Physics

PHYSICS (see also biophysics

)

fourth, fifth, and sixth dimensions : these extra spatial dimensions can be inferred from the perplexing behaviour of dark matter. This mysterious stuff cannot be seen, but its presence in galaxies is betrayed by the gravitational tug that it exerts on visible stars. In the smaller galaxies, dark matter seems to be attracted to itself quite strongly. But in the large galactic clusters this doesn't seem to be the case: strongly interacting dark matter should produce cores of dark material bigger than those that are actually there, as deduced from the way the cluster spins. One explanation is that 3 extra dimensions, in addition to the 3 spatial ones to which we are accustomed, are altering the effects of gravity over very short distances of about 1 nm^ref. The team argues that such astronomical observations of dark matter provide the first potential evidence for extra dimensions. Others are supportive, but unconvinced. Even if their idea works, which it probably does, it may be an overstatement to use these observations as evidence of extra dimensions. The proposal is extremely speculative. Physicists have suspected for years that 'hidden' dimensions exist, largely because they seem to be predicted by string theory, the current favourite for a theory of fundamental subatomic particles. These extra dimensions are generally thought to be tiny: many billions of times smaller than atoms. This would make these dimensions very hard to detect, explaining why the Universe looks as if it has just 3. Physicists, however, have proposed that some extra dimensions might be relatively big, but inaccessible to us. The extra dimensions are likewise 'big', at about 1 nm across. In other words, the Universe is only about 1 nm wide in these 3 'directions'. They argue that the force of gravity does not obey Isaac Newton's famous laws over small distances, where these dimensions come into play. This has never been tested experimentally: no one has measured how gravity behaves over distances below about a hundredth of a millimetre. This variation in gravity could be why dark matter behaves differently in different galactic environments. According to one interpretation of the astronomical observations, dark matter, which is thought to account for 85% of all the mass in the Universe but not to be made from the known fundamental particles, seems to attract itself through some unknown force. And this attraction seems to be stronger in dwarf galaxies than in galactic clusters. This is very odd: it is rather as if apples were to fall faster from single trees than from trees in an orchard. But the attraction isn't due to an unknown force, but to the effect of extra dimensions on gravity. And because dark matter particles are accelerated to higher speeds in massive galactic clusters than in dwarf galaxies, they spend less time close to each other, so the effects of these extra dimensions are felt less. There are other ways of explaining the puzzling dark-matter distributions. For example, one could assume that the rate at which stars explode, as supernovae, was quite different in the past. Changing the supernovae rate is more conservative than changing the number of spatial dimensions, but invoking extra dimensions is such an exciting idea that it is worth investigating, even if it is a long shot. The most popular versions of string theory suggest that there are as many as 8 extra dimensions, not just 3. But thankfully this needn't be a problem. There's no reason why, in addition to the 3 large extra dimensions predicted by Silk and colleagues, there might not be several other small ones too^ref.
just a little heat is enough to turn a waterproof block of foam into a sponge. The waterproof-sponge hybrids have been made to perform their trick at 400 °C or less, so the most likely application might be as a temperature sensor in an oven, they suggest. Regardless of use, the materials are an interesting chemical oddity. The material is made from a matrix of methyltriethoxysilane filled with holes: more than 75% of the block's volume is air. The inside and outside surfaces are covered in a rough layer of water-hating methyl groups formed from carbon and hydrogen. This makes it so hydrophobic that water beads into little droplets on top of the foam, just as mercury does on a table. But heating the foam beyond a critical temperature removes the methyl groups, leaving behind hydroxyl groups made from oxygen and hydrogen, which love water. This irreversibly turns the material into a sponge that sucks up drops of liquid. The remarkable part is that the transition goes straight from waterproof to sponge with no middle step, says Carole Perry, who is part of the team. If a sample is heated to 390°C and then cooled, a drop of water will sit on top of the block. But a sample that has reached 400 °C before cooling will fully absorb the water^ref. The researchers have already made several materials with different switching temperatures, the lowest being 275 °C. Perry says that these materials could be used to record the maximum temperature reached in an oven: a strip of materials that switch at a range of temperatures could be soaked in coloured water after being taken out of the oven. Those that had been activated would suck up the liquid, while those whose switching temperature was higher than the oven's maximum would remain dry. It is not clear, however, when such a thermometer would be better than the traditional sorts already used in ovens. The group is now trying to find out whether the same chemistry could be applied to make a surface switch between being slippery and non-slippery.
aneutronic fusion : Russian scientists have managed to use lasers to create a billion-degree nuclear fireball. The resulting fusion reaction is far cleaner than the kind currently being investigated to generate nuclear power. Sadly, the team's efforts are no good for power generation at the moment as the laser takes so much energy to run. But achieving this kind of laser-driven fusion in the lab will give scientists a better way to investigate the phenomenon, which could one day be used to create cleaner energy. Currently, the main contender for generating fusion power uses strong magnetic fields to confine a fiery plasma of atomic nuclei: fusion experts hope that the International Thermonuclear Experimental Reactor (ITER), to be built in Cadarache, France, will fuse deuterium and tritium nuclei together in this way to create energy. But this reaction also produces copious amounts of neutrons. When these neutrons hit the reactor walls they generate radioactive isotopes that will eventually have to be disposed of. And although this radioactive waste is cleaner than the by-products created by fission, the reaction used by today's nuclear power plants, it isn't perfect. So some physicists have suggested using aneutronic fusion, which forces protons and boron nuclei together in a reaction that generates virtually no neutrons^ref. Although this sounds safer, kick-starting proton-boron fusion requires temperatures of a billion degrees, > 10 times the heat needed by the deuterium-tritium reaction. Deuterium-tritium fusion is the reaction of choice simply because it's easier to achieve. Now a team of Russian scientists have topped the billion-degree mark in a system that does not need huge magnets to confine the reaction. A neutronless proton-boron reaction has been ahieved for the first time using a laser. The team blasted polythene pellets containing boron atoms with laser pulses that last for just over a trillionth of a second (10-12 seconds). This creates an intensely hot plasma where protons from the polythene merge into boron atoms, which then fall apart to release a stream of helium nuclei, also known as a particles. Lumbering alpha particles tend to stay within the reaction mixture rather than escaping to make surrounding equipment radioactive. Crucially, the team detected no neutrons coming from the reaction at all. The success opens the door to "an ecologically pure technology of nuclear energy production. An added advantage to the system is that the charged alpha particles could be directly tapped as a source of electric current. A power plant based on ITER would simply use the heat from fusion to turn electrical turbines, much as coal-fired power stations do today. And laser-driven fusion might be easier to sustain than the reaction inside magnetic bottles used by projects such as ITER. In theory, once the reaction is going, all one would have to do is keep dropping fuel pellets into the beam. In contrast, ITER will use giant magnets to keep a turbulent, burning ball of plasma confined. Unfortunately confinement is the least understood process in fusion. Other labs, including the National Ignition Facility at Lawrence Livermore National Laboratory in California, similarly use lasers to investigate fusion, but of the dirtier deuterium-tritium type. Belyaev now hopes to see a wider international project to investigate the proton-boron reaction^ref.
plasma pencil, a handheld device that generates a thin plume of charged gas that can kill bacteria, and could one day etch away tumours without damaging surrounding tissue. It allows you to treat an area in a very precise way :Laroussi developed the 12-cm-long device with his colleague Xinpei Lu. It's small, portable, and can operate for at least 8 hours at a time. Before this, our devices were large boxes that sat on a table. Plasmas are soups of charged ions and electrons. They are generated anywhere that atoms are stripped of their electrons: in solar flares or around lightning bolts, for example. Their violent birth means that the ions move very quickly, so plasmas have temperatures of thousands of degrees. But Laroussi's device produces a room-temperature plasma that can be used safely on patients. I have put my hand in the plume many times without anything happening. Although the beam has no effect on skin, previous experiments in Laroussi's lab have shown that Escherichia coli bacteria are killed when the plasma breaks open their cell walls. Now the team hopes to use the pencil to clean up the plaque-generating bugs that lie in the nooks and crannies of our mouth. It's very early in the project, but there's no reason why it shouldn't kill mouth bacteria. The 5-cm-long plasma plume is generated when a stream of helium gas containing a trace of oxygen passes between 2 high-voltage copper electrodes. Helium is very difficult to ionize, but the plume's oxygen molecules break into 2 highly reactive oxygen atoms, which then attack the bacteria. Previous plasma plumes have come with all sorts of difficulties: they have been just a few mm long, at least tens of degrees warmer than room temperature, and sometimes carried the risk of electrical arcing. The key to keeping the plasma pencil cool is its kV electric field, which switches on and off thousands of times a second. This kicks the light electrons into high speeds, while the heavier ions are too weighty to be moved much by each zap of voltage. It gives the electrons a lot of energy very quickly. Because electrons are so light, they impart very little heat energy to the material they crash into. You get this plasma where the electrons are very hot, but the gas is very cool. Unlike conventional chemical treatments that kill bacteria, there are no residues to wash away afterwards. It's essentially a chemical etching process where the reactive chemicals are being generated at the flick of a switch. The plasma pencil might eventually be used by doctors to kill off tumour cells. Surgical blades can often damage surrounding tissue, but the plasma pencil could be adapted to eat away at the cancer cells a few layers at a time. Laroussi is now trying to understand at a biochemical level how the plasma breaks up bacteria, and how the plume could be tuned to attack specific types of cells^ref.
time travel : trips in time have been theoretically possible ever since Einstein worked out that heavy masses can warp both time and space, and that objects travelling close to the speed of light tend to experience the passage of time more slowly. Moving forwards in time is therefore easy. Certain short-lived cosmic particles, for example, can be seen on Earth. Their journey looks to us as if it has taken thousands of years, but the particle feels as though it has whipped across space in just a few minutes, and arrives on Earth before it has had time to decay. In effect, the particle has travelled into the future, living beyond its years. But getting back to the past is more problematic. Researchers thought you would need all kinds of strange things to do this, including a neutron star (which we know to exist), worm holes (which we don't), and a kind of exotic matter that we can only imagine. This is where Amos Ori from Technion, the Israel Institute of Technology in Haifa, comes in. According to Einstein's theories, space can be twisted enough to create a local gravity field that looks like a doughnut of some arbitrary size. The gravitational field lines circle around the outside of this doughnut, so that space and time are both tightly curved back on themselves. Crucially, this does away with the need for any hypothetical exotic matter. Although it is difficult to describe what this would look or be like in real life, the mathematics reveal that every period of time between when the doughnut was created and the present moment would be somewhere in the vacuum inside the doughnut. All you need to do is work out how to get there. In theory, it should be possible to travel back to any point in time after the time machine was built^ref. One slight snag is that he has not worked out how to generate the gravitational doughnut, although he has some ideas. It's wild speculation, but you may need to move large masses rapidly in a circular motion. The leading model of travel into the past involves zipping through a wormhole, which offers a shortcut between 2 distant points in space. If you could connect a wormhole between Earth and something very heavy, such as a neutron star, this would set up a time difference between the 2 ends. This is thanks to the fact that mass can warp space and time, such that a clock on the surface of a dense neutron star would run about 30% slower than it does on Earth. But wormholes are tricky beasts, and need something to stop them collapsing under their own intense gravity. Some form of exotic antigravity matter would be needed to keep the wormhole open. Unfortunately for eager time lords, physicists have never seen anything like this. There are still difficulties to overcome with the doughnut model, however. Davis thinks that the instability of the compact vacuum core might be an insurmountable problem. Closed time-like curves are inherently unstable against quantum fluctuations. He expects a huge energy surge inside the doughnut would probably destroy it. Energy fluctuations might be problematic, but thinks this is probably soluble. Unfortunately it's not going to be in existence in our generation, or maybe ever

