While they are more noticeable at the microscopic level, quantum effects are not confined to it.
I have always wondered if the White Queen’s belief in “six impossible things before breakfast” was a subtle reference to the strangeness of quantum physics. The quantum realm is not just filled with peculiarities from Alice in Wonderland; it requires navigating through a cloud of probability and uncertainty and deviates significantly from the common-sense understanding of reality in the macroscopic world.
Exploring the quantum realm is akin to entering Alice’s Wonderland, where common sense is regularly challenged. (Photo Credit: Pushkin/Shutterstock)
A significant difference between the classical and quantum realms is that direct observation is not possible in the quantum realm. Instead, it can only be explored through the use of instruments. While the smaller components (microscopic entities) that make up familiar objects (macroscopic entities) do not conform to classical rules and exhibit behavior that can only be described by quantum theory, familiar objects behave classically and appear with definite values and properties, rather than as a superposition of states.
Since our understanding of reality is based on our experiences, it is not surprising that the quantum and classical realities seem incompatible.
When a physical system reaches a certain size, its behavior deviates from the predictions of quantum theory. The larger the system, the more closely its behavior aligns with classical physics rather than quantum physics.
Although quantum properties are significant at the microscopic level, it would be incorrect to infer that quantum physics only operates at the atomic and subatomic scales. There are several examples of macroscopic quantum behavior, such as superconductivity, superfluidity, and BE condensation.
The reason we do not observe quantum effects like superposition in daily life is not because quantum theory breaks down for larger objects, but because decoherence occurs due to inevitable interaction with the environment. Decoherence causes the loss of all quantum aspects and the disappearance of wave-like behavior.
The Measurement Problem
Classical physics operates under the epistemological assumption that the properties of any object can be accurately measured. It offers a reality unaffected by the measurement process, where information can be obtained without influencing the information itself. The acquired information is independent of the measurement process and the observer. Thus, the knowable world is limited to the physical realm.
However, according to quantum physics, any attempt (by the external world) to measure or acquire knowledge of quantum superpositions, whether through interaction with the environment or even a single photon, collapses the superposition states into a single classical state with definite values. It also eliminates the ability of individual states to interfere with each other.
The cup and ball game (Photo Credit: Maksim Mazur/Shutterstock)
Let’s consider the cup and ball game. This popular sleight of hand trick involves a ball or small object hidden under one of three inverted cups. Rapid shuffling is done to confuse the spectator about the ball’s position.
One firmly believes that the cup under which the ball is found is the cup it has been under all along. However, in the quantum realm, this poses a problem. When we consider the ball as a quantum entity described by a wave function, many questions arise.
Was the ball truly under that specific cup the entire time? Was it the act of looking that caused the ball to appear where it was observed?
Measurement is therefore responsible for transforming an entity into an object with definite outcomes, rather than defining its existence as a range of possibilities. Whether an entity exhibits particle or wave behavior depends on the measurement performed on it. A measurement in physics involves any interaction of the quantum system with the environment and does not require an observer to make an observation, which would result in the collapse of the wave function.
Quantum entities exhibit wave or particle behavior depending on the circumstances and the observed effect.
Why do we not observe wave behavior for large objects?
Macroscopic objects cannot be described by coherent wave functions because it is difficult to isolate them from the environment. Therefore, in everyday experiences, we are unable to observe macroscopic objects as a superposition of states, and their behavior is reduced to that of particles.
If we were able to isolate a macroscopic object, would it behave like a quantum object? The de Broglie equation, which applies to objects of all sizes, uses the extremely small Planck’s constant. When divided by a large mass, the wavelength associated with macroscopic objects becomes so minuscule that interference effects are negligible. Therefore, macroscopic objects like cats or humans cannot behave like quantum objects.
Coherence
Quantum effects such as interference can be attributed to the wave behavior of matter. In order for these effects to be observable, the wave functions of smaller components of macroscopic objects must be precisely aligned with each other. Otherwise, the wave behavior is averaged out and only the classical behavior remains.
A simple analogy is synchronized swimmers who perform in a way that all the small waves created by their movements combine to form one large water wave. This system is considered coherent. On the other hand, the waves created by children randomly splashing water in a pool cancel each other out and the system is considered non-coherent.
