Physics in the 21st Century

The following sections are included:

• Contents

• Preface

• Opening Address

I review the developement of particle physics from the viewpoint of theoretical methodology. Particle physics has gone through certain characteristic periods in the course of discoveries and accumulation of knowledge, both factual and theoretical. To anlyze this situation, I draw on the theses of three stages and three modes, and illustrate the points with historical examples, up to the present Standard Model and the speculative theories of unification. As an exercise in this analysis, I propose a possible regularity in the quark masses.

A brief review is given of quantum transport phenomena in mesoscopic systems. The topics include ballistic transport such as the conductance quantization in quantum point contacts, magnetic focusing, and various phenomena appearing in quantum wire junctions, universal conductance fluctuations arising from quantum interferences, and single electron tunneling due to the charging energy in quantum dots.

Complex spatial or temporal patterns emerge when simple systems are driven from equilibrium in ways that cause them to undergo instabilities. Dendritic (i.e. snowflake-like) crystal growth is an especially clear example. We now have a well tested and remarkably detailed theory of how dendrites form and grow. We understand the role of intrinsically small effects such as crystalline anisotropy, and we even can trace the growth of dendritic sidebranches all the way back to their origins as microscopic thermal fluctuations. This extreme sensitivity to perturbations explains many long-standing puzzles about snowflakes. A similar sensitivity is emerging in the physics of dynamic fracture, a phenomenon which is similar in some ways to dendritic growth but which now seems to be governed by entirely different mechanical and mathematical principles. Extreme sensitivity to perturbations and system parameters is likely to be a characteristic feature of pattern-forming systems in general, including those that occur in biology. If so, we shall not succeed in reducing the physics of complexity to a small number of universality classes. On the contrary, we shall have to be prepared for a wide variety of challenges and surprises in this field.

Beams of β- radioactive nuclei, having a lifetime as short as 1 ms have been used for studies of the nuclear structure and reaction relevant to nucleosynthesis in the universe. In nuclear-structure studies, decoupling of the proton and neutron distributions in nuclei has been discovered. The decoupling appeared as neutron halos and neutron skins on the surface of neutron-rich unstable nuclei. In astrophysics, reaction cross sections have been determined for many key reactions of nucleosynthesis involving short-lived nuclei in the initial and final states. One such important reaction, ¹³N+p → ¹⁴O +γ, has been studied using beams of unstable ¹³N nuclei. Such studies became possible after the invention of beams of radioactive nuclei in the mid-80's. Before that, the available ion beams were restricted to ions of stable nuclei for obvious reasons. In the next section the production method of radioactive beams is presented, then a few selected studies using radioactive beams are discussed in the following sections. In the last section, some useful properties of radioactive nuclei for other applications is shown.

In the first part of the talk, I give a low-resolution overview of the current state of particle physics — the triumph of the Standard Model, and its discontents. I review and re-endorse the remarkably direct and (to me) compelling argument that existing data, properly interpreted, point toward a unified theory of fundamental particle interactions and toward low-energy supersymmetry as a major component of the near-term future of high energy physics. I then briefly discuss other challenges and opportunities in the field.

The following sections are included:

• Introduction

• What Have We Learned?
  • Quantum Field Theory Works
  • The Secret of Nature is Symmetry
  • Effective Dynamics
  • Questions

• What Are the Challenges?
  • Within the Standard Model
  • Beyond the Standard Model

• String Theory

• What Will the Future Bring?

In this article I would like to describe a) major goals in the field of relativistic heavy-ion physics, b) three most recent data, and c) future prospects in particular plans at Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory.

Present status and future plans of high energy physics with hadron colliders are reviewed. Observation of top quark, recent results on bottom quark physics and QCD phenomena are discussed. Current mass limits and future plans of search for Higgs particle, supersymmetric and other new particles are described.

According to the Friedmann models without the cosmological constant, structure and time evolution of the Universe are characterized by the two parameters, the Hubble constant H_o and the density parameter Ω_o. Recent observational determinations of the Hubble constant is reviewed and H_o ~ 73 ± 10 kms⁻¹ Mpc⁻¹ is shown to be the present best estimate of the local (≲ 100 Mpc) value. The determination of the density parameter is shown to be coupled with the evolutionary history of galaxies, which is not well understood yet. The density parameter is not constrained by the observation very well at present and 0.1 < Ω_o ≤ 1.0 would be a reasonable estimate. The age of the Universe is discussed in terms of the parameter values.

Two fundamental laws are needed for prediction in the universe: (1) a basic dynamical law and (2) a law for the cosmological initial condition. Quantum cosmology is the area of basic research concerned with the search for a theory of the initial cosmological state. The issues involved in this search are presented in the form of eight problems.