This is a Small Book about a Big Question, not a textbook of known physics. Or perhaps it's about a Big Opinion – or a small opinion, depending on one's perspective. It's a book about unknown physics. Every scientific fact was born as an opinion about the unknown, often called a ‘hypothesis’. Opinion gradually becomes fact when evidence piles up. By perceptive and diligent work, it is

… possible to attain a degree of probability that quite often is hardly less than complete certainty. Namely, when the things that one has deduced from the supposed principles correspond perfectly to the phenomena that observations show us,

as Huygens wrote. It has been so ever since, except that instead of ‘supposed principles’ we now say ‘theory’. But what if there are two theories, each of which has produced a myriad of ‘things that correspond perfectly to the phenomena’ but that cannot be combined? One theory replaced the mystery of gravity by a precise picture of space and time. The other replaced the mystery of matter by a description of quantum particles that is so exact that some of its predictions have been verified to eleven decimal places. At the present time in our Universe, we may keep these two separate, each in its own domain: space and time for very large things, particles for the world of the very small. However, 13.8 billion years ago, these two incompatible theories referred to a single realm. Many scientists think that they can be united only by a minuscule group of hyper-specialists. I think differently. The mathematics of the ultimate answer will be as arcane as always, but that formulation will have to follow upon some original perception. Insight is freely distributed; all you’ve got to do is pick it up. I hope that somewhere a girl or boy will do so, because the generations of physicists who made the existing brilliant theories will soon be extinct. We will never understand the beginnings of our Universe until this puzzle has been cracked. That is why I hold the opinion that this is not just a big question, but the Biggest Question in physics of the 21st century.