A golden age for precision observational cosmology

Yin-Zhe Ma gave a nice physics colloquium at UQ last week, A Golden Age for Cosmology

I learnt a lot. Too often, colloquia are too specialised and technical for a general audience.

There are three pillars of experimental evidence for the Big Bang model: Hubble expansion of the universe, relative abundance of light nuclei due to nucleosynthesis in the first few minutes, and the Cosmic Microwave Background.

Ma showed Hubble's original data from 1929 for redshift versus distance of galaxies. There was a lot of noise in the data. Nevertheless, Hubble was right.

Big Bang Nucleosynthesis

This was first proposed in 1948 by Ralph Alpher and George Gamow. (Hans Bethe was an honorary author of the paper as a joke so that the author list would sound like the first three letters of the Greek alphabet. Gamow had a mischievous sense of humour.)

The chain of nuclear reactions that will produce the lightest elements and isotopes is shown below.

Because the binding energy of 4He is so large, it could have only been formed at an extremely high temperature of about 10^10 K. (Or is the issue activation energy for formation, not binding energy?)

Detailed calculations using parameters from terrestrial nuclear physics give the observed relative abundances of the elements. In particular, the universe is 74% hydrogen and 24 per cent helium.

The astrophysicist's periodic table showing the origin of the different chemical elements is rather cute.

Giving credit to George Gamow

Gamow, who died in 1968, made impressive contributions to theoretical physics. His Wikipedia page is worth reading. He claimed that he predicted the Cosmic Microwave Background in the late 1940s and did not receive sufficient credit when it was discovered in 1964. The 2019 Nobel Prize citation for James Peebles also minimises Gamow's early contributions. Whether this is fair or not can be debated.

Anisotropies in the Cosmic Microwave Background.

The past two decades have seen amazing advances in precision measurements of these anisotropies. The radiation is isotropic to one part in 25000, with a temperature of 2.72548±0.00057 K.

Measurements of the anisotropies have allowed precise determinations of key cosmological parameters by fitting theoretical predictions to the data shown below from the 2018 Planck collaboration. Different peaks have different physical origins. 

The level of precision in the data is truly amazing.

The solid line is a fit to theory involving six parameters. What would Enrico Fermi say? This is not "making the tale of an elephant wiggle" because the fit parameters are all consistent with independent determination of the cosmological parameters from Hubble expansion and the relative abundance of the light elements.

Aside. The paper from the Planck 2 collaboration has been cited 19000 times, but has almost 200 authors. How does one use that information in evaluating individual authors in job and promotion applications? How are they to be compared to a single-author paper with 100 citations or a five-author paper with 500 citations?

Is this a golden age for cosmology? 

Yes, in terms of precision measurements. 

On the theoretical side, the golden age may have passed. It is not clear that new concepts or theories will emerge. The outstanding questions are:

What is the nature and origin of dark matter? of dark energy? 

Why is the cosmological constant so small? Why is it so fine-tuned?

Can the validity of inflation be pinned down?

Does quantum gravity matter?

A lot of smart people have spent decades on these problems and made little progress. That fact does not preclude the possibility of a theoretical breakthrough. However, it does not make me optimistic. I hope I am wrong.

Colloquium on 2024 Nobel Prizes

This friday I am giving a colloquium for the UQ Physics department.

2024 Nobel Prizes in Physics and Chemistry: from biological physics to artificial intelligence and back

The 2024 Nobel Prize in Physics was awarded to John Hopfield and Geoffrey Hinton “for foundational discoveries and inventions that enable machine learning with artificial neural networks.” Half of the 2024 Chemistry prize was awarded to Dennis Hassabis and John Jumper for “protein structure prediction” using artificial intelligence. I will describe the physics background needed to appreciate the significance of the awardees work. 

Hopfield proposed a simple theoretical model for how networks of neurons in a brain can store and recall memories. Hopfield drew on his background in and ideas from condensed matter physics, including the theory of spin glasses, the subject of the 2021 Physics Nobel Prize.

Hinton, a computer scientist, generalised Hopfield’s model, using ideas from statistical physics to propose a “Boltzmann machine” that used an artificial neural network to learn to identify patterns in data, by being trained on a finite set of examples. 

For fifty years scientists have struggled with the following challenge in biochemistry: given the unique sequence of amino acids that make up a particular protein can the native structure of the protein be predicted? Hassabis, a computer scientist, and Jumper, a theoretical chemist, used AI methods to solve this problem, highlighting the power of AI in scientific research. 

