Who Ordered That?

• 2010-02-07

  The biomechanics of barefoot running : Nature

  A paper in Nature on the mechanics of human running. In the study, runners tended to land on the ball of the foot or flat when barefoot but on the heel—with a greater impact— when shod in modern running shoes.

  A comparison by Daniel Lieberman and colleagues of the biomechanics of habitually shod versus habitually barefoot runners now suggests that the collision-free way that barefoot runners typically land is not only comfortable but may also help avoid some impact-related repetitive stress injuries. Kinematic and kinetic analyses show that modern shoes allow runners to land on the heel, as they do when they walk. Runners who don’t wear shoes land more often on the ball of the foot or with a flat foot.

• 2010-02-04

  Technology Review: Blogs: arXiv blog: How to Build a Dark Energy Detector

  The arXiv blog on a suggested experiment for detecting dark energy:

  Of course there are some important differences between an electric field and the dark energy field that make measurements tricky. Not least of these is that you can’t turn off dark energy. Another is that there is no known reference against which to measure it.

  That leaves the possibility of a gradient in the dark energy field. If there is such a gradient, then it ought to be possible to measure its effect and the best way to do this is with atom interferometry, say Perl and Mueller.

• 2010-01-31

  It all works because Avogadro’s number is closer to infinity than to ten.

  — Ralph Baierlein, American Journal of Physics 46, 1045 (1978).

• 2010-01-29

  Giant laser reaches key milestone for fusion - physics-math - 28 January 2010 - New Scientist

  New Scientist reports on progress at Lawrence Livermore National Laboratory, using the world’s largest laser to compress a spherical pellet in order to ignite a fusion reaction:

  The team used targets that did not contain the key ingredients for fusion – two isotopes of hydrogen known as deuterium and tritium. But the symmetrical implosion of the targets suggests that NIF should be able to ignite fusion with laser pulses of 1.2 to 1.3 megajoules – well below its full 1.8-megajoule capacity.

  Researchers spent last year slowly cranking up the output of the laser, ultimately reaching a total energy of more than 1 megajoules. Now they’re pausing to mount new instruments on the 10-centimetre-thick aluminium target chamber and to install giant concrete doors to contain neutrons they expect to produce in future fusion experiments.

• 2010-01-28

  Single-Photon Cooling: Making Maxwell's Demon

  Chad Orzel at Uncertain Principles explains an experiment from a group at Texas, putting Maxwell’s Demon to work:

  The key to their scheme is the position of the “demon,” which is right along the outer edge of the trap. The only atoms that get far enough out to encounter the demon are necessarily very close to their turning point, and thus moving very slowly. When they drop into the laser trap, they have almost no kinetic energy, and thus a very low temperature. This is exactly the cooling effect you get with Maxwell’s demon.

  The experiment described in the paper is basically a proof-of-principle demonstration of the technique. They only loaded a small fraction of the atoms into their laser trap (their highest claimed efficiency was about 2%), and only really cooled the atoms in one dimension. Still, they were able to select out a sample of a few hundred thousand atoms at 4.3μK, starting with a sample at 53μK. This also increased the “phase space density,” which is a combination of position and momentum used as the figure of merit for attempts to get Bose-Einstein Condensation, by a factor of 350.

• 2010-01-26

  Asteroid and comet bombardment melted one of Jupiter's moons

  John Timmer in Ars Technica describes a new model for formation in Jupiter’s moons:

  The authors of the new paper focus on a period early in our Solar System’s history called the Late Heavy Bombardment. The record of impacts on the Moon suggest that gravitational disturbances sent about 1.6 x 10²²g of material into its surface, equally divided between comets and asteroids. That implies the disruption of a planetesimal disk that contained about 20 times the Earth’s mass, most likely via gravitational interactions with the gas giants. Obviously, a gravity well like Jupiter’s would attract a significant number of these bodies. More significantly, the planet would focus their trajectories such that the closer the moon to Jupiter, the more impacts it would receive. According to the authors, the Late Heavy Bombardment would strike Ganymede with about 80 times the mass than Callisto was hit with. They estimate that the total energy imparted to Ganymede could be up to 10³⁶erg [10³⁰ joules], enough to melt its ice five times over and allow all of the rocky material to sink to the core.

• 2010-01-25

  ESA Portal - Inside the dark heart of the Eagle

  Beautiful images returned from the European Space Agency’s new Herschel Space Observatory, orbiting the L2 Lagrange point of Sun-Earth, show star birth inside a dark cloud in the Eagle Nebula.

  Embedded within the dusty filaments in the Aquila image are 700 condensations of dust and gas that will eventually become stars. Astronomers estimate that about 100 are protostars, celestial objects in the final stages of formation. Each one just needs to ignite nuclear fusion in its core to become a true star. The other 600 objects are insufficiently developed to be considered protostars, but these too will eventually become another generation of stars.

  This cloud is part of Gould’s Belt, a giant ring of stars that circles the night sky – the Solar System just happens to lie near the centre of the belt. The first to notice this unexpected alignment, in the mid-19th century, was England’s John Herschel, the son of William, after whom ESA’s Herschel telescope is named. But it was Boston-born Benjamin Gould who brought the ring to wider attention in 1874.

• 2010-01-23

  symmetry breaking » Blog Archive » Strongly interacting dark matter ruled out by observations

  Symmetry Breaking reports on neutrino observations from the IceCube detector in Antarctica, which appear to rule out strongly interacting dark matter:

  Although the majority of physicists favor models of dark matter that only rely on weakly interacting massive particles, or WIMPs, some theoretical models allow for strongly interacting massive particles, or SIMPs. Massive versions of each of these particles are whimsically referred to as wimpzillas or simpzillas.

  Prior to this study, this plot represents the excluded types of simpzilla based on mass and interaction strength. The colored regions are all excluded by experiment with just the white triangles remaining as possible types of SIMPs. Like the other plot, this shows the SIMP types ruled out but with new data added. It cuts out the white triangles, leaving only a patch in the lower right, which does not correspond to any favored theories as the dark matter would be too massive.

  During recent years, a range of underground and space-borne experiments have ruled out various combinations of interaction strengths and masses for dark matter particles, but there had always been a few openings left for particles of just the right combinations of interaction strength and mass. If simpzillas did exist, they would become captured by the gravity of massive objects such as the Sun. Then they would bump into each other, lose energy, and accumulate near the center of the Sun where they would annihilate, giving off neutrinos. Those extra neutrinos should be detectable by a neutrino telescope such as IceCube in Antarctica. Using data collected from 22 strings of IceCube’s optical modules, a fraction of the 80 strings that make up the entire detector, the physicists determined there was no excess of neutrinos coming from the Sun with enough confidence to rule out the existence of simpzillas.

• 2010-01-22

  Humans are to a large degree sensitive to energy fluxes rather than temperatures, which you can verify for yourself on a cold, dark morning in the outhouse of a mountain cabin equipped with wooden and metal toilet seats. Both seats are at the same temperature, but your backside, which is not a very good thermometer, is nevertheless very effective at telling you which is which.

  — Craig F. Bohren and Bruce A. Albrecht, Atmospheric Thermodynamics (Oxford University Press, 1998).

• 2010-01-19

  On two occasions I have been asked, ‘Pray Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?’ In one case a member of the Upper, and in another a member of the Lower, House put this question. I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.

  — Charles Babbage, on politicians’ lack of understanding of (and refusal to fund) his work on the difference engine.