Beyond Spacetime: The Foundations of Quantum Gravity is one of two edited volumes stemming from a three-year research project led by Nick Huggett and Chris Wuthrich on the philosophy of quantum gravity, with the goal of “explor[ing] the idea that attempts to quantize gravity either significantly modify the structures of classical spacetime or replace them—and spacetime itself—altogether” (1). The background thought here is that such attempts naturally involve an admixture of philosophy and physics, and scrutinizing them is bound to shed new light on the philosophy of space and time, on the notion of “emergence,” and, more generally, on the philosophy of the empirical discipline that we call “physics.” The editors—Huggett, Keizo Matsubara, and Wuthrich—are to be congratulated for bringing together an impressive list of contributors in this 357-page volume (eight philosophers and eight physicists) to participate in a genuinely interdisciplinary conversation and, moreover, for undertaking the challenging task of piecing together these different articles and perspectives into something of a coherent whole.Beyond Spacetime comprises three parts. The chapters in part 1 discuss how the classical gravitational degrees of freedom of general relativity (GR)—which GR models by means of the mathematical formalism that we call “classical spacetime”—can be understood as emergent from a quantum theory of gravity (QTG), given that the latter is manifestly not in the business of modeling the (classical gravitational) degrees of freedom of the former. The chapters in part 2 turn their attention to the question of how one might think about a “time” parameter in QTG, and how it is related to the use of an analogous parameter in classical theories in order to model the dynamics of various empirical scenarios (some of these chapters also discuss the relationship between the use of such a parameter in physical models and certain metaphysical conceptions of time). Save one, the chapters in parts 1 and 2 are contributed by physicists; on the other hand, part 3—which is primarily contributed to by philosophers—concerns the “interpretation” of the relevant physical theories. As Ruetsche (2011: 7) notes, however, every textbook presentation of a physical theory already provides a partial interpretation, so in that sense parts 1 and 2 are no less an exercise in interpretation than the contributions of part 3: perhaps the correct thing to say here is that part 3 concerns some aspects of interpretation that are especially controversial or interesting to a philosophical audience (then again, the black hole information loss paradox is interesting and controversial to physicists for reasons of physical practice and not abstract metaphysics, as David Wallace convincingly argues in this section). At any rate, the distinction between part 3 and the previous parts seems to be primarily a matter of emphasis.The editors provide a long and comprehensive—indeed, heroic—introduction to Beyond Spacetime that summarizes each of the chapters in some detail and that subfield specialists will find very helpful. It would thus be superfluous for us to rehearse such a summary in a book review of this length and scope. Instead, we shall adopt the tack of providing some considerations—and an overall framing—that might help the nonspecialist reader relate the material in this volume to more general themes in the philosophy of science, especially the philosophy of scientific modeling and idealization.With that end in mind, let us pause to recall that one of the most central and enduring themes within general philosophy of science is that of “idealization” and the role that it plays within scientific practice, especially its justification and its relationship to scientific understanding. In emphasizing “idealization,” practice-oriented philosophers are, of course, following the lead of physicists (and other scientists), who intuitively understand that idealization is essential to doing good physics; witness Feynman (2013, chap. 12), who writes that “in order to understand physical laws you must understand that they are all some kind of approximation.” Philosophers have further advanced different views about what idealization is, and what role it plays in scientific practice. For instance, consider the act of idealizing away certain disturbances to a relatively stable subsystem (for example, on a short time-scale: idealizing away the effects of friction on an air hockey puck so that it can be described as moving uniformly). Some philosophers such as Michael Weisberg (2007) have characterized such idealization as being justified only insofar as one can de-idealize and add back the “full detail” into one’s description of the system. On this view, the goal of physics is to achieve a “complete” description of reality, from which all idealizations have been banished. By contrast, Nguyen, Shields, and Teh (forthcoming) argue that this kind of idealization is best understood by attending to the fact that the relevant empirical scenarios (which includes a specification of the relevant time and measurement scales) form what Nancy Cartwright (1999: 50) calls a nomological machine—a stable (enough) arrangement of components whose operation in a particular empirical context give rise to regular behavior—and that, when understood correctly, the idealization represents this stability because various other models flow towards the idealized model when we take the limit of some parameter (typically a ratio of the relevant scales). Thus, the idealization provides what physicists call an “attractive fixed point”: a stable (in the above sense) model about which one can perturb in order to provide an empirically adequate description of the target subsystem; notice that this strategy is not justified by the possibility of adding “all the detail” back in (assuming that there is anything to mean by that form of words); rather, the empirical adequacy of the strategy is justified by the “universal” role of the idealized model in some space of models, given the regime that is under consideration. Readers who are familiar with contemporary physics will immediately see that we are describing a very simplified version of the renormalization group (RG) approach to describing the effective physics of empirical scenarios in some regime: by first flowing (coarse-graining over degrees of freedom) to arrive at some special idealized models, and then by perturbing about these models to obtain the “effective field theory” (EFT) description characterizing that regime. It is this connection that will provide a helpful framing device for the articles in this volume.We would now like to consider the extent to which the volume’s chapters are amenable to being framed in terms of the RG and EFT approach to idealization that we have just outlined above. To put things somewhat polemically, which of these pieces are amenable to viewing QTG as a tool in order to produce effective models of physics at regimes in which both ℏ (that is, reduced Planck’s constant) and G come into play (much as we might use the formally UV-complete property of Yang–Mills theory as a tool to describe interesting physics at various lower-energy scales)? Somewhat unsurprisingly, we find that most of the physicists’ pieces are somewhat amenable to this point of view, perhaps for the simple reason that physics is an empirical discipline, and even in its most rarefied forms, its telos is still to describe (perhaps hypothetical) empirical scenarios—which require some kind of (possibly nonspatiotemporal) stable “target system”—environment relationship—and to understand the various scales (including measurement scales) characterizing such scenarios.For instance, consider Daniele Oriti’s piece in this volume, which