In spite of its spectacular success, quantum mechanics tends to defy the common sense notions of causality, locality and realism.

Let’s first consider what each of these ideas mean in everyday life.

Realism concerns with the philosophical questions such as: does the moon exist even when you’re not looking at it? The macroscopic world described by classical physics is the world of our daily experience. Here an object exists regardless of its observer: the moon exists whether one looks at it or not. Realism is the philosophical position ascribed to Descartes: “the physical world has objectivity that transcends direct experience, and that propositions are true or false independent of our ability to discern which they are”1.

Causality, on the other hand, is a deep rooted concept in everyday life and in classical physics where events are ordered in time: a cause can influence an effect only in its future, and not in its past. For example, you can turn on a light bulb by flipping a switch. The act of flipping the switch is the “cause” whose “effect” is to illuminate the bulb. Thus, if an event A is a cause of an effect B, then B cannot be a cause of A.

The “principle of locality” (Wikipedia) requires that “for an action at one point to have an influence at another point, something in the space between the points, such as a field, must mediate the action”. The theory of relativity limits the speed at which such an action, interaction, or influence can be transmitted between distant points in space. This speed cannot exceed the speed of light. “Einstein locality” or “local relativistic causality” is often stated as “nothing (energy or information) can propagate faster than light”. “Local realism” is the idea that objects have definite dynamical properties whether or not they are measured, and that measurements of these properties are not affected by events taking place sufficiently far away. Objects can affect other objects only if it is adjacent to it (or via the influence of some kind of field, like an electric field, which can send impulses no faster than the speed of light).

However, these principles break down in the quantum realm.

While classical mechanics is purely “local”, quantum mechanics, in contrast, is inherently nonlocal. The most famous example of a nonlocal effect is a phenomenon called quantum entanglement— one that Einstein considered, back in the 1930s — where a pair of particles are connected in such a way that a measurement of one particle appears to affect the state of another, distant particle. The pair seems to be intrinsically linked so as to share information instantly, thus shattering the principal of locality. This “spooky action at a distance” disturbed Einstein who attributed it as a fundamental conceptual weakness in quantum theory. But numerous experiments, first conducted in the 1980s, have confirmed that this spooky action is indeed how our universe works at the micro level.

Perhaps the most bizarre notion is the standard view of quantum mechanics according to which objects (particles) do not have locations until they are observed. Known as the Copenhagen interpretation, the standard view is the most widely accepted interpretation of quantum mechanics, today (we will shortly see that there are other interpretations of what the mathematical formalism of quantum mechanics implies). Albert Einstein, among others, objected to this idea. His biographer Abraham Pais remembered: “We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it.” The well-defined positions and movements of classical physics is replaced in quantum mechanics by a haze of probabilities described by a mathematical structure known as a wave function. The wave function evolves over time. Its evolution is governed by a partial differential equation called the Schrödinger equation. The act of observing a particle causes the wave function to “collapse,” prior to which event we can say nothing about its location. We will explore more about the wave function collapse later in this article.

“In the quantum world, particles seem to be in two places at the same time, information appears to travel faster than light, and cats can be dead and alive simultaneously”2 (see page 7). And recently, theoretical physicists from the University of Vienna and the Université Libre de Bruxelles have shown that in quantum mechanics it is possible to conceive of situations in which a single event can both be a cause and an effect of another one. The findings were recently published in Nature Communications.

On the other hand, classical physics, which is based on the doctrines of causality, locality, and realism, has demonstrated an astonishing success in describing events of the macroscopic world.

Newton’s theory of gravitation, Maxwell’s theory of electromagnetism, and Einstein’s general relativity theory all fit into the classical framework. So does classical mechanics, where the state of a particle at any moment of time is described by its position and its momentum—we can think of that state as specified by a collection of numbers. Thus the macroscopic world is made of objects characterized by their positions and momenta, acted upon by a certain sets of forces. The job of physics is to use information to predict what will happen next. “If Laplace’s Demon3 knew all of the positions and momenta of every particle in the universe, it could predict the future and the past with perfect fidelity; we know that this is outside of our capabilities, but we can imagine knowing the positions and momenta of a few billiard balls on a frictionless table, and at least in principle we can imagine doing the math. After that it’s just a matter of extrapolation and courage to encompass the entire universe” (Carroll, S. 2010).

With the emergence of the atomic model at the beginning of the twentieth century, physicists tried to understand the behavior of matter on atomic and subatomic scales based on their knowledge of classical physics which was the natural thing to do since that was the only “flavor” of physics known at the time. Eventually they were forced to conclude that the rules of classical physics did not make any sense in the quantum world. Quantum mechanics was born out of necessity to explain the behavior of the large volume of observational data (obtained from spectroscopy and associated with atomic transitions) that scientists had accumulated by now. But it offered an image of a world that is fundamentally different from that of classical mechanics. Today, quantum mechanics has successfully passed a variety of experimental tests, and most researchers are convinced that the ultimate laws of physics are quantum-mechanical in nature.

In this article I will explore the oddities in quantum mechanics, the strange features that bothered Einstein throughout his life. During the Solvay conferences of the late 1920s and early 30s, Einstein tried to expose the “weaknesses” of quantum mechanics. His intellectual adversary was Niels Bohr, who in many ways was its guardian angel. Bohr was able to counter most of Einstein’s criticisms and is widely regarded as the winner of the famous Bohr-Einstein debates that happened nearly 90 years ago. Since then, quantum mechanics not only managed to survive Einstein’s severe critiques but also the subsequent scrutiny of probing experiments to become one of the most successful branches of physics. But there was one clever thought experiment which Einstein famously concocted with two of his Princeton colleagues that was not easy to ignore, whose consequences seemed utterly paradoxical. Dubbed as the “EPR paradox”, this gedanken experiment eventually led to the concept of quantum entanglement. In 1964, physicist John Stewart Bell conclusively proved that contrary to Einstein’s belief, quantum physics was in fact a complete, workable theory. His results, called Bell’s Theorem, effectively proved that quantum properties like entanglement are as real as the moon in the night sky. Today the bizarre behaviors of quantum systems are being exploited for use in a variety of real-world applications.

This article is written for scientists in non-quantum mechanical disciplines as well as for non-scientists. It emphasizes the philosophical aspects of quantum mechanics by avoiding the mathematical side, although I could not resist the temptation of citing a few equations, which, in my view are necessary to illustrate key concepts that are best conveyed through the rigor of an equation instead of words. My goal is to show how the various attempts by scientists to come to grips with philosophical underpinnings of quantum mechanics opened the doors of a new quantum information age. In the words of Roger Penrose “…it was Einstein’s continually innovative probing that has led to burgeoning areas of fascinating and potentially practical research, largely encompassed within the scope of what is now referred to as quantum information theory”.

1. Quoted in Home, D., & Robinson, A. (1995).
2. von Baeyer, (2013).
3. The idea of “Laplace’s Demon” was introduced by the French physicist Pierre-Simon Laplace. It concerns with the idea of determinism, namely the belief that the past completely determines the future. By knowing the precise location and momentum of every atom in the universe, the Demon can calculate from the laws of classical mechanics their past and future values for any given time.

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