from this paper that I enjoyed:

Feynman, apparently aiming his comments at mathematical physicists, once said “Don’t be so rigorous or you will not succeed.”

Zeh then closes with this remark: “How can anybody do quantum field theory or cosmology at all nowadays without first stating clearly whether he/she is using Everett’s interpretation or some kind of collapse mechanism (or something even more speculative)?”

May 9, 2008 at 1:43 pm

How can anybody do quantum field theory or cosmology at all nowadays without first stating clearly whether he/she is using Everett’s interpretation or some kind of collapse mechanism (or something even more speculative)?”

And I think that’s still the case today. Only now one can add “Explain Quantum Computing” to quantum field theory or cosmology.

May 9, 2008 at 5:30 pm

Honestly, you could probably say that about almost anything with the word “quantum” in it.

May 10, 2008 at 4:05 am

The success of QFT demonstrates that QM does not need interpretation, and the one that less needs it is that of Everett, that really is not an interpretation but a new theory, and a BIZARRE one.

May 10, 2008 at 8:02 am

Yes, QFT has been very successful, but there is still that glaring problem known as gravity. Whether QM needs an interpretation or not I think will depend upon its relation to gravity (which I believe was Feynman’s point, though I’d be curious to read the full transcript of that meeting).

May 12, 2008 at 11:30 am

I was talking about the Zeh’s remark “How can anybody do quantum field theory…”. We need a quantum theory of gravity not an interpretation of QM. We have some attempts to construct such a theory: string theory, loop quantum gravity and attempts like that of Hardy to reach quantum gravity from foundationals principles. These are true theories not interpretations.

May 12, 2008 at 11:49 am

Ah, I see what you meant now. Yes, that is absolutely true. Even though I will, upon occasion, think about interpretations, I find that they are more useful from a pedagogical standpoint than anything else and are not terribly helpful for furthering our understanding. So, in that sense I am with you. On the other hand, I firmly believe that any theory needs to be directly testable. I’m not a particular fan of string theory for that precise reason (guess I’m in the Lee Smolin camp somewhat). I think Hardy’s work looks pretty promising, though I’ll admit I have not given it my fullest attention.