We assume theories need experimental proof to be credible. But philosopher of science Richard Dawid argues that string theory has earned trust through a different route: meta-empirical assessment. When decades of searching turn up no viable alternatives, and unexpected predictions keep emerging, a theory's viability can become probable even without testing. Empirical confirmation isn't the only path to scientific credence.

For many decades, theoretical physicists have aimed at developing a theory that jointly describes quantum phenomena and gravity. String theory, which is widely considered to be the most promising candidate, takes gauge field theory, the conceptual framework for contemporary particle physics, as its starting point and develops a theory that includes gravity from there. Other approaches, pursued by smaller numbers of physicists, choose a more modest starting point and deploy the basic principles of quantum mechanics for developing a theory of gravity.      

Theories of quantum gravity face two serious problems that set them apart from well-established theories like gauge field theories or general relativity.  First, their characteristic empirical predictions are so difficult to test by experiment that no such empirical tests have been done or can be expected in the foreseeable future. Second, the theories are conceptually and mathematically so difficult that decades of intense work have not even come close to formulating a complete theory. 

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Arguments in support of string theory’s viability don’t look weaker today than 25 years ago.

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In light of these problems, the question is: what can be the status of the current theories of quantum gravity, in particular of its leading exponent, string theory? Views on this question have varied a lot during recent decades. The 1990s saw a high level of confidence among string theorists. They presented their theory as a decisive breakthrough in the human understanding of fundamental physics, had high confidence in the theory’s viability, and were optimistic about imminent substantial steps towards a full understanding of the theory. The early 2000s witnessed a highly visible pushback by physicists from rivaling research programs who doubted the scientific basis for string theorists’ optimism and promoted the idea that, in the absence of empirical testing, all theories of quantum gravity should equally be considered mere speculations.

The last 20 years saw substantial changes in the understanding of what string theory amounted to, but also a significant slowdown of conspicuous conceptual progress towards a full understanding of the theory. Among many string theorists, what was in retrospect viewed as previous overconfidence with respect to completing the theory has led to a more hedged approach towards characterizing the theory’s status in public. Today, the dominant attitude is to avoid any impression that string theorists judge their theory by standards that differ from the canonical principles of empirical confirmation. Working on the theory, therefore, is often motivated by pointing at its pursuit-worthiness rather than by emphasizing the theory’s prospects of being correct.

It seems plausible to say, however, that, while expectations in the late 1990s regarding the prospects of approaching the completion of string theory have turned out overly optimistic, arguments in support of string theory’s viability don’t look weaker today than 25 years ago.