4. Geometry of Wavefunctions
4.1. Wavefunction as a dual-process
Wavefunction is the heart of the quantum mechanics (Dirac, 1930/1958). Von Newman (1932) regards that the wavefunction in quantum mechanics involves two processes. One is the U-process (unitary) and the other is the R-process (reduction). During the U-process, the wavefunction evolves by following the Schrödinger equation:
In this equation, the imaginative i makes the U-process not directly observable. The equation contains the mass term m such that the U-process produces superposition states. Superpositions occur when the particle is massive and spins half, which is the intrinsic property of a quantum particle with the internal space.
The R-process is based on observations. Quantum observations involves the so-call Yes/No measurement (von Newman, 1932; Penrose, 2004). There is a long-standing controversy about the measurement problem, which Penrose also calls the measurement paradox. It is still an open conjecture. The Yes/No measurement can be seen as the polarization of the wavefunction.
Penrose denotes Schrödinger evolution by U and state reduction by R. He wrote, “This alternation between the two completely different-looking procedures would appear to be a distinctly odd type of way for a universe to behave!” The figure Penrose provided is roughly given below.
Penrose wrote further, “Why is R mathematically inconsistent with U? Perhaps the most obvious reason is that R represents a discontinuous change in the state vector, whereas U always acts continuously. But even if we imagine that the ‘jump’ induced by R is not absolutely instantaneous, there would be trouble with uncertainty because of the lack of determinism in R. Different alternative outcomes can result from the same input, which is something that never happens with U. Moreover, a theory that makes R into a real process cannot ever be unitary when a (non-trivial) quantum jump – in accordance with R – actually takes place.”
In this section, we provide an alternative view for such a dual-process of the wavefunction.
4.2. The R-process and the Yes/No measurement
“Once animals begin to approach certain things and avoid others, the ability to adjust their judgement of good and bad becomes a matter of life and death. … “Learning was not the core function of the first brain; it was merely a feature, a trick to optimize steering decisions. Association, prediction, and learning merged for tweaking goodness and badness of things.” (Max Bennet, A brief History of Intelligence. 2024). The Good/Bad distinction is the origin of the Yes/No measurement.
Von Neumann pointed out (1932, 1955/2018) that experimental observations in quantum mechanics can be reduced to a kind of "Yes/No" measurement. Penrose also elaborates on this idea in details. In short, in a quantum experimentation, the particle detector is referred to as the "Yes gate." When the particle source excites a particle and the detector receives it, it is said that the particle has entered the Yes gate. If the detector does not receive the particle, rather than saying the particle was not excited, it is said that the excited particle has entered the "No-gate." This description differs from classical mechanics and is counterintuitive, but it is a key feature of quantum mechanics.
Such "Yes/No" type observations are mathematically represented by the Dirac δ-function (Griffel, 2002):
This function consists of two formulas. The first formula states that if an excited particle enters the Yes gate (the detector, the correct answer, or the predicted future event), the function value is infinite; if the excited particle enters the No gate (not detected, the question is answered incorrectly, or the prediction is wrong), the function value is zero. The second formula is the indefinite integral of the first formula, and its value equals a constant. Here, the second formula tells us that regardless of whether the particle enters the Yes gate or the No gate, the particle has been excited. In philosophical terms, the first formula of the Dirac function can be considered its epistemological support, while the second formula represents its ontological commitment. The Dirac function almost perfectly characterizes "Yes/No" type observations, but it is not a mathematically well-defined function.
It was not until later that the measurement theory in mathematics was developed. In this theory, starting from the second formula of the original Dirac function (the integral formula) to make an ontological commitment, and using the first formula as the integrand, (the test function) to provide the epistemological path, requires that this test function must have at least one "support point." This support point is the excited particle detected by the original quantum observation detector.
4.3. Projections as polarizations
Let us review a few key concepts concerning the measurement paradox.
