Section 37–8 The uncertainty principle

 Indeterminacy principle / Thought experiment / Protect quantum mechanics

 

In this section, Feynman discusses the uncertainty (or indeterminacy) principle, using thought experiments to illustrate its role in quantum mechanics. He presents a modified version of Bohr’s thought experiment to demonstrate how the uncertainty principle protects quantum mechanics from Einstein’s critiques. The section could aptly be titled The Indeterminacy Principle, aligning with Heisenberg’s original terminology.

 

1. Indeterminacy principle:

“This is the way Heisenberg stated the uncertainty principle originally: If you make the measurement on any object, and you can determine the x-component of its momentum with an uncertainty Δp, you cannot, at the same time, know its x-position more accurately than Δx≥ℏ/2Δp (Feynman et al., 1963, p. 37–11).”

 

In his paper titled The actual content of quantum theoretical kinematics and mechanics, Heisenberg (1927) originally states the principle as “According to the basic equations of the Compton effect, the relation between p₁ and q₁ is then p₁q₁ ~ h.” This relation, using Planck’s constant (h), was an approximation and lacked the precise inequality ΔxΔp≥ℏ/2. The formal mathematical expression emerged later through Kennard (1927) and Weyl (1928), who defined Δx and Δp as the standard deviations of position and momentum, providing precision to the concept. Heisenberg originally termed this principle as the indeterminacy principle (German: Unbestimmtheit), reflects its meaning morning accurately than “uncertainty,” which is often associated with measurement inaccuracies. The principle is not merely a consequence of measurement limitations but it is intrinsic to the very nature of quantum systems.

Note: In Fundamentals: Ten Keys to Reality, Wilczek (2021) writes: “[t]he two principles we mentioned above - that observation is an active process and that observation is invasive - were bedrock foundation of Heisenberg analysis (p. 211).”

 

“This is a special case of the uncertainty principle that was stated above more generally. The more general statement was that one cannot design equipment in any way to determine which of two alternatives is taken, without, at the same time, destroying the pattern of interference (Feynman et al., 1963, p. 37–11).”

Feynman’s phrasing—“one cannot design equipment in any way”— may give the impression that the principle arises from practical limitations like equipment design or observer interference. This interpretation aligns with Heisenberg’s initial framing of the principle, which attributed it to the physical disturbance caused by measurement. In Heisenberg’s gamma-ray microscope thought experiment, he proposed that attempting to measure an electron’s position would inevitably disturb its momentum due to the recoil imparted by the interacting photon. In section 38-2 of The Feynman Lectures on Physics, Feynman revisits the uncertainty principle (or bandwidth theorem) from the perspective of wave mechanics. He explains that the uncertainties associated with a wave packet, such as the trade-off between position and momentum precision, are intrinsic to the wave theory, but have nothing to do with quantum mechanics.

 

Historical Context: Pauli’s correspondence with Heisenberg inspired much of the latter’s work. In a letter dated October 19, 1926, Pauli writes: “one can look at the world with the p-eye and one can look at it with the q-eye. But if one wants to open both eyes at the same time, one goes crazy (Hermann et al., 1979).” In other words, he metaphorically described the uncertainty principle as akin to observing the world with the “p-eye” (momentum) and “q-eye” (position). It also succinctly captures the essence of quantum indeterminacy.

 

2. Thought experiment:

“We imagine a modification of the experiment of Fig. 37–3, in which the wall with the holes consists of a plate mounted on rollers so that it can move freely up and down (in the x-direction), as shown in Fig. 37–6. By watching the motion of the plate carefully we can try to tell which hole an electron goes through (Feynman et al., 1963, p. 37–11).”

 

Feynman’s thought experiment builds upon Bohr’s defense of quantum mechanics at the 1927 Solvay Conference. It modifies Bohr’s thought experiment, in which the wall (or screen) with two slits as a plate mounted on rollers. While this thought experiment is a conceptual exercise designed to explore quantum principles, it is practically impossible to perform. The recoil momentum of the plate would be extremely small due to the electron’s negligible mass and measuring such changes in momentum with the necessary precision is beyond current experimental capabilities. This illustrates how any attempt to determine “which-path” information inevitably disrupts interference patterns, affirming the principle of quantum indeterminacy.

