Few things in the Universe keep the beat as reliably as an atom’s pulse.
Yet even the most advanced ‘atomic’ clocks based on variations of these quantum timekeepers lose count when pushed to their limits.
Physicists have known for some time that entangling atoms can help tie particles down enough to squeeze a little more tick from every tock, yet most experiments have only been able to demonstrate this on the smallest of scales.
Not everything needs to be seen to be believed; certain things are more readily heard, like a train approaching its station. In a recent paper, published in Physical Review Letters, researchers have put their ears to the rail, discovering a new property of scattering amplitudes based on their study of sound waves through solid matter.
Be it light or sound, physicists consider the likelihood of particle interactions (yes, sound can behave like a particle) in terms of probability curves or scattering amplitudes. It is common lore that when the momentum or energy of one of the scattered particles goes to zero, scattering amplitudes should always scale with integer powers of momentum (i.e., p1, p2, p3, etc.). What the research team found however, was that the amplitude can be proportional to a fractional power (i.e., p1/2, p1/3, p1/4, etc.).
Why does this matter? While quantum field theories, such as the Standard Model, allow researchers to make predictions about particle interactions with extreme accuracy, it is still possible to improve upon current foundations of fundamental physics. When a new behavior is demonstrated—such as fractional-power scaling—scientists are given an opportunity to revisit or revise existing theories.
A matter-wave interferometer can probe the magnetism of a broad range of species, from single atoms to very large, weakly magnetic molecules.
This year marks the centenary of the ground-breaking experiment of Otto Stern and Walther Gerlach that demonstrated the quantization of the spin angular momentum of an atom [1]. The evidence came from the observation that a beam of silver atoms, upon traversing a spatially varying magnetic field, split into two beams. The spatial splitting of the spin-up and spin-down atoms corresponded to an atomic magnetic moment of 1 Bohr magneton—the magnetic moment of a single spinning electron. The deflection of particle beams in a spatially varying magnetic field remains the basis of techniques for characterizing the magnetic properties of isolated atoms and molecules. Such techniques, however, aren’t sufficiently sensitive to study very large, weakly magnetic molecules, including many biological molecules.
The energy of the vacuum should have a gravitational effect on large atoms. But physicists’ attempts to measure it have puzzlingly come up empty.
Topological materials that possess certain atomic-level symmetries, including topological insulators and topological semi-metals, have elicited fascination among many condensed matter scientists because of their complex electronic properties. Now, researchers in Japan have demonstrated that a normal semiconductor can be transformed into a topological semi-metal by light irradiation. Further, they showed how spin-dependent responses could appear when illuminated with circularly-polarized laser light. Published in Physical Review B, this work explores the possibility of creating topological semi-metals and manifesting new physical properties by light control, which may open up a rich physical frontier for topological properties.
Most ordinary substances are either electrical conductors, like metals, or insulators, like plastic. In contrast, topological insulators can exhibit unusual behavior in which electrical currents flow along the surface of the sample, but not inside the interior. This characteristic behavior is strongly connected to topological properties inherent in the electronic state. Furthermore, a novel phase called a topological semi-metal provides a new playground for exploring the role of topology in condensed matter. However, the underlying physics of these systems is still being pondered.
Researchers at the University of Tsukuba studied the dynamics of excitations in zinc arsenide (Zn3As2) when irradiated with a laser with circular polarization. Zinc arsenide is normally thought of as a narrow-gap semiconductor, which means that electrons are not free to move around on their own but can be easily propelled by energy from an external light source. Under the right conditions, the material can show a special topological state called a “Floquet-Weyl semi-metal,” which is a topological semi-metal coupled with light. In this case, the electrical current can be carried in the form of quasiparticles called Weyl fermions. Because these quasiparticles travel as if they have zero mass and resist becoming scattered, Weyl fermions can move easily through the material.
By redesigning how fluids are simulated, researchers have demonstrated an order of magnitude speed increase on the previous state-of-the-art for slow-flowing viscous liquids.
face_with_colon_three circa 2018.
Understanding the fundamental constituents of the universe is tough. Making sense of the brain is another challenge entirely. Each cubic millimetre of human brain contains around 4 km of neuronal “wires” carrying millivolt-level signals, connecting innumerable cells that define everything we are and do. The ancient Egyptians already knew that different parts of the brain govern different physical functions, and a couple of centuries have passed since physicians entertained crowds by passing currents through corpses to make them seem alive. But only in recent decades have neuroscientists been able to delve deep into the brain’s circuitry.
On 25 January, speaking to a packed audience in CERN’s Theory department, Vijay Balasubramanian of the University of Pennsylvania described a physicist’s approach to solving the brain. Balasubramanian did his PhD in theoretical particle physics at Princeton University and also worked on the UA1 experiment at CERN’s Super Proton Synchrotron in the 1980s. Today, his research ranges from string theory to theoretical biophysics, where he applies methodologies common in physics to model the neural topography of information processing in the brain.
“We are using, as far as we can, hard mathematics to make real, quantitative, testable predictions, which is unusual in biology.” — Vijay Balasubramanian
Posted in particle physics
The Standard Model is our best theory for how the universe operates, but there are some missing pieces that physicists are struggling to find.
The Standard Model of physics is the theory of particles, fields and the fundamental forces that govern them.
It tells us about how families of elementary particles group together to form larger composite particles, and how one particle can interact with another, and how particles respond to the fundamental forces of nature. It has made successful predictions such as the existence of the Higgs boson, and acts as the cornerstone for theoretical physics.
Even in the driest climates, though, there is a considerable amount of moisture in the air. The researchers note that even in places like the Sahel desert, relative humidity is still around 20 percent on average. So they set about finding a way to use this untapped water resource to produce hydrogen.
Their device consists of a water harvesting unit that houses a sponge soaked in a water-absorbing liquid that can pull moisture from the air. On either side of this reservoir are electrodes that can be powered by any renewable energy source. When a current runs through the circuit, the water is split via electrolysis into its constituent oxygen and hydrogen atoms, which can then be collected as gas.
The team showed that the device could run efficiently for 12 consecutive days and produced hydrogen with 99 percent purity. What’s more, the device continues to work in relative humidity as low as four percent.
At a fundamental level, only a few particles and forces govern all of reality. How do their combinations create human consciousness?