What on earth and off earth is dark energy?

TL:DR – A reprint of a feature article of mine on Dark Energy that was published in StarDate magazin in July 2007.


Type 1a Supernova Credit: NASA/Swift/S. Immler)Forget the Large Hadron Collider (LHC), with its alleged ability to create earth-sucking microscopic black holes, its forthcoming efforts to simulate conditions a trillionth of a second after the Big Bang 100 metres beneath the Swiss countryside. There is a far bigger puzzle facing science that the LHC cannot answer: What is the mysterious energy that seems to be accelerating ancient supernovae at the farthest reaches of the universe?

In the late 1990s, the universe changed. The sums suddenly did not add up. Observations of the remnants of stars that exploded billions of years ago, Type Ia supernovae, showed that not only are they getting further away as the universe expands but they are moving faster and faster. It is as if a mysterious invisible force works against gravity and pervades the cosmos accelerating the expansion of the universe. This force has become known as dark energy and although it apparently fills the universe, scientists have absolutely no idea what it is or where it comes from, several big research teams around the globe are working with astronomical technology that could help them find an answer.

Until type Ia supernovae appeared on the cosmological scene, scientists thought that the expansion of the universe following the Big Bang was slowing down. Type Ia supernovae are very distant objects, which means their light has taken billions of years to reach us. But, their brightness could be measured to a high degree of accuracy that they provide astronomers with a standard beacon with which the vast emptiness of space could be illuminated, figuratively speaking.

The supernovae data, obtained by the High-Z SN Search team and the Supernova Cosmology Project, rooted in Lawrence Berkeley National Laboratory, suggested that not only is the universe expanding, but that this expansion is accelerating. to make On the basis of the Type Ia supernovae, the rate of acceleration of expansion suggests that dark energy comprises around 73% the total energy of the universe, with dark matter representing 24% of the energy and all the planets, stars, galaxies, black holes, etc containing a mere 4%.

HETDEX, TEX STYLE

Professor Karl Gebhardt and Senior Research Scientists Dr Gary Hill and Dr Phillip McQueen and their colleagues running the Hobby Eberly Telescope Dark Energy Experiment (HETDEX) based at the McDonald Observatory in Texas are among the pioneers hoping to reveal the source and nature of dark energy. Those ancient supernovae are at a “look-back time” of 9 billion years, just two-thirds the universe’s age. HETDEX will look back much further to 10 -12 billion years.

HETDEX DomeHETDEX will not be looking for dark energy itself but its effects on how matter is distributed. “In the very early Universe, matter was spread out in peaks and troughs, like ripples on a pond, galaxies that later formed inherited that pattern,” Gebhardt explains. A detailed 3D map of the galaxies should reveal the pattern. “HETDEX uses the characteristic pattern of ripples as a fixed ruler that expands with the universe,” explains Senior Research Scientist Gary Hill. Measuring the distribution of galaxies uses this ruler to map out the positions of the galaxies, but this needs a lot of telescope time and a powerful new instrument. “Essentially we are just making a very big map [across some 15 billion cubic light years] of where the galaxies are and then analyzing that map to reveal the characteristic patterns,” Hill adds.

“We’ve designed an upgrade that allows the HET to observe 30 times more sky at a time than it is currently able to do,” he says. HETDEX will produce much clearer images and work much better than previous instruments, says McQueen. Such a large field of view needs technology that can analyze the light from those distant galaxies very precisely. There will be 145 such detectors, known as spectrographs, which will simultaneously gather the light from tens of thousands of fibers. “When light from a galaxy falls on one of the fibers its position and distance are measured very accurately,” adds Hill.

The team has dubbed the suite of spectrographs VIRUS. “It is a very powerful and efficient instrument for this work,” adds Hill, “but is simplified by making many copies of the simple spectrograph. This replication greatly reduces costs and risk as well.”

McQueen adds that after designing VIRUS, the team has built a prototype of one of the 145 unit spectrographs. VIRUS-P is now operational on the Observatory’s Harlan J. Smith 2.7 m telescope, he told us, “We’re delighted with its performance, and it’s given us real confidence in this part of our experiment.”