is explicitly amenable to this perspective. Like many others, Oriti takes the degrees of freedom of QTG to be very different from the classical gravitational degrees of freedom of GR; it follows that QTG’s formalism (for example, group field theory models) will not be “spatiotemporal” in the latter sense. However, he also thinks that the standard picture of QTG needs to be supplemented with a parameter N representing the number of QTG degrees of freedom, and upon using the RG flow to change the scale in the direction of increasing N, one expects to recover an effective description of the classical gravitational degrees of freedom of GR, as well as other regimes in which one has turned on both G and ℏ. As Oriti writes, The important insight is that a lot of what the theory [QTG] contains, in principle, does not affect the relevant physics, at least not significantly. The relevant collective variables and observables may differ from those in the initial definition of the same theory (and that enter the computation of the same collective quantities). Such approximate, coarse-grained description, alongside other forms of truncation, is thus not just a technical tool that physicists have to adopt for lack of computational power or skills but is where they show or test their physical understanding of the system. These approximations, moreover, are both a prerequisite for the understanding of the renormalization group flow of the theory and directly suggested by it, since the renormalization group flow itself gives indications on which dynamics and which observables (e.g., order parameters) are relevant in different regimes (scales). (40)In other words, here QTG is being used as a tool to obtain empirically effective descriptions in various regimes, including ones in which quantum gravity effects are present to some extent.Various pieces in this volume show a similar concern for using QTG to obtain an effective description of empirical scenarios. For instance, Suddhasattwa Brahma works within the framework loop quantum gravity (LQG) to demonstrate the effective emergence of the kind of temporal parameter used in classical cosmology, and Robert Brandenberger applies string theory to obtain an effective theory that accounts for the observational data of early universe cosmology; similarly (and pace some recent philosophical discussions), Daniel Harlow is concerned to emphasize the important role of the low-energy effective field theory in the AdS/CFT correspondence, especially in probing the physics of black holes. Even when considering the role of “time” in a QTG itself, there are resonances with the philosophy of science literature on modeling—for example, with Cartwright’s “nomological machines”: for instance, Carlo Rovelli makes the point that even in LQG, one is concerned to describe physical processes, and thus how the system’s variables change (relative to an environment, or some set of degrees of freedom that it is interacting with)—in other words, here, too, there is temporality in Aristotle’s sense (in Physics 4) (of course, describing such a nomological machine in no way commits one to a preferred time variable, as Rovelli emphasizes). Finally, Wallace’s preferred form of the black hole information loss paradox, which results from the incompatibility of thermal Hawking radiation with ordinary thermodynamic evaporation, demonstrates keen attention to the relevant scales at which the thermodynamic and QFT descriptions are expected to apply to black hole evaporation. Indeed, the framing that Wallace gives to this version of the paradox leads to an inconsistency in a regime for which quantum gravitational effects ought not to be a problem for either effective description—for example, before a time at which the black hole reaches a size comparable to the Planck length. Wallace brings out the genuinely puzzling nature of this discrepancy and points