The principle underlying all evolution, biological, geophysical, social and technological.

This article addresses the main research areas identified in the call for contributions to this special issue. With examples from published articles and books, the present article shows that all the identified areas are already covered by the universal principle underlying all evolution: the constructal law (1996), i.e. the physics law of design evolution in nature (free morphing, flowing, moving systems). The universal principle of evolution belongs in thermodynamics because thermodynamics is a universal science and evolution is a universal phenomenon. The principle unites the natural sciences with the social sciences, and the living with the non-living. It unifies the world of science and its languages (energy, economy, evolution, sustainability, etc.), and brings together the natural and artificial flow architectures, the human made and the not human made. The principle establishes firmly in physics the reality that humans are part of nature. With the principle, physics extends its coverage over phenomena that were previously considered out of reach: social organization, economics and human perceptions. Such phenomena are physical, i.e. facts. The entire world depends on the science of useful things, and benefits greatly from a physics discipline with freedom, life, wealth, time, beauty and future. This article is part of the theme issue 'Thermodynamics 2.0: bridging the natural and social sciences (Part 1)'.

Inflected wings in flight: Uniform flow of stresses makes strong and light wings for stable flight.

Flying animals morph and flex their wings during their flight. Their wings morph with the turbulent flow created around them. The wings of modern airplanes do not have this ability. In this study we show that the ability to flex the wings leads to greater stability (higher flutter speed), and that this is due to the more uniform distribution of stresses in the flexing wing. This way the flexing wing becomes the lightest per unit of flapping force, or the strongest per unit of weight.

Medical imaging dose optimisation from ground up: expert opinion of an international summit.

As in any medical intervention, there is either a known or an anticipated benefit to the patient from undergoing a medical imaging procedure. This benefit is generally significant, as demonstrated by the manner in which medical imaging has transformed clinical medicine. At the same time, when it comes to imaging that deploys ionising radiation, there is a potential associated risk from radiation. Radiation risk has been recognised as a key liability in the practice of medical imaging, creating a motivation for radiation dose optimisation. The level of radiation dose and risk in imaging varies but is generally low. Thus, from the epidemiological perspective, this makes the estimation of the precise level of associated risk highly uncertain. However, in spite of the low magnitude and high uncertainty of this risk, its possibility cannot easily be refuted. Therefore, given the moral obligation of healthcare providers, 'first, do no harm,' there is an ethical obligation to mitigate this risk. Precisely how to achieve this goal scientifically and practically within a coherent system has been an open question. To address this need, in 2016, the International Atomic Energy Agency (IAEA) organised a summit to clarify the role of Diagnostic Reference Levels to optimise imaging dose, summarised into an initial report (Järvinen et al 2017 Journal of Medical Imaging 4 031214). Through a consensus building exercise, the summit further concluded that the imaging optimisation goal goes beyond dose alone, and should include image quality as a means to include both the benefit and the safety of the exam. The present, second report details the deliberation of the summit on imaging optimisation.

Animal design advantage from the analogy between friction and body heat loss.

When a fluid accelerates as it sweeps a solid surface there are two consequences: the friction and the heat transfer (thermal contact) between fluid and solid increase simultaneously. This is known as the universal analogy between fluid friction and heat transfer. In thermal engineering these two effects are problematic because improved thermal contact is beneficial, and increased friction (i.e., pumping power) is detrimental to overall performance. In the present article we question whether the 'analogy' between these conflicting effects hampers the performance of animal movement. The theory focuses on warm-blooded swimmers and the effects (friction, heat transfer) that result from one change in the configuration of the body. Selected for analysis is a breaststroke swimmer. During gliding while reaching forward the 'one change' is from (a) legs spread apart, to (b) legs held tight together. The theory shows that the change from (a) to (b) has two consequences: greater swimming speed, and reduced body heat loss. In animal design both effects are beneficial, unlike in engineered flow systems. The analogy between fluid friction and heat transfer accelerated the evolution of animal design, and accounts for the 'divergent evolution' of fish and mammals.

Perfection is the enemy of evolution.

The universal phenomenon of evolution consists of change after change in flow configuration in a time direction that is perceptible to the observer. This reality clashes with the doctrine of precise optima, minima, and maxima, now rigidly in place because of calculus and computational simulations of all kinds of flowing and changing configurations. With two dissimilar examples, access on an area (a human settlement) and along a line (animal locomotion), it is shown that even a 1-percent imperfection in performance is accompanied by a sizable bandwidth of freedom to hit the 'target', that is, an easily accessible design with close to perfect performance. The evolutionary designs reveal the physics behind the phenomenon of diminishing returns in the vicinity of the mathematical optimum. In evolution what works is kept.