VIRUS will make observations of 10,000 galaxies every night. So, after just 100 nights VIRUS will have mapped a million galaxies. “We need a powerful telescope to undertake the DEX survey as quickly as possible,” adds McQueen. Such a map will constrain the expansion of the universe very precisely. “Since dark energy only manifests itself in the expansion of the universe, HETDEX will measure the effect of dark energy to within one percent,” Gebhardt says. The map will allow the team to determine whether the presence of dark energy across the universe has had a constant effect or whether dark energy itself evolves over time.

“If dark energy’s contribution to the expansion of the universe has changed over time, we expect HETDEX to see the change [in its observations],” adds Gebhardt, “Such a result will have profound implications for the nature of dark energy, since it will be something significantly different than what Einstein proposed.”

SLOAN RANGER

Scientific scrutiny of the original results has been so intense that most cosmologists are convinced dark energy exists. “There was a big change in our understanding around 2003-2004 as a triangle of evidence emerged,” says Bob Nichol of the University of Portsmouth, England, who is working on several projects investigating dark energy.

SDSS M51

First, the microwave background, the so-called afterglow of creation, showed that the geometry of the universe has a mathematically “flat” structure. Secondly, the data from the Type Ia supernovae measurements show that the expansion is accelerating. Thirdly, results from the Anglo-Australian 2dF redshift survey and then the Sloan Digital Sky Survey (SDSS) showed that on the large scale, the universe is lumpy with huge clusters of galaxies spread across the universe.

The SDSS carried out the biggest galaxy survey to date and confirmed gravity’s role in the expansion structures in the universe by looking at the ripples of the Big Bang across the cosmic ocean. “We are now seeing the corresponding cosmic ripples in the SDSS galaxy maps,” Daniel Eisenstein of the University of Arizona has said, “Seeing the same ripples in the early universe and the relatively nearby galaxies is smoking-gun evidence that the distribution of galaxies today grew via gravity.”

But why did an initially smooth universe become our lumpy cosmos of galaxies and galaxy clusters? An explanation of how this lumpiness arose might not only help explain the evolution of the early universe, but could shed new light on its continued evolution and its ultimate fate. SDSS project will provide new insights into the nature of dark energy’s materialistic counterpart, dark matter.

As with dark energy, dark matter is a mystery. Scientists believe it exists because without it the theories that explain our observations of how galaxies behave would not stack up. Dark matter is so important to these calculations, that a value for all the mass of the universe five times bigger than the sum of all the ordinary matter has to be added to the equations to make them work. While dark energy could explain the accelerating acceleration our expanding universe, the existence of dark matter could provide an explanation for how the lumpiness arose.

“In the early universe, the interaction between gravity and pressure caused a region of space with more ordinary matter than average to oscillate, sending out waves very much like the ripples in a pond when you throw in a pebble,” Nichol, who is part of the SDSS team, explains. “These ripples in matter grew for a million years until the universe cooled enough to freeze them in place. What we now see in the SDSS galaxy data is the imprint of these ripples billions of years later.”

Colleague Idit Zehavi now at Case Western University adds a different tone. Gravity’s signature could be likened to the resonance of a bell she suggests, “The last ring gets forever quieter and deeper in tone as the universe expands.” It is now so faint as to be detectable only by the most sensitive surveys. The SDSS has measured the tone of this last ring very accurately.”

“Comparing the measured value with that predicted by theory allows us to determine how fast the Universe is expanding,” explains Zehavi. This, as we have seen, depends on the amount of both dark matter and dark energy.

The triangle of evidence – microwave background, type Ia supernovae, and galactic large-scale structure – leads to only one possible conclusion: that there is not enough ordinary matter in the universe to make it behave in the way we observe and there is not enough normal energy to make it accelerate as it does. “The observations have forced us, unwillingly, into a corner,” says Nichol, “dark energy has to exist, but we do not yet know what it is.”

The next phase of SDSS research will be carried out by an international collaboration and sharpen the triangle still further along with the HETDEX results. “HETDEX adds greatly to the triangle of evidence for dark energy,” adds Hill, “because it measures large-scale structure at much greater look-back times between local measurements and the much older cosmic microwave background,” says Hill. As the results emerge, scientists might face the possibility that dark energy has changed over time or it may present evidence that requires modifications to the theory of gravity instead.

