The Whole Truth

A Cosmologist's Reflections on the Search for Objective Reality

P. J. E. Peebles

• Verlag: Princeton University Press
• Jahr: 2022
• ISBN:
  9780691231365
• Thema: Physical Sciences
• Thema: Cosmology

From the Nobel Prize–winning physicist, a personal meditation on the quest for objective reality in natural science A century ago, thoughtful people questioned how reality could agree with physical theories that kept changing, from a mechanical model of the ether to electric and magnetic fields, and from homogeneous matter to electrons and atoms. Today, concepts like dark matter and dark energy further complicate and enrich the search for objective reality. The Whole Truth is a personal reflection on this ongoing quest by one of the world's most esteemed cosmologists.What lies at the heart of physical science? What are the foundational ideas that inform and guide the enterprise? Is the concept of objective reality meaningful? If so, do our established physical theories usefully approximate it? P. J. E. Peebles takes on these and other big questions about the nature of science, drawing on a lifetime of experience as a leading physicist and using cosmology as an example. He traces the history of thought about the nature of physical science since Einstein, and succinctly lays out the fundamental working assumptions. Through a careful examination of the general theory of relativity, Einstein's cosmological principle, and the theory of an expanding universe, Peebles shows the evidence that we are discovering the nature of reality in successive approximations through increasingly rigorous scrutiny.A landmark work, The Whole Truth is essential reading for anyone interested in the practice of science.

CHAPTER ONE

On Science and Reality

The physical sciences that have grown out of curiosity-driven research have given us the enormous range of technology that so broadly affects our everyday lives, from electric power to the dubious benefit of cell phones. But are the theories that inform all this technology to be considered handy summaries, ways to remember useful experimental results? Or might we accept the assumption that most of us working on research in natural science take for granted, that our well-tested theories are good approximations to a reality that is objective, independent of our attempts to look into what this reality might be?

What is the meaning of reality? It is easy to say that it is what I experience when I wake up. But maybe I am still dreaming, or maybe, as the philosopher Gilbert Harman (1973, page 5) put it, “a playful brain surgeon might be giving [me] these experiences by stimulating [my] cortex in a special way.” The thought is playful but the lesson is serious: natural scientists cannot prove they are discovering the nature of objective reality. The argument I am presenting in this book is that physical scientists are in a position to make a persuasive case about what they feel they have learned about a postulate: reality.

You will not often hear the case for reality made by scientists; they would rather go on with research conducted by the precepts they learned from what others are doing and by what they find works for them. This inattention has aided misunderstandings. Scientists point to the demonstrated power of theories that bring to order large ranges of phenomena, and successfully predict a lot more, down to cell phones. But philosophers and sociologists can point out that the best of our scientific theories are incomplete and rest on evidence that is limited by inevitable measurement uncertainties. How then can scientists claim to be discovering absolute truths? When scientists make such claims they should not, cannot, really mean it. The argument instead is that the predictive power of science, demonstrated by all the technology surrounding us, is what one would expect if objective physical reality operated by rules, and if we were discovering useful approximations to these rules.

We should pause for a little closer consideration of the thinking about the predictive power of theories. Suppose a theory is devised to fit a given set of observations. If the theory is a good approximation to what we are assuming is the reality underlying the observations, then we expect that applications of the theory to other, different, situations successfully predicts results of observations of the new situations. The greater the variety of successful predictions, that is, the greater the predictive power, the better the case that the theory is a good approximation to reality. It cannot be a proof; scientists will never be able to claim that the predictive power of their theories demonstrates that they are exact representations of reality. We can only assert that the impressive success of physical science, the broad predictive power of our theories, makes a case that our science is a good approximation to reality that is difficult to ignore.

You may say that these successful predictions are easy to ignore; just do it. But if you do, I urge you to pause to consider the technology you see operating around you. Scientists and engineers can make electrons do their bidding in your cell phone, by the operation of electric and magnetic fields that manipulate electrons and solid and liquid crystals. Does this look like the application of a myth peculiar to our culture? I put it that the many examples of technology of this sort that you see in operation around you make a case for culture-free physics that is very hard to ignore.

If it is accepted that the results of natural science are useful approximations to objective facts, then are our physical theories simply ways to remember these facts? The position taken in natural science is that the predictive power of the well-established theories demonstrates that they are more than that; they are useful approximations to the way reality operates. This is best explained by an example. The one to be considered in the following chapters is physical cosmology, the study of the large-scale nature of the observable universe.

Thinking about the power and limitations of natural science, about empirical facts and their unification and theoretical predictions, is not new. A century ago the American philosopher and scientist Charles Sanders Peirce emphasized the impressive predictive power of the physical theories of the time, what we now term the classical theories of electromagnetism, mechanics, and gravity. But there were expressions of doubt. Another excellent physicist, the Austrian Ernst Mach, asked whether these theories of mechanics and electromagnetism, and of heat and light, are overelaborate, maybe a little artificial. He preferred to think of theories as means of remembering facts. At that time the German-British philosopher F. C. S. Schiller went further, asking whether these facts are only constructions peculiar to eventualities of choices made by our particular society. The predictive power of our theories is even greater now but some things have not changed; we still hear such doubts. This is at least in part because scientists do not usually acknowledge that they work with some constructions that owe more to society than empirical evidence, to say nothing about even less well grounded speculations that physicists occasionally take more seriously than calm consideration would recommend. I will be discussing examples, and will argue that we find common ground by considering the many results from applications of theory and practice in natural science that certainly look like good approximations to reality, while bearing in mind lessons scientists can draw from what sociologists and philosophers observe scientists doing.

