Plasma (physics) - Wikipedia, the free encyclopedia

Plasma (physics) - Wikipedia, the free encyclopedia

In physics and chemistry, a plasma is typically an ionized gas. Plasma is considered to be a distinct state of matter, apart from gases, because of its unique properties. "Ionized" refers to presence of one or more free electrons, which are not bound to an atom or molecule. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields.

Plasma typically takes the form of neutral gas-like clouds (e.g. stars) or charged ion beams, but may also include dust and grains (called dusty plasmas).[1] They are typically formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move more freely.
Contents
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* 1 History
* 2 Common plasmas
* 3 Plasma properties and parameters
o 3.1 Definition of a plasma
o 3.2 Ranges of plasma parameters
o 3.3 Degree of ionization
o 3.4 Temperatures
o 3.5 Potentials [This is a link to part of http://en.wikipedia.org/wiki/Plasma_%28physics%29]
o 3.6 Magnetization
o 3.7 Comparison of plasma and gas phases [This is a link to part of http://en.wikipedia.org/wiki/Plasma_%28physics%29]
* 4 Complex plasma phenomena
o 4.1 Filamentation
o 4.2 Shocks or double layers
o 4.3 Electric fields and circuits
o 4.4 Cellular structure
o 4.5 Critical ionization velocity
o 4.6 Ultracold plasma
o 4.7 Non-neutral plasma
o 4.8 Dusty plasma and grain plasma
* 5 Mathematical descriptions [This is a link to part of http://en.wikipedia.org/wiki/Plasma_%28physics%29]
o 5.1 Fluid model
o 5.2 Kinetic model
* 6 Fields of active research
* 7 Footnotes
* 8 See also
* 9 External links

Quantum secrets of photosynthesis

Through photosynthesis, green plants and cyanobacteria are able to transfer sunlight energy to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. Speed is the key - the transfer of the solar energy takes place almost instantaneously so little energy is wasted as heat. How photosynthesis achieves this near instantaneous energy transfer is a long-standing mystery that may have finally been solved.
A study led by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley reports that the answer lies in quantum mechanical effects. Results of the study are presented in the April 12, 2007 issue of the journal Nature.

"We have obtained the first direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis," said Graham Fleming, the principal investigator for the study. “This wavelike characteristic can explain the extreme efficiency of the energy transfer because it enables the system to simultaneously sample all the potential energy pathways and choose the most efficient one.

Fleming is the Deputy Director of Berkeley Lab, a professor of chemistry at UC Berkeley, and an internationally acclaimed leader in spectroscopic studies of the photosynthetic process. In a paper entitled, Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems, he and his collaborators report the detection of “quantum beating” signals, coherent electronic oscillations in both donor and acceptor molecules, generated by light-induced energy excitations, like the ripples formed when stones are tossed into a pond.

Electronic spectroscopy measurements made on a femtosecond (millionths of a billionth of a second) time-scale showed these oscillations meeting and interfering constructively, forming wavelike motions of energy (superposition states) that can explore all potential energy pathways simultaneously and reversibly, meaning they can retreat from wrong pathways with no penalty. This finding contradicts the classical description of the photosynthetic energy transfer process as one in which excitation energy hops from light-capturing pigment molecules to reaction center molecules step-by-step down the molecular energy ladder.

"The classical hopping description of the energy transfer process is both inadequate and inaccurate," said Fleming. "It gives the wrong picture of how the process actually works, and misses a crucial aspect of the reason for the wonderful efficiency."

Co-authoring the Nature paper with Fleming were Gregory Engel, who was first author, Tessa Calhoun, Elizabeth Read, Tae-Kyu Ahn, Tomas Mancal and Yuan-Chung Cheng, all of whom held joint appointments with Berkeley Lab’s Physical Biosciences Division and the UC Berkeley Chemistry Department at the time of the study, plus Robert Blankenship, from the Washington University in St. Louis.

The photosynthetic technique for transferring energy from one molecular system to another should make any short-list of Mother Nature’s spectacular accomplishments. If we can learn enough to emulate this process, we might be able to create artificial versions of photosynthesis that would help us effectively tap into the sun as a clean, efficient, sustainable and carbon-neutral source of energy.

Towards this end, Fleming and his research group have developed a technique called two-dimensional electronic spectroscopy that enables them to follow the flow of light-induced excitation energy through molecular complexes with femtosecond temporal resolution. The technique involves sequentially flashing a sample with femtosecond pulses of light from three laser beams. A fourth beam is used as a local oscillator to amplify and detect the resulting spectroscopic signals as the excitation energy from the laser lights is transferred from one molecule to the next. (The excitation energy changes the way each molecule absorbs and emits light.)