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a form of silicon that is bloated with extra neutrons has revealed a 'magic number' for the protons in its nucleus. Atomic nuclei are only stable when packed with certain combinations of positive protons and uncharged neutrons. Some combinations are more stable than others, and the numbers of protons or neutrons in such cases are called 'magic'. This extra stability is achieved when the subatomic particles fill up certain energy levels within the nuclei, leaving no stray particles hanging around at higher energies. Physicists have long known of a series of such magic numbers, including 2, 8, 20, 28, 50, 82 and 126. Some elements have quantities of subatomic nuclei that match these numbers and are more stable. Some even hit these scores in both their neutrons and protons, making them doubly magic. But not every magic number is detailed in a textbook^ref; researchers are still finding more. Radioactive silicon-42, an artificial element with 14 protons and 28 neutrons, also seems to be doubly magic. Between 8 and 20 there are some energy sublevels. Normally, these sublevels are very close together, so they don't obviously stand out as a magic number. This is the case in the most common form of silicon, which has 14 protons and 14 neutrons. But nuclei grow and change shape as more subatomic particles are packed in, and this changes the relative location of their energy levels. As silicon gets beefed up with neutrons, this would alter the energy levels in a way that would make 14 a magic number. To test this idea, the team fired a high-energy beam of sulphur-44 at a beryllium target. This forced the sulphur nuclei to lose two protons, transmuting them to silicon. They counted how much silicon-42 was produced by the collisions, and compared this with quantum mechanical calculations that assumed 14 was magic. The numbers matched up perfectly. The calculations throw up some surprises. It seems odd, for example, that 28 remains a magic number for neutrons in the silicon-42 nucleus. Finding out where the magic stops working is the key to exploring the most neutron-rich isotopes. Understanding how the energy levels of nuclei are arranged is an important test of quantum theory. It could help to reveal the sequence of nuclear reactions that occur in supernova explosions, he adds. These violent stellar deaths are the source of all elements heavier than iron. Huge amounts of neutrons are released in these explosions, which collide with atoms to make very short-lived, neutron-rich isotopes similar to silicon-42. Understanding neutron-rich nuclei helps you to understand the fundamentals of the creation of the Universe

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at first, it sounds like the biggest science story of the century: scientists have invented a desktop fusion machine. If nuclear fusion can be made to happen at room temperatures and pressures in an average lab, then one might think the world's energy crisis is over. But the inventors of the device stress that their gadget cannot generate power at all, because it does not support a self-sustaining thermonuclear reaction. Instead, it has a whole host of other applications, from treating cancer to powering spacecraft. The inventors are led by Seth Putterman, a physicist from the University of California, Los Angeles. Putterman is known for debunking claims of 'bubble fusion' and 'cold fusion' that promised revolutionary advances in energy production. His toaster-sized device relies on a pyroelectric crystal of lithium tantalate, which produces a strong electric field when heated to room temperature from freezing. This field is focused until it is powerful enough to accelerate a beam of deuterium ions (proton-neutron pairs) to about 1% of the speed of light. When these ions hit a target containing deuterium nuclei, they fuse to form helium-3, a combination of 2 protons and a neutron. The process emits about 1,000 neutrons a second, and by allowing the crystal to heat up slowly, fusion can be sustained for as long as 8 hours. This type of fusion is already used in commercially available instruments that determine the chemical composition of materials at a distance. Such devices blast neutrons down to the bottom of oil wells, for example, to determine the quality of oil. They are also used at airports to study in detail the contents of suspicious luggage. However, such applications currently require bulky, expensive particle accelerators with large electricity supplies. Replacing those with a small crystal is a big step. "The amazing thing is that the energy fields of a crystal can be used without plugging it in to a power station. They've built a really neat little accelerator. It will probably make its first big splash in labs looking for an easy neutron source. But there will be a lot of spin-offs from this technology. Everyone will be talking about the fusion, but this crystal can also give off X-rays as it accelerates electronsn. This effectively creates a tiny radioactive source that can be turned on and off at will. Such a device could one day be used to target radiation at cancerous cells: a smaller version could be injected into the body and directed towards a tumour before being switched on. In contrast, today's radiation therapies tend to blast healthy cells along with cancerous ones. Rocket propulsion could benefit. Space probes such as the European Space Agency's SMART-1, which recently arrived at the Moon, already use ion engines that eject a stream of charged xenon gas to produce a gentle forward thrust. The pyroelectric accelerator could produce a similar beam of ions moving at much greater speed, which would increase the thrust considerably. The team is now trying to boost the number of neutrons generated by the machine, as well as miniaturizing the device even further^ref.
when a liquid droplet hits a solid surface, it throws up a crown-shaped rim of liquid, which breaks up into tiny drops. Harold Edgerton, the pioneer of high-speed photography, captured this corona in his famous photo of a milk splash. But splashing can be a nuisance, as it scatters the liquid. Without splashing, each droplet sprayed from an ink-jet printer would produce a smooth circular dot, rather than one with a ragged edge. Now Sidney Nagel and his colleagues at the University of Chicago, Illinois, have shown that lowering the air pressure in which a spray is projected can reduce and even eliminate splashing. They fired drops of alcohol on to a glass slide and found that the splashy corona disappears completely if the air pressure is reduced to around 33% of normal atmospheric pressure^ref. The pressure at which splashing stops depends on the speed of the droplets: the faster they hit the glass, the more they splash. But pressures that are low enough eliminate splashing regardless of droplet speed. This is because, as a droplet spreads out from its impact, the edge of the liquid film pushes against the air. Air resistance bends the rim of the film upwards, forming a corona, which then fragments. As air pressure drops, so does the push on the rim. So the droplet can spread over the surface without fragmenting. Another way to reduce splashing is to replace air with helium. Helium atoms are lighter than the oxygen and nitrogen in air, and so they push less against the droplet rim. Makers of desktop printers are unlikely to want to exploit this effect, but it might aid attempts to use ink-jet technology to draw miniaturized electronic circuits in molten metal on circuit boards. Reducing splashes would make circuits cleaner and would allow higher jet speeds, improving the process. Other industries might actually seek to increase splashing, by using jets in a high-pressure atmosphere. For example, this might create a smoothly dispersed mist of fuel for fuel-injected combustion engines
new materials for advanced electronics are usually expensive, high-tech substances. But a team of researchers in France has shown that energy-storage components called supercapacitors can be made from a remarkably cheap and humble material: baked seaweed. Seaweed, when burned to a charcoal-like form, is just the right stuff for making the electrodes in state-of-the-art supercapacitors. It performs as well as the carbon-based substances currently used in commercial devices. People working on carbons are always looking for improved properties. Coconut shells are already used as a source of porous carbon for water filtration and other applications. Low-tech routes are commonly used when they do the job. The seaweed-derived polymer that Béguin has hit upon, called alginate, is non-toxic, and is widely used as a thickener in foods and cosmetics: 20,000 tonnes of it are extracted from seaweed for this purpose every year. This makes the material very cheap^ref. Supercapacitors provide an alternative to batteries for power storage in portable electronics. They consist of a pair of plates, or electrodes, loaded with electrical charge that can be subsequently tapped, producing a current. Capacitors can provide more power — higher voltages or currents — than batteries, but tend to store less total electrical energy. They are predicted to find applications as emergency power sources for computers, or as supplementary sources in electric vehicles where, for example, they might store energy captured during braking. The amount of energy stored in a capacitor depends on how much charge can be built up on the electrodes without the material they are made from breaking down. Many supercapacitors currently available have electrodes made of a porous form of graphite-like substance, called activated carbon, which is cheap and can store charge. But the porosity is a drawback, because storing a lot of charge in a low-density substance requires a large volume of material, and that's bad news for making miniaturized power sources. Béguin and his colleagues reasoned that what is needed is a form of carbon that is relatively dense, electrically conductive, and capable of holding a lot of charge. The researchers thought that cellulose, the basis of plant matter, might be suitable. This carbohydrate contains plenty of charge-holding oxygen atoms, but most of the oxygen is lost when cellulose is heated. They then struck upon alginate, abundant in brown seaweeds, which is chemically similar to cellulose but holds on to its oxygen when heated. The French team cooked alginate in an air-free enclosure, turning it into a black powder. They then combined this with a polymer binder to make a hard material, which they shaped into electrodes for supercapacitors. The amount of electrical charge and energy that these devices can hold is comparable to that of capacitors made from commercial activated carbons. But the seaweed capacitors can be charged to voltages twice as high without breaking down, and the material is twice as dense. They hold up well over time, too: their charge-storage capacity declines by only 15% after 10,000 cycles of charging and discharging. When can we expect to see burnt seaweed helping to drive your laptop? Commercialization of the carbon material could happen very quickly. We are in touch with a company interested in this development
star in a jar (sonochemistry) : when sound waves crush bubbles of gas in a liquid, energy is released in a dramatic burst of heat and light. Now the first detailed measurements of the phenomenon have shown that the molecules in the gas really do create a pinpoint of plasma, the energetic soup of ions and electrons found in every star. The research raises hopes that the effect, called sonoluminescence, might one day be used as an almost limitless source of energy. The bubbles reach temperatures > 15,000°C, which is 4 times hotter than the surface of the Sun. In 2002, a group of researchers controversially claimed they had seen similar bubbles triggering fusion^ref, the process that is the source of the Sun's energy. Any confined fusion reaction requires a plasma^ref. The researchers used sound waves between 20 and 40 kHz, above the range of human hearing, on a sample of concentrated sulphuric acid that contained traces of argon gas. The sound waves produce areas of high and low density within the liquid, making pressure at any one point oscillate between 2 extremes. Bubbles of gas in the liquid swell rapidly at lower pressures before being squeezed tight by the high pressure that follows. The change in pressure is so fast that the bubble effectively implodes with enough force to generate tremendous heat, in a process called acoustic cavitation. The heat separates electrons from their atoms, and as they snap back into position the energy they acquired is released as a burst of light. Proof of the plasma comes from the presence of an ionic oxygen molecule (O₂⁺). Some process must remove an electron from the molecule without breaking the chemical bond holding the 2 atoms together. Heating alone would break the molecule in 2, so the molecule was ionized after it collided with high-energy electrons or other ions in a hot plasma core. Previous measurements of acoustic cavitation have looked at bubbles in water, and were always stymied by the fact that most of the sonoluminescent energy is absorbed by molecules of water vapour in the bubble. But sulphuric acid is much less volatile than water, so bubbles contained very few sulphuric acid molecules, consisting almost entirely of argon. And because argon is an atom rather than a molecule, it contains no chemical bonds to vibrate or break, so there is less opportunity for energy to be absorbed. The collapsing bubbles released about 2,700 times more light than the equivalent bubbles in water, making it significantly easier to measure the temperature accurately.

