Dear PFer's, I just finished reading the first chapter of Lee Smolin's famous "The Trouble with Physics". The first chapter of the book is titled "The Five Great Problems in Theoretical Physics" and is the source of the questions I want to post. I searched PF for similar posts, but I found nothing, so I will post here. In case you do not have access to the book, you can find more details and the full quote of the problems here: http://physics.about.com/od/physics101thebasics/a/fiveproblems.htm. Here follows the description of the problems that Prof. Smolin gives. I have remarks on Problems 2 and 5 only. Problem 2: Resolve the problem of the foundations of quantum mechanics, either by making sense or by inventing a new theory that does make sense. Here professor Smolin seems to be concerned with the issue of realism in QM - sorry I cannot post the two pages he devotes to the topic. I do not quite understand why realism is a QM issue rather than a general one. Unless one brings in the so called problem of hidden variables, in which case I believe we'd better talk of locality/causality alternatives. Then, I suspect the issue is solved by the results of the various EPR experiments - possibly in favor of non-locality. Again, pardon me if my opinions are a bit naive or over-simplified. Problem 5: Explain dark matter and dark energy. Or, if they don't exist, determine how and why gravity is modified on large scales. More generally, explain why the constants of the standard model of cosmology, including the dark energy, have the values they do. I do not see why the problems of dark matter and dark energy would be put together. Unless one is sure they have one and the same solution, which I suspect is not very likely. Also, I am not sure why inflation (and the associated field) and vacuum energy (i.e., the discrepancy between the QED prediction and the measured value) do not deserve a place in the list, au pair with dark matter and dark energy. I would appreciate the point of view of someone (much) more knowledgeable than I am. I am sure I gave you a lot of material for discussion ;-).