 

Bohr’s thought experiment involved a screen suspended by springs, where reading the scale on the screen required illumination (see below). The scattering of photons during illumination led to an uncontrollable transfer of momentum, introducing uncertainty in the position of the slit in the first screen. This destroys the coherence of the particle’s associated wave, effectively erasing the interference pattern. In Principles of the Quantum Theory, Bohr writes, “It would seem that any theory capable of an explanation of the photoelectric effect as well as the interference phenomena must involve a departure from the ordinary theorem of conservation of energy as regards the interaction between radiation and matter (Kragh, 2012).” Bohr’s words suggest that, in quantum mechanics, the conservation of energy might be momentarily violated.

 

(Source: Bohr, 1996)

Einstein’s thought experiment involves a particle passing through a slit in the first screen, and the screen’s movement is observed to determine the particle’s deflection direction. This observation helps infer which slit in the second screen the particle passed through. By observing the particle’s position on a third screen, its path through the entire setup could be reconstructed. According to Einstein, this setup would reveal particle-like properties (a definite trajectory) and wave-like properties (interference patterns), challenging the validity of the uncertainty principle. Einstein’s argument was based on both the conservation of energy and his philosophical position on realism, the view that physical properties exist independently of measurement.

 

3. Protect quantum mechanics:

“The uncertainty principle “protects” quantum mechanics. Heisenberg recognized that if it were possible to measure the momentum and the position simultaneously with a greater accuracy, the quantum mechanics would collapse. So he proposed that it must be impossible. Then people sat down and tried to figure out ways of doing it, and nobody could figure out a way to measure the position and the momentum of anything—a screen, an electron, a billiard ball, anything—with any greater accuracy (Feynman et al., 1963, p. 37–12).”

 

Is the uncertainty principle necessary?

Feynman’s assertion that the uncertainty principle “protects” quantum mechanics highlights its role in preserving the theory’s consistency. However, in a footnote of his book titled QED, Feynman writes: “I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas…… If you get rid of all the old-fashioned ideas and instead use the ideas that I’m expounding in these lectures—adding arrows for all the ways an event can happen—there is no need for an uncertainty principle!’’ (Feynman 1985, pp. 55, n. 3). This reflects Feynman’s belief that the uncertainty principle is not fundamental in his path integral formulation of quantum mechanics. In this framework, the probability amplitude for an event is calculated as the sum over all possible paths, each weighted by a phase factor. However, it contrasts with Bohr’s insistence on the need of uncertainty principle because he also doubted the legitimacy of “paths” in quantum mechanics.

 

Did Feynman really protect quantum mechanics?

In Lectures on Gravitation delivered to graduate students, Feynman proposed a double-slit thought experiment involving a gravity detector to determine which slit an electron passes through. In Feynman’s words, “I would like to suggest that it is possible that quantum mechanics fails at large distances and for large objects…… We must therefore not neglect to consider that it is possible for quantum mechanics to be wrong on a large scale, to fail for objects of ordinary size” (Feynman et al., 1995, pp. 12–13). Feynman’s suggestion shows an important point: while quantum mechanics has been successful within its domain of applicability, its prediction for a large scale could be unreliable. This open-mindedness is significant, particularly regarding the unresolved challenges of reconciling quantum mechanics with general relativity.

 

Source: (Feynman et al., 1995)

The Second Quantum Revolution

The interpretation of quantum mechanics has been a subject of intense debate among physicists since the beginning of the quantum revolution (see figure below). This lack of consensus is not a weakness but a driving force behind deeper inquiry. Einstein’s critiques of quantum mechanics, for example, helped catalyze what is now called the second quantum revolution. His famous Einstein-Podolsky-Rosen (EPR) paradox questioned whether quantum mechanics could provide a complete description of reality. In response to the EPR paradox, Bell (1964) showed that if quantum mechanics is correct, nature must allow for non-local correlations. This has since driven advances in quantum technologies—such as computing, cryptography, and teleportation—that extend far beyond Bohr’s original vision and, ironically, fulfill Einstein’s critical role in pushing the theory toward some of its most profound and practical consequences.