The first key concept is the Observable Q. The measurement paradox can be simply characterized as U-procedure versus R-procedure. Here U stands for unitary, and R stands for reduction. On one hand, the U procedures work so supremely well for simple enough systems, whereas on the other, we have to give up on U and abruptly, yet stealthily, interpose the R process from time to time. The two quantum processes, U and R, are in conflict. On the one hand, U-procedure is the deterministic process of unitary evolution which can be described by Schrödinger’s Equation which controls the clear-cut temporal evolution of a definite mathematical quantity, namely the state vector The wavefunction in the U process is single-valued, continuous, differentiable, and square-integrable.R-Procedure is the quantum state reduction which takes place when a ‘measurement’ is performed. The R process is a discontinuous random jumping of this same , where only the probabilities of the different outcomes are determined.
The observable operator Q is responsible to transfer from U-process to R-process. Q has two eigenstates, say, one is YES and the other is NO. How the Q operator works is a mystery. Penrose (2004) reviewed and discussed six approaches toward this problem. An interested reader may read his book, particularly its §29.
The second key concept is the Projector E. The projector E was originally introduced by von Neuman (1955). Penrose (2004) provides a thorough characterization of E. Indeed, his §22.5 is titled: Yes/No measurement. It is the view of von Neuman and Penrose that all the quantum measurements are the Yes/No type measurements. Once Q turns a quantum process into the R-procedure, then E projects the quantum state to Yes or else No. In other words, the projector E has exactly two eigenstates.
The third key concept is the Yes/No measurement. The Yes/No type measurement originally proposed by von Neuman (1955) and outlined in detail by Penrose (2004). Both authors stated that all the quantum theoretic experiments are Yes/No type measurements, characterized by the projector E. Examples of experiments in physics, e.g., the Stern-Gerlach experiment, can be found in Sakurai.
E-projector is defined as follows. Consider an any given wavefunction , ranges over all space points of X. For any given space point Q where stands for a testing point. Then, E projects to be Yes or No. We call it the E operation, which stands for the any given the Yes/No observations. In measurement theory (, in (4.2) is called a testing function, and in (4.1) is called the supporting point of . Now we look at an important property of the -function.
Let be an any given one-dimensional wavefunction. Assume () is a R-interval. Then we have
(4.3)
It is held simply by the well-known selectivity of the Dirac -function.
To solve the measurement paradox, the existentiality is not enough. It needs to further formulate a constructive proof. The Yes/No type measurement enables us to recapture paradox of U-process vs. R-process. From mathematical perspectives, it is a single-valued vs. two-valued problem. During the U-process, the wavefunction is single valued. While during the R-process, any measurement of the wavefunction becomes two-valued. However, the two processes must share the same semantics, namely the squared modulus, i.e., the Born probability. Thus, how to generate the required Born probability from R-process is a sensitive issue.
We can see from the above discussion that the projection operator polarizes the wavefunction to two poles: Yes or No. There are two issues for the measurement problem. The first is the semantic one. The wavefunction semantics is the amplitude semantic. The meaning of the U-process of a wavefunction is its amplitude, which is the Born probability that is single-valued. While the R-process is two-valued process, resulting in Yes/No measurement. The second issue is syntactic. The U-process is characterized by the Schrödinger equation, in which the wavefunction is a continuous function, while the R-process is discrete. We solve the first issue in the next section and the second issue in the following section.
4.4. Stochastic sampling
Yang (2024) proposed a stochastic sampling method to generate Born Probability from the R-process. Let us consider an any given wavefunction (x), where x ranges over all of the space points. We assume that (x) is one-dimensional without loss of generality for multi-dimensions. Thus, the corresponding Hilbert space we are currently discussing is one-dimensional, denoted by H. Hence, we may treat all the vectors in H as space points also without loss of generality. Now, it introduces an observation operator Q, which is defined below.
Definition 4.1 For any given a, , . We call that is the observational conjugate of . Accordingly, we define | ranges over all possible observational . Call the observational dual space of .
The necessity of the distinction between the space points and the observational points is analytical to the distinction between of the intuitive natural numbers and the set-theoretic enumerers in Gödel’s work (Yang, 2022).
