P. J. E. Peebles - The Whole Truth: A Cosmologists Reflections on the Search for Objective Reality

THE WHOLE TRUTH

The Whole Truth

A COSMOLOGISTS REFLECTIONS ON THE SEARCH FOR OBJECTIVE REALITY

P. J. E. Peebles

PRINCETON UNIVERSITY PRESS

PRINCETON & OXFORD

Copyright 2022 by Princeton University Press

Princeton University Press is committed to the protection of copyright and the intellectual property our authors entrust to us. Copyright promotes the progress and integrity of knowledge. Thank you for supporting free speech and the global exchange of ideas by purchasing an authorized edition of this book. If you wish to reproduce or distribute any part of it in any form, please obtain permission.

Requests for permission to reproduce material from this work should be sent to

Published by Princeton University Press
41 William Street, Princeton, New Jersey 08540
99 Banbury Road, Oxford OX2 6JX

press.princeton.edu

All Rights Reserved

Library of Congress Cataloging-in-Publication Data

Names: Peebles, P. J. E. (Phillip James Edwin), author.

Title: The whole truth: a cosmologists reflections on the search for objective reality / P. J. E. Peebles.

Description: Princeton: Princeton University Press, [2022] | Includes bibliographical references and index.

Identifiers: LCCN 2021051638 (print) | LCCN 2021051639 (ebook) | ISBN 9780691231358 (hardback) | ISBN 9780691231365 (ebook)

Subjects: LCSH: SciencePhilosophy. | Physics. | Cosmology. | Reality. | BISAC: SCIENCE / Space Science / Cosmology | SCIENCE / History

Classification: LCC Q175.P372 2022 (print) | LCC Q175 (ebook) | DDC 501dc23/eng/20220128

LC record available at https://lccn.loc.gov/2021051638

LC ebook record available at https://lccn.loc.gov/2021051639

Version 1.0

British Library Cataloging-in-Publication Data is available

Editorial: Ingrid Gnerlich and Whitney Rauenhorst

Production Editorial: Mark Bellis

Jacket Design: Chris Ferrante

Production: Danielle Amatucci

Publicity: Matthew Taylor and Kate Farquhar-Thomson

Copyeditor: Bhisham Bherwani

Jacket Credit: Galaxy orbits flowing out of voids and impinging on regions of high density. Constructed by Edward Shaya (University of Maryland), Brent Tully (University of Hawaii), Daniel Pomarede (University of Paris-Saclay), and Yehuda Hoffman (Hebrew University, Jerusalem)

To Alison, my best friend

1. xi

What is at the core of what we do in natural science? I say that we are searching for the nature of reality, but what does that mean? You will not find ready answers from most working scientists; they would rather work on the problems arising in the research at hand. But there are lines of thought drawn from the philosophy, sociology, and history of science that are good descriptions of what I see is at the heart of what we are doing in science. I mean to explain, with illustrations from cosmology, the theory of the expansion of the universe from a hot dense state.

I began to work on cosmology a half century ago, when ideas were speculative and evidence scant, and I have seen it grow into one of the best-established of the branches of physical science. The experience has led me to reflect on what my colleagues and I have been up to, and what we have learned, apart from how to work out more or less well posed problems as they arise. The result is presented in this book.

I mean this book to be accessible to those who are not familiar with the language of physics but are interested in what scientists are doing and why. Colleagues in science may be surprised to find me discussing aspects of sociology and philosophy, and the evidence from natural science for objective physical reality. These are not familiar topics of conversation in my crowd, but I think they are essential for a more complete picture of what we are learning from research in natural science. I am not attempting an assessment of thinking about reality in the philosophy and sociology of science, but I hope authorities in those fields will find it interesting to see how some ideas drawn from their disciplines find resonance with the practice of natural science.

Scientists have different ways of thinking about our subject. I am on the empiricist side of the spectrum; I love to see debates settled by measurements. But despite my empiricist inclination to confine attention to the state of the experimental or observational tests of physical theories, and leave it at that, I have been led to think that curiosity-driven science has produced good evidence of an abstract notion: objective reality. The notion as it is applied in science can be falsified, by failure of the scientific method. It can never be proved, because the accuracy of empirical evidence always is limited. The theme of this book is that the empirical results from natural science, presented in the worked example of physical cosmology, add up to a persuasive case for observer-independent reality. This is the best we can do in science.

How did we arrive at the present state of natural science, and the case for objective reality? A turning point in my thinking about this grew out of recognition of a commonplace experience in physics: when an interesting idea turns up the odds are fairly good that someone else has already remarked on it, or will, independently, if word does not travel fast enough. An example from another branch of science that even I have known for a long time is the independent recognition of the concept of evolution by natural selection, by Charles Darwin and Alfred Wallace. I have observed other examples in physical science since I was a graduate student, and until recently gave no thought to them. I dont imagine my experience is special in this regard, and indeed I have never heard a physicist remark that the occurrence of multiple discoveries is a phenomenon that could teach us something. But it dawned on me that the fact that Darwin and Wallace both hit on the idea of natural selection reenforces the case that this idea is sensible. After all, two people with feet on the ground independently noticed the evidence. In a similar way, the remarkably large number of multiple discoveries in explorations of the large-scale nature and evolution of the universe suggests that the evidence was offering sensible motivation for the directions in which our thinking was taking us.

My search for who recognized these multiples as a phenomenon to be considered in an assessment of natural science led me to sociologists. They recognize the phenomenon, and I saw that I had something to learn about the practice of physical science from books on sociology, and from that to philosophy, books I had never before thought to consult.

One result was my belated recognition that the sociologists concept of constructions, social and empirical, accounts for a curious situation in a book I treasure, The Classical Theory of Fields, by Landau and Lifschitz (1951; my copy is the English translation of the 1948 second Russian edition). The first two-thirds of the book is a careful analysis of the basics of the classical theory of electricity and magnetism. This is a well tested and broadly useful theory. The last third of the book presents Einsteins general theory of relativity. I knew from the time I joined Bob Dickes Gravity Research Group as a graduate student in 1958 that this theory had scant empirical support, quite unlike electromagnetism. So why was general relativity given near equal time to electromagnetism in