I will briefly consider some issues these awards raise, including the blurring of boundaries between scientific disciplines, tensions between public and corporate interests, research driven by curiosity versus technological advance, and the limits of AI in scientific research.

Here is my current draft of the slides.

Gravitational waves and ultra-condensed matter physics

In 2016, when I saw the first results from the LIGO gravitational wave interferometer my natural caution and skepticism kicked in. They had just observed one signal in an incredibly sensitive measurement. A lot of data analysis was required to extract the signal from the background noise. That signal was then fitted the results of numerical simulations of the solutions to Einstein's gravitational field equations describing the merger of two black holes. Depending on how you count about 15 parameters are required to specify the parameters of the binary system [distance from earth, masses, relative orientations of orbits, .... The detection events involve displacement of the mirrors in the interferometer by about 30 picometres!

What on earth could go wrong?!

After all, this was only two years after the BICEP2 fiasco which claimed to have detected anisotropies in the cosmic microwave background due to gravitational waves associated with cosmic inflation. The observed signal turned out to be just cosmic dust! It led to a book, by the cosmologist Brian Keating, Losing the Nobel Prize: A Story of Cosmology, Ambition, and the Perils of Science’s Highest Honor

Well, I am happy to be wrong, if it is good for science. Now almost one hundred gravitational wave events have been observed and one event GW170817 has been correlated with an x-ray observation.

But detecting some gravitational waves is quite a long way from gravitational wave astronomy, i.e, using gravity wave detectors as a telescope, in the same sense as the regular suite of optical, radio, X-ray, ... detectors. I was also skeptical about that. But it does not seem that gravity wave detectors are providing a new window into the universe.

A few weeks ago I heard a very nice UQ colloquium by Paul Lasky, What's next in gravitational wave astronomy?

Paul gave a nice overview of the state of the field, both past and future. 

A key summary figure is below. It shows different possible futures when two neutron stars merge.

The figure is taken from the helpful review

The evolution of binary neutron star post-merger remnants: a review, Nikhil Sarin and Paul D. Lasky

A few of the things that stood out to me.

1. One stunning piece of physics is that in the black hole mergers that have been observed the combined mass of the resulting black hole is three solar masses less than the total mass of the two separate black holes. The resulting loss of mass energy (E=mc^2) of three solar masses is converted into gravitational wave energy within seconds. During this time the peak radiant power was more than fifty times the power of all the stars in the observable universe combined!

I have fundamental questions about a clear physical description of this energy conversion process. First, defining "energy" in general relativity is a vexed and unresolved question with a long history. Second, is there any sense in which needs to describe this in terms of a quantum field theory: specifically conversion of neutron matter into gravitons?

2. Probing nuclear astrophysics in neutron stars. It may be possible to test the equation of state (relation between pressure and density) of nuclear matter. This determines the Tolman–Oppenheimer–Volkoff limit; the upper bound to the mass of cold, non-rotating neutron stars. According to Sarin and Lasky

The supramassive neutron star observations again provide a tantalising way of developing our understanding of the dynamics of the nascent neutron star and the equation of state of nuclear matter (e.g., [37,121,127–131]). The procedure is straight forward: if we understand the progenitor mass distribution (which we do not), as well as the dominant spin down mechanism (we do not understand that either), and the spin-down rate/braking index (not really), then we can rearrange the set of equations governing the system’s evolution to find that the time of collapse is a function of the unknown maximum neutron star mass, which we can therefore infer. This procedure has been performed a number of times in different works, each arriving at different answers depending on the underlying assumptions at each of the step. The vanilla assumptions of dipole vacuum spin down of hadronic stars does not well fit the data [37,127], leading some authors to infer that quark stars, rather than hadronic stars, best explain the data (e.g., [129,130]), while others infer that gravitational radiation dominates the star’s angular momentum loss rather than magnetic dipole radiation (e.g [121,127]).

As the authors say, this is a "tantalising prospect" but there are many unkowns. I appreciate their honesty. 

3. Probing the phase diagram of Quantum Chromodynamics (QCD)

This is one of my favourite phase diagrams and I used to love to show it to undergraduates.

Neutron stars are close to the first-order phase transition associated with quark deconfinement.

