Category: A Level Physics Chapters

3D Invisibility Cloak unveiled

3-D invisibility cloak hides gold “bump” The first device to hide an object in three dimensions has been unveiled by a group of physicists in the UK and Germany. While the design only cloaks micro-scale objects from near-infrared wavelengths, the researchers claim that there is nothing in principle to prevent their design from being scaled up to hide much larger artefacts from visible light. The origins of this design date back to 2006, when David Smith and colleagues at Duke University in North Carolina created a cloak that could bend microwaves around an object, like water flowing around a smooth stone.

This early cloak was made using a metamaterial – an artificially constructed material with unusual electromagnetic or other properties – which consisted of a cylinder built up from concentric rings of copper split-ring resonators. This first cloak, however, only worked in two dimensions – in other words, looking at the cylinder from above revealed the presence of the shielded object.

Carpet cloak Now Tolga Ergin and colleagues at Karlsruhe Institute of Technology in Germany, together with John Pendry of Imperial College in London, have overcome this problem by creating a “carpet cloak”. Proposed in 2008 by Pendry and Jensen Li, this involves hiding an object underneath a bump on the surface of an otherwise smooth material – just as something might be hidden under a carpet – and then smoothing out the resulting bump. This is achieved by creating a bump on a flat mirror and then placing onto the mirror a layer of metamaterial with optical properties such that light appears to reflect off the mirror as if the bump were not there. This technique was demonstrated experimentally at two different wavelengths last year, with Smith’s group showing that it worked in the microwave region while researchers at Berkeley and Cornell University near New York obtained similar results at infrared wavelengths. However, these cloaks were also limited to just two dimensions.

Ergin’s group has made a carpet cloak in three dimensions by stacking nanofabricated silicon wafers on top of one another in a “woodpile” matrix and then filling in the gaps between the wafers with varying amounts of polymer. This achieves the desired distribution of refractive indices within the structure. Hiding the bump The cloak structure was then placed on top of a reflective gold surface containing a bump, leading to a cloaking effect using unpolarized light with wavelengths between 1.4 and 2.7 µm – the near-infrared. Importantly, this effect held for viewing angles up to 60 degrees (with zero degrees representing viewing in just two dimensions).

 The bump, however, was very small – just 30 µm (10–6 m) × 10 µm × 1 µm. Team member Martin Wegener says it should be possible to use existing technology to make the cloak bigger in order to hide larger objects, but that this approach would be extremely time-consuming. “Faster nanofabrication tools will have to be developed allowing for three-dimensional structures,” he adds. For Wegener the aim of the work is not about focusing all efforts on creating invisibility cloaks, but is about exploring a range of applications in transformation optics.

This involves calculating what kind of material is needed to bend light in a certain way, by considering light trajectories as the result of the warping of space. Wegener says that transformation optics should lead, for example, to the design of better antennas or smaller optical resonators. Smith describes the latest work as “very exciting” and agrees that its real importance lies in the development of transformation optics. “Demonstrations like these are paving the way for transformation optical design to become an established design methodology, like ray-tracing,” he says. The research is published in Science.

Permanent link to this article: https://www.animatedscience.co.uk/2010/3d-invisibility-cloak-unveiled

NASA’S Chandra Finds Youngest Nearby Black Hole

WASHINGTON — Astronomers using NASA’s Chandra X-ray Observatory have found evidence of the youngest black hole known to exist in our cosmic neighborhood. The 30-year-old black hole provides a unique opportunity to watch this type of object develop from infancy.

The black hole could help scientists better understand how massive stars explode, which ones leave behind black holes or neutron stars, and the number of black holes in our galaxy and others.

The 30-year-old object is a remnant of SN 1979C, a supernova in the galaxy M100 approximately 50 million light years from Earth. Data from Chandra, NASA’s Swift satellite, the European Space Agency’s XMM-Newton and the German ROSAT observatory revealed a bright source of X-rays that has remained steady during observation from 1995 to 2007. This suggests the object is a black hole being fed either by material falling into it from the supernova or a binary companion.

“If our interpretation is correct, this is the nearest example where the birth of a black hole has been observed,” said Daniel Patnaude of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. who led the study.

The scientists think SN 1979C, first discovered by an amateur astronomer in 1979, formed when a star about 20 times more massive than the sun collapsed. Many new black holes in the distant universe previously have been detected in the form of gamma-ray bursts (GRBs).

However, SN 1979C is different because it is much closer and belongs to a class of supernovas unlikely to be associated with a GRB. Theory predicts most black holes in the universe should form when the core of a star collapses and a GRB is not produced.

“This may be the first time the common way of making a black hole has been observed,” said co-author Abraham Loeb, also of the Harvard-Smithsonian Center for Astrophysics. “However, it is very difficult to detect this type of black hole birth because decades of X-ray observations are needed to make the case.”

The idea of a black hole with an observed age of only about 30 years is consistent with recent theoretical work. In 2005, a theory was presented that the bright optical light of this supernova was powered by a jet from a black hole that was unable to penetrate the hydrogen envelope of the star to form a GRB. The results seen in the observations of SN 1979C fit this theory very well.

Although the evidence points to a newly formed black hole in SN 1979C, another intriguing possibility is that a young, rapidly spinning neutron star with a powerful wind of high energy particles could be responsible for the X-ray emission. This would make the object in SN 1979C the youngest and brightest example of such a “pulsar wind nebula” and the youngest known neutron star. The Crab pulsar, the best-known example of a bright pulsar wind nebula, is about 950 years old.

“It’s very rewarding to see how the commitment of some of the most advanced telescopes in space, like Chandra, can help complete the story,” said Jon Morse, head of the Astrophysics Division at NASA’s Science Mission Directorate.

