Universality in snowflake aggregation

Aggregation of ice crystals is a key process governing precipitation. Individual ice crystals exhibit considerable diversity of shape, and a wide range of physical processes could influence their aggregation; despite this we show that a simple computer model captures key features of aggregate shape and size distribution reported recently from cirrus clouds. The results prompt a new way to plot the experimental size distributions leading to remarkably good dynamical scaling. That scaling independently confirms that there is a single dominant aggregation mechanism at play, albeit our model (based on undeflected trajectories to contact) does not capture its form exactly

Snapchat\u27s Gift: Equity Culture in High-Tech Firms

Snap, Inc., the company that owns the platform Snapchat, controbersially offered nonvoting common shares to the public in 2017. This Article asks what it means to invest in Snap or other (mostly technology-based) companies in which common shareholders collectibely have little or no power to influence corporate policy. In particular, why do such investors expect to be compensated? This Article explores the familiar rationales for equity investing, including stock appreciation and dividends, and the logical shortcomings of those rationales in these circumstances. Adopting Henry Manne\u27s two systems approach to corporate affairs through both law and economics, we show that corporation law fails to ensure that corporations return business profits to shareholders. A similar analysis of the market for corporate control concludes that, without shareholder boting, the market for corporate control also fails to ensure a return to shareholders. Shareholders who invest in firms in the absence of legal or market mechanisms to secure a return on their inbestment, howeber, are not irrational. Instead, inbestors rely on cultural understandings of appropriate reciprocity. This Article employs Marcel Mauss\u27s cultural anthropology classic, The Gift, to explain the equity culture in which shareholders invest in Snap and other high-technology firms, and in which such firms operate. This Article concludes by suggesting some ramifications of understanding shareholding, and consequently management, in terms of equity culture. This Article also complements the substantial work of behabioral economics in explaining inbestor choice and organizational behabiors. The field of corporate finance traditionally has been organized around the figure of the rationally self-interested indibidual. Behabioral economics argues that people are not as rational as orthodox corporate finance assumes. This Article argues that people in markets are not as individual as corporate finance assumes

Enthusiastic Enforcement, Informal Legislation: The Unruly Expansion of the Foreign Corrupt Practices Act

The Foreign Corrupt Practices Act (FCPA) was enactedover thirty years ago to prohibit bribery of foreign officialsby U.S. persons. In the last few years, the Department ofJustice (DOJ) and the Securities and ExchangeCommission (SEC) have dramatically expanded the FCPAthrough a surge in its enforcement. Responding tocomplex developments in law, the global economy, andagency politics, the DOJ and the SEC have brought tentimes as many cases as in prior years, and assessedhundreds of millions of dollars in penalties. At the sametime, the substantive reach of the law has been extendedthrough the increased enforcement. Thus, ad hocenforcement actions, rather than legislation, regulation, or judicial decision, have transformed the FCPA. In theabsence of formal process or reasoned articulation, thescope of the law is currently unclear. Businesses havelittle official guidance in designing effective complianceprograms and may be more likely to violate the FCPA.Unruly enforcement, and the resulting lack of clarityabout what the FCPA requires, may compromise the law\u27sefficacy. Therefore the DOJ and the SEC shouldencourage compliance by providing clear, generalguidance about the scope of the FCPA. The Articleconcludes with specific questions about the FCPA thatsuch guidance should resolve

Theory and observations of ice particle evolution in cirrus using Doppler radar: evidence for aggregation

Vertically pointing Doppler radar has been used to study the evolution of ice particles as they sediment through a cirrus cloud. The measured Doppler fall speeds, together with radar-derived estimates for the altitude of cloud top, are used to estimate a characteristic fall time tc for the `average' ice particle. The change in radar reflectivity Z is studied as a function of tc, and is found to increase exponentially with fall time. We use the idea of dynamically scaling particle size distributions to show that this behaviour implies exponential growth of the average particle size, and argue that this exponential growth is a signature of ice crystal aggregation.Comment: accepted to Geophysical Research Letter

Estimating drizzle drop size and precipitation rate using two-colour lidar measurements

A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution. The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions

Bose-Einstein condensation of metastable helium: some experimental aspects

We describe our recent realization of BEC using metastable helium. All detection is done with a micruchannel plate which detects the metastables or ions coming from the trapped atom cloud. This discussion emphasizes some of the diagnostic experiments which were necessary to quantitatively analyse our results.Comment: 5 pages, 3 figure

Bose-Einstein Condensation and Spin Mixtures of Optically Trapped Metastable Helium

We report the realization of a BEC of metastable helium-4 atoms (4He*) in an all optical potential. Up to 10^5 spin polarized 4He* atoms are condensed in an optical dipole trap formed from a single, focused, vertically propagating far off-resonance laser beam. The vertical trap geometry is chosen to best match the resolution characteristics of a delay-line anode micro-channel plate detector capable of registering single He* atoms. We also confirm the instability of certain spin state combinations of 4He* to two-body inelastic processes, which necessarily affects the scope of future experiments using optically trapped spin mixtures. In order to better quantify this constraint, we measure spin state resolved two-body inelastic loss rate coefficients in the optical trap

A study of atom localization in an optical lattice by analysis of the scattered light

We present an experimental study of a four beam optical lattice using the light scattered by the atoms in the lattice. We use both intensity correlations and observations of the transient behavior of the scattering when the lattice is suddenly switched on. We compare results for 3 different configurations of the optical lattice. We create situations in which the Lamb-Dicke effect is negligible and show that, in contrast to what has been stated in some of the literature, the damping rate of the 'coherent' atomic oscillations can be much smaller than the inelastic photon scattering rate.Comment: An old pape