5. Describing the Behaviour of a 4 + 1 Dimensional Wave. The Emergence of the Hilbert Space Structure of Quantum Mechanics in Our Framework.
In this Section, continuing the study of the behaviour of a 4 +1 dimensional wave in our framework, we will direct our efforts toward describing how a 3 + 1 dimensional observer can model the behaviour of a quantity that changes in reference to the 5th dimension.
In order to do that and in accordance with
Section 3, we turn our attention on a specific point in 4D space
= (
). If the 4
th dimension is treated as space, then at any such point the amplitude A of the 4th dimensional wave (not to be confused with the wave’s angular frequency in the 5th dimension which we also expressed as
above) will oscillate in the 5
th dimension with frequency f′ = ω′/2π as shown in
Figure 2.
This means that for a specific point in space and for a specific moment in time, as a 3D observer perceives it, a 4 - dimensional wave would seem to possess many values of A which cannot be known in advance since the observer doesn’t have access to the 5th dimension.
From the very first moment we try to mathematically model a dynamic spacetime, in the sense that we explained above, problems start to arise. More specifically, is it possible for a 3D observer to describe changes that happen in the same moment in time (4th dimension), like the Amplitude oscillation mentioned above?
In order to answer this question and start giving our dynamic spacetime a mathematical foundation, we once again turn to the 3 - dimensional wave of the form given in (1) and we ask a different question which may give us some insight into our problem. Can we model some aspects of the interactions and interferences of 3 dimensional waves without the need of time, only by using space?
Not surprisingly the answer is yes. If these waves all travel with the same speed (
) and all obey the equation:
= f
λ, then we can make predictions about the Amplitude of the wave on a specific point in space in correlation with its Amplitude on another point in space and also make predictions about interference patterns if we know the geometry of the sources and the relative phases of the waves [
2]. This is where complex numbers come into play. For example, in a single wave if we measure the Amplitude (A
1) of the wave in one point in space we can know the amplitude (A
2) of another point at distance dx from the first point by multiplying it with a phase factor in the form:
Also, in the case of the double slit experiment for light (
Figure 3), we know that the Amplitude of the interference pattern for any point on the screen is analogous to
+
, where
,
the distances of the slits from the point measured on the screen and
[
2,
9].
If there was an observer oblivious to the concept of our time (4th dimension) in any point of the screen of the double slit experiment, it would seem to him that the Amplitude of the wave can take many possible values. The only way any conclusion or correlation about the wave and its behavior can arise is with the use of complex numbers. Still some information is lost to the observer (like the exact value of the Amplitude because it oscillates with time, which the observer can’t measure or understand) but at least a great portion of the total information of the system would be accessible (for example if there is a constructive or destructive interference like in the double slit experiment for light).
Taking that into account, the observer who can’t understand and measure time would have to make use of complex functions and associate them with observables which the observer can measure and understand such as wavelength λ or energy (if the energy of a wave is proportional to its frequency which is the case for electromagnetic radiation – photons and free fundamental particles). Also, such waves can not be entirely described only by spatial functions (for example ). Using the complex plane gives us a necessary extra degree of freedom, essential for our correlations.
Complex numbers are also essential for quantum mechanics. Experiments have shown that it is impossible to predict experimental results with real-number quantum theory. Also, the use of complex numbers is apparent in the fact that we can’t derive both Planck-Einstein and deBroglie relations (E=hf and p=h/λ) in quantum mechanics without their use.
A 3D wave is oscillating both in space and in time. For two different points (x
1, y
1, z
1, t
1) and (x
2, y
2, z
2, t
2), making precise correlations about the Amplitude in different times is impossible without any information about the time separation t
2-t
1. Analogous to this, if we (the 3D observer) wanted to describe a 4+1 dimensional wave and model its behaviour, the only way we could achieve this would be through the use of complex numbers, using them for correlations together with quantities measurable in the 3D plane (observables) such as distance, time separation or energy. This is where the connection with quantum mechanics in our framework starts to arise, since in quantum mechanics there is also a need for operators (which are measurable quantities) in order to determine the evolution of the quantum state and its expectation values, in reference with the values this quantum state possesses in a different point in space or in time [
4,
5].
More specifically, if we consider a 4 + 1 dimensional wave with a constant angular frequency in the 5th dimension, meaning a constant mass in our framework (which is logical since we try to draw conclusions about the similarity of these waves and quantum mechanical particles with constant mass), the wave’s amplitude would take the form:
where x′ is the 4 dimensional space (3 + 1 dimensional spacetime for the observer) and τ is the 5
th dimensional time for this wave.
Analogous to monochromatic classical waves expressed as:
where one can calculate interference and interactions without involving time, only by knowing the spatial part, which is also complex.