a: at low pressure, a gas bubble expands dramatically, until
b: an increase in sound-wave pressure triggers its collapse. As the temperature inside the bubble soars to over 10,000 K, the gas becomes partly ionized, forming a plasma.
c: recombination of electrons and ions results in light emission.

scientists have created the world's first LASER made from silicon. This is an important step in the effort to build computers that process information using light, rather than electricity. Although much of the data in the world is now carried by light along optical fibres, the information processing and handling is usually still done after converting the signals into electrical currents, which are easier to manipulate. This conversion slows the whole process down, and also requires extra components, raising the cost of the circuitry. So scientists have been striving to make devices that can process light directly. Silicon is preferable because it can be mass-produced using conventional chip-making techniques. Unfortunately, it is also very poor at controlling light. Conversely, materials such as gallium arsenide, which are great at dealing with light, are very difficult to make into chips. Lasers are useful for carrying out optical calculations because all the packets of light they produce have the same wavelength, and are precisely marshalled to maximize the amount of energy they deliver in a tight beam. To make a good laser, you need a material that can take an energy input and turn it into light energy in a regular rhythm. But silicon has always been problematic because it loses much of this energy as vibrations within its atomic lattice. Jalali and Boyraz's breakthrough makes a virtue of this, using the vibrations themselves to generate or amplify laser light. Rather than turning electricity directly into light, Jalali's silicon laser is powered by another laser. It's a common method of investigating new laser materials, but the jump to electricity will definitely have to be made before the silicon laser can be used in an optical computer. They also need to develop the laser into a self-contained component that can be incorporated on to a silicon chip. A silicon laser that produces a continuous beam of laser light has been unveiled. This is an important milestone in the quest to create computers that can easily switch from using electrical currents to using light. Laser light is already used to carry information along optical fibres, but data crunching inside computers relies on electrical circuits. Turning a current into light requires expensive components that slow the computing process, so researchers want to make a silicon laser than can be included in microchips during normal manufacturing. But silicon has always stubbornly refused to generate a steady laser beam. Semiconductor lasers have been made using relatively exotic materials such as gallium arsenide, but these devices are expensive and incompatible with silicon-based circuits. Intel silicon laser runs continuously at room temperature for as long as the power is on^ref. The chip produces laser light when it is 'pumped' with another laser. Silicon tends to absorb most of the laser light as soon as it is generated, dispersing the energy as vibrations between the atoms in the crystal. So the researchers added a 1.5-mm-wide ridge that snakes in an S-shape through the chip. This confines the light without absorbing it. The ridge is embedded in a diode that draws away electrons that might otherwise disrupt the beam. Much more research is needed before a commercial silicon laser could be produced. The energy needed to create the laser beam, for example, must eventually come from an electrical signal and not an external laser. But the device is an important proof of principle. It also opens up the possibility of creating low-cost infrared silicon lasers that could be useful in medical imaging. The team's breakthrough follows hot on the heels of its previous paper which showed that silicon could produce very short pulses of laser light, mere nanoseconds long^ref. The first report of a pulsed silicon laser came just before that in October 2004, but the device required 8 m of optical fibre cable to loop part of the laser light back into the front of the silicon crystal^ref.
laser accelerators : physicists have had high hopes for more than a decade, but there have been two main obstacles. The beam of electrons produced by the devices always had a very wide range of energies, and they were physically spread into a broad fan. Both characteristics make the beams useless for physicists wanting to use them to probe the structure of matter. 3 teams have now overcome this problem, producing tightly focused beams of electrons within a very narrow energy range. New devices use laser pulses to drag electrons into tightly focused bursts, rather than relying on the varying electric fields that squeeze particles along lengthy tunnels in atom-smashing facilities such as that at CERN in Geneva, Switzerland. With lasers, the same acceleration can now be achieved over much shorter distances. device fires extremely short laser pulses, each lasting just 40 femtoseconds (4 x 10^-14 s), into a 1-mm-wide chamber of helium gas. The pulse blasts the helium atoms into a plasma, an energetic soup of free electrons and helium nuclei. The laser pulse, like all electromagnetic radiation, is partly composed of a changing electric field. It's not powerful enough to bother the relatively heavy nuclei, but it pushes the electrons around just like a boat creating a wake of water behind it as it cuts through water. Pulled along in that wake is a tight packet of electrons travelling at just a fraction slower than the speed of light. device fires extremely short laser pulses, each lasting just 40 femtoseconds (4 x 10-14 s), into a 1-millimetre-wide chamber of helium gas. Mangles explains that the secret to the team's success was being able to tune the laser so that the trailing electrons actually enhance the pulse's intensity, creating a positive-feedback effect that gathers more and more electrons together until they are delivered in a single, concentrated burst as they emerge from the gas chamber. The electrons eventually break like waves on a sea shore. The electrons each have about 100 MeV of energy - the same as if they were delivered by a 100-million-volt battery. Although this is a thousand times less energetic than particles shot out by the most powerful accelerators, it is similar to those being used to determine the structure of biological molecules. Mangles thinks this will be the first use for his new tool. Accelerators for this kind of research, for example the Swiss Light Source linear accelerator in Villigen, are currently the size of an aircraft hangar. But Mangles is confident that, within two years, laser accelerators of the equivalent power but just two metres long will be commercially available, for around US$1.8 million each. It'll take a bit longer to replace the most powerful machines, such as the Large Hadron Collider currently being built at CERN. This uses a 27-kilometre-long tunnel and will have cost $2.5 billion by the time it opens in July 2007. But within 20 years or so, Mangles believes lasers could achieve the equivalent power with an accelerator just metres across. In the meantime, if biologists and chemists can use these mini-accelerators for studying molecules, it will free up more time for physicists to use the really big machines to tackle fundamental subatomic problems that need higher energies^{ref1,
ref2,
ref3}.