When the neutron stars merge it may be that the phase boundary is crossed.

Colloquium on 2021 Nobel Prize in Physics II

 Here are the slides from today's talk.

Colloquium on 2021 Nobel Prize in Physics

 Every year the UQ Physics Department has a colloquium where someone describes the science behind the latest Nobel Prize. This year I am going to talk about Parisi and the spin glass problem. My colleague Henry Nourse will talk about the climate modelling part.

In preparation, I have found the book, Spin Glasses and Complexity by Daniel L. Stein and Charles M. Newman, very helpful. It is at the level of a colloquium and has a nice chapter on applications to other areas of science (e.g. proteins, simulated annealing, optimisation, computer science, ...) It enabled me to finally "understand" the background and significance of Hopfield's famous paper from 1982, "Neural networks and physical systems with emergent collective computational abilities".

Thinking about replica symmetry breaking has brought back memories of when I was a graduate student at Princeton. When I started Anderson was thinking about spin glasses a lot and had people working on it. I heard lots of talks about spin glasses, replica symmetry breaking, travelling salesmen, ultrametricity, ... Even David Gross gave a colloquium about work he did on spin glasses, with a very warm introduction by Phil. ["I introduce David Gross the condensed matter theorist"] However, once the cuprates happened at the end of 1986, Anderson seemed to largely drop the spin-glass work. Except for Dan Stein, everyone started working on cuprates. In hindsight, I wonder if that was a mistake. In particular, it might have been better for many of his students if they had worked on complexity rather than cuprates.

Next week I will post a draft of my slides. In the meantime, two questions for readers:

1. What are some specific questions you might like answered in such a colloquium?

2. What are some specific resources you may have come across about this year's prize that you found helpful or interesting?

Here are the slides.

Mathematics, biology, and emergence

Last night I heard a model public lecture about science. The School of Mathematics and Physics at UQ hosted a public lecture at the Queensland State Library. Holly Krieger, a pure mathematician at Cambridge, spoke on the Mathematics of Life. This is part of a biannual lecture series endowed by Kurt Mahler.

The lecture was amazing, both in content and presentation. It was engaging for high school students, and stimulating for experts. I wish I had a video or a copy of the slides. Krieger is well known to some through her Numberphile videos on YouTube. Here are a few things I learned in the lecture.

Mathematics is the language of relationships and patterns.

We forget how even the concept of numbers is abstract. The notion of functions is even more so.

An underlying theme of the lecture was that of emergence: a simple rule describing the interactions between the components of a system lead to collective behaviour (complexity) of the whole system.

Examples were given from biological systems that raise the question: how does the system know to do this?

Swarms of starlings were shown in the short film, The art of flying by Jan van IJken.
How do they move in concert when there is no leader?



Other examples included ant bridges, an experiment with a slime mould that was able to replicate the Japanese transport network (here is the Canadian version), stripes and spots on animals (pattern formation explained with coupled reaction-diffusion equations by Alan Turing).

To illustrate how simple rules lead to complex behaviour, several cellular automata were demonstrated starting with Pascal's triangle and Sierpinski triangle. The latter was connected to biology through the pattern on the shell of a (poisonous) cone snail.

Rule 30 produces patterns similar to those found on the shell. It has periodic patterns such as stripes and aperiodic chaotic patterns.
It seems the new Cambridge train station also has this pattern!

Rule 184 can describe traffic including jamming for medium traffic densities.
The occurrence of a traffic jam does not depend on the initial state or a particular car, but only depends on the density of cars and the interaction (rule) between cars.

A nice video was shown of a traffic shockwave.
When water flows from a tap (faucet) and hits a flat sink bottom at right angles it may produce a "hydraulic jump" such as that shown below.

That is just the first half of the lecture. I may blog later about the second half which concerned chaos, defined as small initial changes leading to significant changes in outcome.

One of the most interesting things for me about the lecture was Krieger's claim that "Emergent complexity isn't everywhere. It can be hard to detect or confirm.'' i.e., just because we see complex behaviour (patterns) does not mean that it is due to emergence. In question time she said that this was in response to some of Wolfram's grand claims in A New Kind of Science, along the lines that everything (consciousness, gravity, continuity, free will, ...) could be explained in terms of discrete computational models such as cellular automata.

