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Part II: Speed
Article1
Superfast laser pulses could pave way for beam weapons
BY ANDREW GRANT 10:06AM, MARCH 5, 2014
Time 2
Laser pulses lasting tiny fractions of a second have created superhighways in the air that are potentially capable of transporting megawatts of laser power. The advance should help scientists detect pollution in the atmosphere. It could also enable more exotic applications such as redirecting lightning and building practical laser weapons.
Lost in the hype surrounding President Reagan’s Strategic Defense Initiative and other laser-based weapon systems was the fact that it’s difficult to deliver large amounts of energy through the atmosphere via laser. Air absorbs laser energy, heats up and expands. That low-density air acts like a defocusing lens, causing the beam to spread apart and weaken.
To traverse meters or kilometers through the atmosphere intact, laser beams have to be released in short, intense pulses. But at about 50 quadrillionths of a second in duration, such pulses can’t deliver enough sustained energy to remotely power an aircraft or burn a hole through an incoming intercontinental ballistic missile.
Howard Milchberg, who leads the intense laser-matter interactions group at the University of Maryland in College Park, wondered if he could use those rapid, low-energy pulses to clear the way for a longer-duration, higher-energy laser beam. A single pulse wouldn’t do the trick, his team found, but multiple adjacent pulses fired simultaneously just might.
In a recent experiment, Milchberg and his team fired four quick laser pulses in a square configuration. The quartet of pulses cut through the air, heating and disturbing molecules in its wake. The result was a single high-density region surrounded by a shell of lower-density air. Essentially, the pulses carved out a conducting wire for light in the air: a laser-friendly core enclosed by an insulating layer.
The researchers followed up the air-preparation pulses with a comparatively high-energy beam released over the course of seven billionths of a second. The beam’s energy barely diminished over the course of 70 centimeters, the researchers report February 26 in Physical Review X.
[330 words]
Time 3
“It’s a really intriguing experiment,” says Alexander Gaeta, a Cornell University physicist. He’s most intrigued by the finding that the thoroughfare in the air remained stable for a few milliseconds. That’s analogous to discovering that a baseball thrown by a major league pitcher leaves an imprint in the air for nearly 500 years. “It’s kind of astonishing,” Gaeta says.
The millisecond gap provides plenty of time for a high-energy laser beam to travel. “In the laser world,” Milchberg says, “milliseconds is infinity.” He says that his team’s technique could eventually allow lasers to deliver megawatts of power over kilometers through the atmosphere. For the time being, he plans to test his apparatus over tens of meters.
The new technique could improve efforts to remotely detect polluting aerosols and other particles in the atmosphere, Gaeta says. Currently scientists use quick-pulse lasers that cause certain airborne molecules to fluoresce. Soon scientists may be able to achieve a more complete survey by probing for longer periods of time.
The setup could also protect population centers from lightning, Milchberg says. Just as the airborne thoroughfare provides a path of least resistance for lasers, it could also coax lightning to take a desired path from cloud to ground during a thunderstorm.
Then there’s the possibility of death rays – or directed-energy weapons, the more formal term for lasers designed to burn or destroy a target. Milchberg isn’t shy about saying the new study brings such technology closer to reality; Gaeta agrees. And while the Cold War is over, interest in laser weapons is going strong: The U.S. Navy reportedly will deploy a drone-killing laser weapon system on one of its ships.
Milchberg receives funding from the Navy and Air Force, but it is for basic research with no specific application in mind.
[319 words]
Source: Science
https://www.sciencenews.org/
Article2
Material’s magnetism tuned by temperature
Layered substance is candidate for future hard drives 5:09PM, MARCH 5, 2014
Time 4
DENVER — A small change in temperature can alter a newly fabricated material’s magnetic properties. It’s a unique feature that holds promise for building more dependable data storage devices. “No magnetic material known to man is known to do this,” said Ivan Schuller, a condensed matter physicist at the University of California, San Diego. He described the material March 3 at a meeting of the American Physical Society.
