Facultad de Ciencias
Carrera de Física

Textos selecionados del libro The art of the experimental physics,
D.W. Preston y E.R. Dietz, Wiley and sons, 1991
Versión 20 agosto 2002, RMM

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    Taken as a story of human achievement, and human blindness, the discoveries in the sciences are among the great epics. Robert Oppenheimer

    A basic quest of humans, regardless of culture and nationality, is to know where they are in the universe, how they came to. be, and what the future holds for them. Physics is a highly developed manifestation of the desire to determine our origin and our future. For instance, quantum mechanics, in principle, allows us to understand the macromolecules that make up our bodies, and the general theory of relativity is fundamental to our understanding of stellar and galactic evolution.

    The theories of physics are, in general, abstract and mathematical, and the physical systems that they describe are often far removed from everyday experiences. For these reasons nonscientists often consider physics as devoid of the characteristics referred to as human, namely, feelings, sentiments, and emotions: What they do not recognize is that the laws of physics are an expression of the relations between humans and the universe.

    Although some people may view physics as lacking human characteristics, all agree that the observation of natural phenomena is often an awe-inspiring event. Examples are observation of a lunar or solar eclipse, or simply the observation of a moonlit mountain covered with snow. There is also a deeper beauty in nature that scientists see which is usually not observed by others.

    Nature-from crystals to flowers-is full of ever recurring characteristic shapes and symmetries. There must be a fundamental reason for the typical properties of materials and forms that we observe in the flow of natural events. This reason is found in quantum mechanics: The wave nature of electrons forces them into typical patterns, the shapes of standing waves in the spherically symmetric Coulomb field. These shapes are the fundamental patterns of Nature, which are the basis of all the shapes we observe. . . . Victor F. Weisskopf, Oersted Medal recipient, 1976

    Commonplace as such experiments (nuclear magnetic resonance) have become in our laboratories, I have not yet lost a feeling of wonder, and of delight, that this delicate motion (precession of nuclear spins in a magnetic field) should reside in all the ordinary things around us, revealing itself only to him who looks for it. I remember, in the winter of our first experiments, just seven years ago, looking on snow with new eyes. There the snow lay around my doorstep-great heaps of protons quietly precessing in the earth's magnetic field. To see the world for a moment as something rich and strange is the private reward of many a discovery. Edward M. Purcell, Nobel Lecture, 1952

    The human element of scientific investigation is rarely brought to the attention of the general public. The form of scientific publications is an example of this kind of omission. A publication typically includes an abstract, introduction, results, and conclusion, and suggests a straightarrow path from the conception of the project to its completion.

    We have a habit in writing articles published in scientific journals to make the work as finished as possible, to cover the tracks, to not worry about the blind alleys or to describe how you had the wrong idea first, and so on. So there isn't any place to publish, in a dignified manner, what you actually did in order to get to do the work, although, there has been in these days, some interest in this kind of thing. Since winning the [Nobel] prize is a personal thing, I thought I could be excused in this particular situation, if I were to talk personally about my relationship to quantum electrodynamics, rather than to discuss the subject itself in a refined and finished fashion. Richard P. Feynman, Nobel Lecture, 1965

    The form of a scientific publication suggests that scientists have a method that always leads to the final result. Indeed, there is a method, called the scientific method, which may be summarized as the following sequence of steps:

    1. Carry out observations /

    2. Correlate the observations

    4. Develop a theory /

    3. Develop a model

    These four steps are shown in Figure 1.1. It is not always possible to correlate all of the observations, and one observation is not correlated in the figure.

    It is hindsight that allows us to claim that a scientific method exists. From a historical perspective it is possible to identify the existence of this method, but rarely can we discern it clearly from the work of an individual scientist. Planetary astronomy provides a clean-cut example. Tycho Brahe carried out careful observations. Johannes Kepler correlated the observations and developed a model for planetary motions (usually stated as Kepler's three laws). Sir Isacc Newton discovered laws of motion and a gravitational force law that he used to predict Kepler's laws. Newton also predicted new observations-for example, the orbital speed of artificial satellites.


    Predictions 66

    New observations

    FIGURE I.] Schematic diagram of the scientific method.

    In addition to the form of a scientific publication, the scientific method does not suggest a human side to science. In reality, the path to the completion of a scientific project is usually long and arduous and, as indicated by Feynman, is filled with wrong ideas, blind alleys, and the like. Therefore, it is not surprising that scientists experience the complete spectrum of human emotions, ranging from joyous excitement to severe depression, in the pursuit of scientific investigations.

