Source: https://www.youtube.com/watch?v=LJ_l_DQ1AnU
In this interview, Sean addresses a fundamental question in quantum mechanics that remains unresolved even after 100 years.I focused specifically on the question of whether the wave function is a tool or a reality. This is because I was struck by the fact that the term “measurement” remains undefined within the foundational theory. I hope to convey how our perception of the world shifts when we set aside the idea of separating the observer from the classical realm.
Taking a Comprehensive Look at the Two Gaps Left by the Copenhagen Interpretation
Sean cites the double-slit experiment as a quick way to demonstrate the strangeness of quantum mechanics. He confirms the wave interference—first demonstrated by Young with light in 1801—by sending electrons one at a time. Although they appear as points at the detector, when repeated many times, interference patterns emerge.Jönsson demonstrated electron interference in 1961, and Akira Tonomura and his colleagues visualized the accumulation of single electrons in 1989. When a measurement is taken to determine which slit the electron passed through, the interference pattern disappears. The contrast between the wave-like behavior when not observed and the particle-like behavior when observed serves as the foundation.
“The answer is that it’s a wave when you’re not looking at it and a particle when you are.”
Translation: The answer is that it is a wave when you are not looking at it, and it becomes a particle when you do.
Sean notes that this single statement accurately captures the essence of the experiment. The wave-like spread when not observed and the particle-like localization at the moment of measurement stand side by side as two manifestations of the same object.The term “wave function” was introduced as a simple label to describe this spread. According to the probabilistic interpretation proposed by Born in 1926, a relationship is established whereby the square of the absolute value of the wave function gives the probability of the measurement result; however, the wave function itself cannot be observed directly. What can be observed is always only the measurement result.
According to Sean’s summary, the position of the Copenhagen interpretation becomes clear here. It is described as a concept originating with Bohr, Heisenberg, and others against the backdrop of the 1927 Solvay Conference.The picture is that the wave function spreads out as a wave until measurement occurs, and at the instant of measurement, it collapses to a specific value. While the probability of the measurement outcome is given, this approach involves a pragmatic acceptance that we do not inquire into what is happening when the system is not being observed.
“It is a way of calculating measurement outcomes. And all that is real are the measurement outcomes. There’s no such thing as what is happening in the quantum world when you are not looking at it.”
Translation: It is nothing more than a tool for calculating measurement outcomes. Only the measurement outcomes are real. There is no such thing as what is happening in the quantum world when you are not looking at it.
This position leaves two gaps. One is the problem that measurement is not defined; the other is the question of whether the wave function is real or merely a tool. The former will be addressed in the next section. As for the latter, the Many-Worlds Interpretation offers a solution by positing the wave function as real.This is based on the line of thought that Everett formulated as “relative states” in 1957 and that DeWitt named the “many-worlds” interpretation in 1970. Here, we’ll focus only on the contrast; the details will be explored further.
When the Boundary of “Measurement” Unravels
According to Sean’s explanation, the measurement problem in the Copenhagen interpretation begins with a surprisingly simple question. He points out that there are no ready answers to the questions: What is a measurement? When does it occur? And what counts as a measurement?He argues that even if we try to draw boundaries based on the size of the apparatus or the type of observer, without a fundamental definition, the theory cannot hold up as a foundational theory.
“What is a measurement? When does that happen? What counts? No one knows. Nobody has any idea. It’s an undefined concept in the Copenhagen interpretation of quantum mechanics.”
Translation: What is a measurement? When does it occur? What counts as a measurement? No one knows. Not a single person understands. It is an undefined concept in the Copenhagen interpretation.
The example of whether it is a person, a cat, a video camera, or a rock is cited to show that no matter where the boundary is drawn, it becomes arbitrary. While in classical mechanics, measurement was treated as an external operation, in quantum mechanics, the concept of measurement is said to be embedded within the fundamental laws themselves.The absence of a definition in this context means there is a gap in the theory’s vocabulary. I have interpreted this gap not as a matter of interpretive preference, but as an incompleteness of the theory itself. What troubles me is that the very act of drawing the line is not derived from the principles.