Fleming has compared 2-D electronic spectroscopy to the technique used in the early super-heterodyne radios, where an incoming high frequency radio signal was converted by an oscillator to a lower frequency for more controllable amplification and better reception. In the case of 2-D electronic spectroscopy, scientists can track the transfer of energy between molecules that are coupled (connected) through their electronic and vibrational states in any photoactive system, macromolecular assembly or nanostructure.Read on...

Fleming and his group first described 2-D electronic spectroscopy in a 2005 Nature paper, when they used the technique to observe electronic couplings in the Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting protein, a molecular complex in green sulphur bacteria.

Said Engel, "The 2005 paper was the first biological application of this technique, now we have used 2-D electronic spectroscopy to discover a new phenomenon in photosynthetic systems. While the possibility that photosynthetic energy transfer might involve quantum oscillations was first suggested more than 70 years ago, the wavelike motion of excitation energy had never been observed until now."

As in the 2005 paper, the FMO protein was again the target. FMO is considered a model system for studying photosynthetic energy transfer because it consists of only seven pigment molecules and its chemistry has been well characterized.

"To observe the quantum beats, 2-D spectra were taken at 33 population times, ranging from 0 to 660 femtoseconds," said Engel. "In these spectra, the lowest-energy exciton (a bound electron-hole pair formed when an incoming photon boosts an electron out of the valence energy band into the conduction band) gives rise to a diagonal peak near 825 nanometers that clearly oscillates. The associated cross-peak amplitude also appears to oscillate. Surprisingly, this quantum beating lasted the entire 660 femtoseconds."

Engel said the duration of the quantum beating signals was unexpected because the general scientific assumption had been that the electronic coherences responsible for such oscillations are rapidly destroyed.

"For this reason, the transfer of electronic coherence between excitons during relaxation has usually been ignored," Engel said. "By demonstrating that the energy transfer process does involve electronic coherence and that this coherence is much stronger than we would ever have expected, we have shown that the process can be much more efficient than the classical view could explain. However, we still don’t know to what degree photosynthesis benefits from these quantum effects."

Engel said one of the next steps for the Fleming group in this line of research will be to look at the effects of temperature changes on the photosynthetic energy transfer process. The results for this latest paper in Nature were obtained from FMO complexes kept at 77 Kelvin. The group will also be looking at broader bandwidths of energy using different colors of light pulses to map out everything that is going on, not just energy transfer. Ultimately, the idea is to gain a much better understanding how Nature not only transfers energy from one molecular system to another, but is also able to convert it into useful forms.

"Nature has had about 2.7 billion years to perfect photosynthesis, so there are huge lessons that remain for us to learn,” Engel said. “The results we’re reporting in this latest paper, however, at least give us a new way to think about the design of future artificial photosynthesis systems."

Source: Lawrence Berkeley National Laboratory

The silver lining revealed

Philip Ball

Clouds have unseen portions that stretch for many kilometres.

Clouds are bigger than they look, according to new measurements by atmospheric scientists in Israel and the United States. They say that clouds are surrounded by a 'twilight zone' of diffuse particles, invisible to the naked eye, extending for tens of kilometres around the cloud's visible portion.

These vast, sparse haloes of droplets may have been overlooked in atmospheric studies, the researchers say. And they think that this could have skewed attempts to understand how clouds influence climate.

Clouds are one of the biggest sources of uncertainty in efforts to measure and predict global warming. They have two opposite effects: increasing warming by absorbing heat radiated from the planet's surface (which is why cloudy nights are warmer), while offsetting this by reflecting sunlight back into space from cloud tops.

Most atmospheric scientists now think that clouds have an overall global cooling effect. Measurements of warming trends therefore have to take into account whether the skies are cloudy or not, and model forecasts of future warming may hinge on whether they predict more or less cloudiness.Read on...

Cloudy distinction

Such modelling studies typically try to distinguish between cloudy and cloud-free regions of the atmosphere. But the new results show that this distinction is less clear-cut than has been thought, say Ilan Koren of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues, who publish their discovery in Geophysical Research Letters1.

Right now there is a discrepancy between what global models predict for aerosol effects and what satellites measure.

Lorraine Remer
NASA Goddard Space Flight Center

Clouds are formed when floating solid particles called aerosols — dust, for example — act as 'seeds' on which water droplets grow. Aerosols reflect light, and do so more strongly as they grow by accumulating water. The large droplets in clouds reflect most visible light, which is what makes clouds look white and opaque.

Koren and his colleagues first demonstrated that it is relatively easy to see from digital photographs that clouds are surrounded by an invisible haze, made up of these water-coated, or humidified, aerosols. If the parts of the photo containing visible white stuff are masked out, the surrounding haze comes into view.

This haze extends far further than anyone has ever imagined. "People may have seen these extended haloes anecdotally," says Koren's colleague Lorraine Remer of the NASA Goddard Space Flight Center in Greenbelt, Maryland. "But thanks to a new generation of instruments, the satellite observations have got much better, and we can look on larger scales, with more sensitivity and at finer resolution."

