The 'Hard Problem' of why physical processes are sometimes associated with experiences may best be seen as a confused rewrite of a problem Newton described as 'not so easie'. He gave the example of 'how light â€¦ produceth in our minds the Phantasms of Colours'. It is a practical problem within natural science rather than an intractable mystery but, as Newton admitted, not covered by his 'physics'. It is not covered by his physics for a reason relating to the ways we describe the world, which Niels Bohr's quantum theory most clearly illustrates but which has been known for centuries, at least to people like Leibniz, and probably Newton himself.
Implicit within natural science, and even everyday life, are two complementary accounts of the world. The first is an account of our experience â€“ a phenomenal or experiential account. The second is an account of the rules that predict the way one experience follows another â€“ a dynamic or process account. Unfortunately, much of our language implies that these two accounts can be considered the same, or just differing in reliability or accessibility. This has led to a hybrid account of a 'real' world that is treated like a series of frames from a movie â€“ a geometric or kinematic account. This 'physical reality' actually has no validity. No serious physicist or neuropsychologist should have space for it. It is a folklore concept. The word 'reality' has two separate meanings. There is the reality of experience, which has no internal dynamics, and the reality of the dynamic rules that link experiences, which (the rules) have no experiential or phenomenal qualities â€“ they have no appearance.
Thus it is invalid to ask why physical processes are associated with experiences. Physical processes are nothing more than the rules that link experiences. People find this counterintuitive. However quantum theory shows that these rules cannot have any sort of reality with an appearance or 'physical form'. The operation of the rules is 'concrete' in philosophical terms because it occurs in defined instances, but the only form is that of dynamic relations. Moreover, studies of perception show that our concepts of form, space, time, and movement are internal concoctions full of paradoxical dodges, like the 'specious present', which is 'now' but has duration. They are internally generated experiential accounts, not 'physical' realities.
The 'not so easie' problem is the cracking of the code that links our dynamic account of the world to the experiences we have. Part of the solution (Chalmers's 'Easy' part) is to trace the dynamics right down to the point at which experience occurs. The last 400 years has got us close. But physicists like Leibniz have always known that this 'reductionism' must stop short of the whole answer. The final link between a dynamic and an experiential account can be neither dynamic (mechanistic) nor experiential. A final link between mechanism and experience has no mechanism by definition. When we ask 'why?' we ask what is the mechanism or dynamic causal chain. 'Why?' does not apply to a link with no mechanism, which cannot be reduced, so cannot be a mystery. Nevertheless, there is a mystery, which is that further 'not so easie' set of rules that governs this link. (In fact modern physics casts doubt on the idea of 'cause' or mechanism even within dynamics. One could argue that the extra 'linking' account is just a new section of 'dynamics'. However, its job is so different from that of conventional dynamics that it seems best to make the distinction.)
This hard part of our task of linking dynamics with experience is bedevilled by problems of accessibility and consistency but there is no reason to think it is intractable. It ought to relate to basic biophysical processes that are affected by anaesthetics and sleep. The solution will rely heavily on inference rather than direct observation, but it is hard in a technical and logistic sense, not in Chalmers's sense.