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I'm wondering about the way physicists develop "unified theories" and models about the universe, like string theory, or quantum mechanics. People (on this forum for example) often say things like "at the current state of the development of string theory...", as if predicting further development of the theory, or rather as if understating that such theories are continuously developed.

My questions are : how can "unified theories" be incomplete and need to be developed further if they are trusted by scientists to be true? How does development of such theories occur, through continuous small progress or rather through rare epiphanies (eurêka)?

Sorry for somewhat vague title question, but the subject of the question is to me quite vague as well --- my studies are rather far from hardcore theoretical physics. I'd just like to understand better how research is done in physics.

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  • $\begingroup$ This may be better suited to Philosophy; a discussion of classic concepts of the philosophy of science seems to be relevant. $\endgroup$
    – Danu
    Commented Sep 13, 2015 at 0:36
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    $\begingroup$ Why do you think that "unified theories" are trusted by scientists to be true? So far only electroweak unification has been empirically confirmed, and the rest is seen as work in progress. Scores of different string theories have been developed, most of them just toy models that no one expects to be true, much work is done on exploring relationships between them and finding ways to do efficient computations within them. There is active search for both more realistic theories and experimental ways of confirming them. $\endgroup$
    – Conifold
    Commented Sep 13, 2015 at 0:49
  • $\begingroup$ Theories are being developed by scientists who know where the existing ones are failing. That's a very hard business because one has to understand the structure of our current theories in great detail first, before one can even think about adding to an old theory or before one can suggest a structurally different one. Just to give you an idea... the last fundamental theory that we have added to physics was quantum mechanics and that was in the 1930s... before that it was general relativity in 1915 and special relativity in 1905. Everything else is really just models within these theories. $\endgroup$
    – CuriousOne
    Commented Sep 13, 2015 at 4:07
  • $\begingroup$ @Conifold "the rest is seen as work in progress" : is it expected such work eventually is completed ? (this is also quite related to philosophy, sry) $\endgroup$
    – user92644
    Commented Sep 13, 2015 at 15:30
  • $\begingroup$ @lepiment The expectation is that SM extends from currently probed energy scales all the way to the Planck scale, it is a "desert" for new physics. If that is the case I wouldn't expect anything definitive for decades at least until probing those directly becomes technologically feasible. There are some soft ways of probing Planck scale, but they are unlikely to be sharp enough to discriminate between different versions of QG say en.wikipedia.org/wiki/… $\endgroup$
    – Conifold
    Commented Sep 14, 2015 at 22:53

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(I presume that you meant "Physicist" not "Physician" as in one who practices medicine)

Scientific theories are usually a group of ideas that are linked either through a common phenomenon or a common method. Usually, for more developed theories, these are in the form of postulates(think of quantum mechanics or statistical mechanics), with other ideas developed from them. For newer theories (such as string theory) these postulates are still being figured out (and continually tested against experiments). As far as regarding them as "true", in some cases it means that it fits mathematically with the rest of known physics(if there are no tests able to yet be done in the field), or fits experimental evidence better than other theories do.

And all fields of science have continous improvement. Even the theory of classical mechanics is still being developed mathematically.

The development of these theories is small incremental progress in the right direction. It usually is that someone notices that the current theories have some problem, and multiple theories (most of them wrong) are proposed to correct it, then someone (or a group of people) notice a way to unify the evidence in a new way that is better than all of the others. A great example is the development of Relativity and Quantum Mechanics in the early 20th century.

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  • $\begingroup$ Ok I understand better now. But aren't experimental conditions limits to the development of a theory ? I presume not everything can be verified through experiment, perhaps fundamental postulates of a theory. Does that mean that those postulates will be left unverified, only generally accepted as "true", and that the theory they form basis to is built upon unverified principles ? (This question could be related to philosphy, as @Danu suggested, but I think it fits in the discussion) (Correct, physicist is what I meant) $\endgroup$
    – user92644
    Commented Sep 13, 2015 at 15:28
  • $\begingroup$ @lepiment Experimental conditions are not limits to the development of a theory, they are the way that the theory gets verified. In science, experiment is the final arbiter of truth; if a theory disagrees with experiment, it is incorrect. If the postulates predict phenomenon better than anything we have, we accept them as true. But if buy "Accept as true" you mean "be 100% sure" then we accept very little (perhaps only the metaphysical and epistemological assumptions needed for science) as true. Every theory in science is open to being proven wrong by the facts. $\endgroup$
    – JDThinking
    Commented Sep 13, 2015 at 18:37
  • $\begingroup$ Thank you for your answers, they're very appreciated. I actually believed (or rather wanted to believe) that science was binary, as if every theory was either "reality" or "not reality". And that the things we were taught in physics class corresponded exactly to the reality of the universe -- if not, why would we teach them and use them to work? Well I realised recently this wasn't at all what science was aimed to, and I wanted some information to clarify. Thanks. $\endgroup$
    – user92644
    Commented Sep 14, 2015 at 13:11
  • $\begingroup$ If you provide specific examples to illustrate your statements would improve the answer, such as the one "And all fields of science have continuous improvement. Even the theory of classical mechanics is still being developed mathematically." $\endgroup$ Commented Jun 26, 2020 at 11:05