What is Physics: The individual and the universal, and seeing past the noise

The gist The individual and the universal, and seeing past the noise explores how physics has continuously refined its base concepts by looking past obscuring noises, such as decoherence, to understand underlying reality beyond classical intuition.

Historical Development of Physics

Physics has continuously refined its base concepts on which the whole framework is built over the past five centuries by weaving together observations, experiments, and theoretical constructs into a coherent mesh of understanding of the world around us <ref:1605.03106#pg2>. The beginnings of physics, classical mechanics with Galileo and Newton, required seeing past friction to get at the basic laws of inertia and motion <ref:1605.03106#pg3>. Later, in Einstein’s Special Theory of Relativity, ideas of location in space and time, and relative velocities in motion were modified <ref:1605.03106#pg3>. The recognition that all inertial observers are on par with respect to a common physics necessitated a re-examination of the concepts of space and time <ref:1605.03106#pg3>.

The Role of Quantum Physics and Decoherence

Quantum physics went further regarding even familiar concepts of position, momentum, wave or particle as derived constructs from the classical limit in which we live but not intrinsic to the underlying physics <ref:1605.03106#pg2>. These concepts are not elements of the underlying reality of the Universe only near-universal presence of decoherence making them appear so <ref:1605.03106#pg3>. It is this noise of decoherence that so shapes our intuition and the very definition of terms with which we experience our world that makes it difficult to understand the quantum nature of that world at its base <ref:1605.03106#pg3>. Most crucially, the very idea of the individual, whether an object or an event, distinguished only in a mere label of identity from others identical to it in all the physics, exists only as an approximation in what is called the classical limit <ref:1605.03106#pg2>.

Classical Concepts vs. Underlying Reality

Physics deals with the universal not the individual, as an example in a physical system such as an atom in a particular state, physics deals with the universal system of all such atoms but makes no meaningful prediction of the position of an electron or the time of decay of any specific atom <ref:1605.03106#pg3>. In dealing with large aggregates or degrees of freedom as is most often the case in the classical limit, earlier ideas of position or lifetime are realized as limiting cases <ref:1605.03106#pg3>. Trajectory is tied to the observer, and there is infinity in possible observers at least in principle <ref:1605.03106#pg5>. In Hamilton’s Action Principle formulation of classical mechanics, that physics does not invoke time at all <ref:1605.03106#pg6>.

The Nature of Physical Systems and Observables

Physics studies not an individual particle or event or motion even if we answer a question of the height reached by a specific ball thrown up just once with a specific speed <ref:1605.03106#pg10>. Individual identity is a throwback to our classical intuition, not an element of the underlying reality <ref:1605.03106#pg13>. In quantum mechanics, the physical system is characterized by values for each of that set [of mutually commuting operators] <ref:1605.03106#pg13>. Position and momentum are often incompatible with energy for many systems such as atoms and slits <ref:1605.03106#pg13>.

The Classical Limit and Measurement

The classical limit emerges when an apparatus averages over most or all these phases <ref:1605.03106#pg24>. The very height of a mountain or star is ultimately expressible in terms of fundamental constants such as ħ <ref:1605.03106#pg25>. The very height of a mountain or star is ultimately expressible in terms of fundamental constants such as ħ <ref:1605.03106#pg25>.

The Limits of Speculative Physics

Anthropic arguments and theories like the Many Worlds Interpretation are attempts to account for the nature of the world around us and thus of ‘physics’ from the premise that we are here to ask such questions <ref:1605.03106#pg8>. These attempts fail on their own premises as well <ref:1605.03106#pg8>. Theory, especially when beautiful, suffices <ref:1605.03106#pg8>. Armchair theorizing by the best among us cannot be accepted as truth or physics <ref:1605.03106#pg9>.

Quantum Descriptions of Identity and State

The Bohr-Heisenberg philosophy that a physical state be described only by the arrangements of the apparatus that at least in principle describe it, not extraneous (to physics) entities such as mere ‘labels of individuality,’ was formalized by Dirac in a notation that has come to be universally accepted by physicists <ref:1605.03106#pg19>. Independent of any representation, the state of a system is indicated by a ‘ket’ >, within that bracket symbol standing all the labels measured by those apparatuses <ref:1605.03106#pg25>.