The Heisenberg Cut
The “Heisenberg cut”—a dividing line—was introduced to temporarily plug this gap. As von Neumann discussed in 1932 in terms of a “measurement chain,” and as Wigner sharpened the problem in 1961 through a thought experiment involving a friend, this involves an operation that treats the observer classically and the object quantum mechanically.It is a two-layer structure in which the object’s wave function is treated quantum mechanically, while the measuring apparatus and the observer are treated classically.
However, Mr. Sean argues that since the observer itself is composed of atoms, this cut does not hold in principle. This is because no matter where the cut is made, the opposite side can still be treated quantum mechanically. The fact that predictions appear unchanged even when the location of the cut is moved indicates that the cut is not a physical boundary.This is the point that Sean rejects most strongly. He argues that treating an observer made of atoms as an exception cannot be consistently justified. I sensed a paradox here: the more exceptions we introduce, the more complex the theory becomes; the more we remove them, the simpler the picture becomes.
The Universe’s Wave Function and Entanglement
Sean argues that removing the cut reveals a different picture. It is the view that there is not an independent wave function for each subsystem, but rather a single wave function for the entire universe.In this concept, known as the wave function of the universe, a single function is posited to collectively specify the probabilities of how the world appears. While we observe only a part of it, the probabilities of the whole are folded into a single function behind the scenes.
From this, the correlation known as entanglement naturally emerges. When we have only the knowledge that two particles are moving in opposite directions, it is sometimes explained as a relationship in which measuring the motion of one immediately determines the other.The sequence of events—first pointed out by EPR in 1935, given verifiability as an inequality by Bell in 1964, confirmed experimentally by Aspect et al. in 1982, and ultimately leading to the 2022 Nobel Prize in Physics—demonstrates that this correlation is not merely a result of a lack of knowledge, but is intrinsic to the structure of the world.
Sean’s Many-Worlds Interpretation directly applies this entanglement to explain measurement. It is understood that a measurement occurs when the observer’s state becomes entangled with the object. In this view, it is not just the electron but the combined system of the electron and the observer that enters a superposition; measurement itself is not accorded any special status.
“Wave functions are real. They represent reality precisely and completely. […] Measuring apparatuses and observers obey the laws of physics. They are quantum mechanical systems. They obey the Schrödinger equation—the same equation that everything else obeys. And there’s nothing special about measurement.”
Translation: Wave functions are real and represent reality precisely and completely. Both measuring instruments and observers are quantum systems that obey the laws of physics; they follow the Schrödinger equation, just like everything else. There is nothing special about measurement.
This position fully embraces the reality of the wave function and removes the observer from the category of exceptions. The consistency of the Schrödinger equation—which applies equally to all systems—is said to serve as the key to unraveling these boundaries.The concept of decoherence, developed by Zeh in 1970 and refined by Zurek in 1981, can be understood as a helpful framework for explaining how the entanglement of numerous particles with the environment obscures the interference patterns we observe in everyday life.The line of reasoning is that it is not a single line of separation, but rather the entanglement with countless degrees of freedom that gives rise to behavior that appears classical.
Why Probability and Interpretation Coexist
Sean points out that another gap regarding the wave function is that the philosophical foundation of the term “probability” has not been firmly established. He presents an analysis showing that two perspectives coexist: one that interprets probability as frequency and another that interprets it as a degree of belief. The former views probability as an objective chance defined in the limit of an infinite number of trials.The classic example of rolling a die an infinite number of times is typical, and Copenhagen-style quantum probability is compatible with this interpretation.
The latter views probability as a degree of certainty regarding single events, such as tomorrow’s weather or predictions of who will win a competition. We hold varying degrees of certainty even regarding events that cannot be repeated.Epistemological approaches such as QBism, developed by Fuchs et al. around 2002, as well as debates surrounding von Mises’s frequentist theory and de Finetti’s subjective theory, demonstrate that these two interpretations each have their own roots.Which is correct remains philosophically unresolved, and interpretations of quantum mechanics coexist while accepting this unresolved status.
This coexistence is sharply questioned in contexts where the Copenhagen interpretation treats the wave function as a tool. The point raised is that something that is supposed to be merely a tool interferes with itself in the double-slit experiment.
“Well, when I sent the wave function through these two slits, it interfered with itself. How does something that isn’t real interfere with itself?”
Translation: When the wave function passed through the two slits, it interfered with itself. How can something that does not exist interfere with itself?
