Posts Tagged ‘Physics’

Super-Dense Stars May Squash Neutrons Into Cubes

16 Aug

Deep inside the super-dense hearts of exploding stars, gravity may squash neutron particles from spheres into cubes.

The idea could mean that neutron stars, as researchers call the stellar corpses, are denser than anyone expected. It could also question what stops them from collapsing into black holes and out of existence.

“If you take this result purely at face value, it means neutron star theoreticians are in trouble. [Neutron stars] should collapse into black holes at lower masses,” said theoretical physicist Felipe Jose Llanes-Estrada of Complutense University of Madrid, co-author of a study published Aug. 9 on the prepublication server arXiv.

“But that’s not what we observe. It’s possible there’s an additional repulsive interaction [between neutrons] to counter a collapse that we just haven’t thought of yet,” he said.

A star between nine and 20 times the sun’s mass detonates as a supernova toward the end of its life. At that weight, a star isn’t heavy enough to create a critical, ultra-dense state and shrink into a black hole. Instead, its core collapses into a sphere no bigger than 15 miles wide and so dense that a single teaspoon of it weighs as much as everyone on Earth, multiplied by 18.

Late last year, astronomers discovered the biggest-ever neutron star, called J1614-2230, that weighed in at 1.97 times the sun’s mass.  Prior to its discovery, the most massive neutron star weighed 1.67 solar masses.

The find left more than a few astrophysicists scratching their heads. Its existence ruled out some models of neutron stars that relied on exotic forms of matter and can’t explain the halt in the collapse of such a heavy object. Instead, the discovery supported models of neutron stars as containing only neutrons and protons.

When Llanes-Estrada and his university colleague Gaspar Moreno Navarro heard of J1614-2230, they wanted to know what might be happening inside of it.

The duo knew of a model from the 1970s suggesting pure neutrons could form a crystal lattice under incredible pressure (similar to how carbon forms diamonds in the bowels of the Earth). When they tweaked a familiar computer model to incorporate the idea, they discovered that — at the pressures anticipated deep in neutron stars — neutrons deformed from spheres into cubes.

“There’s an optimum packing density with spheres, including neutrons. It’s about 74 percent. No matter how efficiently you arrange them, like oranges on display at a supermarket, there’s always space in between,” Llanes-Estrada said. “If you want to be most efficient, you distort the oranges. Pack them a mile high and squish the ones on the bottom.”

Gravity shapes aggregate particles of matter into the simplest, most efficiently-packed object possible, normally a sphere like the Earth. The particles themselves, though, remain individually unaffected; gravity is too weak to overcome the strong interactions that hold neutrons and other particles together. But if gravity becomes intense enough, it might overpower the interactions.

So deep within the newly discovered neutron star — which may have a core pressure two times higher than the rest — a neutron’s most efficient shape may be a cube. “They’ll be flattened on all sides, like dice” starting at pressures found about 2.5 miles below the surface, Llanes-Estrada said.

So far, responses to the study have proven lukewarm.

Particle physicist Richard Hill of the University of Chicago, for example, noted the study looks at a neutron in isolation, not in aggregate.

“It’s an interesting idea, but what happens among the neutrons isn’t clear,” said Hill, who wasn’t involved in the study. At the densities in neutron stars, he noted, the “identities of individual neutrons may be blurred out.”

Llanes-Estrada acknowledged the criticism, which a second physicist who wished to remain anonymous also shared. But Llanes-Estrada said that pushing boundaries was, in part, the point.

“I think there is a large uncertainty of what happens to neutrons at very high compressions,” he said. “We should keep studying all of the possibilities.”

Updated: Aug. 17, 2011; 8:45 a.m. EDT

Images: 1) Illustration of a neutron star. (NASA/JPL-Caltech) 2) As pressure and density in a neutron star go up, normally sphere-like neutrons might take on an increasingly cubic shape. (F.J. Llanes-Estrada and G.M. Navarro/

Via: MIT Technology Review

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A test for multiple universes finds four . . . maybe [Mad Science]

04 Aug
Multiple universes are accessed, in fiction, through portals in space or mystical necklaces or sometimes just in dreams, but always when characters break the rules of space time. In reality, alternate universes are not in other dimensions. They're just far, far away. And the reason they are alternate universes is that they can't be reached no matter what. But can they be tested for? More »

How an argument with Hawking suggested the Universe is a hologram

31 Jul

The proponents of string theory seem to think they can provide a more elegant description of the Universe by adding additional dimensions. But some other theoreticians think they've found a way to view the Universe as having one less dimension. The work sprung out of a long argument with Stephen Hawking about the nature of black holes, which was eventually solved by the realization that the event horizon could act as a hologram, preserving information about the material that's gotten sucked inside. The same sort of math, it turns out, can actually describe any point in the Universe, meaning that the entire content Universe can be viewed as a giant hologram, one that resides on the surface of whatever two-dimensional shape will enclose it.