I think a more nuanced view is necessary. I agree, along with many others, that Wolfram's grand claims are not justified. But, I do not equate emergent complexity solely with simple rule-based computational models such as cellular automata. Different people do define emergence differently. For example, Sophia Kivelson and Steve Kivelson propose the following definition.

An emergent behavior of a physical system is a qualitative property that can only occur in the limit that the number of microscopic constituents tends to infinity.

This would rule out classifying most of the phenomena described in the lecture as emergent. I disagree with this definition. On the other hand, I am not sure I agree with Krieger's claim. I do think almost anything interesting is emergent: consciousness, critical phenomena, the vacuum in quantum field theory, superconductivity, ...

Universality, probability, and the growth of rough surfaces

On Friday there was a nice UQ Maths and Physics Colloquium, Beyond the Gaussian Universality Class, given by Ivan Corwin,
The talk was a very nice example of synergy between fundamental physics and maths research.
There are interesting connections with simple one-dimensional models for surface growth, the Kardan-Parisi-Zhang equation, the KPZ universality class, traffic models, random matrix theory, directed polymers in random media, ....

What needs to be said about mental health issues in universities?

On friday I am giving the UQ Physics Department colloquium on mental health issues for scientists. The talk may be similar to one I gave a few years ago.

I will update my talk incorporating some recent reading and the articles below.

A recent Editorial in Nature declared
Time to talk about why so many postgrads have poor mental health 
An outpouring on Twitter highlights the acute pressures on young scientists.

[I thank Tanglaw Roman for bringing the editorial to my attention. I never look at luxury journals unless someone refers me to a specific article.]

The Editorial was in response to the Twitter response to an article in a baby Nature
Evidence for a mental health crisis in graduate education

Poisonous science: the dark side of the lab 
The bullying and subsequent suicide of a talented Ivy League scientist exposes ugly truths about the cruelty and dysfunction at the heart of academic science

Mindfulness won't fix bad management

It also conveniently shifts the burden of wellbeing from the employer causing stress to the employee trying to deal with it. Worse, it allows what you might call "well-washing": employers who cloak themselves in a veneer of caring for their workers while hurting them with bad management practices. 

Five tips to get a good nights sleep

However, I would like some feedback and suggestions from readers.

What do you think needs to be said?

Update. The colloquium was postponed to avoid a scheduling conflict and to make it accessible to a broader audience. Thus, there is still time to send in your suggestions.

Are theoretical physics and chemistry amenable to online collaboration?

Last week at UQ we had a very nice mathematics colloquium, "Crowdsourcing mathematics problems" by Timothy Gowers.
He first talked about the Polymath project, including its successes and marginal contributions.
He then talked about a specific example of a project currently underway on his own blog, concerning transitive dice. This was pretty accessible to the general audience.

This is where a well defined important problem is defined on a blog and then anyone is free to contribute to the discussion and solution. A strength of this approach is that it makes use of the complementary strengths, experience, and expertise of the participants. Specifically, solving problems includes:

• selecting a problem that is important, interesting, and potentially ripe for solution
• defining the problem clearly
• breaking the problem down into smaller parts including conjectures
• sketching a possible heuristic argument for the truth of the conjecture
• giving a rigorous proof of the conjecture
• finding possible counter-examples to the conjecture
• connecting the problem to other areas of mathematics

This can be efficient because dead ends and mistakes are found quicker than someone working in isolation. 

People are more motivated and engaged because they are excited to be working on something bigger than themselves and what they might normally tackle. And they enjoy the community.

What about assigning credit in such group work? There is a clear public record of who has contributed what. Obviously, this does not work for bean counters looking at traditional metrics.

This approach mostly attracts senior people who are secure in themselves and their career stage and more interested in solving problems than getting individual credit.

The cultural differences of pure mathematics and physics was striking. The talk was given on whiteboards and blackboards without notes. No powerpoint! The choice of research problems was purely based on curiousity, not any potential practical value or the latest fashion. It is fascinating and encouraging that the pure mathematics community is still "old school" with the focus on quality not quantity.

Aside: Gowers is also well known for initiating a boycott of Elsevier journals.

Now, my question. 

What is stopping theoretical chemistry and physics from such a "crowd sourcing" approach? 

Is it that the problems are not amenable? 

Or is it largely that we are too driven by a system that is fixated on individual credit?

My post on why I don't worry about getting scooped may be relevant.

