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'What a long strange trip it's been'

By Hamish Johnston

‘What a long strange trip it’s been’…The Grateful Dead’s famous lyric describes exactly how I feel after reading a paper on the arXiv preprint server about correlations between the band’s live performances and how many times its most popular songs are listened to online.

The paper is called “A Grateful Dead Analysis: The Relationship Between Concert and Listening Behavior“.

Marko Rodriguez at the Los Alamos National Laboratory in the US and colleagues have gone through over 1500 “set lists” — the names of songs played at a particular gig — from Dead concerts between 1972 and 1995 to work out the total number of times the group played individual tunes. This was possible because the band’s fans (Deadheads) are a fanatical bunch who have documented the group’s every performance and all this information is available on the Internet.

The team then compared the frequency of live performances with the frequency at which the same songs are downloaded from the website last.fm.

Their conclusion — the songs most played by the band also tend to be the most downloaded tunes, with some important exceptions the significance of which is not obvious from the paper.

You are probably wondering “what possible use is this information”?

All I can think of is this: in a strange way the Dead were ahead of their time in terms of their “business model”. They did 30 years ago what some bands are starting to do today — they gave their music away for free (by allowing fans to tape-record concerts) and made their money from touring.

I suppose it’s possible that by analysing the data lovingly archived by Deadheads, a band could come up with clever strategy to give their music away on the Internet and cash in at the box office.

Or, it could just be a joke!

Nanotube cantilever weighs up

Physicists in the US have built a device that can determine the mass of objects as light as a single gold atom.

The device works by attaching an object of interest to a “double-walled” nanotube, which consists of two concentric rolled-up carbon sheets, each just one atom thick. The object reduces the frequency at which the nanotube oscillates, with the change being proportional to the mass of the object.

The device, called a “nanomechanical mass spectrometer”, has been built by Kenny Jensen, Alex Zettl and colleagues at the University of California at Berkeley (Nature Nanotechnology doi:10.1038/nnano.2008.200).

One end of the double-walled nanotube, which is about 2 nm in diameter and some 200 nm long, is attached to a negatively-charged electrode, while the other end is free to vibrate like the tip of a diving board.

A positively-charged electrode is located near to the free end of the nanotube and electrons are able to flow from the tip of the nanotube to the positive electrode. The size of this current of electrons depends on the frequency at which the nanotube is vibrating. By measuring this current, the team is able to monitor changes in frequency and therefore tell if any tiny objects have become attached to the nanotube.

Light-weight resonator

Although this is not the first such nanomechanical device for measuring mass, most other devices are made from silicon-based materials and have resonators that are much thicker and heavier than a carbon nanotube. This means that they are much less sensitive to very small changes in mass.

The team tested their device by introducing a small number of gold atoms into the vacuum chamber in which it was held, knowing that some of them would stick to the vibrating nanotube.

Exactly where and when each individual gold atom sticks to the nanotube is a random process, which means that the frequency changes in irregular jumps. The team used statistical analysis to study these changes, from which they could calculate the mass of a single gold atom.

The analysis revealed that the gold atoms have a mass of 0.29 +- 0.05 zg, where zg is a zeptogram (10-24 kg). This is consistent with the accepted mass of 0.327 zg as measured by a conventional mass spectrometer.

‘Best-ever’ sensitivity

The team measured the sensitivity of their device to be 0.13 zg Hz-1/2, which they claim is the best ever value for a nanomechanical mass spectrometer.

Although this is nowhere near as sensitive as a conventional mass spectrometer (in which an atom or molecule is ionized and then accelerated using electric and magnetic fields) there is no need to ionize the atom or molecule of interest. It could therefore be used to study large and fragile molecules that would be destroyed in a mass spectrometer.

The team are currently working on integrating the device into a chip.

ISS looks to the future

iss%20new.jpg

By Belle Dumé

The Heads of the International Space Station (ISS) agencies from Canada, Europe, Japan, Russia and the US met at European Space Agency (ESA) Headquarters in Paris yesterday as part of the ongoing meetings to celebrate the Space Station’s 10th anniversary, which is this year. I went along to find out what they have planned for ISS from now until 2015, and perhaps beyond.