Wiggle-Z

The Anglo-Australian team is also undertaking its own cosmic ripple experiment, Wiggle-Z. “This program is measuring the size of ripples in the Universe when the Universe was about 7 billion years old,” Brian Schmidt at Australian National University says. Schmidt was leader of the High-Z supernovae team that found the first accelerating evidence. SDSS and 2dF covered 1-2 billion years ago and HETDEX will measure ripples at 10 billion years. “Together they provide the best possible measure of what the Universe has been doing over the past several years,” Schmidt muses.

INTERNATIONAL SURVEY

The Dark Energy Survey, another international collaboration, will make any photographer green with envy, but thankful they don’t have to carry it with them. The Fermilab team plans to build an extremely sensitive 500 Megapixel camera, with a 1 meter diameter and a 2.2 degree field of view that can grab those millions of pixels within seconds.

The camera itself will be mounted in a cage at the prime focus of the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, a southern hemisphere telescope owned and operated by the National Optical Astronomy Observatory (NOAO). This instrument, while being available to the wider astronomical community, will provide the team with the necessary power to conduct a large scale sky survey.

Over five years, DES will use almost a third of the available telescope time to carry out its wide survey. The team hopes to achieve exceptional precision in measuring the properties of dark energy using counts of galaxy clusters, supernovae, large-scale galaxy clustering, and measurements of how light from distant objects is bent by the gravity of closer objects between it and the earth. By probing dark energy using four different methods, the Dark Energy Survey will also double check for errors, according to team member Joshua Frieman.

WFMOS

Subaru 51

According to Nichol, “The discovery of dark energy is very exciting because it has rocked the whole of science to its foundations.” Nichol is part of the WFMOS (wide field multi-object spectrograph) team hoping to build an array of spectrographs for the Subaru telescopes. These spectrographs will make observations of millions of galaxies across an enormous volume of space at a distances equivalent to almost two thirds the age of the universe. “Our results will sit between the very accurate HETDEX measurements and the next generation SDSS results coming in the next five years,” he explains, “All the techniques are complimentary to one another, and will ultimately help us understand dark energy.”

DESTINY’S CHILD

If earth-based studies have begun to reveal the secrets of dark energy, then three projects vying for attention could take the experiments off-planet to get a slightly closer look. The projects all hope to look at supernovae and the large-scale spread of matter. They will be less error prone than any single technique and so provide definitive results.

SNAP, SuperNova/Acceleration Probe, is led by Saul Perlmutter of Lawrence Berkeley National Laboratory in Berkeley, California, one of the original supernova explorers. SNAP will observe light from thousands of Type Ia supernovae in the visible and infra-red regions of the spectrum as well as look at how that light is distorted by massive objects in between the supernovae and the earth.

Adept, Advanced Dark Energy Physics Telescope, is led by Charles Bennett of Johns Hopkins University in Baltimore, Maryland. This mission will also look at near-infrared light from 100 million galaxies and a thousand Type Ia supernovae. It will look for those cosmic ripples and so map out the positions of millions of galaxies. This information will allow scientists to track how the universe has changed over billions of years and the role played by dark energy.

Destiny, Dark Energy Space Telescope, led by Tod Lauer of the National Optical Astronomy Observatory, based in Tucson, Arizona, will detect and observe more than 3000 supernovae over a two-year mission and then survey a vast region of space looking at the lumpiness of the universe.

LIGHTS OUT ON DARK ENERGY

So, what is dark energy? “At this point it is pure speculation,” answers Hill, “The observations are currently too poor, so we are focusing on making the most accurate measurements possible.” Many scientists are rather embarrassed but equally excited by the thought that we understand only a tiny fraction of the universe. Understanding dark matter and dark energy is one of the most exciting quests in science. “Right now, we have no idea where it will lead, adds Hill.

Supernovae (NASA collage)

“Despite some lingering doubts, it looks like we are stuck with the accelerating universe,” says Schmidt. “The observations from supernovae, large-scale structure, and the cosmic microwave background look watertight,” he says. He too concedes that science is left guessing. The simplest solution is that dark energy was formed along with the universe. The heretical solution would mean modifying Einstein’s theory of General Relativity, which has so far been a perfect predictor of nature. “Theories abound,” Schmidt adds, “whatever the solution, it is exciting, but a very, very hard problem to solve.”

