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, how 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.
FIALA REPLIES: My numbers assume that all cow production occurs in CAFOs for two reasons. First, CAFOs produce most of the beef consumed in the U.S. The total percentage of CAFO cows is unclear, but in testimony given to the House Judiciary Committee in 2000, the U.S. National Farmers Union's then president Leland Swenson claimed that four companies produce 81 percent of cows in the country. Companies of such size can only be using the CAFO system, which creates large quantities of beef cheaply. More environmentally friendly production systems will never yield enough food for Americans to eat the current amount of beef, almost 100 pounds a year per person.
Second, to meet the demand for increased consumption worldwide, CAFOs are the fastest-growing production method in developing countries, and they most likely are the future of beef production for everyone around the globe.
Also, focusing on CAFOs in many ways actually underestimates cows’ impact. Multiplication of my CAFO figures shows that assuming all animals are grown in CAFOs produces aggregate carbon dioxide (CO2) numbers that are at least one half to one third as large as those from the Food and Agriculture Organization, which found that livestock contribute about 18 percent of world greenhouse emissions.
Pastoral systems can sometimes be responsible for producing more CO2 than CAFOs, mainly because many communities, mostly in Latin America, require deforestation for pastoral land. And CAFO cows live only about one year before slaughter. In the July 1999 Ecological Economics, Susan Subak did find that a good pastoral system generates just more than half the CO2 of a feedlot, but she assumed that the pastoral animals live for just less than three years. If the animals are allowed to live more than three times as long, the difference narrows considerably.
My work is not intended to convert people to vegetarianism, only to help them understand how consumption choices can have major effects on the environment. Given the incredible quantities of meat Americans and others eat, even a small decrease in beef consumption (to, say, three or four times a week) can have a big impact.
In discussing the woes of the U.S. auto industry in “Transforming the Auto Industry” [Sustainable Developments], Jeffrey Sachs misses the essential failing that has led to decline of its long-term market share and sales volume: mediocre cars.
Having worked in the industry in the 1970s and having followed it since then, I observed at least 15 years go by before U.S. auto executives acknowledged their products’ quality deficiencies. They have been playing catch-up since then. American vehicles have been improving, but they still lag. For the U.S. auto industry to truly succeed, management needs a paradigm shift that will result in the best vehicles in their class by any measurement.
Grass Valley, Calif.