High energy physics, at the risk of oversimplification, represents the physics of the 'very small.' It's a field of physics where researchers probe matter at ever-smaller scales and ever-closer distances to better understand the intricate yet elegant laws of nature.
Astrophysics and cosmology, on the other hand, represent the physics of the 'very large' where researchers seek to understand the forces of nature that drive and shape our seemingly endless universe.
Standing between these two 'physics poles,' at mid-scale so to speak, is condensed matter physics. Researchers in this subfield are concerned neither with the 'very small' nor with the 'very large.' Instead, they focus on matter at the human or 'macroscopic' scale, which literally means 'visible to the naked eye.' Put another way, the field of condensed matter physics is the study of the complex behaviour of mid-scale material systems comprised of many interacting particles or constituents.
The behaviour of individual particles that form the basis of study for condensed matter physics are now well understood. However, when a large number of macroscopic particles cross paths and interact, they show surprising, often unexpected, properties.
Condensed matter physicists have come to call such unexpected behaviour 'emergent properties'--professional jargon for the collective behaviour of biological and material systems that is qualitatively different than the behaviour of each of the system's constituent parts. The challenge for condensed matter physics, then, lies in understanding the complex emergent phenomena that are characteristic of interacting particles.
Let me give you an example. If you have a cluster of atoms in a confined field or system, under certain conditions, the atoms are a gas. However, if you cool the system, the gas may condense into a liquid. And if you cool it even more, it might solidify.
From the perspective of condensed matter physicists, such increasing rigidity is an emergent property. That's because the atoms themselves are not rigid but become so only after organising themselves in a particular manner--a phase transition that has been fostered by a particular set of conditions (namely, temperature and pressure).
Superconductivity, which allows an electrical current to flow through materials with no detectable resistance, is another emergent property. Individual electrons are not superconducting but they sometimes assume this property upon organising themselves in a certain manner under certain conditions. In other words, it's not the particles themselves but their interaction with one another that enables this pattern of behaviour to take shape.
Biology may offer the most spectacular example of emergent phenomena. Indeed the ultimate emergent phenomena are life and consciousness. That's because individual molecules don't exhibit life-like properties. It's only when complex assemblies of these molecules become a cell, that life is formed. And it's only when a complex assembly of cells becomes a human being that consciousness takes place.
Condensed matter physicists not only study complex interactive phenomena but also examine a vast diversity of systems--for example, granular matter or soft matter (sands, foams, fluids and proteins), and hard matter (metals, semiconductors, magnets and superconductors).
Condensed matter physicists, moreover, are often trained in departments other than physics--for example, materials science, chemistry and electrical engineering. Even within the discipline of physics itself, condensed matter physics overlaps with studies of biophysics, atomic physics, optics, and quantum field theory.
Finally, theories in condensed matter physics are not only explored for the intellectual challenges that they pose but also for the practical applications that may emerge from the theoretical frameworks which are created.
The best known application of theories in condensed matter physics has involved the development of semiconductors, the 'heart and soul' of computers.
Think of our world today. Now think of our world without computers. Although their widespread use dates back less than a quarter century, virtually no aspect of life, in either the North or South, would be the same without them. Certainly not communication. But the same is true of international politics, science, culture and entertainment.
Our lives have been touched and forever transformed by computers. Yet what we have come to call the 'computer age' in the eyes and minds of condensed matter physicists should be more appropriately called the 'semiconductor age.'
No semiconductors, no computers. No computers, no internet. No global communication and the world, as we know it in the early 21st century (both for better and worse), would be a different place. This is not a theory; it's a fact.
Direct applications of condensed matter physics research have led to enormous capital investments in the field. While the money is welcome, it sometimes obscures the reality that condensed matter physics carries enormous intrinsic intellectual content. Even my colleagues sometimes need to be reminded that such fundamental concepts in physics as symmetry breaking and renormalisation group theory grew from studies in condensed matter physics.
Given the enormous range of fields and materials covered by condensed matter physics, observers sometimes lose sight of the fact that this enterprise has an overarching theme and a common goal: that is, to gain insight into the emergent properties of complex materials and systems, and to understand, predict and control these unexpected behavioural patterns. That is what condensed matter physics--whether applied to the study of sandpiles or superconductors--is all about.
High temperature superconductivity, the field that I have concentrated on for the past 10 to 15 years, offers an excellent example of this general theme.
Scientists have known about the existence of conventional superconducting materials for about a century and have understood the intricacies of their behaviour for about 50 years. These conventional materials exhibit their unique property of zero resistance to electrical currents at about 10 degrees Kelvin--an extremely low temperature. By way of contrast, the temperature of the room in which you are reading this article is probably about 300 degrees Kelvin.
New high temperature superconducting materials, first uncovered by scientists about two decades ago, lose their resistance to electrical current at 100 degrees Kelvin, still cold but not nearly as cold conventional superconducting materials. Indeed these new materials display their unique zero-resistance qualities at temperatures 10 times warmer than their predecessors.
Why have the new superconducting materials commanded the attention of condensed matter physicists for the past two decades? One reason lies in their potential applications. For example, if scientists could find a way to produce superconducting high-tension transmission wires, electricity could be transported without line loss, saving money and improving reliability. That is likely a long way off; yet, superconducting materials are currently being used for filters in cell phone relay stations, helping to enhance the sound quality and minimise the breaks in transmission.
So, commercial applications of superconducting materials are emerging. Yet, regardless of such lucrative applications, there is another, equally important, reason for studying this subject: Before the emergence of research and studies on new superconductors, condensed matter physicists had developed a set of theories, ideas and even language to describe the behaviour of conventional metals and superconductors. Research into the new high temperature superconductors, however, has challenged conventional paradigms in the field by raising fundamental questions about how systems of strongly interacting electrons organise themselves into novel phases and give rise to unusual phase transitions.
I am sure that I speak not only for myself but for my colleagues as well when I say it is such intellectual challenges that have sparked our curiosity as scientists in the past and will continue to light the way in the future as we continue to uncover the mysteries of our natural world.

