Scientists find answer to supernova riddle

The discovery of a supernova only hours after its explosion has probably solved a long-standing mystery on the origin of the brightest known phenomena in the Universe, scientists reported Wednesday.
On August 24, scientists witnessed the spectacular eruption of light and energy thrown off by the birth of SN 2011fe, the brightest and -- at a mere 20.9 million light years away -- closest-to-Earth supernova in over 25 years.
One light year is the distance that light travels in 365 Earth days, about 9.46 trillion kilometres or 5.87 trillion miles.
"We caught the supernova just 11 hours after it exploded, so soon that we were later able to calculate the actual moment of the explosion within 20 minutes," said Peter Nugent of the US Lawrence Berkeley National Laboratory and lead author of one of two studies, both published in Nature.
"With this close-up look, we found things nobody had dreamed of," he said in a statement.
Brightness thrown off by the violent stellar blast of type 1a supernovas is the yardstick for measuring distances across space.
This was crucial in this year's Nobel Prize winning discovery that the expansion of the Universe is accelerating and not slowing down, as once thought.
Up to now, astronomers had advanced three competing scenarios to explain the origin of 1a supernovas, rare events that only happen once every couple of hundred years in a galaxy such as our own.
In all three, a so-called white dwarf -- an ageing star in decline -- steals matter, and energy, from a companion star until the white dwarf explodes in a flash a billion times brighter than our Sun.
A supernova is so powerful it can outshine an entire galaxy such as the Milky Way, which contains some 200 billion stars, for several weeks.
White dwarfs are essentially Earth-sized diamonds. A single tablespoon weighs about ten tonnes.
Most white dwarfs die not with a bang but a whimper, gradually leaking away heat over billions of years.
But a few interact with other stars to create supernova detonations that are, in essence, colossal thermonuclear explosions.
The question was this: what species are these progenitor companion stars that feed white dwarfs until they burst?
Candidates included a red giant, a second white dwarf, and a so-called subgiant star, of which our Sun is a specimen.
By looking at pre-blast images captured by the Hubble Space Telescope, one team of scientists led by Li Weidong was able to exclude the red giants, which are 10,000 times more luminous that the Sun.
They were not, however, able to narrow things down more.
Nugent's team, which observed SN 2011fe exploding in the Pinwheel Galaxy from the California's Palomar Observatory, took the analysis a step further.
Sifting through the spectrum of the supernova as it expanded, they determined that the companion start was most likely a subgiant, also known as a main-sequence star.
"If there was a giant companion star orbiting nearby, we should have seen some fireworks when the debris from the supernova crashed into it," said co-author Daniel Kasen, a professor at the University of California at Berkeley.
"Because we didn't observe any bright flashes like that, we determined that the companion star could not have been much bigger than our Sun."
"Although the studies do not yet provide a definitive answer to this question, they are a reassuring step forward," said Mario Hamuy, in a commentary, also published in Nature.
But scientists may have to wait another 30 years before another supernova explodes in our neighbourhood or, if we are lucky, in our very own Milky Way, he said.

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Why has Britain done a U-turn on plutonium?

Why does the UK government want to convert its 112 tonnes of plutonium — the largest civilian stockpile in the world — into mixed-oxide fuel (MOX), when just a few months ago it announced the closure of the only MOX production facility in the country because of a lack of demand? Energy minister Charles Hendry announced last week that converting the plutonium to MOX was still the plan, but a new £3-billion (US$4.7-billion) plant would only be built if it could be shown that it was both affordable and offered value for money.

Although at first glance this may seem fiscally prudent, when taken with other recent events it starts to look nonsensical. Just a few weeks ago, the Nuclear Decommissioning Authority (NDA), responding to the government’s own request to consider how to deal with the stockpile, advised that the cost of making MOX is greater than its value as a fuel. And therefore, the most cost-effective plan would be to continue storing the plutonium until a geological disposal facility becomes available for it to be buried underground. So what’s going on? To make sense of it all, Nature peers through the looking glass of the British nuclear industry.

Why would the UK government want to reprocess plutonium into fuel?

Partly, because the plutonium is a huge embarrassment. To continue storing it is not only a constant reminder of how ill-thought-out British nuclear policy has been over the past decades, but it also poses a risk to non-proliferation. So converting it into MOX would transform a huge pile of waste — which the NDA euphemistically calls a “zero value asset” — into something potentially useful, while making the plutonium far harder to weaponize were it to fall into the wrong hands.

So why is it closing the only plant that can do that?Ostensibly, because, in the wake of the Fukushima nuclear disaster, the Sellafield MOX Plant no longer has any customers. But in truth it never really worked. It was built to convert the UK plutonium stockpile into MOX, but, having been plagued by technical failures and delays, it has managed to produce only around 2.5% of its intended quota since it started operating ten years ago. With such a low throughput, it would have been unable to convert the UK stockpile fast enough to provide MOX for new reactors. And so in 2010 the NDA did a deal with ten Japanese utility companies to process their smaller amounts of plutonium and supply them with MOX, a deal that ultimately sank beneath a tsunami.

Why build a new one?