This David Bradley special feature article originally appeared on Sciencebase last summer, having been published in print in StarDate magazine – 2007-07-01-21:12:X1

Balloon Party Tricks

Balloon trick

Unlike Monday’s wind power video, this one is no joke. In fact it’s testament to the strength of balloon rubber, the force of gravity, fluid mechanics, and high-speed photography. The clip lasts about 32 seconds, but the actual action is taking place in a fraction of that time. Recorded on a Photron
ultima APX at 2000 frames per second. If you view the original high-quality clip you can use the play controller to scroll through the video slowly and observe each stage of the process at your leisure.

“I do so love it when water balloons distort themselves for our viewing pleasure. This one certainly did not disappoint in that regard. The water balloon can be seen undulating in a very odd fashion prior to its equally odd compression and explosion.”

There are a lot of similarly high-speed clips on the makers’ site at http://www.lucidmovement.com/, including a burning lightbulb, a cannonball landing in a pond, compressed air blasted into water, a scanner being dropped from a great height, a jiggling electric light filament, a gasoline (petrol) fireball, a woman running etc etc. There’s a whole category on balloon fun, you get the picture (pardon the pun).

Manes, Brains and Branes

Why the Lion Grew Its ManeI’m playing catch up, after some offline time last week (holidays, families, and illness), so today’s post is a grab-bag of the various items (mainly books) sitting in a large pile on my desk that I thought deserved a quick mention and a link or two for more information.

First up: Why the Lion Grew Its Mane – a big floppy book with a glossy cover and some wonderful nature photography. Author Lewis Smith (a science reporter for The Times (London) describes it as a miscellany of recent scientific discoveries from astronomy to zoology, and that’s pretty much what you get. Somewhat more esoteric is the cosmic book – The Origins of the Universe for Dummies – from Financial Times writer Stephen Pincock and sci-tech writer Mark Frary. Apparently, this is an easy book about a tough question, written at a time when dark energy, dark matter and the validity of the Big Bang are all offering humanity an array of new questions about the nature of reality. Although I didn’t see mention of ‘branes.

My old friend Kyriacos “KC” Nicolaou and colleague T Montagnon are next up. I have written widely about KC’s organic odyssey over the last (almost) two decades of my career as a science writer, having become fascinated by the incredible ways in which he and his team turn simple starting materials into some of the most complex natural products. In Molecules That Changed The World, Nicolaou and Montagnon provide a brief history of the art of chemical synthesis and its impact on society from aspirin and penicillin (sample PDF chapter) to the anticancer compound Taxol.

Nothing is static in the world of health, and as Brian L Syme suggests in Seasonally Fit, improving fitness and health is not just about diet and exercise, it’s about understanding the “rules of the game”. I must confess to agreeing with many of the critics of this book that it is aimed at too wide an audience but fails to hit the spot for any single group – whether health practitioners, academics, or lay people. Nevertheless, there is a nugget of an idea here – that our health is affected by the seasons – and with a decent ghost writer could become a useful book to add to the library of anyone hoping to understand their health more fully.

Also on my desk – Achieving Sustainable Mobility by Erling Holden (a scientific study of the impact of the European Commission’s 1992 motion), Darwin’s Paradox a novel by Nina Munteneanu (about an intelligent virus), Neuromatrix from Morphonix Inc (a PC game based around rogue nanobots, a kind of Lemmings for the 21st Century).

Finally, I’m thoroughly enjoying Brain Rules by John Medina in which he presents 12 principles for surviving and thriving at work, home, and school. This is not just a book, but has an interactive and augmentative website as well as an accompanying DVD to help you get the most out of your brain.

Lemon Battery

Lemon BatteryThe lemon battery, it’s a perennial kids science favourite and perfect for a rainy Saturday morning (if it’s not raining why aren’t you kids outside playing instead of surfing the Pipes on the InterWebs, huh?) Anyway, with a single lemon, a few bits of wire, a copper penny, and a zinc-galvanized nail you can generate electricity (just over one volt).

However, one lemon is not enough to light an LED or power a pocket calculator, for that you’ll need not only more voltage but a higher current, which means more power – Power (Watts) equals voltage (in Volts) multiplied by current in Amps. Four lemons produce enough power to make an LED glow dimly. But, that low current is probably not going to be enough to power your iPod, which is a higher current device. For that you will need what is called a lithium-ion battery and iPods (other mp3 players are available) usually come with such a battery built in, so there’s no need to worry about carrying a dozen lemons and a bag of nails with you for portable music.