Development and CT image-domain validation of a computational lung lesion model for use in virtual imaging trials.

PURPOSE: Computational abnormalities (e.g., lesion models) for use in medical imaging simulation studies are frequently generated using data collected from clinical images. Although this approach allows for highly-customizable lesion detectability studies on clinical computed tomography (CT) data, the ground-truth lesion models produced with this method do not provide a sufficiently realistic lesion morphology for use with current anthropomorphic simulation studies. This work is intended to demonstrate that the new anatomically-informed lesion model presented here is not inferior to the previous lesion model under CT imaging, and can therefore provide a more biologically-informed model for use with simulated CT imaging studies. METHODS: The lesion model was simulated initially from a seed cell with 10 µm diameter placed in an anatomical location within segmented lung CT and was allowed to reproduce locally within the available solid angle in discrete time-intervals (corresponding to synchronous cell cycles) up to a size of ∼200 µm in diameter. Daughter cells of generation G were allowed also to reproduce on the next available time-step given sufficient space. At lesion sizes beyond 200 µm in diameter, the health of subregions of cells were tracked with a Markov chain technique, indicating which regions were likely to continue growing, which were likely stable, and which were likely to develop necrosis given their proximity to anatomical features and other lesion cells. For lesion sizes beyond 500 µm, the lesion was represented with three nested, triangulated surfaces (corresponding to proliferating, dormant, and necrotic regions), indicating how discrete volumes of the lesion were behaving at a particular time. Lesions were then assigned smoothly-varying material properties based on their cellular level health in each region, resulting in a multi-material lesion model. The lesions produced with this model were then voxelized and placed into lung CT images for comparison with both prior work and clinical data. This model was subject to an observer study in which cardiothoracic imaging radiologists assessed the realism of both clinical and synthetic lesions in CT images. RESULTS: The useable outputs of this work were voxel- or surface-based, validated, computational lesions, at a scale clearly visible on clinical CT (3-4 cm). Analysis of the observer study results indicated that the computationally-generated lesions were indistinguishable from clinical lesions (AUC = 0.49, 95% CI = [0.36, 0.61]) and non-inferior to an earlier image-based lesion model-indicating the advantage of the model for use in both hybrid CT images and in simulated CT imaging of the lungs. CONCLUSIONS: Results indicated the non-inferiority of this model as compared to previous methods, indicating the utility of the model for use in both hybrid CT images and in simulated CT imaging.

Evolution, physics, and education.

Evolution is a universal phenomenon of nature, bio, and non-bio. Evolution belongs in physics and general education. This step clarifies and enhances the transmission and retention of knowledge. Teaching and learning are the mental activity of grasping a new subject as something familiar, like an older and wider body of knowledge. The new subject is evolution, and the older and wider is physics. The older is accepted because it is based on morphing images available for observation and testing during a human lifetime (e.g., dendrites of river basins, lightning, snowflakes). Physical images can be predicted, tested, confirmed, and condensed into principles with the power to predict directionality and future evolution. The teaching of this method is illustrated with examples from animal locomotion and vehicle configuration, speed, and path.

Freedom and evolution in the dynamics of social systems.

This is a review of recent work on how the physics of evolution accounts for the origin, changes and future of social systems. Evolution means the changes (spatial, temporal) that happen with discernible direction in time. Changes are everywhere because physical features of 'freedom to change' are in every moving and flowing system. With freedom comes evolution, and with evolution come all these visible things, movement, change, configurations, diversity, multiple scales, and the universality of social organization phenomena such as human settlements, hierarchy, economics, economies of scale and diminishing returns.

Human evolution is biological & technological evolution.

We are the human & machine species. The homo is just the inner part. The evolution of the human & machine species is oriented in time from the naked man to the man with progressively more powerful add-ons (artifacts, contrivances, devices, machines, science). In evolution, the flow system morphs with freedom in the discernible direction identified as the constructal law. In all evolutionary designs, what works is kept.

Horizontal extent of the urban heat dome flow.