Emphasis was clearly on collaboration with a capital “C”. ISS Partners seem to be very happy with what is the world’s biggest peacetime scientific cooperation to date, which is something that they can be proud of.

The Heads said ISS will continue in its role as “test-bed” for future space exploration and research and development in space.

New modules, including Japan’s H-2 Transfer Vehicle, US Commercial Orbital Transportation Services and US Orion Crew Exploration Vehicle, together with current vehicles, US Shuttle (up to 2010), Russian Soyuz and Progress, and ESA Automated Transfer Vehicle (ATV) were discussed.

The Heads then spoke about new initiatives, such as ESA’s plan for an ATV-Advanced Return Vehicle system for down-mass from ISS and Russia-ESA joint preparatory activities on an advanced Crew Space Transportation System.

They are also considering how best to take advantage of an increased six-man crew in 2009.

ISS should be complete by 2010 but the Heads are already looking to post-ISS times — and mentioned a possible international lunar colony.

And to round off the press conference, a surprise for us journalists: a live audio link with astronauts onboard the ESA Jules Vernes ATV, which is cargo carrier, storage facility and “tug” vehicle that raises the Space Station’s orbit every so often. The astronauts had taken original manuscripts from the pioneering 19th century science fiction writer, after whom the spacecraft is named, with them into space and proudly showed these to us in a separate video.

The audio link with ESA headquarters and the Jules Verne was also a link between the past and future, and dreams and reality, said ESA Director General Jean-Jacques Dordain as he referred to Verne’s extraordinary vision in books like From the Earth to the Moon (1865) and Around the Moon (1870).

A pawn in the string wars?

By Hamish Johnston

The July 21 issue of The New Yorker landed on my doormat this morning and I tore open the wrapper knowing that it contained a profile of the controversial theoretical physicist Garrett Lisi. The profile is by the journalist Benjamin Wallace-Wells.

Lisi, who works independently and is not affiliated with a university or research institute, burst on the scene last year when he published “An Exceptionally Simple Theory of Everything” on the arXiv preprint server.

As the title suggests the paper tackles one of the big questions of physics — how to unify the Standard Model of particle physics with gravitation. However, it is anything but simple. Lisi’s paper is 31 dense pages of equations, diagrams and tables and is concerned with an 8D lattice called “E8”.

Stephen Maxfield of the University of Liverpool has just written an article on E8 in the July issue of Physics World and Matin Durrani touches on the Lisi controversy in his editorial.

Those with the mathematical expertise (and patience) to get to the bottom of Lisi’s theory have expressed very mixed views on its merit — and the work has yet to pass muster with any peer-review process.

According to The New Yorker, Lee Smolin of the Perimeter Institute described it as “one of the most compelling unification models I’ve seen in years”.

On the other hand, loop quantum gravity“, an alternative theory of everything.

Also, instead of beavering away at a reputable institute, Lisi has spent the last few years snowboarding in the Rockies and surfing in Hawaii. “One can’t deny that the particular romance of this surfer dude played a part,” Distler is quoted as saying.

So is Lisi’s story one of “a surfer in search of credibility and a movement in search of a poster boy,” as The New Yorker suggests?

NB: I’m afraid you will have to buy The New Yorker to read this particular article because it is not available online.

Football fans cheer to the science of solitons

Around lunchtime tomorrow, 50,000 fans at the Feyenoord stadium in Rotterdam, the Netherlands, will instigate two Mexican waves. But they won’t be cheering football players — it will all be in the name of physics.

The stunt is the idea of GertJan van Heijst, a physicist at Eindhoven University of Technology and fellow of the Royal Netherlands Academy of Arts and Sciences. To celebrate the 200th anniversary of the Academy, Van Heijst and his colleagues thought they would take the opportunity to stage an experiment at the Feyenoord grounds — which, coincidentally, is celebrating a 100-year anniversary.

It’s an open question whether Mexican waves are solitons GertJan van Heijst, Eindhoven University of Technology

Van Heijst told physicsworld.com that he is looking forward to doing the experiment in front of all the families who will be there for the jubilee. “We want to show the public how interesting science is,” he says.