 

Source: (Gibney, 2025).

The moral of the lesson: Heisenberg’s uncertainty principle remains a cornerstone of quantum mechanics, but Feynman’s path integral formulation offers an alternative perspective, challenging its necessity.

(The evolution of quantum theory—from foundational debates to breakthroughs in quantum entanglement—demonstrates that quantum mechanics advances not by “protecting” it, but by probing its boundaries and exploring new horizons.)

 

Key Takeaways (In Feynman’s style?): The uncertainty principle was a clever way Heisenberg kept quantum mechanics from collapsing under old-fashioned ideas, but it need not be the heart of the matter. In the path integral view, you don’t need it—you just add up all the possible ways something can happen, and nature takes care of the rest. What really drives physics forward isn’t guarding the theory with protective walls, but pushing it until it breaks—or surprises us with something new.

 

Review Questions:

1. How would you state the uncertainty (or indeterminacy) principle)?

2. How does Feynman’s thought experiment reinforce the principle?

3. Why might the uncertainty principle be considered essential for protecting quantum mechanics?

 

References:

1. Bell, J. S. (1964). On the einstein podolsky rosen paradox. Physics Physique Fizika, 1(3), 195-200.

2. Bohr, N. (1996). Discussion with Einstein on epistemological problems in atomic physics. In Niels Bohr Collected Works (Vol. 7, pp. 339-381). Amsterdam: Elsevier.

3. Feynman, R. P. (1985). QED: The strange theory of light and matter. Princeton: Princeton University Press.

4. 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.

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

6. Gibney, E. (2025). Physicists disagree wildly on what quantum mechanics says about reality, Nature survey shows. Nature, 643(8074), 1175-1179.

7. Heisenberg, W. (1983). The actual content of quantum theoretical kinematics and mechanics (No. NAS 1.15: 77379).

8. Hermann, A., v. Meyenn, K., & Weisskopf, V. F. (Eds.). (1979). Wolfgang Pauli: Wissenschaftlicher Briefwechsel mit Bohr, Einstein, Heisenberg ua Band I: 1919–1929. Berlin, Heidelberg: Springer Berlin Heidelberg.

9. Kennard, E. H. (1927). Zur quantenmechanik einfacher bewegungstypen. Zeitschrift für Physik, 44(4), 326-352.

10. Kragh, H. (2012). Niels Bohr and the quantum atom: The Bohr model of atomic structure 1913-1925. Oxford: OUP.

11. Wilczek, F. (2022). Fundamentals: Ten keys to reality. New York: Penguin.

Section 37–7 First principles of quantum mechanics

 (Ideal experiment / Probabilistic predictions / Hidden variables)

 

In this section, Feynman discusses an ideal experiment, probabilistic predictions, and hidden variables in quantum mechanics. Perhaps a more fitting title would be “Probabilistic Determinism in Quantum Mechanics” or “Probabilistic Predictions and Hidden Variables.” The section addresses the conceptual shift from classical determinism to probabilistic determinism in quantum mechanics.

 

1. Ideal experiment:

“We can write our summary more simply if we first define an “ideal experiment” as one in which there are no uncertain external influences, i.e., no jiggling or other things going on that we cannot take into account. We would be quite precise if we said: ‘An ideal experiment is one in which all of the initial and final conditions of the experiment are completely specified’ (Feynman et al., 1963, p. 37–10).”