Satellite images of clouds over the Atlantic Ocean show that the sky's reflectance — a measure of how much humidified aerosol it contains — falls very gradually with increasing distance from the edge of a cloud, and is still declining at least 20-30 kilometres away, Koren's team says.

Into the twilight zone

To study these twilight zones further, the researchers studied several years' worth of images collected by a global network of ground-based lightmeters called AERONET, usually used to ethmonitor the brightness of the Sun.

Sudden dips in the light detected by these instruments are automatically logged as indicating the passage of a cloud. Koren and colleagues discovered that it can take well over an hour for light levels to recover fully after a cloud has passed, indicating that their haloes are very broad.

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Not all clouds will have a big twilight zone, the researchers say. For example, the halo might be tightly reined in around the sharp-edged white cumulus clouds that form when moist, warm air rises and cools. But they estimate that for typical global cloud coverage, the halo could encompass as much as two-thirds of the sky usually classed as cloud-free.

Remer says that some climate models might already include these extended cloud haloes — they should 'grow' them automatically if they do a good job of capturing the humidity variations of the air. But other, simpler, models might neglect the effect.

As a result, Remer suspects that the overall cooling effect of aerosols may have been underestimated. But she admits that it is too early to say whether that is really the case, or how significant an impact it might have on climate predictions.

"Right now there is a discrepancy between what global models predict for aerosol effects and what satellites measure," she says. "This might be part of the reason for that."

Door to another universe

* 20:42 27 April 2007
* NewScientist.com news service
* David Shiga

The objects scientists think are black holes could instead be wormholes leading to other universes, a new study says. If so, it would help resolve a quantum conundrum known as the black hole information paradox, but critics say it would also raise new problems, such as how the wormholes would form in the first place.

A black hole is an object with such a powerful gravitational field that nothing, not even light, can escape it if it strays within a boundary known as the event horizon. Einstein's theory of general relativity says black holes should form whenever matter is squeezed into a small enough space.

Though black holes are not seen directly, astronomers have identified many objects that appear to be black holes based on observations of how matter swirls around them.

But physicists Thibault Damour of the Institut des Hautes Etudes Scientifiques in Bures-sur-Yvette, France, and Sergey Solodukhin of International University Bremen in Germany now say that these objects could be structures called wormholes instead.

Wormholes are warps in the fabric of space-time that connect one place to another. If you imagine the universe as a two-dimensional sheet, you can picture a wormhole as a "throat" connecting our sheet to another one. In this scenario, the other sheet could be a universe of its own, with its own stars, galaxies and planets.

Damour and Solodukhin studied what such a wormhole might look like, and were surprised to discover that it would mimic a black hole so well that it would be virtually impossible to tell the difference.
Hawking radiation

Matter would swirl around a wormhole in the same way as for a black hole, since both objects distort the space around them in the same way.

One might hope to distinguish the two by something called Hawking radiation, an emission of particles and light which should only come from black holes and would have a characteristic energy spectrum. But this radiation is so weak that it would be completely swamped by other sources, such as the background glow of microwaves left over from the big bang, making it unobservable in practice.

Another difference one might hope to exploit is that unlike black holes, wormholes have no event horizon. This means that things could go in a wormhole and come back out again. In fact, theorists say one variety of wormhole wraps back onto itself, so that it leads not to another universe, but back to its own entrance.
Daring plunge

But this does not provide a foolproof test either. Depending on the detailed shape of the wormhole, it could take billions of years or more for things to pop back out after falling in. With the right shape, even the oldest wormholes in our universe would not have had time to spit anything back out yet.

It seems the only way to decide the issue for sure with astronomical black holes is to make a daring plunge inside. That would be a dangerous gamble, because if it is a black hole, the incredibly strong gravitational field inside would tear apart every atom in your body. Even if it turns out to be a wormhole, the forces inside could still be deadly.

Assuming you could survive, and the wormhole was not symmetric, you might find yourself in another universe on the other side. Without further intervention, the wormhole would tend to suck you back in and carry you back to the opening in your universe.
Yo-yo motion

"The spaceship would do this yo-yo motion," Damour told New Scientist. "[But] if you use your fuel, then you can escape from the attracting power of the wormhole and explore" the space on the other side, he says.

But a friend in either universe might have to wait billions of years to hear back from you, since the transit time could be excruciatingly long.

Such a delay would make meaningful communication with anyone on the other side impossible. But the delay gets smaller with smaller wormholes. If a microscopic wormhole could be found or constructed, the delay across it could be as short as a few seconds, Solodukhin says, potentially making two-way communication possible.