The aim of the experiment is to explore whether Mexican waves behave in the same way as solitons, a type of wave that was first described mathematically by erstwhile Academy fellow Diederik Korteweg and his student in the late 19th century. Unusually for waves that travel in a physical medium, solitons travel at a constant speed without dispersing or changing shape. They are also unaffected when they collide with each other — and it is this property that van Heijst is looking for in human-borne Mexican waves.

“It’s an open question whether Mexican waves are solitons,” he explains.

‘Hard to predict’

Van Heijst plans to get the ball rolling on his experiment at around 1:15 pm (central European time) when he will ask fans in a small part of the terraces to momentarily stand up and throw their arms in the air. In time-honoured fashion, neighbouring fans on both sides will copy the movement so that two Mexican waves ripple clockwise and anticlockwise around the stadium.

When the waves meet at the opposite side of the stadium — which van Heijst expects will take no more than 20–seconds — cameras will monitor the interaction to see if the waves emerge unscathed.

If the Mexican waves deteriorate after their collision, van Heijst says he will repeat the experiment but this time ask the fans to look further afield and launch themselves upwards when they see a wave approach as close as, say, 10 people. This increases the “interaction length”, which — according to a mathematical model created by van Heijst and his team — must be a sufficient size for soliton characteristics to come about.

“The circumstances for such an experiment will be perfect,” says Jan van Merwijk, the Feyenoord stadium director, noting that the fans will be in high spirits. “We’ve seen a lot of waves circling through our venue but we have never experienced opposite waves. We’re very curious for the results and hope that the experiment will be a success.”

First observation

The first account of a soliton was published over 150 years ago by the UK engineer John Scott Russell. As the story goes, Scott Russell was watching a boat being drawn along a canal when the boat suddenly stopped and created a large wave some 30–m long.

We’re curious about the results and hope the experiment will be a success Jan van Merwijk, director, Feyenoord stadium

As he would later write, the wave was “a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued along the channel apparently without change of form or diminution of speed.”

But will the observation of a solitary wave in a crowd of people reveal any new physics? The answer, perhaps surprisingly, is yes. Van Heijst is eager to find out if crowds of people can behave as a continuum — in other words, if they can demonstrate macroscopic properties based on microscopic behaviour. Physicists already know, for example, that granular media like sand behave as a continuum and act as fluids on large scales.

Although Van Heijst’s stunt has been mistakenly touted to have the most people ever involved in a scientific experiment — that award probably goes to the alien-searching “SETI@home” project, which currently has some three million participants worldwide — van Heijst thinks it will still be impressive.

“It’s good to show that science is fun,” he adds.

Graphene has record-breaking strength

Graphene is the strongest material in the world, according to new experiments done by researchers at Columbia University in the US. The secret to the material’s extraordinary strength, says the team, lies in the robustness of the covalent carbon-carbon bond and the fact that the graphene monolayers tested were defect-free.

Since “wonder material” graphene – sheets of carbon just one atom thick – was discovered in 2004, it has been shown to be an extremely good electrical conductor; a semiconductor that can be used to create transistors; and a very strong material. But now, Columbia University’s James Hone, Jeffrey Kysar, Changgu Lee and Xiaoding Wei have shown that it is the strongest material ever (Science 321 385).

The researchers measured the intrinsic strength of the material — that is the maximum stress that a pristine (or defect-free) material can withstand just before all the atoms in a given cross-section are pulled apart at the same time. This was found to be 42 Nm–1 and represents the intrinsic strength of a defect-free sheet.

Essentially all materials contain defects, such as microscopic cracks or scratches, which are “weaker” than surrounding material. As a result, the breaking stress of a macroscopic material depends mainly upon the number and sizes of defects it contains, rather than its intrinsic strength.

The researchers began by exfoliating individual atomic layers of graphene from a graphite source using transparent sticky tape — the most popular way of preparing monolayer graphene. Next, they placed the graphene flakes over a series of holes on a silicon wafer – rather like placing plastic cling film over a tiny “muffin tin”. Each hole measured either 1.0 or 1.5 µm across.

Tiny drums

“Each graphene film is like a small drum,” explained Kysar, “except that the drumhead is only one atomic layer thick.” The team then indented the graphene film using an atomic force microscope with a diamond tip that has a radius of about 20 nm. It was necessary to use a diamond tip because conventional silicon tips would break before the graphene breaks.