The results will appear in the New Astronomy journal in a paper by Patnaude, Loeb, and Christine Jones of the Harvard-Smithsonian Center for Astrophysics. NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for the agency’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge.

For more information about Chandra, including images and other multimedia, visit:

https://www.chandra.nasa.gov

and

https://www.chandra.harvard.edu

Permanent link to this article: https://www.animatedscience.co.uk/2010/nasas-chandra-finds-youngest-nearby-black-hole

The Hydrogen Bomb…Teller–Ulam

File:Teller-Ulam device 3D.svg

This is part of a great Wikipedia Article, read more here….

The Teller–Ulam design is the nuclear weapon design concept used in most of the world’s nuclear weapons.  It is colloquially referred to as “the secret of the hydrogen bomb” because it employs hydrogen fusion to generate neutrons. However, in most applications the bulk of its destructive energy comes from uranium fission, not hydrogen fusion. It is named for its two chief contributors, Edward Teller and Stanisław Ulam, who developed it in 1951 for the United States. It was first used in multi-megaton-range thermonuclear weapons. As it is also the most efficient design concept for small nuclear weapons, today virtually all the nuclear weapons deployed by the five major nuclear-armed nations use the Teller–Ulam design.

Its essential features, which officially remained secret for nearly three decades, are:

  1. separation of stages into a triggering “primary” explosive and a much more powerful “secondary” explosive.
  2. compression of the secondary by X-rays coming from nuclear fission in the primary, a process called the “radiation implosion” of the secondary.
  3. heating of the secondary, after cold compression, by a second fission explosion inside the secondary.

The radiation implosion mechanism is a heat engine exploiting the temperature difference between the hot radiation channel, surrounding the secondary, and the relatively cool interior of the secondary. This temperature difference is briefly maintained by a massive heat barrier called the “pusher”. The pusher is also an implosion tamper, increasing and prolonging the compression of the secondary, and, if made of uranium, which it usually is, it undergoes fission by capturing the neutrons produced by fusion. In most Teller–Ulam weapons, fission of the pusher dominates the explosion and produces radioactive fission product fallout.

The first test of this principle was the “Ivy Mikenuclear test in 1952, conducted by the United States. In the Soviet Union, the design was known as Andrei Sakharov‘s “Third Idea“, first tested in 1955. Similar devices were developed by the United Kingdom, China, and France, though no specific code names are known for their designs

Permanent link to this article: https://www.animatedscience.co.uk/2010/the-hydrogen-bomb-teller%e2%80%93ulam

Neutron Stars – from BBC…

Neutron star packs two Suns’ mass in London-sized space 

Artist's concept of a pulsar (SPL) Pulsars are so-called because of the way their radio emission is detected at Eart. Astronomers have discovered what they say is the mightiest neutron star yet. The super-dense object, which lies some 3,000 light-years from Earth, is about twice as massive as our Sun. That is 20% greater than the previous record holder, the US-Dutch team behind the observation tells the journal Nature.

Like all neutron stars, the object’s matter is packed into an incredibly small space probably no bigger than the centre of a big city like London. “The typical size of a neutron star is something like 10km in radius,” said Dr Paul Demorest from the National Radio Astronomy Observatory (NRAO), Charlottesville, US. The size is easy to understand but the densitiy is much more extreme than anything we know here on Earth.

“It’s approximately the size of a city, which for an astronomical object is interesting because people can conceive of it pretty easily; and yet in that space it has the mass in this case about two times our Sun. So the size is easy to understand but the densitiy is much more extreme than anything we know here on Earth,” the study’s lead author told BBC News.

Green Bank Telescope in West Virginia (NRAO)The finding is important, says Dr Demorest’s team, because it puts constraints on the type of exotic material that can form a neutron star. Such objects are thought to be the remnant cores of once giant stars that blew themselves apart at the ends of their lives. Theory holds that all atomic material not dispersed in this supernova blast collapses to form a body made up almost entirely of neutrons – the tiny particles that appear in the nuclei of many atoms. As well being fantastically compact, the cores also spin incredibly fast. This particular object, classified as PSR J1614-2230, revolves 317 times a second. It is what is termed a pulsar – so-called because it sends out lighthouse-like beams of radio waves that are seen as radio “pulses” every time they sweep over the Earth.

  The observations were made using the Green Bank Telescope in West Virginia. The pulses are akin to the ticks of a clock, and the properties of stable neutron stars make for ultra-precise time-pieces. This was how the team, observing with the Green Bank Telescope in West Virginia, was able to measure the object’s mass. Because PSR J1614-2230 also circles a companion star, its pulses – as received at Earth – are disturbed by the neighbour’s gravity.

“The way it works is that as the pulses travel from the neutron star past the companion, they slow down a little bit. And how we see that on Earth is that the pulses arrive a little later than we would otherwise expect when the neutron star is lined up behind the companion,” Dr Demorest said.

The team could use this effect to calculate the masses of both bodies. The group reports a pulsar mass 1.97 times that of our Sun – significantly greater than the previous precise record of 1.67 solar masses. The result is said to put limits on the type of dense matter that can make up the cores of these bizarre objects. Some scientists had suggested exotic particles such as hyperons, kaon condensates or free quarks could exist deep inside neutron stars. But Dr Demorest and colleagues believe their observations preclude this possibility. “It’s simply that if those particles were formed, the star would get too dense and collapse into a black hole prior to this point,” the NRAO researcher said.

Permanent link to this article: https://www.animatedscience.co.uk/2010/neutron-stars-from-bbc

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