That was the premise of panel at this summer's World Science Festival, which described how the idea developed, how it might apply to the Universe as a whole, and how they were involved in its development.

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Plutonium Is Hot Suspect in Pioneer Spacecraft Mystery

26 Jul

By Duncan Geere, Wired UK

A 30-year mystery as to why the Pioneer spacecraft have slowly been drifting off course is close to being explained — the latest analysis pins the blame on heat.

The Pioneer 10 and 11 spacecraft, launched in 1972 and 1973 respectively, were designed to fly past the asteroid belt and investigate the Jovian and Saturnian systems. Both are now located in the interstellar medium, and radio contact has been lost, but each is slowing gradually due to the influence of the sun’s gravity.

In 1980, an algorithm to study gravitational effects in the outer solar system was concocted by astronomer John Anderson, but it didn’t quite work. There was a discrepancy — small but noticeable — between the readings predicted by the algorithm and those actually observed from the Pioneer spacecrafts’ radio signal. The phenomenon was named the Pioneer Anomaly.

Physicists struggled to explain the discrepancy, proposing complex theories that even included a suggestion that gravity might behave differently at large distances from the Earth. However, the most recent analysis suggests a rather more mundane explanation — heat from the plutonium inside the spacecrafts’ generators.

If heat is radiating evenly in all directions from the spacecraft, then there will be no effect on its course, but if there’s a difference of as little as five percent between the front and the back then that could explain the difference between the predicted course and the course that’s actually been observed. The smoking gun is that the level of deceleration seems to be decreasing at an exponential rate, which tallies with the radioactive decay of the plutonium-238 that powers the two spacecraft.

It’s not completely out of the question that there might be another culprit, but it’s looking increasingly unlikely. NASA is performing its own investigation to compare to the data, but if that comes out with heat as the cause too, then the 30-year mystery may finally be over.

Image: NASA


Via: Discovery News

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Physicists: Universe Almost Certainly Not a Hologram

05 Jul

By Duncan Geere, Wired UK

An astrophysicist’s attempt to measure quantum “fuzziness” — to find out if we’re living in a hologram — has been headed off at the pass by results suggesting that we’re probably not.

In October 2010, reported on Craig Hogan’s experiments with two of the world’s most precise clocks, which he was using to try and confirm the existence of Planck units — the smallest possible chunks of space, time, mass and other properties of the universe.

Hogan’s interpretation of results from the GEO600 gravitational wave experiment had shown a quantum fuzziness — a sort of pixelation — at incredibly small scales, suggesting that what was perceive as the universe might be projected from a two-dimensional shell at its edge.

However, a European satellite that should be able to measure these small scales hasn’t found any quantum fuzziness at all, contradicting the interpretation of the GEO600 results and indicating that the pixelation of spacetime, if it exists, is considerably smaller than predicted.

By examining the polarisation of gamma-ray bursts as they reach Earth, we should be able to detect this graininess, as the polarisation of the photons that arrive here is affected by the spacetime that they travel through. The grains should twist them, changing the direction in which they oscillate so that they arrive with the same polarization. Also, higher energy gamma rays should be twisted more than lower ones.

However, the satellite detected no such twisting — there were no differences in the polarization between different energies found to the accuracy limits of the data, which are 10,000 times better than any previous readings. That means that any quantum grains that exist would have to measure 10^-48 meters or smaller.

“This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories,” says Philippe Laurent, a physicist at France’s Atomic Energy Commission who analyzed the data, in a press release.

