Section 38–6 Philosophical implications

Conscious observer / Unmeasurable concept / Classical indeterminism

In this section, Feynman addresses several philosophical issues in quantum mechanics, including the role of the conscious observer, unmeasurable concepts, and classical indeterminism. This indicates that he did not adhere to the “shut up and calculate” mindset—a phrase popularized by David Mermin and sometimes mistakenly attributed to Feynman. While the slogan reflects a pragmatic attitude of many physicists toward philosophical debates, it overlooks Feynman’s willingness to engage with foundational questions, even as he dismissed metaphysical speculation.

       Feynman’s colleague Murray Gell-Mann was more openly contemptuous of philosophy. As described in George Johnson’s (1999) biography Strange Beauty, Gell-Mann considered philosophy as a waste of time—for instance, debating over the “reality” of quarks. In one (possibly apocryphal) anecdote, he is said to readily provide a “doctor’s note” when asked about philosophical issues, saying, “I’m sorry, I cannot talk about philosophy. My doctor said it’s bad for my blood pressure.” Ironically, Gell-Mann playfully named his quark classification scheme the “Eightfold Way,” referencing Buddhist philosophy.

       Feynman’s attitude toward philosophy was possibly ambivalent. He famously said, “Philosophy of science is about as useful to scientists as ornithology is to birds”—implying that scientists don’t need it to do good science. Yet he included a “Philosophy” section in his paper on gauge theories, though he likely meant physical significance rather than philosophical inquiry. In a 1962 letter to his wife Gweneth from a Warsaw conference, Feynman wrote, “…it is not good for my blood pressure...,” expressing frustration with unproductive research on gravitational theory—not philosophy per se.

1. Conscious observer

“The observer was sometimes important in prequantum physics, but only in a rather trivial sense. The problem has been raised: if a tree falls in a forest and there is nobody there to hear it, does it make a noise? A real tree falling in a real forest makes a sound, of course, even if nobody is there (Feynman et al., 1963, p. 38-8).”

 

Feynman’s view assumes a mind-independent reality: physical processes—such as the generation of sound waves—occur regardless of conscious observers. In his lectures on gravitation for postgraduates, Feynman (1995) comments, “…are you the observer? Then there is no reality to the world after you are dead? I know a number of otherwise respectable physicists who have bought life insurance. By what philosophy will the universe without man be understood? (p. 14).” This passage lightly mocks the idea that consciousness is necessary for reality to exist. While thinkers such as von Neumann, Wigner, and Penrose have proposed consciousness-based interpretations of quantum mechanics, their ideas remain outside mainstream physics. Feynman’s mentor, John Wheeler, pushed the discussion further by suggesting an active role for observers—though not necessarily linking it directly to human consciousness.

 

In the same Lectures on Gravitation, Feynman (1995) examines the distinction between external and internal observers in quantum mechanics:

“… what may properly be described by an amplitude to an external observer, is not necessarily well described by a similar amplitude when the observer is part of the amplitude. Thus the external observer of the usual quantum mechanics is in a peculiar position. In order to find out whether the cat is alive or dead, he makes a little hole in the box and looks; it is only after he has made his measurement that the system is in a well-defined final state; but clearly, from the point of view of the internal observer, the results of this measurement by the external observer are determined by a probability, not an amplitude (p. 13).”

This passage explores central ideas from the Wigner’s friend thought experiment, which raises questions about observer-dependent reality. In such scenarios:

1.      The external observer (Wigner) describes the entire system—including the friend and the cat—using a wavefunction that remains in superposition until measurement is made.

2.      The internal observer (the friend) experiences a definite outcome—either the cat is alive or dead—not a superposition.

While these two descriptions may appear contradictory, they illustrate the challenge of reconciling quantum mechanics across different observational perspectives. Rather than exposing a logical inconsistency in the theory, the contrast points to controversies in defining objective reality in quantum mechanics. Potential resolutions depend on one’s philosophical position—whether one emphasizes consciousness, decoherence, or treats the wavefunction as a mathematical tool.

