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Physics on film

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Recently in APS 2008 March Meeting Category

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This is a slide from a talk given by David Bader of Lawrence Livermore on behalf of Brian Soden of the University of Miami. It shows the four main feedback mechanisms that are believed to play a role in climate change.

They are the temperature and water content of the atmosphere; ice and snow cover; and cloud cover. Physicists are fairly certain that the first three are have a positive-feedback effect — that is they tend to increase the rate of global warming — but they are not so sure about clouds.

In particular, the effect of stratocumulus clouds on climate has been very difficult to understand. The problem is that these puffy clouds are very thin and turbulent, making it hard to understand the physics of how they participate in the transfer of radiation into and out of the atmosphere.

According to Bader “uncertainty of cloud feedback is the primary cause of uncertainties in climate models”. That sounds like a challenge to the physics community.

“You can flood a city, but you can’t drown a university”, says Greg Seab, a physicist at the University of New Orleans who was speaking at a press conference on the impact of Katrina on local physics departments.

Although the university was above the high water mark when Katrina flooded much of the city in September 2005, the campus was without electricity for six months. Indeed, the power only came on three days before the campus was scheduled to reopen in 2006.

But instead of cowering in the dark, the University re-invented itself online. Just a month after the disaster, faculty were delivering lectures and course work to 7000 students. However, one third of the university’s faculty eventually left after Katrina — something that Seab blames in part on “abysmal support from the state [of Louisiana].

The Xavier University campus suffered a direct blow, with many of its lecture halls underwater. The institute managed to reopen in January 2007, extending its academic year until August. Repairs have so far cost the university $50 million according to physicist Murty Akundi. 75% of students returned that January and Akundi says that enrolment is expected to be back to 80% of pre-Katrina levels by September 2008.

On a more cheerful note, David Hoagland of the University of Mass. at Amherst explained how he received a call from a colleague at New Orleans’s Tulane University asking if he could move his entire research group to Amherst. Hoagland said yes and the team were up and running in a month — and apparently “flourished with no scientific loss”.

I naturally assumed that these were theorists — but no, these intrepid experimentalists managed to clone their Tulane lab using borrowed equipment, much of it coming from scientific equipment makers. Where there is a will, there is a way!

One of the most sought after theories in condensed matter physics is that of high temperature superconductivity. It is hard to walk into these kind of talks and understand what is going on. If it is theory, it’s next to impossible as the first slide almost jumps into a large Hamiltonian, and as I guess these Hamiltonian’s have been discussed ad nauseum by now, people have started to not even describe any of the terms.

In a talk given by Doug Scalapino from the University of California at Santa Barbara, he discussed the question of a pairing ‘glue’ in high temperature superconductors. From what I understand this goes back to last year when Phil Anderson wrote a perspectives in science with the point that if “we have a mammoth and an elephant in our refrigerator - do we care much if there is also a mouse?”

Here the mammoth and the elephant are U, the on site repulsion and J, the exchange interaction in the Hubbard model which describes the transition between conducting and insulating systems. So Anderson says as these interactions are so large why do we need a mouse or a much smaller interaction that is the ‘glue’ that pairs electrons. In ordinary superconducting metals, like lead or Tin, these exchanged particles are phonons (lattice vibrations) that act like a bosonic “glue” to hold the electron pairs together. But what is the ‘glue’ for high temperature superconductors, like the cuprates? Anderson contends that the pairing interaction is coming from J which is instantaneous, attractive and large, so why do we need a smaller interaction to describe the bosonic glue?

Well, the glue that Scalapino was describing was not very clear to me, and it seems that was also the case with some audience members. Indeed, one person did ask at the end of the talk what is the glue and Scalapino’s answer was, well, spin fluctuations. But as Anderson points out these are just a natural consequence of the exchange interaction, J.

I think the jury is still out.