KITP seminars online

A wonderful thing about the web is that now there is so much material online. A pioneer in putting all their seminars and colloquia online is the KITP at Santa Barbara. I know some people who regularly watch seminars (both old and recent). Others do not know it exist. This is a particularly valuable resource for students and those of us in distant countries.

I have to confess that until yesterday I have never actually watched a talk; just occasionally skimmed some slides. Generally I find I don't have the patience to watch talks online. I just seem to prefer to look at papers. However, yesterday I was forced to do this because at the weekly UQ condensed matter theory group meeting we watched a nice talk by Antoine Georges on Hund's metals. Although, I have read and blogged about some of the relevant papers, I really found it helpful seeing what was highlighted and going through the material at a "slow pace". Hopefully, I will do this more often.

What do you think about online talks or lectures? How often do you watch them? Are there any that you would particularly recommend?

Am I too black and white?

I don't like people who are very black and white about things. Life, science, teaching, and politics are complicated. There are shades of grey. Simplistic analysis leads to simplistic "solutions". Hopefully I often bring that out in this blog, in posts such as a political metaphor for the correlated electron community.

Yet there is an issue where I am quite black and white and adverse to technicolour solutions (the rainbow coalition?!): power point slides.
Recently I sat through a talk where the speakers slides had every dot point in a different colour.
I really find this hard to read and distracting. 
Yet this is not unusual.

What is wrong with plain old black and white and traditional fonts?
Below is a random choice of one of my slides. It is not very creative or glamorous but it is easy on the eyes and brain.

Am I alone in my aversion to fancy colours, fonts, and backgrounds?

I believe in irreproducible results

At UQ we just had an interesting colloquium from Signe Riemer-Sorensen about Dark matter emission - seeing the invisible. Central to the talk was the data below. Focus on the red data around 3.6 keV.

This has stimulated more than 100 theory papers!
This reminds me of the faster than speed of light neutrinos and the 17 keV neutrino, 500 GeV particles seen by the Fermi gamma ray telescope, BICEP2 "evidence" for cosmic inflation, ....

The above data is discussed in detail here.

I don't want to just pick on my astrophysics and high energy physics colleagues as this happens in condensed matter and chemistry too... remember cold fusion... think about periodic reports of room temperature superconductors!

The painful reality is that cutting edge science is hard. One can be incredibly careful about noise, subtracting background signals, statistical analysis, sample preparation, .... but in the end there is Murphy's law .... things do go wrong .... and crap happens...

Skepticism and caution should always be the default reaction; all the more so the greater the possible significance or surprise of the "observed" result.

I believe in irreproducible results.

Update (14 December).
Clifford Taubes brought to my attention two relevant papers on the possible 3.5 keV line. The first paper rules out a dark matter origin of the line and even mentions Occam's razor. The second has a mundane alternative explanation of the line in terms of charge exchange between hydrogen gas and sulfur ions.

Rebutting the historical conflict thesis

Last friday we had Peter Harrison give the Physics colloquium on "The Progress of Science and the Decline of Religion?"
He is a historian, who prior to coming back to UQ, held a chair at Oxford, and in 2011 gave The Gifford Lectures, which were recently published.
He is probably best known for arguing that changing approaches to Biblical interpretation, associated with the Reformation [moving away from an emphasis on allegorical interpretations towards more literal and historical interpretations] changed peoples conception of "nature" and had a significant influence on the development of modern science.
Peter is director of the Centre for the History of European Discourses at UQ and attracts many stimulating and distinguished seminar speakers, some of whom I have blogged about before.

One issue Peter addressed head on is the "conflict thesis" which claims that science and religion have always been in conflict and particularly that religion has impeded the progress of science. This view is popular in the public realm but not among historians of science. [The Wikipedia page is worth reading]. Like most issues the reality is much more complex.

Peter mentioned several widely cited historical "conflict" incidents such as Galileo and Darwin. In both cases there were people who opposed them and who supported them using religious and scientific arguments. For the Galileo affair the main contention was about competing scientific models and different philosophical perspectives. Furthermore, Galileo's scientific case was hardly solid; no stellar parallax had been observed and his argument using the tides was (in hindsight) wrong. In Darwin's case he was opposed by Lord Kelvin (who had miscalculated the age of the earth) and supported by Asa Gray and some conservative theologians.
John Heilbron's book, The Sun in the Church "illuminates the niches protected and financed by the Catholic Church in which science and mathematics thrived."
Stephen Gaukroger's book, The Emergence of a Scientific Culture: Science and the Shaping of Modernity emphasised how science had religious sanctions.