out the fruitfulness of such an arena for carefully thinking through our best low-energy effective theories of quantum gravity.What of the pieces that are less amenable to the view of idealization that we outlined above—that is, pieces whose primary understanding of (or aspiration for) QTG is as providing a “completely detailed” view of the world? Sebastian De Haro’s chapter on when duality ought to be understood as physical equivalence is one locus for this sort of view. In particular, De Haro assumes a framework for interpretation of physical theories in which a “bare” theory is some set-theoretic structure of states, quantities, and dynamics, while an “interpreted” theory is simply a bare theory equipped with a map that connects elements of the formalism to physical quantities and values of quantities. The domain of these quantities and values is understood as that of a possible world that represents the real world “in a straightforward way” (263). This formal set-theoretic understanding of interpretation in terms of possible worlds is shared by Tiziana Vistarini in her contribution to the volume, as she considers connections between QTG and Lewisian modal metaphysics. Allied with questions about “extendability” in De Haro’s chapter, this picture of interpretation assumes the possibility of a full, or exact, description of physical reality by our physical theories. To be clear, De Haro does not suppose that any QTG under consideration is a “final theory,” in the sense that it tells us everything we could ever want to know about the real world, but he does suppose that they are “candidate descriptions of an entire (possible) physical world” (267). It is less clear how to apply lessons about idealization, RG, and EFT in this case, because a totalizing, literalistic interpretation of this kind obscures the modeling relationship between theory and reality, and thus of the empirical character of physical theories. It also seems to exclude the possibility of understanding the role of the environment of a target system, as there is nothing outside of the target system in an unextendable theory. Interestingly, while at first blush Richard Dawid’s conception of string theory as a “final theory” might also seem to fit nicely with totalizing or literalistic views, this is less than clear upon further reflection. Dawid’s notion of finality rests upon a theory’s viability at all scales, where “viability in a regime” is agreement with all data that could be collected in that regime, but this does not commit him to the idea that a final theory must be a “complete description.” In fact, one of Dawid’s core arguments for string theory’s finality is precisely the idea that string theory seems to resist completion. This view also relies on careful consideration of the correct relationship between effective theories at energy scales below the Planck scale and string theory as a theory of quantum gravity.The other chapters in this volume are also of great interest and will repay careful study. We especially encourage readers to read the entire volume with an eye toward the recent literature on idealization, modeling, and scientific understanding, an effort to which we hope this review will serve as a propaedeutic.

中文翻译：

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超越时空：量子引力的基础


