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arXiv:gr-qc/0508001

Excess Baggage

James B. Hartle

Published Jul 30, 2005• 0 citations
arXivHTMLPDF

Abstract

Many advances in physics have in common that some idea which was previously accepted as fundamental, general, and inescapable was subsequently seen to be consequent, special, and dispensable. The idea was not truly a general feature of the world, but only perceived to be general because of our special place in the universe and the limited range of our experience. It was excess baggage which had to be jettisoned to reach a more a more general perspective. This article discusses excess baggage from the perspective of quantum cosmology which aims at a theory of the universe's quantum initial state. We seek to answer the question `Which features of our current theoretical framework are fundamental and which reflect our special position in the universe or its special initial condition?' Past instances of cosmological excess baggage are reviewed such as the idea that the Earth was at the center of the universe or that the second law of thermodynamics was fundamental. Examples of excess baggage in our current understanding of quantum cosmology are the notion that measurement is central to formulating quantum mechanics, a fundamental quantum mechanical arrow of time, and the idea that a preferred time is needed to formulate quantum theory. We speculate on candidates for future excess baggage. (This article appeared in 1991 but is posted here for accessibility and possible current interest.)

DOI
10.48550/arxiv.gr-qc/0508001

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Excess Baggage

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Many advances in physics have in common that some idea which was previously accepted as fundamental, general, and inescapable was subsequently seen to be consequent, special, and dispensable. The idea was not truly a general feature of the world, but only perceived to be general because of our special place in the universe and the limited range of our experience. It was excess baggage which had to be jettisoned to reach a more a more general perspective. This article discusses excess baggage from the perspective of quantum cosmology which aims at a theory of the universe's quantum initial state. We seek to answer the question `Which features of our current theoretical framework are fundamental and which reflect our special position in the universe or its special initial condition?' Past instances of cosmological excess baggage are reviewed such as the idea that the Earth was at the center of the universe or that the second law of thermodynamics was fundamental. Examples of excess baggage in our current understanding of quantum cosmology are the notion that measurement is central to formulating quantum mechanics, a fundamental quantum mechanical arrow of time, and the idea that a preferred time is needed to formulate quantum theory. We speculate on candidates for future excess baggage. (This article appeared in 1991 but is posted here for accessibility and possible current interest.)
Many advances in physics have in common that some idea which was previously accepted as fundamental, general, and inescapable was subsequently seen to be consequent, special, and dispensable. The idea was not truly a general feature of the world, but only perceived to be general because of our special place in the universe and the limited range of our experience. It was excess baggage which had to be jettisoned to reach a more a more general perspective. This article discusses excess baggage from the perspective of quantum cosmology which aims at a theory of the universe's quantum initial state. We seek to answer the question `Which features of our current theoretical framework are fundamental and which reflect our special position in the universe or its special initial condition?' Past instances of cosmological excess baggage are reviewed such as the idea that the Earth was at the center of the universe or that the second law of thermodynamics was fundamental. Examples of excess baggage in our current understanding of quantum cosmology are the notion that measurement is central to formulating quantum mechanics, a fundamental quantum mechanical arrow of time, and the idea that a preferred time is needed to formulate quantum theory. We speculate on candidates for future excess baggage. (This article appeared in 1991 but is posted here for accessibility and possible current interest.)
QUANTUM COSMOLOGY It is an honor, of course, but also a pleasure for me to join in this celebration of Murray Gell-Mann's sixtieth birthday and to address such a distinguished audience. Murray was my teacher and more recently we have worked together in the search for a quantum framework within which to erect a fundamental description of the universe which would encompass all scales – from the microscopic scales of the elementary particle interactions to the most distant reaches of the realm of the galaxies – from the moment of the big bang to the most distant future that one can contemplate. Such a framework is needed if we accept, as we have every reason to, that at a basic level the laws of physics are quantum mechanical. Further, as I shall argue below, there are important features of our observations which require such a framework for their explanation. This application of quantum physics to the universe as a whole has come to be called the subject of quantum cosmology. The assignment of the organizers was to speak on the topic “Where are our efforts leading?” I took this as an invitation to speculate, for I think that it is characteristic of the frontier areas of science that, while we may know what direction we are headed, we seldom know where we will wind up.
