# Schild’s Ladder

## Decoherence

**Decoherence** is a phenomenon that plays a role in many of the events in
*Schild’s Ladder*. Beyond the novel, understanding decoherence is
essential to understanding how classical physics emerges from quantum mechanics.

The basic idea is this: a quantum system, *A*, in isolation, behaves in a
characteristically quantum-mechanical fashion, exhibiting interference
effects that reflect the **phase difference** between the various components of its
state vector. For example, if *A* consists of an electron in a state that is a
superposition of equal parts **spin up** and **spin down**, there will be
measurements that can be performed on the electron that will be sensitive to the phase
relationship between these two components. This is quite different from the classical
notion of probability: there isn’t merely a 50% chance for the electron’s spin to be up
or down; rather, both possibilities exist simultaneously, and the phase describes
a relationship between them that would be meaningless if either was absent.

If system *A* then interacts with another system, *B*, in such a manner that
different components of *A*’s state vector influence *B* differently, the two systems become
**entangled**, and observations on *A* alone will no longer exhibit quantum effects.
System *A* appears to have “collapsed” down to just one component
of its original state vector. In the example of the electron, it now acts as if there
were merely a 50/50 chance for its spin to be either purely up or purely down.

However, no such “collapse” has really taken place. Measurements on the
combined system, *A*+*B*, reveal that it is in a pure quantum state, and none of the original
components of *A*’s state vector have been lost. Classical physics emerges,
essentially, from the inability to observe everything we’d need to in order to
detect quantum phenomena in the world at large.
More details

This applet demonstrates three experiments that
illustrate how quantum behaviour is hidden when a system becomes entangled, but can
be recovered by observations on the complete system.

Schild’s Ladder / Decoherence / created Tuesday, 1 January 2002 / revised Saturday, 5 January 2002
**If you link to this page, please use this URL**: http://www.gregegan.net/SCHILD/Decoherence/Decoherence.html

*Copyright © Greg Egan, 2002. All rights reserved.*