here is the first post in a physicsforums.com thread that is relevant to our discussion:
http://www.physicsforums.com/showthread.php?t=49742&page=1&pp=15. Harv, you may find it interesting, as well as the remainder of the thread. I wonder how you would characterize that post.
Ontology: what's really there
In the history of physics there are moments of crisis now and then when questions of ontology get confronted. Mostly ontology is ignored and it is business as usual (the business is largely predicting experiment outcomes) without worry about what is really there. Ontology comes from the greek word for Being.
Ontology is about Being----what really is there.
Phenomena (from greek word for appearance) is about what you see, how it looks, what you measure.
Ontology is about the basic underlay that gives rise to the appearance.
It is a philosophical quagmire so wear hip-boots and approach with caution. Working physicists shun philosophical discussion nearly all the time (but really good ones, like Newton and Einstein, now and then grapple with those issues during times of crisis)
In another thread--serving as a collection thread for links to sourcematerial---Mike asked what i think is an ontological question.
Are "gravitons" really there, or are they just convenient approximations that you can use in certain limited circumstances to approximate the field?
If "gravitons" really exist, could the world be made of them?
This was my reply. I want to make a separate thread for any discussion that might ensue because otherwise it clogs the collection of reference-links.
----here's to start an Ontology discussion---
I believe not, Mike. That is, this paper does not indicate that gravitons are real----rather that they are a mathematical fiction which plays a part in a possibly useful approximation. Analysis of the gravitational field using gravitons would be appropriate, as this paper suggests, in the weak field case.
We should start a thread about ontology. You seem to me to have an abiding interest in ontological questions---what is really there, what is reality made of. I would say, in answer to such questions, "the field is the field."
To say it in a lot more words: the field is the field and it is not made of anything more basic---it is not made of gravitons or anything else, it is really there. It may or may not sometimes be useful (as an approximation) to describe it in some special case in terms of the mathematical device of postulated gravitons and then it may or may not be helpful to talk among ourselves about "gravitons". But that is an overlay of mathematics, it is not ontological. Ontologically speaking there is nothing more basic than the field.
The field is not something defined "in space and time" because space and time arise from the field. space and time are appearances or phenomena which emerge from the field. We must arrive at ways of describing the field which do not depend on imagining a prior space and time in which to define it.
The field is a quantum animal, living in a Hilbertspace of all possible fields. The central problem in physics today is to find a satisfactory formal way of to describe the gravitational field, and the Hilbertspace of its possibilities.
When and if that is found, everything else will be describable relative to, and on top of, the field.
----let's continue this in a separate thread, so as not to choke the linkbasket---
you are cordially invited to agree or disagree or put forth alternative ontological views
Daniele Colosi, Carlo Rovelli
Global particles, local particles
brief sample from conclusions section:
...the distinction between global and local states can therefore be safely neglected in concrete utilizations of QFT. However, the distinction is conceptually important because it bears on three related issues: (i) whether particles are local or global objects in conventional QFT; (ii) the extent to which the quantum field theoretical notion of particle can be extended to general contexts where gravity cannot be neglected; and furthermore, more in general, (iii) whether particles can be viewed as the fundamental reality (the “ontology”) described by QFT. Let us discuss these three issues separately. ...
...Can we base the ontology of QFT on local particles? Yes, but local particle states are very different from global particle states. Global particle states such as the Fock particle states are defined once and for all in the theory, while each finite size detector defines its own bunch of local particle states. Since in general the energy operators of different detectors do not commute ([HR, HR'] nonzero) there is no unique “local particle basis” in the state space of the theory, as there is a unique Fock basis. Therefore, we cannot interpret QFT by giving a single list of objects represented by a unique list of states. In other words, we are in a genuine quantum mechanical situation in which distinct particle numbers are complementary observables. Different bases that diagonalize different HR operators have equal footing. Whether a particle exists or not depends on what I decide to measure. In such a context, there is no reason to select an observable as “more real” than the others.
The world is far more subtle than a bunch of particles that interact.