What We Do Not Know

Throughout this book I have tried to convey the incredible amount of knowledge that we have obtained about our universe and our physical world. The cumulative effort of scientists over several thousand years has led to this understanding. There is still a final chapter on this to come, but before contemplating the ultimate fate of the universe and of life itself, let us look from a more critical viewpoint at the knowledge that we have acquired and ask the question, What do we really know? I have made quite a big deal of the fact that, at this moment in our history, we have determined our place in space and time within our universe. And that is a truly incredible achievement from which I do not wish to distract in this chapter. But it is good to question things at an even deeper level and to reveal some of the basic things that we do not completely understand, which include space and time itself.