《超越时空：量子引力的基础》是由 Nick Huggett 和 Chris Wuthrich 领导的关于量子引力哲学的为期三年的研究项目的两本编辑卷之一，其目标是“探索试图量子化引力要么显着改变经典时空的结构，要么完全取代它们——以及时空本身”（1）。这里的背景思想是，这种尝试自然地涉及哲学和物理学的混合，仔细研究它们必然会对空间和时间的哲学、“涌现”的概念，以及更普遍的哲学产生新的启发。我们称之为“物理学”的经验学科。值得祝贺的编辑——Huggett、Keizo Matsubara 和 Wuthrich——在这本 357 页的书中汇集了令人印象深刻的贡献者名单（八位哲学家和八位物理学家），参与真正的跨学科对话，此外，他们还承担了将这些不同的文章和观点拼凑成一个连贯的整体是一项具有挑战性的任务。《超越时空》包括三个部分。第 1 部分中的章节讨论了如何将广义相对论 (GR) 的经典引力自由度（GR 通过我们称为“经典时空”的数学形式主义进行建模）理解为源自量子引力理论 (QTG) ），考虑到后者显然不涉及对前者的（经典引力）自由度进行建模。 第 2 部分的章节将注意力转向这样一个问题：人们如何思考 QTG 中的“时间”参数，以及它如何与经典理论中类似参数的使用相关，以便对各种经验场景的动态进行建模（其中一些章节还讨论了物理模型中此类参数的使用与某些形而上学的时间概念之间的关系）。除此以外，第 1 部分和第 2 部分中的章节均由物理学家撰写；另一方面，第三部分——主要由哲学家贡献——涉及相关物理理论的“解释”。然而，正如 Ruetsche (2011: 7) 指出的那样，物理理论的每本教科书介绍都已经提供了部分解释，因此从这个意义上来说，第一部分和第二部分与第三部分的贡献一样，都是解释练习：也许是正确的事情这里要说的是，第三部分涉及对哲学读者来说特别有争议或有趣的解释的某些方面（话又说回来，黑洞信息丢失悖论对物理学家来说是有趣且有争议的，因为物理实践而不是抽象形而上学的原因，正如大卫华莱士在本节中令人信服地论证）。无论如何，第三部分和前面的部分之间的区别似乎主要是强调的问题。编辑们为《超越时空》提供了一篇长而全面的——实际上是英雄式的——介绍，其中详细总结了每一章以及该子领域专家会发现非常有帮助。因此，我们在如此长度和范围的书评中排练这样的总结是多余的。 相反，我们将采取提供一些考虑因素和总体框架的策略，这可能会帮助非专业读者将本书中的材料与科学哲学中更普遍的主题联系起来，特别是科学建模和理想化的哲学。考虑到这一点，让我们停下来回顾一下，一般科学哲学中最核心和最持久的主题之一是“理想化”及其在科学实践中发挥的作用，特别是它的正当性及其与科学理解的关系。在强调“理想化”时，实践导向的哲学家当然是在追随物理学家（和其他科学家）的领导，他们直观地理解理想化对于做好物理学至关重要；证人 Feynman（2013 年，第 12 章）写道，“为了理解物理定律，你必须明白它们都是某种近似值。”哲学家们对理想化是什么以及它在科学实践中扮演什么角色进一步提出了不同的观点。例如，考虑理想化消除对相对稳定的子系统的某些干扰的行为（例如，在短时间尺度上：理想化消除空气曲棍球上的摩擦力的影响，以便可以将其描述为均匀移动）。一些哲学家，如迈克尔·韦斯伯格（Michael Weisberg，2007）将这种理想化描述为，只有在人们能够去理想化并将“完整细节”添加回系统描述中的情况下，这种理想化才是合理的。根据这种观点，物理学的目标是实现对现实的“完整”描述，并消除所有理想化。 相比之下，Nguyen、Shields 和 Teh（即将发表）认为，最好通过关注这样一个事实来理解这种理想化：相关的经验情景（包括相关时间和测量尺度的规范）形成了 Nancy Cartwright（1999）的内容。 ：50）称之为法则机器——一种稳定（足够）的组件排列，其在特定经验背景下的操作会产生常规行为——并且，如果正确理解，理想化就代表了这种稳定性，因为各种其他模型都流向理想化模型当我们取某些参数的极限（通常是相关尺度的比率）时。因此，理想化提供了物理学家所说的“有吸引力的固定点”：一个稳定的（在上述意义上）模型，人们可以对其进行扰动，以便提供对目标子系统的经验充分的描述；请注意，这种策略并不能通过重新添加“所有细节”的可能性来证明（假设这种形式的单词有任何含义）；相反，考虑到所考虑的制度，该策略的经验充分性是由理想化模型在某些模型空间中的“普遍”作用所证明的。熟悉当代物理学的读者会立即看到，我们正在描述重正化群（RG）方法的一个非常简化的版本，用于描述某种体系中经验场景的有效物理学：通过首先流动（自由度上的粗粒度）得出一些特殊的理想化模型，然后通过扰动这些模型来获得表征该状态的“有效场论”（EFT）描述。正是这种联系为本卷中的文章提供了一个有用的框架装置。现在我们想考虑本书的章节在多大程度上适合按照我们上面刚刚概述的 RG 和 EFT 理想化方法来构建。有点争议性地说，这些部分中的哪一个适合将 QTG 视为一种工具，以便在 ℏ（即简化的普朗克常数）和 G 都发挥作用的情况下产生有效的物理模型（就像我们可能的那样）使用杨-米尔斯理论的正式 UV 完全属性作为工具来描述各种低能尺度下有趣的物理）？毫不奇怪的是，我们发现大多数物理学家的文章在某种程度上都符合这种观点，也许原因很简单，物理学是一门经验学科，即使在其最稀有的形式中，它的目的仍然是描述（也许是假设的） ）经验场景——需要某种（可能是非时空的）稳定的“目标系统”——环境关系——并理解表征此类场景的各种尺度（包括测量尺度）。例如，考虑本卷中 Daniele Oriti 的作品，它是明确服从这一观点。和许多其他人一样，Oriti 认为 QTG 的自由度与 GR 的经典引力自由度有很大不同；由此可见，QTG 的形式主义（例如群场论模型）不会是后一种意义上的“时空”。 但他也认为QTG的标准图需要补充一个代表QTG自由度数的参数N，利用RG流向增加N的方向改变尺度，期望能恢复出有效描述 GR 的经典引力自由度，以及同时打开 G 和 ℏ 的其他状态。正如 Oriti 所写，重要的见解是理论 [QTG] 所包含的很多内容原则上不会影响相关物理学，至少不会显着影响。相关的集体变量和可观测量可能与同一理论的初始定义中的变量和可观测量不同（并且进入相同集体量的计算）。因此，这种近似的、粗粒度的描述以及其他形式的截断不仅是物理学家因缺乏计算能力或技能而必须采用的技术工具，而且是他们展示或测试他们对系统的物理理解的地方。此外，这些近似既是理解理论的重整化群流的先决条件，也是由该理论直接暗示的，因为重整化群流本身指示了哪些动力学和哪些可观测量（例如，阶次参数）在不同的情况下是相关的。制度（规模）。 (40)换句话说，这里 QTG 被用作一种工具，用于在各种情况下获得经验有效的描述，包括在某种程度上存在量子引力效应的情况。本卷中的各个部分都显示了使用 QTG 的类似关注获得经验情景的有效描述。 例如，Suddhasattwa Brahma 在圈量子引力（LQG）框架内工作，证明了经典宇宙学中使用的时间参数的有效出现，罗伯特·布兰登伯格（Robert Brandenberger）应用弦理论获得了解释早期观测数据的有效理论。宇宙宇宙学；类似地（并加快了最近的一些哲学讨论），丹尼尔·哈洛（Daniel Harlow）关注强调低能有效场论在 AdS/CFT 对应中的重要作用，特别是在探测黑洞物理方面。即使在考虑“时间”在 QTG 本身中的作用时，也与关于建模的科学哲学文献存在共鸣——例如卡特赖特的“法则机器”：例如，卡洛·罗维利 (Carlo Rovelli) 指出，即使在 LQG 中，一个关注于描述物理过程，以及系统的变量如何变化（相对于环境，或与之相互作用的一组自由度）——换句话说，这里也存在亚里士多德意义上的暂时性（在物理学 4）（当然，描述这样一个法则机器决不会让人们相信一个首选的时间变量，正如罗韦利所强调的那样）。最后，华莱士优选的黑洞信息丢失悖论形式是由热霍金辐射与普通热力学蒸发的不相容性导致的，这表明了对热力学和 QFT 描述预计适用于黑洞蒸发的相关尺度的强烈关注。 事实上，华莱士对这个版本的悖论的框架导致了一种制度的不一致，对于这种制度，量子引力效应不应该成为任何有效描述的问题——例如，在黑洞达到一定尺寸之前与普朗克长度相当。华莱士揭示了这种差异的真正令人费解的本质，并指出了通过我们最好的低能有效量子引力理论进行仔细思考的这样一个舞台的成果。哪些部分不太适合我们概述的理想化观点上面的——也就是说，那些对 QTG 的主要理解（或愿望）是提供“完全详细”的世界观的作品？塞巴斯蒂安·德哈罗（Sebastian De Haro）关于何时应将二元性理解为物理等效性的章节是这种观点的一个核心。特别是，德哈罗假设了一个解释物理理论的框架，其中“裸”理论是状态、数量和动力学的某种集合论结构，而“解释”理论只是一个配备了映射的裸理论，将形式主义元素与物理量和量值联系起来。这些数量和值的领域被理解为“以直接的方式”代表现实世界的可能世界（263）。 Tiziana Vistarini 在她对该书的贡献中分享了这种对可能世界解释的正式集合论理解，她考虑了 QTG 和刘易斯模态形而上学之间的联系。与德哈罗章节中有关“可扩展性”的问题相结合，这种解释的图景假设了我们的物理理论对物理现实进行完整或精确描述的可能性。 需要明确的是，德哈罗并不认为正在考虑的任何 QTG 都是“最终理论”，因为它告诉我们关于现实世界我们可能想知道的一切，但他确实认为它们是“候选描述”。整个（可能的）物理世界”（267）。在这种情况下，如何应用理想化、RG 和 EFT 的经验教训还不太清楚，因为这种总体化、字面主义的解释模糊了理论与现实之间的建模关系，从而模糊了物理理论的经验特征。它似乎也排除了理解目标系统环境的作用的可能性，因为在不可扩展的理论中，目标系统之外没有任何东西。有趣的是，虽然乍一看理查德·戴维德将弦理论视为“最终理论”的概念似乎也非常适合总体化或字面主义的观点，但经过进一步思考后，这一点就不太清楚了。大卫的最终性概念依赖于理论在所有尺度上的可行性，其中“政权中的可行性”与该政权中可以收集到的所有数据一致，但这并不意味着他认为最终理论必须是“完整的描述。”事实上，戴维德关于弦理论终结性的核心论据之一正是弦理论似乎无法完成的观点。这一观点还依赖于对普朗克尺度以下能量尺度的有效理论与作为量子引力理论的弦理论之间的正确关系的仔细考虑。本书中的其他章节也很有趣，值得仔细研究。 我们特别鼓励读者在阅读整本书时着眼于最近关于理想化、建模和科学理解的文献，我们希望这篇评论能够起到一种宣传作用。