Wing Sheung Chan

Summary “Mysterious things – when you were a child you used to think: ‘They exist for sure!’, but all of a sudden it changes to, ‘It would be nice if they existed.’ I wonder when and why you stop believing.” — Kozue Amano, “Akari Mizunashi”, ARIA Nature’s mysteries Since the beginning of history, humans have always been fascinated by Nature and trying to solve all its mysteries. We make observations, find patterns in them, and then create theories that we hope could describe and predict Nature. But more often than not, solving a mystery would only lead us to discover even more mysteries. Yet, this seemingly never ending dialectic cycle of theory – problem – solution – new theory has always been how we advanced our understanding of the world. A problem is but an opportunity for us to break through and expand our knowledge. Today, probably the best theory we have to describe the physical world is the Standard Model (SM) of particle physics. The SM is a theory that postulates a set of elementary particles (Figure S.1) and uses their interactions to accurately describe many observed phenomena. It has withstood the scrutiny of countless experiments to date. Nevertheless, there are still many unsolved mysteries of Nature that the SM could not explain, including: • Why is there so much more matter than antimatter in the universe? (the matter- antimatter asymmetry problem) • What is dark matter – the invisible matter in our universe that can only be “observed” indirectly through gravitational phenomena? (the dark matter problem) • Within the same theoretical framework that the SM is based on (quantum field theory), what is gravity? (the quantum gravity problem) • Why is gravity so much weaker than the other fundamental forces? Or related to this, why is the Higgs boson so light compared to the Planck mass? (the hierarchy problem) 157