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Zulu time (Z) / Universal Time Coordinate (UTC) / Greenwich Mean Time (GMT) : the 24 hour time scale that is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, which is the Prime Meridian of Longitude. The globe is divided into 24 time zones of 15° of arc, or 1 hour in time apart. To the east of this meridian, time zones are numbered 1 to 12 and prefixed with a minus (-), while to the west, the time zones are also numbered 1 through 12 but prefixed with a plus (+).
atomic clocks^ref get their exceptional accuracy from the unvarying vibration of atoms. For example, caesium atoms vibrate 9.2 billion times each second, so a clock that generates its ticks from these vibrations could in theory be accurate to just one second in 30 million years. But until now atomic clocks have been two metres tall and required huge amounts of energy. Small devices have relied on a vibrating quartz crystal, which can gain or lose up to 2 seconds per week. The miniature atomic clocks would not drop a second for 300 years. The device still lacks some of the parts needed to measure time by atomic vibrations. But the biggest challenge, making a tiny caesium-filled chamber, has been met. Computer-chip technology was used to etch a tiny chamber into a template made of silicon and glass. The cavity is about the size of a grain of sand, and holds about a billion atoms of caesium gas. The remaining components should be relatively easy to develop, and hopes to have a marketable product in around 3 years. one of their first uses could be in GPS units. At the moment, a GPS tracker accesses four satellites to get a reading: 3 to triangulate position based on how long signals take to bounce there and back, and a fourth for a baseline time. A unit that took its baseline from an atomic clock would need only 3 satellites. This would mean better coverage, especially in urban areas, where buildings often get in the way. Another application may be in cell phones. These are relatively easy to listen in on, but the super-synchronization provided by internal atomic clocks could let two bounce between frequencies too quickly for eavesdroppers to keep up with^ref. Scientists have used a single mercury atom to build the world's most precise clock. The clock is proof that optical clocks, which count miniscule fractions of a second using visible or ultraviolet laser light, can outperform the current generation of atomic timepieces. It could also open the door to a new era of precision measurements of fundamental constants (Oskay W. H., et al. Phys. Rev. Lett., 91. 020801 (2006)). Time is measured in beats, and the most important part of measuring time accurately is having the fastest-beating pulse possible. The pulse in today's best atomic clocks comes in the form of oscillating laser-light waves. Just like the beating of the human heart, the laser is the beat of the atomic clock. But even the best-built lasers can fall victim to a slight drift in the light's frequency, which would play havoc with a clock based on the light. To keep that from happening, the laser is constantly checked by shining it on an atom that can only be stimulated into an excited state by a very specific frequency. This calibrates the laser and keeps it in check. Most current atomic clocks work by shining microwave lasers on to clusters of cesium atoms. The clocks count 9,192,631,770 oscillations of the laser for every second, and they are expected to only be out of true by up to 1 second after running for 60 million years. But now Bergquist and his team have outdone this by building a clock with an ultraviolet laser (which has a higher frequency than both visible light and microwaves) and a single atom of mercury. Their calibrating mercury atom is suspended in a cryogenic trap and cooled further by lasers, to reduce any unwanted wobbling from heat. Thanks to the brightness of mercury's atomic emission upon stimulation by the laser, the calibrator can use a solitary atom rather than a cluster, reducing any odd effects that might come of bouncing a laser off a number of atoms. This clock ticks 5 times more per second than the best cesium clock, making it five times more accurate. And it's more reliable too: it would only be expected to drift out by a second over 400 million years. The fine precision comes with its own problems, however. The clock is so sensitive to things such as gravitational fields that a clock at height will tick differently from one at sea level. Such differences must be carefully corrected for. The clock and others like it may also be sensitive to changes in the fundamental constants of nature. By comparing a mercury clock with a clock of another atom, such as aluminium, it should be possible to see shifts in natural constants, such as the strength of the electromagnetic force. Any shifts that are theoretically possible in such constants are thought to only shift subtly over millions or billions of years. But the high precision of optical clocks would allow you to see such changes in a few seconds. It's a pretty impressive paper : the team made numerous improvements to existing single-atom setups and carefully measured systematic errors. Still, it's unlikely that the mercury clock will supersede existing cesium technology, at least for the time being. The second has been defined by 9,192,631,770 oscillations of a cesium clock for years, and any switch in unit would require engineers to re-jig global positioning system (GPS) receivers, spacecraft and other gadgets that depend on precise time measurements. The standard won't change overnight. It could be up to another decade before the metrology community redefines the second.
nanomaterials : nanotechnology is the science of the very small, involving particles that are < 100 nm across. Nanomaterials are the molecule-sized wires, tubes and cages that could one day be used to build everything from drug delivery systems to more efficient computers. But researchers are finding that their orders are frequently contaminated or damaged on arrival. Both big and small nanomaterial suppliers are guilty and the situation is unlikely to change until standards are imposed on the industry. Nanotechnology research is booming in the USA; almost $1 billion of public money will be spent in 2004 as part of the country's National Nanotechnology Initiative. But the Lux Research Inc. report suggests that the surge in nanotechnology projects has outpaced the ability of companies to supply the basic materials needed by researchers reliably. One semiconductor company found that 33% of a sample of carbon nanotubes it had purchased was actually iron left over from the production process. There was lots of silicon contamination : the time spent removing the impurities set research back 2 weeks. University labs were some of the worst offenders in terms of reliable service, but big companies are far from blameless. Because some firms regularly upgrade their equipment, materials ordered just weeks apart can have widely different characteristics. Part of the problem may be attributable to poor handling by researchers (e.g. tubes inadvertently vaporized while attempting to purify them. Before an external body introduces standards that suppliers can follow, which is unlikely to happen before 2007, companies should shop around, buying and studying small quantities from different suppliers before deciding where to place their main order.

Health effects

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in vitro

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nanowires : although researchers can shrink individual components of circuits to the nanoscale, they cannot wire them together without conventional connections, which are hundreds of times bigger than the components themselves. Scientists can already make minuscule silicon wires that are just 10 nanometres wide, or 10,000 times thinner than a sheet of paper. Lieber's team covered one of these nanowires with a temporary mask that obscured alternating sections. Then they blasted the wire with nickel vapour, which transformed the uncovered sections into nickel silicide. The resulting structure was equivalent to a string of transistors, all pre-connected by conducting nickel silicide^ref. Such devices are unlikely to compete directly with the well-established semiconductor industry any time soon. Instead they will find niches that exploit their extreme sensitivity : for example, a nanotransistor network could be triggered by the electrical charge carried by a single protein, forming a powerful detection system.