Stephen Hsu of the University of Oregon in Eugene, US, who has studied the formation of black holes and the properties of wormholes, says he agrees that distinguishing between the two types of object with observations is practically impossible, at least with current technology.
Exotic matter

"The most important property of a black hole – that there is a 'point of no return' for an object falling in – is not something we can test at the moment," he told New Scientist.

Still, he says the objects out there suspected to be black holes probably really are black holes rather than wormholes. There are plausible scenarios for forming black holes, he says, such as the collapse of a massive star, but it is not clear how you would form a wormhole.

"Wormholes that might be confused with a macroscopic black hole require some kind of exotic matter to stabilise them, and it is not known whether such exotic matter exists," he says.

Solodukhin says that a wormhole might form in much the same way that black holes form, such as from a collapsing star. Physicists normally expect in these situations that a black hole would be produced, but Solodukhin says that quantum effects may stop the collapse just short of producing a black hole, leading to a wormhole instead.
Microscopic black holes

He says this mechanism might be inevitable in a more complete picture of physics that unites gravity and quantum mechanics – a longstanding goal of physics. If he is right, then wherever we used to expect black holes to form, wormholes would form instead.

And there might be a way to test the conjecture. Some physicists say that future particle accelerator experiments could produce microscopic black holes (see Atom smasher may give birth to 'Black Saturns').

Such tiny black holes would emit measurable amounts of Hawking radiation, proving that they are black holes rather than wormholes. But if Solodukhin is right, and microscopic wormholes are formed instead, no such radiation would be expected. "In that case, you would actually see if it is a black hole or a wormhole," he says.

An added benefit of wormholes is that they could resolve the so-called black hole information paradox. The only way anything can exit a black hole is in the form of Hawking radiation, but it is not clear how the radiation carries information about the original object that was swallowed. This scrambling effect conflicts with quantum mechanics, which forbids such erasing of information (see Black holes: The ultimate quantum computers?).

"Theoretically, wormholes are much better than black holes because all these problems with information loss don't exist in this case," Solodukhin says. Since wormholes have no event horizons, things are free to leave without first being converted into Hawking radiation, so there is no problem with lost information.

Physicists bid farewell to reality

Quantum mechanics just got even stranger.
In the quantum world, it is meaningless to imagine which cars are which colours, or what might happen if you step into a busy road.
Getty
There's only one way to describe the experiment performed by physicist Anton Zeilinger and his colleagues: it's unreal, dude.
Measuring the quantum properties of pairs of light particles (photons) pumped out by a laser has convinced Zeilinger that "we have to give up the idea of realism to a far greater extent than most physicists believe today."
By realism, he means the idea that objects have specific features and properties —that a ball is red, that a book contains the works of Shakespeare, or that an electron has a particular spin.
For everyday objects, such realism isn't a problem. But for objects governed by thelaws of quantum mechanics, like photons and electrons, it may make no sense to think of them as having well defined characteristics. Instead, what we see may depend on how we look.
This notion has been around ever since the advent of quantum mechanics in the early twentieth century. The theory seemed to show that, in the quantum world, objects are defined only fuzzily, so that all we can do is work out the probability that they have particular characteristics — such as being located in a specific place or having a specific energy.
Allied to this assault on reality was the apparent prediction of what Albert Einstein, one of the chief architects of quantum theory, called 'spooky action at a distance'. Quantum theory suggests that disturbing one particle can instantaneously determine the properties of a particle with which it is 'entangled', no matter how far away it is. This would violate the usual rule of locality: that local behaviour is governed by local events.
We have a little more evidence that the world is really strange.
Anton Zeilinger
University of Vienna
Einstein could not believe that the world was really so indeterminate. He supposed that a deeper level of reality had yet to be uncovered — so-called 'hidden variables' that specified an object's properties precisely and in strictly local terms.
Failed test
In the 1960s the Irish physicist John Bell showed how to put locality and realism to the test. He deduced that if both ideas applied to the quantum world, then two particular quantities calculated from measurements made on a pair of entangled photons would be equal to one another. If so, there would be nothing 'spooky' about entanglement after all.
Experiments were done to test his prediction in the ensuing two decades, and results showed that Bell's equality was violated. Thus, either realism or locality, or possibly both of these ideas, do not apply in the quantum world.
But which is it? That's what Zeilinger, based at the University of Vienna in Austria, and his colleagues tried to find out.
They came up with a similar test to Bell's, to see whether quantum mechanics obeys realism but not locality. Again the experiment involves comparing two quantities calculated from measurements on entangled photons, to see if they are equal. But whereas in Bell's test these quantities are derived from the so-called 'linear' polarization of the photons — crudely, whether their electromagnetic fields oscillate in one direction or the other — Zeilinger's experiment looks at a different sort of polarization, called elliptical polarization.
Like Bell's, Zeilinger's equality proved false. This doesn't rule out all possible non-local realistic models, but it does exclude an important subset of them. Specifically, it shows that if you have a group of photons that all have independent polarizations, then you can't ascribe specific polarizations to each. It's rather like saying that you know there are particular numbers of blue, white and silver cars in a car park — but it is meaningless even to imagine saying which ones are which.
Truly weird
If the quantum world is not realistic in this sense, then how does it behave? Zeilinger says that some of the alternative non-realist possibilities are truly weird. For example, it may make no sense to imagine what would happen if we had made a different measurement from the one we chose to make. "We do this all the time in daily life," says Zeilinger — for example, imagining what would have happened if you had tried to cross the road when a truck was coming. If the world around us behaved in the same way as a quantum system, then it would be meaningless even to imagine that alternative situation, because there would be no way of defining what you mean by the road, the truck, or even you.
Another possibility is that in a non-realistic quantum world present actions can affect the past, as though choosing to read a letter or not could determine what it says.
Zeilinger hopes that his work will stimulate others to test such possibilities. "Our paper is not the end of the road," he says. "But we have a little more evidence that the world is really strange."

