Volume 14 Issue 5, May 2018

The criteria by which the validity of theories of complex systems are judged are more nuanced than a naive understanding of ‘the scientific method’ suggests.

Bringing next-generation atomic clocks out of the lab is not an easy task, but doing so will unlock many new possibilities. As a crucial first step, a portable atomic clock has now been deployed for relativistic geodesy measurements in the Alps.

Quantum tomography infers quantum states from measurement data, but it becomes infeasible for large systems. Machine learning enables tomography of highly entangled many-body states and suggests a new powerful approach to this problem.

Many particles — both fundamental and emergent — carry angular momentum or spin. Experiments have now demonstrated that phonons can transport angular momentum, showing that they may have spin too.

The folded structure of the human brain is a hallmark of our intelligence — an optimized packing of neurons into a confined space. Similar wrinkling in brain-on-a-chip experiments provides a way of understanding the physics of how this occurs.

An atomic clock has been deployed on a field measurement campaign to determine the height of a mountain location 1,000 m above sea level, returning a value that is in good agreement with state-of-the-art geodesy.

The roton energy spectrum, originally introduced by Landau, explains the thermodynamic behaviour of strongly interacting liquid helium at low temperature. Now, a similar spectrum has been observed in weakly interacting dipolar quantum gas.

Unsupervised machine learning techniques can efficiently perform quantum state tomography of large, highly entangled states with high accuracy, and allow the reconstruction of many-body quantities from simple experimentally accessible measurements.

Magnetotransport measurements show that ZrTe₅ exhibits an anomalous Hall effect without magnetic ordering, a signature of Berry curvature introduced by Weyl nodes. This indicates that ZrTe₅ may be a Weyl semimetal, even though this was not predicted.

By means of a sensitive neutron spectroscopy approach the magnetic excitations in the heavy fermion superconductor CeRhIn₅ are probed, revealing a uniaxial anisotropy that can be tuned with an external magnetic field.

Nodal chains are observed for the first time in a photonic crystal with accompanying drumhead surface states. This will stimulate further study of topological nodal lines with non-trivial connectivity.

The observation of three new classes of domain wall demonstrates the importance of topological disclination and dislocation defects in the helimagnet iron germanium, via analogy with grain boundaries in cholesteric liquid crystals.

The cluster size distribution of cells’ progeny in developing organs is found to be universal. A new theory inspired by the physics of aerosols suggests that collective cell dynamics leads to a critical state balancing merger with fragmentation.

Electrons can be accelerated by astrophysical shocks if they are sufficiently fast to start with. As laboratory laser-produced shock experiments reveal, this can be achieved by lower-hybrid waves generated by a shock-reflected ion instability.

The first observational evidence of plasma heating through the dissipation of Alfvén-wave energy in tenuous regions of solar magnetism provides fresh insight into heating processes in the solar atmosphere, and in other magnetohydrodynamic systems.

Photoexcitation circular dichroism generates an ultrafast response in chiral molecules, with a much higher sensitivity than standard circular dichroism.

The Gilbert damping constant, a fundamental parameter to describe magnetization dynamics, is an isotropic scalar for most magnetic materials. Now, at a metal/semiconductor interface, the emergence of anisotropic magnetic damping has been observed.

A method for resolving the spin texture of the surface state of a topological insulator using a transport measurement is developed. Understanding the spin texture will help engineer spintronic devices.

Experiments on a magnetostrictive material reveal the conversion between coherent magnons and phonons that have spin.

Canonical pattern formation relies on a system being close to an instability and stabilized by nonlinearities — but real systems seldom conform to these conditions. A new theory solves the problem by recasting it in terms of moving local equilibria.

Wrinkling in human brain organoids suggests that brain development may be mechanically driven, a notion supported only by model gels so far. Evidence in this simple living system highlights roles for cytoskeletal contraction and nuclear expansion.

Michael Jentschel and Klaus Blaum explain why the most famous equation of physics needs checking — and how to do it.