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BEIJING, July 26 (Xinhuanet) -- Hong Kong scientists announced that they had determined the idea of time travel is impossible by proving nothing can travel faster than the speed of light.The finding is contained in a study done by a research team from Hong Kong University of Science and Technology. The study was published Monday in a scientific journal "Physical Review Letters" in the United States."The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality, that is, an effect cannot occur before its cause," the university said on its website."By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon." said Professor Du Shengwang, who led the study.The possibility of time travel was raised 10 years ago when scientists discovered the optical pulses in some specific medium might propagate information in a faster-than-light speed."Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information." Du said.

WASHINGTON, June 15 (Xinhua) -- Using the deepest X-ray image ever taken, astronomers found the first direct evidence that massive black holes were common in the early universe, the U.S. National Aeronautics and Space Administration (NASA) said Wednesday in a statement.The discovery from NASA's Chandra X-ray Observatory shows that very young black holes grew more aggressively than previously thought, in tandem with the growth of their host galaxies.By pointing Chandra at a patch of sky for more than six weeks, astronomers obtained what is known as the Chandra Deep Field South (CDFS). When combined with very deep optical and infrared images from NASA's Hubble Space Telescope, the new Chandra data allowed astronomers to search for black holes in 200 distant galaxies, from when the universe was between about 800 million to 950 million years old."Until now, we had no idea what the black holes in these early galaxies were doing, or if they even existed," said Ezequiel Treister of the University of Hawaii, lead author of the study to appear Thursday in journal Nature. "Now we know they are there, and they are growing like gangbusters."The super-sized growth means that the black holes in the CDFS are less extreme versions of quasars -- very luminous, rare objects powered by material falling onto supermassive black holes. However, the sources in the CDFS are about a hundred times fainter and the black holes are about a thousand times less massive than the ones in quasars.The observations found that between 30 and 100 percent of the distant galaxies contain growing supermassive black holes. Extrapolating these results from the small observed field to the full sky, there are at least 30 million supermassive black holes in the early universe. This is a factor of 10,000 larger than the estimated number of quasars in the early universe."It appears we've found a whole new population of baby black holes," said co-author Kevin Schawinski of Yale University. "We think these babies will grow by a factor of about a hundred or a thousand, eventually becoming like the giant black holes we see today almost 13 billion years later."

coastline in the U.S. State of California sampled by the state water board harbored fish with mercury in such high concentrations that they shouldn't be eaten by young women and children, a newly released survey has found.Fourteen percent of locations had similarly elevated levels of PCBs, according to the survey published by The Los Angeles Times on Sunday.The most elevated concentrations of mercury and PCBs were found in San Francisco Bay and San Diego Bay, said the survey funded by the state water board.The findings, part of a two-year inquiry that is the largest statewide survey of contaminants in sport fish along the California coast, examined more than 2,000 fish from three dozen species gathered in 2009 from waters near Los Angeles, San Francisco and San Diego, the paper said.The survey highlights the health problem of lingering mercury, a poisonous metal that is found in fish globally, and of PCBs, toxic chemicals the United States banned in the 1970s, the report said.Both substances continue to pose a risk to people who eat fish caught along the California coast because they can lead to nervous system damage and developmental problems in children and can cause cancer, liver damage and reproductive harm, the report quoted researchers as saying."Unfortunately, we're not seeing many areas that are totally clean," said Jay Davis, a senior scientist for the San Francisco Estuary Institute and lead author of the study. But a catalog of where and in what fish the substances abound should help anglers make better choices, Davis said. "With good information, people can reduce their exposure significantly."Sharks had some of the highest levels of mercury because of their unusual tendency to accumulate contaminants in their flesh, while chub mackerel had the lowest levels of contamination, according to the survey.The survey results were used in part to help craft new fish consumption guidelines issued earlier this week for anglers in San Francisco Bay, the first update there by state health officials in 17 years. The advisory identifies shiner perch and other surf perches as unsafe to eat in any quantity and warns young women and children not to eat white sturgeon, striped bass and sharks caught in the bay, The Times said.The buildup of metals and other chemicals in fish is such a problem along the Southern California coast that health officials two years ago expanded the number of fish species on the "do not eat" list from one to five because of high levels of PCBs, mercury and the banned pesticide DDT, the report noted.

SAN FRANCISCO, Sept. 27 (Xinhua) -- Apple on Tuesday announced an invitation-only special event featuring iPhone on Oct. 4, 2011.With a simple headline reading "Let's talk iPhone," the invitation said that there will be an "executive presentation at 10 a.m.." This could be the first big product introduction by the newly installed Chief Executive Officer Tim Cook after he took over the reins of Apple from Steve Jobs in August.The event, which will be held at Apple campus in Cupertino, California, has long been reported to be the introduction of iPhone 5, the next generation iPhone.According to U.S. media reports, the iPhone 5 is expected to have revamped hardware design with a larger screen, a higher resolution camera and a faster dual-core A5 chip on which the iPad 2 runs. Apple is also reported to release a low-cost and contract- free iPhone model bundled with its cloud computing service iCloud.