Change The Past

Scientific American: A Do-It-Yourself Quantum Eraser

A Do-It-Yourself Quantum Eraser

Using readily available equipment, you can carry out a home experiment that illustrates one of the weirdest effects in quantum mechanics

By Rachel Hillmer and Paul Kwiat

Notoriously, the theory of quantum mechanics reveals a fundamental weirdness in the way the world works. Commonsense notions at the very heart of our everyday perceptions of reality turn out to be violated: contradictory alternatives can coexist, such as an object following two different paths at the same time; objects do not simultaneously have precise positions and velocities; and the properties of objects and events we observe can be subject to an ineradicable randomness that has nothing to do with the imperfection of our tools or our eyesight.

Gone is the reliable world in which atoms and other particles travel around like well-behaved billiard balls on the green baize of reality. Instead they behave (sometimes) like waves, becoming dispersed over a region and capable of crisscrossing to form interference patterns.

Yet all this strangeness still seems remote from ordinary life. Quantum effects are most evident when tiny systems are involved, such as electrons held within the confines of an atom. You might know in the abstract that quantum phenomena underlie most modern technologies and that various quantum oddities can be demonstrated in laboratories, but the only way to see them in the home is on science shows on television. Right? Not quite.

We will show you how to set up an experiment that illustrates what is known as quantum erasure. This effect involves one of the oddest features of quantum mechanics--the ability to take actions that change our basic interpretation of what happened in past events.

Before we explain what we mean by that and outline the experiment itself, we do have to emphasize one caveat in the interest of truth in advertising. The light patterns that you will see if you conduct the experiment successfully can be accounted for by considering the light to be a classical wave, with no quantum mechanics involved. So in that respect the experiment is a cheat and falls short of fully demonstrating the quantum nature of the effect.

Nevertheless, the individual photons that make up the light wave are indeed doing the full quantum dance with all its weirdness intact, although you could only truly prove that by sending the photons through the apparatus and detecting them one at a time. Such a procedure, unfortunately, remains beyond the average home experimenter. Still, by observing the patterns in your experiment and by thinking about what they mean in terms of the individual photons, you can get a firsthand glimpse of the bizarre quantum world.

If you want to go straight to the home experiment, it is detailed in the sidebar. The discussion that follows here delves into the science of quantum erasers in general. This explanation will help you understand what the do-it-yourself eraser demonstrates, but you might want to come back to it after seeing what that specific kind of eraser does.

What a Quantum Eraser Erases
One of the strange features of quantum mechanics is that the behavior that something exhibits can depend on what we try to find out about it. Thus, an electron can behave like a particle or like a wave, depending on which experimental setup we subject it to. For example, in some situations particlelike behavior emerges if we ascertain the specific trajectory that an electron has followed and wavelike behavior transpires if we do not.

A standard demonstration of this duality relies on what is called a two-slit experiment (your do-it-yourself quantum eraser is similar to this experiment in that it involves two pathways, but not two slits). A source emits particles, such as electrons, toward a screen that has two slits they can pass through. The particles ultimately arrive at a second screen where each one produces a spot. Where each particle lands is to some extent random and unpredictable, but as thousands of them accumulate, the spots build up into a definite, predictable pattern. When the conditions are right for the particles to behave as waves, the result is an interference pattern--in this case a series of fuzzy bars, called fringes, where most of the particles land, with very few hitting the gaps between them.
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The particles will generate the interference pattern only if each particle could have traveled through either of the two slits, and there is no way of ascertaining which slit each one passed through. The two pathways are then said to be indistinguishable and each particle acts as if it actually traveled through both slits. According to the modern understanding of quantum mechanics, interference occurs when indistinguishable alternatives are combined in this way.

