Pub Date : 2021-01-01DOI: 10.1142/9789811216404_0017
J. Hartle, M. Srednicki
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Pub Date : 2019-01-13DOI: 10.1142/9789811216404_0023
J. Hartle
When quantum mechanics was developed in the '20s of the last century another revolution in physics was just starting. It began with the discovery that the universe is expanding. For a long time quantum mechanics and cosmology developed independently of one another. Yet the very discovery of the expansion would eventually draw the two subjects together because it implied the big bang where quantum mechanics wasimportant for cosmology and for understanding and predicting our observations of the universe today. Textbook (Copenhagen) formulations of quantum mechanics are inadequate for cosmology for at least four reasons: 1) They predict the outcomes of measurements made by observers. But in the very early universe no measurements were being made and no observers were around to make them. 2) Observers were outside of the system being measured. But we are interested in a theory of the whole universe where everything, including observers, are inside. 3) Copenhagen quantum mechanics could not retrodict the past. But retrodicting the past to understand how the universe began is the main task of cosmology. 4) Copenhagen quantum mechanics required a fixed classical spacetime geometry not least to give meaning to the time in the Schr"odinger equation. But in the very early universe spacetime is fluctuating quantum mechanically (quantum gravity) and without definite value. A formulation of quantum mechanics general enough for cosmology was started by Everett and developed by many. That effort has given us a more general framework that is adequate for cosmology --- decoherent (or consistent) histories quantum theory in the context of semiclassical quantum gravity. Copenhagen quantum theory is an approximation to this more general quantum framework that is appropriate for measurement situations. We discuss whether further generalization may still be required.
{"title":"The Impact of Cosmology on Quantum Mechanics","authors":"J. Hartle","doi":"10.1142/9789811216404_0023","DOIUrl":"https://doi.org/10.1142/9789811216404_0023","url":null,"abstract":"When quantum mechanics was developed in the '20s of the last century another revolution in physics was just starting. It began with the discovery that the universe is expanding. For a long time quantum mechanics and cosmology developed independently of one another. Yet the very discovery of the expansion would eventually draw the two subjects together because it implied the big bang where quantum mechanics wasimportant for cosmology and for understanding and predicting our observations of the universe today. Textbook (Copenhagen) formulations of quantum mechanics are inadequate for cosmology for at least four reasons: 1) They predict the outcomes of measurements made by observers. But in the very early universe no measurements were being made and no observers were around to make them. 2) Observers were outside of the system being measured. But we are interested in a theory of the whole universe where everything, including observers, are inside. 3) Copenhagen quantum mechanics could not retrodict the past. But retrodicting the past to understand how the universe began is the main task of cosmology. 4) Copenhagen quantum mechanics required a fixed classical spacetime geometry not least to give meaning to the time in the Schr\"odinger equation. But in the very early universe spacetime is fluctuating quantum mechanically (quantum gravity) and without definite value. A formulation of quantum mechanics general enough for cosmology was started by Everett and developed by many. That effort has given us a more general framework that is adequate for cosmology --- decoherent (or consistent) histories quantum theory in the context of semiclassical quantum gravity. Copenhagen quantum theory is an approximation to this more general quantum framework that is appropriate for measurement situations. We discuss whether further generalization may still be required.","PeriodicalId":416124,"journal":{"name":"The Quantum Universe","volume":"126 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128218115","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-01-25DOI: 10.1142/9789811216404_0020
J. Hartle
A quantum theory of the universe consists of a theory of its quantum dynamics (H) and a theory of its quantum state (Ψ). The theory (H,Ψ) predicts quantum multiverses in the form of decoherent sets of alternative histories describing the evolution of the universe’s spacetime geometry and matter content. A small part of one of these histories is observed by us. 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 (H,Ψ) or not. (c) Quantum multiverses are generic for simple (H,Ψ). (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 having different values of the fundamental constants 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. ∗ A pedagogical essay. †Electronic address: hartle@physics.ucsb.edu 1 ar X iv :1 80 1. 08 63 1v 1 [ gr -q c] 2 5 Ja n 20 18
宇宙的量子理论由其量子动力学理论(H)和量子态理论(Ψ)组成。该理论(H，Ψ)预测了量子多重宇宙以不同历史的退相干集合的形式来描述宇宙的时空几何和物质含量的演化。我们可以观察到其中一个历史的一小部分。这些结果如下:(a)宇宙通常表现出不同层次的不同量子多重宇宙和各种粗粒化。(b)量子多重宇宙不是一种选择或假设，而是(H，Ψ)的结果。(c)量子多重宇宙是一般的(H，Ψ)。(d)人为选择是自动的，因为观察者是宇宙内部的物理系统，而不是宇宙之外的。(e)量子多重宇宙可以为有效理论(如宇宙学常数)中的变异常数提供不同的机制，从而实现人类选择。(f)由于退相干性，不同级别的粗粒度多元提供了不同的计算途径。我们通过分析各种量子宇宙学模型的量子多重宇宙来支持这些结论，这些模型旨在预测我们宇宙的可观测特性。特别地，我们展示了多元宇宙的例子是如何由一个巨大的经典时空组成的，其中包含许多具有不同基本常数值的口袋宇宙，作为量子多元宇宙的一部分，它描述了一个永恒膨胀的假真空，它通过真正的真空气泡的量子成核而衰减。在一个常见问题解答中，我们认为宇宙的量子多重宇宙是科学的、真实的、可测试的、可证伪的，即使它们不能在任意大尺度上直接观察到，也与其他科学领域相似。一篇教学论文。†电子地址:hartle@physics.ucsb.edu 1 ar X iv:1 801。[au:] [au:] [au:] 2 5 Ja n 2018 .