Urban heat dome flow, which is also referred to as urban heat island circulation, is important for urban ventilation and pollutant transport between adjacent cities when the background wind is weak or absent. A "dome-shaped" profile can form at the upper boundary of the urban heat island circulation. The horizontal extent of the heat dome is an important parameter for estimating the size of the area it influences. This study reviews the existing data on the horizontal extent of the urban heat dome flow, as determined by using either field measurements or numerical simulations. A simple energy balance model is applied to obtain the maximum horizontal extent of a single heat dome over the urban area, which is found to be approximately 1.5 to 3.5 times the diameter of the city's urban area at night. A linearized model is also re-analysed to calculate the horizontal extent of the urban heat dome flow. This analysis supports the results from the energy balance model. During daytime, the horizontal extent of the urban heat dome flow is found to be about 2.0 to 3.3 times the urban area's diameter, as influenced by the convective turbulent plumes in the rural area.

Rolling stones and turbulent eddies: why the bigger live longer and travel farther.

Here we report the discovery that even the simplest, oldest and most prevalent forms of evolutionary movement--rolling bodies and whirls of turbulence--exhibit the same body-size effect on life time and life travel as the evolutionary movement united by the body-size effect so far: animals, rivers, vehicles, jets and plumes. In short, the bigger should last longer and travel farther. For rolling bodies, the life span (t) and the life travel (L) should increase with the body mass (M) raised to the powers 1/6 and 1/3, respectively. The number of rolls during this movement is constant, independent of body size. For an eddy of turbulence, t should increase with the eddy mass (M) raised to the power 2/3, while L should increase with M(2/3) times the bulk speed of the turbulent stream that carries the eddy. The number of rolls during the eddy life span is a constant independent of eddy size.

Life and evolution as physics.

What is evolution and why does it exist in the biological, geophysical and technological realms - in short, everywhere? Why is there a time direction - a time arrow - in the changes we know are happening every moment and everywhere? Why is the present different than the past? These are questions of physics, about everything, not just biology. The answer is that nothing lives, flows and moves unless it is driven by power. Physics sheds light on the natural engines that produce the power destroyed by the flows, and on the free morphing that leads to flow architectures naturally and universally. There is a unifying tendency across all domains to evolve into flow configurations that provide greater access for movement. This tendency is expressed as the constructal law of evolutionary flow organization everywhere. Here I illustrate how this law of physics accounts for and unites the life and evolution phenomena throughout nature, animate and inanimate.

Why humans build fires shaped the same way.

Here we see why humans unwittingly build fires that look the same: edifices of fuel, as tall as they are wide. The pile of fuel is permeable, air invades it by natural convection and drives the combustion. I show that the hottest pile of burning fuel occurs when the height of the pile is roughly the same as its base diameter. Future studies may address the shape effect of wind, material type, and packing. Key is why humans of all eras have been relying on this design of fire "unwittingly". The reason is that the heat flow from fire facilitates the movement and spreading of human mass on the globe.

Cerebral oxygenation and optimal vascular brain organization.

The cerebral vascular network has evolved in such a way so as to minimize transport time and energy expenditure. This is accomplished by a subtle combination of the optimal arrangement of arteries, arterioles and capillaries and the transport mechanisms of convection and diffusion. Elucidating the interaction between cerebral vascular architectonics and the latter physical mechanisms can catalyse progress in treating cerebral pathologies such as stroke, brain tumours, dementia and targeted drug delivery. Here, we show that brain microvascular organization is predicated on commensurate intracapillary oxygen convection and parenchymal diffusion times. Cross-species grey matter results for the rat, cat, rabbit and human reveal very good correlation between the cerebral capillary and tissue mean axial oxygen convective and diffusion time intervals. These findings agree with the constructal principle.

Maxwell's demons everywhere: evolving design as the arrow of time.

Science holds that the arrow of time in nature is imprinted on one-way (irreversible) phenomena, and is accounted for by the second law of thermodynamics. Here I show that the arrow of time is painted much more visibly on another self-standing phenomenon: the occurrence and change (evolution in time) of flow organization throughout nature, animate and inanimate. This other time arrow has been present in science but not recognized as such since the birth of thermodynamics. It is Maxwell's demon. Translated in macroscopic terms, this is the physics of the phenomenon of design, which is the universal natural tendency of flow systems to evolve into configurations that provide progressively greater access over time, and is summarized as the constructal law of design and evolution in nature. Knowledge is the ability to effect design changes that facilitate human flows on the landscape. Knowledge too flows.

Why the bigger live longer and travel farther: animals, vehicles, rivers and the winds.