Sean points out that this is where the problem lies—it cannot be explained by simply adding probabilities. With ordinary probabilities, adding the probabilities for each path would yield the total, but since the wave function can take both positive and negative values, cancellation occurs. The alternating dark and light fringes do not arise from a simple sum of probabilities.As shown by the classification of ψ functions systematized by Harrigan and Spekkens in 2010 and the PBR theorem of 2012, interference pulls us toward the idea that the wave function has a physical substance. A paradox remains: even though it cannot be directly observed, it behaves exactly like a physical entity.
Another Path Suggested by the Pilot Wave
Let’s also consider the perspective of another interpretation. The pilot wave—or Bohmian mechanics—suggested by de Broglie in 1927 and formalized by Bohm in 1952, provides the most straightforward answer: that an electron is both a wave and a particle.This model—in which a wave passes through a slit and a particle is guided by the wave to produce interference patterns—is sometimes cited as appealing because it can be mathematically defined with precision in the non-relativistic realm.
On the other hand, achieving consistency with relativistic quantum field theory is not straightforward. It has been pointed out that if the pilot wave is simply carried over into field theory, the structure immediately becomes overly complex. Rather than dismissing this complexity as a technical shortcoming, I interpreted it as a manifestation of the ongoing process of organizing the space for interpretation.Attempts to incorporate contraction itself as a physical process—as GRW did in 1986 with the objective contraction model—can also be situated within this coexistence.
From the Disappearance of Time in Equations to the Emergence of Arrows
Sean argues that the relationship between quantum mechanics and gravity boils down to the fact that standard quantization does not work straightforwardly in the context of gravity.Although quantum mechanics is a framework that replaces classical mechanics, in physics education, it is often presented as a process of quantizing classical systems. While quantum theories for electromagnetism and the nuclear force have been constructed using this approach and are considered consistent, the same procedure does not work well for gravity as described by general relativity.Sean suggests that a correct theory of quantum gravity may not be simply a quantization of existing classical theories. This is a reversed perspective: it is not that gravity is difficult to quantize, but rather that we were fortunate that other forces were easy to quantize.
The Wheeler–DeWitt Equation and the Problem of Time
Applying canonical quantization to gravity yields the Wheeler-DeWitt equation. This equation, first presented by DeWitt in 1967, states that the rate of change of the universe’s wave function is zero.
“Do that for gravity, and you get this equation called the Wheeler-DeWitt equation, named after two famous physicists. And the Wheeler-DeWitt equation says the rate at which the wave function is changing is zero. That’s all it says. It’s not changing. And so, there is no time.”
Translation: If you apply this to gravity, you get an equation called the Wheeler-DeWitt equation—an equation named after two famous physicists. And this equation states that the rate at which the wave function changes is zero. That’s all it says. It isn’t changing. Therefore, there is no time.
Whereas the standard Schrödinger equation describes time evolution, here we encounter the paradox that time itself disappears. This is a topic that has been referred to in the literature as “the problem of time.”This raises the question of how the time we experience emerges from an equation that contains no time. While the term “emergence” may seem to resolve the issue, it has been pointed out that attempting to derive its specific manifestation leads to different physical laws depending on the chosen derivation path.It has been pointed out that the results appear to vary depending on which derivation is chosen, and it is argued that this leaves room for arbitrariness in the choice of derivation. I interpreted this arbitrariness as a gap that has opened up between the broad concept of “emergence” and the specific details of the actual derivations.Both the no-boundary proposal presented by Hartle and Hawking in 1983 and the relational description discussed by Page and Wootters in 1983 can be situated within the context of the same difficulty: how to extract time.
The Initial Condition of Low Entropy
Sean points out that the problem of the arrow of time runs parallel to the problem of time itself. Whether it is Newtonian mechanics, Maxwell’s equations, the general theory of relativity, or the Schrödinger equation, the fundamental laws themselves are symmetric under time reversal. Yet, in our experience, the past and the future are clearly asymmetric.This asymmetry—where memories belong to the past, and while causality extends into the future, it does not reach back into the past—permeates our daily lives.
