Black Hole Evaporation and Complementarity

About twenty years ago Hawking made the remarkable suggestion that the black hole evaporation process will inevitably lead to a fundamental loss of quantum coherence. The mechanism by which the quantum radiation is emitted appears to be insensitive to the detailed history of the black hole, and thus it seems that most of the initial information is lost for an outside observer. However, direct examination of Hawking's original derivation (or any later one) of the black hole emission spectrum shows that one inevitably needs to make reference to particle waves that have arbitrarily high frequency near the horizon as measured in the reference frame of the in-falling matter. This exponential red-shift effect associated with the black hole horizon leads to a breakdown of the usual separation of length scales, and effectively works as a magnifying glass that makes the consequences of the short distance, or rather, high energy physics near the horizon visible at larger scales to an asymptotic observer.