Quantum physics, brought to life.

In the physics of everyday objects, waves and particles are completely different things. Particles follow paths whereas waves spread out and interfere. Quantum mechanics was born when Louis de Broglie proposed that particles also have a wave-like aspect which determines how they behave. In standard quantum mechanics, the motion of a particle is described by the evolution of its associated wave under the Schroedinger equation. The intensity of the wave in a region indicates the chance of finding a particle there.

This animation shows a quantum wave hitting two slits in an otherwise impenetrable screen. As the wave passes through the slits, two beams emerge and overlap, producing bright and dark interference bands. The bright points on the screen, appearing one at a time, mark where individual particles arrive, only in regions where the beam is bright. The particles' particle-like nature is revealed in the localised spots; their wave-like nature is revealed in the interference pattern. When a detector records which slit each particle passes through, the interference disappears, as if the unused branch of the wave has collapsed.

Text by Chris Dewdney.

The Copenhagen interpretation forbids any picture of how particles move from source to screen. In stark contrast, Bohm's theory describes the way particles move along trajectories while still accounting for every observation predicted by quantum theory. In the two-slit case, each particle passes through one slit, but its motion is guided by the interfering waves from both slits through the medium of the quantum potential.

Here we see the particles moving over the quantum potential surface, shown in ghostly white, dashing across the valleys and lingering on the plateaux to arrive at the bright regions of the interference pattern. Information about the whole experimental environment is encoded in the form of the quantum potential, which informs the particle how to move. This is the origin of quantum wholeness, the behaviour of individual particles depends sensitively on their whole environmental context.

Text by Chris Dewdney.

A detector is placed between the slits and can tell which slit each particle passes through. When it is switched on, the interference disappears, the paths no longer wriggle into bright fringes. Richard Feynman claimed no hidden variable theory could ever account for this loss of interference, yet Bohm's theory does. The quantum wave function lives in a multidimensional abstract mathematical space known as configuration space.

The box on the right of the animation illustrates a simplified three dimensional configuration space. The horizontal plane represents the two dimensions in which the particle moves through the slits; the vertical dimension represents the detector particle. A single point describes both the slit particle's location and the detector's state. When the detector fires, the trajectories are pulled apart along the detector dimension, the beams no longer overlap in configuration space and the interference vanishes. The real space motion is recovered by projecting from configuration space back into real space.

Text by Chris Dewdney.

The animation shows two possible motions for a single spinning particle moving through a Stern Gerlach device, as calculated in Bohm's theory. The device measures a quantum property known as spin. For electrons, quantum mechanics correctly predicts the probabilities that a given particle will appear in one of two separated spots on the detecting screen. In any given case, quantum mechanics cannot predict which spot the particle will land in.

While passing through the device, the particles have no location according to ordinary quantum mechanics, there are no trajectories. The wave function instead develops two branches corresponding to the two outcomes. Bohm's theory has no measurement problem, it gives a clean account of our definite perceptions. A particle ends up in a particular location because it always has a definite location, following a trajectory from beginning to end. The role of the wave function is to determine how that definite state evolves.

Text by Chris Dewdney.

A two particle quantum system is described by a six dimensional wave function. Under some circumstances this factors into two independent functions, one per particle. In other circumstances it cannot be factored, it is then entangled, and the two particles' behaviour shows correlations that cannot be explained by signals passing between them in three dimensional space, nor by any local hidden variables associated with each particle in the past. From inside our three dimensional view, the result looks like a spooky action at a distance.

The animation shows two spin entangled particles. Two Stern Gerlach devices are widely separated, one on Earth and one on Andromeda for example, and the two entangled particles travel one to each device. When both devices are aligned along the same direction, if one particle is measured to be spin up, the other is always measured to be spin down. Bohm's theory accounts for the correlation through nonlocal variables, the motion of one particle can depend on the motion of the other even though no signal can pass between them. This is probably the deepest mystery of quantum mechanics.

Text by Chris Dewdney.