PRIORITIZING POPULATION
In “A Puzzle for the Planet,” Michael E. Webber discusses the need to integrate three key factors (energy, water and food) to make it possible to meet the needs of a growing population. A critical point is that we have to stabilize that population in the first place. As long as it continues to grow, all other efforts are merely stopgaps.

AVI ORNSTEIN
New Britain, Conn.

WEBBER REPLIES: Population growth is indeed important, but it turns out that economic growth is a bigger deal: demand for food, energy and water are growing faster than population because people tend to demand more meat and electricity (both of which are water-intensive) as they are elevated out of poverty. The average Chinese citizen, for instance, consumes about a fourth of the energy of a typical U.S. citizen, and as the former becomes richer, that gap narrows. (Meanwhile urbanization, which reduces birth rates, is increasing.)

Therefore, making sure that people have the energy, water and food they need for a free and prosperous life, without all the environmental and security challenges that plague our old approaches, is the most effective place to start. Plus, the policy levers (investing in new technologies, reinventing markets and pushing for a culture of conservation) for solving this nexus are more straightforward and palatable than population controls, which are objectionable on many levels.

BLACK HOLE BIZARRENESS
I agree with Adam Brown's crazy but fun article—“Can We Mine a Black Hole?”—that it is physically impossible to rapidly mine black holes for energy. There are many other “catches” that he does not mention, however. One is that the more massive a black hole is, the colder it is, and a black hole with a mass greater than about one tenth of that of Earth will have a temperature lower than that of the cosmic microwave background (CMB), which is about 2.73 kelvins. Any black hole with greater mass will therefore gain energy from the CMB and get more massive (and hence, oddly, colder). Only lighter, smaller black holes radiate, getting lighter and hotter as they do so, until they explode in a sudden burst of particles. So find a micron-size black hole but do not go close to it!

MICHAEL ALBROW
Fermi National Accelerator Laboratory

BROWN REPLIES: The CMB is indeed much hotter than a solar-mass black hole, but it won't be for long. Because the universe is expanding, the temperature of the CMB is falling; thanks to dark energy, it is falling exponentially, halving every 10 billion years or so, and will soon (relatively speaking!) be much colder than any black hole.

OUR MURDEROUS ANCESTORS
Kate Wong's suppositions about what brought about Neandertals' extinction in “Neandertal Minds” are contrary to the known history of anatomically modern Homo sapiens (that is, us). Her assertions that Neandertals were just outcompeted and that the 1.5 to 2.1 percent Neandertal DNA within people outside of Africa is the result of occasional “dalliances” would be historically unlikely.

The most likely scenario would involve waves of immigrating anatomically modern humans taking over land and causing death by plunder and disease, as Europeans discovering the New World did. And it would be naive to think that our Neandertal DNA was the result of consensual dalliances when rape went hand in hand with the pillage of every other civilization.

It would be wise for us to give up the notion that we are, or our ancestors before us were, a benevolent and sharing species.r

ROBERT E. MARX
University of Miami Miller School of Medicine

NEANDERTAL SPELLING
The cover of the issue and Wong's article refer to “Neandertals.” So is the spelling for “Neanderthal” now without an “h”?

STEVE LARIOS
via e-mail

THE EDITORS REPLY: German linguistic reforms in the early 1900s changed the spelling of the name of the Neander Valley from “Neander Thal” to “Neander Tal” (the “h” was silent). Today the common name of these extinct humans can be spelled with or without the “h.” SCIENTIFIC AMERICAN has long favored “Neandertal.”

MEMRISTOR NETWORKS
In “Just Add Memory,” Massimiliano Di Ventra and Yuriy V. Pershin talk about how a network of memristors—computing components that change electrical resistance in response to the amount of current and retain that change—can solve a maze problem in one step. They fail to mention that to appropriately “wire” the memristors in the maze, so that an input is connected to an output, each square of it would need to be visited and a memristor placed where needed. Doing so may require more of the maze to be visited than a random drunkard's walk solution or the classic right-hand-to-the-wall solution.

DAVE BRUMLEY
San Diego

THE AUTHORS REPLY: To create a maze, the only thing you need to know is the maze topology—namely the position of the walls and openings. But the knowledge of the topology does not mean that the maze solution is known or even exists. As an example, let us consider a maze drawing in a magazine. The maze solver knows the topology at the outset but not the maze solution and needs time to find it. The same is true for finding the maze solution with memristor networks.

IS VS. OUGHT
A Moral Starting Point,” by Michael Shermer [Skeptic], reminds us that although science and morality depend on each other, they are distinct. Science is about what “is”; morality is about what “ought” to be. When we make moral arguments, we act as humans equipped with a capacity for empathy toward human and nonhuman life and informed by science. We can contrast science-informed, empathy-based morality with religion-based morality, but science is not what makes us moral human beings; empathy is.

ANNE DENTON
Fargo, N.D.

SHERMER REPLIES: The most controversial section of my book The Moral Arc is my assertion that science's description of the way something “is” can tell us what we “ought” to do. Ever since the scientific revolution, when scientists such as Copernicus, Kepler, Galileo and Newton discovered that the world is governed by natural laws that can be understood and used to make predictions and test hypotheses, thinkers in other fields have sought to understand the laws and principles that govern political, economic, legal, social and moral systems. They have then used these tenets to make predictions and test hypotheses about how best we should live. I contend that the “is-ought” fallacy is itself a fallacy.

ERRATA
Neandertal Minds,” by Kate Wong, erroneously describes Gibraltar as the southernmost tip of the Iberian Peninsula. The southernmost part is Cape Tarifa.

A Weakness in Bacteria's Fortress,” by Carl Zimmer [January 2015], incorrectly refers to penicillin grabbing onto a protein that aids in building cell membranes in bacteria. The protein helps to build bacterial cell walls.