>String theory has not made predictions that are in any sense meaningful to science.
You're welcome to set the bar of meaningful at any level you like; such a statement cannot in principle be disagreed with. However the point stands that string theory has made more accurate empirical predictions than any other theory of quantum gravity. That doesn't mean no other theory of quantum gravity will ever beat it, just that the focus on string theory among people and funders who are precomitted to studying quantum gravity is rational.
I think the above is using "meaningful" to refer to "falsifiable". The fact that string theory is so much more advanced theoretically and still doesn't make any falsifiable predictions makes me less optimistic about it than less mature theories like LQG, which (according to some experts) already seems closer to making falsifiable claims.
String theory leads to the falsifiable claims of gravity and quantum mechanics. I don't know why this non-falsifiability argument keeps getting repeated! It would be nice for string theory to make more claims than just that, but they're still working on it. LQG is an example of a framework that truly makes no claims, because there isn't even a LQG setup for three dimensions of space. To my knowledge it hasn't even been proven that there is an LQG setup for three dimensions of space.
Yes. It has "falsifiable claims" in the sense that, if existing theories of e.g. general relativity and quantum mechanics were proven false, it would also be proven false. Generally when people refer to "falsifiable claims" they're referring to falsifiable claims that are not already made by a more parsimonious theory. As far as I understand the field, string theory has made no such claims.
And yes, that's what I meant by LQG being a less mature theoretical space. Either LQG will fail to find a solution for 3D space (so sad), or it will and its solution will make falsifiable predictions that are not part of existing theory (yay!), or it will propose a set of solutions that make no new falsifiable predictions (in which case it enters the same state as string theory). Both of those first two possibilities would be forward progress.
Look around you and note that there appear to be about three dimensions, and furthermore there's gravity. Also, quantum mechanics is true. String theory is the only theory that satisfies both of those conditions at once. Other approaches to quantum gravity aren't that far yet, but could conceivably yield those predictions at some point in the future. Today, though, there's only string theory.
You are still using “prediction” in a sense that I would say is not at all “risky”.
Put another way, so far string theory is an elaborate kind of curve-fitting — a mathematical construction that matched the empirical data available at its inception, but one that to this day has never predicted a novel measurement. Unencumbered by the theory, we would not have expected any recent experiment to produce different results.
Maps are literally curves fit to surveyed datapoints, and nobody questions their value. So what if string theory is nothing more than a map of physical laws? GR can't predict quantum behavior, the standard model doesn't have gravity, string theory does both and so is presently the only "theory of physics" in existence.
I thought I remember string theory positing more than three dimensions. Does string theory explain our perception of three at human length and time scales?
Yes, in string theory the extra dimensions are very small, leading to three macroscopic dimensions the same way the thinness of a piece of paper leads to two macroscopic dimensions.
Considering that it started with 11, or was it 26 dimensions, and had to go through some hoops to get it down to three, that statement is almost absurdly comical.
You're welcome to set the bar of meaningful at any level you like; such a statement cannot in principle be disagreed with. However the point stands that string theory has made more accurate empirical predictions than any other theory of quantum gravity. That doesn't mean no other theory of quantum gravity will ever beat it, just that the focus on string theory among people and funders who are precomitted to studying quantum gravity is rational.