Web resources

Nanowires at Wikipedia
Royal Society nanoscience enquiry

electric currents are being used to move bacteria around silicon chips and trap them at specific locations. The technique could help to assemble nanomachines from miniature parts, and to create a new generation of biological sensors. Nanodevices are typically built by connecting tiny components. But such a delicate task is not easy. So, many researchers are exploring ways to fix components in place using the binding properties of biological molecules, notably DNA. Robert Hamers and his colleagues from University of Wisconsin-Madison propose using entire microbes instead. The cells have surface proteins that attach to certain biological molecules. Once the cells are placed at specific sites on a silicon wafer, nanoparticles tagged with these molecules can bind to the cells in those locations. This is easier than dragging the nanoparticles themselves to the right spot, because their high density makes them harder to move through fluid media than the less dense living cells. The technique gives one a way to fix components such as quantum dots or carbon nanowires at very precise locations. The researchers use Bacillus mycoides, rod-shaped bacteria that are about 5 mm long. They pass a solution containing the cells over a silicon wafer with gold electrodes on its surface. The charge on the electrodes captures the bacteria, which flow along the electrodes' edges like luggage on a conveyor belt. The electrodes have tiny gaps between them. When a bacterium reaches a gap, it is trapped there by the electric field. It can be released by reducing the field between the electrodes, or permanently immobilized by increasing the voltage enough to break its cell wall. Cells have been manipulated using electric currents before but it is typically done using larger cells, which are moved around as they are observed under a microscope. Hamers' work is unique because the locations of the bacteria are detected electrically. When a cell bridges the gap between 2 electrodes, it acts like a wire and increases the current, signalling the bacterium's presence.You can collect the cell, measure it and then if you want you can release the field and let it go again. Electrical detection will allow the method to be used on organisms that are too small to be seen with an optical microscope. It should also help the automation of nanoscale assembly : you don't want to have to visually inspect every electrode to see what's happening. You could have a computer detect it electrically. As well as providing the glue for miniature devices, the system could also be used to detect harmful biological agents such as anthrax spores or certain strains of Escherichia coli bacteria. The electrodes on the chip could be coated with biomolecules designed to bind to particular pathogens and hold them in place, and other pathogens would flow away when the electrode voltage was reduced below a certain threshold^ref.
the first electrical switch made entirely from carbon nanotubes has been unveiled. Its inventors hope that it could help to replace silicon chips with faster, cheaper, smaller components. The device is a Y-shaped nanotube that behaves like a transistor, such as those found in every electronic device in your home. Current flowing from one branch to another can be switched on and off by applying a voltage to the third. The switching is perfect - the current is either on or off, with nothing in between. The small size and dramatic switching behaviour of these nanotubes makes them candidates for a new class of transistor. The scientists make their Y-shaped nanotubes by adding a titanium-iron catalyst to a pot of straight nanotubes while they are growing. When a catalyst particle sticks to the side of a nanotube, it forms the base of a new branch. Conventional transistors are built from layers of semiconducting materials, such as silicon. Better manufacturing methods have led to ever-smaller chips, packing enormous amounts of computing power into desktop machines. But as the components shrink, they start to leak electrical current. This causes overheating, wastes power, and can make some switches read 'on' when they should be 'off'. It seems that the silicon chip cannot get much smaller. So scientists are looking for ways to make carbon nanotubes do the same job. These rolled up sheets of carbon atoms conduct electricity and take up a lot less room than silicon circuits, measuring just a few billionths of a metre across. Nanotubes can also be made using cheaper chemical methods that avoid the laborious layering and etching used to make today's circuits. This allows us to go for devices with much smaller size but much more complex functionalities^ref. Scientists have already made logic circuits using nanotubes^{ref1,
ref2,
ref3}, but these required metal 'gates' to control the flow of charge. Making such devices requires several steps, so it is unlikely that they could compete economically with conventional electronics. But the gates in the new device are part of the nanotube's structure, making it fully self-contained. The catalyst particle sitting at the centre of the nanotube could be tweaked to change the switching properties of the device - making it switch at different voltages, for example. The team is now trying to extend the alphabet of branched nanotubes with T- and X-shapes that could allow different functions. What really keeps me enthused is that there are so many possibilities. Large, transparent sheets of carbon nanotubes can now be produced at lightning speed. The new technique should allow the nanotubes to be used in commercial devices from heated car windows to flexible television screens. Rarely is a processing advance so elegantly simple that rapid commercialization seems possible^ref. Nanotubes are tiny cylinders of carbon atoms measuring just billionths of a metre across. They are light, strong, and conductive. But for years their promise has outweighed their utility, because the complicated processes involved in making devices from nanotubes were too slow and expensive to be used in large-scale manufacturing. But now, nanotubes have gone into warp drive. Baughman's team can churn out up to ten metres of nanoribbon every minute, as easily as pulling a strip of sticky tape from a reel^ref. This ribbon can be up to five centimetres wide, and after a simple wash in ethanol compacts to just 50 nm thick, making it 2,000 times thinner than a piece of paper. The ribbons are transparent, flexible, and conduct electricity. Weight for weight, they are stronger than steel sheets, yet a square kilometre of the material would weigh only 30 kg. This is basically a new material. Scientists have been weaving carbon nanotubes into fibres and sheets for several years. But until now, the most common way of making large sheets of nanotubes relied on a labour-intensive technique much the same as that used by the ancient Egyptians to make papyrus. Nanotubes suspended in a solvent were slowly filtered to create a mat, which was then dried and peeled off the filter. Baughman's team instead start with a 'forest' of 0.5-mm-long nanotubes sticking upright on an iron-based platform. Pulling gently from the edge of the forest with an adhesive strip, such as a Post-It note, uproots a row containing millions of nanotubes. As these nanotubes pull out, they tangle with the next row, and so on. The nanotubes tangle together just enough to keep a ribbon growing, without jumbling up into a huge ball. A lot of people will now try this out with a Post-It in their own labs. A 1-cm-long forest of nanotubes can produce three metres of nanoribbon. The researchers had previously used a similar method to draw strings of nanotubes from a forest^ref. Getting them to knit into a wider fabric is a bit trickier, but that scaling the work up to produce large sheets will now be easily do-able. Nanotubes are already replacing graphite in certain commercial devices such as batteries. But this technique could now propel many more nanotube products into the marketplace. The team has already proved the sheets' usefulness in several applications, filing patents as they go. They have sandwiched a nanoribbon between 2 Plexiglass plates, for example, using the heat of a domestic microwave oven to weld the layers. This forms a transparent, conductive sheet ideal for a heated car window. And since bending does not change the electrical properties of the nanotubes they could be used to carry current in a 'rollable TV screen', something that has long been promised by nanotechnologists. Things move quickly if you can prove that the supply of the material is good
nanoparticles carrying siRNA able to stop Ewing's sarcoma and coated with a cyclodextrin-derived polymer are designed to be injected into the bloodstream and taken up primarily by tumour cells. Without treatment, 8 out of 8 study mice developed widespread cancer after 3.4 weeks. But out of 10 mice given twice-weekly injections of nanoparticles carrying a gene-silencing agent, only 2 showed cancerous growth, and this was relatively weak. The nanoparticles, which are small enough to pass through blood vessels into the surrounding tissue, are taken up by cancerous cells because they carry a molecular tag that binds to receptors found on tumours. Other attempts to carry siRNA to tumours have used nanoparticles made of lipids (Protiva Biotherapeutics in Burnaby, Canada), but in some mouse studies, these particles provoked an immune response, which would make the approach difficult to use in humans
imagine glasses that never fog up or reflect light. US scientists think they have the technology to make this happen, along with fogless, reflection-free ski goggles, windscreens, bathroom mirrors and more. Michael Rubner of the Massachusetts Institute of Technology in Boston and his team have developed a nanoparticle coating that reflects just 0.2% of light hitting its surface. That's much less than the 2-3% that is reflected by current antireflective coatings. The coating also soaks up the tiny water droplets that cause fogging. It consists of multiple layers of polymer fibres and glass nanoparticles that form a network filled with tiny gaps. Water is sucked into these spaces, which act like a sponge. The result is a thin film of water, rather than a number of droplets, which doesn't scatter light and cloud the glass. Rubner likes to call them 'molecular diapers'. The particles involved are just 7 nm in diameter and this keeps the coating transparent. As a bonus, the process is so simple that the team got local high-school students to whip up the coatings. The scientists soak the surface of choice in a series of solutions, which alternate between being positively and negatively charged to help glue the layers together. They then bake the coating at 500ºC to make it harder and more scratch resistant. This step means that, at the moment, the coating can be applied only to surfaces, such as glass, that can withstand high temperatures. We are still working to apply this to low melting-point materials like plastics. Rubner has used similar technology to create a coating that has the opposite effect: it repels water extremely well. This material is covered in an additional layer of wax-like polymers, mimicking the surface of the lotus flower. The lotus repels water so strongly that droplets roll off, taking dirt with them. Mixtures of these 2 materials might come in handy. The carapace of a beetle from the Namib Desert (Stenocara), for example, has parts that attract water and others that repel it. This helps the bug to attract tiny droplets from the air, concentrate them, and direct them to its mouth to drink. So what's next for Rubner? Commercialization of course. Rubner says he has had visits from 3 car manufacturers interested in the technology.

official response

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report

Web resources

Nanotechnology news

space warp is a consequence of Einstein's general theory of relativity, which describes gravity as a curvature in space produced by objects sitting in it. It also implies that a rotating mass will drag space around it like a spinning top placed in treacle - an effect known as the Lense-Thirring effect, or more commonly as 'frame-dragging'. The effect becomes important in understanding extreme situations like spinning quasars, and the rotation of jets of gas around black holes. The effect was first predicted by Austrian physicists Joseph Lense and Hans Thirring in 1918, but until now scientists haven't had sufficiently accurate instruments to measure its tiny perturbations in the fabric of our Universe. One of the last untested predictions of general relativity has been confirmed by the first reasonably accurate measurement of how the rotating Earth warps the fabric of space. The experiment, carried out for virtually no cost with Earth-based laser range-finders, scoops Gravity Probe B, the US$700 million orbiting craft launched in April 2004 to test exactly the same effect. However, the Gravity Probe B team has questioned whether the result is really quite as accurate as it seems. The effect dragged the path of 2 NASA satellites, LAGEOS and LAGEOS 2, out of position by about 2 metres each year when charted over 11 years with laser range-finders with the precision of a few millimetres. The result is 99% of the value predicted by relativity, with an error of up to 10%. One of the difficulties is extracting the frame-dragging effect from the huge gravitational effect of the Earth. If the Earth were perfectly symmetrical, frame-dragging would be easy to measure. But the lumpy Earth generates an uneven gravity field, Will points out, which moves the satellites about far more than frame-dragging. To tease the two effects apart, a map of the Earth's gravity field provided by a NASA mission called GRACE, launched in March 2002, was used. This relies on 2 satellites orbiting Earth about 220 kilometres apart, measuring the tiny changes in that distance as they pass through different parts of the Earth's gravity field. Ciufolini's previous attempt^ref at measuring frame-dragging was less than 20% accurate, because it did not have the benefit of the GRACE gravity model. The laser-ranging method can deliver the accuracy, but it is still uncertain if the GRACE gravity models are good enough. Will adds that the Gravity Probe B team is also sceptical, and thinks that Ciufolini may have drastically underestimated his errors. Either way, the Gravity Probe B experiment is expected to deliver a measurement of frame-dragging with 1% accuracy very soon. Physicists did not expect either of these experiments to overturn relativity, but insist that confirmation was still essential. The last major prediction of general relativity requiring confirmation is the existence of gravity waves. The LIGO experiment, run by the California Institute of Technology and the Massachusetts Institute of Technology, is already searching for these on Earth, while NASA's LISA probes are expected to launch in 2010^ref.