 

Feynman’s definition of an ideal experiment—where all initial and final conditions are completely specified and free of uncertain influences—has notable issues in quantum mechanics: 1. The act of measurement itself alters the system, making it impossible to exactly specify final conditions without disturbing the initial state. 2. The uncertainty principle limits the simultaneous specification of complementary variables, such as position and momentum. 3. The external influences (such as thermal fluctuations) cannot be entirely eliminated or accounted for in practice. While Feynman’s definition is an idealization, the concept remains a valuable theoretical construct for analyzing probabilistic relationships in quantum systems.

 

In quantum mechanics, an ideal experiment would need to account for probabilistic nature of quantum systems. It can be redefined to include the following:

• Experimental Design: Control conditions (e.g., laser voltage), manipulated variables (e.g., slit separation), and environmental factors (e.g., a dark room) are specified.
• Minimization of External Influences: The experiment must minimize or account for unintended environmental interactions that disturb the quantum state.
• Probabilistic Outcomes: The experiment tests quantum probabilities rather than deterministic causality.

Importantly, the experiment must be highly accurate and repeatable, i.e., the probability of an event remains constant across repeated measurements.

 

Note: In one of his Messenger Lectures, Feynman (1965) added: “A philosopher once said 'It is necessary for the very existence of science that the same conditions always produce the same results'. Well, they do not. You set up the circumstances, with the same conditions every time, and you cannot predict behind which hole you will see the electron…...” In short, Feynman emphasizes that in quantum mechanics, the deterministic predictability of outcomes gives way to probabilistic predictions. Furthermore, he clarifies that we do not know how to predict what would happen in a given circumstance and the only thing that can be predicted is the probability of different events. This shift from classical determinism to probabilistic determinism underscores the need for redefining the ideal experiment in quantum mechanics, acknowledging its fundamentally probabilistic nature.

 

2. Probabilistic predictions:

“Yes! Physics has given up. We do not know how to predict what would happen in a given circumstance, and we believe now that it is impossible, that the only thing that can be predicted is the probability of different events. It must be recognized that this is a retrenchment in our earlier ideal of understanding nature. It may be a backward step, but no one has seen a way to avoid it (Feynman et al., 1963, p. 37–10).”

 

Feynman’s statement that “physics has given up” may convey an unintended sense of pessimism, which is potentially misleading. In reality, quantum mechanics represents a profound advancement in our understanding of nature. Unlike classical physics, quantum mechanics does not predict specific outcomes of individual events but instead provides precise probabilistic predictions.

For instance, in the double-slit experiment:

• We cannot predict the exact position where a single photon or electron will land.
• However, we can accurately predict the probability distribution of many such particles, which manifests as an interference pattern.

Thus, quantum mechanics can be described as probabilistically deterministic: while individual outcomes appear random, the probabilistic behavior of a large number of events is determined by the wavefunction.

 

Feynman’s explanations should not be viewed as pessimistic; quantum mechanics underpins integrated circuits (ICs) technologies, particularly at the nanoscale. For example:

• In ICs, the probabilistic behavior of electrons determines quantum tunneling through barriers in transistors as devices shrink toward quantum limits.
• Accurate quantum models help optimize transistor performance and improve IC yield rates—the proportion of functional chips produced. Even minor yield improvements can translate into substantial profits, especially in high-demand chips used for consumer electronics, data centers, and AI systems.

Thus, far from “giving up,” quantum mechanics has enabled us to harness the probabilistic nature in ways that profoundly affect both science and industry.

 

3. Hidden variables:

“We make now a few remarks on a suggestion that has sometimes been made to try to avoid the description we have given: “Perhaps the electron has some kind of internal works—some inner variables—that we do not yet know about. Perhaps that is why we cannot predict what will happen (Feynman et al., 1963, p. 37–11).”

 

The term “inner variables” is not a standard term in quantum mechanics. The correct term in this context is “hidden variables,” which refers to theoretical variables not accounted for in the conventional quantum mechanical framework. In his Messenger lectures, Feynman (1965) explains: “One theory is that the reason you cannot tell through which hole you are going to see the electron is that it is determined by some very complicated things back at the source: it has internal wheels, internal gears, and so forth, to determine which hole it goes through ... … That is called the hidden variable theory.” Furthermore, in the Audio Recordings* [48 min: 35 sec] of this lecture, Feynman says: “internal conditions— hidden variables—…” instead of inner variables. Thus, this seems to be an editorial problem.

