Q: What exactly is referred to by the "hard problem".
A: The hard problem of consciousness, according to Chalmers and the majority of philosophers that use this term, is the problem of how and why there is conscious experience occurring in a physical process. (1-5) Again, let’s examine Chalmers’ frequently quoted words:
“The hard problem, as I understand it, is that of explaining how and why consciousness arises from physical processes in the brain.” Chalrmers 1997(1)
“The hard problem of consciousness is the problem of experience. …There is no question that experience is closely associated with physical processes in systems such as brains. … But how and why do physical processes give rise to experience? Why do not these processes take place "in the dark," without any accompanying states of experience? This is the central mystery of consciousness.” Charlmers 2002(2)
And by the term experience or subjective experience, he means “Humans beings have subjective experience: there is something it is like to be them. … There is something it is like to see a vivid green, to feel a sharp pain, to feel a deep regret, .... Each of these states has a phenomenal character, with phenomenal properties (or qualia) characterizing what it is like to be in the state.” Charlmers 2002(2)
For example, our digestive system, our endocrine system, our cerebellar system, and all other unconscious systems in the brain are all complex physical processes that manage a lot of complex information, yet there are no conscious experiences (e.g. no what it is like to sense protein or fat in the stomach, no what is like to sense plasma testosterone at various levels, no what it is like to sense various signals from spinocerebellar tract, etc.) occurring in these systems. They all operate purely mechanically – in the dark – just like computers, and all their operations are completely explicable by existing physical laws.
On the other hand, the consciousness system operates with additional phenomena occurring – the conscious experiences: what it is like to see the red color in our mind, to hear in the sound of a song in our mind, to smell the sweetness of a rose in our mind, to feel happy in our mind, to relive past events in our mind, etc. Why does not the consciousness system just operate in the dark like all subconscious processes above – how and why do these conscious experiences occur in this particular physical process? So far, no known physical laws can explain their occurring. This is the essence of the problem.
So, the hard problem of consciousness is not the problem of whether we can replicate the consciousness system (its complex neural circuits and their orderly firing) in another system so that conscious experience occurs in the physical process of the latter system. If we can do that but still cannot answer the problem of how and why there is conscious experience occurring in such a physical process, the hard problem is still not resolved. Even if we cannot replicate such a feat because of some technical obstacles but know exactly what neural circuits and their firing that create conscious experience are, we still do not solve the hard problem if we cannot explain how and why there is conscious experience occurring in such a physical process.
On the contrary, even if we cannot replicate neural circuits and their firing in a physical process so that conscious experience occurs (because of some technical obstacles or any other things) but we can explain how and why a certain physical process has conscious experience occurring, we will have the done the task of solving the hard problem of consciousness.
Q: Does Chalmers believe that the hard problem can never be solved?
No, Chalmers does not espouse Mysterianism (which holds that the hard problem of consciousness cannot be resolved). In fact, he has a hope that the solution will be found and that some of the ideas needed for this are already present but only need to be developed:
“It is often held that even though it is hard to see how materialism could be true, materialism must be true, since the alternatives are unacceptable. As I see it, there are at least three prima facie acceptable alternatives to materialism on the table, each of which is compatible with a broadly naturalistic (even if not materialistic) worldview, and none of which has fatal problems. So given the clear arguments against materialism, it seems to me that we should at least tentatively embrace the conclusion that one of these views is correct. ...” Chalmers 2002(2)
Q: Does Chalmers believe that neuroscience cannot solve this hard problem?
I’ve never seen Chalmers concluded anywhere that neuroscience will not be able to do that. For him, to solve the hard problem requires some natural principles, which are not known at present:
“A solution to the hard problem would involve an account of the relation between physical processes and consciousness, explaining on the basis of natural principles how and why it is that physical processes are associated with states of experience.” Chalmers 2002(2)
And, as far as I know, he did not assert that this cannot come from neuroscience.
P.S. The following is not one of the questions directly asked in this thread, but it is an interesting and exciting question: Chalmers’ ideas and beliefs put aside, “Has the hard problem of consciousness actually been solved?” I believe the consensus at present among the majority of philosophers is “no”. But, to neuroscientists, quite some of them believe that they have solved it. Of course, their theories have not been accepted widely as correct, and the solution to the hard problem remains controversial and is still being widely discussed, debated, and researched. But, for those who are interested in exploring these new ideas, please try browsing references 6-10. At least, you’ll get some ideas of how neuroscientists are currently trying to solve this hard problem.
Chalmers DJ. Moving forward on the problem of consciousness. J Conscious Stud. 1997;4(1):3-46. http://consc.net/papers/moving.html
Chalmers DJ. Consciousness and its place in nature. In: Chalmers DJ, editor. Philosophy of mind: Classical and contemporary readings. Oxford: Oxford University Press; 2002. ISBN-13: 978-0195145816 ISBN-10: 019514581X. http://consc.net/papers/nature.html
Weisberg J. The hard problem of consciousness. The Internet Encyclopedia of Philosophy. https://www.iep.utm.edu/hard-con/
Gennaro RJ. Consciousness. Internet Encyclopedia of Philosophy. Retrieved 2017 Apr 18 from http://www.iep.utm.edu/consciou/
Van Gulick R. Consciousness. In: Zalta EN, editor. The Stanford Encyclopedia of Philosophy (Summer 2017 Edition). Retrieved 2017 Sep 8 from https://plato.stanford.edu/archives/sum2017/entries/consciousness/
Grossberg S. Towards solving the hard problem of consciousness: The varieties of brain resonances and the conscious experiences that they support. Neural Netw. 2017 Mar;87:38-95.
Loorits K. Structural qualia: A solution to the hard problem of consciousness. Front Psychol. 2014;5:237.
McFadden J. The Conscious Electromagnetic Information (Cemi) Field Theory. The hard problem made easy? J Conscious Stud. 2001;9(8):45–60.
Tononi G, Koch C. Consciousness: Here, there and everywhere? Philos Trans R Soc Lond B Biol Sci. 2015 May 19;370(1668):20140167.
Ukachoke C. The Basic Theory of the Mind. 1st ed. Bangkok, Thailand; Charansanitwong Printing Co. 2018.