Source: Wired UK

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Image: Gamma ray burst. (ESA/SPI Team/ECF)


Flawed Diamonds Could Store Quantum Data

25 Mar

DALLAS — Scientists have developed a new way to manipulate atoms inside diamond crystals so that they store information long enough to function as quantum memory, which encodes information not as the 0s and 1s crunched by conventional computers but in states that are both 0 and 1 at the same time. Physicists use such quantum data to send information securely, and hope to eventually build quantum computers capable of solving problems beyond the reach of today’s technology.

sciencenewsFor those developing this quantum memory, the perfect diamonds don’t come from Tiffany & Co. — or Harry Winston, for that matter. Impurities are the key to the technology.

“Oddly enough, perfection may not be the way to go,” said David Awschalom of the University of California, Santa Barbara. “We want to build in defects.”

One of the most common defects in diamond is nitrogen, which turns the stone yellow. When a nitrogen atom sits next to a vacant spot in the carbon crystal, the intruding element provides an extra electron that moves into the hole. Several years ago, scientists learned how to change the spin of such electrons using microwave energy and put them to work as quantum bits, or qubits.

In search of a more stable way to store quantum information, Awschalom has now figured out how to link the spin of a electron to the spin of the nearby nitrogen’s nucleus. This transfer, triggered by magnetic fields, is fast — about 100 nanoseconds, comparable to how long it takes to store information on a stick of RAM.

The technique has “a fidelity of 85 to 95 percent,” Awschalom said March 22 in Dallas at a meeting for the American Physical Society.

In contrast to some other quantum systems under development, which require temperatures close to absolute zero, this diamond memory works at room temperature. The spins inside the diamond can be both changed and measured by shining laser light into the diamond. This could make diamond an attractive material for scientists developing nanophotonic systems designed to move and store information in packets of light.

Unlike a diamond itself, this quantum memory isn’t forever. But it lasts for a very long time by quantum standards. The nuclear spin remains coherent for more than a millisecond, with the potential to improve to seconds.

“You can only do your quantum magic as long as you have coherence,” said Sebastian Loth, a physicist at IBM’s Almaden Research Center in San Jose, Calif. “If you have a lifetime of milliseconds, that lets you do millions of operations.”

In addition to stability, diamond may also overcome another hurdle that has faced quantum computing — it can be scaled up to larger sizes. In a paper published last year in Nano Letters, Awschalom developed a technique for creating customizable patterns of nitrogen atoms inside a diamond, using lasers to implant thousands of atoms in a grid.

Awschalom’s diamond quantum memory could also be useful for building large quantum networks. Currently, quantum information is transmitted by connecting, or entangling, qubits. This scheme is limited to distances of kilometers. Quantum repeaters could potentially use small chips of diamond to catch, store and retransmit this information to extend the range, enabling quantum networks to work over much longer distances.

Image: Jurvetson/Flickr

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Stephen Hawking to give a public lecture in Texas

23 Mar
The world's most famous living scientist, Stephen Hawking, will make his fourth visit to Texas in April. As part of his visit Hawking will give a lecture (see details) on the origin of the universe at Texas A&M University on...

Six science selections

16 Mar
  • How Radiation Threatens Health – Why and how does exposure to radiation make you ill? What levels of exposure are dangerous and what levels are lethal?
  • Fukushima is a triumph for nuke power – Quake + tsunami = 1 minor radiation dose so far, says El Reg. Tragic as recent events in Japan have been. We should be building more nuclear reactors not fewer. Global warming caused by burning more and more fossil fuel in coming decades will have a far more detrimental effect on many more people than minor nuclear leaks.
  • Dog walking ‘is good exercise’ – Owning a dog but not walking it is bad for the dog’s owner as well as the dog. NHS Choices unravels the spin on recent headlines proclaiming dog ownership good for health.
  • Top banana – Atomic absorption spectroscopy is being used to assess how well banana peel can filter heavy metals, such as copper, from waste water. Preliminary results look promising and could lead to an ecologically sound method of industrial cleanup that uses a renewable but otherwise wasted source material.
  •″>Toxic robot – A new high-speed robotic screening system for chemical toxicity testing was recently unveiled by collaborating US federal agencies, including the National Institutes of Health. The system will screen some 10,000 different chemicals for putative toxicity in what represents the first phase of the "Tox21" program aimed at protecting human health and improving chemical testing.
  • Crystal unknowns – Frank Leusen and his co-workers at the University of Bradford, England, have turned to a quantum mechanical approach to help them predict the three known possible polymorphic structures of a sulfonimide. The work could assist crystallographers in structure determination of unknowns

My latest selection of six science stories, picked up by David Bradley Science Writer @sciencebase.