The Wave-Particle Duality

The very dichotomy between particle and wave, so familiar in everyday experience and throughout classical physics, is not an element of physics or of the underlying reality of our Universe <ref:1605.03106#pg26>. Often phrased as a ‘wave-particle duality,’ as if every physical system is simultaneously both, they are again representations, alternative representations through which we access that reality itself which is out of reach <ref:1605.03106#pg26>.

The Classical Limit in Quantum Mechanics

The very acceptance of position as an intrinsic variable in physics is seen to result from ‘decoherence,’ that is, the blurring of the many phases that underlie a quantum system <ref:1605.03106#pg24>. This blurring over or ignoring degrees of freedom accounts in large measure for the way we approach our world through seemingly self-evident concepts such as position (or time or momentum) <ref:1605.03106#pg25>. The classical limit emerges when we average over most or all these phases <ref:1605.03106#pg25>.

The Nature of Physical Observables

Energy, momentum, and angular momentum do have intrinsic significance <ref:1605.03106#pg25>. These quantities are conserved in every decay of an atom or nucleus <ref:1605.03106#pg25>. The laws of conservation are intrinsic elements of physics <ref:1605.03106#pg25>.

The Role of Observation in Defining Reality

Any individual excited atom or Ra nucleus can display any number for the value of how long it lived, a lifetime not an intrinsic property of it, but as with the trajectory of a ball in Sec. II also involves the observation <ref:1605.03106#pg25>. The very ascribing of a time, a lifetime, is necessarily tied to observation <ref:1605.03106#pg25>. Absolute space was realized early to depend on the choice of the origin and, therefore not an element of intrinsic physics <ref:1605.03106#pg25>.

The Classical Limit as a Constraint

Relativistic quantum physics precludes defining a position to better than the Compton wavelength h/mc <ref:1605.03106#pg25>. Only entities with m = 0, and that too bosons (integer spin) have a wave as a classical limit, all others being particles in that limit <ref:1605.03106#pg25>.

The Role of the Observer

Every observer/observation doing so has no question of a single spin s being simultaneously in more than one of them <ref:1605.03106#pg25>. The Stern-Gerlach apparatus that gives one or the other of two values constitutes the entire spectrum <ref:1605.03106#pg25>.

The Final Arbiter of Truth

That natural world is the final arbiter of truth for our physics <ref:1605.03106#pg25>. Physics is ultimately an experimental subject, and our observations and experiments involve veils that we must see past to get closer to the essence of underlying reality <ref:1605.03106#pg25>.

The Classical Limit as a Result of Observation

It is in this that the ‘classical limit’ emerges <ref:1605.

1. Bold headers for concepts of observation: The AI system can be designed to explicitly model decoherence, recognizing that it is this noise of decoherence that so shapes our intuition and the very definition of terms with which we experience our world. This allows the system to distinguish between classical approximations and underlying quantum reality.

2. Bold headers for state representation: The system should utilize the Dirac notation "|>, signifying a state, which restricts labels to all those physical observables that are mutually compatible, that is, whose operators commute. This ensures that only physically meaningful states are used in calculations, avoiding the use of labels like position or momentum when they are not elements of the underlying reality.

3. Bold headers for observational context dependence: The system must be architected to recognize that concepts like trajectory or lifetime are tied to observation, as stated: the very ascribing of a time, a lifetime, is necessarily tied to observation. This prevents the system from making irrevocably separated claims about intrinsic properties without specifying the experimental setup.

4. Bold headers for representation selection: The AI can employ partial wave analysis to make contact with experimental arrangements by choosing between representations based on context, recognizing that the experimental context that determines which one we use. This allows the system to dynamically switch between spherical and parabolic representations depending on the interaction geometry (e.g., whether it is a scattering experiment or a static field scenario).

5. Bold headers for classical limit identification: The system can be programmed to identify when it is operating in the classical limit, noting that this occurs when we are in that limit and where concepts like position and momentum become classical concepts, however intuitive and inevitable they may seem. This prevents the system from misinterpreting classical results as intrinsic quantum elements.