 

Feynman (1965) describes hidden variables as “internal wheels, internal gears, and so forth” that determine which path an electron takes, without references to properties like spin or angular momentum. Formally, hidden variables (local or non-local) are hypothetical, unobservable parameters introduced to explain the outcomes of quantum mechanics. Experiments testing Bell’s inequalities have conclusively ruled out the local hidden variables, which assume no faster-than-light communication between entangled particles. However, the term “hidden” itself is arguably misleading or even a misnomer. In the derivation of Bell’s inequality, there is no hidden pre-established agreement among particles (Scarani et al., 2010). Interestingly, some interpret the term “hidden variables” as variables that are hidden from the eyes of quantum pioneers (Belinfante, 2014).

 

According to Belinfante (2014), hidden-variable theories can be categorized:

1. First Kind: Deterministic theories (e.g., Bohmian mechanics) have the same probability predictions as a conventional quantum theory.

2.  Second Kind: They aim for theories that look like causal theories when applied to spatially separated systems that interacted in the past. Each theory (e.g., “local” theory) has a deterministic mechanism underlying quantum phenomena.

3. Zeroth Kind: They include non-conventional approaches that attempt to explain quantum phenomena. For example, von Neumann defines an “impossible” hidden variables theory.

 

Review Questions:

1. How would you redefine an ideal experiment in the context of quantum mechanics?

2. How would you explain that quantum mechanics predicts probabilities rather than specific outcomes? Do you agree that physics has given up?

3. What are hidden variables and how do they relate to quantum theory?

 

The moral of the lesson: In quantum mechanics, we cannot predict the outcome of an individual event. The only thing that can be predicted—reliably and precisely—is the probability of different events. This shift from classical determinism to probabilistic determinism marks a fundamental change in how we view the natural world.

 

References:

1. Belinfante, F. J. (2014). A Survey of Hidden-Variables Theories: International Series of Monographs in Natural Philosophy (Vol. 55). Philadelphia: Elsevier.

2. Feynman, R. P. (1965). The character of physical law. Cambridge: MIT Press Feynman.

3. 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.

4. Scarani, V., Lynn, C., & Liu, S. (2010). Six quantum pieces: A first course in quantum physics. Singapore: World Scientific.

Section 37–6 Watching the electrons

 (Observer effect / Complementarity principle / Complementary descriptions)

 

In this section, Feynman discusses Proposition A, uncertainty principle, and logical tightropes in communicating quantum mechanics. However, the three main points of the section could be observer effect, complementarity principle, and complementary descriptions in quantum mechanics.

 

1. Observer effect:

“Here is what we see: every time that we hear a “click” from our electron detector (at the backstop), we also see a flash of light either near hole 1 or near hole 2, but never both at once! And we observe the same result no matter where we put the detector. From this observation we conclude that when we look at the electrons we find that the electrons go either through one hole or the other. Experimentally, Proposition A is necessarily true (Feynman et al., 1963, p. 37–7).”

 

According to Feynman, Proposition A—"each electron either goes through hole 1 or hole 2"—becomes true only under experimental conditions that forces the electron to choose a definite path. This reveals a deeper truth: the validity of Proposition A is dependent on the act of observation. In the absence of observation, the electron is described by a wavefunction representing a superposition of paths, leading to interference. When an observation is made, the act of measurement restricts the wavefunction to a new quantum state (a different function), allowing the electron to exhibit particle-like behavior and follow a specific path—either through slit 1 or slit 2. This transition from wave-like to particle-like behavior upon observation is often attributed to the observer effect, a concept sometimes linked to Heisenberg’s microscope. It challenges classical notions of objective reality, illustrating that the properties of quantum systems are not pre-determined but emerge through interaction with the apparatus.

