The so-called “measurement problem” of quantum physics continues to vex scientists, but our experience of art shows us that “problem” may not be the right word.
The French artist Marcel Duchamp said that the artist does only 50 percent of the work in creating a piece of art; the remaining 50 percent is completed in the viewer’s brain. This latter part is often referred to as “the beholder’s share.”
This concept of subjectivity, inherent to art, contrasts with the traditional view of science as an objective and unbiased accounting of how things really are.
In the hierarchy of sciences, physics is often seen as the ultimate “hard” science, the solid foundation upon which the other sciences rest. But quantum physics blurs these lines. Developed a century ago to explain phenomena that classical physics could not, quantum mechanics challenged the clear-cut realism that many scientists, including Einstein, held dear.
Quantum mechanics suggests that reality, much like art, is not fixed but is influenced by observation. Werner Heisenberg’s uncertainty principle showed that one cannot precisely know both the position and momentum of a particle simultaneously.
Scientific realism
This notion of uncertainty troubled scientific realists, who believed that the properties of objects should exist independently of observation.
Physicalism, materialism, and naturalism (synonymous for our purposes) assert that everything in existence is part of the physical world, governed by physical laws and explainable by science.
This outlook holds that subjective phenomena, such as the experience of art, can ultimately be understood as interactions of matter obeying the laws of physics – brain synapses, chemicals, and stimuli.
Scientific realism goes a step further, positing that the entities described by scientific theories – whether observable or unobservable – exist independently of our perception or understanding. Scientific realists assert that when we talk about fundamental particles, we are referring to real entities in the world, not just abstract concepts.
In Einstein’s time, most physicists were scientific realists who, like him, believed classical physics could be modified or reimagined to incorporate the quantum effects into a single, simple, and beautiful theory of reality that explains every phenomenon.
In 1925, Erwin Schrödinger introduced his namesake equation, which appeared to unify quantum physics into a single mechanical framework. But Heisenberg complicated matters in 1927 when his uncertainty principle demonstrated that quantum mechanics forbids knowledge of both the position and momentum of particles.
Einstein eventually admitted that quantum physics was a logically consistent theory with numerous successes, but maintained that it was not a complete theory. For a realist, a scientific theory must specify an objective reality.
Is the Moon a quantum thing?
A colleague of Einstein’s, Abraham Pais, wrote in a biography: “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.”
Of course, Einstein knew that the Moon, like anything visible to the naked eye, is too big for quantum uncertainty to be relevant, but he was testing logical boundaries. The vexing truth is that the Moon behaves much differently than the elemental particles that make up the Moon.
Although observing the Moon does not change it, observing the particles that create the Moon (or anything else) would affect them. This “measurement problem” of quantum physics is the topic of countless theories and courses and papers.
But modern advancements in quantum measurement theory and experiments repeatedly demonstrate that there is no problem. Like colours in a rainbow, where red seamlessly blends into orange without a distinct boundary, the quantum and the classical are coexistent frameworks without a clear-cut division.
So what is reality, really?
With no definitive “cut” between the quantum and the classical, they represent different extreme aspects of the same underlying experience of humans. The measurement problem presupposes the existence of the dual realities of classical and quantum, but there is no quantum reality – this is precisely what quantum uncertainty rules out.
Reality cannot refer to some collection of things with observer-independent properties. Like art, it is not static but dynamic – a collaborative creation that emerges from the interplay between the observer and the observed.
This will never stop us from inventing realistic analogies – spinning electrons, particulate photons, and so on – as our mental models of this inaccessible part of nature. That’s all we can do to infer the behaviour of isolated quantum systems we periodically probe. When we finally observe them, they cease to be isolated, changing the very conditions which define them.
In the end, while quantum physics may represent the limits of our ability to understand reality on our terms, it does not represent the limit of our imagination.
Chris Ferrie is an associate professor at the University of Technology Sydney and Centre for Quantum Software and Information. He is the author of the successful Baby University series, including the breakout success Quantum Physics for Babies.