 

To avoid the philosophical problems surrounding consciousness, some physicists prefer the term “agent” or “participant” instead of “observer.” In modern interpretations such as Quantum Bayesianism (QBism), an agent is an active participant who assigns probabilities based on their personal expectations and experiences. In this framework, quantum events (e.g., an atom’s decay) do not possess definite outcomes until the agent interacts with the system. That is, unmeasured properties are not merely unknown; they are fundamentally undefined. QBism redefines the observer not as a metaphysical entity or conscious mind, but as a decision-maker who updates their knowledge through measurement. The focus shifts from an objective wavefunction collapse to subjective process of Bayesian probability updating. Notably, the framework allows even an AI robot: it can perform experiments and revise assigned probabilities, without raising questions about consciousness.

 

2. Unmeasurable concept

“The situation in the sciences is this: A concept or an idea which cannot be measured or cannot be referred directly to experiment may or may not be useful. It need not exist in a theory. In other words, suppose we compare the classical theory of the world with the quantum theory of the world, and suppose that it is true experimentally that we can measure position and momentum only imprecisely (Feynman et al., 1963, p. 38-10).”

 

In Discussion with Einstein on Epistemological Problems in Atomic Physics, Bohr (1949) writes “Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems.” His statement aligns closely with empiricism and shares some tenets of logical positivism, but it does not strictly adhere to either philosophy. Specifically, logical positivists would argue that a statement is cognitively meaningful only if it is either analytically true (true by definition or logic, like mathematics) or empirically verifiable (testable through observation or experiment). For instance, Bohr rejected the notion of an “electron path” in quantum mechanics because it lacked direct experimental verification. On the contrary, in his development of quantum electrodynamics (QED), Feynman revived the concept of "paths," relating them to abstract mathematical amplitudes to be summed over all possible paths a particle might take.

 

During an interview, Feynman recounts his encounter with Bohr’s objections of his idea: “Bohr got up and said: ‘Already in 1925, 1926, we knew that the classical idea of a trajectory or a path is not legitimate in quantum mechanics; one could not talk about the trajectory of an electron in the atom, because it was something not observable.’ In other words, he was telling me about the uncertainty principle. It became clear to me that there was no communication between what I was trying to say and they were thinking. Bohr thought that I didn’t know the uncertainty principle … Bohr was concerned about the uncertainty principle and the proper use of quantum mechanics. To tell a guy that he doesn’t know quantum mechanics—well, it didn’t make me angry, it just made me realize that he [Bohr] didn’t know what I was talking about, and it was hopeless to try to explain it further. I gave up, I simply gave up…  (Mehra, 1994, p. 248).” Feynman’s approach to quantum mechanics was pragmatic, treating even unmeasurable concepts as useful computational tools for generating accurate predictions.

 

Feynman’s path integral approach to quantum mechanics is based on the principle of summing over all possible histories, treating a particle as if it explores every conceivable path between two points. These include not only classical trajectories but also mathematically useful—though physically implausible—constructs, such as those that move backward in time (often associated with antiparticles) or imaginary-time paths used in certain calculations. Although the sum over all paths yields experimentally verifiable probabilities, the individual paths themselves are not observable. A related example arises in Feynman diagrams, where internal lines represent virtual particles. These entities cannot be directly detected, yet they play a crucial role in calculating observable quantities and explaining interaction processes. Such examples highlight a key feature of theoretical physics: concepts that cannot be directly measured or empirically verified may still have significant explanatory and predictive value.

 

3. Classical indeterminism

“It is therefore not fair to say that from the apparent freedom and indeterminacy of the human mind, we should have realized that classical ‘deterministic’ physics could not ever hope to understand it, and to welcome quantum mechanics as a release from a ‘completely mechanistic’ universe. For already in classical mechanics there was indeterminability from a practical point of view (Feynman et al., 1963, p. 38-10).”

