Thursday, March 23, 2017

Units! Units! Units!

I am spending more time with undergraduates lately: helping in a lab (scary!), lecturing, marking assignments, supervising small research projects, ...

One issue keeps coming up: physical units!
Many of the students struggle with this. Some even think it is not important!

This matters in a wide range of activities.

  • Giving a meaningful answer for a measurement or calculation. This includes canceling out units.
  • Using dimensional analysis to find possible errors in a calculation or formula.
  • Writing equations in dimensionless form to simplify calculations, whether analytical or computational.
  • Making order of magnitude estimates of physical effects.

Any others you can think of?

Any thoughts on how we can do better at training students to master this basic but important skill?

Tuesday, March 21, 2017

Emergence frames many of the grand challenges and big questions in universities

What the big questions that people are (or should be) wrestling within universities?
What are the grand intellectual challenges, particularly those that interact with society?

Here are a few. A common feature of those I have chosen is that they involve emergence: complex systems consisting of many interacting components produce new entities and there are multiple scales (whether length, time, energy, the number of entities) involved.

Economics
How does one go from microeconomics to macroeconomics?
What is the interaction between individual agents and the surrounding economic order?
A recent series of papers(see here and references therein) have looked at how the concept of emergence played a role in the thinking of Friedrich Hayek.

Biology
How does one go from genotype to phenotype?
How do the interactions between many proteins produce a biochemical process in a cell?


The figure above shows a protein interaction network and taken from this review.

Sociology
How do communities and cultures emerge?
What is the relationship between human agency and social structures?

Public health and epidemics
How do diseases spread and what is the best strategy to stop them?

Computer science
Artificial intelligence.
Recently it was shown how Deep learning can be understood in terms of the renormalisation group.

Community development, international aid, and poverty alleviation
I discussed some of the issues in this post.

Intellectual history
How and when do new ideas become "popular" and accepted?

Climate change

Philosophy
How do you define consciousness?

Some of the issues are covered in the popular book, Emergence: the connected lives of Ants, Brains, Cities, and Software.
Some of these phenomena are related to the physics of networks, including scale-free networks. The most helpful introduction I have read is a Physics Today article by Mark Newman.

Given this common issue of emergence, I think there are some lessons (and possibly techniques) these fields might learn from condensed matter physics. It is arguably the field which has been the most successful at understanding and describing emergent phenomena. I stress that this is not hubris. This success is not because condensed matter theorists are smarter or more capable than people working in other fields. It is because the systems are "simple" enough and the presence (sometimes) of a clear separation of scales that they are more amenable to analysis and controlled experiments.

Some of these lessons are "obvious" to condensed matter physicists. However, I don't think they are necessarily accepted by researchers in other fields.

Humility.
These are very hard problems, progress is usually slow, and not all questions can be answered.

The limitations of reductionism.
Trying to model everything by computer simulations which include all the degrees of freedom will lead to limited progress and insight.

Find and embrace the separation of scales.
The renormalisation group provides a method to systematically do this. A recent commentary by Ilya Nemenman highlights some recent progress and the associated challenges.

The centrality of concepts.

The importance of critically engaging with experiment and data.
They must be the starting and end point. Concepts, models, and theories have to be constrained and tested by reality.

The value of simple models.
They can give significant insight into the essentials of a problem.

What other big questions and grand challenges involve emergence?

Do you think condensed matter [without hubris] can contribute something?

Saturday, March 18, 2017

Important distinctions in the debate about journals

My post, "Do we need more journals?" generated a lot of comments, showing that the associated issues are something people have strong opinions about.

I think it important to consider some distinct questions that the community needs to debate.

What research fields, topics, and projects should we work on?

When is a specific research result worth communicating to the relevant research community?

Who should be co-authors of that communication?

What is the best method of communicating that result to the community?

How should the "performance" and "potential" of individuals, departments, and institutions be evaluated?

A major problem for science is that over the past two decades the dominant answer to the last question (metrics such as Journal "Impact" Factors and citations) is determining the answer to the other questions. This issue has been nicely discussed by Carl Caves.
The tail is wagging the dog.

People flock to "hot" topics that can produce quick papers, may attract a lot of citations, and are beloved by the editors of luxury journals. Results are often obtained and analysed in a rush, not checked adequately, and presented in the "best" possible light with a bias towards exotic explanations. Co-authors are sometimes determined by career issues and the prospect of increasing the probability of publication in a luxury journal, rather than by scientific contribution.

Finally, there is a meta-question that is in the background. The question is actually more important but harder to answer.
How are the answers to the last question being driven by broader moral and political issues?
Examples include the rise of the neoliberal management class, treatment of employees, democracy in the workplace, inequality, post-truth, the value of status and "success", economic instrumentalism, ...

Thursday, March 16, 2017

Introducing students to John Bardeen

At UQ there is a great student physics club, PAIN. Their weekly meeting is called the "error bar." This friday they are having a session on the history of physics and asked faculty if any would talk "about interesting stories or anecdotes about people, discoveries, and ideas relating to physics."

I thought for a while and decided on John Bardeen. There is a lot I find interesting. He is the only person to receive two Nobel Prizes in Physics. Arguably, the discovery associated with both prizes (transistor, BCS theory) are of greater significance than the average Nobel. The difficult relationship with Shockley, who in some sense became the founder of Silicon Valley.