Here we show that constructal-law physics unifies the design of animate and inanimate movement by requiring that larger bodies move farther, and their movement on the landscape last longer. The life span of mammals must scale as the body mass (M) raised to the power 1/4, and the distance traveled during the lifetime must increase with body size. The same size effect on life span and distance traveled holds for the other flows that move mass on earth: atmospheric and oceanic jets and plumes, river basins, animals and human operated vehicles. The physics is the same for all flow systems on the landscape: the scaling rules of "design" are expressions of the natural tendency of all flow systems to generate designs that facilitate flow access. This natural tendency is the constructal law of design and evolution in nature. Larger bodies are more efficient movers of mass on the landscape.

The constructal law and the evolution of design in nature.

The constructal law accounts for the universal phenomenon of generation and evolution of design (configuration, shape, structure, pattern, rhythm). This phenomenon is observed across the board, in animate, inanimate and human systems. The constructal law states the time direction of the evolutionary design phenomenon. It defines the concept of design evolution in physics. Along with the first and second law, the constructal law elevates thermodynamics to a science of systems with configuration. In this article we review the more recent work of our group, with emphasis on the advances made with the constructal law in the natural sciences. Highlighted are the oneness of animate and inanimate designs, the origin of finite-size organs on animals and vehicles, the flow of stresses as the generator of design in solid structures (skeletons, vegetation), the universality and rigidity of hierarchy in all flow systems, and the global design of human flows. Noteworthy is the tapestry of distributed energy systems, which balances nodes of production with networks of distribution on the landscape, and serves as key to energy sustainability and empowerment. At the global level, the constructal law accounts for the geography and design of human movement, wealth and communications.

The constructal law of design and evolution in nature.

Constructal theory is the view that (i) the generation of images of design (pattern, rhythm) in nature is a phenomenon of physics and (ii) this phenomenon is covered by a principle (the constructal law): 'for a finite-size flow system to persist in time (to live) it must evolve such that it provides greater and greater access to the currents that flow through it'. This law is about the necessity of design to occur, and about the time direction of the phenomenon: the tape of the design evolution 'movie' runs such that existing configurations are replaced by globally easier flowing configurations. The constructal law has two useful sides: the prediction of natural phenomena and the strategic engineering of novel architectures, based on the constructal law, i.e. not by mimicking nature. We show that the emergence of scaling laws in inanimate (geophysical) flow systems is the same phenomenon as the emergence of allometric laws in animate (biological) flow systems. Examples are lung design, animal locomotion, vegetation, river basins, turbulent flow structure, self-lubrication and natural multi-scale porous media. This article outlines the place of the constructal law as a self-standing law in physics, which covers all the ad hoc (and contradictory) statements of optimality such as minimum entropy generation, maximum entropy generation, minimum flow resistance, maximum flow resistance, minimum time, minimum weight, uniform maximum stresses and characteristic organ sizes. Nature is configured to flow and move as a conglomerate of 'engine and brake' designs.

The evolution of speed, size and shape in modern athletics.

In the present study, we show that the fastest runners and swimmers are becoming not only faster but also heavier, taller and more slender. During the past century, the world record speeds for 100 m-freestyle and 100 m-dash have increased with body mass (M) raised to the power 1/6, in accordance with the constructal scaling of animal locomotion. The world records also show that the speeds have increased in proportion with body heights (H) raised to the power 1/2, in accordance with animal locomotion scaling. If the athlete's body is modeled with two length scales (H, body width L), the (M, H) data can be used to calculate the slenderness of the body, H/L. The world records show that the body slenderness is increasing very slowly over time.

The constructal unification of biological and geophysical design.

Here we show that the emergence of scaling laws in inanimate (geophysical) flow systems is analogous to the emergence of allometric laws in animate (biological) flow systems, and that features of evolutionary "design" in nature can be predicted based on a principle of physics (the constructal law): "For a finite-size flow system to persist in time (to live) it must evolve in such a way that it provides easier and easier access to its currents", meaning that the configuration and function of flow systems change over time in a predictable way that improves function, distributes imperfection, and creates geometries that best arrange high and low resistance areas or volumes. This theoretical unification of the phenomena of animate and inanimate flow design generation is illustrated with examples from biology (lung design, animal locomotion) and the physics of fluid flow (river basins, turbulent flow structure, self-lubrication). The place of this design-generation principle as a self-standing law in thermodynamics is discussed. Natural flow systems evolve by acquiring flow configuration in a definite direction in time: existing configurations are replaced by easier flowing configurations.