When two or more alternatives coexist, the situation is called a superposition. Erwin Schrödinger highlighted the oddity of quantum superpositions in 1935, when he proposed his now infamous concept of a cat that is simultaneously alive and dead, sealed inside a hermetic box where it cannot be observed. When quantum interference happens, something in the experiment is like a kind of Schrödinger's cat. But instead of being alive and dead at the same time, the cat may be walking by a tree, passing on both sides of it simultaneously.

Schrödinger's cat ceases to be in a superposition as soon as we look inside its box: we always see it to be either alive or dead, not both (although some interpretations of quantum mechanics have it that we become in a superposition of having seen a dead or a live cat). If a spotlight is shining near the tree, we see the quantum cat go one way or the other. Similarly, we can add a measurement tool to watch each particle as it passes the slits. One could imagine having a light shining on the slits so that as each particle comes through we can see a flash of light scatter from where the particle went. The flash makes the two alternative pathways distinguishable, which destroys the superposition, and the particles arrive at the final screen not in a pattern of fringes but in one featureless blob. Experiments analogous to this scenario have been conducted, and, as predicted by quantum mechanics, no interference pattern builds up.

We need not actually "do the looking." We do not have to detect the light flashes and ascertain which way each particle went. It suffices that the information is available in the flashes and could have been observed in that way.

Now we finally get to the quantum eraser. The eraser is something that can erase the information indicating which path each particle has followed, thereby restoring the indistinguishability of the alternatives and restoring interference.

How might an eraser do that? Imagine that the "flash of light" that scatters from each particle is a single photon. For the photon to reveal the "which path?" information of the particle, it must be possible (even if only in principle) to tell which slit the photon came from. That means we must be able to measure the position of where each photon scattered accurately enough to tell the slits apart. Heisenberg's uncertainty principle, however, tells us that if we instead measure the momentum of each photon with great accuracy, then the photons' positions become less well defined. So if we pass the photons through a lens that makes their momentum information available, the information about their positions is erased. When that happens, the two paths the particles can follow are again indistinguishable and interference is restored.

We have omitted one last tricky detail, but we will come back to that. First, stop and think a bit more about what is happening in the erasing process we just described, because that is where the weirdness lies. When we detect the position where one of the photons scattered, we learn which slit its corresponding particle went through, which means the particle did go through one slit or the other, not both. If we instead detect the photon's momentum, however, we cannot know which slit the particle went through. What is more, when we do many momentum measurements and see an interference pattern, we infer that in those cases the particles went through both slits (interference would be impossible otherwise).

In other words, the answer to the question, "Did the particle go through one slit or both slits?" depends on what we do with its corresponding photon long after the particle has gone through. It is almost as if our actions with the photons influence what has happened in past events. We can find out which slit the particle went through, or with our quantum eraser we can delete that information from the universe.

Strangest of all, we can decide which measurement to make after the particle has passed through the slits--we can have the apparatus for both alternative measurements in place, with a switch that we flick one way or the other just before each photon arrives. Physicists call this variation a delayed-choice experiment, an idea introduced by John A. Wheeler of the University of Texas at Austin in 1978 that extends a scenario that Niels Bohr and Albert Einstein used in their arguments about quantum mechanics and the nature of reality in 1935.

At this point, some particularly clever readers will be worrying about a fundamental problem that seems to undermine what we have just described: Why can't we delay the choice of our photon measurement until after we have seen if the particles form an interference pattern? We could, in fact, arrange to do just that by having the final screen not too far from the slits and the photon detector much farther away. So what would happen if we saw the particles form fringes but then chose to do photon position measurements that should prevent such fringes from forming? Wouldn't we have created a paradox? Surely we would not expect the already registered interference pattern to vanish! Similar reasoning suggests we could use the delayed-choice effect to transmit messages instantaneously over arbitrary distances (thereby circumventing the speed of light).

That tricky detail that we omitted earlier is what saves the day: to see the interference of the particles after applying the quantum eraser, we first have to divide them into two groups and observe the groups separately. One group will display the original pattern of fringes; the other will display the inverse of that pattern, with particles landing on what were originally the dark bands and avoiding the places where the bright fringes were. The two groups combined fill in all the gaps, hiding the interference.

The paradox is avoided because we need data from the photon measurement to know which group each particle belongs to. Thus, we cannot observe the fringes until after we have done the photon measurements, because only then do we know how to split the particles into groups. In the home experiment, dividing particles into groups is done for you automatically because one group gets blocked by a polarizing filter, and you can therefore see the interference pattern of the group that gets through with your own eyes. In the final step you can see the interference patterns of the two groups right next to each other.

From a practical standpoint, the inability to send messages faster than the speed of light and create a paradox is perhaps disappointing, but physicists and logicians consider it to be a very good feature.