Here are my slides.


In preparing the talk I read the interesting articles in the April 1992 issue of Physics Today that was completely dedicated to Bardeen. In his article David Pines, says
[Bardeen's] approach to scientific problems went something like this: 
  • Focus first on the experimental results, by careful reading of the literature and personal contact with members of leading experimental groups. 
  • Develop a phenomenological description that ties the key experimental facts together. 
  • Avoid bringing along prior theoretical baggage, and do not insist that a phenomenological description map onto a particular theoretical model. Explore alternative physical pictures and mathematical descriptions without becoming wedded to a specific theoretical approach. 
  • Use thermodynamic and macroscopic arguments before proceeding to microscopic calculations. 
  • Focus on physical understanding, not mathematical elegance. Use the simplest possible mathematical descriptions. 
  • Keep up with new developments and techniques in theory, for one of these could prove useful for the problem at hand. 
  • Don't give up! Stay with the problem until it's solved. 
In summary, John believed in a bottom-up, experimentally based approach to doing physics, as distinguished from a top-down, model-driven approach. To put it another way, deciding on an appropriate model Hamiltonian was John's penultimate step in solving a problem, not his first.
With regard to "interesting stories or anecdotes about people, discoveries, and ideas relating to physics," what would you talk about?

Wednesday, March 15, 2017

The power and limitations of ARPES

The past two decades have seen impressive advances in Angle-Resolved PhotoEmission Spectroscopy (ARPES). This technique has played a particularly important role in elucidating the properties of the cuprates and topological insulators. ARPES allows measurement of the one-electron spectral function, A(k,E) something that can be calculated from quantum many-body theory. Recent advances have included the development of laser-based ARPES, which makes synchrotron time unnecessary.

A recent PRL shows the quality of data that can be achieved.

Orbital-Dependent Band Narrowing Revealed in an Extremely Correlated Hund’s Metal Emerging on the Topmost Layer of Sr2RuO4 
Takeshi Kondo, M. Ochi, M. Nakayama, H. Taniguchi, S. Akebi, K. Kuroda, M. Arita, S. Sakai, H. Namatame, M. Taniguchi, Y. Maeno, R. Arita, and S. Shin

The figure below shows a colour density plot of the intensity [related to A(k,E)] along a particular direction in the Brillouin zone.  The energy resolution is of the order of meV, something that would not have been dreamed of decades ago.
Note how the observed dispersion of the quasi-particles is much smaller than that calculated from DFT, showing how strongly correlated the system is.

The figure below shows how with increasing temperature a quasi-particle peak gradually disappears, showing the smooth crossover from a Fermi liquid to a bad metal, above some coherence temperature.
The main point of the paper is that the authors are able to probe just the topmost layer of the crystal and that the associated electronic structure is more correlated (the bands are narrower and the coherence temperature is lower) than the bulk.
Again it is impressive that one can make this distinction.

But this does highlight a limitation of ARPES, particularly in the past. It is largely a surface probe and so one has to worry about whether one is measuring surface properties that are different from the bulk. This paper shows that those differences can be significant.

The paper also contains DFT+DMFT calculations which are compared to the experimental results.

Monday, March 13, 2017

What do your students really expect and value?

Should you ban cell phones in class?

I found this video quite insightful. It reminded me of the gulf between me and some students.



It confirmed my policy of not allowing texting in class. Partly this is to force students to be more engaged. But it is also to make students think about whether they really need to be "connected" all the time?

What is your policy of phones in class?

I think that the characterisation of "millennials" may be a bit harsh and too one dimensional. Although I did encounter some of the underlying attitudes in a problematic class a few years ago. Then reading a Time magazine cover article was helpful.
I also think that this is not a good characterisation of many of the students that make it as far as an advanced undergraduate or Ph.D programs. By then many of the narcissistic and entitled have self selected out. It is just too much hard work.

Friday, March 10, 2017

Do we really need more journals?

NO!

Nature Publishing Group continues to spawn "Baby Natures" like crazy.

I was disappointed to see that Physical Review is launching a new journal Physical Review Materials. They claim it is to better serve the materials community. I found this strange. What is wrong with Physical Review B? It does a great job.
Surely, the real reason is APS wants to compete with Nature Materials [a front for mediocrity and hype] which has a big Journal Impact Factor (JIF).
On the other hand, if the new journal could put Nature Materials out of business I would be very happy. At least the journal would be run and controlled by real scientists and not-for-profit.

So I just want to rant two points I have made before.

First, the JIF is essentially meaningless, particularly when it comes to evaluating the quality of individual papers. Even if one believes citations are some sort of useful measure of impact, one should look at the distribution, not just the mean. Below the distribution is shown for Nature Chemistry.


Note how the distribution is highly skewed, being dominated by a few highly cited papers. More than 70 per cent of papers score less than the mean.

Second, the problem is that people are publishing too many papers. We need less journals not more!
Three years ago, I posted about how I think journals are actually redundant and gave a specific proposal of how to move towards a system that produces better science (more efficiently) and more accurately evaluates the quality of individuals contributions.

Getting there will obviously be difficult. However, initiatives such as SciPost and PLOS ONE, are steps in a positive direction.
Meanwhile those of us evaluating the "performance" of individuals can focus on real science and not all this nonsense beloved by many.