Letters

Only in the north It is not clear in the fine article on volcanoes (“Disaster goes global,” SN: 8/30/08, p. 16) how dust from the eruption of Huaynaputina, well south of the equator, in 1600 could affect only the Northern Hemisphere. David Bronson, Biddeford Pool, Maine For one thing, there’s less real estate in the Southern Hemisphere to have been affected. Also, the apparent lack of agricultural effects probably stems from population distribution at the time this eruption popped off. Australia was inhabited only by Aborigines until 1787 or so, and Cape Town, South Africa, wasn’t settled until the mid-1600s. There’s also a dearth of written records for the hemisphere from that time. Research examining tree rings and other climate proxies at Southern Hemisphere sites would probably show effects from the eruption in that half of the world too. — Sid Perkins Not so small Contrary to the title and the first sentence of the article, hydrogen is not “the smallest atom of them all” (“Spotting the smallest atoms,” SN: 8/16/08, p. 7). Atoms with more numerous electrons display decreasing radii. As there is one proton for each electron and because the size of the proton is negligible compared with the atomic radius, the increasing number of protons in the nucleus increases the positive charge of the nucleus, drawing the electrons into tighter orbitals. Thus helium, with two protons, has the smallest atomic radius and is followed by hydrogen. Simcha Pollack, Jamaica, N.Y. The size of atoms can be measured or calculated in different ways, says Jannik Meyer of the University of Ulm in Germany. Under a transmission electron microscope, atoms “look” larger if their nuclei have more electric charge, which makes the atoms scatter the microscope’s electrons. “In our work all that matters is how much the atom scatters electrons,” Meyer says. “Hydrogen is the lightest element, and it has the smallest charge.” — Davide Castelvecchi

Just checking In regards to the phone conversation between Alice and Bob during which they compare the keys they have received from Charlie (“Welcome to the quantum Internet,” SN: 8/16/08, p. 24): If the keys do not agree, Alice and Bob conclude that someone (Eve) has interfered, and it is clear that they cannot use the key. But what if someone listens in on the telephone conversation? Steven Africk, Newton, Mass. Alice and Bob only use a random subset of the strings of 0s and 1s for a spot check. (They agree to check the same subset. Bit No. 7, bit No. 21, bit No. 33 and so on, for example.) Eve doesn’t know in advance which subset they’re going to check. If the spot check shows no signs of eavesdropping, Alice and Bob use the remaining bits — the ones they haven’t compared over the phone — as their key. —Davide Castelvecchi The migrating eye Regarding “The wandering fish eye” (SN: 8/2/08, p. 11): As I understand Darwinian theory, it basically says that a random change in an organism, if it confers a survival advantage, will tend to be passed on to that individual’s offspring, thus gradually increasing the population of that organism with that trait. I can understand why the present one-sided eye position is advantageous for flatfish. But how did the slightly migrated eye confer an advantage? Clark Waite, Hurst, Texas Adaptive questions are always tricky to answer. Some scientists speculate that a slightly migrated fish eye could have benefited fish that did not lie completely flat on the seafloor. These fish could have propped themselves up a bit with their fins, tilting their bodies. In this case, any movement of the eye would give the fish a better view. Another possibility is that a shifted eye would avoid contact with the seafloor after a fish already began lying flat, preventing damage to the eye. In this scenario, only later in evolutionary history would the eye shift become important for improved vision. —Ashley Yeager