APS Physics Tip Sheet – May 20, 2014
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The Universality of Free Fall
Einstein’s assertion that free-fall is the same for all objects has now been confirmed by a quantum test on two different atoms.
The equivalence principle states that free fall is universal, i.e. all objects in a given gravitational field will undergo the same acceleration, independent of their properties. But certain quantum gravity theories predict that gravitational pull may depend on an object’s atomic makeup: different combinations of neutrons and protons would fall at slightly different rates. A new experiment, carried out by researchers in the US and Germany, has looked for such an effect by comparing, for the first time, how different atoms accelerate in free fall. The researchers cooled rubidium and potassium atoms to a temperature at which they behave as matter waves and compared their free fall in an ultrasensitive matter-wave interferometer. The measured accelerations were the same to within about 1 part in 10 million, confirming the universality of free fall and constraining alternative gravity models.
* Dennis Schlippert (contact author) et al., “Quantum test of the universality of free fall,” Physical Review Letters (expected publication date:May 22)
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Sailing on Convection
Convection—the motion induced in a fluid by temperature differences—can be exploited to controllably propel macroscopic objects
Convection is one of the major mechanisms of heat transport, driving, for instance, the circulation of the oceans and the atmosphere. But convection can also be “tamed” to propel macroscopic floating objects, as shown by experiments carried out at MIT’s ENDLab. By heating the surfaces of a floating object, the researchers were able to control the convective fluid motion around the object, and thus steer the object in a programmed direction. The scheme is particularly suitable to move small masses for applications ranging from bioengineering to microfluidics.
* Matthieu J. Mercier (contact author) et al., “Propulsion via natural convection,” Physical Review Letters (expected publication date: May 21)
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How Bacteria Regulate Their Size
A new model explains how bacteria know when to stop growing and start dividing.
All organisms must regulate the dimensions of their cells, since size affects all basic physiological processes, from nutrition to cell division. Bacteria are very successful in solving this challenge - each cell looks remarkably similar to another in size and shape. But how do they know they have reached the size at which they should divide? A researcher at Harvard University has proposed an explanation based on a model built using tools of statistical mechanics: without having a way to “measure” their absolute size, bacteria can add a constant volume from one replication to the next thanks to a a simple biochemical mechanism that doesn't get disrupted by random fluctuations in cell size and growth rate. His model can reproduce many experimentally observed features of bacterial growth, from the statistical distribution of bacterial sizes to the correlation between size at birth and size at division.
* Ariel Amir (contact author), “Cell size regulation in bacteria,” Physical Review Letter (expected publication date: May 23)
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Journal articles and preprints are available to journalists on request.
Matteo Rini, PhD
Deputy Editor, Physics