The force-displacement response of the monolayer graphene films allowed the scientists to determine the elastic properties of the graphene film. The force at which the film breaks and the statistical distribution of the breaking force of many films allowed them to calculate the intrinsic strength of graphene.

“The stiffness of graphene is literally ‘off the chart’ when compared to other classes of material,” Hone told physicsworld.com. “This is thanks to both the covalent carbon-carbon bonds in graphene as well as the absence of any defects in the highest stressed portion of the graphene films.”

The graphene monolayers used in the experiments are defect-free because they are so small, something that precludes the existence of flaws — a condition that cannot be satisfied in macroscopic materials. Given the known robustness of the covalent carbon-carbon bond (that also gives carbon fibres used in high-performance composites their remarkable stiffness and strength), it is not unreasonable to claim that pristine graphene is the strongest material,” said Kysar.

Upper bound on strength

The new result will also serve as an experimental ‘benchmark’ James Hone, Columbia University

“The intrinsic strength of graphene can be considered as an ‘upper bound’ for the strength of materials — rather like diamond is for hardness — that could serve as a goal for engineers who design materials,” added Hone. “The new result will also serve as an experimental ‘benchmark’ to validate various theories and computer models that predict the elastic properties of materials at very high strains.”

“To put things in perspective: if a sheet of cling film (which typically has a thickness of around 100 µm) were to have the same strength as pristine graphene, it would require a force of over 20,000 N to puncture it with a pencil,” he explained. “That is the force exerted by a mass of 2000 kg, or a large car!”

The team is now performing more experiments to determine the friction properties of freestanding monolayer graphene, as well as quantifying the van der Waals forces between the graphene and underlying substrates.

Putting the 'Warp' into Warp Drive

By Hamish Johnston

My favourite episode of The Simpsons begins with Homer returning to college to retake the course “Nuclear Physics 101”. He manages to get three fellow physics students expelled and they all move into the Simpson home. They proceed to drive Marge crazy with their geeky ways, including tieing up the telephone line by downloading “Top ten reasons why Captain Kirk is better than Captain Picard”.

Funny as it may be, portraying physicists as Trekies is a stereotype that does the physics community no good — which is why someone should have a quiet word with Richard Obousy and Gerald Cleaver at Baylor University, who have posted a paper on the arXiv preprint server called “Putting the ‘Warp’ into Warp Drive“.

The paper describes how to create a “warp bubble”, which would surround a spacecraft and allow it to “effectively travel faster than the speed of light”. Instead of being powered by a cantankerous engineer with a bad Scottish accent, this warp drive harnesses the Casimir effect.

However, Cleaver and Obousy calculate that the total mass/energy contained in the planet Jupiter would be needed to propel a starship the size of the Enterprise to beyond the speed of light.

That’s a lot of dilithium crystals, and the inevitable tabloid headlines like “Physicists want to annihilate Jupiter to reach Rigel 7” will do the physics community no good.

Indeed, many readers will recall Lawrence Krauss’s 1996 book 20 articles will come up.

A plea to the physics community — no more Star Trek!

Oh, just to set the record straight — Captain Kirk is light years better than Captain Picard.

Pumped atom laser brings high-precision measurements in sight

Physicists in Australia have demonstrated the first “pumped” atom laser. Their achievement marks another step on the road to a continuously operating atom laser, which should enable high-precision measurements of rotations, accelerations and magnetic fields.

An atom laser is made from a Bose–Einstein condensate (BEC), a collection of atoms that have been cooled until they fall into the same quantum state. Normally a BEC is contained completely within magnetic fields, but if some of a BEC’s atoms are allowed to escape, they stream away in a coherent state — just like the photons in a conventional laser.

Eventually the atoms in an atom laser’s BEC will run out, and the lasing will stop. The idea of the pumped atom laser, therefore, is to continuously replenish the lasing BEC with atoms from a similar BEC a short distance away. “The pumped atom laser has been a goal of our group for five years,” says John Close , the lead author of the research at the Australian National University.