Web resources

terahertz radiation

mini-lenses made out of water droplets have been made to bulge and contract in response to temperature and pH changes in their environment. The clever trick makes for a tiny sensor whose changing focal length could one day be used to quickly and simply monitor all sorts of things, from blood samples to miniature chemical reactors. Miniature lenses were built using millimetre-wide rings of hydrogel, a material that expands and contracts when exposed to different temperatures or acidities. A drop of water within the ring then bulges or contracts in response, changing the focus of the lens. The whole thing is sandwiched within a glass case, with tiny channels allowing any fluid outside to leak in and affect the hydrogel ring. Adjustable lenses have been designed before, but mainly for optical use, in things such as spectacles, so that users can switch from near to far focus. These lenses, in contrast, act as tiny sensors of the world around them. So far the achievement is just a proof of principle. The lenses only react to fairly major swings of about 10-20 ºC or a change in pH from acidic to alkaline. And there are already well-established ways to measure temperature and pH, without the need for a novel mini-lens system. But the team plans to develop hydrogels that react to other conditions — to different proteins or salts from the body, for example. The lenses could then be scaled down to the mm range and set into thousand-strong arrays to instantly and easily monitor a slew of substances in any given sample. These lab-on-a-chip applications should come pretty soon^ref. But it should also be possible to produce larger versions, where the focusing is visible to the naked eye. A scaled-up lens might change its focus when detecting a specific protein in a blood sample; the sample might test positive, for example, if some text placed beneath the lens in the blood comes into focus
terahertz waves, which nestle between microwaves and infrared light in the electromagnetic spectrum, are able to penetrate materials such as plastic and cardboard, which are opaque to other wavelengths. But technology using them has been slow to take off because it is difficult to guide the waves from one place to another. For example, hollow metal pipes can carry terahertz waves, but they absorb the radiation very quickly, turning it into heat. This means that most of the terahertz waves are lost in translation. Pipes also transmit waves of different frequencies at different speeds, making it impossible to reconstruct detailed information from a broad frequency scan. Now it seems that a humble length of stainless steel wire is enough to carry the waves to a receiver, in the way that a coat-hanger jammed into a portable radio helps it to pick up a signal. The researchers were working on a new way of producing terahertz images that involved bringing a metal rod very close to a metal surface. When they pointed the terahertz waves at the metal rod, rather than the surface, they found that they could still pick up the radiation, but with a time delay as the wave travelled down the rod. The new method is comparable to a radio aerial, in which electrons are sloshed from one end to the other as the long wavelength radio waves change the electromagnetic field in the wire. At shorter wavelength terahertz frequencies, the electrons only wobble by a few millimetres, but this creates areas of high and low electron density. These areas of high and low density then travel down the wire, just like sound waves passing through air. When they hit the end of the wire, they regenerate electromagnetic terahertz waves that cross a short distance through air before being picked up by a receiver. This allows to use wires to guide terahertz waves to wherever you want. To prove the point, they made an endoscope that can see inside confined spaces using just 2 wires and a metal plate. Terahertz waves travel down one wire, bounce off the angled plate and the interior of the container, and then travel back up the second wire. Amplifying this signal generates a crude image of the container's insides. Once they can be manipulated more easily, terahertz waves could be used to see inside suspect packages at airports, Mittleman says. They will not replace X-rays, because they cannot penetrate metal boxes. But they can distinguish between different chemicals inside a package, each of which absorbs a characteristic set of terahertz frequencies^ref. A terahertz device could be inserted through tiny holes in cargo containers to check for residues of explosives. Because the wire can carry a very broad range of different terahertz frequencies at once, it allows a general chemical analysis.
surface-scaling superpowers : animals such as insects and lizards employ an impressive range of tools, including everything from flat attachment pads, used by grasshoppers, to microscopic hairs, which cover the feet of geckos. Whereas some animals, including flies and beetles, produce sticky secretions to glue themselves to vertical surfaces, others, such as lizards, use a dry system. A very sticky glue is harder to release from a surface, so animals sometimes opt for dry systems that make it easier for them to detach when required. Geckos use intermolecular attractions, known as van der Waals forces, to climb walls^ref. The lizards take advantages of these normally weak forces by maximizing the surface area between their feet and the wall. The bottoms of their feet are covered in millions of microscopic hairs, each of which splits off into hundreds of tips just nanometres across. The increased area of contact makes the intermolecular attractions between hair and wall significant enough to support the gecko. Different shapes of contact element used by the grasshopper (top left), fly (top right) and beetle (bottom left and right).

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quantum theory describes the Universe at the tiniest possible scale - about 10-35 metres (about 1020 times smaller than the radius of a proton). It predicts that on this scale the apparently smooth fabric of space and time must degenerate into a kind of 'foam' in which connections between different points are constantly appearing and vanishing. Physicists have long been trying to figure out how the fuzzy nature of space-time at this tiny scale can give rise to the large 4-dimensional Universe we see around us, as described by Einstein's theory of relativity. Scientists studying the problem assume that each tiny piece of the foam is a kind of four-dimensional triangle, with three dimensions of space and one corresponding to time. The smooth fabric of space-time can be built up by gluing these triangular tiles together, just as a smoothly curved surface can be made from flat, 2-dimensional tiles. Because the quantum foam fluctuates through all kinds of configurations, constructing the physical Universe means adding up all the possible tiling patterns. You might think that this would inevitably generate a 4-dimensional Universe - but it doesn't. Earlier researchers found that they got a space-time with either an infinite number of dimensions or just two. Neither of these looks at all like our Universe. Renate Loll of Utrecht University in the Netherlands and her co-workers have now found a way to assemble the pieces so that they inevitably produce a four-dimensional Universe. Instead of assuming that all tilings are allowed, they impose two constraints. First, the theory of relativity must apply within each individual tile (so that nothing can travel through it faster than light) and second, the assembly must preserve causality. This means that a piece of space-time cannot be constructed in such a way that an 'event' - some change in the Universe - precedes its cause. When they enforced these criteria on their calculations, the researchers ended up with universes with three spatial dimensions and one time dimension - just like our own^ref. Even more startling, they found that typical universes generated this way started off small and got bigger - they expanded, just like the real Universe has done since the big bang. This was completely unexpected - there was nothing in the tiling rules that seemed to demand it. There's no a priori reason to demand that quantum space-time has to observe causality: the researchers put it into their equations by hand. But that, it seems, is the only way to end up with a realistic Universe
Cliff Reiter, a mathemathician of Lafayette College in Easton, Pennsylvania has been making indoor snowflakes that are surprisingly similar to nature's fractal beauties and have the classic 'dendrite' snowflake form, in which 6 central stems divide and taper to increasingly fine fronds. There are a handful of mathematical models of snowflake growth, but most involve fiendishly complicated differential equations. Reiter tried to make snowflakes using mathematical processes called cellular automata, sets of simple rules that can generate extremely complex forms when applied to a system over and over again. Unlike a differential equation, which tries to describe the whole snowflake, a cellular automaton just looks at a tiny part of the whole structure, and describes it in relation to areas that have already been built. Stephen Wolfram, a British mathematician, reignited interest in cellular automata 20 years ago^ref, but Reiter says that his snowflake automata are an improvement on Wolfram's attempts, because they are able to generate realistic snowflakes from just 2 parameters^ref. Reiter is unsure whether his model helps to explain how real snowflakes grow, but is trying to invent new cellular automata that can generate 3D snowflakes. Physicists currently describe how fluids move using the complicated Navier-Stokes equations, but Reiter hopes that a cellular model could describe the motion just as effectively yet much more simply. Until then... it seems that the snow shows no sign of stopping, so buy some corn for popping, and click on the videos to let it snow on your screen.

