Quantum Mechanics (QM) stands as the most comprehensive theory available to physicists today, especially concerning all realms except gravity. However, grasping its true implications is notoriously challenging. This is the first time in the field of physics that the mathematical framework we use to articulate a theory is effective, yet it simultaneously defies intuitive interpretation. Richard Feynman’s famous remark is often cited to justify the idea that pondering interpretations of QM might be futile, as comprehending it fully seems out of reach.

This article begins a two-part series, as I realized it would be overwhelming to condense all this material into a single piece. I am also conscious of the anxiety that often accompanies studying QM, a sentiment I recall vividly from my own undergraduate experience.

In this piece, I will concentrate on the measurement process within QM and what it reveals about the fundamental entities that comprise reality, referred to as quantum systems. The second part will build on this foundation to discuss the interpretative challenges surrounding quantum mechanics.

Why are measurements crucial in QM? Measurements forge the connection between the world and us, the inquisitive observers. Each interaction between reality and scientists occurs through measurement. The measurement problem brings ontology—the study of existence—into conflict with epistemology, which concerns our understanding of the world. Are the entities we measure genuinely existent, or do they merely symbolize our limited knowledge of them? In a Kantian sense, is the "Ding an sich" eternally concealed from our perception, with all knowledge filtered through our subjective frameworks, or are we genuinely observing an objective reality? Alternatively, could the only reality we accept be the structure our theories reflect, a concept aligned with contemporary philosophical approaches like structural realism?

When we measure an observable property—such as charge or position—of a quantum system, we connect it with a measuring device that we can interpret. For instance, measuring room temperature involves using a thermometer, where the mercury level corresponds to the temperature.

In quantum mechanics, a similar process occurs: we can couple a measuring device to an observable property like spin. This interaction results in what is termed an entangled state, a distinctive characteristic of QM that I will revisit in the next article.

For now, let’s focus on the curious results that arise even from the simplest quantum measurements.

A basic illustration of a quantum measurement involves a spin system. Spin represents a uniquely quantum property found in particles like electrons and photons, often described as intrinsic angular momentum. If this concept seems unfamiliar, consider it akin to an electron spinning around its own axis.

Visualize the spin as an arrow directed in a specific spatial orientation. It can be aligned upwards in the z-direction or downwards in the x-direction, among other possibilities.

Should we encounter an unknown electron and decide to measure its spin in the z-direction, we will ascertain whether it points either upwards (spin-up) or downwards (spin-down).

At this juncture, everything appears straightforward. However, ponder this: why does the spin point specifically up or down in the z-direction when it could have oriented in any direction? Remember, we had no prior knowledge about the electron. If we consider a rotating sphere, the symmetry is disrupted, revealing a distinct axis around which it spins. Hence, the angular momentum consistently points in a defined spatial direction, independent of measurement.

Assuming for now that our spin measurement results in:

spin = up z

We can restart the measurement process, this time evaluating the spin in the x-direction, leading to a result of either upward or downward in that direction, potentially yielding:

spin = down x

In each measurement, the spin can only be definitively up or down. Herein lies the strangeness: we can combine our findings from the two measurements. After measuring the spin in the z-direction, we know its orientation.

Upon measuring in the x-direction, can we predict the spin's direction? No! It's purely a matter of chance, akin to a coin flip—a true random generator.

We remain oblivious to the spin's orientation in advance.

Bell’s theorem indicates that no hidden variables could provide additional insight into the spin's direction, confirming that we possess all conceivable knowledge about it.

After measuring the spin in both directions, one might assume we have enhanced our understanding of the electron’s spin. For instance, if we measure spin-up in the z-direction followed by spin-down in the x-direction, we might express our findings as:

spin = up z + down x.

If we proceed with a third measurement in the z-direction, logic suggests we should obtain a spin-up.

However, this only occurs 50% of the time. In the other half, the spin may point downward—again, a random generator, and we remain ignorant of its orientation.

The assertion that "God plays dice" arises from these observations.

This puzzling situation raises questions about causality. At the quantum level, phenomena seem to contradict our fundamental notions of cause and effect. Why is there no discernible rationale behind the spin's orientation? How can randomness be so intrinsic to the fabric of physics?

Mathematically, we note that spin observables do not commute, implying that the sequence of measurements holds significance. Therefore, the outcomes differ based on the order of measurements.

In one scenario, measuring:

1. Spin in the z-direction (yielding either up or down)
2. Spin in the x-direction (offering a 50% chance for either outcome)
3. Spin in the z-direction (again with a 50% chance)

contrasts with:

1. Spin in the z-direction (resulting in either up or down)
2. Spin in the z-direction (repeating the first outcome)
3. Spin in the x-direction (with a 50% chance).

In the latter case, after measuring the spin in the z-direction and obtaining a result, a repeated measurement will consistently yield the same outcome, thus supporting the notation:

spin = up z.

However, as previously noted, this encapsulates all we can understand.

For the mathematically inclined, spins are represented by two-dimensional, non-commuting matrices known as Pauli matrices instead of traditional numbers to reflect this phenomenon (the order of matrix multiplication matters, so for matrices A and B, ABA = AAB only holds if A and B commute).

Yet, it’s essential to recognize that it's not entirely random. An underlying structure exists. If we conduct this spin measurement process a thousand times, we can expect roughly 500 up spins and 500 down spins. The law of large numbers applies in quantum mechanics: possessing complete knowledge of the spin allows for statistical predictions of measurement outcomes, and repeated measurements will yield results approximating our predictions.

This underlying structure is encapsulated in the wave function, the core element of quantum mechanics.

These wave functions reside in Hilbert space—a fun tidbit to mention at your next gathering.

The wave function conveys all we can know about spin, integrating the statistical properties of measurements into the essence of reality (as previously mentioned, ontology and epistemology intersect in a perplexing manner within quantum mechanics).

If we simply express our understanding, a spin wave function might appear as follows:

spin = up x (with 50%) + down x (with 50%)

This resembles a probability sum in statistical theory. For instance, describing a dice roll could be formulated as:

dicethrow = 1*(1/6) + 2*(1/6) + 3*(1/6) + 4*(1/6) + 5*(1/6) + 6*(1/6)

However, a critical distinction persists between rolling dice and measuring spin. In dice throwing, we, as observers, could theoretically know which number would land face up: with complete knowledge of the dice and the process, we could perfectly simulate the throw and predict the result with precision.

In contrast, quantum physics defies such perfect simulations. We cannot foresee the outcome of a measurement, no matter how precise our approach, and so far, no clear rationale explains why one measurement yields one result while another produces a different one.

This challenges Leibniz's principle of sufficient reason, which posits that every occurrence should have a comprehensive explanation. We assume that a full understanding of all mechanisms in a physical process will clarify its outcome. But it ain't necessarily so.

This represents just one of the many counterintuitive aspects of QM, yet it lies at the core of the enigma that has perplexed thinkers for the past century. It's a peculiar dilemma—so peculiar that Feynman remarked, "Quantum Mechanics is so confusing that I don’t even know if there’s a problem." The mathematics holds true and functions flawlessly, yet for reasons beyond our comprehension, it eludes a coherent understanding the more we ponder it.

Indeed, this encapsulates the inherent strangeness of Quantum Mechanics.

(The continuation of this discussion can be found in the subsequent article.)