Condensed matter physics in 15 minutes!

Oxford University Press has a nice podcast on Very Short Introductions. 

In each episode, an author of a specific volume has 10-15 minutes to introduce themself and answer several questions.

What is X [the subject of the VSI]?

What got you first interested in X?

What are the key aspects of X that you would like everyone to know?

The ones I have listened to and particularly liked are Infinity, Philosophy of Science, Evangelicalism, Development, Consciousness, Behavioural Economics, and Modern China.

Tomorrow, I am recording an episode for Condensed Matter Physics: A Very Short Introduction.

Here is a practise version of the audio and the draft text is below. 

I welcome feedback.

VSI Podcast 

I am Ross McKenzie. I am an Emeritus professor of physics at the University of Queensland in Brisbane, Australia. I have spent the past forty years learning, teaching, and researching condensed matter physics. I really love the Very Short Introduction series and so I am delighted to share my experience by writing Condensed Matter Physics: A Very Short Introduction.

What is condensed matter physics? It is all about states of matter. At school, you were probably taught that there are only three states of matter: solid, liquid, and gas. This is wrong. There are many more states such as liquid crystal, glass, superconductor, ferromagnet, and superfluid. New states of matter are continually, and often unexpectedly, being discovered. Condensed matter physics investigates how the distinct physical properties of states of matter emerge from the atoms of which a material is composed.

What first got me interested in condensed matter physics?

After I finished an undergraduate degree in theoretical physics in Australia in 1982, I would not have been able to answer the question, “what is condensed matter physics?”, even though it is the largest sub-field of physics. I then went to Princeton University in the USA to pursue a Ph.D. in and I took an exciting course on the subject and began to interact with students and faculty working in the field. 

At Princeton was Phil Anderson, who had won a Nobel Prize in physics for work in condensed matter. At the time I did not appreciate his much broader intellectual legacy. In his recent biography of Anderson, Andrew Zangwill states “more than any other twentieth-century physicist, he [Anderson] transformed the patchwork of ideas and techniques formerly called solid-state physics into the deep, subtle, and intellectually coherent discipline known today as condensed matter physics.” Several decades later, my work became richer as Anderson gave me an appreciation of the broader scientific and philosophical significance of condensed matter physics, particularly its connection to other sciences, such as biology, economics, and computer science. When do quantitative differences become qualitative differences? Can simple models describe rich and complex behaviour? What is the relationship between the particular and the universal? How is the abstract related to the concrete?

So what are the key aspects of condensed matter physics that I would like everyone to know?

First, there are many different states of matter. It is not just solid, liquid, and gas. Consider the “liquid crystals” that are the basis of LCDs (Liquid Crystal Displays) in the screens of televisions, computers, and smartphones. How can something be both a liquid and a crystal? A liquid crystal is a distinct state of matter. Solids can be found in many different states. In everyday life, ice means simply solid water. But there are in fact eighteen different solid states of water, depending on the temperature of the water and the pressure that is applied to the ice. In each of these eighteen states, there is a unique spatial arrangement of the water molecules and there are qualitative differences in the physical properties of the different solid states.

Condensed matter physics is concerned with characterising and understanding all the different states of matter that can exist. These different states are called condensed states of matter. The word “condensed’’ is used here in the same sense as when we say that steam condenses into liquid water. Generally, as the temperature is lowered or the pressure is increased, a material can condense into a new state of matter. Qualitative differences distinguish the many different states of matter. These differences are associated with differences in symmetry and ordering.

Second, condensed matter physics involves a particular approach to understanding properties of materials. Every day we encounter a diversity of materials: liquids, glass, ceramics, metals, crystals, magnets, plastics, semiconductors, and foams. These materials look and feel different from one another. Their physical properties vary significantly: are they soft and squishy or hard and rigid? Shiny, black, or colourful? Do they absorb heat easily? Do they conduct electricity? The distinct physical properties of different materials are central to their use in technologies around us: smartphones, alloys, semiconductor chips, computer memories, cooking pots, magnets in MRI machines, LEDs in solid-state lighting, and fibre optic cables. Why do different materials have different physical properties? 

Materials are studied by physicists, chemists, and engineers, and the questions, focus, goals, and techniques of researchers from these different disciplines can be quite different. The focus of condensed matter physics is on states of matter. Condensed matter physics as a research field is not just defined by the objects that it studies (states of matter in materials), but rather by a particular approach to the study of these objects. The aim is to address fundamental questions and to find unifying concepts and organizing principles to understand a wide range of phenomena in materials that are chemically and structurally diverse. 