It is often said that this arrow is explained by the initial conditions. The explanation is that since the universe began with extremely low entropy near the Big Bang, the direction of increasing entropy from that point manifests as an arrow.This is based on Boltzmann’s 1877 definition of entropy as S = k log W. The understanding is that entropy represents the number of microscopic arrangements that can produce states appearing macroscopically identical. Since a mixed state allows for an overwhelmingly greater number of possible arrangements, entropy is said to proceed in the direction of increase.
However, the question of why the entropy of the early universe was so low remains unresolved. As Penrose discussed in his Weyl curvature hypothesis, there are suggestions that the key may lie in how we count degrees of freedom—including those of gravity—but a definitive explanation has not yet been established.I identified a two-tiered structure—the symmetry of the laws and the asymmetry of the initial conditions—as the core of the explanation for the direction of the arrow. The arrow does not emerge from the laws alone; rather, it is embedded in how the beginning is defined.
An Attempt at a Cyclic Universe
The problem of initial conditions is directly linked to the question of how to treat the beginning of the universe.While the Big Bang appears as a singularity in the predictions of classical general relativity, if quantum mechanics is correct, the description of the singularity itself may be nothing more than an extrapolation from classical theory. There is hope that our description of the beginning might change once quantum gravity is understood.
This brings us back to the long-standing idea of a cyclic universe.Apart from the Hindu view of the universe and Nietzsche’s concept of the “eternal recurrence,” it is sometimes argued that solutions involving repeated expansion and contraction can be mathematically formulated within the framework of general relativity. However, since the equations break down at both ends—the Big Bang and the Big Crunch—efforts have been made to replace the singularity with a “bounce.”The issue lies in the treatment of the direction of time. Many cyclic models have uniformly set the direction of time from the past to the future and fixed it as an initial condition. As Hawking once discussed the reversal of the direction of time—and later reportedly acknowledged his error—it is not straightforward to determine whether entropy decreases or continues to increase during the contraction phase.
The model mentioned in the interview—which Sean is developing together with Nadia Dyachkova and Sakshi Dulani—proposes a framework that starts with the Schrödinger equation and branches into two paths depending on whether the state space is bounded or not.In the bounded case, the reasoning is that if the system evolves smoothly and deterministically over infinite time within a finite set of possibilities, periodic solutions will emerge. The picture depicted is of a universe that begins with low entropy, undergoes expansion and cooling toward heat death, and—after a long period of equilibrium during which the arrow of time vanishes—moves toward contraction in a time-reversed manner.At the bounce, entropy returns to nearly zero, suggesting a relationship in which the expanding and contracting sides each regard the other as the “future.” I interpreted this image as an attempt to weave low entropy into a cycle without fixing the arrow in a single direction.While acknowledging this as an unfinished concept, I sense in the approach—which seeks to consider emergence and the origin of the arrow on the same plane—the same commitment to consistency that I observed in the measurement problem.
A New Way of Viewing the World
If we do not assign special status to the term “measurement” and treat the wave function as a reality through to the very end, our view of the world becomes much clearer. It is this clarity that I took away with me. Instead of agonizing over where to draw the line, we treat both the observer and the apparatus as quantum systems governed by the same equations, thereby achieving consistency by describing observation as a superposition of coupled systems.The idea that branching is not an added claim but rather a consequence of taking the equations seriously made perfect sense to me as a way to avoid mystifying measurement.
At the same time, the mystery of the initial condition of low entropy remains.The explanation that attributes the direction of the arrow not to a law but to how the system was initially set up allows us to understand the arrow’s direction without postponing the question of why the initial conditions were so orderly. I also felt that the arbitrariness remaining in the derivation of emergent time must be faced head-on as a form of diversity, rather than being glossed over with words.
Even while grappling with these two issues, I still want to take a chance on the perspective that views the world as a single wave function. This is a vision in which there exists a function that collectively bears the probabilities of the entire universe, and through entanglement, our observations are linked to a part of it. Observation is repositioned not as a ritual that bestows reality upon the world, but as a physical process within the world itself.What remains as a shift in perspective is the sense that the world does not come into existence only when it is observed, but continues to spread out as a wave even when unobserved. Within that expanse, observation can be understood as an event in which we find ourselves at one of the branches. Questions that remain unresolved even after 100 years seem to reveal the depth of the world through the very fact that they remain unsolved.
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