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So as not to be outdone by the APS, here’s a photo of a cake baked in honour of the 10th anniversary of IOP Publishing’s New Journal of Physics. The first-ever multi-discipline open access physics journal.

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That’s some cake!

Yesterday evening my IOP Publishing colleagues and I managed to blag our way into a posh reception celebrating 50 years of the journal Physical Review Letters. I forgot to take my camera, so the photo is courtesy of James Riordon at the APS.

And yes, we did sing:
Happy birthday Physical Review Letters,
Happy birthday to you.

At last year’s March Meeting in Denver, Ian Appelbaum gave a ten-minute talk about how he had injected spin-polarized electrons into a piece of silicon, transported them micrometres and then detected a spin-polarized current at the other end. It was just one of thousands of talks given that year.

But then Appelbaum published his results in Nature and this year he has been invited back to speak for 30 minutes — which he did today in a packed session that focused on spin injection in silicon.

The ultimate goal of Appelbaum’s research is to find practical ways to make “spintronic” devices, which in principle, could use the spin of the electron to process information much more efficient ways than coventional electronics.

To make a spintronic device, you need a material through which electrons can flow without losing their spin polarization — and it would be nice if that material was compatible with chip-making processes. Silicon fits the bill on both accounts, but is also has several drawbacks — it is difficult to inject spin-polarized electrons into the material; and once they are there it is difficult to measure their polarization.

Working at the University of Delaware Appelbaum’s team were the first to overcome these problems and you can find out how here.

I spoke with Appelbaum before his talk about how the fledgling field of silicon spin injection was shaping up. He described his breakthrough as a “clarification of the technologies that are needed”, and added that at least one more year of work by his team and others was needed before it would be possible to take a broader view of where the field was going.

Also speaking at the session was Berry Jonker of the Naval Research Lab. While Appelbaum detected spin polarization electrically, Jonker has worked out ways to detect it using light — something that is not usually possible thanks to silicon’s poor optical properties. Jonker finished his talk by declaring “There is a bright future for silicon spintronics”.

The future could also be bright for spintronics based on pieces of graphene — which are tiny flakes of carbon just one atom thick. It turns out that graphene shares many of silicon’s spin-friendly properties including weak spin-orbit and hyperfine interactions.

Speaking at a session on graphene, Bart van Wees of the University of Groningen, described a similar experiment to Appelbaum’s — but with graphene as the conductor. The experiment revealed that graphene is a good conductor of spin — but nowhere as good as silicon. The Groningen team found that spin polarization decays after the electrons travelled about 2 micrometres — a tiny distance compared to silicon. Indeed, Appelbaum told me that he hopes to transmit spins through a centimetre of silicon by the end of the year.

Van Wees described this shortcoming as a “mystery”.

Graphene has earned a reputation as a “wonder material” thanks to its outstanding electrical, thermal and mechanical properties. It’s comforting to know that graphene has been beaten by humble silicon when it comes to spintronics — at least for now!

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This is the tale of Alice, Bob and a black hole.

Alice and Bob are a couple with a big communication problem — they only talk using quantum information systems. Usually this involves sending encrypted messages via quantum dots or entangled photons, which are unreliable at the best of times.

But now Caltech’s John Preskill believes that they should try to exchange messages via a black hole. The idea is that Alice sends her message into the black hole where it gets mixed up in whatever goes on inside the event horizon. The event horizon marks the distance from the black hole from which nothing — not even light — can escape, so you would be forgiven for thinking that her message would be lost for ever.

Not so says Preskill — the information is slowly transmitted out of the black hole in the form of Hawking radiation. This is radiation that is thought to slowly leak out of a black hole, eventually causing the black hole to vanish.

All Bob has do is gather the Hawking radiation and use it to build a quantum state that is entangled with the state of the black hole inside the event horizon. Eventually, Alice’s message will leak out with the Hawking radiation, and the entanglement will allow Bob to read it —or something like that!

