More highlights from the physics arXiv this week:
Archive for September, 2008
Art ‘n’ crafts (part 2)
Sunday, September 7th, 2008Art ‘n’ crafts (part 1)
Saturday, September 6th, 2008The other highlights from the arXiv this week:
Maximum Size of Drops Levitated by an Air Cushion
How Cells Tiptoe on Adhesive Surfaces Before Sticking
The First-Mover Advantage in Scientific Publication
Primordial Nucleosynthesis: from Precision Cosmology to Fundamental Physics
Image of Another Universe Being Observed Through a Wormhole Throat
A short history of computer art in Soviet bloc countries
Friday, September 5th, 2008
Many of us think of little else than the history of computer art in former Soviet bloc countries and today our prayers are answered in the form of a curious paper that examines just this topic.
The paper says that the Soviet bloc country most advanced in this respect appears to have been Yugoslavia. The author, Eric Engle, focuses on the role of computer art in the New Tendencies movement that grew up in and around Zagreb the late 1960s and early 70s in Yugoslavia.
Why was Yugoslavia the most advanced? One important factor that Engle identifies is the extent to which Soviet bloc countries embraced capitalism, which gave them access to more advanced western computers.
For various reasons, Yugoslavia was more open to the west than say East Germany or the USSR and so its computer art became more advanced.
Ref: arxiv.org/abs/0809.0524: Computer Art in the (Former) Soviet Bloc
Could life have come from other stars?
Thursday, September 4th, 2008
Late in the last century, researchers calculated that an asteroid impact on Mars could jettison rocks towards Earth in a way that preserved bacterial life within them; the implication being that life could have evolved first on a warmer wetter Mars and later seeded life on Earth.
Now Mauri Valtonen from Turku University in Finland and colleagues have worked out whether bacteria might have been able to make a similar journey from a planet orbiting another star.The answer is probably not.
But there is with one important caveat.The sun almost certainly formed in a star-birthing nursery with various other stars which later dispersed. It’s quite possible, say the team, that bacteria could have passed from one system to another while all these star systems were close together.
Nobody knows which stars are the sun’s sisters and brothers but various groups are looking to solve this conundrum. In particular, the European Space Agency’s Gaia observatory (launch date 2011) is designed to create a 3D map of our galaxy that should allow us to work out what came from where. Studying these stars will then become an obsession for various astronomers.
Equally likely, of course, is the possibility that Earth seeded planets around these stars with life.So if you work on Gaia, don’t be surprised to find somebody staring right back at us.
Ref:arxiv.org/abs/0809.0378 : Natural Transfer of Viable Microbes in Space from Planets in the Extra-Solar Systems to a Planet in our Solar System and Vice-Versa
How fast could Usain Bolt have run the 100m?
Wednesday, September 3rd, 2008
You’ll probably have seen footage of “the greatest 100m performance in the history of the event” as Michael Johnson put it. But if not, here’s short description:
In the Olympic final of the 100 metres in Beijing, the Jamaican sprinter Usain Bolt accelerated away from the field and then, with victory assured but with 20 metres still to run, started waving his arms and high-stepping in celebration. When he eventually crossed the line ahead of his rivals, he appeared to be dancing.
Despite easing up up, Bolt broke the world record by 0.03 seconds in a time of 9.69 seconds. But his performance raised the question: how fast could he have run?
Bolt’s coach later said he could have done it in 9.52 seconds. Perhaps.
To settle the question, Hans Kristian Eriksen and colleagues at the University of Oslo decided to apply a little physics.
They used video footage of the race to first work out Bolt’s acceleration profile during the 100 metres and that of the silver medallist Richard Thompson.
Bolt clearly slows down towards the end of the race. So the Norwegian team calculated what Bolt’s time would have been had he matched Thompson’s acceleration in the final 20 metres. The answer is a stunning 9.61 seconds.
But they also say he could have done better. Had Bolt outpaced Thompson’s acceleration by 0.5 m/s^2, as he did in the earlier part of the race, the Norwegians say he could have run 9.55 seconds (plus or minus 0.04 seconds). That makes the coach’s assessment look eminently achievable.
Perhaps he knows something we don’t.
Ref: arxiv.org/abs/0809.0209: Velocity Dispersions in a Cluster of Stars: How Fast Could Usain Bolt Have Run?
Predicting mine collapse
Tuesday, September 2nd, 2008
Northern France is riddled with limestone mines that occasionally collapse creating a ring-shaped crater on the surface that can cause serious damage to nearby buildings.
Is there any way to predict these failures and thereby attempt to prevent them?
If there is, Siavash Ghabezloo and Ahmad Pouya from the Laboratoire Centrale des Ponts et Chaussées in Paris, seem determined to find it and publish their initial efforts today.
Their approach is to assume that mine failure is the result of weathering in limestone rock, in other words the reaction of carbon dioxide, water with calcium carbonate. This dissolves the rock, severely weakening it until it collapses.
Ghabezloo and Pouya introduce a set of equations that govern the different hydro/chemo/ mechanical aspects of this weathering phenomenon. They then attempt to model the process numerically.
That’s all very well but does it get us any closer to predicting collapse? Unfortunately not. Validating their model will be difficult to say the least and applying it to a real mine almost impossible.
And even if it could predict how fast such weathering might occur, how would it predict where it was happening?
Nice try but no cigar.
Ref: arxiv.org/abs/0808.4081: Numerical Modelling of the Effect of Weathering on the Progressive Failure of Underground Limestone Mines
Orbiting observatory finds dark matter, but what kind?
Monday, September 1st, 2008
The world of cosmology is abuzz with rumours that an orbiting observatory called PAMELA has discovered dark matter. Last month, the PAMELA team gave a few selected physicists a sneak preview of their results at a conference in Stockholm.
Here’s the deal. The PAMELA people say their experiment has seen more positrons than can be explained by known physics and that this excess exactly matches what dark matter particles would produce if they were annihilating each other at the center of the galaxy.
What makes this particularly exciting is that other orbiting observatories have also seen similar, but less clear cut, evidence of dark matter annihilations.
Since then, the shutters have come down. With the prospect of a major discovery on their hands and with publication in a major journal at stake, the team has closed ranks to re-analyse their data and prepare it for exclusive publication. Not a word has leaked from the PAMELA team since their preliminary announcement.
That hasn’t stopped physicists speculating for themselves. Today Marco Cirelli from the CEA near Paris in France and Alessandro Strumia from the Università di Pisa in Italy present their own analysis of the PAMELA data.
Cosmologists have long speculated on the nature of dark matter and dreamt up all manner of models and particles to explain it. The big question is which type of particle does the PAMELA data point towards.
Today, Cirelli and Strumia stake their own claim. They say the data agrees with their own model called Minimal Dark Matter in which the particle responsible is called the “Wino” (no, it really is called the wino).
But given the PAMELA team’s reluctance to publish just yet, where did Cirelli and Strumia get the data? The answer is buried in a footnote in their paper.
“The preliminary data points for positron and antiproton fluxes plotted in our figures have been extracted from a photo of the slides taken during the talk, and can thereby slightly differ from the data that the PAMELA collaboration will officially publish.”
Can’t fault them for initiative.
Ref: arxiv.org/abs/0808.3867: Minimal Dark Matter Predictions and the PAMELA Positron Excess