When the wave properties of the constituent parts of matter are aligned, the overall wave function of the object becomes coherent. The more coherent an object is, the more prominent its wave behavior.
The classical world is the world as viewed by physics before the emergence of quantum mechanics. In this world, a physical system consists of objects with definite properties. In the quantum realm, however, these properties do not have definite values and are often involved in interactions with the environment.
As long as a measurement is not taken on the system, the various values that a property can have exist together. One could say that the behavior of the system is the combined result of many different systems that are all interfering with each other. However, there is still a consistency that needs to be maintained in terms of how these properties are connected to each other. Coherence does not mean that a system lacks structure; it means that the system has more freedom in how it can evolve over time.
Recovering superpositions becomes even more difficult for macroscopic objects, which interact extensively with their surroundings, because quantum effects are fragile and easily disrupted by interactions with the environment. This occurs through a process called Entanglement, which makes the quantum system and the environment inseparable. The interacting entities become entangled in a combined state where the quantum state of any individual entity cannot be described independently of the state of the other entities it is interacting with. As a result, the superposition of the original particle spreads into the environment.
When multiple particles are interacting, this scrambling process occurs almost instantaneously. As the superposition extends into the environment, the wave function of the initial particle becomes mixed with those of its surrounding particles, causing the loss of the phase relationship that needs to be preserved.
According to quantum theory, particles can exist in a state of superposition until a measurement is made on the system. The uncertainty principle is significant for individual quantum entities or small collections of quantum entities because in systems consisting of a large number of quantum entities, the uncertainty of the constituent elements may average out, making the system as a whole appear less quantum in its mechanics.
The behavior of x quantum entities cannot be assumed to be the same as the behavior of one entity multiplied by x. For a single particle, uncertainty plays a significant role, but since quantum theory is inherently statistical, for a statistically significant collection of particles, the uncertainty becomes insignificant and the system follows the equations of quantum mechanics with reasonable predictability.
Whether or not there is a strict limit for the transition between the quantum and the classical is still an intriguing question, and if such a limit exists, it has not been discovered yet. In 1999, a team at the University of Vienna conducted a double-slit experiment using fullerene (C60) molecules to investigate how large an object could be while still exhibiting wave behavior. They observed a clear interference pattern, indicating the occurrence of superposition even in molecules as large as C60 (0.7 nanometers across). They also noticed that the interference pattern gradually disappeared when a background gas was introduced into the chamber, as predicted. The collision of the gas molecules with the fullerene molecules caused the wave nature to diminish.
In 2011, another team demonstrated coherence in carbon-based organic molecules consisting of approximately 430 atoms, measuring up to 6 nanometers across. In 2019, the interference experiment was performed using interfering particles with an increased mass of up to 2,000 atoms. In 2020, interference patterns were studied in a biological molecule called gramicidin A1.
Scientists have successfully observed quantum entanglement on a larger scale by creating two aluminum drums the size of red blood cells and causing them to vibrate in sync using microwave frequency. The drums were found to share an entangled state, meaning that their vibrations were correlated. In a recent experiment, there is a possibility that a living organism, the resilient Tardigrade, was entangled with two superconducting qubits.
Roger Penrose suggested that gravity may have a role in the collapse of the wave function, which describes a system in multiple values of position and momentum. According to Penrose, while electrons can remain in superposition for extended periods, gravity imposes a limit on massive particles, leading to an immediate collapse on a macroscopic scale. Therefore, quantum behavior disappears for systems above a certain mass.
The Ghirardi-Rimini-Weber theory, also known as the objective collapse theory, proposes that a particle’s wave function collapses spontaneously and randomly. While the collapse would be rare for a single particle, it becomes more frequent and rapid for macroscopic systems like Schrodinger’s cat, which consist of numerous entangled particles. As a result, superpositions quickly fade away as one collapse triggers the collapse of the entire system.
Although quantum physics applies to everything, its effects are typically unobservable in practice for objects larger than microscopic particles. However, quantum behavior on a macroscopic scale does exist and has been observed.