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arXiv:gr-qc/9601046 · WORLD SCIENTIFIC eBooks

Scientific Knowledge from the Perspective of Quantum Cosmology

James B. Hartle

Published Dec 9, 2020• 12 citations
arXivHTMLPDF

Abstract

Existing physical theories do not predict every feature of our experience but only certain regularities of that experience. That difference between what could be observed and what can be predicted is one kind of limit on scientific knowledge. Such limits are inevitable if the world is complex and the laws governing the regularities of that world are simple. Another kind of limit on scientific knowledge arises because even simple theories may require intractable or impossible computations to yield specific predictions. A third kind of limit concerns our ability to know theories through the process of induction and test. Quantum cosmology -- that part of science concerned with the quantum origin of the universe and its subsequent evolution -- displays all three kinds of limits. This paper briefly describes quantum cosmology and discusses these limits. The place of the other sciences in this most comprehensive of physical frameworks is described.

DOI
10.1142/9789811216404_0005

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Existing physical theories do not predict every feature of our experience but only certain regularities of that experience. That difference between what could be observed and what can be predicted is one kind of limit on scientific knowledge. Such limits are inevitable if the world is complex and the laws governing the regularities of that world are simple. Another kind of limit on scientific knowledge arises because even simple theories may require intractable or impossible computations to yield specific predictions. A third kind of limit concerns our ability to know theories through the process of induction and test. Quantum cosmology -- that part of science concerned with the quantum origin of the universe and its subsequent evolution -- displays all three kinds of limits. This paper briefly describes quantum cosmology and discusses these limits. The place of the other sciences in this most comprehensive of physical frameworks is described.
A quantum theory of the universe consists of a theory of its quantum dynamics and a theory of its quantum state The theory predicts quantum multiverses in the form of decoherent sets of alternative histories describing the evolution of the universe's spacetime geometry and matter content. These consequences follow: (a) The universe generally exhibits different quantum multiverses at different levels and kinds of coarse graining. (b) Quantum multiverses are not a choice or an assumption but are consequences of the theory or not. (c) Quantum multiverses are generic for simple theories (d) Anthropic selection is automatic because observers are physical systems within the universe not somehow outside it. (e) Quantum multiverses can provide different mechanisms for the variation constants in effective theories (like the cosmological constant) enabling anthropic selection. (f) Different levels of coarse grained multiverses provide different routes to calculation as a consequence of decoherence. We support these conclusions by analyzing the quantum multiverses of a variety of quantum cosmological models aimed at the prediction of observable properties of our universe. In particular we show how the example of a multiverse consisting of a vast classical spacetime containing many pocket universes arises automatically as part of a quantum multiverse describing an eternally inflating false vacuum that decays by the quantum nucleation of true vacuum bubbles. In a FAQ we argue that the quantum multiverses of the universe are scientific, real, testable, falsifiable, and similar to those in other areas of science even if they are not directly observable on arbitrarily large scales.
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
Modern scientists describe the universe in terms of two basic partial theories: firstly, the general theory of relativity proposed by Albert Einstein, and secondly, the theory of quantum mechanics. This theory introduces an immense reality and simultaneously creates complex questions for the world of physics. Quantum mechanics is an incompletable knowledge however scientists are finding the basic theories from this theory. They propose complex mathematical expressions from quantum mechanics to prove the basic law of atoms. Quantum mechanics is the entire knowledge of subatomic particles such as electrons, protons, and neutrons. Quantum mechanics reveals that what we see is just a part of a deeper reality. Then we can grasp from this theory that the material world is not the completed reality; there are other things beyond human physical senses. The concept of quantum mechanics describes particles that can exist in multiple states. The concept of quantum mechanics is directly proportional to contemporary world technologies. The super specialty of this theory is that scientists cannot be explained universe without quantum mechanics. The uncertainty principle of Heisenberg in quantum mechanics suggests that there are limits to what we can know about a particle state of an atom. Humans cannot yet find out what it looks like and how we can imagine the existence of subatomic particles in an atom. That was an unseen dynamite for humans' perspective observation of the material world.
Modern science has held unprecedented success in describing the workings of the universe. Quantum electrodynamics – the capstone of 20th century scientific endeavour – is one of the most accurately demonstrated theories in history. It predicts the behaviour of the physical world to unparalleled precision, and it has driven the most revolutionary of our modern technological innovations. But this most fundamental of physical theories is far from complete. To the contrary, its many successes do well to conceal its limitations. The more deeply we peer into the fundamental workings of the universe, the more we find that we have to impose limitations not only upon what we know, but on what we can know. In over 100 years since its inception, there has been little progress in reconciling the illogical, often counterintuitive implications of quantum theory. This lack of progress in forming a coherent interpretation suggests either a theory that is incomplete in its description of reality, or a fundamental flaw underlying the manner in which we perceive the world around us.
Measurement outcomes provide data for a physical theory. Unless they are objective they support no objective scientific knowledge. So the outcome of a quantum measurement must be an objective physical fact. But recent arguments purport to show that if quantum theory is universally applicable then there is no such fact. This calls for a reappraisal of the notions of fact and objectivity. If quantum theory is universally applicable the facts about the physical world include a fact about each quantum measurement outcome. These physical facts lack an ideal kind of objectivity but their more modest objectivity is all that science needs.
One of the fundamental insights of quantummechanics is that complete knowledge of the state of a quantum system is not possible. Such incomplete knowledge of a physical system is the norm rather than the exception. This is becoming increas-ingly apparent as we apply scientific methods to increasingly complex situations. Empirically intensive disciplines in the biological, human, and geosciences all operate in situations where valid conclusions must be drawn, but deductive com-pleteness is impossible. This paper argues that such situations are emerging exam-ples of Open World Science. In this paradigm, scientific models are known to be acting with incomplete information. Open World models acknowledge their incompleteness, and respond positively when new information becomes available. Many methods for creating Open World models have been explored analytically in quantitative disciplines such as statis-tics, and the increasingly mature area of machine learning. This paper examines the role of quantum theory and quan-tum logic in the underpinnings of Open World models, ex-amining the importance of structural features of such as non-commutativity, degrees of similarity, induction, and the im-pact of observation. Quantum mechanics is not a problem around the edges of classical theory, but is rather a secure bridgehead in the world of science to come.