The central question of condensed matter physics is, how do the properties of a state of matter emerge from the properties of the atoms in the material and their interactions? 

Let’s consider a concrete example, that of graphite and diamond. While you will find very cheap graphite in lead pencils, you will find diamonds in jewelery. Both graphite and diamond are composed solely of carbon atoms. They are both solid. So why do they look and feel so different?  Graphite is common, black, soft, and conducts electricity moderately well. In contrast, diamond is rare, transparent, hard, and conducts electricity very poorly. We can zoom in down to the scale of individual atoms using X-rays and find the spatial arrangement of the carbon atoms relative to one another. These arrangements are qualitatively different in diamond and graphite.. Diamond and graphite are distinct solid states of carbon. They have qualitatively different physical properties, at both the microscopic and the macroscopic scale. 

Third, I want you to know about superconductivity, one of the most fascinating states of matter. I have worked on it many times over the past forty years. Superconductivity occurs in many metals when they are cooled down to extremely low temperatures, close to absolute zero (-273 ºC). In the superconducting state, a metal can conduct electricity perfectly; without generating any heat. This state also expels magnetic fields meaning one can levitate objects, whether sumo wrestlers or trains. 

The discovery of superconductivity in 1911 presented a considerable intellectual challenge: what is the origin of this new state of matter? How do the electrons in the metal interact with one another to produce superconductivity? Many of the greatest theoretical physicists of the twentieth century took up this challenge but failed. The theoretical puzzle was only solved 46 years after the experimental discovery. The theory turns out also to be relevant to liquid helium, nuclear physics, neutron stars, and the Higgs boson. New superconducting materials and different superconducting states continue to be discovered. A “holy grail” is to find a material that can superconduct at room temperature. 

I find superconductivity even more interesting when considering quantum effects. By 1930 it was widely accepted that quantum theory, in all its strangeness, describes the atomic world of electrons, protons, and photons. However, this strangeness does not show itself in the everyday world of what we can see and touch. You cannot be in two places at the same time. Your cat is either dead or alive. However, condensed matter physicists have shown that the boundary between the atomic and macroscopic worlds is not so clear cut. A piece of superconducting metal can take on weird quantum properties, just like a single atom, even though the metal is made of billions of billions of atoms. It is in two states at the same time, almost like Schrodinger’s famous cat.

Fourth, condensed matter physics is all about emergence; the whole is greater than the sum of the parts. A system composed of many interacting parts can have properties that are qualitatively different from the properties of the individual parts. Water is wet, but a single water molecule is not. Your brain is conscious, but a single neuron is not. Such emergent phenomena occur in many fields, from biology to computer science to sociology, leading to rich intellectual connections. Condensed matter physics is arguably the field with the greatest success at understanding emergent phenomena in complex systems, particularly at the quantitative level. This is not because condensed matter physicists are smarter than sociologists, economists, or neuroscientists. It is because the materials we study are much “simpler” than societies, economies, and brains. 

Finally, condensed matter physics is one of the largest and most vibrant sub-fields of physics. For example, in the past thirty years, the Nobel Prize in Physics has been awarded thirteen times for work on condensed matter. In the past twenty years, eight condensed matter physicists have received the Nobel Prize in Chemistry. 

I hope I have sparked your interest in condensed matter physics. I invite you to learn more about why I consider this field of science significant, beautiful, and profound. 

Opening the door for women in science

 I really liked reading Transcendent Kingdom by Yaa Gyasi. She is an amazing writer. I recently reread some of it for an extended family book club. Just check out some of these quotes. 

A colleague suggested I might like Lessons in Chemistry, a novel by Bonnie Garmus. I have not read the book yet, but I have watched the first two episodes of the TV version on AppleTV. I watched the first episode for free.

The show contains a good mix of humour, love of science, and feminism. The chemistry dialogue seems to be correct. The show chronicles just how in the 1950s how awful life was for a young woman who aspired to be a scientist. Things have improved. But there is still a long way to go... 

Could faculty benefit from a monastery experience?

A few months ago, the New York Times ran a fascinating Guest Opinion, Why Universities Should Be More Like Monasteries by Molly Worth, a historian at University of North Carolina. She describes a popular undergraduate course, the "monk" class, at the University of Pennsylvania. 