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a tiny cavity inside a crystal makes an ideal laboratory for quantum experiments, entangling light and matter inside a solid for the first time. Their miniature laboratories should make it easier to study quantum entanglement, an important effect that could one day help to build a quantum computer. Whereas conventional computers store information as bits, which can be 'on' or 'off', a quantum computer takes advantage of the inherent uncertainty in the quantum world by storing data in qubits that can be on or off at the same time. In theory, this should allow a quantum computer to perform many calculations simultaneously. Entanglement is another quantum quirk that would allow data to be shared between different parts of the computer at the same time. When 2 objects are entangled they start to behave like a single entity, carrying the same information even if they are physically separated. Tweaking one instantaneously changes the nature of the other. Holes inside the semiconductor material gallium arsenide (and aluminium arsenide or atom by atom), a pillar just 800 nm across, are used to house a quantum dot, a ball of just a few million atoms. A laser pulse directed at the dot jolts it into spitting out a particle of light, which is entangled with both the quantum dot and the electric field of the cavity itself.

flying qubit

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Web resources

a team of US physicists has proved a theorem that explains how our objective, common reality emerges from the subtle and sensitive quantum world. If, as quantum mechanics says, observing the world tends to change it, how is it that we can agree on anything at all? Why doesn't each person leave a slightly different version of the world for the next person to find? Because certain special states of a system are promoted above others by a quantum form of natural selection, which they call quantum darwinism. Information about these states proliferates and gets imprinted on the environment. So observers coming along and looking at the environment in order to get a picture of the world tend to see the same 'preferred' states. If it wasn't for quantum darwinism the world would be very unpredictable: different people might see very different versions of it. Life itself would then be hard to conduct, because we would not be able to obtain reliable information about our surroundings... it would typically conflict with what others were experiencing^ref. The difficulty arises because directly finding out something about a quantum system by making a measurement inevitably disturbs it. After a measurement, the state will be what the observer finds out it is, but not, in general, what it was before. They survive monitoring by the environment to leave 'descendants' that inherit their properties. Because the Universe is quantum to the core, this property seems to undermine the notion of an objective reality. In this type of situation, every tourist who gazed at Buckingham Palace would change the arrangement of the building's windows, say, merely by the act of looking, so that subsequent tourists would see something slightly different. Yet that clearly isn't what happens. This sensitivity to observation at the quantum level (which Albert Einstein famously compared to God constructing the quantum world by throwing dice to decide its state) seems to go away at the everyday, macroscopic level. God plays dice on a quantum level quite willingly but, somehow, when the bets become macroscopic he is more reluctant to gamble. How does that happen? The Los Alamos team define a property of a system as 'objective', if that property is simultaneously evident to many observers who can find out about it without knowing exactly what they are looking for and without agreeing in advance how they'll look for it. Physicists agree that the macroscopic or classical world (which seems to have a single, 'objective' state) emerges from the quantum world of many possible states through a phenomenon called decoherence, according to which interactions between the quantum states of the system of interest and its environment serve to 'collapse' those states into a single outcome. But this process of decoherence still isn't fully understood. Decoherence selects out of the quantum 'mush' states that are stable, that can withstand the scrutiny of the environment without getting perturbed. These special states are called 'pointer states', and although they are still quantum states, they turn out to look like classical ones. For example, objects in pointer states seem to occupy a well-defined position, rather than being smeared out in space. The traditional approach to decoherence was based on the idea that the perturbation of a quantum system by the environment eliminates all but the stable pointer states, which an observer can then probe directly. But he and his colleagues point out that we typically find out about a system indirectly, that is, we look at the system's effect on some small part of its environment. For example, when we look at a tree, in effect we measure the effect of the leaves and branches on the visible sunlight that is bouncing off them. But it was not obvious that this kind of indirect measurement would reveal the robust, decoherence-resistant pointer states. If it does not, the robustness of these states won't help you to construct an objective reality. Now, Zurek and colleagues have proved a mathematical theorem that shows the pointer states do actually coincide with the states probed by indirect measurements of a system's environment. The environment is modified so that it contains an imprint of the pointer state. Yet this process alone, which the researchers call 'environment-induced superselection' or einselection^ref, isn't enough to guarantee an objective reality. It is not sufficient for a pointer state merely to make its imprint on the environment: there must be many such imprints, so that many different observers can see the same thing. Happily, this tends to happen automatically, because each individual's observation is based on only a tiny part of the environmental imprint. For example, we're never in danger of 'using up' all the photons bouncing off a tree, no matter how many people we assemble to look at it. This multiplicity of imprints of the pointer states happens precisely because those states are robust: making one imprint does not preclude making another. This is a Darwin-like selection process. One might say that pointer states are most 'fit'. They survive monitoring by the environment to leave 'descendants' that inherit their properties. This work shows that the environment is not just finding out the state of the system and keeping it to itself. Rather, it is advertising it throughout the environment, so that many observers can find it out simultaneously and independently.
the Materials Library at King's College is a growing unique collection of substances. Its curator, Mark Miodownik, hopes that his 'materials library' will help artists looking for inspiration, and scientists seeking substances with particular properties. He founded the library in 2003 after seeing colleagues faced with lack of space simply chucking interesting materials away. His collection now includes > 300 samples, including artificial skin made of rubber composites, and a material known as a superslurper that absorbs 400 times its own weight in water. Financial support for the materials library has come from the National Endowment for Science, Technology and the Arts, and the Engineering and Physical Sciences Research Council, both UK institutions. So far, Miodownik's collection has only had around 60 visitors, mostly curious artists. But he hopes that, as the library's fame grows, more and more scientists, architects and students will come for a look around, and ultimately adopt some of the materials for use in their own work. Among his collection, for example, is a kit called the 'DNA of a city' that consists of samples of common construction matter. It's important to have a physical library : people can touch the materials and that's a great advantage. Other reference tools, such as one provided by the New York-based company Material Connexion, give details about different substances without letting people touch samples or learn their history. Miodownik will present his project at London's Tate Modern Engineering Art gallery on April 2005, a move that he hopes will catapult materials science into the spotlight^ref

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MatWeb

physicists have come home empty-handed from a thorough hunting expedition for pentaquarks. The lack of evidence has led some to doubt that these odd subatomic particles, first sighted in 2002, actually exist. The pentaquark was discovered at the SPring-8 synchrotron in Harima, Japan^ref. The particle, thought to be made up of five quarks, is so unstable that physicists inferred its existence from the debris of collisions between gamma rays and carbon atoms. Trios of quarks make up the protons and neutrons that are the basic building blocks of atomic nuclei. Particles made up of five quarks would be extremely unusual, and were hailed as a new form of matter. But experiments at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, now suggest that the discovery was a mistake. We just didn't see it in these experiments. I think this is the only lab in the world to repeat the experiments with at least ten times the sensitivity. It's the first time we can look at this problem with really good statistics. The results fuel a controversy that has raged since the first pentaquark sighting was confirmed by more than ten other labs, which looked back through the results of similar collisions to search for evidence of the particle. In 2003, there were lots of positive results, everybody found something. But in 2004, results from higher energy collisions showed no trace of the pentaquark, throwing the original discovery into doubt. Higher energy collisions generally give more statistically significant results, but some scientists argued that these conditions were obliterating any evidence of the pentaquark. So the Jefferson team mounted a rigorous safari of the debris from a gamma-ray collision on a liquid hydrogen target, repeating an experiment from the Electron Stretcher and Accelerator in Bonn, Germany, that claimed to have found the pentaquark. The data for the existence of pentaquarks do not look convincing. After the SPring-8 discovery was announced many groups prematurely jumped on the pentaquark bandwagon. The original sightings were probably just background noise from the experiments. Scientists study how quarks stick together because they make up most of the matter we see around us. Theorists believe that in its first seconds, the newborn Universe was filled with a plasma of quarks and gluons, the particles that bind quarks to each other. Understanding this quark soup could reveal why the Universe evolved into the form we see today. Efforts to find the pentaquark have not been a wild goose chase. It really caught the attention of the community for a few years, and delivered a lot of good theoretical work. Experiments at the Jefferson lab later this year will probably convince any remaining doubters that the particles do not in fact exist.
Officials have announced that an extra second will be added to 2005, to accommodate a slowing down of the Earth's rotation. The announcement is by no means unprecedented. We have been adding leap seconds to years since the 1970s, but, owing to unpredictable quirks in our planet's rotation, we haven't needed one since 1998. The International Earth Rotation and Reference Systems Service in Paris will sneak the extra time in on 31 December 2005, making the countdown to the new year one second longer than some might expect. The advent of atomic clocks in the 1950s allowed for extremely accurate measurement of periods of time. Starting at a particular point in 1958, an international array of these clocks has been counting out seconds, the length of which was defined at that point. This representation of time is the standard by which the public sets their watches. But people have been keeping an eye on changes in the length of seconds, as fractions of the Earth's daily rotation, using astronomical measurements. Over the years, the time expressed by atomic clocks has diverged slightly from that determined by the astronomic approach. This difference results from changes to the Earth's rotation, due to gravitational forces from celestial bodies such as the Moon and the Sun. These forces are thought to cause heavy matter to shift within the Earth's core. When heavy materials move toward the planet's centre, it speeds up its rotation just like an ice skater bringing in her (or his) arms for a swift twirl. Other events, including the large earthquake that preceded the Asian tsunami of December 2004, can also alter the Earth's spin by a tiny amount. To smooth out wrinkles between atomic and astronomic time, experts introduced the practice of adding and subtracting leap seconds. Leap years come at regular intervals and correct for the slight difference between the time it takes our planet to travel around the Sun and a full 365 days, but leap seconds are more capricious. Rotation is much less predictable than our orbit around the Sun. There was a spate of leap seconds in the 1970s. So far 32 leap seconds have been added to years and there will be 33 by the end of 2005. Desktop computers will adjust to the added second by talking to other units on the Internet. Computers switched off for weeks often have to make the same type of time correction. But some experts believe that highly specialized technologies that involve Global Positioning System tools will lack the capability to cope with such a change. For this reason, a movement has emerged to abolish the practice of adding leap seconds. Some are not convinced that this would be a good idea : the problem is exaggerated. We can just develop a standard for how computers adjust to the leap second. So how will Kuhn spend his extra second: it's quite possible I might do a very geeky thing and watch how my computer behaves.