It’s not a case of doing it slowly, as you might if you were trying to fill a bucket of water without splashing John Close, Australian National University

‘Subtle’ technique

Close and colleagues situate the lasing BEC some 8 µm beneath the second BEC, so that it can be “drip fed” with atoms from above. But as the atoms from the second BEC are falling they are slowed by an optical laser pointed upwards, which forces each atom to absorb a photon. Shortly after, the atoms emit their own photon downwards, thereby slowing their descent further.

The advantage of this mechanism, which is known as a Bose-stimulated irreversible transition, is that the total upwards momentum produced by the photon absorption and emission exactly cancels the downwards momentum. This means that the atoms from the second BEC can join those in the lasing BEC without disturbing them, and therefore without creating any noise (Nature Physics doi:10.1038/nphys1027).

“It’s not a case of doing it slowly or carefully, as you might if you were trying to fill a bucket of water without splashing it,” explains Close. “It is more subtle than that.”

Wolfgang Ketterle, the Nobel Prize-winning physicist who, together with his team at the Massachusetts Institute of Technology, invented the atom laser in 1996, told physicsworld.com that Close and colleagues have “cleverly” implemented their mechanism. “It is nice to see how the analogy with an optical laser is getting closer,” he adds.

Towards a continuous atom laser

The next step for Close’s team is to combine the new pumping mechanism with a previously realised “conveyor belt” mechanism that takes the BEC atoms into the lasing mode. Once they have achieved this, says Close, they will have a “truly” continuous atom laser that can perform measurements of moving systems and magnetic fields.

“The sensitivity of an atom-based measurement ultimately depends on the flux of atoms, or how many atoms per second go through the measurement system,” he explains. “More atoms gives you higher precision.”

Electron microscope sees single hydrogen atoms

Physicists in the US claim to have used a transmission electron microscope (TEM) to see a single hydrogen atom – the first time that a TEM has been used to image such a light atom. The breakthrough was made by supporting the atom on graphene — a sheet of carbon just one atom thick. The team has also been able to watch hydrocarbon chains move across the graphene surface, suggesting that the technique could be used to study the dynamics of biological molecules.

There is nothing new in using TEMs to see individual atoms, but until now such instruments could only be used to image heavy atoms. One reason is that a TEM creates an image by shining an electron beam on a sample and measuring how much it is deflected by atoms of interest. Lighter atoms deflect electrons less than heavier atoms, which means that only the latter show up on an image.

Another problem is that a sample in a TEM has to be supported on a substrate that is durable enough not to be damaged by the electron beam, but thin enough for most of the electrons to pass straight through. Thin metal films or semiconductor foils are usually chosen as substrates, but these are still much thicker than single atoms and contain atoms heavier than carbon or hydrogen. Scattering from the substrate therefore tends to swamp the already weak signal from lighter atoms.

Thinnest and toughest

Now, however, Jannik Meyer, Alex Zettl and colleagues at the University of California, Berkeley have found away around this problem by using graphene, the thinnest and toughest known material, as a TEM substrate ( Nature 454 319 ).

The team came up with the idea while using a TEM to study defects in graphene. However, they also discovered that they could identify individual carbon and hydrogen atoms — as well as hydrocarbon chains — that had contaminated the surface of the graphene.

A crucial feature of the technique is that carbon atoms within the graphene lattice are invisible to the TEM — even though the technique can clearly see a single carbon atom on the surface of the graphene.

The carbon sheet provides a uniform background that shows no structure on its own Jannik Meyer, University of California

“The carbon atoms are packed in a regular arrangement with a spacing that is not resolved in this microscope,” explained Meyer, adding “thus, the carbon sheet provides a uniform background that shows no structure on its own”.

In addition to seeing individual atoms, the team was able to watch as the electron beam created the occasional hole in the graphene substrate. They even saw one such hole being repaired as the graphene absorbed carbon atoms from the surrounding environment.

Dynamical behaviour

The team was also able to study the dynamical behaviour of hydrocarbon chains (thought to be alkanes) that attached themselves to the graphene. These molecules were being imparted with energy by the electron beam, and so the researchers were able to watch them move around on the surface.

Zettl told physicsworld.com that the team is particularly interested in using the technique in the development of functionalized nanostructures — tiny objects that are engineered to perform a specific function. These are often hybrid materials — say a carbon nanotube decorated with biologically active molecules — and Zettl believes that TEM could be used to understand the real-time chemical binding or molecular dynamics processes that make such materials function.