On the first day of class — officially called Living Deliberately — Justin McDaniel, a professor of Southeast Asian and religious studies, reviewed the rules. Each week, students would read about a different monastic tradition and adopt some of its practices. Later in the semester, they would observe a one-month vow of silence (except for discussions during Living Deliberately) and fast from technology, handing over their phones to him.

This got me wondering about whether universities and funding agencies might experiment with similar initiatives for faculty. It might be a bit like the Aspen Center for Physics and Gordon Research Conferences were before the internet. Faculty would surrender their phones and have the internet disabled on their computers. For one week they would be required to spend their time reading, writing, and thinking. Exercise, daydreaming, doodling, and just having fun are to be encouraged. No administrative work or grant writing. The emphasis would be coming up with new ideas, not finishing off old work. For one hour per day, participants could meet with others and talk about their new ideas. I think it might be refreshing, reinvigorating, and highly productive (in the true sense of the word).

After trialling the program for a week, why not then try it for month-long periods.

These are the conditions under which Newton wrote the Principia and Darwin The Origin of Species. In their case, they did it over a period of several years.

What do you think?

Emergent abilities in AI: large language models

The public release of ChatGPT was a landmark that surprised many people, both in the general public and researchers working in Artificial Intelligence. All of a sudden it seemed Large Language Models had capabilities that some thought were a decade away or even not possible. It is like the field underwent a "phase transition." This idea turns out to be more than just a physics metaphor. It has been made concrete and rigorous in the following paper.

Emergent Abilities of Large Language Models

Jason Wei, Yi Tay, Rishi Bommasani, Colin Raffel, Barret Zoph, Sebastian Borgeaud, Dani Yogatama, Maarten Bosma, Denny Zhou, Donald Metzler, Ed H. Chi, Tatsunori Hashimoto, Oriol Vinyals, Percy Liang, Jeff Dean, William Fedus

They use the following definition, "Emergence is when quantitative changes in a system result in qualitative changes in behavior," citing Phil Anderson's classic "More is Different" article. [Even though the article does not contain the word emergence]. 

In this paper, we will consider a focused definition of emergent abilities of large language models: 

An ability is emergent if it is not present in smaller models but is present in larger models.

How does one define the "size" or "scale" of a model? Wei et al., note that "Today’s language models have been scaled primarily along three factors: amount of computation, number of model parameters, and training dataset size."

The essence of the analysis in the paper is summarised as follows.

We first discuss emergent abilities in the prompting paradigm, as popularized by GPT-3 (Brown et al., 2020). In prompting, a pre-trained language model is given a prompt (e.g. a natural language instruction) of a task and completes the response without any further training or gradient updates to its parameters.

An example of a prompt is shown below 

Brown et al. (2020) proposed few-shot prompting, which includes a few input-output examples in the model’s context (input) as a preamble before asking the model to perform the task for an unseen inference-time example. 

 The ability to perform a task via few-shot prompting is emergent when a model has random performance until a certain scale, after which performance increases to well-above random. 

An example is shown in the Figure below. The horizontal axis is the number of training FLOPs for the model, a measure of model scale. The vertical axis measures the accuracy of the model to perform a task, Modular Arithmetic, for which the model was not designed, but just given two-shot prompting. The red dashed line is the performance for a random model. The purple data is for GPT-3 and the blue for LaMDA. Note how once the model scale reaches about 10^22 there is a rapid onset of ability.

The Figure below summarises recent results from a range of research groups studying five different language model families. It shows eight different emergent abilities.

Wei et al., point out that "there are currently few compelling explanations for why such abilities emerge the way that they do".

The authors have encountered some common characteristics of emergent properties. They are hard to predict or anticipate before they are observed. They are often universal, i.e., they can occur in a wide range of different systems and are not particularly sensitive to the details of the components. Even after emergent properties are observed, it is still hard to explain why they occur, even when one has a good understanding of the properties of the system at a smaller scale. Superconductivity was observed in 1911 and only explained in 1957 by the BCS theory.

On the positive side, this paper presents hope that computational science and technology are at the point where AI may produce more exciting capabilities. On the negative side, there is also the possibility of significant societal risks such as having unanticipated power to create and disseminate false information, bias, and toxicity.

