Thursday, September 23, 2010

Lunchtime reading: The Observational Foundations of Physics

I have started reading "The Observational Foundations of Physics" by Sir Allan Cook during my lunch breaks. The book's purpose, as Cook states in the first sentence of Section 1.1, "is to attempt to unravel some ways in which the practice of physics determines the form and content of physics and physical theory." In other words, Cook wishes to understand how the practices found in physics affect physical theories and the practices themselves. It is as if there existed a feedback loop such that performing experiments changed not simply the theory used to describe a phenomenon but the nature of theory itself.

Further in Section 1.1, he poses these questions that are central to his analysis:
  1. "Why should physics be so effective, and what does that tell us about the world of physics and our ways of gaining knowledge of it?"
  2. "Is there a real world that exists independently of whether I or anyone else is looking at it, or are all the ideas I have about a world external to me just the construction of my mind?"
He defers a thorough answer to the second question until the end of the book, but does offer that he believes that most physicists, while working at the bench or on a computer, act as if an external world existed independent of their attention.

Section 1.2 deals with observations and sets many of the premises of his arguments. Observation and experiment are decided to be equivalent. Observations also consist of two aspects: objective and subjective. Of the subjective aspect, only the communal nature of observation is of consequence to his arguments. Science is a social construct and scientists hold great influence over each other such that the act of observation is never truly independent of people other than the experimenter.

Cook goes to some length to explain that physics is empirical, "with observation primary and theory secondary," but he concedes that rarely can observation be performed without some theory underlying the act of observing. He gives the example of reading a voltage from a digital multimeter. The direct observation is of figures on a LCD readout, a consequence of numerous electronic circuits that respond to potential differences between two probes and relates to the potential energy difference of electrons between two points in a circuit. Of course, electrons are theoretical constructs. The theories underlying an observation can in some ways assure an experimenter that the results are telling us something of the real world and not subject to some extraneous errors or misinterpretations. For simplicity, an observation is defined as the operations that lead to a measurement and result in  "raw data." The data is considered "raw" regardless of the complexity of the measurement.

Finally, theories are models of observations, not a model of the real world itself. "I take a theory to be a mathematical realisation of an abstract system that has properties corresponding to those of a set of observations... It is in that sense that I take a theory to be a model of the world of observations, with the implication that there is a more fundamental correspondence than just giving the right answers..." Theory is an abstraction of the real world, not vice versa.

Friday, September 3, 2010

Consistency vs. Accuracy

Here's an interesting footnote from Chap. 3 of Goodman's Introduction to Fourier Optics:
"The fact that one theory is consistent and the other is not does not necessarily mean that the former is more accurate than the latter."
The footnote is in reference to the Kirchhoff diffraction integral which was derived under two inconsistent assumptions for the boundary conditions on the field. Despite these inconsistencies, the theory gives a very good prediction for the diffracted field far from a large aperture.

Kirchhoff's theory is also a good demonstration of the fact that mathematical consistency and exactness does not mean that a theory makes good predictions or can be used to calculate physical quantities. Experimentation must validate a theory's ability to do so.

Wednesday, September 1, 2010

Osmotic fun

In my readings on how to properly maintain cell cultures underneath a microscope, I came across the entry for osmotic pressure on Wikipedia. In the introduction, a thought experiment is described as such:
In order to visualize this effect, imagine a U shaped clear tube with equal amounts of water on each side, separated by a membrane at its base that is impermeable to the sugar molecules (made from dialysis tubing). Sugar has been added to the water on one side. The height of the water on each side will change proportional to the pressure of the solutions.
Just like my post on a proposed experiment dealing with entropic elasticity in rubber bands, this looks like an interesting and easy demonstration to perform. That is, if I have time to devise the setup (I'm also still working on the entropic elasticity experiment and a demonstration of Schlieren photography for CREOL's student group's outreach program, CAOS).