 

“You remember that when we discussed the microscope we pointed out that, due to the wave nature of the light, there is a limitation on how close two spots can be and still be seen as two separate spots. This distance is of the order of the wavelength of light. So now, when we make the wavelength longer than the distance between our holes, we see a big fuzzy flash when the light is scattered by the electrons (Feynman et al., 1963, p. 37–9).”

 

Traditionally, physicists would use Heisenberg’s microscope to explain the uncertainty principle involved in the experiment. However, Feynman’s mention of microscope is related to Rayleigh criterion, which states the minimum angular resolution of an image-forming system. Notably, the experiment could be analyzed using the concepts of visibility and distinguishability: 1. Visibility refers to the clarity of the interference pattern, with high visibility indicating sharp fringes and low visibility indicating blurred or no patterns. 2. Distinguishability refers to the ability to determine which hole a particle passed through. There is a quantitative relation, sometimes known as distinguishability-visibility relation, which expresses a trade-off between the visibility of interference pattern and the distinguishability of the particle’s path.

 

2. Complementarity principle:

“He proposed, as a general principle, his uncertainty principle, which we can state in terms of our experiment as follows: ‘It is impossible to design an apparatus to determine which hole the electron passes through, that will not at the same time disturb the electrons enough to destroy the interference pattern’ (Feynman et al., 1963, p. 37–9).”

 

Feynman’s statement of the uncertainty principle suggests that the principle arises from the disturbance caused by the act of measurement. Furthermore, it implies that the main problem is one of apparatus design, but one may consider the possibility of determining the electron’s path using an observation system (instead of physical apparatus). More important, the uncertainty principle is now understood as intrinsic uncertainties of quantum systems, that are independent of any disturbance. Bohr’s complementarity principle offers an alternative explanation: we cannot observe both the wave and particle properties simultaneously. When we observe the electron’s path (particle-like behavior), the interference pattern (wave-like behavior) disappears, and vice versa.

 

Feynman’s perspective is related to Bohr-Einstein debates on the epistemological problems of quantum mechanics. In Bohr’s (1949) words, “[I]t is only the circumstance that we are presented with a choice of either tracing the path of a particle or observing interference effects, which allows us to escape from the paradoxical necessity of concluding that the behavior of an electron or a photon should depend on the presence of a slit in the diaphragm through which it could be proved not to pass. We have here to do with a typical example of how the complementary phenomena appear under mutually exclusive experimental arrangements and are just faced with the impossibility, in the analysis of quantum effects, of drawing any sharp separation between an independent behavior of atomic objects and their interaction with the measuring instruments which serve to define the conditions under which the phenomena occur (p. 46).” Remarkably, Bohr explains the experiment using the complementarity principle. Complementarity is the concept that a single entity can exhibit different (or seemingly contradictory) properties depending on the perspectives or context in which it is observed.

 

Strictly speaking, Feynman’s statement is not a general formulation of the uncertainty principle but rather a specific application to the double-slit experiment. An alternative phrasing of the principle, avoiding terms like “apparatus” and "disturb," might be: It is impossible to determine which hole the electron passes through without affecting the visibility of the interference pattern. Specifically, the distinguishability-visibility relation in a modern double slit experiment may be expressed as: D² + V² ≤ 1 (Jaeger, 2007). This inequality means that as path distinguishability (D) increases, the visibility (V) of the interference pattern must decrease, and vice versa. In the double-slit experiment, when D = 1 (complete distinguishability or complete path knowledge), V = 0, meaning no interference pattern will appear. Conversely, when D = 0 (no path knowledge), V = 1, meaning a full interference pattern.

 

3. Complementary descriptions:

“If one looks at the holes or, more accurately, if one has a piece of apparatus which is capable of determining whether the electrons go through hole 1 or hole 2, then one can say that it goes either through hole 1 or hole 2. But, when one does not try to tell which way the electron goes, when there is nothing in the experiment to disturb the electrons, then one may not say that an electron goes either through hole 1 or hole 2. If one does say that, and starts to make any deductions from the statement, he will make errors in the analysis. This is the logical tightrope on which we must walk if we wish to describe nature successfully (Feynman et al., 1963, p. 37–9).”