My worked example of all this is the growth of physical cosmology, the study of the nature of the universe on the largest scales we can observe. I start with the situation a century ago, when Einstein was thinking about this and he and others were contemplating the broader question of the nature of physical science: how is it done, and what do the results mean? My account begins in Chapter 3 with considerations of Einstein’s theory of gravity, general relativity. The evidence we have now is that this theory gives a good description of the expansion of our universe. The idea was discussed in the 1930s, but until the 1960s the evidence for the expanding universe was meagre, the idea of cosmic expansion largely speculative. We can term it a social construction, to take a term from sociology. This with other assessments of science from the perspective of sociology is the subject of Chapter 2. The rest of the present chapter reviews thoughts about the nature of science, now and a century ago. These first two chapters are meant to introduce the considerations that are illustrated by the examples from general relativity and cosmology presented in Chapter 3 and on.

1.1 Thinking a Century Ago

A century ago Albert Einstein was thinking about about the nature of a satisfactory theory of gravity, and that led him to wonder how a philosophically sensible universe might be arranged. What were others thinking about physical science then? We see one line of thought in an essay published in The Popular Science Monthly,¹ by the American philosopher and scientist Charles Sanders Peirce² (1878a pages 299–300). Peirce wrote that³

all the followers of science are fully persuaded that the processes of investigation, if only pushed far enough, will give one certain solution to every question to which they apply it. One man may investigate the velocity of light by studying the transits of Venus and the aberration of the stars; another by the oppositions of Mars and the eclipses of Jupiter’s satellites; a third by the method of Fizeau; a fourth by that of Foucault; a fifth by the motions of the curves of Lissajoux; a sixth, a seventh, an eighth, and a ninth, may follow the different methods of comparing the measures of statical and dynamical electricity. They may at first obtain different results, but, as each perfects his method and his processes, the results will move steadily together toward a destined centre. So with all scientific research. Different minds may set out with the most antagonistic views, but the progress of investigation carries them by a force outside of themselves to one and the same conclusion. This activity of thought by which we are carried, not where we wish, but to a foreordained goal, is like the operation of destiny. No modification of the point of view taken, no selection of other facts for study, no natural bent of mind even, can enable a man to escape the predestinate opinion. This great law is embodied in the conception of truth and reality. The opinion which is fated¹ to be ultimately agreed to by all who investigate, is what we mean by the truth, and the object represented in this opinion is the real. That is the way I would explain reality.

The footnote in Peirce’s second last sentence explains that

Fate means merely that which is sure to come true, and can nohow be avoided. It is a superstition to suppose that a certain sort of events are ever fated, and it is another to suppose that the word fate can never be freed from its superstitious taint. We are all fated to die.

We see Peirce’s pragmatic endorsement of the idea of objective facts. It is in line with common experience: hit a wine glass and it will break. The idea of facts is taken seriously if usually implicitly in the normal practice of natural science. Also usually implicit in natural science, though Peirce makes it explicit, is the assertion or, better, the postulate that there is “truth and reality” that would be “ultimately agreed to by all who investigate.”

In other versions of this essay, which I think appeared later, the first sentence reads “all the followers of science are animated by a cheerful hope …,” and the third from the last begins “This great hope is embodied …”⁴ Misak (2013, pages 50–52) makes it clear that Peirce took the word “hope” seriously. An example is his comment that⁵

the only assumption upon which he can act rationally is the hope of success … it is always a hypothesis uncontradicted by facts and justified by its indispensibleness for making any action rational.

This is a good way to characterize the pragmatic study of physical science and the search for reality. We assume reality operates by rules so we can hope to discover them. It has worked well so far.

Peirce points out that values of the speed of light derived from quite different methods of observation, and using different theories for the reduction of the data from the observations, agree within reasonable allowance for measurement uncertainties. That is, given the result of one method, you could successfully predict what the results from the other methods would be. This demonstration of successful predictions is at the core of the meaning of results of research in physical science, so important that we should take the time to review the experiments and observations Peirce had in mind.

A transit of Venus is observed as a small black dot that moves across the sun along a chord of the face of the sun. Observers at different latitudes on Earth see the transit at chords of different length, meaning they find different times of transit as Venus enters and then leaves the face of the sun. Surveyors had measurements of the radius of the earth, so the distances between observers at different latitudes were known. Newton’s theory of the motions of the planets gave the ratio of distances to Venus and the sun. With these data, trigonometry gives the earth-sun distance and the speed of the earth around the sun.⁶ The former is known in the jargon as the solar parallax: the angular size of the earth at the sun. The latter is checked by the time it takes for the earth to complete one orbit around the sun, one year, given the solar parallax. Finally, the time of passage of Venus across the face of the sun gives the speed of Earth relative to the sun. The speed of Earth relative to the speed of light gives the angle, or aberration, by which the apparent positions of stars move as the earth swings around the sun. (Motion at speed v perpendicular to the direction to the star causes its angular position to shift in the direction of motion by v∕c radians, where c is the speed of light.) Since our speed around the sun was measured, this ratio gave a measure of the speed of light.

Peirce mentioned a second measure of the solar parallax, derived from observations when Mars, Earth, and the sun are nearly in a line and Mars is on the opposite side of Earth from the sun. Mars is said to be in opposition to the sun. Since Mars is closest to us then, this is a good time to measure the distance to Mars by measuring angular positions of Mars relative to distant stars from different places on Earth, or from one place in the morning and evening when Mars and stars close to it in the sky are just visible. Then again trigonometry with the radius of the earth translates these angles to the Mars-Earth distance. Newtonian physic...