 

Feynman points out a subtle but profound insight about classical physics: although it is deterministic in principle, it can be indeterminate in practice. According to classical mechanics, perfect knowledge of a system's initial conditions would theoretically enable exact prediction of its future behavior. However, in real-world scenarios, even infinitesimal measurement uncertainties can result in exponentially diverging outcomes over time—a phenomenon known as the butterfly effect. This sensitivity to initial conditions means that while the present strictly determines the future in theory, an approximate present fails to reliably predict an approximate future. As Lorenz (1963) succinctly put it: “When the present determines the future, but the approximate present does not approximately determine the future.” This fundamental limitation, central to chaos theory, reveals how deterministic systems can nevertheless produce effectively unpredictable behavior.

 

Classical indeterminism refers to the practical unpredictability of systems governed by deterministic laws. While classical mechanics is deterministic in principle, several factors undermine our ability to predict outcomes with certainty in practice. At least four key reasons contribute to this indeterminism:

1. Sensitivity to Initial Conditions: Even infinitesimal uncertainties lead to exponentially diverging outcomes over time.

2. Measurement Precision limits: Absolute precision is physically unattainable and these microscopic uncertainties grow through interactions.

3. Computational Intractability: While the behavior of individual particles in macroscopic systems is theoretically governed by classical mechanics, solving equations for systems involving something on the order of Avogadro’s number of particles becomes computationally unfeasible.

4. Analytical Unsolvability: Even simple systems (like the three-body problem) defy closed-form solutions, requiring numerical approximations that accumulate errors.

This shows that unpredictability isn’t unique to quantum mechanics—it emerges even in purely classical physics contexts.

 

Perhaps Feynman could have acknowledged the pioneering insights of Poincaré into classical indeterminism. Decades before chaos theory's formalization, Poincaré's study of the three-body problem revealed a profound truth in celestial mechanics: deterministic systems can exhibit inherent unpredictability. As Poincaré (1908) presciently noted in Science and Method: “…it may happen that small differences in the initial conditions produce very great differences in the final phenomena. A small error in the former will produce an enormous error in the latter. Prediction becomes impossible, and we have the fortuitous phenomenon (p. 68).” In articulating this sensitivity to initial conditions, Poincaré anticipated what would much later be recognized as a hallmark of chaotic systems. Poincaré’s work, long underappreciated, thus serves as a conceptual bridge between classical determinism and the modern understanding of dynamical chaos.

 

Review Questions:

1. Does quantum mechanics require the concept of an observer?

2. Should physicists use concepts that cannot be directly connected to experiment?

3. How would you explain classical indeterminism?

 

The Moral of the Lesson: Although Feynman is often associated with the pragmatic “shut up and calculate” mindset, this section reveals that he engaged with philosophical issues of quantum mechanics. He was not dismissive of philosophy itself, but of poor or misguided philosophy. Feynman sought to clarify the implications of quantum theory while rejecting pseudoscientific speculation and conceptual overreach —a lesson in how to think critically about profound ideas.

 

References:

Bohr, N. (1949). Discussion with Einstein on epistemological problems in atomic physics. In Niels Bohr Collected Works (Vol. 7, pp. 339-381). Amsterdam, Netherlands: Elsevier.

Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

Feynman, R. P., Morinigo, F. B., & Wagner, W. G. (1995). Feynman Lectures on gravitation (B. Hatfield, ed.). Reading, MA: Addison-Wesley.

Johnson, G. (2000). Strange beauty: Murray Gell-Mann and the revolution in twentieth-century physics. New York, NY: Vintage.

Lorenz, E. N. (1963). Deterministic nonperiodic flow. Journal of the Atmospheric Sciences, 20(2), 130–141.

Mehra, J. (1994). The Beat of a Different Drum: The life and science of Richard Feynman. Oxford: Oxford University Press.

Poincaré, H. (1908). Science and Method (original French: La science et la méthode). London, UK: Thomas Nelson & Sons.