The following video explains the ins and outs, quite literally, of making a lemon battery, it’s very methodical and shows you the precise steps needed even if the narration is a bit stiff.

More science videos from the same labs available here

Einstein Meets Hendrix

Einstein meets Hendrix

Well, not quite, but the wonderfully named Dr Mark Lewney puts on a great show not only as an axe hero extraordinaire but as a high-flying physicist who can explain why his nifty chops and runs sound the way they do. I had a quick e-chat with him the other day and we obtained permission to post his Famelab video from Channel4 on Youtube. So turn your speakers up to 11 and get ready to rock, harmonically, to the physics of heavy metal geetar!

The one thing that lets Dr Rock down is the total lack of a Justin Out of off of The Darkness jumpsuit and chest wig. Oh well, can’t have everything…

Jonny Wilkinson, Physicist Extraordinaire

Jonny Wilkinson

On this side of The Atlantic, there is growing interest this week in Jonny Wilkinson’s balls, and more to the point how he kicks them. Wilkinson’s drop goals are testament to his keen understanding of the physics of aerodynamics, fluid mechanics, and possibly even the Bernoulli effect. Perfect fodder for a physics science project.

However, it’s not all about the shape of the ball nor the swing of the leg, according to UK research published this month. The prodigious kicking success of England rugby player Johnny Wilkinson may rely more on what he does with his arms than his legs, according to a paper published in the journal Sports Biomechanics. Scientists at Bath University analysed the kicking techniques of professional and semi-professional rugby players to see which technique is most successful.

They found that players who swing their non-kicking-side arm across their chest as they make contact with the ball are the most accurate kickers, particularly over longer distances. It could be that the increased momentum produced by this arm movement helps the kicker control the amount of rotation in their bodies so that when they kick the ball their body is facing the target for longer.

Although Wilkinson’s trademark posture in lining up for the kick is well known, it is his arm movement you should watch out for in Saturday’s Rugby World Cup final, it might just signal defeat for the Springboks. Or, maybe that’s just wishful thinking on my part. Two RWC victories in a row, could it happen, could England swing it?

Nobel Prize for Physics 2007

This year’s Nobel Prize for Physics went to Albert Fert (France) and Peter Grünberg (Germany), who share the prize fifty:fifty for their discovery of giant magnetoresistance in which a very weak magnetic change gived rise to a major difference in electrical resistance of a system.

This effect underpins the technology that is used to read data on hard disks. It is thanks to their discovery that it has been possible to miniaturize hard disks so radically in recent years. Sensitive read-out heads are needed to be able to read data from the compact hard disks used in laptops and mp3 players, for instance.

You can read more details on the Nobel site here

There is Iron in Them There Bills

Have you ever wondered what it would be like to make a dollar bill smoothie? Well popular science guru Steve Spangler certainly did and with the help of a super powerful neodymium magnet he demonstrates in the video below just how much iron you would get if you were stupid enough to drink the smoothie. The iron is present in certain magnetic inks used to print a fistful of dollars.

There’s iron in them there bills…you might say!

Dollar bill video

So, now you’ve watched the video, you’re probably wondering, what are neodymium, or rare earth magnets and why is it so much stronger than a standard fridge magnet? Well, unlike conventional ferric or iron magnets, neodymium magnets are composed of iron, boron, and as the name would suggest, neodymium. The general chemical formula for this alloy is Nd2Fe14B which basically means for every 14 iron atoms in the material there are two neodymium atoms and one boron atom. That special blend (pardon the pun), however, means they can be up to about twenty times stronger than conventional ceramic magnets. Check out the HowStuffWorks site for a simplistic explanation of magnetism.

I asked magnetic expert (soon to be) Dr James Stephenson, who has probably forgotten more about magnets than I ever knew, why it is that the neodymium, or neo, magnets are so much stronger. The strength of a permanent magnet is down to how strong are the individual magnetic moments of the atoms from which it is composed and that’s down to how many electrons can be aligned in each atom, he explains.

Put simply, “Rare-earth magnets, also known as nib magnets, are stronger because the individual atomic magnetic moments are stronger and that adds up to a stronger magnetic field overall,” he says. Taken individually, an isolated atom of a rare earth element, such as neodymium, has gaps in its so-called d [electron] f-shell. When Nd is alloyed with boron and iron those gaps get filled up to a maximum of 14 electrons in the f-shell of each Nd atom, this results in a very strong dipole. “In other words,” Stephenson adds, “more electrons means more current and as a result the magnetic field due to each dipole is higher.” So, there you have it.