 

Feynman’s “logical tightrope” could be related to complementary descriptions, which include classical and quantum descriptions. Proposition A—"each electron either goes through hole 1 or hole 2" is a classical description, which can be determined by an experiment. Perhaps Feynman could have included Proposition B: “each electron exists as a superposition of different states (or possible paths),” which is a quantum description. This describes the electron’s quantum state, where its path remains undefined until measured. In essence, the classical description captures the electron’s particle-like behavior, emphasizing a definite path when observed, while the quantum description highlights its wave-like nature, characterized by an indefinite path when unobserved. This duality illustrates the principle of complementarity, where the wave and particle descriptions are not contradictory but dependent on the experimental setup.

 

In the Audio Recordings* [39 min: 30 sec] of this lecture, Feynman says something like: “The world must be entirely quantum mechanical. It cannot be half classical and half quantum mechanical. If, for example, shmootrinos, a new particle, would all go exactly like waves according to classical physics, we can make any intensity we want. Then, we can just use them to watch electrons, we can cut the intensity down because they don’t have the rules of quantum mechanics, then it would be a paradox...” Feynman also used the word shmootrinos in his lecture on gravitation delivered to advanced graduate students. In Feynman’s (1995) words, “These we might think of as photons or gravitons or neutrinos, or maybe some new particles, some shmootrinos which don't worry about baryon conservation. When they meet another shmootrino dropping in from the other side with opposite momentum, these can have enough energy to create a hydrogen atom.” This playful invention serves to emphasize the necessity of quantum mechanical rules, suggesting the inconsistency that would arise if a new (classical?) particle existed in a quantum world.

*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

“In Fig. 37–5 we have tried to indicate schematically what happens with large-scale objects. Part (a) of the figure shows the probability distribution one might predict for bullets, using quantum mechanics. The rapid wiggles are supposed to represent the interference pattern one gets for waves of very short wavelength. Any physical detector, however, straddles several wiggles of the probability curve, so that the measurements show the smooth curve drawn in part (b) of the figure (Feynman et al., 1963, p. 37–10).”

 

It might seem surprising that the double-slit experiment involving bullets could produce an interference pattern as shown in Fig. 37–5(a). This can be explained by the de Broglie wavelength of bullets, which is extremely small compared to objects like electrons. The rapid wiggles in the interference pattern predicted by quantum mechanics for bullets could be observed if a sufficiently large number of bullets is fired under highly controlled conditions. In practical experiments, however, real-world factors such as slight imperfections in the slit edges (e.g., sharp versus smooth edges) and the granularity of the bullet impacts would likely introduce significant noise, obscuring the interference pattern. If the separation between the two slits were increased, the probability distribution might display two distinct peaks instead of one. Feynman’s analysis also implicitly assumes that the bullets are highly correlated—that is, they possess nearly identical energies or de Broglie wavelengths. Without this assumption, the underlying quantum mechanical effects would be overwhelmed by classical randomness.

Review Questions:

1. Would you explain the uncertainty principle using the observer effect? (Do you agree with Feynman’s statement of uncertainty principle)?

2. Would you explain the double slit experiment using the uncertainty principle or complementarity principle?

3. How would you explain the logical tightrope on which we must walk if we wish to describe nature successfully?

 

The moral of the lesson: The test of a first-rate intelligence is the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function (Fitzgerald, 2009).

 

References:

1. Bohr, N. (1949). Discussion with Einstein on Epistemological Problems in Atomic Physics, In N. Bohr, Philosophical Writings of Niels Bohr, 3 vols. Woodbridge: Ox Bow Press.

2. 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.

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

4. Jaeger, G. (2007). Quantum information. New York: Springer.

5. Fitzgerald, F. S. (2009). The crack-up. New Directions Publishing.