Coming from psychology this feels alien to me. In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics. There is no smooth transition between the two, either you describe the individual or you describe a group, you cannot do both (even thought debates about IQ here on HN will have you believe otherwise) and there is certainly no smooth transition.
How is the boundary between GR and QM different from the boundary of psychology and sociology?
I would say it is completely different. In physics, quantum mechanics gives extremely precise and verifiable predictions of the outcomes of experiments within a certain range of physical conditions. So does general relativity. In contrast, psychology has no mathematical model that will, for example, accurately predict what I am going to eat for lunch, nor is there a model that predicts the exact outcomes of elections. Since physics has two models which are exquisitely precise in different size regimes, but which are mutually incompatible, you have a definite puzzle about what happens in situations where the effects of both theories should be important. There are no exquisitely precise and accurate mathematical models of anything in the social sciences, as far as I am aware.
Im neither a psychologist nor a physicist, but I think one can analogize population vs individual behaviour to condensed matter physics versus particle physics. Condensed matter physics finds emergent behaviour in large clumps of stuff that would, in principle, be totally predictable from first principles (the standard model), but which in practice are quite difficult to guess a priori. Different scales lend themselves to different tools, since nonlinear dynamics (chaos) makes it intractable to apply bedrock reductionist formulae to large systems. The higher order behaviour of complex systems is in no way less interesting or true, I would argue, than the seemingly simpler behaviour of very small systems.
In contrast to condensed matter vs the standard theory (QM basically), QM vs GR has fundamental incongruities, since both theories make claims about what happens at the same scale. Only one (or most likely neither) can be correct at the event horizon and center of a black hole.
The difference would be that the laws of physics are much more rigorous than the 'laws' of psychology/sociology.
Outliers in the latter aren't necessarily indicative of anything wrong with theory in general, while even a single outlier in anything in the former is indicative of an incomplete model.
There are various results in physics which should be predictable via both GR and QM independently. The results should be the same as both models are supposed to be describing the same thing, so it follows that there should be some sort of gradual transition as one set of effects gradually comes to dominate over the other. Otherwise we'd see a single point in the data where QM stops being accurate and GR takes over, but despite investigating so many different scales, we have not seen any such cutoff point.
> In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics.
I'm aware of my own behavior as an individual being influenced by social context, is that not the kind of bleed over you might look for? Maybe you're referring to specific concepts I'm not actually even understanding.
You still use theories from psychology to describe the interaction. This is precisely what social-psychology does (admittedly spectacularly often without replication). And a good social psychology theory is consistent with other fields of psychology, like cognitive, or—more often—behavioral psychology. You don’t use population statistics to predict how you as an individual will behave in a certain situation.
If you could precisely and reliably describe and predict individual behavior for any individual, then population behavior would follow directly from those laws.
> In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics.
> either you describe the individual or you describe a group
In gravitational physics, with respect to a flow in a dynamical system (an example is galaxies in an expanding universe) we can use a Lagrangian observer (e.g., one galaxy, drifting along with the flow, tracing out a pathline/worldtube that depends on features like its mass-evolution and proper motion within a cluster of galaxies) or a Eulerian observer (e.g. a notional observer with no spatial motion at all, watching alllll the galaxy clusters jiggle, swirl, turn, and age a little differently in relation to her). One can convert observations of each type of observer to the other in a rigorous mathematical procedure, since they are just two (families of) the infinity of different observers allowed by even just Special Relativity. See e.g. <https://en.wikipedia.org/wiki/Lagrangian_and_Eulerian_specif...> for more detail.
>> There are boundaries between where GR and QM are predictive
> this feels alien to me
You can do both quantum matter and classical General Relativity in one of several effective field theories, which I'll return to below.
GR and relativistic quantum field theories (QFT) purport to make accurate predictions in strong gravity, which one only finds deep within black holes (i.e., not on our side of any horizon), but they make very different predictions in that regime pretty generically. Generically in the sense that choosing different behaviours of particle-particle interactions (and self-interactions) do not really move the needle on GR's prediction of a collapse to a core of infinite density. However, in various approaches which convert GR's classical gravitational waves into large number of gravitons, one can write down a matter QFT in which charged particles' self-interaction can lead to a degeneracy pressure (a repulsive force) that increases at higher particle energies such that they overwhelm gravitational collapse at very high but finite density in black holes of arbitrary mass.
In weak gravity, like we have in our solar system, QFTs allow us to prepare significant masses in superpositions of (spatial) position. General relativity does not allow for such superpositions. We are approaching lab-testability, with results from sensitive accelerometers allowed to point at tiny superposed masses.
However, in regimes far from (non-negligibly) gravitating superpositons and strong gravity, GR and QFT are usefully (and possibly fully) compatible. We get good results in astrophysics from semi-clasical gravity, where the classical curved spacetime of General Relativity couples with the expectation value of QFT matter (i.e., we average out some quantum weirdness and justify this by the weak gravitational effects of the "lumpy" weirdness being practically impossible to measure; superpositions and ultra-high-energy/ultra-high-denisty systems might be too lumpy).
We also get good results from perturbative quantum gravity and canonical quantum gravity, for example. Neither of these latter two is really classical General Relativity so they can deal with the gravitation of superposed matter (otherwise they give for all practical purposes the same answers as semiclassical gravity). These approaches do not work in strong gravity, however. Essentially they become calculationally intractable or they crash into unresolved problems splitting spacetime into space and time (in order to do time-dependent quantum mechanics).
I can't say anything about promising. The hard part seems to be building an apparatus that works, and I don't know how to do that. I hear of short-lived superpositions with increasing amus or daltons but I can't deal with those units without the newest SI prefixes (fun facts, 666 Yamu is a bit more than 1 kg; and there is maybe about 0.666 YMsun locked up in observable galaxy clusters).
How is the boundary between GR and QM different from the boundary of psychology and sociology?