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given that the movement of electrons underlies all chemical reactions, the team hopes that its technique could help to design catalysts or even speed developments in a futuristic form of computing known as spintronics. Short X-ray pulses were used to watch an electron moving away from a sulphur atom stuck to a ruthenium metal surface. They saw that it took just 320 quintillionths of a second to make the jump, that's 320 attoseconds or 320 x 10^-18 seconds. The speed fits with theories of electron movement. Researchers have managed to measure even quicker electron processes in the past, such as the movement of an electron within a single atom. "The electron of the hydrogen atom takes just 150 attoseconds to make it around the nucleus. But this is the first time they have spied on an electron moving between two different atoms that are tightly bound to each other. Watching this sort of process has much more relevance to real-world chemistry. Watching an electron move is tricky, given that electrons are not single objects. Instead chemists think of them as existing within clouds of probability, and have to track such 'clouds' as they move between atoms. To do this, the team used X-rays to boost the energy of an electron close to the core of a sulphur atom. This pushed the electron towards the ruthenium surface, leaving the remaining electrons in the sulphur atom to shuffle around. By watching how the X-ray pulses were absorbed, the researchers tracked how long it took for the electron's cloud of probability to shift entirely from the sulphur to the ruthenium. The team chose to study a sulphur atom because the element is notorious for clogging up catalysts based on metals such as ruthenium. Sulphur in gasoline can deactivate the metals in a vehicle's catalytic converter, for example. Chemists use computer simulations to predict how catalysts will behave, so measurements of electron motion may improve these models and help to build better catalysts. The scientists have added to their technique by using a polarized X-ray wave that rotates as it travels, in order to excite electrons that 'spin' in a specific direction. Electrons with different spins will move through magnetic materials at different speeds. Clocking these speeds is important for scientists working on 'spintronic' computing, in which the binary data in your computer is carried in the spin state of electrons, hopefully leading to faster, smaller processors. Of course it's also just fun to know how fast the electron actually goes^ref.
ultra-hard metals : a team of researchers in the USA and Switzerland simulated the atomic-scale structure of a slab of copper, made up of a patchwork of grains about 20 nm across. Their study showed that the material becomes harder and stronger after a shock wave has passed through. An explosion could produce such a shock, suggest the team, led by Eduardo Bringa at the Lawrence Livermore Laboratory in California. Ultra-hard metals are needed not just for military armour but for applications such as nuclear fusion. Researchers are looking into initiating fusion reactions using laser blasts, and very strong materials are needed to contain these reactions. Metals are a patchwork of grains stuck together. These materials bend and deform when misalignments of rows of atoms, called dislocations, slip through a grain. This allows the material to adapt itself to different shapes when under stress, making the metal soft. Smaller grains make for harder metals, because dislocations tend to get stuck when they reach the edge of a grain. So in grains just a few tens of nanometres across, dislocations can move travel a very small distance, limiting how much the material can change shape. But there is a limit to the strengthening effects of shrinking crystalline grains. If stressed far enough, the grains themselves may slip and slide against each other, deforming the material. Bringa and colleagues sought to stop these slips by investigating what happens when one subjects a metal with nanoscale grains to sharp shocks. A shock wave creates a very high pressure over a very narrow region as it travels through a material. As the region of stress is on the same size scale as a grain itself, the pressure can't force grains to slide over each other. Instead, the material can only accommodate the deformation by dislocations appearing within the grains. But this time, that hardens the metal^ref. The dislocations produce kinks on the grain edges that knit them together, providing extra strength. The dislocations hook the grain boundaries a little bit and stop them sliding. That way, we can take nanocrystalline metals even further along the hardening path than people thought we could. The researchers have preliminary evidence that same thing happens in real life, as well as simulations. A piece of nanocrystalline nickel, after being subjected to an explosive shock, becomes peppered with dislocations inside the grains. Normally you never see that, because the grains slide first. The team hasn't tested the strength of their shocked nickel, because such measurements are tricky with such tiny amounts of material. The results are interesting and could be useful for fusion research. But for large-scale industrial applications such as aerospace engineering, she says, the question is whether one can make enough of the material to be useful. There are already industrial techniques for shocking large volumes of materials. These strengthened metals might make ultra-hard coatings for other materials. Hardness comes at the cost of brittleness. Dislocations serve a purpose in engineering. They make things more forgiving. That's why we make aircraft out of bendy metal instead of brittle ceramics. What you'd like is something that is ductile and strong at the same time.
The time-honoured trick for efficiently packing grains into a container, be they sand in a jar or wheat in a silo, is to give the thing a good tap. But alternate cycles of heating and cooling will do the job too. The conclusions could help explain why storage silos sometimes split apart after being exposed to extreme temperatures. How do grains pack together? Imagine putting some marbles in the bottom of a container — you expect the next layer to go on top of the dimples. But when you actually pour grains into a container, you tend to get jams and arches building up. A tap usually sorts out most of these disparities and settles everything down. Vibrations induce small rearrangements, helping objects to settle. Temperature changes affect industrial storage silos, sometimes contributing to dramatic accidents. But the whole idea of temperature changes having an effect on granular mechanics hadn't been properly analysed. The researchers decided to study the situation by pouring plastic or glass spheres into plastic or glass containers. They placed their sphere-laden containers into ovens, heating and then cooling them over a few hours between room temperature and 160 °C. The hot/cold treatment cycles caused the spheres to pack more efficiently, regardless of factors such as the heating rate or the height to which the cylinders were filled. The fraction of cylinder filled by spheres rather than empty space (the 'packing fraction') typically increased by 1 or 2% — a fair amount, given that the typical packing proportion for such spherical objects spans from 57 to 64%. The hot/cold treatment works because the containers and the spheres expand slightly differently. If the container walls expand more than the particles, this allows the grains to settle into free space; if the grains out-expand the container, this pushes the grains inwards and shrinks the space between them. Equal expansion doesn't have much of an effect, they note: glass spheres in a glass container show much smaller changes in packing. These simulations, involving moving walls of a container in and out, reach much the same conclusions. But he warns that the mechanism behind this kind of packing might not be as simple as Schiffer proposes. Other material properties of the spheres might change with temperature too. The results are unlikely to revolutionize how farmers go about stacking their grain. You can already use motors to vibrate the tops of silos or you can push in high-pressure air if you want to pack in grains efficiently. Still, this research will trigger more detailed experiments — there will be more interesting results coming. The work could potentially help researchers to better understand the dangerous effects of heat cycling on silos. Silo manufacturers are already aware of a phenomenon known as ratcheting — the process by which a silo expands during the day and then contracts again in the cool of the night. Material that settles into the wider daytime space can resist being forced back up the silo at night, putting pressure on the silo walls instead. In places such as the Arizona desert, where temperatures can reach 50 °C during the day and fall to 5 °C at night, such repeated settlings can cause a poorly designed silo to split open. Our model can give us a possible timescale for this type of behaviour^ref.
A seaside conundrum has been solved: what shape is a pebble? The answer, of course, is 'pebble-shaped'; but now, thanks to research by a team in France and the USA, it's possible to define what that means. Technically speaking, says material physicist Doug Durian of the University of Pennsylvania in Philadelphia and his colleagues, a pebble is a rounded body with a near-gaussian distribution of curvatures. While no two pebbles are exactly alike, all seem to end up with this mathematical form^ref1,
ref2. And once this shape is attained, it never changes (in lab experiments, at least). As a large pebble is ground down to a tiny grain, it retains this classic pebble shape. Aristotle proposed that the rounded shapes of pebbles arise because of faster erosion of parts furthest from the centre, which neatly explains why sharp corners get worn away. But although various proposals have been made for how to define a pebble's shape, including simply measuring its width, breadth and height, our understanding has not advanced much since Aristotle's time. The best way to define rounded shapes is in terms of their distributions of curvature (curvature being the radius of a circle that would match the contour at a given point). A sphere has equal curvature everywhere. But the curvature of a pebble varies from place to place. Durian's team looked at pebbles of hardened mud formed at Mont St-Michel bay on the coast of northern France, where the tides produce flattish, dried mud fragments that are progressively eroded over many months. As erosion proceeds, the pebbles 'mature' from sharp-edged fragments to rounded forms. They took 2D pictures of > 60 pebbles at various states of erosion, and calculated the distribution of curvatures around the circumference of each pebble. They then plotted this distribution on a graph. On such a graph, the curvature distribution of a circle would appear as a single spike. But the pebbles all showed a broad bell curve, with the more mature pebbles approaching a shape that can be described by a gaussian curve. Although the exact shapes of the mature pebbles varied, they all showed the same curvature distribution once past a certain stage of erosion. The researchers saw precisely the same thing for pebbles made in the laboratory from clay, moulded initially into various sharp-edged polygons (including triangles, squares and odd-shaped chunks) and then bashed about in a square metal pan to erode them. In this case, the curvature distribution was a little closer to the ideal gaussian form. The distribution of curvatures doesn't seem to depend on the pebble's starting shape, but rather solely on the erosion process itself. Maybe softer, gentler erosion might lead to a narrower bell curve and a shape closer to a perfect circle. This raises the possibility that experts might be able to tell whether a pebble was worn away by one kind of erosion (in a raging river, for example) or another (such as the slow wear in a glacier, or from the wind), just by looking at its shape. A trained geologist might be able to tell the difference, but it has been hard to check that intuition

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American Institute of Physics
FusEdWeb from Lawrence Livermore National Laboratory in California
PhysOrg.com : the latest physics and technology news
The Virtual Library - Physics
2005 World Year of Physics
Physics @ Nature.com
Journals : Nature Physics

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