Letters from the November 27, 2004, issue of Science News

Dark Secrets

Astronomers and physicists seem to speak of black holes as though they took matter completely out of the universe (“Information, Please,” SN: 9/25/04, p. 202: Information, Please). An evaporating black hole would not fizz away into nothingness. It would lose energy and reappear in normal space as a very dense object (complete with information). Someone might consider this when discussing quasars.

Nancy Parker
Caldwell, Idaho

There are many obvious ways to destroy information, one of which is thermodynamically irreversible processes. The article says, “Conservation of information, it turns out, is tantamount to saying you can always run a film backward.” But you can’t. That’s what irreversible processes are all about.

John Myers
San Diego, Calif.

The supposed “paradox” of information being in two places at once occurs only when the infalling observer reaches the event horizon. Prior to that, general relativity provides a consistent model for the experiences of both the infalling and the distant observers. The distant observer sees the infalling observer slow so that it would require an infinite amount of time for him to reach the event horizon. The infalling observer would see his partner speed up correspondingly.

Michael McGinnis
Seattle, Wash.

Asked to comment on these and many other letters on this story, Lenny Susskind of Stanford University replied: “I imagine the reader [Nancy Parker] is referring to the discredited ‘remnant theory,’ in which the black hole decays to an ultimate tiny [visible] remnant that contains all the information that ever went into it.” However, “as long as the [black hole] remains significantly larger than [a certain subatomic length], classical relativity would require it to remain a black hole.” The scenario John Myers describes “is true, but it’s really an issue of information getting scrambled, not lost.” In the black hole paradox, the problem is that the information appears to be truly lost, not merely scrambled, yet “the foundations of classical mechanics and statistical mechanics are based on the exact conservation of information.” Finally, “a number of the letters express a very common misconception, namely, that because an outside observer sees an infalling observer slow down, that the infalling observer sees the outsider speed up. This is simply not so. The infalling observer looks back and sees nothing unusual.”—R. Cowen