Quantum gravity approaches (in particular, string theory) receive again and again the label of `unscientific’ endeavors.\footnote{Influential voices for both the academic and popular domain have been Peter Woit, and, a bit later, Sabine Hossenfelder, with their respective blogs and book projects.} Much of this criticism focuses on string theory, and, within string theory, on falsification: the naive charge usually is that string theory cannot be falsified as there is no connection to experiment. Upon the rejoinder that the scales relevant for string theory are simply so far away that tremendous amount of work (and time) is needed to make string theory phenomenologically accessible, the charge gets varied into a pessimistic meta-inductive one: string theorists have again and again claimed to be close to experimental verification, and yet they could never deliver. When thus (or rather why not) stop believing them then?
As far as I can tell, this variant is still a false understanding of falsificationism (itself a highly oversimplified take on how theories get tested in the face of evidence — just take a look at Smith’s Closing the loop if you are in search of the actual sophistication with which researchers found theories) — as if falsifiable hypotheses had to be produced within a certain time-frame to make a theory worth investigating. As a matter of fact, given that string theory is a final theory candidate, one might very well think that it is hard to probe it at its native scales.\footnote{See \cite{DawidFinal} for an argument to this effect.}
More interestingly though, one may want to judge string theory to be a stalling research program in the spirit of Lakatos: similar questions get discussed again and again without major breakthroughs (think for instance of the evergreen of that even though we live rather in a dS than in a AdS world, there is so far only a AdS/CFT not a dS/… correspondence to work with). Another charge is that string theory has degenerated into some general (albeit perhaps not rigorous\footnote{\cite{Wait} likes to even condemn the status of string theory as mathematics but that strikes me as — often pointed out — as a too narrow conception of what mathematics is.}) mathematical framework: applications of string theory’s technique in condensed matter physics for instance support such conception of string theory as effectively only a formal toolbox.
There are, however, also charges pressed against string theory with very strong sociological components: that the approach is kept central and free of larger competition by a large interest group (with motifs such as jobs for the juniors and awards for the seniors, and incentives such as low costs for departments), which corroborates its views among each other in a form of group think; and`mafia-like’ keeps other parties from entering the stage (string theory is described as the `only game in town’ by its proponents; proactively so, by its opponents).
Now, this book has little focused on string theory specifically. Similar charges can (at least partly) be heard against other approaches (with clear exception of the unjust domination-charge that is usually used by proponents of other approaches to quantum gravity against string theory). What we can, however, resist in any case based on the findings of this book, is that the typical quantum gravity approaches are not physics (or science, more generally speaking). That they are just mathematics. Here’s why.
The status quo is that there is no experimental data for (high-energy) quantum gravity — or rather that we have no concrete predictions from high-energy quantum gravity that we can compare to any known data. As long as this is the case, our approaches are still in an a decisive sense under construction. (This is not to say that discovery necessarily entails the lack of data; historically, even discovery, not to say motivation for a new theory was driven by data, of course and acknowledged earlier. At the same time, discovery and construction of theories in theoretical physics did happen without empirical data — a central example being general relativity.)
The process of discovery is scientific from accounts of science: following Hoyningen-Huene, the hallmark of the scientific project is systematicity; and, with sympathies for Hoyningen, Bartels thinks of theoreticity, i.e., that one works with refined terminology and reasoning complexes, as the central mark of the scientific endeavor. Both systematicity and theoreticity in the development of quantum gravity approach have arguably been worked out with the the central strategies found — viewpoint search, analogies (including more specific schemes such as a quantisation), and physical principles for guideline and weak corroboration. (Notably, these methods are also standard methods of discovery of 20th century physics more generally.) Arguably, even biting the bullet that only falsifiability is sufficient ultimately (something which Hoyningen-Huene and Bartels also respectively stress), the point remains that theories of quantum gravity are perfectly fine theories physics \textit{at the stage of discovery} — and that it is just mistaken to ask more from them (at this stage).
Arguably, more charitably, what Woit and Hossenfelder (for instance) want to put in question is not that string theory (or other approaches) are science; but that they operate on good standards, to which we can attach much promise. Leaving aside that some of their charges do attempt to call into question the basic premises for good science (such as intellectual honesty and freedom of speech).s
As far as I can tell, this variant is still a false understanding of falsificationism (itself a highly oversimplified take on how theories get tested in the face of evidence — just take a look at Smith’s Closing the loop if you are in search of the actual sophistication with which researchers found theories) — as if falsifiable hypotheses had to be produced within a certain time-frame to make a theory worth investigating. As a matter of fact, given that string theory is a final theory candidate, one might very well think that it is hard to probe it at its native scales.\footnote{See \cite{DawidFinal} for an argument to this effect.}
More interestingly though, one may want to judge string theory to be a stalling research program in the spirit of Lakatos: similar questions get discussed again and again without major breakthroughs (think for instance of the evergreen of that even though we live rather in a dS than in a AdS world, there is so far only a AdS/CFT not a dS/… correspondence to work with). Another charge is that string theory has degenerated into some general (albeit perhaps not rigorous\footnote{\cite{Wait} likes to even condemn the status of string theory as mathematics but that strikes me as — often pointed out — as a too narrow conception of what mathematics is.}) mathematical framework: applications of string theory’s technique in condensed matter physics for instance support such conception of string theory as effectively only a formal toolbox.
There are, however, also charges pressed against string theory with very strong sociological components: that the approach is kept central and free of larger competition by a large interest group (with motifs such as jobs for the juniors and awards for the seniors, and incentives such as low costs for departments), which corroborates its views among each other in a form of group think; and`mafia-like’ keeps other parties from entering the stage (string theory is described as the `only game in town’ by its proponents; proactively so, by its opponents).
Now, this book has little focused on string theory specifically. Similar charges can (at least partly) be heard against other approaches (with clear exception of the unjust domination-charge that is usually used by proponents of other approaches to quantum gravity against string theory). What we can, however, resist in any case based on the findings of this book, is that the typical quantum gravity approaches are not physics (or science, more generally speaking). That they are just mathematics. Here’s why.
The status quo is that there is no experimental data for (high-energy) quantum gravity — or rather that we have no concrete predictions from high-energy quantum gravity that we can compare to any known data. As long as this is the case, our approaches are still in an a decisive sense under construction. (This is not to say that discovery necessarily entails the lack of data; historically, even discovery, not to say motivation for a new theory was driven by data, of course and acknowledged earlier. At the same time, discovery and construction of theories in theoretical physics did happen without empirical data — a central example being general relativity.)
The process of discovery is scientific from accounts of science: following Hoyningen-Huene, the hallmark of the scientific project is systematicity; and, with sympathies for Hoyningen, Bartels thinks of theoreticity, i.e., that one works with refined terminology and reasoning complexes, as the central mark of the scientific endeavor. Both systematicity and theoreticity in the development of quantum gravity approach have arguably been worked out with the the central strategies found — viewpoint search, analogies (including more specific schemes such as a quantisation), and physical principles for guideline and weak corroboration. (Notably, these methods are also standard methods of discovery of 20th century physics more generally.) Arguably, even biting the bullet that only falsifiability is sufficient ultimately (something which Hoyningen-Huene and Bartels also respectively stress), the point remains that theories of quantum gravity are perfectly fine theories physics \textit{at the stage of discovery} — and that it is just mistaken to ask more from them (at this stage).
Arguably, more charitably, what Woit and Hossenfelder (for instance) want to put in question is not that string theory (or other approaches) are science; but that they operate on good standards, to which we can attach much promise. Leaving aside that some of their charges do attempt to call into question the basic premises for good science (such as intellectual honesty and freedom of speech).s