“Furthermore, we are expanding our studies of the mechanical and electronic properties of graphene itself, and these TEM methods are expected be highly useful for that too,” added Zettl.

The team is also confident that others will use graphene substrates in their TEMs. “Chances are that these membranes will soon be used in TEM labs around the world,” said Meyer.

Contamination concerns

Debbie Stokes, an electron microscopist at the UK’s University of Cambridge agrees that graphene is a good substrate for supporting a TEM specimen, because it has minimal influence when imaging the overlying material of interest. “A single graphene layer helps to increase imaging sensitivity compared to other substrates,” she said. Stokes cautioned that contamination could be a problem, for which “carbon is notorious”.

As for studying atoms, Stokes believes the technique would be of limited use because it is “difficult to do and open to misinterpretation”.

BaBar gets to the bottom of bottomonia

Physicists working on the BaBar experiment at the Stanford Linear Accelerator Center in California are the first to see the lightest member of the “bottomonia” family of mesons — 30 years after the first of its heavier siblings was discovered. physicsworld.com talks to BaBar team member Tim Gershon of the University of Warwick about the significance of the discovery.

So what exactly is bottomonia?

It is a family of mesons comprising a bottom quark and an anti-bottom quark, which can pair up to form a number of different mass states. The Υ meson was the first bottomonia particle to be seen (at Fermilab in 1977) and is an excited or heavier mass state. Since then physicists have detected 12 other excited states — but never the lowest mass (or ground state) meson called ηb.

Why is it important to observe the ground state?

It is particularly important to know the ground-state mass because it relates directly to the theory used to understand the strong force

Measuring the masses of the bottomonia mesons tells us a great deal about their constituent quarks and the strong force that binds them together. It is particularly important to know the ground-state mass because it relates directly to the theory used to understand the strong force. The bottom quark is of great interest to those studying the very early universe because it would have been abundant in the quark-gluon primordial soup that existed for several microseconds after the Big Bang. Understanding the forces acting on this quark could shed light on important mysteries of this period, such as why the universe evolved to contain much more matter than antimatter.

Why did it take 30 years to make the measurement?

The ground state has zero spin, whereas the excited states can have non-zero integer values of spin. This means that the excited states will often decay with distinctive “signatures”, which can be detected in an experiment. An example is the decay of the Υ into a pair of muons with opposite charges. The ηb on the other hand is expected to decay mainly to multiple lighter particles, making it much more difficult to spot. Indeed, millions, if not billions, of ηb particles are believed to have already been produced in accelerators such as the Tevatron at Fermilab. However, it has not been possible to find its decay products amongst other all the other particles produced in the collisions.

How did the BaBar collaboration get around this problem?

We used an approach that does not require reconstructing the ηb from its decay products (arXiv:0807.1086v1). Instead, we used data accumulated during special runs of the PEP-II accelerator at the Stanford Linear Accelerator Laboratory (SLAC), in which about 100 million Υ(3S) mesons were produced by smashing together electrons and positrons. The Υ(3S) can decay to produce an ηb by emitting a photon with a characteristic energy.

About 20,000 such photons were detected by BaBar and, after careful analysis, we were able to conclude without reasonable doubt that we had observed the ηb meson. We were also able to determine that its mass is about 9340 MeV, making it about ten times the mass of the proton.

Was this the mass you expected?

Theoretical models gave a range within which the mass (or actually, the so-called “mass splitting” — the difference between the ηb mass and the Υ mass) was expected to be found. The measured value is slightly above most expectations, but consistent with some recent calculations.

Are there any more missing bottomonia states to be found?

There are several waiting to be found, and physicists at BaBar are now scouring the Υ(3S) data in search of them. We expect more exciting results to come from the analysis of this data.

However, it is likely that the sensitivity of the BaBar data will not be enough to answer all questions about the bottomonia and a more precise experiment will be needed. A proposal for an experiment called SuperB is now being discussed in Italy. While the primary goal of this experiment is to study particle physics beyond the Standard Model, it will also provide the ideal environment to study quarkonia physics, which includes bottomonia and charmonia — the latter being mesons comprising charm and anti-charm quark pairs.

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