North

Philosopher Jill North ponders what quantum mechanics has done to reality.

In a post-Einstein world where wave/particles apparently exist in multidimensions, why do we seem to live in a space of only three dimensions?

North, associate professor of philosophy, is a proponent of wave function realism, which posits that quantum mechanics' wave function is real and fundamental, but occupies a space very different from the one we seem to live in, as she writes in her essay “The Structure of a Quantum World,” published in “The Wave Function: Essays on the Metaphysics of Quantum Mechanics” (Oxford University Press, 2013). North defines a quantum mechanic wave function as “the mathematical object that represents the state of a system at a time.”

There is a way of reconciling this high-dimensional wave space with the three-dimensionality of our experience, North says. It requires understanding the dynamical laws that govern the fundamental level of reality.

“When I say that the laws ‘are a guide to the fundamental nature of a world,’” writes North, “I mean that we infer the fundamental nature of a world from the dynamical laws. We do not directly observe the fundamental level of reality; we infer it from the dynamics.”

Physics, she continues, “is first and foremost about how and why physical objects move around and interact with one another, and the dynamical laws are generalizations describing this behavior. We thus infer the fundamental nature of reality from the dynamical laws, which are themselves inferred from the observable behaviors of physical objects.”

By looking at the dynamical laws, we are able to infer the structure that is needed to support the laws. Such an approach provides its own assessment: “A match in structure between the dynamical laws and the world is evidence that we have inferred the correctspace-time structure to a world governed by those laws,” writes North.

Ordinary experience leads us to assume that 3-D space accurately represents the world’s fundamental physical space. But quantum mechanical systems are more complex; the possibility of entanglement in such systems means that the wave function is intrinsic. Since a wave function can’t be broken down into particles in 3-D space but requires “high dimensional” space to contain it, “we are led to conclude that the fundamental space of a world governed by these dynamics is the high-dimensional one,” according to North.

But ordinary 3-D space is not just an illusion, says North. “I think that 3-D space exists, and our talk about it is true. It’s just that this space is nonfundamental. Similarly, ordinary particles exist but are nonfundamental. They are more like tables and chairs: made up out of fundamental stuff, not themselves in the fundamental inventory.” 3-D space is a separate thing that is in some sense “made up out of” the higher-dimensional space, just as particles are ultimately “made up out of” the wave function.

In North’s view, the wave function’s space is fundamental and ultimately responsible for ordinary space, while at the same time allowing for the reality of ordinary space. What we observe, notes North, “is a parochial reflection of our own situation: we are familiar with tables and chairs. It is then no great coincidence that we end up with a fundamental theory that has the power to predict the appearances [of 3-D tables and chairs] for us.”

Linda B. Glaser is staff writer for the College of Arts and Sciences.