In Roman mythology, Janus is the two-faced god of transition and change. One of his faces looks toward the past; the other looks toward the future. In The Janus Point: A New Theory of Time, Julian Barbour offers not a new theory of time, as the subtitle suggests, but a new perspective on the arrow of time, one that builds on the theory he expounded in The End of Time: The Next Revolution in Physics (1999).The two-faced Roman god Janus, as depicted in a miniature from a 15th-century illuminated manuscript.ALBUM/ALAMY STOCK PHOTOPPT|High resolutionBarbour challenges the conventional wisdom that the one-way nature of physical processes—such as ripples emanating from a stone thrown into a pond—is best accounted for by postulating that the universe began in a special initial condition. According to that standard worldview, the ever-increasing entropy predicted by the second law of thermodynamics eventually leads to a featureless, cold universe with no meaningful structure.In The Janus Point, Barbour aims to present an alternative to that picture, one in which the universe’s starting point is not so atypical and the unidirectionality of physical processes is a consequence of either the universe’s expansion or its increasing complexity. He associates that increasing complexity with what one might call the finer things: life, humanity, art, and science. That vision of inexorable progress echoes Gottfried Leibniz’s view that we live in the best of all possible worlds. Barbour contrasts that vision with what he sees as the bleak pessimism of the traditional explanation.After a long introductory critique of the history of thermodynamics, Barbour turns to N-body theory. Drawing on the results of Joseph Louis de Lagrange and Carl Jacobi, he shows that isolated systems of gravitating point masses with a nonnegative total energy have a finite minimum size at some point in time. He calls that minimum the Janus point because at that point one can face toward the past or toward the future and “see” an expanding universe.Because Barbour wants to defeat what he sees as the pessimism of the second law, and because he needs a quantity more plausibly associated with time asymmetry than just the expansion of the universe, he introduces the term “shape complexity,” which he defines as −IV/M2, in which I is the moment of inertia about the center of mass, V is its potential energy, and M is its total mass.Away from the Janus point, I increases monotonically, which means it is plausible that the shape complexity will too—just like entropy does in the traditional worldview. That increase reflects the tendency of gravitating systems both to expand (given sufficient kinetic energy) and to form what Barbour calls “Kepler pairs” (the result of gravitational attraction).Barbour presents the example of three bodies that nearly collide. As a result of their interaction, two of the particles wind up orbiting each other and the other heads off to infinity, thus increasing the shape complexity. It is nontrivial to show that the complexity increases monotonically as the system moves away from the Janus point, but Barbour and collaborators have managed to put bounds on the amount it deviates from monotonic increase. Those bounds get narrower as the number of particles increases. That is certainly an interesting result.The next part of the book involves a technical demonstration of another intriguingly suggestive result: If one assumes that both the energy and the total angular momentum of the universe are equal to zero, one can show that the Janus point is a point of total collision or total explosion akin to our Big Bang singularity. It thus follows that particle configurations become highly symmetric as the Janus point approaches, which suggests that the “special” initial conditions that seem to dominate in the early universe might actually be generic features of the early stages of a gravity-dominated universe.To show that increasing complexity is a good proxy for time’s arrow, Barbour must demonstrate that it not only strongly tends to increase monotonically but also that the increase manifests in the myriad temporally asymmetric processes that provide the observational basis for our arrow of time. At times he fully embraces that idea and argues that the growth of complexity, not the growth of disorder, “puts the direction into time—and us into the universe to witness its forward march.” Elsewhere he is content to concede that purely dissipative processes in which complexity decreases are also part of the arrow of time.Be that as it may, making a precise connection between complexity or cosmological expansion and the observed arrow of time is of secondary interest to Barbour. More important for him is to overcome what he and others, including Bertrand Russell and Steven Weinberg, regard as the bleak prospect of heat death. Although he acknowledges that energy is continually dissipated in an expanding universe in accordance with the second law, Barbour wants to explain why structure, complexity, life, and art nevertheless continue to emerge. As he says on the penultimate page, The Janus Point is “in part, a song of thanks to the cosmos and the fact that I, like you, am a participant in whatever it does.”One could hardly find a more romantic view of the cosmos.© 2022 American Institute of Physics.

