In these lectures I propose to discuss some of the results of modern study of the physical world which give most food for philosophic thought. This will include new conceptions in science and also new knowledge. In both respects we are led to think of the material universe in a way very different from that prevailing at the end of the last century. I shall not leave out of sight the ulterior object which must be in the mind of a Gifford Lecturer, the problem of relating these purely physical discoveries to the wider aspects and interests of our human nature. These relations cannot but have undergone change, since our whole conception of the physical world has radically changed. I am convinced that a just appreciation of the physical world as it is understood to-day carries with it a feeling of open-mindedness towards a wider significance transcending scientific measurement, which might have seemed illogical a generation ago; and in the later lectures I shall try to focus that feeling and make inexpert efforts to find where it leads. But I should be untrue to science if I did not insist that its study is an end in itself. The path of science must be pursued for its own sake, irrespective of the views it may afford of a wider landscape; in this spirit we must follow the path whether it leads to [Pg xvii] the hill of vision or the tunnel of obscurity. Therefore till the last stage of the course is reached you must be content to follow with me the beaten track of science, nor scold me too severely for loitering among its wayside flowers. That is to be the understanding between us. Shall we set forth? CONTENTS Preface v Introduction ix Chapter I. The Downfall of Classical Physics 1 II. Relativity 20 III. Time 36 IV. The Running-Down of the Universe 63 V. “Becoming” 87 VI. Gravitation—the Law 111 VII. Gravitation—the Explanation 138 VIII. Man’s Place in the Universe 163 IX. The Quantum Theory 179 X. The New Quantum Theory 200 XI. World Building 230 XII. Pointer Readings 247 XIII. Reality 273 XIV. Causation 293 XV. Science and Mysticism 316 Conclusion 343 Index 355 THE NATURE OF THE PHYSICAL WORLD [Pg 1] Chapter I THE DOWNFALL OF CLASSICAL PHYSICS