Additionally, translations (which refer to shifting a system in 3 dimensional space and time) would only be possible by correlating the 4 dimensional spatial function Φ0(x′) at a 4D point x′ with the 4 dimensional spatial function at another 4D point, with the spatial difference between these two points and a measurable quantity that encodes the dynamics of the system in that direction. This is analogous what happens in classical waves where we multiply with phase factors in order to correlate the Amplitude of a wave in one point with the Amplitude of that wave in another point.
Mathematically, such translations would have to be implemented through unitary operators generated by observable quantities. For a small displacement in 4D space (
) the function Φ
0(x′) transforms as:
where
would be an operator that encodes the dynamics of the system in the direction of the displacement.
Since Φ0() is not the full description of our wave solution, which evolves also in the 5th dimension, its evolution must be state-dependent and must be generated by operators not simple numbers. These operators contain the dynamical rules (e.g., frequency, mass, momentum) that determine how the wave transforms when shifted, just as the momentum operator generates phase shifts in ordinary wave mechanics.
Also, in the classical case some observed physical quantities are dependent on the square of the complex spatial part ψ(r) like intensity (the energy per unit area per unit time transported by the wave).
This is also the case with quantum mechanics.
All the above show that in our framework since we intend to model the behaviour of 4 + 1 dimensional waves with only spatial components (4 dimensional space for the wave), the most effective thing to do would be to use complex 4 dimensional functions. For this purpose, the natural formalism would be a Hilbert space of complex-valued functions, where:
- -
Observable quantities, which help us make correlations, would be treated as Operators
- -
Expectation values would encode measurable quantities we are interested in measuring and would be calculated by:
To summarize, the oscillatory wave solution of a harmonic 4 + 1 dimensional wave (which implies a particle with constant mass in our framework):
naturally suggests that the space of all such solutions forms a complex vector space. We can define a complex vector space
, where Φ
0(x′) ∈
and endow the space with an inner product (the superscript “2” comes from the type of integrability condition imposed on the functions in that space – real valued square-integrable functions). Observables are then modeled as linear operators acting on this space, with measurable quantities obtained via:
This formalism aligns with the Hilbert space structure of quantum mechanics, allowing us to define observables as self-adjoint operators and extract physical quantities through expectation values.
Furthermore, any interaction of such a wave which results in an irreversible exchange of energy or an irreversible change in one or more of the wave’s characteristics would have to be interpreted as a discontinuous update of the 4-dimensional complex-valued function Φ0(x′) ∈ This update can be modeled via projection operators associated with the eigenstates of a self-adjoint observable . Upon obtaining the outcome, the complex-valued function Φ0(x′) collapses to the new state’s corresponding eigenfunction and all future 5th dimensional evolution proceeds from this new state. This process parallels the standard collapse postulate of quantum mechanics referring to the quantum measurement problem.
All of the above demonstrate that the use of quantum formalism in our framework is not merely an analogy, but a mathematically necessary structure. Moreover, quantum behavior emerges naturally from the underlying dynamics, rather than being introduced through external postulates. This is also a key difference between our framework and other classical higher dimensional theories where quantum correlations are not natural outcomes and have to be imposed by turning classical fields into quantum fields, promoting classical observables into operators and then defining probabilistic behavior through a Hilbert space, that does not emerge naturally from the theory.
Finally, we will try to give the most basic form of an equation in our framework by attempting to model a pure 4 +1 dimensional wave of a Scalar Field (Φ), which does not interact with lower dimensional disturbances of itself or any other field and propagates in a harmonic way. The magnitude of its rate of transmition is taken to be equal to the magnitude of the speed of light. For this wave the 5th dimension is acting as time and the 4th dimension (our time dimension) is acting as another spatial dimension. We are interested in modelling this wave in a way that makes sense to us, the 3 + 1 dimensional observer, following the same rules we imposed on the previous Sections.
This equation would take the form:
The following apply:
- -
The wave is 4 + 1 dimensional which means that time for this wave is the 5th dimension
- -
For this wave our time (the 4th dimension) is behaving as a spatial dimension. For this reason, our time dimension will be included in the spatial derivative terms
- -
Since we are modelling the wave’s behaviour in the reference frame of a 3 + 1 dimensional observer, all quantities and relations must apply to that reference frame.
Taking all these into account our equation should have the form:
Considering that the wave propagates in a harmonic way in the 5
th dimension and taking a sinusoidal solution Φ, the second 5
th dimensional derivative term will be in the form:
(where
is the wave’s angular frequency in the 5th dimension)
The second spatial derivative terms will now include 3-dimensional time (4
th dimension) and we will again make use of the Minkowski metric (metric tensor for flat spacetime) because we want the results to have a physical meaning to us the 3 + 1 dimensional observer. This means that the spatial derivative terms will take the form [
6]:
Combining (10), (11) and (12) we get:
which is equal to the Klein – Gordon equation if we consider that:
The same result for as the one we derived earlier!
