End without Horizons?
In “Naked Singularities,” Pankaj S. Joshi argues that models for stellar collapse can produce naked singularities, or singularities without the event horizon that surrounds a black hole. According to quantum theory, black holes emit thermal radiation and evaporate because of the separation of particle-antiparticle pairs near their event horizon. Will a naked singularity ever disappear?
If an event horizon has an extreme but finite spacetime curvature and gravity, and in a singularity these are infinite, whow can there be any path between a low-gravity and curvature region and a singularity without passing through a horizon?
Villa Park, Ill.
Joshi replies: Regarding Chamudot’s question, the event horizon is a crucial factor in the evaporation of a black hole through quantum effects, but in a naked singularity case, it is still possible for the event horizon to disappear or evaporate through quantum or classical processes. The effects of quantum gravity, for example, could generate a huge negative pressure, causing the star to emit most of its mass in late collapse stages. Further, classical processes such as powerful shock formations caused by inhomogeneities in matter densities near the naked singularity could cause it to explode.
With respect to Anderson’s letter, it is not just the local density or curvature values that determine the behavior of light paths in general relativity. Aspects such as the causal structure of spacetime and the global properties of light cones are crucial factors. These factors arise mainly as a result of the nonlinearity of Einsteinian equations, and detailed studies of collapse models imply that gravity can be arbitrarily large and dense in a stellar collapse but still not inescapable. Large density or curvature values do not necessarily mean an event horizon is present.
In Newtonian gravity, density is the sole parameter that determines the behavior of a gravitational field. But in general relativity, there are 10 gravitational metric potentials, and these elements can and do give rise to many novel features for gravity and its interactions in the universe.
Gaming the Neural System
In “Childhood Recovered” [News Scan], Gary Stix notes that adult amblyopia patients have achieved substantial improvements after video game–like exercises. He states that “Grand Theft Auto IV or Medal of Honor may retrain the brain in ways its developers never imagined.”
I found this assertion personally ironic: I was the initial designer and executive producer on the Normandy Beach game that later became the first Medal of Honor and am now working primarily in games that have a function beyond entertainment. Designers are increasingly aware of games’ power to influence neural pathway formation and have designed programs to build cognitive reserve through brain training, to treat attention-deficit hyperactivity disorder and to help patients with Parkinson’s disease. We may not have thought of those applications years ago, but we are learning to rewire our brains to do so now!
Cows and Carbon
“The Greenhouse Hamburger,” by Nathan Fiala, argues that beef production is a major cause of global warming. But the data and articles Fiala cites assume that the total amount of beef produced is all grown in concentrated animal feeding operations (CAFOs). In my beef operation, cows never eat or see a pound of harvested grain. They spend their lives (average of 10 years) eating grass from native pastureland. I am not the exception; all my neighbors raise their cattle the same way.