If you have a few hours (and $200!) you can watch some very nice lectures on the above historical issues. I highly recommend a course given by Lawrence Principe (Johns Hopkins) and sold by The Great Courses [The Learning Company in Australia]. [Peter Harrison recommended these to me and I bought them on special for A$52 including shipping].

Peter also discussed more recent history including some sociological studies, which seemed to attract the most questions from the audience.
He gave a similar talk at BrisScience a few years ago and can be viewed here.

Physicists are bosons; mathematicians are fermions

The first observation is that each mathematician is a special case, and in general mathematicians tend to behave like “fermions” i.e. avoid working in areas which are too trendy whereas physicists behave a lot more like “bosons” which coalesce in large packs and are often “over-selling” their doings, an attitude which mathematicians despise.

Alain Connes, Advice to beginning mathematicians

I learnt this quote today, courtesy of Elena Ostrovskaya, who gave todays Physics Colloquium at UQ.

An alternative to cosmic inflation

On Friday Robert Mann gave a very nice colloquium at UQ, The Black Hole at the Beginning of Time. The video is below.

The (end of) the talk is based on the recent paper
Out of the white hole: a holographic origin for the Big Bang 
Razieh Pourhasan, Niayesh Afshordi, and Robert B. Mann

The key idea is to consider our universe as the 4-dimensional boundary (brane or hologram) of a 5-dimensional space-time in which there is a black hole.
In our universe one then has not just 4D gravity and matter, but also induced gravity and an effective fluid from the 5D "bulk".

(For better or worse) this work was recently featured on the cover of Scientific American.

Robert covered a massive amount of material moving through special relativity, general relativity, black holes, big bang, cosmology, recent results from the Planck satellite,  inflation, the multiverse,... and finally his alternative model.
I took several pages of notes.
He went overtime. I think this was one of the rare cases where I did not mind the speaker doing it.

Besides learning some interesting physics, what was most interesting to me was the refreshing way the work was presented. The tone was something like, "cosmology has some amazing successes but there are a few paradoxes, inflation is an interesting idea but also presents some problems, ...fine tuning is a challenge, ... so let me throw out a different idea.... it is a bit weird... but lets see where it goes ... it also has some strengths and weaknesses .... I am not sure this is better than inflation, but it is worth looking at." There was no hype or sweeping things under the rug.

Many in the audience were undergrads. I thought it was a great talk for them to hear. It was largely tutorial, there was some fascinating physics, connections to experiment were emphasised, healthy skepticism was modelled, and there was no hype.

I also liked the talk because it confirms my prejudice that people need to work harder, more creatively, and more critically, on foundational problems in cosmology. Dark matter, dark energy, inflation, and fine tuning are all really weird. They may be right. But they may not be. I think just accepting them as the only option and regressing to even weirder ideas like the multiverse is a mistake. [Of course, it is easy as an outsider to tell colleagues to work harder and more creatively.]

The physical model of the early universe that was presented was completely different to inflation. Yet it solves most of the same problems (horizon, flatness, and no monopoles). Its biggest problem is that it does not predict the observed 4 per cent deviation from scale invariance.

The most important and interesting bits are from about 52:00 to 58:00.

The art and discipline of a good colloquium

There is a helpful and challenging article The Physics of Physics Colloquia by James Kakalios on The Back Page of the APS News. It is based around old notes Suggestions for giving talks by Robert Geroch.

Both Kakalios and Geroch are worth reading in full, but here are a few random things that stood out to me. [Things I need to keep working on].

"What is the key take-away point that you want to impress on everyone when they leave your talk?"

Divide the talk up, centred around 3 or 4 key messages.

"Figures are easier to understand than words."

"You have been staring at these data and plots for years, but many in the audience have not."

Don't include more than five non-trivial equations.

"It is almost always a disaster to run over time".

Much of this may seem "common sense". However, as management guru Steven Covey said, "Common sense is not common practise." Preparing and giving a good talk requires a lot of discipline, particularly with regard to cutting out material.

Colloquium on Emergent states of quantum matter

Here are the slides for the talk I am giving today at the UQ Physics colloquium.
I will show the video Quantum levitation, and discuss what is and isn't quantum about it.