Aside: One thing I found surprising is that the authors did not reference John Holland, a computer scientist, and his book, Emergence.

I thank Gerard Milburn for bringing the paper to my attention.

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.

Listing mistakes in Condensed Matter Physics: A Very Short Introduction

Someone told me that the day after your book is published you will start finding errors. They were correct.

Here are the first errors I have become aware of.

On Page 2 I erroneously state that diamond "conducts electricity and heat very poorly."

However, the truth about conduction of heat is below, taken from the opening paragraph of this paper.

Diamond has the highest thermal conductivity, L, of any known bulk material. Room-temperature values of L for isotopically enriched diamond exceed 3000 W/m-K, more than an order of magnitude higher than common semiconductors such as silicon and germanium. In diamond, the strong bond stiffness and light atomic mass produce extremely high phonon frequencies and acoustic velocities. In addition, the phonon-phonon umklapp scattering around room temperature is unusually weak.

Figure 2 on page 4 has a typo. Diamond is "hard" not "hand".

On page 82 I erroneously state that for the superfluid transition, the "critical exponent alpha was determined to have a value of -0.0127, that is to five significant figures."  The value actually has three significant figures. 

I thank my engineering friend, Dave Winn, for pointing out the first and third errors.

Update. August 27, 2024.

On page 40, I erroneously state that shear sound waves exist in a liquid. I thank Jean-Noel Fuchs for pointing out this error. Below, I have drafted a corrected paragraph

"In a fluid (gas or liquid) one way to distort a cubic volume of the fluid is to compress the cube into a shape (a rectangular prism) where the lengths of the sides are not identical, but the angles between the sides of the shape are still 90 degrees. Sound waves in air consist of this type of compression: oscillations in the density and pressure of the air occur in the same direction that the sound wave travels. 

      In an isotropic solid there is a second type of distortion: the shape of the cube is changed to that of a rhombohedron, the angles are no longer 90 degrees, but the lengths of the sides remain the same. Associated with these two types of distortions, there are two distinct ways in which sound can travel through a solid. A second type of sound wave corresponds to the second type of distortion, and is called a shear wave. The two types of sound travel at different speeds. An earthquake produces both types of waves: pressure waves and shear waves, the latter travelling slower. Observing and comparing the two types of waves plays an important role in seismology and in the detection of earthquakes."

There are some subtle issues here that go beyond what is appropriate in a VSI. Damped shear waves can exist in a liquid for wavevectors larger than some critical value. I will discuss the issues in a separate blogpost.

Please do write other errors in the comments below. This will help with future revisions.

Amazing things about Chandrasekhar's white dwarf mass limit

This is ultra-condensed matter physics!

In 1931, Subrahmanyan Chandrasekhar published a seminal paper, for which he was awarded the Nobel Prize in 1983. He showed that a white dwarf star must have a mass less than 1.4 solar masses, otherwise it will collapse under gravity. White dwarfs are compact stars for which the nuclear fuel is spent and electron degeneracy pressure prevents gravitational collapse. 

The blog Galileo unbound has a nice post about the history and the essential physics behind the paper.

There are a several things I find quite amazing about Chandrasekhar's derivation  and the expression for the maximum possible mass. 

m_H is the mass of a proton. M_P is the Planck mass. The value of the mass limit is about 1.4 solar masses.

Relativity matters

If the electrons are treated non-relativistically then there is no mass limit. However, when the star becomes dense enough the Fermi velocity of the electrons approaches the speed of light. Then relativistic effects must be included.

A macroscopic quantum effect

Degeneracy pressure is a macroscopic quantum effect. The expression above involves Planck's constant.

Quantum gravity

The mass formula involves the Planck mass, M_P.  On the one hand, this phenomena does not involve quantum gravity because there is no quantisation of the gravitational field. On the other hand, the effect does involve the interplay of gravity and quantum physics.

A "natural" scale

Formula that involve Planck scales usually represent scales of length, energy, mass, time, and temperature that are "unreal", i.e., they are vastly different from terrestial and astrophysical phenomena. For example, the temperature of the Hawking radiation from a black hole of one solar mass is 60 nanoKelvin. 

In contrast, the limiting mass is on the same scale as that of our own sun!

It agrees with astronomical observations

Determinations of the masses of hundreds of white dwarfs show most have a mass of about 0.5 solar masses and the highest observed value is 1.3 solar masses.