APS Physics Tip Sheet – Apr 29, 2014
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Sliding Sand
Adding a small amount of water to sand can significantly reduce the sliding friction.
Pouring some water—but not too much—on sand makes it easier to slide objects thanks to the formation of capillary bridges. This is the conclusion of a study by a team of researchers from the Netherlands, Iran, France, Germany and India. The authors tested the sliding friction of dry and wet sand when a weighted sled was pulled across the surface. As water was added, both the force needed to pull the sled and the friction coefficient were found to decrease below that of the dry sand, reaching a minimum before increasing again as more water was added. The authors explain that this is due to capillary liquid bridges that start to form between the sand grains when water is added, facilitating the sliding. But if too much water is added the capillary bridges merge and disappear.
* A. Fall, Daniel Bonn (contact author) et al., “Sliding Friction on Wet and Dry Sand,” Physical Review Letters (published Apr 29)
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A Tractor Beam Made of Sound
Sound waves can be engineered to realize a tractor beam – a device able to pull centimeter-size objects.
A “tractor beam”—a term coined in the 1931 science-fiction novel Spacehounds of IPC by E.E. Smith—is a device that can attract one object from a distance. Researchers have had some success at a microscopic level, demonstrating optical tweezers that can pull nanoparticles opposite to the light propagation direction. Now a team of researchers from the UK and the US has reported a step towards a macroscopic device. The scheme is based on a sound beam that generates low-pressure zones towards which an object can be attracted. The authors were able to scale up the tractor beam to handle centimeter-size objects, a million times larger than those manipulated by previous schemes.
* Christine E.M. Démoré (contact author) et al., “Acoustic tractor beam,” Physical Review Letters (expected publication date: Apr 30)
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Neutron Star Goes Wobbly
The x-ray emission from a magnetar has revealed that the star’s huge magnetic field has distorted its shape, causing it to wobble.
Magnetars are neutron stars with a colossal magnetic field. Having magnitudes up to 1011 tesla, their fields are a billion times stronger than the most powerful magnets on Earth. Possible evidence of an even stronger field comes from recent x-ray observations of one of the brightest magnetars (4U 0142+61, located in the constellation Cassiopeia at 13000 light-years from Earth). A team of researchers from Japan and the US has reported the detection of a time-varying x-ray signal, indicating a wobble, or precession, in the magnetar’s rotation caused by an internal field of about 1012 tesla. The authors suggest that a strong internal toroidal field deforms the magnetar into a prolate shape, like a football, which wobbles as it spins.
* Kazuo Makishima (contact author) et al., “Possible evidence for free precession of a strongly magnetized neutron star in the magnetar 4U 0142+61,” Physical Review Letters (expected publication date: Apr 30)
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Element Z=117 Confirmed
An international collaboration has used an intense isotope beam provided by the GSI research facility in Darmstadt, Germany, and a target material of radioactive berkelium supplied by Oak Ridge National Lab in Tennessee to produce two atoms of the superheavy element with atomic number Z=117 (ununseptium). The result confirms the initial observation published in 2010 –an essential step towards the recognition of the discovery of the element. In the process, the researchers also discovered a new isotope (lawrencium-266).
* J. Khuyagbaatar (contact author) et al., “Study of the 48Ca+249Bk fusion reaction leading to element Z =117: long-lived-decaying 270Db and discovery of 266Lr,” Physical Review Letters (expected publication date: May 01)
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Remote Controlled Entanglement
Researchers from California and India have created entangled states of two superconducting qubits separated by more than a meter of coaxial cable. While it is now routine to entangle photons at a distance, the authors have managed to entangle two superconducting circuits using microwave radiation transported by 1.3 meters of coaxial cable. The result shows that quantum entanglement can be established between distant systems that interact only through a signal propagating along low-loss electrical wires. This functionality may be exploited in future quantum networks.
* Nicolas Roch (contact author) et al., “Observation of measurement-induced entanglement and quantum trajectories of remote superconducting qubits,” Physical Review Letters (published Apr 28)
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The Arrow of Time in Physics and Psychology
Researchers from Caltech and University of South California have presented a conjecture of why we perceive time as flowing from past to future—the same direction determined by the laws of thermodynamics. According to their theory, the psychological arrow of time is determined by a principle they call “generality”: this states that what defines a memory is the correlation with the things it remembers. A memory can thus be correlated with our past, but not with our future, since the correlation with the future would be contingent on different possible outcomes. This, suggest the authors, is what makes a prediction psychologically different from a true memory, forming the basis of our perception of time.
* Leonard Mlodinow, Todd A. Brun (contact author), “On the Relation between the Psychological and Thermodynamic Arrows of Time,”Physical Review E (expected publication date: May 02)
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Journal articles and preprints are available to journalists on request.
Matteo Rini, PhD
Deputy Editor, Physics