We successfully model the behavior of two-spin systems using neural networks known as conditional Restricted Boltzmann Machines (cRBMs) which encode physical information in the properties of a thermal ensemble akin to an Ising model. The result gives local "hidden" variable models for product states and entangled states, including the singlet state used in the EPR-Bohm experiment. Bell's theorem is circumvented because the state of the system is dependent not only on the preparation but also on the measurement setup (the detector settings). Though at first glance counterintuitive, the apparent "retrocausality" in these models has a historical precedent in the absorber theory of Wheeler and Feynman and an intuitive analog in the simple AC circuit of an electric guitar.

We construct a hidden variable model for the EPR correlations using a Restricted Boltzmann Machine. The model reproduces the expected correlations and thus violates the Bell inequality, as required by Bell's theorem. Unlike most hidden-variable models, this model does not violate the $locality$ assumption in Bell's argument. Rather, it violates $measurement$ $independence$, albeit in a decidedly non-conspiratorial way.

From classical mechanics to quantum field theory, the physical facts at one point in space are held to be independent of those at other points in space. I propose that we can usefully challenge this orthodoxy in order to explain otherwise puzzling correlations at both cosmological and microscopic scales.

However, because some model components are representational and some are not, a complete analysis of their use in science must take this into account. In chapter 13, Pincock attempts to ‘flesh out’ his claims in chapter 2 concerning the way in which physical and mathematical concepts relate to the contents of scientific representations. And finally, in chapter 14, Pincock summarizes the claims he made earlier in the book into a single argument that the reason ‘mathematics is so central to our best contemporary science’ is because ‘it is an ideal tool for arriving at well-confirmed and widely applicable scientific representations’ (280). Overall, Pincock’s book is an excellent analysis of some of the most important topics in philosophy of science and philosophy of mathematics, and is well worth a read for any philosopher interested in the issue of mathematical application.

It is a common place to note that in a world governed by special or general relativity, an observer has access only to data within her past lightcone (if that). The significance of this for prediction, and thus for confirmation, does not however seem to have been appreciated. In this paper we show that what we regard as our most well-confirmed relativistic theory, Maxwell's theory of electromagnetism, is not at all well-confirmed in the absence of an additional assumption, the assumption that all fields have sources in their past. We conclude that we have reason to believe that there is a lawlike time-asymmetry in the world.

We study the initial value problem for the wave equation and the ultrahyperbolic equation for data posed on initial hypersurfaces surface of arbitrary space–time signature. We show that, under a non-local constraint, the initial value problem posed on codimension-one hypersurfaces—the Cauchy problem—has global unique solutions in the Sobolev spaces H m . Thus, it is well-posed. However, we show that the initial value problem on higher codimension hypersurfaces is ill-posed due to failure of uniqueness, at least when specifying a finite number of derivatives of the data. This failure is in contrast to a uniqueness result for data given in an arbitrary neighbourhood of such initial hypersurfaces, which Courant deduces from Asgeirsson’s mean value theorem. We give a generalization of Courant’s theorem that extends to a broader class of equations. The proofs use Fourier synthesis and the Holmgren–John uniqueness theorem.

Bell's theorem is purported to demonstrate the impossibility of a local "hidden variable" theory underpinning quantum mechanics. It relies on the well-known assumption of 'locality', and also on a little-examined assumption called 'statistical independence' (SI). Violations of this assumption have variously been thought to suggest "backward causation", a "conspiracy" on the part of nature, or the denial of "free will". It will be shown here that these are spurious worries, and that denial of SI simply implies nonlocal correlation between spacelike degrees of freedom. Lorentz-invariant theories in which SI does not hold are easily constructed: two are exhibited here. It is conjectured, on this basis, that quantum-mechanical phenomena may be modeled by a local theory after all.

It has been claimed that decoherence of open quantum systems explains the tendency of macroscopic systems to exhibit quasiclassical behavior. We show that quasiclassicality is in fact an unremarkable property, characterizing generic subsystems of environments even in the absence of dynamical decoherence. It is suggested that decoherence is best regarded as explaining the persistence of true classicality, rather than the emergence, rather than the emergence of quasiclassicality.

We consider the claim that decoherence explains the emergence of classicality in quantum systems, and conclude that it does not. We show that, given a randomly chosen universe composed of a variety of subsystems, some of which are macroscopic and subject to decoherence-inducing interactions, and some of which are microscopic, the macroscopic subsystems will not display any distinctively classical behavior. Therefore, a universe in which macroscopic and microscopic do display distinct behavior must be in a very special, highly nongeneric quantum state.

