Dark matter

Mystery of the Hidden Cosmos,” by Bogdan A. Dobrescu and Don Lincoln, discusses the possible forms of dark matter.

Could a potential explanation of dark matter not involve new particles? Could, say, gravity decrease with distance slightly less than described in Newton's laws, noticeable only on a scale of light-years? And have scientists tried to quantify this possibility in a way that could explain galaxies' rotational speed without dark matter?

Cornellà del Terri, Spain

The universe is believed to have additional mass, which the article assumes exists in the form of unknown particles. Maybe the search shouldn't be limited to our own universe. We might be merely constrained to a subuniverse of X, Y, Z and T, whereas nearby similar subuniverses exist along a fifth axis of W. A weak,balanced gravitational coupling between the subuniverses could create an increase in the apparent mass of our own structures, undetectable at small scales but significant at very large ones.

Lexington, Ky.

THE AUTHORS REPLY: Regarding Licka's question: The possibility of modifications to the equations relating force and inertia (called MOND, for modified Newtonian dynamics) can explain the rotation curves of galaxies and some other discrepancies but not all of them. New MOND theories still require some residual dark matter to explain observations, so the dark matter hypothesis accurately describes the data with fewer assumptions. Per Occam's razor, the simpler theory is likelier correct.

Both of us once wondered if the scenario Krimm suggests might be true, but it is not if the additional dimension is large. If there exist parallel, overlapping and mostly noninteracting universes, then each one consists of four infinite spatial dimensions. We have measured the dimensional behavior of gravity and have seen that it expands to fill our three spatial dimensions. To look like dark matter, gravity would have to expand into all four dimensions, which would mean that the gravitational force between two objects would decrease as the third power of separation.

Police who kill

In “Outrageous” [Skeptic], Michael Shermer credits the brain's rage circuitry to explain “why cops kill.” But there's a more direct explanation: police can employ lethal force and are trained to use it by default to protect themselves rather than making the safety of the citizenry their highest priority. A partial solution is to equip and train police primarily with nonlethal weapons and require a prime directive that citizens' lives are to be preserved even at the risk of their own safety. Police who are driven to kill unarmed citizens through fear should find other employment.

Fairfax, Va.

I am worried about Shermer's skepticism if it's his understanding that these killings of black men by police are a “rash” of incidents or that prejudice “to whatever extent ... still percolates in the minds of a few cops in a handful of pockets.”

What has always been the harshest, most feared reality for some can now be witnessed by all of us on television. Video cameras are the only new actors in these scenes—which happen all the time nationwide and, if no civilians are around, are still almost always covered up.

Berkeley, Calif.

SHERMER REPLIES: There is no national database on how many people are killed by police each year, and reports by the nation's 18,000 individual police departments are voluntary. With a reporting rate of less than 3 percent, estimates are considered unreliable by scholars who study violence. Thus, using reliable data to assess whether police violence is increasing is not currently possible.

A study by the Washington Post recorded 694 people killed by police by mid-September this year, compared with a crude estimate (possibly an underestimate) of about 400 a year during the past decade. Blacks make up 13.2 percent of the population but 26 percent of this cohort. Updated data, including many more details, can be seen at http://wapo.st/1LFzepU.

It is possible that there is an upward trend line that confirms our intuitions, but an unfortunate by-product of the moral panic in response to the media coverage of the topic is that the police have been hampered from doing their job, and in many cities we have seen an uptick in homicide rates after a 23-year decline. In any case, we need more and better data. In the meantime, police training in how to defuse potentially violent situations and the use of body cameras are a good start.

Under pressure

In reporting on exploration below the West Antarctic Ice Sheet in “Life at Hell's Gate,” Douglas Fox refers to “extreme pressure” exerted on the water by the mass of the ice above the grounding zone. But does not hydrostatics dictate that the pressure of the water in this zone depends only on its depth? The ice above the water should have no influence on the pressure.


El Cerrito, Calif.

FOX REPLIES: The patch of Antarctic coastal seafloor that was explored in January sits about 700 meters below sea level (much of the continent's hidden rock-and-sediment surface sits far below sea level, in part because of the weight of the ice pressing on the earth's crust). Based on that depth alone, the pressure would be about 70 atmospheres.

Depth below sea level does indeed provide a good shortcut for estimating subglacial pressure in coastal areas of Antarctica, where the ice is floating. Further inland, however, where the ice is thick enough that it rests directly on the continent rather than floating, the best way to estimate pressure under the ice is to begin with the thickness of the ice itself, corrected for its lower density relative to water.

Perovskites for solar

Outshining Silicon,” by Varun Sivaram, Samuel D. Stranks and Henry J. Snaith, discusses how perovskite could replace crystalline silicon in the manufacture of solar cells. Was it deliberate that the article avoided explaining what perovskite is?

Lexington, Ky.

SIVARAM REPLIES: Photovoltaic researchers use “perovskite” for compounds that share the crystal structure of a calcium titanate (CaTiO3) mineral that was discovered in Russia in the 19th century.

The solar perovskites we discuss are organic-inorganic hybrid compounds that crystallize from organic halide and metal halide salts to form crystals in an ABX3 structure—A is the organic cation (positively charged ion), B is the metal cation and X is the halide anion (negatively charged ion). Adjusting the chemical composition of the perovskite layer enables researchers to tune the electronic, optical and physical properties of the solar cell.


Blitzkrieg Basics,” by Tim Palucka [Advances], wrongly states that Germany engaged in a blitzkrieg of Stalingrad in 1941. It should have referred to Germany's invasion of the Soviet Union as whole, which began in 1941. The Battle of Stalingrad began in 1942.