The astute reader may have noticed that in the main article we assiduously—and somewhat sneakily—avoided saying that the interfering quantum particle was actually in two places at once (using such phrases as "seemingly" and "as if"). The reason for using such legalistic-sounding verbiage is that we simply don't know what the particle "really" did.

If we measure which of two ways the particle went, we will only ever measure that it went exactly one of those ways. But how should we interpret the situation when we don't make such a measurement and as a result we end up with an interference pattern? A classical wave can only produce interference if there were waves going in both ways. So does that mean the particle really went both ways? Quantum theory does not tell us.

In some respects, the quantum mechanical description is quite similar to the classical description of interfering waves. The particle is described mathematically by a special quantum kind of wave called a wavefunction. When interference occurs, each particle's wavefunction includes parts corresponding to the particle going each way, much like the way that a classical wave that interferes has parts going both ways. Unfortunately, eight decades after Erwin Schrdinger introduced the idea of a wavefunction, physicists still do not agree about the nature of the physical reality that it describes.

Physicists do agree completely about how to use wavefunctions to predict the results of measurements, sometimes with extraordinary precision and accuracy. But how one should interpret the physical meaning of a wavefunction prior to its being measured remains uncertain—even controversial—and quite likely always will. If the wavefunction describes the particle as being in a superposition of going both ways, does that mean the particle "really" did? No one knows.

For example, according to the so-called Copenhagen interpretation of quantum mechanics, one is simply not allowed to ask what happened in a situation where no measurement was made. In the "many-worlds" interpretation, the particle indeed went both ways. What's more, it still went both ways even if we measured that it went one particular way—in effect the universe bifurcated and in our branch the particle went one way and in the other branch (which has an otherwise identical copy of us) it went the other. And in the Bohm-DeBroglie "guiding-wave" interpretation there is a quantum wave which does go both paths, but the interfering particle is more like a surfer on that wave, choosing a particular (but unknown) path through the slits.

Note that, despite the different models of what happens "behind the curtain," all these interpretations make exactly the same predictions about what will actually be measured. In other words, there is no experimental way to distinguish them and therefore no way to proclaim that one interpretation is "right."

Perhaps it is because of the disquieting features of all of these interpretations that noted physicist Richard Feynman described the double-slit experiment as the true mystery of quantum mechanics.

Curiously, in the new field of quantum computing, quantum physicists seem less hesitant to claim that the qubits (quantum bits) are in a state of being "0" and "1" at the same time. Such statements are completely equivalent to saying that a particle passed through one slit and the other slit simultaneously.

More to Explore:

View the slideshow of quantum erasure in action

Discuss the experiment in the blog

What You Will Need For the Experiment

What Polarizers Do To Photons

How A Quantum Eraser Works

Notes on Polarizing Film

Troubleshooting the Experiment

More Experiments

Answer to the 3-Polarizer Puzzle Featured in the Print Edition

Whither Waves? More About Interference

Cutting-Edge Experiments: Interfering Soccer Balls

Delayed-Choice Experiments

What Do the Quantum Particles Really Do?

What is Being Erased?