Mohit Randeria
Tata Institute of Fundamental Research, Mumbai, India

THE MATTER IN INDIA
After living in the United States for about 15 years, my wife and I moved back to India in 1995.
How do I feel about my move eight years later?

Overall, it's been a wonderful experience, in large part because I have been privileged to work at Tata Institute of Fundamental Research (TIFR) in Mumbai, an extraordinary institution, where I have been shielded from most of the problems that people working in developing countries often face. The Institute has an excellent infrastructure-a first-rate library and excellent computational facilities and internet access. I have outstanding colleagues and beautiful surroundings in which to work. Although I have few students, they are often outstanding, and I have complete freedom to work on anything I want, which may not have been the case if I had remained in the United States. I also have enjoyed a sense of satisfaction for contributing something of value to my own country through my research and teaching.
Has life in India been a bed of roses? Of course not. Pursuing cutting-edge research has not been easy. Even with all of the privileges that I enjoy at TIFR (I am painfully aware that many of my fellow researchers in India face much worse conditions), the research environment could be improved substantially. I have had to learn to accept the obstacles posed by India's bureaucracy. I have had to learn to work in an environment where there are few outstanding condensed matter experimentalists. And I have not enough funds for travel, a factor that has reinforced my sense of isolation.
On balance, however, my return to India has been an extraordinary experience. As far as the problems are concerned, it only heightens in my mind the critical role that institutions such as ICTP continue to play for scientists who choose to return home to continue their careers as well as for their students. The direct exposure that our visits to ICTP gives us to current research and the environment it provides for exchanging ideas with colleagues, whom we would otherwise never meet in person, allows us to continue to do quality work at home. So, in a sense, I must extend my thanks not only to TIFR, my home institute, but also to ICTP.

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