Schuller’s creation consists of nickel layered atop a vanadium oxide compound. Researchers had previously shown that the oxide is an electric conductor at high temperatures and an insulator at low ones. But by adding nickel, the researchers found that the hybrid material’s magnetism also became linked to temperature. Schuller and his team manipulated the material’s coercivity, a measure of how difficult it is to switch the magnetic state, by adjusting the temperature over a 20-degree-Celsius range.
Schuller envisions this material as the foundation of next-generation magnetic hard drives. An ideal device would apply small magnetic fields to write data, and then when not in use, it would raise the requisite magnetic field strength to prevent accidental overwriting. A nickel-vanadium oxide device, combined with a modest heating system to control the temperature, could have those features
[215 words]
Source: Science
https://www.sciencenews.org/
Article3
You are what your dad ate, perhaps
BY BETHANY BROOKSHIRE 2:05PM, DECEMBER 13, 2013
Time 5
“You are what you eat.”
We’ve all heard that one. What we eat can affect our growth, life spanand whether we develop disease. These days, we know that we also are what our mother eats. Or rather, what our mothers ate while we were in the womb. But are we also what our father eats? A new study shows that in mice, a dietary deficiency in dad can be a big downer for baby.
The dietary staple in the study was folic acid, or folate. Folate is one of the B vitamins and is found in dark leafy greens (eat your kale!) and has even been added to some foods like cereals. It is particularly essential to get in the diet because we cannot synthesize it on our own. And it plays roles in DNA repair and DNA synthesis, as well as methylation of DNA. It’s particularly important during development. Without adequate folate, developing fetuses are prone to neural tube disorders, such asspina bifida.
Some of the neural tube disorders caused by folate deficiency could result from breaks in the DNA itself. But folic acid is also important in the epigenome. Epigenetics is a mechanism that allows cells to change how genes are used without changing the genes themselves. Instead of altering the DNA itself, epigenetic alterations put chemical “marks” or “notes” —methyl or acetyl groups — on the DNA and the proteins associated with it. The marks can either make a gene more accessible (acetylation) or less accessible (methylation), making it more or less likely to be made into a protein. This means that each cell type can have a different epigenome, allowing a neuron to function differently than a muscle cell, even though they contain the same DNA.
[301 words]
Time 6
Folate affects DNA synthesis, but it can also affect DNA methylation. In fact, DNA methylation requires the presence of folate. So low folate could affect whether genes are turned off or on and by how much. In a developing fetus, that could contribute to developmental problems.
Luckily, taking prenatal vitamins loaded with folic acid greatly reduces a woman’s chances that her developing baby will have these defects. But most of the time we think of folate as a “woman thing.” It turns out, though, that folate matters in men as well. And it doesn’t just matter to men, it matters to their offspring. We are what our father eats.
To look at how deficiencies in folate in fathers might affect offspring, Romain Lambrot and colleagues at McGill University in Quebec, Canada, mated a male mouse with two females. One female was on a normal diet with normal levels of folic acid (two milligrams per kilogram per day). The other was on a diet with very low folate (0.3 milligrams per kilogram per day). Both gave birth to pups, and the male pups from the low-folate female continued on a low-folate diet all the way through adulthood. Then the low-folate males were mated with a female who had normal levels of folic acid in her diet.
Lambrot and colleagues found that folate-deficient fathers themselves had delays in their first production of sperm. When the sperm caught up in growth, they looked normal and swam normally. But the sperm from low-folate fathers had a lot more DNA breaks in it. And when the folate-deficient fathers were mated, they were less fertile than their high-folate counterparts. Males raised on a normal folate diet produced pregnancies with an 85 percent success rate, while males on a low folate diet only had a 52 percent success rate. The mothers mated with low-folate dads also had increased loss of fetuses during pregnancy, theresearchers report December 10 in Nature Communications.
[329 words]
Source: Science
https://www.sciencenews.org/blog/scicurious/you-are-what-your-dad-ate-perhaps?mode=blog&context=131
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发表于 2014-3-10 21:15:54
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