By the way, it’s not illegal to blend a dollar bill unless you plan on trying to spend it later, but to be on the safe side bring a friend along, not only can you make sure it’s their dollar bill you blend, but you can claim it was their idea when the FBI turn up at the door too!

Two Slits Are Better Than One

Sciencebase Exclusive – Careful experimentation and theoretical analysis of a double-slit experiment have finally quashed a controversy in fundamental physics — the complementarity-uncertainty debate.

Ever since the catflap to the quantum world was opened up to us and Schrödinger’s feline friend was idiomatically let out of the bag, to mix a metaphor or two, there have been more questions and controversies raised than conundrums solved in the world of the very, very small. How can something be both particle and wave, for instance? What allows particles of matter to tunnel through solid objects? And, how is the interference pattern destroyed in a double-slit experiment when measurements are performed on the path traversed by a particle?

What is a double slit experiment, you ask? Well, traditionally, Young’s double-slit experiment consists of shining a light through two narrow, closely spaced slits and observing the results on a screen placed beyond the slits.

Intuitively, you might think that the result would simply be two bright lines, aligned with the slits, representing where the light passes through the slits and hits the card. However, this is not seen in practice, instead, the light is diffracted by the slits and produces fringes corresponding to wave-like interference pattern. The fringes of light and dark regions correspond to where either the light waves constructively (add) and destructively (subtract) from each other. Two peaks in the light wave meet to make a brighter fringe whereas a dark fringe is formed when a peak and a trough coincide. This result seemingly settles a three-century conundrum about whether light is particle or wave, showing apparently that it is a wave.

However, a similar experiment carried out with beams of electrons or atoms fired through the slits produces a very similar interference pattern. How could that be? Particles are solid objects, surely? Well, the double-slit experiment shows that they are not. They produce an interference pattern, which suggests that the particles behave as waves.

The double-slit experiments work perfectly well and reveals interference patterns with light, electrons, and beams of other particles, but only if the experimenter does not try to find out through which slit a particular wave-particle passed before hitting the screen. Try to fire particles through the slits one at a time and as illustratd in the 5-minute video below, you will still see an interference pattern. It is as if each particle passes through both slits simultaneously, each slit individually and together and neither slit all at the same time; behaving some as waves…

As if this were not complicated enough, physicists reasoned that if they could discover which slit the individual particle really goes through each time in this experiment, they could solve the problem. So, they put a measuring device next to one slot and observed what happens as particles are fired through the slits one at a time. Astoundingly, the interference pattern disappears, simply having a measuring device present to observe the route taken by the particles somehow disturbs their wave-like nature and they revert to being tiny, solid objects and produce just two bands on the screen as if they were tiny marbles rather than wave. How could the particles know they were being watched.

This loss of interference has been explained by several of the biggest names in twentieth century physics, among them Niels Bohr and Richard Feynman. They suggested that whenever the path is measured within the double-slit, the momentum of the wave-particle is uncontrollably and irreversibly disturbed. Think about it, it has to be affected by the observer somehow because the very act of observing involves some kind of sharing of information either via photons, charge, energy or matter. This process “washes out” the interference fringes.

Most physicists simply accept this as being precisely what happens. It is a little vague and some might say “handwaving” because it does not pin down the nature of this washing out nor say anything about how the momentum is disturbed by the transaction between observer and observed. More precisely, it is simply what happens because of the back-reaction resulting from the Heisenberg uncertainty relation that says we cannot know simultaneously both the energy and position of any quantum wave or particle with absolute precision. While that kind of folds the argument into a loop, Feynman famously pointed out that, ‘No one has ever thought of a way around the uncertainty principle.’

But, not everyone was happy with this. In 1991, Marlan Scully, Berthold-Georg Englert, and Herbert Walther (Nature 1991, 351, 111) suggested that a microscopic pointer could be used to carry out the observation in such a way that the very act of observation would not disturb the momentum of the particle and so bypass the uncontrollable and irreversible effects suggested by Bohr that leads to interference breakdown. However, Pippa Storey, Sze Tan, Matthew Collett, and Daniel Walls (Nature, 1994, 367, 626), countered this argument, demonstrating that no matter how small the observer nor how the measurements are made, momentum is affected and the interference pattern would disappear. A long and controversial debate has raged between the two scientific factions that back either the Scully or Walls teams.