Because the problem hasn’t gone away. Britain still needs to get rid of the plutonium and, despite the failings of the Sellafield plant, MOX production is a proven technology. The one other plant in the world, the Melox plant in Gard, France, has produced around 1,700 tonnes of MOX since it was built in 1995. So, currently, the only other options are to do nothing — keeping the plutonium in storage until geological disposal becomes possible sometime after 2075 — or to process it so that it can supply a fast reactor, such as the kind that, with almost comic timing, GE Hitachi offered to build at Sellafield last week. The problem with this is that, despite 30 years in development, these reactors are still an unproven technology.

Is it worth it?

According to the NDA, not if you care about making a profit. With the low cost of uranium and the high cost of producing MOX, the reprocessed fuel would probably have to be given away. But profit isn’t everything. Indeed, as long-term plutonium managing solutions go, MOX production gets the NDA’s backing. And that is the message Hendry is putting out, that although the government is not quite ready to do anything with the plutonium yet, when it is ready it will be taking the MOX route. And if that means not making a profit, that’s a small price to pay for getting rid of such an albatross. It’s a brave decision, and one that gets the backing of both the Royal Society and the government’s former chief scientific adviser David King at the University of Oxford’s Smith School of Enterprise and the Environment. 

Will it ever actually happen?

Despite all the caveats, we could see a MOX plant approved sooner rather than later. Tied into the decision is a proposal for Britain’s next-generation reactors, which would use MOX as fuel, thereby requiring a MOX plant to be in production before they come online. What remains to be seen, however, is whether a new MOX plant will be able to show that the UK nuclear industry has learnt from the mistakes and failures of the Sellafield plant and to get rid of the plutonium once and for all.

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Walk-Through-Wall Effect Might Be Possible With Humanmade Object, Physicists Predict

Tunnel vision. A superthin sheet of carbon atoms suspended could quantum tunnel between two positions, one slightly bowed and one so bent it contacts the metal plate. An atomic force microscopy image of a graphene membrane (inset) 

If you've ever tried the experiment, you know you can't walk through a wall. But subatomic particles can pull off similar feats through a weird process called quantum tunneling. Now, a team of physicists says that it might just be possible to observe such tunneling with a larger, humanmade object, though others say the proposal faces major challenges.

If successful, the experiment would be a striking advance toward fashioning mechanical systems that behave quantum mechanically. In 2010, physicists took a key first step in that direction by coaxing a tiny object into states of motion that can be described only by quantum mechanics. Tunneling would be an even bigger achievement.

So how does quantum tunneling work? Imagine that an electron, for example, is a marble sitting in one of two depressions separated by a small hill, which represent the effects of a sculpted electric field. To cross the hill from one depression to the other, the marble needs to roll with enough energy. If it has too little energy, then classical physics predicts it can never reach the top of the hill and cross over it.

Tiny particles such as electrons, however, can still make it across even if they don't have enough energy to climb the hill. Quantum physics describes such particles as extended waves of probability—and it turns out that there is a probability that one of them will "tunnel" through the hill and suddenly materialize in the other depression, even though the electron can't occupy the high ground between the two low spots.

It sounds unlikely, but scientists and engineers have amply demonstrated quantum tunneling in semiconductors in which electrons tunnel through nonconducting layers of material. (In fact, some types of magnetic hard drives rely on tunneling for reading out data.) And the Nobel Prize-winning scanning tunneling microscope relies on electrons tunneling through a forbidden no man's land between a tiny fingerlike probe and a conducting surface. Still, no one has ever seen a macroscopic object tunnel through an obstacle.

But Mika Sillanpää and colleagues at Aalto University in Finland say it might be possible to do just that using a tiny widget that resembles a trampoline as they reported 8 November in Physical Review B. Researchers would fashion the micrometer-wide trampoline out of graphene, a superstrong, superflexible sheet of carbon only one atom thick. They would suspend the membrane—small but much larger than the atoms and molecules that are the usual domain of quantum physics—over a metal plate. When experimenters applied an electrical voltage, the membrane would have two stable positions: one in which it bows slightly in the middle and one in which it bends enough to contact the plate below. In the Finnish team's design, the electrical and mechanical forces on the membrane create an energy barrier between these two positions. If researchers could lower the membrane's energy by cooling it to a temperature of less than a thousandth of a degree above absolute zero, then the only way it could get between the two positions is quantum tunneling. The experimenters could then observe the membrane's change of configuration by looking for a change in the system's capacitance, a measure of how well it can store electrical charge. Sillanpää says achieving the low temperatures required may take several years, but the team is moving forward with an experiment.

Quantum tunneling in a mechanical system is "the kind of holy grail that people are looking for now," says physicist Walter Lawrence of Dartmouth College, but the experiment is likely to be difficult. Gil-Ho Lee, a physicist at Pohang University of Science and Technology in South Korea, says the proposed experiment would be an important first step toward demonstrating quantum tunneling. But he cautions that it might not be conclusive because the membrane might perform similar flip-flops when it absorbs a little extra energy in the form of heat. "A more sophisticated test must be done," Lee says. He says that searches for quantum tunneling in electrical systems known as Josephson junctions faced similar issues in the 1980s before experiments eventually confirmed tunneling.

So why can't you use quantum tunneling to walk through a wall? Quantum mechanical calculations show that for something as big as a person, the probability is so small that you could wait until the end of the universe and most likely still not find yourself on the other side.

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