I have been thinking about these question and if I should post it, because I want it to fit the rules of these site. Let's see if I managed it:
Are the ultimate degrees of freedom of the subatomic realm accessible with a good experiment or are we going to decide their existence with indirect proofs like those of quarks?
I mean, usually, we have learned about the ultimate constituent of the subatomic universe colliding stuff and breaking them into "parts"...I suppose this method could have a limit (I am not sure if due to money of machines or due to the nature of the ultimate degrees of freedom of matter-energy and space-time). Thus, what are the alternative to colliders or detectors of high energy particles? Maybe astronomical, astrophysical and cosmological observations?
And with respect to the issue of the existence of strings, p-branes and other supersymmetric objects, due to duality, can we really test the p-brane degrees of freedom with some class of experiments? After all, for instance, in electromagnetic duality, a strong electric field theory is dual equivalent to a weak theory of magnetic monopoles (even if we have not detect them!)...So, are dual degrees of freedome "testable" or are they a mere tool of calculations? This question remember me a little bit about the dual character of light at the end of the 19th century and the beginning of the 20th...Or is this a different issue?