So it goes

All time is all time. It does not change. It does not lend itself to warnings or explanations. It simply is. Take it moment by moment, and you will find that we are all, as I've said before, bugs in amber. -KV
By Arthur Salm
UNION-TRIBUNE BOOKS EDITOR
April 15, 2007

Listen: Kurt Vonnegut has come unstuck in time.

The American novelist, short-story writer, essayist and would-be curmudgeon died Wednesday at 84 – the age at which he dispatched his fictional alter ego, Kilgore Trout – several weeks after suffering head trauma from a fall.

Fans will recognize the first line, above, as a variation on a line from “Slaughterhouse-Five,” Vonnegut's masterpiece. His protagonist, Billy Pilgrim – “unstuck in time” – ricochets from childhood to old age and back again, from his experiences as a POW in the Second World War to a humdrum life as an optometrist to animals-in-a-zoo-like captivity on the planet Tralfamador with the adult film star Montana Wildhack.

The semi-autobiographical novel opens with “All this happened, more or less,” and it did – the part about being a prisoner of war, at least. Vonnegut survived the firebombing of Dresden, and the unspeakable carnage he witnessed in its aftermath imbued him with a horror of war in all its forms and heartfelt contempt for those who wage it.

It's comforting to imagine the rumpled, latter-day Mark Twain (to whom he has been compared, to neither's detriment) zipping forever, Billy Pilgrim-like, among various stations of his eventful and productive life.

Right now – whatever, in Vonnegut's way of thinking, that means – he could be a young man toiling away as an advertising copywriter; taking in three of his sister's children after her and her husband's tragic deaths; typing a second draft of his breakthrough novel “Cat's Cradle”; absent-mindedly leaving one of his still-smoldering unfiltered Pall Malls in an ashtray, burning down half his East Side Manhattan brownstone, consuming most of his archives (he almost died from smoke inhalation); or – fingers crossed! – frolicking with an adult film star in a pressurized dome, if not on Tralfamador, then maybe on the asteroid 25399 Vonnegut, named in his honor.

Or appearing on “The Daily Show” in September 2005. Speaking on humans' place in the evolutionary scheme of things, he told (is telling?) host Jon Stewart, “Our planet's immune system is trying to get rid of us, and it should.”

But it was always clear, from the gentle nature of his fiction to the warmth that radiated from the cracks and creases in his crusty person and persona, that Vonnegut could never qualify as a misanthrope. His life was not without its tragedies – in addition to the untimely death of his sister, his mother committed suicide – but the bitterness of his later years, unlike Twain's, was not all-consuming.

Though a pessimist to his core, he must have harbored some hope for humanity, as he served for a time as the president of the American Humanist Association. Humanism, he once explained, “is trying to behave decently without expectation of rewards or punishment after you are dead.”

And behave decently, and honorably, he did, despite his self-effacement about things such as his Purple Heart (“a ludicrously negligible wound”) and his AHA presidency (“that totally functionless capacity”). And write well – make that, brilliantly – he did, too, from the early, straight science fiction of the 1950s (“Player Piano,” “The Sirens of Titan”) to the high-watermark novels of the '60s like “Cat's Cradle,” “God Bless You, Mr. Rosewater” and “Slaughterhouse-Five,” to his lesser (by comparison, remember) fiction beginning with 1973's “Breakfast of Champions.”

All this happened.

Or maybe, is happening.

Unified Theory -FINALLY, Somebody gets it!

Wired for sound

: How the brain senses visual illusions

Wired for sound: How the brain senses visual illusions
In a study that could help reveal how illusions are produced in the brain's visual cortex, researchers at the UCSD School of Medicine have found new evidence of rapid integration of auditory and visual sensations in the brain. Their findings, which provide new insight into neural mechanisms by which visual perception can be altered by concurrent auditory events, will be published online in the April 12 edition of the Journal of Neuroscience.
When subjects were shown a single flash of light interposed between two brief sounds, many subjects reported seeing two distinct flashes of light. Investigating the timing and location of the brain processes that underlie this illusory effect – the illusion of seeing two flashes in the presence of two auditory signals, when only one flash actually occurs – can reveal how information from different senses are integrated in the brain.

The study of 34 subjects was carried out in the laboratory of Steven A. Hillyard, Ph.D., UCSD professor of neurosciences. "This type of perceptual illusion has been described before," said first author Jyoti Mishra, graduate student in the Hillyard lab. "The surprising finding we made is that the illusion depends on a rapidly timed sequence of interactions between the auditory and visual cortical areas."

"This is part of a set of new findings by scientists in the field that show how integration of multiple sensations can happen much more rapidly than we thought before," said Mishra. "We show physiological evidence that visual and auditory stimulation might not be processed separately, then merged together, as previously assumed, but that an almost-simultaneous integration of the sensations may actually take place in the brain."

The UCSD scientists measured event-related potentials (ERPs), brain responses that are directly related to the perceptual experiences induced by sensory stimuli, using an electrophysiological or EEG recording procedure that measures electrical activity of the brain through the skull.