The possibility of physics in multiple time dimensions is investigated. Drawing on recent work by Walter Craig and myself, I show that, contrary to conventional wisdom, there is a well-posed initial value problem–deterministic, stable evolution–for theories in multiple time dimensions. Though similar in many ways to ordinary, single-time theories, multi-time theories have some rather intriguing properties which suggest new directions for the understanding of fundamental physics.

BACKGROUND:MAP0004 (a proprietary formulation of dihydroergotamine mesylate [DHE]) for inhaled delivery is being developed for acute migraine treatment. Because asthma and migraine often occur as co-morbid conditions, it is considered important to study the safety of MAP0004 in a population of asthmatic adults and to confirm that the pharmacokinetics of DHE, when inhaled by asthmatic subjects, were comparable to a population of healthy volunteers. The safety, tolerability, and pharmacokinetics of orally-inhaled MAP0004 administered by the Tempo inhaler were studied in adult asthmatics.SCOPE:This was a randomized, double-blind, placebo-controlled study of two doses of inhaled MAP0004. Eligible subjects were randomized in a 2 : 1 ratio to MAP0004 or placebo and observed for 4 h after each dose. Pharmacokinetic parameters were determined pre-dose and up to 36 h post-dose.FINDINGS:Among 19 subjects, geometric mean AUC(0-36) was 6754 pg.h/mL and geometric mean AUC(0-inf) was 7483 pg.h/mL. Geometric mean t(max) was 9.6 min, geometric mean C(max) was 3174 pg/mL, and geometric mean t((1/2)) was 9.5 h. Overall, 13 of 19 (68%) subjects reported at least one adverse event, most commonly nausea, vomiting, dysgeusia, and headache.CONCLUSION:MAP0004 results in rapid and efficient systemic absorption in asthmatic subjects. Systemic DHE concentrations were similar to those previously reported in healthy subjects, and no clinically relevant safety issues were observed. While this small study was suitable for pharmacokinetic analysis and conclusions, MAP0004 use in migraineurs with concomitant stable asthma should be supported by larger studies of longer duration to confirm that it does not present additional safety risks compared to non-asthmatic migraineurs.

Causality is analyzed in the light of modern relativistic theories. It is shown that such theories provide a natural home for causation, and provide a framework for understanding both the time-asymmetry of causation and single-event causation without introducing any further, contingent features of the world.

String theory, like any mathematical theory, presents physicists with a host of technical challenges in how to extract predictions about the real world. As a unified theory of gravity and particle physics, string theory should, on the face of it, be amenable to experimental test because it must at the very least reproduce all observed phenomena. But extracting unambiguous predictions has proved difficult.

Anthropic arguments in multiverse cosmology and string theory rely on the weak anthropic principle (WAP). We show that the principle is fundamentally ambiguous. It can be formulated in one of two ways, which we refer to as WAP1 and WAP2. We show that WAP2, the version most commonly used in anthropic reasoning, makes no physical predictions unless supplemented by a further assumption of ‘typicality’, and we argue that this assumption is both misguided and unjustified. WAP1, however, requires no such supplementation; it directly implies that any theory that assigns a non-zero probability to our universe predicts that we will observe our universe with probability one. We argue, therefore, that WAP1 is preferable, and note that it has the benefit of avoiding the inductive overreach characteristic of much anthropic reasoning.

We argue that computation via quantum mechanical processes is irrelevant to explaining how brains produce thought, contrary to the ongoing speculations of many theorists. First, quantum effects do not have the temporal properties required for neural information processing. Second, there are substantial physical obstacles to any organic instantiation of quantum computation. Third, there is no psychological evidence that such mental phenomena as consciousness and mathematical thinking require explanation via quantum theory. We conclude that understanding brain function is unlikely to require quantum computation or similar mechanisms.

Special relativity is said to prohibit faster-than-light (superluminal) signaling, yet controversy regularly arises as to whether this or that physical phenomenon violates the prohibition. I argue that the controversy is a result of a lack of clarity as to what it means to ‘signal’, and I propose a criterion. I show that according to this criterion, superluminal signaling is not prohibited by special relativity.