But, like all Alice and Bob’s other quantum conversations, there are problems — the process would take a very long time, and no-one has actually been able to see Hawking radiation from a black hole. Alice and Bob need to talk this over before they agree to Preskill’s scheme.

Comparisons

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Being at the APS meeting last year in Denver, I can’t help but think about some comparisons. Although I was at the march meeting last year in a different capacity (as a researcher, giving my 10+2 minute talk), It seems to me that the scale of this year’s event is much smaller compared to last year.

Although people from the APS say this year more than 7000 participants are registered, having chatted to a few people in the exhibition area, where most of the companies selling their equipment are based, they say business is much less than last year, and indeed the number of people just walking around and in sessions, to me at least, seems much less.

Most people who I have talked to seem to think along the same lines. Of course one can come up with their own conclusions about this, but maybe physics departments and institutions are tightening their belts as a result of funding tightening both in the UK and US.

I don’t have any figures to back up my claim about any possible decrease in participant figures, but possibly Bush’s last US science budget request for the financial year 2009 will promise more money for physicists and a return turnout for the APS march meeting 2009.

I just sat in on a press conference about how physicists are contributing to the study of climate change.

As a physicist writing for a physics publication, I often find it very difficult to cover stories about physicists tackling problems outside of “mainstream” physics. The problem is that I usually don’t know enough about the other discipline — be it botany, climatology or whatever — to really know if what the physicist has done is relevant.

One way to find out is to call up a botanist (say) and ask them. But it’s often the case that they are completely unaware that physicists are working in their field, and they speak a completely different language so it can be difficult to understand their take on the work.

Climate change offers rich seams of data and complex systems for physicists to study, and I’m guessing that research funds are not hard to come by. But despite the endless calls for more interdisciplinary collaboration, I fear that many physicists are tempted to look at climate change from a purely physics perspective — and miss the chance to make a more significant contribution.

I raised my concerns with the panel and John Wettlaufer of Yale University said that it was very important for physicists working outside the mainstream “to have a genuine interest in learning about someone else’s problem”. However, he admitted that “not many people want to do this”.

Brad Marston of Brown University added that it was important that physicists try to publish their work in general-interest journals such as the Proceedings of the National Academies of Science, where they will be peer reviewed (and hopefully read) by non-physicists.

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Now for a topic that is close to the heart for many inhabitants of New Orleans - hurricanes. The devastation caused by the events in 2005 by hurricane Katrina led to most of the inhabitants of New Orleans to be displaced and caused news and debate around the world for months - still today, nearly two years on from the events many inhabitants are still homeless.
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A hurricane can be modeled as a vortex, with a depth of only around 50 - 100 ft, but being miles across. Katrina was a Category 5 hurricane which brought water levels to around 30 ft at the coastal lines and was the sixth strongest Atlantic hurricane ever recorded.

Greg Holland from the National Center for Atmospheric Research, Boulder, Colorado, talked about understanding extreme hurricanes. Hurricane activity has increased substantially since the 1970’s, Holland pointed out that in his simulations just a 5 m/s increase in wind speeds can lead to a 100% increase in category 5 hurricanes - which he says seems to correlate well with observed behavior.

Of course the economic costs can be great of the devastation brought from a hurricane such as Katrina, which has now cost around $140 per household in the US. A following talk from Harold Brooks from National Severe Storms Laboratory, in Norman Oklahoma, showed another aspect of climate change - large hail stones. He showed that the frequency of large hail fall (i.e that the size of a baseball - still going on the baseball theme) is increasing by about 6% a year, and the ‘favorable severe environment’ for such weather conditions is increasing by 0.8 % per year. Both exploding just after the 1980’s.

Town planning was also a subject under scrutiny, with Holland pointing out the lemming like way we are build more and more communities next to the coast on some of the dangerous places like on marsh land.

However, it was noted during the session that Katrina is likely a few hundred year event, but this, however, now remains to be seen.

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