This is very promising since the mass term we derived by alternative means in
Section 4 is identical to the mass related component in the Klein – Gordon and Dirac equations [
7,
8]:
Also, in QFT the mass term is recognized as a term in the Lagrangian that is quadratic in the field and has the form
for some
(
∝ m the mass of the particle) [
7,
8]. Since we can not exactly model the behaviour of 4+1 dimensional waves as we showed in this Section and need to make correlations with observables, such a term would make sense to appear in any attempt of the 3D observer to model the dynamics and possible interactions and of those waves with themselves and other lower dimensional waves.
What makes the present work fundamentally depart from other Kaluza – Klein theories, where mass is interpreted as momentum in the extra dimension is that in our approach the fifth dimension is not an extra spatial coordinate but represents a dynamical temporal dimension. This means that we do not need to rely on compactification or gauge unification and then impose quantization in order to make the equation quantum, but instead the formalism derived earlier both provide quantum behavior and compatibility with special relativity which is emergent from the framework itself.
Additionally, in other higher dimensional theories the values of the field in the fifth dimension correspond to different events in 5D spacetime and do not collapse into one 4D event unless some mechanism (e.g. compactification, brane confinement, integral over the 5th dimension) forces that [
16]. This means that in those theories both the deduction of the four-dimensional Klein-Gordon equation from a five-dimensional wave equation of zero (hyper-)mass and the superposition of different values of that field in one 4D event do not work unless we use a methodology or mathematical trick external to the theory. Our framework has no use for that.
Finally, our framework provides a natural avenue for integrating different types of physical equations (including those that treat time and space asymmetrically) due to the dynamic role of the time dimension in our framework. Since time is not a fixed dimension for all phenomena (fields and their disturbances) and interactions but the +1 dimension relative to a field disturbance propagation, the mathematical form of energy dispersion may vary with dimensionality.
Take for example the case of the heat equation or diffusion in general.
It is first-order in time but second-order in space, because it models irreversible energy dispersion that depends on spatial imbalance (curvature) but has no memory or oscillatory behavior. By contrast, the wave equation is second-order in both time and space because waves involve oscillations and the acceleration (2nd time derivative) is tied to spatial curvature, reflecting the symmetric, oscillatory nature of wave propagation.
In our framework, distinctions between how space and time are treated for the same interaction may emerge naturally from the dimensional context of the interaction. For example, a higher-dimensional field could experience diffusion therefore treating lower dimensional time in a second-order, while a lower-dimensional field might also evolve in a diffusion-like, first-order way in its own time coordinate. This opens the door to a geometric reinterpretation of the Schrödinger equation (first-order in time), the Dirac equation (first-order in time and space), and even non-Hermitian dissipative systems.
Therefore, the dynamic time concept allows us to consider hybrid evolution equations (first-order in some dimensions, second-order in others) all governed by how energy propagates and disperses across interacting fields of different dimensionalities. The consideration of different fields of different dimensionalities and their interactions may also eliminate constraints between the gradients along the different coordinates (since time in one field can behave as space in another), potentially accommodating the integration of distinct symmetries and symmetry violations across different interactions, in a manner analogous to what occurs in quantum field theory (QFT). This line of thinking may allow further generalization and unification of quantum, classical, and dissipative dynamics under a common higher-dimensional geometric structure, though this is much beyond the scope of this paper.
Until now we have been focusing on wave solutions in our framework and not on system dynamics. This has been intentional because we want to focus on how both relativistic and quantum mechanical behaviour emerges from these solutions. However, the inherently dynamic structure of our model also provides a natural avenue for examining the emergence of physical laws from first principles. Let’s consider an example:
Suppose that in the context of our framework, every 4 + 1 dimensional field experiences a harmonic restoring force in the fifth dimension, aiming to restore the field to a minimum value. Such a force would take the general form:
providing that our system is conservative and we have harmonic motion in the fifth dimension.
Taking the Minkowski Metric, applying what we said in
Section 4 and demanding that invariance must be conserved we derive:
f = -
, where f is the projection of F onto a unit 4vector
= (
If we require that energy is conserved in the 3 + 1 dimensional reference frame, which means that
= 0, and the evolution of the field is governed by the spatial derivatives of energy, then the integral
δx dt, in a specific direction x, which is the action for a 3 + 1 dimensional observer is:
This means that there exists a minimum non-zero variation in action corresponding to the smallest physical influence a system can undergo in this framework. As a result, the total contribution of all those 4 forces projected to the 3 + 1 dimensional reference frame from all possible directions to a certain 4 dimensional point, would be
and consist of all possible
from all possible directions, resulting in distinct (quantized) units of action.
This postulated quantization aligns with the principles underlying both the Bohr–Sommerfeld quantization rule:
and the path integral formulation of quantum mechanics, where transition amplitudes are weighted by a phase factor:
This means that quantization may not need to be postulated externally in our framework but could emerge from the deeper structure of how higher-dimensional dynamics project into the observable 3+1-dimensional world, requiring that energy and causality are preserved under projection.