Brigitte Falkenburg

What are the phenomena of physics

Depending on different positions in the debate on scientific realism, there are various accounts of the phenomena of physics. For scientific realists like Bogen and Woodward, phenomena are matters of fact in nature, i.e., the effects explained and predicted by physical theories. For empiricists like van Fraassen, the phenomena of physics are the appearances observed or perceived by sensory experience. Constructivists, however, regard the phenomena of physics as artificial structures generated by experimental and mathematical methods. My paper investigates the historical background of these different meanings of “phenomenon” in the traditions of physics and philosophy. In particular, I discuss Newton’s account of the phenomena and Bohr’s view of quantum phenomena, their relation to the philosophical discussion, and to data and evidence in current particle physics and quantum optics.

A Critical Account of Physical Reality

Several methodological a priori assumptions that underlie modern physics are investigated. They are transcendental in Kants sense, i.e. necessary conditions of objective physical knowledge, in particular, of theory formation and experimental practice. In the transition from classical physics to quantum physics, their meaning weakened substantially. (1) The general methodological and metaphysical principles behind modern physics were closely related to rationalist metaphysics, above all the belief in knowledge-independent causal agents and fundamental laws of nature. This belief is an essential ingredient of Plancks and Einsteins metaphysical realism and of current positions of scientific realism, or what is called classical realism in the paper. (2) Empiricism criticises classical realism. However, this criticism misses the methodological indispensability of non-empirical principles such as the principles of substance, mereological and causal analysis, unity, and simplicity. (3) Kant criticised classical realism. His a priori is compatible with a methodological view of these principles. (4) In twentieth century physics, however, Kants a priori turned out to be too strong. The touchstone of his critical account of physical reality is quantum theory. It is shown that a critical account of subatomic reality, which is slightly more liberal, comes close to central ideas of Niels Bohrs complementarity view of quantum mechanics.

From Waves to Particle Tracks and Quantum Probabilities

Here, the measurement methods for identifying massive charged particles are investigated. They have been used from early cosmic ray studies up to the present day. Laws such as the classical Lorentz force and Einstein’s relativistic kinematics were established before the rise of quantum mechanics. Later, it became crucial to measure the energy loss of charged particles in matter. In 1930, Bethe developed a semi-classical model based on the quantum mechanics of scattering. In the early 1930s, he and others calculated the passage of charged particles through matter including pair creation and bremsstrahlung. Due to missing trust in quantum electrodynamics, however, only semi-empirical methods were employed in order to estimate the mass and charge from the features of particle tracks. In 1932, Anderson inserted a lead plate into the cloud chamber in order to determine the flight direction and charge of the ‘positive electron’. In the 1940s, nuclear emulsions helped to resolve puzzles about particle identification and quantum electrodynamics. Later, the measurement theory was extended in a cumulative process by adding conservation laws for dynamic properties, probabilistic quantum formulas for resonances, scattering cross sections, etc. The measurement method was taken over from cosmic ray studies to the era of particle accelerators, and finally taken back from there to astroparticle physics. The measurement methods remained the same, but in the transition from particle to astroparticle physics the focus of interest shifted. Indeed, the experimental methods of both fields explore the grounds of ‘new physics’ in complementary ways.

How Do Quasi-Particles Exist?

Quasi-particles emerge in solids. In the context of the debate on scientific realism, their concept is puzzling. While quasi-particles seem to be fake entities rather than full fledged physical particles, they can nevertheless be used as markers, etc., in crystals. This has led some authors (e.g., Gelfert) to argue that, even though one can use them as technological tools, they cannot be said to ‘exist’ in the way ordinary particles do. Hence they seem to counter Hacking’s reality criterion, “If you can spray them, they exist.” However, this line of reasoning misses the crucial point that quasi-particles are real collective effects of the constituents of a solid. In order to spell out the way in which they do or don’t exist, i.e., their ontological status, their particle properties are discussed in detail. It is instructive to compare them with the field quanta of a quantum field, on the one hand, and subatomic matter constituents, on the other. All these particle concepts differ substantially from the classical particle concept. Not only does this discussion shed light on the specific way in which quasi-particles exist, but it may also clarify the ontological status of quantum phenomena in general.

