The North Pole in your kitchen.
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A window on the deep Earth opened unexpectedly in 2011, when Japan’s nuclear reactors were shut down after the Fukushima disaster. Before the closure, an underground particle detector called KamLAND based in Kamioka, Japan, was monitoring a torrent of neutrinos streaming from dozens of nearby nuclear reactors, seeking clues to the nature of these hard-to-catch subatomic particles. After those plants fell silent, KamLAND scientists could see more clearly a signal that had largely been obscured: a faint trickle of neutrinos produced inside the planet.
These detections are not just curiosities. Geoneutrinos offer the only way to measure one of Earth’s internal heat sources. The total heat flow, measured with sensors in deep mines and amounting to 47 terawatts (TW) of power, drives everything from plate tectonics to Earth’s magnetic field. Some of it comes from the decay of radioactive elements, the rest is primordial heat left over from when Earth was formed by the violent collision of planetary building blocks.
Enter KamLAND and Borexino, which spot geoneutrinos as a sideline to their other neutrino studies. Both experiments use liquid scintillator detectors, in which huge vats of fluid capture the occasional sparkle of light when a passing neutrino interacts with atomic nuclei in the liquid.
The team at Borexino, a vat containing 300 tonnes of liquid buried under the Italian Alps, captured 14 candidate geoneutrinos between December 2007 and August 2012 (ref. 2). Scientists at KamLAND, with 1,000 tonnes of liquid, say that they detected 116 probable geoneutrinos between March 2002 and November 2012 (ref. 3).
Assuming that uranium and thorium are spread uniformly in the mantle, the KamLAND findings suggest that about 11 of the 47 TW come from the radioactive decay of those elements. A similar calculation for Borexino yields about 18 TW.
One challenge is that emissions from uranium and thorium much nearer the surface in the continental crust can mask the geoneutrino signal coming from deeper in the planet (see ‘Under the sea’). Next year, for example, the retrofitted Sudbury Neutrino Observatory (SNO) in Ontario, Canada, will start taking data with a 780-tonne detector that is sensitive to geoneutrinos. But SNO+, as the upgrade is called, sits smack in the middle of continental crust. Separating crustal from mantle geoneutrinos is crucial, says Steve Dye, a physicist at Hawaii Pacific University in Honolulu, as “the mantle is really what contributes to the rate of cooling of the planet”.
Meanwhile, China is working on its Daya Bay II experiment, a 20,000-tonne detector on land that could be ready to hunt for geoneutrinos in 2019. Borexino has funds to run for at least another four years. And KamLAND plans to keep going for at least five more years, says team member Hiroko Watanabe of Tohoku University in Sendai, Japan. Even after Japan’s nuclear reactors restart, the detector will still be able to find geoneutrinos — just not as easily.
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Spore formation is a drastic choice because it requires the cell to kill itself to encase a copy of its genetic code in a tough, impervious shell. Though the living cell dies, the spore acts as a kind of time capsule that allows the organism to re-emerge into the world of the living when conditions improve.
“This time-travel strategy of waiting and safeguarding a copy of the DNA in the spore ensures the survival of the colony,” Ben-Jacob said. “But there are other, less desperate options that B. subtilis can take to respond to stress. Some of these cells turn into highly mobile food seekers. Others turn cannibalistic, and about 10 percent enter a state called ‘competence’ in which they bide their time and bet on present conditions to improve.”
Scientists have long been curious about how bacteria decide which of these paths to pursue. Years of studies have determined that each individual constantly senses its environment and continuously sends out chemical signals to communicate with its neighbors about the choices it is making. Experimental studies have revealed dozens of regulatory genes, signaling proteins and other genetic tools that cells use to gather information and communicate with one another.
Ben-Jacob said the timer has an internal clock that is controlled by cell stress. The noise-intolerant timer typically keeps the competence switch closed, but when the cell is exposed to stress over a long period of time, the timer activates a decision gate that opens brief “windows of opportunity” in which the competence switch can be flipped.
Thanks to its architecture, the gate oscillates during the window of opportunity, he said. At each oscillation, the switch opens for a short time and grants the cell a short window in which it can use noise as a “roll of the dice” to decide whether to escape into competence.
“The ingenuity is that at each oscillation the cell also sends ‘chemical tweets’ to inform the other cells about its stress and attempt to escape,” said Ben-Jacob, the Maguy-Glass Professor in Physics of Complex Systems and professor of physics and astronomy at Tel Aviv University. “The tweets sent by others help regulate the circuits of their neighbors and guarantee that no more than a specific fraction of cells within the colony will enter into competence.”
“What if intelligent behavior doesn’t just correlate with the production of long-term entropy, but actually emerges directly from it?” Alexander D. Wissner-Gross
– extract from the article – The second law of thermodynamics—the one that says entropy can only increase—dictates that a complex system always evolves toward greater disorderliness in the way internal components arrange themselves. In Physical Review Letters, two researchers explore a mathematical extension of this principle that focuses not on the arrangements that the system can reach now, but on those that will become accessible in the future. They argue that simple mechanical systems that are postulated to follow this rule show features of “intelligence,” hinting at a connection between this most-human attribute and fundamental physical laws.
Wissner-Gross teamed up with Cameron Freer of the University of Hawaii at Manoa to propose a “causal path entropy.” This entropy is based not on the internal arrangements accessible to a system at any moment, but on the number of arrangements it could pass through on the way to possible future states. They then calculated a “causal entropic force” that pushes the system to evolve so as to increase this modified entropy.
The researchers interpreted this and other behaviors as indications of a rudimentary adaptive intelligence, in that the systems moved toward configurations that maximized their ability to respond to further changes.
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In a paper submitted to Physical Review Letters, the IceCube Collaboration confirms that those two events are the two highest energy neutrinos ever observed. The events have estimated energies of 1.04 ± 0.16 and 1.14 ± 0.17 PeV, hundreds of times larger than the energy of a proton at the LHC.
Most of the neutrinos reaching the Earth originate either in the Sun or in our atmosphere through the interaction of incoming cosmic rays. However, if we look at neutrino energy, once we reach the PeV scale (the energy equivalent to 1,000,000 times the mass of a proton), neutrinos coming from far off in our galaxy or from more distant places in the visible Universe become dominant.
IceCube researchers have estimated that the probability that these two events are not background, i.e., anything else in the detector besides astrophysical neutrinos, is at the 2.8 sigma level.