From artefacts to atoms - A new SI for 2018 to be based on fundamental constants

At the 26th General Conference on Weights and Measures (CGPM) in November 2018, it is planned to adopt a new definition of the International System of Units, SI, based on the fixed numerical values of seven defining constants, broadly the fundamental constants of physics. This will be the culmination of more than two hundred years of metrology, instantiating for the first time the original ideas of the creators of the metric system. The key is the replacement of the present definition of the unit of mass, the kilogram artefact of platinum-iridium, by one based on a fixed numerical value of the Planck constant. This article outlines how this has come about, how it will work, why we need a system of measurement that is uniform, accessible world-wide and stable in the long term, and the international structures that now exist to achieve it.     

Generation of chick skeletal muscle cells in groups of 16 from stem cells

The commonly accepted hypothesis explaining the control of skeletal muscle differentiation is that all myogenic precursor cells are equivalent and that they differentiate into post-mitotic muscle cells in response to exogenous signals, specifically low mitogen concentrations. Large clones derived from vertebrate myogenic cells, however, consist both of cycling precursors and of terminally differentiated, post-mitotic muscle cells. Here, we count the total number of cells and the number of terminally differentiated cells (or nuclei, in fused cells) in large myogenic clones. The number of terminally differentiated cells per clone was usually equal to or just below a multiple of 16. This finding is not expected from a model postulating a homogeneous population of muscle precursor cells. Rather, our results suggest that a self-renewing stem cell exists in the skeletal muscle lineage. This cell can generate committed precursors which then give rise to cohorts of 16 terminally differentiated muscle cells. This model of myogenesis provides a simple explanation for the protracted and asynchronous nature of muscle differentiation in vertebrate embryogenesis.     

Images of a fourth planet orbiting HR 8799

High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets. Such images were possible because of the wide orbits (>25?astronomical units, where 1?au is the Earth-Sun distance) and youth (<100?Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. An important area of contention in the exoplanet community is whether outer planets (>10?au) more massive than Jupiter form by way of one-step gravitational instabilities or, rather, through a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium. Here we report the presence of a fourth planet, interior to and of about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts, the HR 8799 planetary system is a unique laboratory in which to study the formation and evolution of giant planets at wide (>10?au) separations.     

An earlier origin for the Acheulian

The Acheulian is one of the first defined prehistoric techno-complexes and is characterized by shaped bifacial stone tools. It probably originated in Africa, spreading to Europe and Asia perhaps as early as ～1?million years (Myr) ago. The origin of the Acheulian is thought to have closely coincided with major changes in human brain evolution, allowing for further technological developments. Nonetheless, the emergence of the Acheulian remains unclear because well-dated sites older than 1.4?Myr ago are scarce. Here we report on the lithic assemblage and geological context for the Kokiselei 4 archaeological site from the Nachukui formation (West Turkana, Kenya) that bears characteristic early Acheulian tools and pushes the first appearance datum for this stone-age technology back to 1.76?Myr ago. Moreover, co-occurrence of Oldowan and Acheulian artefacts at the Kokiselei site complex indicates that the two technologies are not mutually exclusive time-successive components of an evolving cultural lineage, and suggests that the Acheulian was either imported from another location yet to be identified or originated from Oldowan hominins at this vicinity. In either case, the Acheulian did not accompany the first human dispersal from Africa despite being available at the time. This may indicate that multiple groups of hominins distinguished by separate stone-tool-making behaviours and dispersal strategies coexisted in Africa at 1.76?Myr ago.     

The case against climate regulation via oceanic phytoplankton sulphur emissions

More than twenty years ago, a biological regulation of climate was proposed whereby emissions of dimethyl sulphide from oceanic phytoplankton resulted in the formation of aerosol particles that acted as cloud condensation nuclei in the marine boundary layer. In this hypothesis--referred to as CLAW--the increase in cloud condensation nuclei led to an increase in cloud albedo with the resulting changes in temperature and radiation initiating a climate feedback altering dimethyl sulphide emissions from phytoplankton. Over the past two decades, observations in the marine boundary layer, laboratory studies and modelling efforts have been conducted seeking evidence for the CLAW hypothesis. The results indicate that a dimethyl sulphide biological control over cloud condensation nuclei probably does not exist and that sources of these nuclei to the marine boundary layer and the response of clouds to changes in aerosol are much more complex than was recognized twenty years ago. These results indicate that it is time to retire the CLAW hypothesis.     

Transparency and secrecy in citizen science: Lessons from herping

In this paper I will outline a worry that citizen science can promote a kind of transparency that is harmful. I argue for the value of secrecy in citizen science. My argument will consist of analysis of a particular community (herpers), a particular citizen science platform (iNaturalist, drawing contrasts with other platforms), and my own travels in citizen science. I aim to avoid a simple distinction between science versus non-science, and instead analyze herping as a rich practice [MacIntyre, 2007]. Herping exemplifies citizen science as functioning simultaneously within and outside the sphere of science. I show that herpers have developed communal systems of transmitting and protecting knowledge. Ethical concerns about secrecy are inherently linked to these systems of knowledge. My over-arching aim is to urge caution in the drive to transparency, as the concepts of transparency and secrecy merit close scrutiny. The concerns I raise are complementary to those suggested by previous philosophical work, and (I argue) resist straightforward solutions.     

Transiting circumbinary planets Kepler-34 b and Kepler-35 b

Most Sun-like stars in the Galaxy reside in gravitationally bound pairs of stars (binaries). Although long anticipated, the existence of a 'circumbinary planet' orbiting such a pair of normal stars was not definitively established until the discovery of the planet transiting (that is, passing in front of) Kepler-16. Questions remained, however, about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we report two additional transiting circumbinary planets: Kepler-34 (AB)b and Kepler-35 (AB)b, referred to here as Kepler-34 b and Kepler-35 b, respectively. Each is a low-density gas-giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 b orbits two Sun-like stars every 289?days, whereas Kepler-35 b orbits a pair of smaller stars (89% and 81% of the Sun's mass) every 131?days. The planets experience large multi-periodic variations in incident stellar radiation arising from the orbital motion of the stars. The observed rate of circumbinary planets in our sample implies that more than ～1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.