Wave–Particle Duality in Quantum Optics

Philosophers of science are inclined to think that wave–particle duality is an obsolete concept, because according to quantum mechanics there are neither waves nor particles in a classical sense. But in physical practice, wave–particle duality is alive. The concept is crucial in order to understand the recent which-way experiments of quantum optics. First, several aspects of the concept will be sketched. Then I explain why the experimenters say that they prepare waves but detect particles. Indeed, their pragmatic attitude helps to understand a prominent thought experiment of Scully, Englert and Walther and the which-way experiments that realised it. Finally, I discuss a simple polarizer experiment. The experiment shows that no realistic interpretation of particles can cope with wave–particle duality, whereas the causal relevance of the quantum waves can not be denied.

Naturverständnis und Menschenbild

Wir sind am Ende unserer Reise durch die Grundprobleme der Philosophie des Geistes, das Methodenarsenal der Naturwissenschaften, die Befunde, Experimente und mechanistischen Erklarungen der Hirnforschung sowie die Erklarungsleistungen und Erklarungslucken der kognitiven Neurowissenschaft angelangt. Zu welchen Einsichten hat uns diese Reise gefuhrt? Zwingen uns die Ergebnisse der neueren Hirnforschung dazu, unser Selbstverstandnis als vernunftige Lebewesen mit einem freien Willen komplett zu revidieren?

Das Bewusstsein im Versuchslabor

Im letzten Kapitel haben wir die grundlegenden Befunde der Hirnforschung gesichtet. Neben rein naturwissenschaftlichen Phanomenen der Gehirnanatomie und der Elektrochemie der Nervenzellen waren zwei Arten von Befunden dabei, die an die Schnittstelle von Geist und Gehirn fuhren – die neuropathologischen Fallgeschichten und die Gehirnbeobachtung mit bildgebenden Verfahren. Kognitive Ausfalle im weitesten Sinn, sei es beim Sprechen, Verstehen, Sehen, Identifizieren von Dingen, Erkennen von Personen oder Handeln, treten als Folge von Hirnschadigungen auf. Die Suche nach den Ursachen dieser Ausfalle fuhrt dazu, unsere sensorischen, motorischen und kognitiven Fahigkeiten im Gehirn zu verorten und sie als Funktion der entsprechenden Gehirnareale zu betrachten. Die bildgebenden Verfahren, d. h. die Aufnahme der Hirnstrome mit Elektroden und die tomographischen Hirnscans, messen die neuronalen Aktivitaten direkt. Diese Techniken erlauben es buchstablich, dem Gehirn bei der Arbeit zuzusehen, wahrend die untersuchte Person bestimmte Aufgaben ausfuhrt und uber ihr Erleben Auskunft gibt.

Befunde der Hirnforschung

Im Folgenden geht es um Gehirn und Geist als naturwissenschaftliche Phanomene; oder: um physische und mentale Phanomene und ihre Beziehungen zueinander, soweit die Hirnforschung sie untersucht. Was sind diese Phanomene? Fuhren wir uns zunachst noch einmal die Ergebnisse des letzten Kapitels vor Augen. Naturwissenschaftliche Phanomene liegen meistens nicht auf der Hand. Anders als die Empiristen von Aristoteles uber Galileis Gegner bis hin zu Ernst Mach und seinen Nachfolgern glaubten, sind sie nichts unmittelbar Gegebenes. Sie werden erst durch Beobachtungsinstrumente, Messgerate und experimentelle Tricks zu Tage gebracht, oder: der Natur durch Werkzeuge vom Seziermesser bis zum Teilchenbeschleuniger abgerungen. Deshalb konnten wir uns im letzten Kapitel erst dann naher mit den Phanomenen der Physik befassen, als wir schon einiges daruber erfahren hatten, wie die Phanomene zerlegt, experimentell erforscht und kausal analysiert werden.

Wieviel erklärt uns die Hirnforschung

Bitte erinnern Sie sich nun wieder an das analytisch-synthetische Methodenarsenal der Physik, das ich Ihnen im 2. Kapitel vorgestellt habe. Physiker, Chemiker, Biologen oder Hirnforscher gehen immer in zwei Richtungen vor: analytisch oder top-down – vom Ganzen zu den Teilen, von den Wirkungen zu den Ursachen; und umgekehrt synthetisch oder bottom-up – von den Teilen zum Ganzen, von den Ursachen zu den Wirkungen. Die naturwissenschaftliche Forschung zielt darauf, das Ganze moglichst vollstandig aus den Teilen zu erklaren und die Wirkungen moglichst luckenlos aus den Ursachen. Der top-down-Ansatz soll den Schluss auf die beste Erklarung der Phanomene ermoglichen, wahrend der bottom-up-Ansatz umgekehrt zeigen soll, ob die Erklarung gut funktioniert – ob sie die Ausgangsphanomene erklart und daruber hinaus neue Phanomene vorhersagt.