A good discussion of some of the issues raised is Laughlin and Pines article The Theory of Everything. A more extensive and introductory discussion by Pines is at Physics for the 21st Century.

Postscript.
Based on comments and questions afterwards, particularly from some undergraduates, there are few things I would do slightly differently.

I should have said what a Hamiltonian is: a function that defines the energy as a function of the system variables, e.g., the position and velocity of all of the particles.

The stratification of reality shown by my boxes is a simplification for schematic purposes. There is no clearly defined boundary between strata. For example, at the boundary between chemistry and physics one has chemical physics and physical chemistry. The boundary between biology and biochemistry is blurred. On the other hand, anatomy is qualitatively different to enzyme mechanisms. Acid-base equilibria is chemistry not physics.

Ben Powell emphasized to me that the claim that "superconductors exhibit broken U(1) gauge symmetry" is problematic and subtle. There is a long detailed paper, Superconductors are topologically ordered that I have read several times but don't really understand.

Draft of Colloquium on Emergent States of Quantum Matter

Next week I am giving the Physics Department Colloquium at UQ. I am working hard at trying to follow David Mermin's advice, and make it appropriately basic and interesting. I am tired of listening too many colloquia that are more like specialist research seminars.

I would welcome any feedback on what I have done so far. Here is the draft of the abstract.

Emergent states of quantum matter 

When a system is composed of many interacting components new properties can emerge that are qualitatively different from the properties of the individual components. Such emergent phenomena leads to a stratification of reality and of scientific disciplines.
Emergence can be particularly striking and challenging to understand for quantum matter, which is composed of macroscopic numbers of particles that obey quantum statistics. Examples included superfluidity, superconductivity, and the fractional quantum Hall effect. I will introduce some of the organising principles for describing
such phenomena: quasi-particles, spontaneously broken symmetry, and effective Hamiltonians. I will briefly describe how these ideas undergird some of my own research on complex molecular materials such as organic charge transfer salts, fluorescent proteins, and hydrogen bonded complexes. The interplay of emergence and reductionism raises issues in philosophy and as to the best scientific strategy for describing complex systems.

Here is a very preliminary version of the slides. [Latest version is here].

Let me know of any ways to make any of this clearer and more interesting.

The key ingredient of a good colloquium?

I still remember the main idea of David Mermin's What is wrong with those talks?
It was published as a Reference Frame in Physics Today in 1992. Mermin says:

Your only goal must be to furnish ordinary physicists with some modest glimpse of what sustains your own interest in your subject.

This past week this idea really shaped the preparation of my quantum science seminar. My main goal was to give the context for my latest paper, rather than talk about the contents of the paper.

I think Mermin's comments are particularly pertinent to colloquiums. But I feel he is a bit too pessimistic about seminars and conference talks for experts.

If you read the article, let me know if you think Mermin is too pessimistic? Or is he realistic? Has Powerpoint made the problem greater or less than 20 years ago?

Learning how bees navigate and make smooth landings

Last friday I attended a very nice physics colloquium by Mandyam Srinivasan on "Honeybees as a Model for the Study of Visually Guided Flight, Navigation, and Biologically Inspired Robotics"

Here are two fascinating things that really stood out to me. The picture below [taken from this review] gives a schematic of the experiment showing that image flow is the key property that bees balance to navigate obstacles.

Building on this led to insights as to how bees make smooth landings.

Analysis of the landing trajectories revealed that the flight speed of the bee decreases steadily as she approaches the surface. In fact, both the forward speed and the descent speed are approximately proportional to the height above the surface (Fig. 8), indicating that the bee is holding the angular velocity of the image of the surface approximately constant as the surface is approached. This strategy automatically ensures that both the forward and the descent speeds are close to zero at touchdown. Thus a smooth landing is achieved by an elegant and surprisingly simple process that does not require explicit knowledge of the bee's instantaneous speed or height (250).

This analysis leads to two linear first order differential equations which can be solved to make predictions that the bee regulates its height to decrease exponentially with time. This is indeed observed to the case. [A colleague commented how the agreement between experiment and theory was much more impressive than the average biophysics research].

Srinivasan then went on to describe how some of these ideas are being implemented in algorithms for automated flight.
He stressed that his view was one should not follow a biomimetic strategy but rather a bio-princip one, i.e. finding what principles are used in nature [e.g. constant image flow] and using appropriately adapted implementations in artificial flight.