Six science selections is a post from: Sciencebase Science Blog


Giant ice caverns lead the hunt for exotic particles [Mad Science]

13 Feb
This is a gigantic hole that's been melted into the South Pole. It's one of the 100 or so such vertical caves that have been punched into the Antarctic surface as part of the IceCube Neutrino Observatory, which is searching for tiny, almost massless particles known as neutrinos. This remarkable image reveals the incredible lengths scientists have to go in order to detect these ultra small particles. More »

Cold, Dead Stars Could Help Limit Dark Matter

15 Oct

Hunting for cold stellar corpses near the center of the galaxy or in star clusters could put new limits on the properties of dark matter.

“You can exclude a big class of theories that the experiments cannot exclude just by observing the temperature of a neutron star,” said physicist Chris Kouvaris of the University of Southern Denmark, lead author of a paper in the Sept. 28 Physical Review D. “Maybe by observations, which come cheaper than expensive experiments, we might get some clues about dark matter.”

Dark matter is the irritatingly invisible stuff that makes up some 23 percent of the universe, but makes itself known only through its gravitational tug on ordinary matter.

There are several competing theories about what dark matter actually is, but one of the most widely pursued is a hypothetical weakly interacting massive particle (WIMP). Physicists in search of WIMPs have placed experimental detectors deep underground in mines and mountains, and are waiting for a dark matter particle to hit them.

Others have proposed looking for the buildup of dark matter in stars like the sun or white dwarfs. But both subterranean and stellar-detection strategies will light up only for WIMPs larger than a certain size. That size is miniscule — about a trillionth of a quadrillionth of a square centimeter — but dark matter particles could be smaller still.

One way to rule out such diminutive particles is to look to neutron stars, suggest Kouvaris and co-author Peter Tinyakov of the Université Libre de Bruxelles in Belgium.

Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter.

“For their size and their temperature, they have the best efficiency in capturing WIMPs,” Kouvaris said. Particles up to 100 times smaller than the ones underground experiments are sensitive to could still make a noticeable difference to neutron stars.

After the fires of their birth, neutron stars slowly cool over millions of years as they radiate photons. But if WIMPs annihilate each other whenever they meet — like a particle of matter meeting a particle of antimatter — as some models suggest they should, dark matter could reheat these cold stars from the inside.

Kouvaris calculated the minimum temperature for a WIMP-burning neutron star, and found it to be about 100,000 kelvins [about 180,000 degrees Fahrenheit]. That’s more than 10 times hotter than the surface of the sun, but more than 100 times cooler than the sun’s fuel-burning interior. It’s also much cooler than any neutron star yet observed.

Dark matter and ordinary matter are thought to clump up in some of the same places, like the center of the galaxy or globular clusters of stars. So Kouvaris and Tinyakov suggest that astronomers try to find a neutron star colder than the minimum temperature in a region with a lot of dark matter floating around.

“If you observe a neutron star with a temp below the one we predict, that excludes a whole class of dark-matter candidates,” Kouvaris said. It could mean the WIMPs are extra-small, or that they don’t annihilate when they meet each other — a property of WIMPs that experiments can’t get at.

“It’s an intriguing idea,” said observational astronomer David Kaplan University of Wisconsin-Milwaukee. “But I’m a little skeptical that it can be done immediately, or even in the near future.”

The center of the galaxy is dusty and difficult to observe, and most globular clusters are so far away that a cold, tiny neutron star hiding inside them would be beyond today’s telescopes. The next generation of ultraviolet telescopes could be up to the task, Kaplan suggests. “But that’s not to say that it will be easy.”

Astronomer Bob Rutledge of McGill University suggests an alternative approach: Rather than squinting for neutron stars’ dim light, astronomers could find them through ripples in space-time called gravitational waves. When two neutron stars merge, they are expected to throw off massive amounts of these waves, and Earth-based detectors like LIGO are already in place to catch them — although no waves have actually shown up yet.

“It would be technically hard, but a sound approach,” Rutledge said. “This sort of thing could become possible in the more distant future.”

Image: Artist’s impression of a neutron star with a powerful magnetic field, called a magnetar. Credit: NASA

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