Dark matter inspiration
On reading Tom Siegfried’s editorial “Dark matter nothing to fear, if it’s there or not” (SN: 5/19/12, p. 2):
As into the universe I did stare
     I met a particle that wasn’t there
     It wasn’t there again today
     Oh, I wish it would go away.
Tom Derderian, Winthrop, Mass.

Reality bits
Regarding “Bits of reality” (SN: 4/7/12, p. 26): There may be a catch in the hypothesis that the rules of information processing determine physical theory. This idea appears to be based upon axioms such as causality — in essence, the notion that signals from the future cannot affect the present. This may break time symmetry in the equations of quantum mechanics.  In fact, an article by Yakir Aharonov and colleagues (“A time-symmetric formulation of quantum mechanics,” Physics Today, November 2010) addresses this question. Simple experiments have been proposed to demonstrate signaling from future to present. By now, some of them may actually have been performed. If this is ever demonstrated to occur, we may have a historic scientific discovery of great importance.
Richard K. Bernstein, Mamaroneck, N.Y.

Smoking out ammunition
Residue analysis will help identify only the make of ammunition fired — not the gun in which it was fired (“Smoking out clues from gun residue,” SN: 5/5/12, p. 16). And if the ammo was hand-loaded (i.e. “homemade”), all it can do is point to the makers of the various components: bullet, powder and primer.
Walter Weller, online comment

Raman spectroscopy revealed chemical signatures for several different calibers of ammunition. If a suspect was found in possession of a firearm and an analysis of gunshot residue at the scene was from a different caliber gun, the data could rule out the suspect’s gun. Such data alone cannot prove that a specific gun was used at a crime scene, but the researchers speculate that with a large enough library of spectral data it may be possible to link residue to an individual gun. Hand-loaded ammunition could make identification difficult. —Rachel Ehrenberg

Defining alien life
I am disappointed in the tone of “Aliens in Antarctica” (SN: 5/5/12, p. 20). Suppose it could be conclusively determined that some of these organisms reached Antarctica without the influence of humans: Would they still be considered unwelcome invaders, or would their arrival be deemed natural, and therefore acceptable?
Dawn Nelson, Henderson, Nev.