    It took almost a full year before a workable scheme (experimental method to measure the fine structure of hydrogen) was clear in my mind. Willis E. Lamb, Nobel Lecture, 1955

    I worked on this problem (quantum electrodynamics) about eight years until the final publication in 1947. Richard P. Feynman, Nobel Lecture, 1965

    My first introduction to superconductivity came in the 1930's My first abortive attempt to construct a theory, in 1940's. . . . It was not until 1950, as a result of the discovery of the isotope effect, that I again began to become interested in superconductivity, . . . . John Bardeen, Nobel Lecture, 1972

    For a few days I was beside myself with joyous excitement. Albert Einstein's response to his predictions that explained anomalies in the orbit of Mercury

    Finally, there is the tragic case of Ludwig Boltzmann, who was born in 1844 and committed suicide in 1906. Today, 'Boltzmann's work in physics is highly regarded, primarily because of his contributions to' kinetic theory and for the statistical interpretation that he gave to classical thermodynamics. A large part of his lifework was related to the atomic theory of matter. Until the I 890s, it was generally agreed among physicists that matter was composed of atoms. But near the end of the nineteenth century, various paradoxes (e.g., specific heats and reversibility) were seen as serious defects of the atomic theory of matter, and Boltzmann came to be referred to as the last 'defender of atomism. The despondency that led to his suicide may have resulted from the rejection of his work by the physics community. It is one of the most tragic ironies in the history of science that Boltzmann ended his .life just before the existence of atoms was finally established by experiments on Brownian motion.

    During any experiment you are likely to experience some of the emotions just described, ranging from the disappointment of equipment failure to immense joy when the wiring, oscillator, amplifier, and computer respond in unison to yield a beautiful spectrum.

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    1. Nobel Lectures. Physics, Vols. 1-4, Elsevier, New York. The four volumes cover from 1901 (the first year the Nobel prize was awarded) to 1970. Unfortunately, 'later volumes did not exist when this book was written.
    2. V. Weisskopf, Physics Today, 23 (June 1976). This article is entitled "Is physics Human?" and is based on the author's response as recipient of the 1976 Oersted Medal of the American Association of Physics Teachers.
    3. J. Bardeen, Physics Today, 41 (July 1973). This paper is a reprint of Professor Bardeen's 1972 Nobel Lecture.
    4. C. Gillispie (Ed. in chien, Dictionary of Scientific Biography, Vol. 2, Charles Scribner's Sons, New York, 1970. The biography of Ludwig Boltzmann is on pp. 260-268.

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    The basic aims of the laboratory are to have the student do the following:

    1. Gain an understanding of some basic physical concepts and theories.

    2. Realize that completely functioning experimental apparatus are rarely encountered, and learn how to recognize and correct ,an equipment malfunction.

    3. Gain familiarity with a variety of instruments and learn to make reliable measurements.

    4. Learn how precisely a measurement can be made with a given instrument and the size of the measurement error. (See the section on "Error Analysis," page 7.)

    5. Learn how to do calculations so that the results have the appropriate number of significant figures. (See the section on "Significant Figures," page 16.)

    6. Learn how to analyze data by calculations and by plotting graphs that illustrate functional relations. (See the. sections on "Graphical Analysis" and "Curve Fitting," pages 18 and 23.)

    7. Learn how to keep an accurate and complete laboratory notebook. (See the following discussion on the "Laboratory Notebook.")

    8. Ultimately, learn how best to approach a new laboratory problem.

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    In general, loose-leaf paper is not appropriate for recording data or doing laboratory calculations. It is recommended that you record data and do calculations directly in your laboratory notebook. General questions to be considered when wnting a lab notebook are "If I pick up this notebook in a year or two, is there enough information in it for me to understand what was done, why, and what the results and conclusions were? Could I reproduce the experiment if I wanted to?"

    With these concepts in mind, it is suggested that for each experiment in your lab notebook you record or perform the following eight procedures:

    1. First write the title, date, partner, and page numbers in the upper right-hand comer of the appropriate notebook page.=

    2. Next: state a general purpose in one or two sentences. Throughout the experiment indicate the purpose of each new set of measurements or calculations. (In this. instance, the purpose may simply. be a statement of exactly what is being measured if the "why" i obvious.)

    3. Sketch the apparatus, free hand, but with the parts labeled.

    4. Record all original data directly in the laboratory notebook, not on scratch paper. The original data readings are the most important pieces of information you have, and their loss should not be risked by recording them on scratch paper. Copying the data wastes valuable time and risks mistakes. Be sure to indicate clearly what is- being measured and in what units. You may cross out data that appear to be us less or wrong, but do not erase them-they may turn out to be valuable.

    5. Make certain that measured quantities include a figure of uncertainty or "error." (See the following section on "Error Analysis.")

    6. Write all calculated values in your notebook with the method of calculation clearly indicated. They will usually appear near the data and may be presented in the form of a table. Each calculated result should include appropriate significant figures. (Refer to the section on "Significant Figures," page 16.)

    7. To graph the data follow the guidelines listed in the section on "Graphical Analysis."

    8. To record your results and conclusion, tell briefly what you did and how it came out. For example, if you measured a physical constant, how does it compare with the "accepted" value in the light of your estimated errors?

    The format of a notebook is not rigid, but it should follow the order in which you worked. As you perform the experiment, you should carry out error, data, and graphical analyses. Such analyses should not be postponed.