The Intricacies of Quantum Physics
A fundamental challenge in physics lies in the fact that it is a construct of human invention. We observe the world around us and utilise those observations to build a mental model of reality. This process begins in childhood and continues as we mature, particularly among physicists. While this approach has led to remarkable successes, such as Newton’s laws of motion, it becomes increasingly complex when we attempt to conceptualise the behaviour of minuscule and invisible entities, like subatomic particles.
This realm is often referred to as the “quantum realm,” a term popularised by the film Ant-Man. In truth, it is not a distant place one can don a special suit to explore; it is an intrinsic part of our existence, existing within us. However, the peculiarities of quantum phenomena are so extraordinary that this profound layer of reality could easily be mistaken for an alternate universe. Renowned physicist Richard Feynman once remarked, “Nobody really understands quantum mechanics.”
Feynman did not imply that we are incapable of explaining quantum behaviour; in fact, we now possess models that can predict quantum outcomes with remarkable accuracy. What he meant is that fully comprehending it from our macroscopic perspective is an impossibility. What follows is a series of truths that defy logic, and they require a willingness to accept the absurdity at face value.
The Double-Slit Experiment
To illustrate this, let us examine one of the most renowned experiments in science: the double-slit experiment. Initially conducted by Thomas Young in 1801, this experiment demonstrated that light propagates in waves. Later, in 1961, Claus Jönsson adapted this setup to show that massive subatomic particles also exhibit wave-like behaviour.
Consider the world of the macroscopic. If you throw a ball into a pond, ripples will form, correct? In the quantum domain, however, it is as though physical matter behaves simultaneously as both a ball and a wave. And from this point, things become even more bewildering.
Imagine a wall with two narrow vertical slits, and a machine launching tennis balls toward it. Some balls hit the wall, while others pass through the left or the right slit. Behind the wall is a second partition covered in Velcro, so when a ball strikes it, it adheres. What do we observe?
As anticipated, the balls create two clusters corresponding to the slits. But what occurs if we replace the tennis balls with electrons and employ minuscule slits? Commonly, we envision electrons as tiny spheres orbiting an atomic nucleus, so one might expect a similar outcome on the screen. Instead, we observe multiple bands of impacts, reminiscent of light passing through two slits, as Thomas Young had discovered.
The Nature of Electrons
Since light behaves as a wave, diffraction occurs when it passes through each opening, expanding like a wave in the sea. As the waves from the two slits overlap, they generate an interference pattern: bright spots appear where the waves are in phase, while dark spots emerge where they are out of phase. This is what we see in reality with a red laser light source.
If electrons yield an effect analogous to that of light, can we model these minuscule particles as waves? Indeed, this notion underpins Schrödinger’s equation, which forms the foundation of quantum mechanics, outlining how a quantum system evolves through time and space.
Understanding Superposition
While this is informative, it remains unsatisfactory. We still yearn to understand what is truly happening. Are electrons genuinely behaving like waves traversing both slits and interfering, or does each electron pass through only one slit like a tennis ball? The surprising answer is… both.
We can set our electron cannon to fire one electron at a time. Even in this scenario, where the electrons cannot interfere with one another, over time, the same interference pattern of multiple bands emerges. In some sense, the electrons are interfering with themselves.
This leads physicists to describe the electron as existing in a state of superposition: it embodies a combination of passing through the left slit and the right slit.
The Paradox of Measurement
But wait! Could we not determine which slit an electron passes through? Certainly. By placing a minuscule light source near each slit, if the light reflects off an electron, we can ascertain its path. However, herein lies the paradox: if such a detector is used, the interference pattern disappears. Instead, we observe results indicating that the electron has traversed a single slit, much like the pattern of the tennis balls. The double-slit pattern only emerges when no measurement is made! We say that measurement “collapses the wave function” of a system, meaning the electron is no longer in a state of superposition.
This notion may seem absurd: how can the act of measurement alter the outcome? Does this imply that physics experiments can reshape reality? Perhaps.
Schrödinger’s Cat and Its Implications
Earlier, I mentioned Erwin Schrödinger, a pioneer of quantum theory who struggled to accept the concept of superposition. Grounded in his macroscopic understanding of the world, he insisted that there must be a flaw in the model, believing that a complete theory of quantum mechanics would ultimately be deterministic, akin to classical physics.
When scientists are confronted with discomforting ideas, they often concoct absurd scenarios to invite ridicule. Schrödinger devised a famous thought experiment, known as “Schrödinger’s Cat,” which posits the following: there is a cat in a box, alongside a radioactive atom, a detector, and a vial of poison. If the atom decays and emits radiation, the detector triggers the vial, resulting in the cat’s demise. If the box remains closed, the atom is in a state of superposition, existing as both decayed and non-decayed (implying the cat is simultaneously alive and dead). Only upon opening the box and observing the cat does its wave function collapse into one of the two states: alive or dead.
However, this scenario was swiftly debunked. Firstly, no one claimed that superposition applied to large objects like cats. Secondly, a measurement had already occurred before the box was opened: the device monitoring the atom’s state was already in operation.
The Reality of Quantum Mechanics
In truth, Schrödinger was mistaken (as was Einstein, who concurred with him). Numerous experiments since then have demonstrated that superposition is a reality and that uncertainty is a fundamental characteristic of our universe. Thankfully, it exists, even if it defies comprehension. Superposition is what grants quantum computers their immense capability for parallel processing.
Understanding Quantum States
It is crucial to recognise that the superposition of states does not imply ignorance. If you toss a coin and cover it as it falls, the coin is already either heads or tails; you simply do not yet know which. In the double-slit experiment, we know: it passes through one slit and through both slits.
Quantum systems possess other properties that may leave you bewildered, such as entanglement, where measuring the state of one particle determines the state of another, regardless of the distance separating them. This occurs instantaneously, without allowing time for a signal to travel between them.
Yet, at this point, these phenomena are well-established. So, is it fair to label them as strange? Are we merely revealing the limitations of our human minds? It is evident that the level of reality in which we exist is only part of the narrative, compelling us to remain sceptical of our intuition. At the very least, quantum mechanics teaches us humility. The pursuit of scientific knowledge continues, and intriguingly, the deeper we delve, the greater the enigma becomes.