Consider the power set of , . Now, we start to select the elements from . Notice that this selection process is countable, but the cardinal number of is an uncountable infinity. We may reasonably assume this selection process is stochastic.
Definition 4.2 We introduce a new variable , . Of course, we also have , so we can introduce another variable , where the superscript j indicates the jth element stochastically selected from , the subscript i indicates that x ranges over only those space points within . It is easy to see that connects and . Accordingly, we have
Definition 4.3 We introduce a new operator , called the sample generator. , . Call the testing adjoint of .
Definition 4.4 Stochastic sampling: 1. For any given once a is stochastically selected, its adjoint becomes a testing sample. 2. For any , if it has not been selected, then its adjoint is not a testable sample yet.
Note, this definition is analytical to the expressibility in Gödel’s work [
6]. (Hint, the notion of expressibility is necessary to bridge the relations in Piano arithmetic and functions in the first order theory.) While here the definition of stochastic sampling process is necessary to bridge any
from sampling perspectives.
Definition 4.5 (R-process). Let stand for an any given sample , denote a YES/No type experiment, and q be a Yes/No type stimuli that can use to test . By Dirac bra-ket formalism, we can write this structure as . When gives the stimulus q to , each operational conjugate in returns a Yes/No type response. Thus, is a function of . This is called the R-procedure of the wavefunction. Note, this idea is from Feynman (1965), who calls the final state and the initial state of a quantum theoretic experiment.
Definition 4.6 (Sample space). The sample space for the Yes/No type measurement is two-valued, i.e., This means the E-projector has two and only two eigenstates, of which the eigenvalues are Yes and No.
Definition 4.7 (Sample phase). Consider projector E, for each proper sample of Yes/No type measurement, produces a pair of the yes-number c and the no-number d, which in turn produces a sample phase with respect to the exponential form of . All the possible sample phases form an group, write it G. From Definitions 1 to 4, it is easy to see that G is originally generated from the wavefunction , so we write G as .
Because symmetry, the stochastic sampling here satisfies the required conservativeness. It is worth mentioning that, in addition to the well-documented dynamic phase and Berry phase in the literature of dynamic analysis, the sample phase introduced here is the third kind of phase. This is one significant character of the R-procedure. For the U-procedure, we have the dynamic phase potential group, write it as .
Definition 4.8 (Linearization). The linearization operator L is defined by ).
Definition 4.9 (Sample Born probability). For any given testing sample , which produces a yes-number and a no-number . The sample Born probability is defined by
. (4.4)
Born probability is a kind of explanation, which serves as a semantics for the evolution of wavefunction. As Penrose pointes out [
1], the U-procedure and R-procedure must share the same semantics, i.e., the squared magnitude of two eigenvalues.
Theorem 4.1 (Born rule). The Born probability defined by Definition 9 obeys Born rule.
Proof. Let be a testing sample. eigenstates, Yes or else No. Assume the eigenvalue for Yes is c and the eigenvalue for No is d. Then, by Definition 8, we have ). Hence, by Definition 9, we have
( . (4.5)
This shows that Definition 8 is conformal with respect to the Born rule.
4.5. Geometry of the wavefunction
Consider the Yes/No measurement (Yes, No) as a Boolean Weyl-spinor
. By Theorem 2.1,
rotates through complexification. The phase of such a rotation is a continuous variable. Call this spinor the R-spinor. As we analyzed earlier in §2 and §3, The decomposition and reassemble of this R-spinor result in a Boolean Dirac-spinor with superpositions. Call this new spinor the U-spinor. Denote the R-spinor as
and the U-spinor as
. To compose the two spinors, it satisfies the twistor equation (2.7),
. Recall the
Figure 4.1, we now have a new geometry of the wavefunction as follows.
In this new picture, the wavefunction is represented as a twistor
, which connects straight paths. It is not hard to characterize the twistor path by the path integral. In addition, the angle
in
Figure 4.2 is actually the dynamic phase from gauge field theoretic perspectives; thus, it would be interesting to address the gauge symmetry issue. These are topics that go beyond the scope of this paper.