You are essentially correct in the third paragraph. Let me elucidate and be more correct. Bear with me. We have found four field theories (electromagnetism, we

You are essentially correct in the third paragraph. Let me elucidate and be more correct. Bear with me. We have found four field theories (electromagnetism, weak theory, strong theory, gravitation)[1]. Each of theories have a domain of applicability in which, in principle, all the results of all experiments can be predicted using that one theory. Domain of applicability includes length scale, time scale, energies involved etc. This is not a satisfying situation because it is extremely common for systems to not respect this human-imposed domain. Therefore, we want a single theory whose domain is the union of the domains of all of these theories. Call this goal the goal of unification. This has been partly been achieved (electroweak theory), and work continues. Then there is another goal. To find a theory that has a even larger domain of applicability than before. This is the problem of scale. The article claims that the problem of scale is never ending because finite experimental data (and philosophical claims not discussed in the article). String theory is an attempt at the unification goal but it comes with a larger domain of applicability as a bonus. I think this makes the two separate issues discussed in the article clear. I make a third point that you asked about. I make the following observations. General relativity under reasonable restrictions reduces to Newton's law of gravity. The correspondence principle claims that quantum mechanics (typically) reduces to classical mechanics if your system has a large number of particles. Special relativity reduces to classical mechanics for low speeds. These are three pairs of micro-macro theories. In all of these cases the micro theory also works at the scale of the macro theory if you choose to use it: In other words the domain of applicability of the macro theory is a strict subset of the domain of applicability of the micro theory. This subset business does not work so nicely for field theories. Or when looking at general relativity and quantum theories at the same time. There are overlapping domains but no nice strict subset. In that case there is not simple notion of micro-macro theory. However, returning to your original question you can sometimes use multiple theories at the same time to make predictions. Eg. particle physics experiments at CERN do this (multiple field theories), or that blackhole physics that keeps making headlines (quantum and gravity). This situation is of course far from satisfactory and work towards the goal of unification continues. [1] All of these theories of fields assume quantum mechanics which is a theory of particles. Our universe is made up of particles and fields, as far as we know.