"In subjects who reported seeing a second flash, the ERP measurements showed a boost of activity within the visual cortex of the brain immediately after hearing the second sound," said Mishra, adding that the second sound amplified the brain activity stimulated by the first sound. Perception of the second illusory flash was also marked by a rapid enhancement of processing in the auditory cortex of the brain. By observing the auditory boost, the researchers could predict when subjects would report seeing the visual illusion of a second flash.

"Our results provide evidence that perception of the illusory second flash is based on a very rapid and dynamic interplay between the auditory and visual cortices of the brain – on a time scale less than one tenth the blink of an eye." Mishra said. Interestingly, the pattern was very different between individuals who did or didn't see the second flash, indicating that the brain's wiring and the strength of integration between the different sensory cortices may differ between individuals, or even vary over time. "It suggests that there are consistent differences in the neural connectivity that are possibly shaped during one's development and through experience," she said.

Next, the researchers plan to look at whether or not attention affects these illusory sensations. These studies could shed light on how people deprived of one sensation often compensate by developing another – for instance, blind people with a more acute sense of hearing.

Source: University of California - San Diego

Remotely Activated Nanoparticles Destroy Cancer

Targeted nanotech-based treatments will enter clinical trials in 2007.
By Kevin Bullis
The first in a new generation of nanotechnology-based cancer treatments will likely begin clinical trials in 2007, and if the promise of animal trials carries through to human trials, these treatments will transform cancer therapy. By replacing surgery and conventional chemotherapy with noninvasive treatments targeted at cancerous tumors, this nanotech approach could reduce or eliminate side effects by avoiding damage to healthy tissue. It could also make it possible to destroy tumors that are inoperable or won't respond to current treatment.

One of these new approaches places gold-coated nanoparticles, called nanoshells, inside tumors and then heats them with infrared light until the cancer cells die. Because the nanoparticles also scatter light, they could be used to image tumors as well. Mauro Ferrari, a leader in the field of nanomedicine and professor of bioengineering at the University of Texas Health Science Center, says this is "very exciting" technology.
"With chemotherapy," Ferrari says, "we carpet bomb the patient, hoping to hit the lesions, the little foci of disease. To be able to shine the light only where you want this thing to heat up is a great advantage."
Although several groups are now working on similar localized treatments, Naomi Halas and Jennifer West have led the way in this area, and their work is the farthest along. (See "Nano Weapons Join the Fight Against Cancer.") Nearly ten years ago, Halas, professor of chemistry and electrical and computer engineering at Rice University, developed a precise and reliable method for making nanoshells, which can be hollow spheres of gold or, in the case of the cancer treatment, gold-coated glass spheres. These spheres are small enough (about 100 nanometers in diameter) to slip through gaps in blood vessels that feed tumors. So as they circulate in the bloodstream, they gradually accumulate at tumor sites.
Halas tuned the nanoparticles to absorb specific wavelengths of light by changing the thickness of the glass and gold. For the cancer treatment, she selected infrared wavelengths that pass easily through biological tissues without causing damage. To destroy a nanoshell-infiltrated tumor, the tumor is illuminated with a laser, either through the skin or via an optical fiber for areas such as the lungs.
"We shine light through the skin, and in just a few minutes, the tumor is heated up," Halas says. "In the studies that were initially reported--and this has been repeated now more than 20 times in at least three different animal models--we have seen essentially 100 percent tumor remission." The tests also suggest the nanoshells are nontoxic. Halas says they are eliminated from the body through the liver over several weeks. The technology was developed at Rice in collaboration with Jennifer West, a professor of bioengineering. It has been licensed by Nanospectra Biosciences, a startup based in Houston, TX, that is beginning the process of getting FDA approval for clinical trials for treating head and neck cancer. In the future, the technology could be used for a wide variety of cancers.
"There is a potential for this to bring a profound change in cancer treatment," Halas says. "For the case of someone discovering a lump in their breast, this would mean that a very simple procedure could be performed that would induce remission." She says that "for many, many cases of cancer, rather than the lengthy chemotherapy or radiation therapy," an individual would have "one simple treatment and very little side effects."
Halas anticipates that approval for the method will come quickly, in part because the nanotechnology is not a drug but a device, for which the approval process is simpler. Also, she expects it will perform the same in humans as in animal models, "because heat and light work in exactly the same way whether you're in a pig, a dog, [or] a human being."
Since their initial experiments, the researchers have been further developing the technology. They've demonstrated the ability to coat the nanoshells with antibodies that latch on to breast-cancer cells, further improving the selectivity of the treatment. They've also attached molecules that make the nanoshells into pH sensors that would be useful for both imaging tumors and as an "optical biopsy" for identifying cancers, Halas says.

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