Will fundamental physics become an art?
According to Popper
scientific theories are never true but are always falsifiable.
...of course there aesthetic considerations play a large part even if
they're not nominally thought of in that way.
The answer bellow was before the change of the question to "...is driven solely by aesthetics?" My answer and time did not deserve this new question.
Fundamental physics has beauty as a selection criterion of a theory, and falsificationism is falsifiable.
Theories or hypotheses can only be subjected to empirical testing in groups or collections, never in isolation. The idea here is that a single scientific hypothesis does not by itself carry any implications about what we should expect to observe in nature; rather, we can derive empirical consequences from an hypothesis only when it is conjoined with many other beliefs and hypotheses, including background assumptions about the world, beliefs about how measuring instruments operate, further hypotheses about the interactions between objects in the original hypothesis' field of study and the surrounding environment, etc. For this reason when an empirical prediction turns out to be falsified, we do not know whether the fault lies with the hypothesis we originally sought to test or with one of the many other beliefs and hypotheses that were also needed and used to generate the failed prediction. It forms a criticism of methodological falsificationism.
Holist underdetermination ensures there cannot be any such thing as a “crucial experiment”: a single experiment whose outcome is predicted differently by two competing theories and which therefore serves to definitively confirm one and refute the other. Our response to the experimental or observational falsification of a theory is always underdetermined in this way. When the world does not live up to our theory-grounded expectations, we must give up something, but because no hypothesis is ever tested in isolation, no experiment ever tells us precisely which belief it is that we must revise or give up as mistaken. All of the beliefs we hold at any given time are linked in an interconnected web, which encounters our sensory experience only at its periphery.
It would be possible for us to preserve it “come what may” in the way of empirical evidence, by making sufficiently radical adjustments elsewhere in the web of belief. It is in principle open to us to revise even beliefs about logic, mathematics, or the meanings of our terms in response to recalcitrant experience; it might seem a tempting solution to certain persistent difficulties in quantum mechanics, for example, to reject classical logic's law of the excluded middle, allowing physical particles to both have and not have some determinate classical physical property like position or momentum at a given time.
Underdetermination then is when the available data do not permit us to make a decision between two or more rival theories. There are two forms of underdetermination: strong and weak. Strong tells us that there is no way to distinguish between theories with the same observable consequences – called empirical equivalence – and points to the existence of an infinity of possible theories consistent with any finite data set. We do not claim to be able to choose between empirically equivalent theories on the basis of empirical criteria, which is impossible by definition. It forms other criticism of methodological falsificationism. Moreover, it relies on an implicit separation of theory and observation: We cannot distinguish between theory and observation in a straightforward fashion, we cannot appeal to or rely on observations without theories and make a choice. All the observations that give us this problem of underdetermination are themselves theory-laden. Then strong underdetermination makes theory choice impossible because we already use theory in obtaining the observatory evidence that leads to underdetermined theories.
To the weaker underdetermination is always possible to construct alternative theories which are empirically equivalent and also choise one with many of the characteristics we desire in scientific theories: parsimony; internal consistency; beauty. Weak underdetermination is the recognition of the limits of evidentialism and falsificationism, the notion that we hold to our ideas insofar as they are supported by evidence. We do not accept or reject theories based solely on the evidence for them but also on account of many non-empirical criteria, such as parsimony; internal consistency; beauty.
The underdetermination is inductive, but science can believe in entities that are not directly observable, such as electrons. Not only the observables are relevant to believing a scientific theory. Simplicity, explanatory power or some other feature of a theory is criterion for it over its rivals.