Computability and physical theories / R. Geroch and J.B. Hartle -- Excess baggage -- Sources of predictability -- The no-boundary wave function of the universe -- Scientific knowledge from the perspective of quantum cosmology -- The physics of 'now' -- The quantum mechanics of closed systems -- The spacetime approach to quantum mechanics -- Quantum physics and human language -- Quantum pasts and the utility of history -- The quasiclassical realms of this quantum universe -- Living in a superposition -- The quantum mechanical arrows of time -- Why our universe is comprehensible -- Quantum cosmology : problems for the 21st century -- Anthropic reasoning and quantum cosmology -- Are we typical? / J.B. Hartle and M. Srednicki -- Science in a very large universe / M. Srednicki and J.B. Hartle -- The observer strikes back / J.B. Hartle and T. Hertog -- Quantum multiverses -- Quantum mechanics at the Planck scale -- The state of the universe -- The impact of cosmology on quantum mechanics -- Murray Gell-Mann : a short appreciation -- Working with Stephen Hawking
Human languages employ constructions that tacitly assume specific properties of the limited range of phenomena they evolved to describe. These assumed properties are true features of that limited context, but may not be general or precise properties of all the physical situations allowed by fundamental physics. In brief, human languages contain `excess baggage' that must be qualified, discarded, or otherwise reformed to give a clear account in the context of fundamental physics of even the everyday phenomena that the languages evolved to describe. The surest route to clarity is to express the constructions of human languages in the language of fundamental physical theory, not the other way around. These ideas are illustrated by an analysis of the verb `to happen' and the word `reality' in special relativity and the modern quantum mechanics of closed systems.
Human languages employ constructions that tacitly assume specific properties of the limited range of phenomena they evolved to describe. These assumed properties are true features of that limited context, but may not be general or precise properties of all the physical situations allowed by fundamental physics. In brief, human languages contain `excess baggage' that must be qualified, discarded, or otherwise reformed to give a clear account in the context of fundamental physics of even the everyday phenomena that the languages evolved to describe. The surest route to clarity is to express the constructions of human languages in the language of fundamental physical theory, not the other way around. These ideas are illustrated by an analysis of the verb `to happen' and the word `reality' in special relativity and the modern quantum mechanics of closed systems.
In discussing the fundamental sources of unpredictability, I shall concentrate on those indeterminacies that are definitely required by theory. Let me therefore begin \nby eliminating from consideration supposed indeterminacies stemming from doubts that some people may entertain about basic principles or from certain kinds of ignorance that I believe to be temporary and likely to be remedied in the relatively near future. I assume the following: \n1. Quantum mechanics is correct. The formulation and interpretation of quantum mechanics are still undergoing some necessary evolution, especially in order to accommodate quantum cosmology in a comfortable way, but the basic character of quantum mechanics has always been the same and we may suppose it will remain unchanged. \n2. The elementary particles and their interactions obey a definite dynamical law, discoverable by inquiring complex adaptive systems such as the human scientific enterprise. Although the process of discovery involves a sequence of \napproximate schemata, there is an endpoint to the process after a finite amount of research. (Of course, it can never be possible to prove that the resulting unified theory is perfect; one can only verify it in the usual way by comparing predictions to available observations.) Humans may already have found this unified quantum field theory in the form of superstring theory, which has, to begin with, no arbitrary parameters. (Of course, spontaneous symmetry \nbreaking may give rise to some parameters and even to a choice of solutions, with probabilities for the various alternatives. I will deal with that possibility further on.) This second assumption is equivalent to stating that there is no necessary fundamental unpredictability stemming from ignorance of the universal dynamical law. \n3. The density matrix (in the Schrödinger picture) of the universe near the beginning of its expansion is also knowable. It must in any case be comparatively simple and very far from equilibrium. The second law of thermodynamics and the other associated arrows of time are explained by these properties of the initial density matrix along with the fact that the universe is still very young—the interval of ten billion years is extremely short compared with the relaxation time from the special initial condition.