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The micro to macro connection (2013)

Page history last edited by Ben Dreyfus 9 years, 3 months ago

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Much of what we have done so far -- the Newtonian framework, describing the properties of solids and liquids, and the concepts of heat and temperature -- are macroscopic concepts: they describe things we see, feel, and experience. They express the regularities and consistencies of the behavior of physical systems. Much of this was well known by the middle of the nineteenth century. But one of the most extraordinary and important pieces of knowledge that humanity has garnered since then is the idea of the microscopic.  By this, physicists don't mean "what you can see in a microscope", but rather the fact that everything we regularly experience is made up of a small number of different kinds of atoms (91 in the natural world, a few more that have been created by humans). The essential point about this is that we believe that all properties of the macroscopic world are ultimately due to the properties and interactions of those 91 distinct elements. Although some phenomena require a description at a higher level (see the discussion of emergent phenomena), at some level (even if it's not convenient or useful for us to explicate), everything we see is a result of atomic properties.


A major component of modern biology is working at the microscopic -- atomic and molecular -- level and learning what are the critical elements that underlie basic biological mechanisms. Much of the research and development that can be expected to transform both biology and medicine over the next few decades will depend on making sense of the micro to macro connection. In this class, we will develop a few of the basic tools needed for making this connection. One set of tools involves statistical physics.  Since there is a huge amount of energy distributed in all objects at common temperatures, and since these energies tend to be randomly distributed among the atoms and molecules of a substance, the science of figuring out the implications of randomness is critical for understanding many biological phenomena.


We will begin our study of the implications of microscopic properties and randomness with two phenomena: kinetic theory and diffusion. Kinetic theory is about understanding thermal phenomena in molecular terms, and diffusion is about what happens when materials are not uniformly distributed. Analyzing both of these using the methods of statistical physics will give us insights into the mechanism of a large class of complex and important phenomena.


So far we have looked at temperature and pressure as macroscopic properties of substances, which we can measure through macroscopic observations.  We defined pressure as force per unit area, and we can observe the pressure of a substance by seeing the forces that it exerts on other objects (such as the buoyant force).  We also defined temperature as related to our intuitive sense of hot and cold and as what we can measure with an object (such as a thermometer) that responds to being warmer and colder in a measurable way. In the follow-ons, we'll build simple molecular models whose properties lead to our observed macroscopic results and give us deeper insights into the mechanisms responsible for the phenomena we observe.






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