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The flow of time is an essential feature of the human experience, and it also lies at the centre of modern physics. Einstein's relativity is concerned with the nature of time as it affects stars and planets, while quantum physics explains that the flow of time in a system may be strongly affected by the actions of an external observer. However, these theories often seem irrelevant for the most striking human experiences of time: we find that living creatures grow old and die, and man-made structures crumble over time. The physical basis of these processes lies in the increase of disorder, or entropy: maintaining ordered structures requires external work, and in the absence of such work, disorder increases inexorably.In the 20th century, deep and elegant theories were built, to quantify entropy, and to understand why we observe an arrow of time pointing from the past to future. However, many important questions remain unanswered. In particular, theories predict which processes will happen in a system, but they do not predict how fast they happen, nor how natural processes might be resisted by humans or machines. My research is concerned with such questions, but most scientists agree that we are still very far from finding full answers to them. For this reason, I consider specific systems in which such questions are relevant. I then aim to combine the results from different systems in order to arrive at general principles.To take one specific example, glass is a material that has fascinated architects, designers, and artists over centuries. On heating, it softens and can flow as a liquid; if it is then rapidly cooled, it hardens into solid glass, retaining the transparency of the liquid, and the flowing shapes that are familiar from vases and ornaments. In this sense, glass lives on the borderline between liquids and solids. For my research, the key point is that liquids obey the arrow of time by flowing downhill, but solids have a fixed shape, and do not flow. If the glass is indeed a liquid, how does it resist flow? If it is a solid, why does it resemble so closely the liquid? These simple-sounding questions are in fact at the core of long-running scientific debates. In particular, it is not known whether there can exist an ideal glass : a liquid that resists time by flowing only infinitely slowly. If it is indeed possible for spontaneous flow to stop completely, this would have fundamental consequences for theories of the arrow of time.For a second example, consider what happens when viruses spread through a population. Inside the cells of infected organisms, molecules assemble spontaneously into ordered structures that are less than a thousandth of a millimetre in size. In turn, these ordered structures will develop into new viruses, allowing infection of other cells or other organisms. Here, time is of the essence: if the virus develops quickly enough then it can kill the cell, while if it develops too slowly, the cell can detect and destroy the virus. These questions may sound essentially biological, but physics has much to contribute here. My research investigates how fast the ordered structures can assemble, and how they might be speeded up or slowed down. The laws of physics seem to set speed limits on assembly, which I aim to exploit and control. Finally, I also consider how insights from biological assembly processes might be applied in man-made structures that are as tiny as biological viruses, with similarly complex structure and functionality. Such structures are difficult to build, but they have been proposed for the next generation of efficient solar cells, or even as building blocks for tiny machines that might be used to fight diseases like cancer. By working towards a theory for the assembly and control of such structures, my research aims both to develop fundamental theories in physics, and to give practical insights in biology and nanotechnology.
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