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Pub Date : 2016-12-06DOI: 10.1142/9789811216404_0014
J. Hartle
Einstein wrote memorably that `The eternally incomprehensible thing about the world is its comprehensibility.' This paper argues that the universe must be comprehensible at some level for information gathering and utilizing subsystems such as human observers to evolve and function.
{"title":"Why Our Universe Is Comprehensible","authors":"J. Hartle","doi":"10.1142/9789811216404_0014","DOIUrl":"https://doi.org/10.1142/9789811216404_0014","url":null,"abstract":"Einstein wrote memorably that `The eternally incomprehensible thing about the world is its comprehensibility.' This paper argues that the universe must be comprehensible at some level for information gathering and utilizing subsystems such as human observers to evolve and function.","PeriodicalId":416124,"journal":{"name":"The Quantum Universe","volume":"35 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2016-12-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125079892","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2015-11-04DOI: 10.1142/9789811216404_0012
J. Hartle
This essay considers a model quantum universe consisting of a very large box containing a screen with two slits and an observer (us) that can pass though the slits. We apply the modern quantum mechanics of closed systems to calculate the probabilities for alternative histories of how we move through the universe and what we see. After passing through the screen with the slits, the quantum state of the universe is a superposition of classically distinguishable histories. We are then living in a superposition. Some frequently asked questions about such situations are answered using this model. The model's relationship to more realistic quantum cosmologies is briefly discussed.
{"title":"Living in a Superposition","authors":"J. Hartle","doi":"10.1142/9789811216404_0012","DOIUrl":"https://doi.org/10.1142/9789811216404_0012","url":null,"abstract":"This essay considers a model quantum universe consisting of a very large box containing a screen with two slits and an observer (us) that can pass though the slits. We apply the modern quantum mechanics of closed systems to calculate the probabilities for alternative histories of how we move through the universe and what we see. After passing through the screen with the slits, the quantum state of the universe is a superposition of classically distinguishable histories. We are then living in a superposition. Some frequently asked questions about such situations are answered using this model. The model's relationship to more realistic quantum cosmologies is briefly discussed.","PeriodicalId":416124,"journal":{"name":"The Quantum Universe","volume":"318 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2015-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116290666","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2015-03-24DOI: 10.1017/9781316535783.010
J. Hartle, T. Hertog
In the modern quantum mechanics of cosmology observers are physical systems within the universe. They have no preferred role in the formulation of the theory nor in its predictions of third person probabilities of what occurs. However, observers return to importance for the prediction of first person probabilities for what we observe of the universe: What is most probable to be observed is not necessarily what is most probable to occur. This essay reviews the basic framework for the computation of first person probabilities in quantum cosmology starting with an analysis of very simple models. It is shown that anthropic selection is automatic in this framework, because there is no probability for us to observe what is where we cannot exist. First person probabilities generally favor larger universes resulting from inflation where there are more places for us to be. In very large universes it is probable that our observational situation is duplicated elsewhere. The calculation of first person probabilities then requires a specification of whether our particular situation is assumed to be typical of all the others. It is the combination of the model of the observational situation, including this typicality assumption, and the third person theory which is tested by observation. We conclude with a discussion of the first person predictions of cosmological observables such as the cosmological constant and features of the primordial density fluctuations, in the no-boundary quantum state of the universe and a dynamical theory in which these are allowed to vary.
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