"Are we particles," Amy wants to know, "or waves?"
.... John Barth, On With the Story

Among the most significant intellectual advances of the twentieth century is "quantum mechanics", a view of the material world unlike any that has existed at any prior time in human history. Quantum mechanics is both sweeping in its implications and sufficiently novel and counter-intuitive so that even those most actively involved in its advancement remain today uncertain about exactly how its essence is best interpreted and conveyed. We here provide a set of experiences aimed at giving you some sense of what is involved in quantum mechanics, so that you too can perhaps participate in the ongoing consideration of how best to think and talk in these new terms, and what their implications are.

Most people comfortably and intuitively deal with and talk about objects. "Particles" are no more and no less than idealizations or abstractions of "objects". They have positions, velocities, bang into and rebound from one another in ways that few would find surprising. "Waves" are, for many people, probably a little harder to make sense of, but one of course knows them as well. A lot of their properties are familiar from ocean beaches and ripples on a pond. So particles, on the one hand, and waves, on the other, yield a picture of the material world with which most people are comfortable. The problem with quantum mechanics is that it requires us to try and imagine something which is simultaneously a wave and a particle.

"Particle/wave duality" is a little hard to swallow, since an obvious question is how can something be both a particle and a wave at the same time? The trick is not to try and imagine some combination of a particle and a wave but rather to conceive of something new, something which would, depending on the circumstances, sometimes exhibit the behaviors of a wave and other times the behaviors of a particle. Richard Feynman, a Nobel Prize physicist, came up with a way to do this that allows one to retain most of the intuitions one has for the behavior of particles (there are other ways to do this which instead retain most of one's intuitions about waves).

In this exhibit, you can work through Feynman's way of thinking about quantum behavior which he summarized as

Explore All Paths!

In addition to providing you the tools to try and develop your own intuitive understanding of quantum behavior along the "explore all paths line", this exhibit will introduce you as well to some other central concepts of quantum mechanics. While retaining much of one's intuitions about particles, Feynman's way of thinking embodies as well two fundamental aspects of quantum mechanics. One is the idea that objects can't generally be described as being at particular places, but rather have to be thought of as having varying probabities of being at a number of different places. And a related idea is that to calculate these probabilities, as one would have to do, for example, to predict the results of interactions of objects, requires the concept of "phase", which can result in processes constructively and destructively interfering with one another.

Feynman's is certainly a different way of thinking about the material world than most people are used to, but so too is any other way of interpreting and conveying quantum mechanics. Does Feynman provide a better understanding of things we thought we understood? What new questions does it raise? Join us and others in thinking about these questions, and leave your thoughts in the Forum Area.