A theoretical solution was posited by Howard Wiseman and colleagues in 2003 (Phys Rev A, 2003, 311, 285) and refined in 2004 (J. Opt. B: Quant. Semiclass. Opt. 2004, 6, S506-S517). Now, in a seminal paper published today in the New Journal of Physics, Aephraim Steinberg together with Wiseman and colleagues Mir, Lundeen, Mitchell, and Garretson have applied the theory in a novel double-slit setup. Their experimental results suggest that, as is the way with all things quantum, both camps are equally correct and equally wrong. Somehow, you can have your quantum cake and eat it.

They found that by using only weak measurements, they can directly observe the momentum transfer that causes interference breakdown but equally do so without disturbing the two-slit superposition. They effectively verify both the Scully and Walls views. In terms of the Scully position, the team shows that there is no change in the mean momentum, or the mean energy, whereas with respect to the Walls work, they show that the momentum is spread, as one would expect given the uncertainty inherent in the quantum world, according to Heisenberg’s principle.

Feynman always held that the double-slit setup was central to quantum theory, but would never be fully understood. This work by Wiseman and colleagues shows that the humble double-slit experiment can still throw up new quantum mysteries to baffle us.

Quantum Dots and Spin Pumps

Spin pumped quantum dotIt is not so long ago, that the first thing that sprang to mind when one read the phrase ‘quantum dot’ was the idea of some rather esoteric and complicated aspect of avant garde physics. This is still partly true, there is some rather complex experimental work underway underpinned by even more complex theoretical work investigating the bizarre properties of tiny devices that can trap a single electron in zero-dimensions.

Practical applications of quantum dots have emerged recently in sensor science but US and Brazilian researchers hope to exploit them in a new kind of electronics, known as spintronics where electron charge and quantum spin add an extra dimension to electronic operations and computation. Spin currents might also be used to allow quantum communications take place “in-chip” in devices so small that light propagation is not practical. Such developments will open up quantum dots that can increase processing speed, storage capacity, and functionality of conventional electronics, communication, and computations and technologies.

Eduardo Mucciolo of the Department of Physics at the University of Central Florida, Orlando and Caio Lewenkopf of the Department of Theoretical Physics at State University of Rio de Janeiro, Brazil, are investigating lateral semiconductor quantum dots. They believe that such devices could be used as pumps to produce spin polarised currents, by exploring quantum phase coherence phenomena. The effect, called pure spin pumping, is analogous to charging a battery in conventional electronics. Such a spin pump might provide the much-needed circuit element for spin-based electronics.

Writing in the International Journal of Nanotechnology (2007, 4, 482-495), Mucciolo and Lewenkopf describe a lateral semiconductor quantum dot. In these systems, electrons within a two-dimensional gas are trapped within small puddles by the application of a voltage; applied voltages control the shape and size of these puddles. Electrodes can be used to vary the width of the point contacts between the electron puddle and the 2D gas. Controlling these point contacts allows quantum dots to be “opened” and “closed”.

Controlling these point contacts allows them to “open” and “close” the quantum dots. This effect dates back to the early 1990s, points out Mucciolo. “Closing and opening the propagation through a constriction, the point contact, can be used to detect spin-polarized currents,” he explains, “This is how Susan Watson and colleagues at Middlebury College managed to see spin currents coming out of their quantum dot pump in 2003.”

“Recently, our spin pump proposal passed its first experimental test,” say the researchers, who now hope that other teams will take up the challenge and investigate the potential of spin pump quantum dots.

“The main idea behind the spin pumping mechanism was actually published for the first time in Physical Review Letters in a paper I co-authored with Claudio Chamon (Boston University) and Charles Marcus (Harvard University),” adds Mucciolo. The main development since that earlier work presented in the current paper with Lewenkopf is that now they have carried out a much more detailed analysis to demonstrate the precise details, this was entirely missing from the PRL paper, Mucciolo told us. “In the J Nanotech paper we also develop a general formalism that could serve as a basis for the theoretical investigation of several aspects of spin pumps which, albeit important, have not yet been considered in the literature,” Mucciolo adds.