> But in gravity, you’re sort of combining a whole bunch of different possible geometries of space-time. And what that means is, you’re not really sure what time is, for one thing, and you’re not really sure where things are in space, because if you don’t know the geometry of space, it is impossible to identify a point in space uniquely throughout all the possible quantum combinations of the geometry of space-time. So, we really, at a fundamental level, have difficulty knowing what we’re talking about, when it comes to quantum gravity.
Stephen Hawking once summarized this as "Gravity shapes the arena in which it acts" and concluded that, therefore, it'd be unlikely that gravity would be a quantum field like any other. It really annoys me that this is so often forgotten by people in high-energy theory and that quite a few of them pretend that
> We have a pretty good idea of what a theory of quantum gravity must look like
when in reality we have no clue at all. (That quote is from one of Sean Carroll's guests on the former's podcast – for better or for worse I don't remember her name. Notably, as becomes clear from the podcast, Sean Carroll is not among the people subscribing to this view.)
> But in gravity, you’re sort of combining a whole bunch of different possible geometries of space-time. And what that means is, you’re not really sure what time is, for one thing, and you’re not really sure where things are in space, because if you don’t know the geometry of space, it is impossible to identify a point in space uniquely throughout all the possible quantum combinations of the geometry of space-time. So, we really, at a fundamental level, have difficulty knowing what we’re talking about, when it comes to quantum gravity.
Stephen Hawking once summarized this as "Gravity shapes the arena in which it acts" and concluded that, therefore, it'd be unlikely that gravity would be a quantum field like any other. It really annoys me that this is so often forgotten by people in high-energy theory and that quite a few of them pretend that
> We have a pretty good idea of what a theory of quantum gravity must look like
when in reality we have no clue at all. (That quote is from one of Sean Carroll's guests on the former's podcast – for better or for worse I don't remember her name. Notably, as becomes clear from the podcast, Sean Carroll is not among the people subscribing to this view.)