Sabine Hossenfelder posted these questions 21 Nov on her blog. http://backreaction.blogspot.com/2013/11/the-five-questions-that-keep-physicists.html They are too good not to copy here for us at Beyond forum. I've been a Bee fan for a long time--admire her commonsense insight into essentials. I'll just copy the questions--you should read the whole post, if you haven't already. ==quote from Bee's post== Is the time-evolution of the universe deterministic, indeterministic or neither? How can we find out? Can we at all? And, based on this, is free will an illusion? This question doesn’t really fall into any particular research area in physics as it concerns the way we formulate the laws of nature in general. It is probably closest to the foundations of quantum mechanics, or at least that’s where it gets most sympathy. Does the past exist in the same way as the present? Does the future? Does a younger version of yourself still exist, just that you’re not able to communicate with him (her), or is there something special about the present moment? The relevance of this question (as Lee elaborated on in his recent book) stems from the fact that none of our present descriptions of nature assigns any special property to the ever-changing present. I would argue this question is closest to quantum gravity since it can’t be addressed without knowing what space and time fundamentally are. Is mathematics the best way to model nature? Are there systems that cannot be described by mathematics? I blame Max Tegmark for this question. I’m not a Platonist and don’t believe that nature ultimately is mathematics. I don’t believe this because it doesn’t seem likely that the description of nature that humans discovered just yesterday would be the ultimate one. But if it’s not then what is the difference between mathematics and reality? Is there anything better? If so, what? If not, what does this mean for science? Does a theory of everything exist and can it be used, in practice (!), to derive the laws of nature for all emergent quantities? If so, will science come to an end? If not, are there properties of nature that cannot be understood or even modeled by any conscious being? Are there cases of strong emergence? Can we use science to understand the evolution of life, the development of complex systems, and will we be able to tell how consciousness will develop from here on? What is the origin and fate of the universe and does it depend on the existence of other universes? That’s the question from my list you are most likely to find on any ‘big questions of physics’ list. It lies on the intersection of cosmology and quantum gravity. Dark matter, dark energy, black holes, inflation and eternal inflation, the nature and existence of space-time singularities all play a role to understand the evolution of the universe. ==endquote== She calls these the "night-time questions" because they are not the practical well-defined sort that are part of a physicists ordinary daytime job (like "what particle constitutes dark matter?"). Instead they are the questions that could keep a physicist awake at night worrying about them. What's the right way to even ask such questions, much less answer them?